Evapotranspiration (ET) is a key component of the hydrologic cycle, accounting for~70% of precipitation in the conterminous U.S. (CONUS), but it has been a challenge to predict accurately across different spatio-temporal scales. The increasing availability of remotely sensed data has led to significant advances in the frequency and spatial resolution of ET estimates, derived from energy balance principles with variables such as temperature used to estimate surface latent heat flux. Although remote sensing methods excel at depicting spatial and temporal variability, estimation of ET independently of other water budget components can lead to inconsistency with other budget terms. Methods that rely on ground-based data better constrain long-term ET, but are unable to provide the same temporal resolution. Here we combine long-term ET estimates from a water-balance approach with the SSEBop (operational Simplified Surface Energy Balance) remote sensing-based ET product for 2000-2015. We test the new combined method, the original SSEBop product, and another remote sensing ET product (MOD16) against monthly measurements from 119 flux towers. The new product showed advantages especially in non-irrigated areas where the new method showed a coefficient of determination R 2 of 0.44, compared to 0.41 for SSEBop or 0.35 for MOD16. The resulting monthly data set will be a useful, unique contribution to ET estimation, due to its combination of remote sensing-based variability and ground-based long-term water balance constraints.Keywords: evapotranspiration; water resources; remote sensing; water balance; evaporation; transpiration
Background and RationaleEvapotranspiration (ET) is the quantity of water that includes net water evaporation and plant transpiration. It plays a significant role in the hydrologic cycle, returning about 70% of precipitation across the conterminous United States (CONUS) to the atmosphere [1,2]. Because it is generally the second largest component of the water budget, following precipitation, researchers and water resource managers require accurate and reliable ET estimates to understand water availability and distribution for both short and long-term water resources management.The use of remote sensing-based methods of estimating ET has increased in recent years [3], and has proven useful in various applications such as agricultural water use monitoring and estimation [4], and hydrologic simulation [5]. These methods are particularly advantageous in their ability to produce estimates on timescales as short as weeks. The remote sensing approaches generally use surface energy balance arguments to partition the net incoming solar radiation energy into sensible heat flux, ground heat flux, and latent heat flux (energy equivalent of ET mass flux) [6][7][8][9][10]. Different algorithms applied to the same input remote sensing data sets can make predictions of the latent heat