Michael Mengistu - Academia.edu (original) (raw)

Papers by Michael Mengistu

Water

Crop yield in rainfed agriculture is directly influenced by rainfall patterns, which vary from on... more Crop yield in rainfed agriculture is directly influenced by rainfall patterns, which vary from one growing season to another. The failure or success of such crops can depend on the amount and distribution of the rainfall and, particularly, on the occurrence of dry- and wet-spells during the growing season. The aim of this study was to investigate the initial and conditional probabilities of dry-spell pentads using the Markov chain model in the western maize-growing region of South Africa, as well as to determine the direction and magnitude of dry-spell trends using the Mann–Kendal monotonic trend test and Sen’s slope estimator. The results revealed that all the rainfall districts are affected by dry-spells during the mid-January-to-end-of-February period. This finding is significant because maize is usually planted during late November to late December in this region, and dry-spells may coincide with the flowering stage of the maize crop. When dry-spells occur during the flowering s...

Water, 2020

This research study evaluated the projected future climate and anticipated impacts on water-linke... more This research study evaluated the projected future climate and anticipated impacts on water-linked sectors on the transboundary Limpopo River Basin (LRB) with a focus on South Africa. Streamflow was simulated from two CORDEX-Africa regional climate models (RCMs) forced by the 5th phase of the Coupled Model Inter-Comparison Project (CMIP5) Global Climate Models (GCMs), namely, the CanESM2m and IPSL-CM5A-MR climate models. Three climate projection time intervals were considered spanning from 2006 to 2099 and delineated as follows: current climatology (2006–2035), near future (2036–2065) and end of century future projection (2070–2099). Statistical metrics derived from the projected streamflow were used to assess the impacts of the changing climate on water-linked sectors. These metrics included streamflow trends, low and high flow quantile probabilities, the Standardized Streamflow Index (SSI) trends and the proportion (%) of dry and wet years, as well as drought monitoring indicators...

Journal of Hydrology, 2010

ABSTRACT Evaporation is one of the main components of the energy and water balance of reservoir w... more ABSTRACT Evaporation is one of the main components of the energy and water balance of reservoir water behind dams and is a major component of water loss. Measurements in winter of sensible heat (H) for the small and shallow reservoir of Midmar Dam, KwaZulu-Natal, South Africa were made using the surface renewal (SR) method, a renewal model method that uses the average cubic air temperature structure function and eddy covariance (EC). The small magnitude of H during winter makes it difficult to test the reliability of the H estimates obtained using instruments and sensors mounted above the water surface using the SR, renewal model and EC methods. Latent energy flux was estimated as a residual of the energy balance using additional measurements of net irradiance Rn above the water surface and the water-stored heat flux. The SR sensible heat flux (HSR) was estimated for heights of 1.0, 1.3, 1.9 and 2.5 m above the water surface using two air temperature time lags r of 0.4 and 0.8 s of the 10-Hz measurements. The SR method depends on a weighting factor α which represents the capability of the atmospheric turbulence to mix the scalar, within the air parcel to be renewed. The factor α was determined for each measurement height and time lag from the slope of a linear regression relationship forced through the origin of measured EC sensible heat flux (HEC) values on the y-axis vs.HSR or renewal model H on the x-axis. All α values obtained using the renewal model method were not statistically different from that obtained using the SR method for z = 1.0 m for both time lags. Using a calibration dataset, an average α value for the 1.0- and 1.3-m heights of 0.198 for r = 0.4 s and 0.245 for r = 0.8 s for the SR and renewal model methods was obtained. The 30-min HSR, renewal model and HEC estimates were often the smallest component of the energy balance (generally −40 to 40 W m−2) and compared reasonably well for the validation dataset. The heat storage flux G was larger in magnitude (0–200 W m−2) compared to the sensible heat flux. The SR, renewal model and EC latent energy fluxes, each calculated as residuals of the energy balance, were almost the same in magnitude as the available energy flux Rn − G due to the relatively small magnitude of the sensible heat flux during the winter measurement period. The daily evaporation ranged between 1.0 and 3.9 mm.

Hydrology and Earth System Sciences, 2012

The surface renewal (SR) method was used to determine the long-term (12 months) total evaporation... more The surface renewal (SR) method was used to determine the long-term (12 months) total evaporation (ET) from the Mfabeni Mire with calibration using eddy covariance during two window periods of approximately one week each. The SR method was found to be inexpensive, reliable and with low power requirements for unattended operation. Despite maximum ET rates of up to 6.0 mm day −1 , the average summer (October to March) ET was lower (3.2 mm day −1) due to early morning cloud cover that persisted until nearly midday at times. This reduced the daily available energy, and the ET was lower than expected despite the available water and high average wind speeds. In winter (May to September), there was less cloud cover but the average ET was only 1.8 mm day −1 due to plant senescence. In general ET was suppressed by the inflow of humid air (low vapour pressure deficit) and the comparatively low leaf area index of the wetland vegetation. The accumulated ET over 12 months was 900 mm. Daily ET estimates were compared to the Priestley-Taylor model results and a calibration α = 1.0 (R 2 = 0.96) was obtained for the site. A monthly crop factor (K c) was determined for the standardised FAO-56 Penman-Monteith. However, K c was variable in some months and should be used with caution for daily ET modelling. These results represent not only some of the first longterm measurements of ET from a wetland in southern Africa, but also one of the few studies of actual ET in a subtropical peatland in the Southern Hemisphere. The study provides wetland ecologists and hydrologists with guidelines for the use of two internationally applied models for the estimation of wetland ET within a coastal, subtropical environment and shows that wetlands are not necessarily high water users.

Hydrological Processes, 2012

Total evaporation (ET) is one of the major components of the water budget of a wetland. Very litt... more Total evaporation (ET) is one of the major components of the water budget of a wetland. Very little research has been conducted on the loss of water to the atmosphere from different wetland vegetation types occurring in southern Africa. This study on the ET of taro (locally known as madumbe) and sedge within the Mbongolwane wetland was conducted to assess the potential impact of madumbe cultivation on the hydrology of the wetland. Sugarcane planted on the contributing catchment outside the wetland was the other crop examined. Two field campaigns were conducted in November 2009 and January 2010 during the growing season of the madumbe crop to quantify ET rates in the Mbongolwane wetland and from sugar cane in the surrounding catchment. ET was measured over two vegetation types in the wetland, namely: madumbe (Colocasia esculenta); sedge (Cyperus latifolius) with some reeds (Phragmites australis); and sugarcane in adjacent terrestrial areas. ET from the madumbes ranged from 1.0 to 6.0 mm day À1. The daily average ET rates in November 2009 were 3.5 and 4.9 mm for the madumbe and sedge sites, respectively, and 4.0 mm for sugarcane grown in the catchment. The daily average ET rates in January 2010 were 3.3 and 3.7 mm for the madumbes and sedge sites, respectively, and 2.4 mm for the sugarcane site. The daily ET was therefore lower at the madumbe site in November 2009 and in January 2010 compared to the sedge site. An average crop factor of 0.6 was obtained from this study during the growth stage of the madumbes.

Biomass and Bioenergy, 2013

Global attention on biofuels and the potential for Jatropha curcas to produce biodiesel from marg... more Global attention on biofuels and the potential for Jatropha curcas to produce biodiesel from marginal land with low inputs has recently created worldwide interest in this species. This paper reports on the water dynamics and productivity of J. curcas in a silvopastoral experiment with Pennisetum clandestinum at the Ukulinga research farm of the University of KwaZulu-Natal near Pietermaritzburg, South Africa. Measurements of daily total evaporation rates during December to February (summer) on clear hot days ranged between 3 to 4 mm day-1. However, due to the deciduous nature of the species, water use was negligible (< 1 mm day-1) during winter (May to August). The results have shown that two to four year-old J. curcas trees were conservative water users. High oil yields are unlikely due to the low seed production. The best seed yield was in 2009 (348.8 kg ha-1) in the J. curcas only plots. The other treatments (where pasture competition was a factor) ranged between 77.8 and 166 kg ha-1. Data collected on the time taken by labourers to harvest and dehusk the seeds, showed that one kilogram of seed took approximately three hours to process, suggesting that mechanical harvesting would be necessary to make seed production economically viable. J. curcas showed a low tolerance to pests and was prone to diseases. This significantly increased the input costs for insecticides and fungicides. The South African results are so unequivocal that J. curcas under the experimental conditions does not fulfil the claims that it is a wonderful biodiesel plant.

Water SA, 2010

A dual-beam surface-layer scintillometer (SLS) was used to estimate sensible heat flux (H) every ... more A dual-beam surface-layer scintillometer (SLS) was used to estimate sensible heat flux (H) every 2 min for a path length of either 50 or 101 m, for more than 30 months in a mesic grassland in eastern South Africa. The SLS method relies on Monin-Obukhov similarity theory, the correlation between the laser beam signal amplitude variances and the covariance of the logarithm of the beam signal amplitude measured using 2 laser detectors. Procedures for checking SLS data integrity in real-time are highlighted as are the post-data collection rejection procedures. From the H estimates, using SLS and measurements of soil heat flux and net irradiance, evaporation rates were calculated as a residual of the shortened energy balance equation and compared with grass reference evaporation rates (ETo). Inconsistent hourly ETo values occur in the late afternoon due to the incorrect assumption that the soil heat flux is 10% of net irradiance. The SLS estimates of H and the estimates of evaporation rate as a residual compared favourably with those obtained using the Bowen ratio and eddy co variance methods for cloudless days, cloudy days and days with variable cloud. There was no evidence for the eddy co variance measurements of H being underestimated in comparison to the Bowen ratio and SLS measurements. On many days, the diurnal variation in SLS H was asymmetrical, peaking before noon.

Sustainability

Notwithstanding the dispersed nature of the water, energy and food (WEF) nexus scholarship in the... more Notwithstanding the dispersed nature of the water, energy and food (WEF) nexus scholarship in the African continent, its strategic importance to the African agenda has gained widespread attention in research and planning circles. In this regard, the bibliometric science mapping and content analysis of the WEF nexus scientific publication trends, the conceptual, intellectual and social structures, as well as the inherent paradigmatic shifts in the WEF nexus body of knowledge in the African continent have been undertaken, using the nexus body of literature accessed from the Web of Science and Scopus core collection databases. The review results confirmed that, whilst the WEF nexus scholarship has expanded since 2013, there is also evidence of growth in the conceptual, intellectual and social structures of the WEF nexus in the African continent. These shifts have resulted in the emergence of hot topics (subfields) including modelling and optimization, climate variability and change, en...

South African Journal of Science

Soil moisture is an important hydrological parameter, which is essential for a variety of applica... more Soil moisture is an important hydrological parameter, which is essential for a variety of applications, thereby extending to numerous disciplines. Currently, there are three methods of estimating soil moisture: ground-based (in-situ) measurements; remote sensing based methods and land surface models. In recent years, the cosmic ray probe (CRP), which is an in-situ technique, has been implemented in several countries across the globe. The CRP provides area-averaged soil moisture at an intermediate scale and thus bridges the gap between in-situ point measurements and global satellite-based soil moisture estimates. The aim of this study was to test the suitability of the CRP to provide spatial estimates of soil moisture. The CRP was set up and calibrated in Cathedral Peak Catchment VI. An in-situ soil moisture network consisting of time-domain reflectometry and Echo probes was created in Catchment VI, and was used to validate the CRP soil moisture estimates. Once calibrated, the CRP wa...

Hydrology and Earth System Sciences

A combination of measurement and modelling was used to find a pragmatic solution to estimate the ... more A combination of measurement and modelling was used to find a pragmatic solution to estimate the annual total evaporation from the rare and indigenous Nkazana Peat Swamp Forest (PSF) on the east coast of Southern Africa to improve the water balance estimates within the area. Actual total evaporation (ET_a) was measured during three window periods (between 7 and 9 days each) using an eddy covariance (EC) system on a telescopic mast above the forest canopy. Sap flows of an understory tree and an emergent tree were measured using a low-maintenance heat pulse velocity system for an entire hydrological year (October 2009 to September 2010). An empirical model was derived, describing the relationship between ET_a from the Nkazana PSF and sap-flow measurements. These overlapped during two of the window periods…

Hydrology and Earth System Sciences

The Bowen ratio surface energy balance method is a relatively simple method to determine the late... more The Bowen ratio surface energy balance method is a relatively simple method to determine the latent heat flux and the actual land surface evaporation. The Bowen ratio method is based on the measurement of air temperature and vapour pressure gradients. If these measurements are performed at only two heights, correctness of data becomes critical. In this paper we present the concept of a new measurement method to estimate the Bowen ratio based on vertical dry and wet bulb temperature profiles with high spatial resolution. A short field experiment with distributed temperature sensing (DTS) in a fibre optic cable with 13 measurement points in the vertical was undertaken. A dry and a wetted section of a fibre optic cable were suspended on a 6 m high tower installed over a sugar beet trial plot near Pietermaritzburg (South Africa). Using the DTS cable as a psychrometer, a near continuous observation of vapour pressure and air temperature at 0.20 m intervals was established. These data all...

E Prinsloo (CSIR) for the technical support and assistance during the setting of the experiments.... more E Prinsloo (CSIR) for the technical support and assistance during the setting of the experiments. My thanks also go to Mr SJ Hilcove the owner of the Bellevue farm (Pietermaritzburg), the farm manager Mr H Ovenstone, and the workers of the farm for allowing the use of the facilities and for providing their grassland field available for this work; the Komatiland Plantations for allowing the use of their facilities and for providing the Outeniqua Yellow wood plot; the research staff of the Hluhluwe Game reserve for providing the Chromolaena odorata site and their facilities available for this work. Last but not least, I would like to express my special thanks to my parents and friends for their continuous encouragement and moral support during my study period. Praise, honour, and glory be to the Lord of heaven and earth! iii

Limited water supplies are available to satisfy the increasing demands of crop production. It is ... more Limited water supplies are available to satisfy the increasing demands of crop production. It is therefore very important to conserve the water, which comes as rainfall, and water, which is used in irrigation. A proper irrigation water management system requires accurate, simple, automated, non-destructive method to schedule irrigations. Utilization of infrared thermometry to assess plant water stress provides a rapid, nondestructive, reliable estimate of plant water status which would be amenable to larger scale applications and would overreach some of the sampling problems associated with point measurements. Several indices have been developed to time irrigation. The most useful is the crop water stress index (CWSI), which normalizes canopy to aIr temperature differential measurements, to atmospheric water vapour pressure deficit. A field experiment was conducted at Cedara, KwaZulu-Natal, South Africa, to determine the non-water-stressed baselines, and CWSI of cereal rye (Secale cereale L.) from 22 July to 26 September 2002, and aImual (Italian) ryegrass (Lolium multiflorum Lam.) from October 8 to December 4, 2002, when the crops completely covered the soil. An accurate measurement of canopy to air temperature differential is crucial for the determination of CWSI using the empirical (Idso et al., 1981) and theoretical (Jackson et al., 1981) methods. Calibrations of infrared thermometers, a Vaisala CS500 air temperature and relative humidity sensor and thermocouples were performed, and the reliability of the measured weather data were analysed. The Everest and Apogee infrared thermometers require correction for temperatures less than 15 QC and greater than 35 QC. Although the calibration relationships were highly linearly significant the slopes and intercepts should be corrected for greater accuracy. Since the slopes of the thermocouples and Vaisala CS500 air temperature sensor were statistically different from 1, multipliers were used to correct the readings. The relative humidity sensor needs to be calibrated for RH values less than 25 % and greater than 75 %. The integrity of weather data showed that solar irradiance, net irradiance, wind speed and vapour pressure deficit were measured accurately. Calculated soil heat flux was underestimated and the calculated surface temperature was underestimated for most of the experimental period compared to measured canopy temperature. The CWSI was

Journal of Applied Remote Sensing, 2015

Report to the Water Research Commission on the project "Implementation of bichromatic scintillati... more Report to the Water Research Commission on the project "Implementation of bichromatic scintillation as an operational tool for the estimation of spatially averaged evaporation" ISBN No 1-77005-247-X WRC Report No 1335/1/04 November 2004 Executive summary 1 Motivation 4 Structure of report Chapter 1 introduces the study, emphasising that currently there is no single acceptable method for evaporation measurement or estimation that is capable of providing both good spatial and temporal data of evaporation that is reliable and has adequate resolution. Additional arguments are presented to justify the need for the investigation. An introduction to turbulence and a brief introduction to scintillometry, and the Monin-Obukhov similarity theory (MOST) upon which the SLS method is based, is presented in Chapter 2. The Monin-Obukhov similarity theory is a widely-accepted body of semi-empirical theory of atmospheric turbulence, assuming similarity between fluxes, that allows the dimensionless stability factors for sensible heat flux density to be estimated from the so-called Monin-Obukhov length. In establishing the link between turbulence and energy, the various radiation and energy balance components existing Theory and practice of evaporation measurement, with special focus on vi SLS as an operational tool for the estimation of spatially-averaged evaporation Savage et al. Executive summary at the surface is discussed first. Following this, the Kolmogorov cascade theory of turbulence is discussed. A discussion on eddy structure and stability functions leads to the Monin-Obukhov stability length and the Monin-Obukhov stability theory. Methods for the measurement or estimation of sensible heat flux density, including the scintillometer method are discussed. Most of these methods involve turbulence theory for the estimation of sensible heat flux density and application of the energy balance which in turn allows evaporation to be estimated. The theoretical basis of various methods, with special emphasis on the scintillometer method, for the estimation of sensible heat flux density, is presented. Chapter 3 is devoted solely to the SLS method and the logistics of estimating evaporation from SLS data. The methodology associated with the use of the dual-beam SLS method is discussed in detail. The procedures associated with the crucial step of beam alignment are presented as are the procedures for error-checking the data post-data collection. The seasonal march of the percentage of error-free data is discussed. The assumption of a horizontal beam above a horizontal surface, required by MOST, is also investigated by comparison of non-horizontal beam SLS measurements with simultaneous EC measurements. Also, for a limited time, SLS measurements at a very low beam height were compared with EC measurements. An error analysis is undertaken to determine the worst-case scenario influence of the input data on the measured sensible heat flux density as well as the influence of net irradiance and soil heat flux density on the estimated evaporation. In a sense, these so-called errors reflect the spatial variability in the surface cover. Chapter 4, the bulk of the report, presents the comparisons between the SLS, BR, EC and SR sensible heat flux density measurements and evaporation estimates at various time scales: 2-min, 20-min, and daily. Peculiarities of the collected data such as the asymmetrical nature of the diurnal sensible heat flux density curve and the so-called counter-gradient latent energy flux densities, also receive attention in this chapter. Also, a footprint [which refers to the relative contribution of upwind surface sources to the measured flux density] model is improved upon and used to calculate three key footprint parameters: the peak location of the footprint, the footprint fraction itself and the 90 % fetch to measurement height ratio. the relative contribution of upwind surface sources to the measured flux density. This chapter is also concerned with BR measurements and in particular the replacement of a cooled mirror dew point sensor by a thin film polymer capacitive sensor. Chapter 5 is concerned with seasonal evaporation aspects of the collected data over the 18-month period. The influence of a fire and two mowings at the site on the evaporation estimations is also discussed. Chapter 6 presents the overall findings and recommendations for future research. 5 Results and conclusions The various methods used for the estimation of evaporation are discussed, particularly with reference to their theoretical basis and instrumentation requirements. The operational procedures for use of the surface layer scintillometer (SLS) method is discussed in detail. The procedures for checking the integrity of the data in real-time are highlighted as are the post-data collection rejection procedures. Surface layer scintillometer measurements were performed at the Hay Paddock site near Pietermaritzburg for most of 2003 until November 2004. The system allowed for the real-time Theory and practice of evaporation measurement, with special focus on vii SLS as an operational tool for the estimation of spatially-averaged evaporation Savage et al. Executive summary The raw data has been archived to one DVD (SLS_DATA1) and to a second as a backup (SLS_BUDATA1). The subdirectory structure for the raw data is contained in the file read_me.txt contained in the root subdirectory. All of the SLS data is space delimited. The EC, BR, SR, energy balance and automatic weather station data are comma delimited. The worked data (that is, Excel spreadsheets, files for the graphics created including Plotit and Origin graphics files) has also been archived to one DVD (SLS_DATA2) and to a second as a backup (SLS_BUDATA1). The report itself has been converted to a number of pdf files. 11 Information about this report and data handling Initially the Microsoft Word (2000) word processing software package was used for entering text for this report. In the final report stages, the desktop publishing program Corel Ventura 10 was used. Virtually all of the computer graphics were generated using Plotit 3.1 and version 3.20k (Scientific Programming Enterprises, Haslett, Michigan). TextPad 4.6.2 was used for combining SLS and data logger files. The data required to generate the graphics was copied from Excel to Plotit using the Windows clipboard and saved in Plotit as txt files. The Plotit graphics were either copied to the Windows clipboard and pasted individually directly into Ventura frames or exported from Plotit as wmf files and then imported into Ventura. Some of the scientific graphics were generated using Origin, version 7.5 from OriginLab Corperation and copied to the Windows clipboard and pasted directly in Ventura frames. The report was printed using a Hewlett Packard LaserJet 4 printer at the 600 dpi by 600 dpi resolution. Most of the calculations for this report were performed using MicroSoft Excel (version 2002). List of Figures, Tables, Abbreviations and Symbols and Table of contents Theory and practice of evaporation measurement, with special focus on a SLS as an operational tool for the estimation of spatially-averaged evaporation Savage et al. List of Figures, Tables, Abbreviations and Symbols and Table of Contents 3.5.2.8 Error in calculating latent energy flux density from SLS sensible heat flux density 3.6 Conclusions Chapter Comparison of evaporation measurements using surface layer scintillometry, eddy covariance, Bowen ratio, and surface renewal methods 4.1 Abstract 4.2 Introduction 4.3 Theoretical considerations 4.3.1 Bowen ratio theory 4.3.2 Surface renewal theory 4.3.3 Eddy covariance 4.3.4 Scintillometer theory 4.3.5 Reference evaporation 4.3.6 Footprint estimation 4.4 Material and methods 4.4.1 Bowen ratio laboratory measurements 4.4.2 Bowen ratio field measurements 4.4.3 Surface renewal measurements 4.4.4 Eddy covariance measurements 4.4.5 Scintillometer measurements 4.4.6 Reference evaporation estimation 4.4.7 Footprint analyses 4.5 Results and discussion 4.5.1 Bowen ratio measurements Theory and practice of evaporation measurement, with special focus on x SLS as an operational tool for the estimation of spatially-averaged evaporation Savage et al. List of Figures, Tables, Abbreviations and Symbols and Table of Contents 4.5.1.1 Error analysis of Vaisala HMP45C and CS500 water vapour pressure measurements 4.5.1.2 Use of Vaisala sensors for water vapour pressure profile difference measurements in the laboratory 4.5.1.3 Field use of a Vaisala sensor for water vapour pressure profile difference measurements 4.5.1.4 Practical issues related to BR measurements 4.5.1.5 Conclusions 4.5.2 Analysis of wind direction 4.5.2.1 Daily wind vector analysis 4.5.2.2 Two-minute wind vector analysis 4.5.3 Footprint analyses 4.5.4 Flux comparisons 4.5.4.1 Averaging periods 4.5.4.2 EC (2-min) vs EC (20-min) sensible heat fl ux density comparisons 4.5.4.3 EC and SLS 2-min sensible heat flux density comparisons 4.5.4.4 Two-minute latent energy flux density estimates using EC and SLS methods and the saga of positive latent energy 4.5.4.5 SLS and SR 2-min sensible heat flux density comparisons 4.5.4.6 BR, EC, SR and SLS 20-min sensible heat and latent energy flux density comparisons 4.5.4.7 EC and SLS 30-min sensible heat flux density comparisons 4.5.4.8 SLS and EC sensible heat density comparisons (daily values) 4.5.5 Latent energy density comparisons 4.6 Conclusions Theory and practice of evaporation measurement, with special focus on y SLS as an operational tool for the estimation of spatially-averaged evaporation Savage et al. List of Figures, Tables, Abbreviations and Symbols and Table of Contents Chapter 5 Estimation of seasonal evaporation using surface layer scintillometer and eddy covariance measurements 5.1 Abstract 5.2 Introduction 5.3 Material and methods 5.4 Results and discussion 5.5 Conclusions Chapter 6 Conclusions and recommendations for...

We have used the technique of distributed temperature sensing (DTS) with a fiber optic cable to d... more We have used the technique of distributed temperature sensing (DTS) with a fiber optic cable to determine actual evaporation over land. The results were compared with measurements using a surface layer scintilometer, surface renewal and eddy covariance techniques. Dry and wetted sections of a fiber optic cable were suspended from a six meter high tower in a sugar beet trial in KwaZulu-Natal, South Africa. From the principle of a psychrometer, a near continuous observation of vapor pressure and temperature at 0.20 m intervals of a vertical column of air above the field could be derived. Subsequently it allowed accurate determination of the ratio of sensible and latent heat, i.e. the Bowen ratio over time and in the vertical. Using measurements of the net radiation, soil heat flux and the Bowen ratio sensible heat flux, the actual evaporation could be determined as the residual of the shortened energy balance equation. The advantage of the DTS method over the traditional Bowen ratio m...

A dual-beam surface-layer scintillometer (SLS) was used to estimate sensible heat flux (H) every ... more A dual-beam surface-layer scintillometer (SLS) was used to estimate sensible heat flux (H) every 2 min for a path length of either 50 or 101 m, for more than 30 months in a mesic grassland in eastern South Africa. The SLS method relies on Monin-Obukhov similarity theory, the correlation between the laser beam signal amplitude variances and the covariance of the logarithm of the beam signal amplitude measured using 2 laser detectors. Procedures for checking SLS data integrity in real-time are highlighted as are the post-data collection rejection procedures. From the H estimates, using SLS and measurements of soil heat flux and net irradiance, evaporation rates were calculated as a residual of the shortened energy balance equation and compared with grass reference evaporation rates (ETo). Inconsistent hourly ETo values occur in the late afternoon due to the incorrect assumption that the soil heat flux is 10% of net irradiance. The SLS estimates of H and the estimates of evaporation ra...

The demand for the world’s increasingly scarce water supply is rising rapidly, challenging its av... more The demand for the world’s increasingly scarce water supply is rising rapidly, challenging its availability for agriculture and other environmental uses, especially in water scarce countries, such as South Africa, with mean annual rainfall is well below the world’s average. The implementation of effective and sustainable water resources management strategies is then imperative, to meet these increasingly growing demands for water. Accurate assessment of evaporation is therefore crucial in agriculture and water resources management. Evaporation may be estimated using different micrometeorological methods, such as eddy covariance (EC), Bowen ratio energy balance (BR), surface renewal (SR), flux variance (FV), and surface layer scintillometry (SLS) methods. Despite the availability of different methods for estimating evaporation, each method has advantages and disadvantages, in terms of accuracy, simplicity, spatial representation, robustness, fetch, and cost. Invoking the shortened su...

Water

Crop yield in rainfed agriculture is directly influenced by rainfall patterns, which vary from on... more Crop yield in rainfed agriculture is directly influenced by rainfall patterns, which vary from one growing season to another. The failure or success of such crops can depend on the amount and distribution of the rainfall and, particularly, on the occurrence of dry- and wet-spells during the growing season. The aim of this study was to investigate the initial and conditional probabilities of dry-spell pentads using the Markov chain model in the western maize-growing region of South Africa, as well as to determine the direction and magnitude of dry-spell trends using the Mann–Kendal monotonic trend test and Sen’s slope estimator. The results revealed that all the rainfall districts are affected by dry-spells during the mid-January-to-end-of-February period. This finding is significant because maize is usually planted during late November to late December in this region, and dry-spells may coincide with the flowering stage of the maize crop. When dry-spells occur during the flowering s...

Water, 2020

This research study evaluated the projected future climate and anticipated impacts on water-linke... more This research study evaluated the projected future climate and anticipated impacts on water-linked sectors on the transboundary Limpopo River Basin (LRB) with a focus on South Africa. Streamflow was simulated from two CORDEX-Africa regional climate models (RCMs) forced by the 5th phase of the Coupled Model Inter-Comparison Project (CMIP5) Global Climate Models (GCMs), namely, the CanESM2m and IPSL-CM5A-MR climate models. Three climate projection time intervals were considered spanning from 2006 to 2099 and delineated as follows: current climatology (2006–2035), near future (2036–2065) and end of century future projection (2070–2099). Statistical metrics derived from the projected streamflow were used to assess the impacts of the changing climate on water-linked sectors. These metrics included streamflow trends, low and high flow quantile probabilities, the Standardized Streamflow Index (SSI) trends and the proportion (%) of dry and wet years, as well as drought monitoring indicators...

Journal of Hydrology, 2010

ABSTRACT Evaporation is one of the main components of the energy and water balance of reservoir w... more ABSTRACT Evaporation is one of the main components of the energy and water balance of reservoir water behind dams and is a major component of water loss. Measurements in winter of sensible heat (H) for the small and shallow reservoir of Midmar Dam, KwaZulu-Natal, South Africa were made using the surface renewal (SR) method, a renewal model method that uses the average cubic air temperature structure function and eddy covariance (EC). The small magnitude of H during winter makes it difficult to test the reliability of the H estimates obtained using instruments and sensors mounted above the water surface using the SR, renewal model and EC methods. Latent energy flux was estimated as a residual of the energy balance using additional measurements of net irradiance Rn above the water surface and the water-stored heat flux. The SR sensible heat flux (HSR) was estimated for heights of 1.0, 1.3, 1.9 and 2.5 m above the water surface using two air temperature time lags r of 0.4 and 0.8 s of the 10-Hz measurements. The SR method depends on a weighting factor α which represents the capability of the atmospheric turbulence to mix the scalar, within the air parcel to be renewed. The factor α was determined for each measurement height and time lag from the slope of a linear regression relationship forced through the origin of measured EC sensible heat flux (HEC) values on the y-axis vs.HSR or renewal model H on the x-axis. All α values obtained using the renewal model method were not statistically different from that obtained using the SR method for z = 1.0 m for both time lags. Using a calibration dataset, an average α value for the 1.0- and 1.3-m heights of 0.198 for r = 0.4 s and 0.245 for r = 0.8 s for the SR and renewal model methods was obtained. The 30-min HSR, renewal model and HEC estimates were often the smallest component of the energy balance (generally −40 to 40 W m−2) and compared reasonably well for the validation dataset. The heat storage flux G was larger in magnitude (0–200 W m−2) compared to the sensible heat flux. The SR, renewal model and EC latent energy fluxes, each calculated as residuals of the energy balance, were almost the same in magnitude as the available energy flux Rn − G due to the relatively small magnitude of the sensible heat flux during the winter measurement period. The daily evaporation ranged between 1.0 and 3.9 mm.

Hydrology and Earth System Sciences, 2012

The surface renewal (SR) method was used to determine the long-term (12 months) total evaporation... more The surface renewal (SR) method was used to determine the long-term (12 months) total evaporation (ET) from the Mfabeni Mire with calibration using eddy covariance during two window periods of approximately one week each. The SR method was found to be inexpensive, reliable and with low power requirements for unattended operation. Despite maximum ET rates of up to 6.0 mm day −1 , the average summer (October to March) ET was lower (3.2 mm day −1) due to early morning cloud cover that persisted until nearly midday at times. This reduced the daily available energy, and the ET was lower than expected despite the available water and high average wind speeds. In winter (May to September), there was less cloud cover but the average ET was only 1.8 mm day −1 due to plant senescence. In general ET was suppressed by the inflow of humid air (low vapour pressure deficit) and the comparatively low leaf area index of the wetland vegetation. The accumulated ET over 12 months was 900 mm. Daily ET estimates were compared to the Priestley-Taylor model results and a calibration α = 1.0 (R 2 = 0.96) was obtained for the site. A monthly crop factor (K c) was determined for the standardised FAO-56 Penman-Monteith. However, K c was variable in some months and should be used with caution for daily ET modelling. These results represent not only some of the first longterm measurements of ET from a wetland in southern Africa, but also one of the few studies of actual ET in a subtropical peatland in the Southern Hemisphere. The study provides wetland ecologists and hydrologists with guidelines for the use of two internationally applied models for the estimation of wetland ET within a coastal, subtropical environment and shows that wetlands are not necessarily high water users.

Hydrological Processes, 2012

Total evaporation (ET) is one of the major components of the water budget of a wetland. Very litt... more Total evaporation (ET) is one of the major components of the water budget of a wetland. Very little research has been conducted on the loss of water to the atmosphere from different wetland vegetation types occurring in southern Africa. This study on the ET of taro (locally known as madumbe) and sedge within the Mbongolwane wetland was conducted to assess the potential impact of madumbe cultivation on the hydrology of the wetland. Sugarcane planted on the contributing catchment outside the wetland was the other crop examined. Two field campaigns were conducted in November 2009 and January 2010 during the growing season of the madumbe crop to quantify ET rates in the Mbongolwane wetland and from sugar cane in the surrounding catchment. ET was measured over two vegetation types in the wetland, namely: madumbe (Colocasia esculenta); sedge (Cyperus latifolius) with some reeds (Phragmites australis); and sugarcane in adjacent terrestrial areas. ET from the madumbes ranged from 1.0 to 6.0 mm day À1. The daily average ET rates in November 2009 were 3.5 and 4.9 mm for the madumbe and sedge sites, respectively, and 4.0 mm for sugarcane grown in the catchment. The daily average ET rates in January 2010 were 3.3 and 3.7 mm for the madumbes and sedge sites, respectively, and 2.4 mm for the sugarcane site. The daily ET was therefore lower at the madumbe site in November 2009 and in January 2010 compared to the sedge site. An average crop factor of 0.6 was obtained from this study during the growth stage of the madumbes.

Biomass and Bioenergy, 2013

Global attention on biofuels and the potential for Jatropha curcas to produce biodiesel from marg... more Global attention on biofuels and the potential for Jatropha curcas to produce biodiesel from marginal land with low inputs has recently created worldwide interest in this species. This paper reports on the water dynamics and productivity of J. curcas in a silvopastoral experiment with Pennisetum clandestinum at the Ukulinga research farm of the University of KwaZulu-Natal near Pietermaritzburg, South Africa. Measurements of daily total evaporation rates during December to February (summer) on clear hot days ranged between 3 to 4 mm day-1. However, due to the deciduous nature of the species, water use was negligible (< 1 mm day-1) during winter (May to August). The results have shown that two to four year-old J. curcas trees were conservative water users. High oil yields are unlikely due to the low seed production. The best seed yield was in 2009 (348.8 kg ha-1) in the J. curcas only plots. The other treatments (where pasture competition was a factor) ranged between 77.8 and 166 kg ha-1. Data collected on the time taken by labourers to harvest and dehusk the seeds, showed that one kilogram of seed took approximately three hours to process, suggesting that mechanical harvesting would be necessary to make seed production economically viable. J. curcas showed a low tolerance to pests and was prone to diseases. This significantly increased the input costs for insecticides and fungicides. The South African results are so unequivocal that J. curcas under the experimental conditions does not fulfil the claims that it is a wonderful biodiesel plant.

Water SA, 2010

A dual-beam surface-layer scintillometer (SLS) was used to estimate sensible heat flux (H) every ... more A dual-beam surface-layer scintillometer (SLS) was used to estimate sensible heat flux (H) every 2 min for a path length of either 50 or 101 m, for more than 30 months in a mesic grassland in eastern South Africa. The SLS method relies on Monin-Obukhov similarity theory, the correlation between the laser beam signal amplitude variances and the covariance of the logarithm of the beam signal amplitude measured using 2 laser detectors. Procedures for checking SLS data integrity in real-time are highlighted as are the post-data collection rejection procedures. From the H estimates, using SLS and measurements of soil heat flux and net irradiance, evaporation rates were calculated as a residual of the shortened energy balance equation and compared with grass reference evaporation rates (ETo). Inconsistent hourly ETo values occur in the late afternoon due to the incorrect assumption that the soil heat flux is 10% of net irradiance. The SLS estimates of H and the estimates of evaporation rate as a residual compared favourably with those obtained using the Bowen ratio and eddy co variance methods for cloudless days, cloudy days and days with variable cloud. There was no evidence for the eddy co variance measurements of H being underestimated in comparison to the Bowen ratio and SLS measurements. On many days, the diurnal variation in SLS H was asymmetrical, peaking before noon.

Sustainability

Notwithstanding the dispersed nature of the water, energy and food (WEF) nexus scholarship in the... more Notwithstanding the dispersed nature of the water, energy and food (WEF) nexus scholarship in the African continent, its strategic importance to the African agenda has gained widespread attention in research and planning circles. In this regard, the bibliometric science mapping and content analysis of the WEF nexus scientific publication trends, the conceptual, intellectual and social structures, as well as the inherent paradigmatic shifts in the WEF nexus body of knowledge in the African continent have been undertaken, using the nexus body of literature accessed from the Web of Science and Scopus core collection databases. The review results confirmed that, whilst the WEF nexus scholarship has expanded since 2013, there is also evidence of growth in the conceptual, intellectual and social structures of the WEF nexus in the African continent. These shifts have resulted in the emergence of hot topics (subfields) including modelling and optimization, climate variability and change, en...

South African Journal of Science

Soil moisture is an important hydrological parameter, which is essential for a variety of applica... more Soil moisture is an important hydrological parameter, which is essential for a variety of applications, thereby extending to numerous disciplines. Currently, there are three methods of estimating soil moisture: ground-based (in-situ) measurements; remote sensing based methods and land surface models. In recent years, the cosmic ray probe (CRP), which is an in-situ technique, has been implemented in several countries across the globe. The CRP provides area-averaged soil moisture at an intermediate scale and thus bridges the gap between in-situ point measurements and global satellite-based soil moisture estimates. The aim of this study was to test the suitability of the CRP to provide spatial estimates of soil moisture. The CRP was set up and calibrated in Cathedral Peak Catchment VI. An in-situ soil moisture network consisting of time-domain reflectometry and Echo probes was created in Catchment VI, and was used to validate the CRP soil moisture estimates. Once calibrated, the CRP wa...

Hydrology and Earth System Sciences

A combination of measurement and modelling was used to find a pragmatic solution to estimate the ... more A combination of measurement and modelling was used to find a pragmatic solution to estimate the annual total evaporation from the rare and indigenous Nkazana Peat Swamp Forest (PSF) on the east coast of Southern Africa to improve the water balance estimates within the area. Actual total evaporation (ET_a) was measured during three window periods (between 7 and 9 days each) using an eddy covariance (EC) system on a telescopic mast above the forest canopy. Sap flows of an understory tree and an emergent tree were measured using a low-maintenance heat pulse velocity system for an entire hydrological year (October 2009 to September 2010). An empirical model was derived, describing the relationship between ET_a from the Nkazana PSF and sap-flow measurements. These overlapped during two of the window periods…

Hydrology and Earth System Sciences

The Bowen ratio surface energy balance method is a relatively simple method to determine the late... more The Bowen ratio surface energy balance method is a relatively simple method to determine the latent heat flux and the actual land surface evaporation. The Bowen ratio method is based on the measurement of air temperature and vapour pressure gradients. If these measurements are performed at only two heights, correctness of data becomes critical. In this paper we present the concept of a new measurement method to estimate the Bowen ratio based on vertical dry and wet bulb temperature profiles with high spatial resolution. A short field experiment with distributed temperature sensing (DTS) in a fibre optic cable with 13 measurement points in the vertical was undertaken. A dry and a wetted section of a fibre optic cable were suspended on a 6 m high tower installed over a sugar beet trial plot near Pietermaritzburg (South Africa). Using the DTS cable as a psychrometer, a near continuous observation of vapour pressure and air temperature at 0.20 m intervals was established. These data all...

E Prinsloo (CSIR) for the technical support and assistance during the setting of the experiments.... more E Prinsloo (CSIR) for the technical support and assistance during the setting of the experiments. My thanks also go to Mr SJ Hilcove the owner of the Bellevue farm (Pietermaritzburg), the farm manager Mr H Ovenstone, and the workers of the farm for allowing the use of the facilities and for providing their grassland field available for this work; the Komatiland Plantations for allowing the use of their facilities and for providing the Outeniqua Yellow wood plot; the research staff of the Hluhluwe Game reserve for providing the Chromolaena odorata site and their facilities available for this work. Last but not least, I would like to express my special thanks to my parents and friends for their continuous encouragement and moral support during my study period. Praise, honour, and glory be to the Lord of heaven and earth! iii

Limited water supplies are available to satisfy the increasing demands of crop production. It is ... more Limited water supplies are available to satisfy the increasing demands of crop production. It is therefore very important to conserve the water, which comes as rainfall, and water, which is used in irrigation. A proper irrigation water management system requires accurate, simple, automated, non-destructive method to schedule irrigations. Utilization of infrared thermometry to assess plant water stress provides a rapid, nondestructive, reliable estimate of plant water status which would be amenable to larger scale applications and would overreach some of the sampling problems associated with point measurements. Several indices have been developed to time irrigation. The most useful is the crop water stress index (CWSI), which normalizes canopy to aIr temperature differential measurements, to atmospheric water vapour pressure deficit. A field experiment was conducted at Cedara, KwaZulu-Natal, South Africa, to determine the non-water-stressed baselines, and CWSI of cereal rye (Secale cereale L.) from 22 July to 26 September 2002, and aImual (Italian) ryegrass (Lolium multiflorum Lam.) from October 8 to December 4, 2002, when the crops completely covered the soil. An accurate measurement of canopy to air temperature differential is crucial for the determination of CWSI using the empirical (Idso et al., 1981) and theoretical (Jackson et al., 1981) methods. Calibrations of infrared thermometers, a Vaisala CS500 air temperature and relative humidity sensor and thermocouples were performed, and the reliability of the measured weather data were analysed. The Everest and Apogee infrared thermometers require correction for temperatures less than 15 QC and greater than 35 QC. Although the calibration relationships were highly linearly significant the slopes and intercepts should be corrected for greater accuracy. Since the slopes of the thermocouples and Vaisala CS500 air temperature sensor were statistically different from 1, multipliers were used to correct the readings. The relative humidity sensor needs to be calibrated for RH values less than 25 % and greater than 75 %. The integrity of weather data showed that solar irradiance, net irradiance, wind speed and vapour pressure deficit were measured accurately. Calculated soil heat flux was underestimated and the calculated surface temperature was underestimated for most of the experimental period compared to measured canopy temperature. The CWSI was

Journal of Applied Remote Sensing, 2015

Report to the Water Research Commission on the project "Implementation of bichromatic scintillati... more Report to the Water Research Commission on the project "Implementation of bichromatic scintillation as an operational tool for the estimation of spatially averaged evaporation" ISBN No 1-77005-247-X WRC Report No 1335/1/04 November 2004 Executive summary 1 Motivation 4 Structure of report Chapter 1 introduces the study, emphasising that currently there is no single acceptable method for evaporation measurement or estimation that is capable of providing both good spatial and temporal data of evaporation that is reliable and has adequate resolution. Additional arguments are presented to justify the need for the investigation. An introduction to turbulence and a brief introduction to scintillometry, and the Monin-Obukhov similarity theory (MOST) upon which the SLS method is based, is presented in Chapter 2. The Monin-Obukhov similarity theory is a widely-accepted body of semi-empirical theory of atmospheric turbulence, assuming similarity between fluxes, that allows the dimensionless stability factors for sensible heat flux density to be estimated from the so-called Monin-Obukhov length. In establishing the link between turbulence and energy, the various radiation and energy balance components existing Theory and practice of evaporation measurement, with special focus on vi SLS as an operational tool for the estimation of spatially-averaged evaporation Savage et al. Executive summary at the surface is discussed first. Following this, the Kolmogorov cascade theory of turbulence is discussed. A discussion on eddy structure and stability functions leads to the Monin-Obukhov stability length and the Monin-Obukhov stability theory. Methods for the measurement or estimation of sensible heat flux density, including the scintillometer method are discussed. Most of these methods involve turbulence theory for the estimation of sensible heat flux density and application of the energy balance which in turn allows evaporation to be estimated. The theoretical basis of various methods, with special emphasis on the scintillometer method, for the estimation of sensible heat flux density, is presented. Chapter 3 is devoted solely to the SLS method and the logistics of estimating evaporation from SLS data. The methodology associated with the use of the dual-beam SLS method is discussed in detail. The procedures associated with the crucial step of beam alignment are presented as are the procedures for error-checking the data post-data collection. The seasonal march of the percentage of error-free data is discussed. The assumption of a horizontal beam above a horizontal surface, required by MOST, is also investigated by comparison of non-horizontal beam SLS measurements with simultaneous EC measurements. Also, for a limited time, SLS measurements at a very low beam height were compared with EC measurements. An error analysis is undertaken to determine the worst-case scenario influence of the input data on the measured sensible heat flux density as well as the influence of net irradiance and soil heat flux density on the estimated evaporation. In a sense, these so-called errors reflect the spatial variability in the surface cover. Chapter 4, the bulk of the report, presents the comparisons between the SLS, BR, EC and SR sensible heat flux density measurements and evaporation estimates at various time scales: 2-min, 20-min, and daily. Peculiarities of the collected data such as the asymmetrical nature of the diurnal sensible heat flux density curve and the so-called counter-gradient latent energy flux densities, also receive attention in this chapter. Also, a footprint [which refers to the relative contribution of upwind surface sources to the measured flux density] model is improved upon and used to calculate three key footprint parameters: the peak location of the footprint, the footprint fraction itself and the 90 % fetch to measurement height ratio. the relative contribution of upwind surface sources to the measured flux density. This chapter is also concerned with BR measurements and in particular the replacement of a cooled mirror dew point sensor by a thin film polymer capacitive sensor. Chapter 5 is concerned with seasonal evaporation aspects of the collected data over the 18-month period. The influence of a fire and two mowings at the site on the evaporation estimations is also discussed. Chapter 6 presents the overall findings and recommendations for future research. 5 Results and conclusions The various methods used for the estimation of evaporation are discussed, particularly with reference to their theoretical basis and instrumentation requirements. The operational procedures for use of the surface layer scintillometer (SLS) method is discussed in detail. The procedures for checking the integrity of the data in real-time are highlighted as are the post-data collection rejection procedures. Surface layer scintillometer measurements were performed at the Hay Paddock site near Pietermaritzburg for most of 2003 until November 2004. The system allowed for the real-time Theory and practice of evaporation measurement, with special focus on vii SLS as an operational tool for the estimation of spatially-averaged evaporation Savage et al. Executive summary The raw data has been archived to one DVD (SLS_DATA1) and to a second as a backup (SLS_BUDATA1). The subdirectory structure for the raw data is contained in the file read_me.txt contained in the root subdirectory. All of the SLS data is space delimited. The EC, BR, SR, energy balance and automatic weather station data are comma delimited. The worked data (that is, Excel spreadsheets, files for the graphics created including Plotit and Origin graphics files) has also been archived to one DVD (SLS_DATA2) and to a second as a backup (SLS_BUDATA1). The report itself has been converted to a number of pdf files. 11 Information about this report and data handling Initially the Microsoft Word (2000) word processing software package was used for entering text for this report. In the final report stages, the desktop publishing program Corel Ventura 10 was used. Virtually all of the computer graphics were generated using Plotit 3.1 and version 3.20k (Scientific Programming Enterprises, Haslett, Michigan). TextPad 4.6.2 was used for combining SLS and data logger files. The data required to generate the graphics was copied from Excel to Plotit using the Windows clipboard and saved in Plotit as txt files. The Plotit graphics were either copied to the Windows clipboard and pasted individually directly into Ventura frames or exported from Plotit as wmf files and then imported into Ventura. Some of the scientific graphics were generated using Origin, version 7.5 from OriginLab Corperation and copied to the Windows clipboard and pasted directly in Ventura frames. The report was printed using a Hewlett Packard LaserJet 4 printer at the 600 dpi by 600 dpi resolution. Most of the calculations for this report were performed using MicroSoft Excel (version 2002). List of Figures, Tables, Abbreviations and Symbols and Table of contents Theory and practice of evaporation measurement, with special focus on a SLS as an operational tool for the estimation of spatially-averaged evaporation Savage et al. List of Figures, Tables, Abbreviations and Symbols and Table of Contents 3.5.2.8 Error in calculating latent energy flux density from SLS sensible heat flux density 3.6 Conclusions Chapter Comparison of evaporation measurements using surface layer scintillometry, eddy covariance, Bowen ratio, and surface renewal methods 4.1 Abstract 4.2 Introduction 4.3 Theoretical considerations 4.3.1 Bowen ratio theory 4.3.2 Surface renewal theory 4.3.3 Eddy covariance 4.3.4 Scintillometer theory 4.3.5 Reference evaporation 4.3.6 Footprint estimation 4.4 Material and methods 4.4.1 Bowen ratio laboratory measurements 4.4.2 Bowen ratio field measurements 4.4.3 Surface renewal measurements 4.4.4 Eddy covariance measurements 4.4.5 Scintillometer measurements 4.4.6 Reference evaporation estimation 4.4.7 Footprint analyses 4.5 Results and discussion 4.5.1 Bowen ratio measurements Theory and practice of evaporation measurement, with special focus on x SLS as an operational tool for the estimation of spatially-averaged evaporation Savage et al. List of Figures, Tables, Abbreviations and Symbols and Table of Contents 4.5.1.1 Error analysis of Vaisala HMP45C and CS500 water vapour pressure measurements 4.5.1.2 Use of Vaisala sensors for water vapour pressure profile difference measurements in the laboratory 4.5.1.3 Field use of a Vaisala sensor for water vapour pressure profile difference measurements 4.5.1.4 Practical issues related to BR measurements 4.5.1.5 Conclusions 4.5.2 Analysis of wind direction 4.5.2.1 Daily wind vector analysis 4.5.2.2 Two-minute wind vector analysis 4.5.3 Footprint analyses 4.5.4 Flux comparisons 4.5.4.1 Averaging periods 4.5.4.2 EC (2-min) vs EC (20-min) sensible heat fl ux density comparisons 4.5.4.3 EC and SLS 2-min sensible heat flux density comparisons 4.5.4.4 Two-minute latent energy flux density estimates using EC and SLS methods and the saga of positive latent energy 4.5.4.5 SLS and SR 2-min sensible heat flux density comparisons 4.5.4.6 BR, EC, SR and SLS 20-min sensible heat and latent energy flux density comparisons 4.5.4.7 EC and SLS 30-min sensible heat flux density comparisons 4.5.4.8 SLS and EC sensible heat density comparisons (daily values) 4.5.5 Latent energy density comparisons 4.6 Conclusions Theory and practice of evaporation measurement, with special focus on y SLS as an operational tool for the estimation of spatially-averaged evaporation Savage et al. List of Figures, Tables, Abbreviations and Symbols and Table of Contents Chapter 5 Estimation of seasonal evaporation using surface layer scintillometer and eddy covariance measurements 5.1 Abstract 5.2 Introduction 5.3 Material and methods 5.4 Results and discussion 5.5 Conclusions Chapter 6 Conclusions and recommendations for...

We have used the technique of distributed temperature sensing (DTS) with a fiber optic cable to d... more We have used the technique of distributed temperature sensing (DTS) with a fiber optic cable to determine actual evaporation over land. The results were compared with measurements using a surface layer scintilometer, surface renewal and eddy covariance techniques. Dry and wetted sections of a fiber optic cable were suspended from a six meter high tower in a sugar beet trial in KwaZulu-Natal, South Africa. From the principle of a psychrometer, a near continuous observation of vapor pressure and temperature at 0.20 m intervals of a vertical column of air above the field could be derived. Subsequently it allowed accurate determination of the ratio of sensible and latent heat, i.e. the Bowen ratio over time and in the vertical. Using measurements of the net radiation, soil heat flux and the Bowen ratio sensible heat flux, the actual evaporation could be determined as the residual of the shortened energy balance equation. The advantage of the DTS method over the traditional Bowen ratio m...

A dual-beam surface-layer scintillometer (SLS) was used to estimate sensible heat flux (H) every ... more A dual-beam surface-layer scintillometer (SLS) was used to estimate sensible heat flux (H) every 2 min for a path length of either 50 or 101 m, for more than 30 months in a mesic grassland in eastern South Africa. The SLS method relies on Monin-Obukhov similarity theory, the correlation between the laser beam signal amplitude variances and the covariance of the logarithm of the beam signal amplitude measured using 2 laser detectors. Procedures for checking SLS data integrity in real-time are highlighted as are the post-data collection rejection procedures. From the H estimates, using SLS and measurements of soil heat flux and net irradiance, evaporation rates were calculated as a residual of the shortened energy balance equation and compared with grass reference evaporation rates (ETo). Inconsistent hourly ETo values occur in the late afternoon due to the incorrect assumption that the soil heat flux is 10% of net irradiance. The SLS estimates of H and the estimates of evaporation ra...

The demand for the world’s increasingly scarce water supply is rising rapidly, challenging its av... more The demand for the world’s increasingly scarce water supply is rising rapidly, challenging its availability for agriculture and other environmental uses, especially in water scarce countries, such as South Africa, with mean annual rainfall is well below the world’s average. The implementation of effective and sustainable water resources management strategies is then imperative, to meet these increasingly growing demands for water. Accurate assessment of evaporation is therefore crucial in agriculture and water resources management. Evaporation may be estimated using different micrometeorological methods, such as eddy covariance (EC), Bowen ratio energy balance (BR), surface renewal (SR), flux variance (FV), and surface layer scintillometry (SLS) methods. Despite the availability of different methods for estimating evaporation, each method has advantages and disadvantages, in terms of accuracy, simplicity, spatial representation, robustness, fetch, and cost. Invoking the shortened su...