Abstract
The current study investigates a MODIS-based remote-sensing-based algorithm for the assessment of daily latent heat flux (LE), or evapotranspiration (ET), with a focus on semi-arid regions. The approach uses the triangle method, relying on remotely sensed inputs and a previously developed net radiation model. A major difference from previous studies is that we utilize only MODIS products for estimation of ET at the daily timestep for clear and cloudy days. The algorithm is evaluated at four flux tower locations in the San Pedro River basin in Arizona. The mean daily LE varies significantly between the sites, ranging from 144 to 179 W/m 2 in the riparian areas to 36 to 76 W/m 2 at the rangeland sites. Comparison of the flux towers shows good correlation and low root mean square error at the riparian sites (0.81 to 0.82 and 37 to 51 W/m 2 , respectively) with slightly larger errors at the upland sites, where ET is strongly correlated to precipitation events. The model assumption of a linear variation in evaporative fraction across the triangular domain (LST/EVI space) results in more uncertainty under water-stressed conditions such as those found at the upland sites. Overall, the proposed MODIS-based algorithm provides reasonable estimates of riparian and upland plant water use and unique spatial and temporal information.
Highlights
Evapotranspiration (ET) is a key parameter in numerous disciplines, including water resource management, agriculture, ecology, and climate change science
The initial net radiation model developed in the Kim and Hogue[23] methodology incorporates the cloud product from MOD08 (1 × 1 deg) using regional fitting coefficients for downward shortwave radiation under cloudy conditions; this study uses the cloud product from MOD06 (1 × 1 km and 5 × 5 km) and does not require regional calibration
The approach is based on the triangle method, which relies on two remotely sensed inputs (EVI and land surface temperature (LST)) and a previously developed net radiation (Rn) model which uses a range of Moderate Resolution Imaging Spectroradiometer (MODIS) products
Summary
Evapotranspiration (ET) is a key parameter in numerous disciplines, including water resource management, agriculture, ecology, and climate change science. ET is arguably one of the most difficult hydrologic components to estimate, given the dependence on a range of climatological parameters (i.e., solar radiation, temperature, wind speed, vapor pressure, etc.) as well as soil physical properties, land cover, and the surrounding environment. Traditional ground-based measurements of land surface heat fluxes (e.g., Bowen ratio system and eddy covariance system) represent a relatively small area or footprint, and spatial interpolation is sometimes used to regionalize these values.[1] A number of ET methods using meteorological and radiosonde observations are available, but the required data are only found typically over small areas of the global land surface. Utilization of reanalysis data has limitations due to its coarser scale.[2,3] Alternatively, remotely sensed data have been recognized as a means to provide broader areal coverage, more frequent estimates, and moderate (or high) spatial resolution.[3,4,5]
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