Evapotranspiration between satellite overpasses: methodology and case study in agricultural dominant semi-arid areas

A backward-averaged iterative two-source surface temperature and energy balance solution (BAITSSS) algorithm was developed to estimate evapotranspiration (ET) during the period between Landsat satellite overpass dates. The METRIC (mapping evapotranspiration at high resolution with internalized calibration) model was used to estimate ET using short wave and thermal data from the Landsat images. METRIC generated ET was used to define initial surface characteristics, soil water conditions and initialize the soil water content for the surface and root zone at the start of the simulation period, and to adjust the results from BAITSSS at the next satellite overpass date if needed. North American Regional Reanalysis (NARR) weather data were used to estimate the surface energy balance components in between the satellite overpasses. The fraction of vegetation cover (f(c)) was used to partition surface energy balance components, as defined by the normalized difference vegetation index. Soil surface resistance (r(ss)) and Jarvis-model-type canopy resistance (r(sc)) were used to calculate latent heat flux using the aerodynamic equations. A water balance was implemented to track the water content at the surface and root zone. An irrigation sub-model was developed to consider the role of irrigation for known irrigated agricultural fields, which is critical when computing ET in an agriculture-dominant area. Any mismatch between the estimated and METRIC ET at the next satellite overpass can be adjusted back over the simulation period with a time-based linear correction to increase the accuracy and reduce computation time. In this study, BAITSSS results from southern Idaho and northern California are presented for 3 h time steps.