Estimating ET Partitioning Using a Thermal-Based Two-Source Energy Balance Model with High Resolution Airborne Imagery.

As water supplies for agricultural production become more restricted due to overuse and  drought,  particularly in arid regions,  there is a concerted effort to improve water use efficiency (WUE), which is defined as amount of carbon fixed per unit of water used .  In other words, WUE is equal to the amount of water transpired (T) by the crop versus its biomass production. For many crops, biomass production is closely tied to crop yield.  The ongoing drought in California significantly impacting crop production is a prime example of where reliably estimating WUE and developing techniques to increase WUE of irrigated cropland are needed for improving water conservation and agricultural sustainability. While a number of measurement techniques have been developed to estimate T and water loss from soil evaporation (E), they are very difficult to extrapolate from the local patch scale to field and certainly to the landscape and regional scales. This study presents the application of the Two-Source Energy Balance (TSEB) model for estimating evapotranspiration (ET) that explicitly partitions ET to T and E.  An overview of the modeling approach is presented along with examples demonstrating the utility of TSEB for partitioning ET between E and T.  Recently the TSEB model has been applied to very high resolution LST and reflectance imagery from an unmanned aerial system (UAS) allowing separation of soil/substrate and vegetation component temperatures for a vineyard site in California.  The results on flux output from TSEB using a composite versus component LST are analyzed. Such high resolution spatial information is being used in precision farming applications to assess the impacts of within variability in soil texture, water availability and other stress factors on plant condition and productivity. Modifications to the original TSEB formulation using such high resolution imagery for estimation of T and E will be discussed.