Estimating Soil Evaporation Using Drying Rates Determined from Satellite-Based Soil Moisture Records.

We describe an approach (ESMAP; Evaporation–Soil Moisture Active Passive) to estimate direct evaporation from soil, E_soil, by combining remotely-sensed soil drying rates with model calculations of the vertical fluxes in and out of the surface soil layer. Improved knowledge of E_soil can serve as a constraint in how total evapotranspiration is partitioned. The soil drying rates used here are based on SMAP data, but the method could be applied to data from other sensors. We present results corresponding to ten SMAP pixels in North America to evaluate the method. The ESMAP method was applied to intervals between successive SMAP overpasses with limited precipitation (<2 mm) to avoid uncertainty associated with precipitation, infiltration, and runoff. We used the Hydrus 1-D model to calculate the flux of water across the bottom boundary of the 0 to 50 mm soil layer sensed by SMAP, q_bot. During dry intervals, q_bot typically transfers water upwards into the surface soil layer from below, usually <0.5 mm day⁻¹. Based on a standard formulation, transpiration from the surface soil layer, E_{T_s}, is usually < 0.1 mm day⁻¹, and, thus, generally not an important flux. Soil drying rates (converted to equivalent water thickness) are typically between 0 and 1 mm day⁻¹. Evaporation is almost always greater than soil drying rates because q_bot is typically a source of water to the surface soil and E_{T_s} is negligible. Evaporation is typically between 0 and 1.5 mm day⁻¹, with the highest values following rainfall. Soil evaporation summed over SMAP overpass intervals with precipitation <2 mm (60% of days) accounts for 15% of total precipitation. If evaporation rates are similar during overpasses with substantial precipitation, then the total evaporation flux would account for ~25% of precipitation. ESMAP could be used over spatially continuous domains to provide constraints on E_soil, but model-based E_soil would be required during intervals with substantial precipitation. [ABSTRACT FROM AUTHOR]