Evaluation of a MODIS triangle-based evapotranspiration algorithm for semi- arid regions

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 semiarid 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 ETat 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 in the riparian areas to 36 to 76 W∕m 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, 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. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.JRS.7.073493]