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Volume 22, issue 3 | Copyright
Hydrol. Earth Syst. Sci., 22, 1649-1663, 2018
https://doi.org/10.5194/hess-22-1649-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 06 Mar 2018

Research article | 06 Mar 2018

Controls on surface soil drying rates observed by SMAP and simulated by the Noah land surface model

Peter J. Shellito1, Eric E. Small1, and Ben Livneh2 Peter J. Shellito et al.
  • 1Geological Sciences, University of Colorado Boulder, Boulder, 80309, USA
  • 2Cooperative Institute for Research in Environmental Science, and Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, Boulder, 80309, USA

Abstract. Drydown periods that follow precipitation events provide an opportunity to assess controls on soil evaporation on a continental scale. We use SMAP (Soil Moisture Active Passive) observations and Noah simulations from drydown periods to quantify the role of soil moisture, potential evaporation, vegetation cover, and soil texture on soil drying rates. Rates are determined using finite differences over intervals of 1 to 3 days. In the Noah model, the drying rates are a good approximation of direct soil evaporation rates, and our work suggests that SMAP-observed drying is also predominantly affected by direct soil evaporation. Data cover the domain of the North American Land Data Assimilation System Phase 2 and span the first 1.8 years of SMAP's operation.

Drying of surface soil moisture observed by SMAP is faster than that simulated by Noah. SMAP drying is fastest when surface soil moisture levels are high, potential evaporation is high, and when vegetation cover is low. Soil texture plays a minor role in SMAP drying rates. Noah simulations show similar responses to soil moisture and potential evaporation, but vegetation has a minimal effect and soil texture has a much larger effect compared to SMAP. When drying rates are normalized by potential evaporation, SMAP observations and Noah simulations both show that increases in vegetation cover lead to decreases in evaporative efficiency from the surface soil. However, the magnitude of this effect simulated by Noah is much weaker than that determined from SMAP observations.

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After soil gets wet, much of the surface moisture evaporates directly back into the air. Recent satellite data show that this process is enhanced when there is more water in the soil, less humidity in the air, and less vegetation covering the ground. A widely used model shows similar effects of soil water and humidity, but it largely misses the role of vegetation and assigns outsized importance to soil type. These results are encouraging evidence that the satellite can be used to improve models.
After soil gets wet, much of the surface moisture evaporates directly back into the air. Recent...
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