Combining the Penman-Monteith equation with measurements of surface temperature and reflectance to estimate evaporation rates of semiarid grassland (original) (raw)
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Remote Sensing, 2014
Rangeland monitoring services require the capability to investigate vegetation condition and to assess biomass production, especially in areas where local livelihood depends on rangeland status. Remote sensing solutions are strongly recommended, where the systematic acquisition of field data is not feasible and does not guarantee properly describing the spatio-temporal dynamics of wide areas. Recent research on semi-arid rangelands has focused its attention on the evaporative fraction (EF), a key factor to estimate evapotranspiration (ET) in the energy balance (EB) algorithm. EF is strongly linked to the vegetation water status, and works conducted on eddy covariance towers used this parameter to increase the performances of satellite-based biomass estimation. In this work, a method to estimate EF from MODIS products, originally developed for evapotranspiration estimation, is tested and evaluated. Results show that the EF estimation from low spatial resolution over wide semi-arid area is feasible. Estimated EF resulted in being well correlated to field ET measurements, and the spatial patterns of EF maps are in agreement with the well-known climatic and landscape Sahelian features. The preliminary test on rangeland biomass production shows that satellite-retrieved EF as a water availability factor significantly increased the capacity of a remote sensing operational product to detect the variability of the field biomass measurements.
Hourly and Daytime Evapotranspiration from Grassland Using Radiometric Surface Temperatures
Agronomy Journal, 2004
Determination of evapotranspiration (E) is needed for many appliter availability. cations in agriculture, hydrology, and meteorology. The spatial variability of leaf area index (LAI) and soil water availability makes it Evapotranspiration (expressed in this paper as a laimpractical to model E over heterogeneous lands using ground-based tent heat flux by multiplying the mass of water evapotechniques. Remote sensing can be a good source for both LAI and rated per unit surface area by the latent heat of evaporaradiometric surface temperature (T s) estimates. However, remotely tion) and surface temperature are linked through the sensed soil moisture content is not suitable for E prediction. In this land surface energy budget: study, we propose a procedure to estimate E using T s. The method uses a dimensionless temperature ⌬ T , defined as (T s-T a)/(T max-T a), R n ϭ H ϩ E [1] where T a is the air temperature and T max is the surface temperature (e.g., Brutsaert, 1982) where R n (W m Ϫ2) is the net inthat would occur if all the net radiation (R n) was converted to sensible coming radiation minus the heat flux into the ground heat flux and no evaporation occurred. This approach has been tested (W m Ϫ2) and H (W m Ϫ2) and E (W m Ϫ2) are the sensible on data from two grassland sites in Oklahoma and Kansas. Root mean and latent (evaporative) heat fluxes into the atmosquare differences between hourly predicted and measured E ranged from 30 to 50 W m Ϫ2. The slope and r 2 for the zero-intercept linear sphere, respectively. For the energy balance to close, regression between hourly estimated and measured E ranged from any part of R n that does not contribute to E must be 1.01 to 1.37 and 78 to 0.94, respectively. Daytime conservation of converted into H. In order for that to happen, the surevaporative fraction (EF ϭ E/R n) was used to extrapolate from hourly face has to have the right temperature (T s). This temperto daytime E. The slope and r 2 of the linear regression between ature is called the aerodynamic surface temperature and daytime estimated and measured E ranged from 0.89 to 1.07 and 0.69 will be discussed later. Although soil water availability to 0.9, respectively. These results demonstrate that, for grassland, the is crucial in controlling E, T s can be used as an indicator model may give good estimates of E when T a and T s are available. of E as will be shown in the Theory section. In some agricultural applications, daily evapotranspiration is often needed more than instantaneous rates.
Actual evaporation estimation from infrared measurement of soil surface temperature
Journal of Agricultural Engineering, 2013
Within the hydrological cycle, actual evaporation represents the second most important process in terms of volumes of water transported, second only to the precipitation phenomena. Several methods for the estimation of the Ea were proposed by researchers in scientific literature, but the estimation of the Ea from potential evapotranspiration often requires the knowledge of hard-to-find parameters (e.g.: vegetation morphology, vegetation cover, interception of rainfall by the canopy, evaporation from the canopy surface and uptake of water by plant roots) and many existing database are characterized by missing or incomplete information that leads to a rough estimation of the actual evaporation amount. Starting from the above considerations, the aim of this study is to develop and validate a method for the estimation of the Ea based on two steps: i) the potential evaporation estimation by using the meteorological data (i.e. Penman-Monteith); ii) application of a correction factor based on the infrared soil surface temperature measurements. The dataset used in this study were collected during two measurement campaigns conducted both in a plain testing site (Grugliasco, Italy), and in a mountain SouthEast facing slope (Cogne, Italy). During those periods, hourly measurement of air temperature, wind speed, infrared surface temperature, soil heat flux, and soil water content were collected. Results from the dataset collected in the two testing sites show a good agreement between the proposed method and reference methods used for the Ea estimation.
EQUILIBRIUM EVAPORATION AND POSITWE EVAPORATION-SURFACE TEMPERATURE RELATIONSHIPS ACROSS A GRASSLAND
Journal of the American Water Resources Association, 1999
Sensible (H) and latent (LE) heat fluxes, soil moisture (SM) and surface temperatures (T8) were analyzed from seven sites at FIFE to evaluate relationships among the spatial variability of evaporative fraction, EF, SM, and the diurnal surface temperature range (Tar). Intersite correlations between EF and Tdr were significantly negative for regional average soil moisture SM < 20 percent, insignificant for 20 < SM < 27 percent, and slightly positive for SMr> 27 percent. Statistical analysis of the pooled correlation coefficient between EF and Tdr for SMr < 20 percent indicates that it is less than zero at a very high level of significance, while the pooled correlation coefficient for regional SMr > 27 percent is greater than zero at the 10 percent level. The positive EF:Td correlations are attributed to increased surface vapor pressure at warmer sites under nearly potential conditions. These results suggest that to characterize the spatial variability of the energy budget partitioning, a variable representing the thermal response of the site should be included. An important application of these findings relates to modeling the subgrid variability of a region by subdividing the region into a few classes within which surface variables and parameters are assumed invariant. The thermal response of the surface should be included as a variable in defining these classes.
Water SA, 2018
Accurately measuring evapotranspiration (ET) is important in the context of global atmospheric changes and for use with climate models. Direct ET measurement is costly to apply widely and local calibration and validation of ET models developed elsewhere improves confidence in ET derived from such models. This study sought to compare the performance of the Penman-Monteith-Leuning (PML) and Penman-Monteith-Palmer (PMP) ET models, over mesic grasslands in two study sites in South Africa. The study used routine meteorological data from a scientific-grade automatic weather station (AWS) to apply the PML and PMP models. The PML model was calibrated at one site and validated in both sites. On the other hand, the PMP model does not require calibration and hence it was validated in both sites. The models were validated using ET derived from a large aperture scintillometer (LAS). The PML model performed well at both sites with root mean square error (RMSE) within 20% of the mean daily observe...
Testing three approaches to estimate soil evaporation through a dry soil layer in a semi-arid area
Journal of Hydrology, 2018
Bare soils and grasslands in arid and semi-arid conditions constitute a large portion of the earth surface. Evaporation, which is the main component of the water balance in these conditions, often takes place through a dry soil layer (DSL). There is no scientific agreement yet on the DSL effects on evaporation rates. The implementations of three conceptual models of DSL-evaporation were tested for the simulation of evaporation rates in a semi-arid study area in Central Spain: (i) the daily-average model, based on the assumption that the daily average vapour transport in a DSL can be represented in analogy to isothermal liquid flow; (ii) the numerical model solving the Richards equation, in this case HYDRUS1D was used; and (iii) the pore-scale model, based on soil column experiments in laboratory conditions. The evaporation rates estimated by the three conceptual models for semi-arid field conditions were compared with the evaporation rates measured by an eddy covariance tower in the same area. The results indicate that the daily-average conceptual model assumption, in which the DSL has no effects on evaporation, does not hold in very dry conditions. The numerical model solving the Richards equation was not able to simulate the effects of the DSL on evaporation rates. The evaporation estimates obtained by the pore-scale conceptual model were closest to the eddy covariance measurements during the dry season, however this model was applicable only to the relatively steady evaporation conditions during afternoons and only assuming spatially constant DSL thickness.
Hydrology and Earth System Sciences, 2014
Instantaneous evapotranspiration rates and surface water stress levels can be deduced from remotely sensed surface temperature data through the surface energy budget. Two families of methods can be defined: the contextual methods, where stress levels are scaled on a given image between hot/dry and cool/wet pixels for a particular vegetation cover, and single-pixel methods, which evaluate latent heat as the residual of the surface energy balance for one pixel independently from the others. Four models, two contextual (S-SEBI and a modified triangle method, named VIT) and two single-pixel (TSEB, SEBS) are applied over one growing season (December-May) for a 4 km × 4 km irrigated agricultural area in the semi-arid northern Mexico. Their performance, both at local and spatial standpoints, are compared relatively to energy balance data acquired at seven locations within the area, as well as an uncalibrated soilvegetation-atmosphere transfer (SVAT) model forced with local in situ data including observed irrigation and rainfall amounts. Stress levels are not always well retrieved by most models, but S-SEBI as well as TSEB, although slightly biased, show good performance. The drop in model performance is observed for all models when vegetation is senescent, mostly due to a poor partitioning both between turbulent fluxes and between the soil/plant components of the latent heat flux and the available energy. As expected, contextual methods perform well when contrasted soil moisture and vegetation conditions are encountered in the same image (therefore, especially in spring and early summer) while they tend to exaggerate the spread in water status in more homogeneous conditions (especially in winter). Surface energy balance models run with available remotely sensed products prove to be nearly as accurate as the uncalibrated SVAT model forced with in situ data.
Agricultural Water Management, 2006
Accurate measurements of the turbulent exchanges of mass and energy at the land surface are necessary for a good understanding of the various components of the hydrological cycle. The two most commonly used methods to measure evapotranspiration rates are the Bowen ratio energy balance (BREB) and the Eddy correlation (EC) methods. These methods are applicable when a number of requirements, mostly with respect to terrain topography and homogeneous fetch extension, are fulfilled. On the other hand, meteorological variables can be used to calculate evapotranspiration rates. The two most frequently used methods for this purpose are the Penman-Monteith (PM) combination equation and the Priestley-Taylor (PT) approximation. The objective of this paper is to compare these different methods under non-ideal conditions, more specifically for a wet sloping grassland. The BREB-based and EC-based latent heat fluxes are intercompared, and a good agreement between the estimates from both methods is found. A comparison between the results of the PM and PT methods and the measured latent heat fluxes is then done. A strong annual cycle in the calculated values for the PT alfa factor (a), with a mean annual average value of 1.21 AE 0.79, has been observed. This annual cycle is related to the annual cycle of the humidity of the soil, which can be evaluated by either the soil moisture or the vapor deficit of the air. A strong annual cycle in the inverted surface resistances has also been observed. A relationship between the inverted surface resistances and a has been found, in which the highest values for a coincide with low surface resistances, and vice versa. The results indicate that the methods to www.elsevier.com/locate/agwat Agricultural Water Management 82 (2006) 1-24 measure and calculate latent heat fluxes are in a good agreement, and that imposing an annual cycle in the surface resistance and a leads to an improvement in the estimated evapotranspiration rates. The results suggest that it is possible to measure or model evapotranspiration rates in situations where the theoretical requirements (more specifically a non-sloping surface) are not met. #
Measurement of grassland evaporation using a surface-layer scintillometer
Water SA (Online …, 2010
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.
Knowledge of soil-vegetation-atmosphere energy exchange processes is essential for examining the response of agriculture to changes in climate in both the short and long term. However, there are relatively few sites where all the flux measurements necessary for evaluating these responses are available; where they exist, data are often incomplete and/or of limited duration. At the same time, there is often an extensive observation network available that has gathered key meteorological data (sunshine, wind, rainfall, etc.) over decades. Simulating the terms of the surface energy balance (SEB) using available meteorological, soil and vegetation data can improve our understanding of how agricultural systems respond to climate and how this response will vary spatially. Here, we employ a physically-based scheme to simulate the SEB fluxes over a mid-latitude, maritime temperate environment using routine weather observations. The latent heat flux is a critical SEB term as it incorporates the response of the plant to environmental conditions including available energy and soil water. This response is represented in modeling schemes through surface resistance (r s), which is usually expressed as a function of near-surface water vapor alone. In this study, we simulate the SEB over two grassland sites, where eddy flux observations are available, representing imperfectly-and poorly-drained soils. We employ three different formulations of r s , representing varying degrees of sophistication, to estimate the surface fluxes. Due to differences in soil moisture characteristics between the sites, we ultimately focused our attention on an r s formulation that accounted for soil water retention capacity, based on the Jarvis conductance model; the results at both hourly and daily intervals are in good agreement, with RMSE values of ≈ 40 W m −2 for sensible and latent heat fluxes at both sites. The findings show the potential value of using routine weather observations to generate the SEB where flux observations are not available and the importance of soil properties in estimating surface fluxes. These findings could contribute to the assessment of past and future climate change on grassland ecosystems.