Using SMAP Satellite Observations to Estimate Terrestrial Evaporation Rates

Direct evaporation from soil is one component of total evapotranspiration from the land surface. Evapotranspiration (ET) is an essential terrestrial water flux, with important linkages between water and energy cycles. Yet, measurement of evapotranspiration over large-scales faces many practical and logistical challenges such that few large-scale observational estimates of ET are currently available. Model-based simulation of ET and its components—soil evaporation, transpiration, and canopy evaporation—subsequently lacks observational constraints. This work seeks to produce a continental-scale estimate of soil evaporation (E) based primarily on soil drying rates from the NASA SMAP satellite mission, termed here: E-SMAP. E-SMAP will improve understanding of the dynamics of the terrestrial water balance and be useful for developing and constraining models.

In the proposed methodology, computation of soil evaporation requires three basic components: (i) soil drying observations from SMAP, (ii) a remotely-sensed estimate of surface transpiration and (iii) a gradient-driven flux of moisture between the surface and root-zone soil. This presentation will evaluate each of the three components, exploring the implications of key assumptions on E-SMAP and identifying periods when uncertainties are sufficiently small to make evaporation estimates most reliable. Spatial estimates of E-SMAP will be presented, as well as point-based estimates at flux-tower locations.