Evaporative fraction is independent of near surface soil moisture measurements for most areas in the U.S. Corn Belt

Abstract

Previous literature concluded that during water-limited periods evaporative fraction (EF), a measure of plant transpiration and consequently latent heat flux as well as photosynthesis, declines simultaneously in both surface (≈ 0-10 cm) and root-zone soil moisture. Thus, surface soil moisture alone, which can be observed via satellite, could describe EF during water-limited periods. However, this hydraulic link between the surface and the root-zone soil moisture may not hold in the U.S. Corn Belt. Tile drainage below much of the Corn Belt (≈ 1-2 m) attempts to prevent shallow groundwater (≤ 2 m) from encroaching up to the surface. Shallow groundwater can also act as a water source because corn and soybean roots extend ≈ 1.5 m deep.

We hypothesized that EF would remain high in the Corn Belt during limited surface soil moisture if crop roots access soil water via capillary rise from groundwater. Therefore, satellite-observed limited surface soil moisture could not infer low EF. To test our hypothesis we analyzed latent and sensible heat flux, surface soil moisture, and meteorological variables at several eddy covariance sites (AmeriFlux and Iowa State University). EF for corn and soybean declined when surface soil moisture became limited for silty loam soil with 20-m groundwater (Rosemount, MN). On the other hand, EF for corn and soybean did not decline with limited surface soil moisture for clay loam soil with shallow groundwater (Ames, IA).

Agro-IBIS VSF, an agro-ecosystem model that accounts for variably-saturated soil water movement, can vary groundwater depth to verify measurements. AgroIBIS-VSF for a corn site in Iowa showed that shallow groundwater (≤ 2 m) caused higher EF than deep groundwater (>2 m) particularly due to increased root-zone soil moisture from groundwater capillary rise. AgroIBIS-VSF, as well as flux data (Ne3), also showed that non-limited root-zone soil moisture kept EF fairly high even for limited surface soil moisture and deep groundwater. We can conclude that satellite observations in the Corn Belt of the top 5 cm limited soil moisture (e.g., from NASA’s SMAP and ESA’s SMOS) cannot infer low EF for shallow groundwater and adequate root-zone soil moisture, contrary to previous work.