Measurement of grassland evaporation using a surface-layer scintillometer (original) (raw)
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Measurement of evaporation using a surface-layer scintillometer
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...
Scintillometry for evapotranspiration estimation over irrigated alfalfa and dry grassland
2012
In recent years scintillometry technology has become a recognized tool for the estimation of spatially-averaged vegetative ET rates. In particular, a Large Aperture Scintillometer (LAS) can provide estimates of the surface sensible heat flux (H) and, in combination with measurements of surface net radiation (Rn) and ground heat flux (G), can be used to solve for evaporative heat flux (λE) using the surface energy balance method. The LAS can operate over path lengths between 250 and 4500 m approximately and reports a path-averaged signal whose largest weight is at the path center. Two separate deployments of a Kipp and Zonen model LAS were considered; the first case (alfalfa deployment, site 1) during the 2010 summer over a fully irrigated alfalfa field and the second case (grassland deployment, site 2) at a dry grassland site during the 2011 summer. In each case, ancillary instrumentation was deployed for capturing necessary meteorological inputs and measurements of Rn and G. For th...
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.
Plant Biosystems, 2009
The objective of the present study was to investigate the potential of a large aperture scintillometer (LAS) combined with a simple available energy model to estimate area-averaged latent heat flux in difficult environmental conditions. The difficulties are related to the sparseness of the vegetation, the heterogeneity of the soil characteristics, and, most importantly, the heterogeneity in terms of soil moisture induced by the ''flood irrigation'' method. In this context, three sites (Agdal, R3 and Sâada) in the Tensift Al Haouz plain (region of Marrakech city, central Morocco) have been equipped with a LAS and eddy covariance (EC) system (local scale measurements). Agdal and R3 are a flood-irrigated olive yard and wheat field, respectively. Sâada is a drip-irrigated orange orchard. Due to the irrigation method applied, the Agdal and R3 sites shifted from being almost homogeneous between two irrigations (dry conditions) and completely heterogeneous during the irrigation events (large variability of soil moisture along the site), while Sâada was always heterogeneous, at least at the scintillometer footprint scale. Consequently, the comparison between the sensible heat fluxes derived from both LAS and EC showed a large scatter during the irrigation events, while a good correspondence was found in between two irrigations. It was also found that combining LAS and an estimate of the available energy (using a simple model) can provide reasonable large-scale evapotransipration estimates, which are of prime interest for irrigation management.
Agricultural and Forest Meteorology, 1996
The Penman-Monteith equation is useful for computing evaporation rates of uniform surfaces, such as dense vegetation or bare soil. This equation becomes less useful for evaluation of evaporation rates at the regional scale, where surfaces are generally characterized by a patchy combination of vegetation and soil. This is particularly true in the arid and semi-arid regions of the world. The approach proposed here is an attempt to use remotely-sensed measurements of surface reflectance and temperature to allow application of the Penman-Monteith theory to partiallyvegetated fields without a-priori knowledge of the percent vegetation cover and canopy resistance. Basically, the Penman-Monteith equation was combined with the energy balance equation to estimate the surface temperature CT,) associated with four states: surfaces characterized by full-cover vegetation and bare soil with evaporation rates at potential and zero. Then, linear interpolations between T, values computed for full-cover and bare soil conditions were used to provide information at intermediate states based on measurements of actual surface reflectance and temperature. The approach was first tested using ground-based measurements of surface reflectance and temperature at a rangeland site; the results compared well with on-site measure-* Corresponding author.
Agricultural and Forest …, 2012
Scintillometry is an established technique for determining large areal average sensible heat fluxes. The scintillometer measurement is related to sensible heat flux via Monin-Obukhov similarity theory, which was developed for ideal homogeneous land surfaces. In this study it is shown that judicious application of scintillometry over heterogeneous mixed agriculture on undulating topography yields valid results when compared to eddy covariance (EC). A large aperture scintillometer (LAS) over a 2.4 km path was compared with four EC stations measuring sensible (H) and latent (L v E) heat fluxes over different vegetation (cereals and grass) which when aggregated were representative of the LAS source area. The partitioning of available energy into H and L v E varied strongly for different vegetation types, with H varying by a factor of three between senesced winter wheat and grass pasture. The LAS derived H agrees (one-to-one within the experimental uncertainty) with H aggregated from EC with a high coefficient of determination of 0.94. Chronological analysis shows individual fields may have a varying contribution to the areal average sensible heat flux on short (weekly) time scales due to phenological development and changing soil moisture conditions. Using spatially aggregated measurements of net radiation and soil heat flux with H from the LAS, the areal averaged latent heat flux (L v E LAS ) was calculated as the residual of the surface energy balance. The regression of L v E LAS against aggregated L v E from the EC stations has a slope of 0.94, close to ideal, and demonstrates that this is an accurate method for the landscape-scale estimation of evaporation over heterogeneous complex topography.
The use of scintillometers to determine latent heat fluxes is now common in studies of land-atmosphere interactions due to their ability to calculate the evapotranspiration (ET) over long distances. The scintillometer measurement is related to evapotranspiration via Monin-obukhov similarity Theory (Mosr), which was developed ror iaeat homogeneous land surfaces. Hence, the extending ôf th.r" empirical funciions to sparse and tall vegetation can generate some error in thi estimation àr the evapotranspirâtion.
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...