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Shellito, Peter J ¹ ; Small, Eric E ²

¹ Department of Geological Sciences, ²ÊÃñ±¦µä, Boulder, Colorado
² Department of Geological Sciences, ²ÊÃñ±¦µä, Boulder, Colorado

Surface soil moisture is measured both by NASA’s SMAP satellite mission and by validation networks of in situ probes. In the days after a rain event, we model the timescale of drying by fitting an exponential curve. The time constant is 20% shorter as observed by SMAP than by in situ probes. However, fitting the model to in situ observations concurrent with SMAP also reduces the time constant by 20%. We conclude that (1) SMAP observations are not frequent enough to characterize the drydown timescale in the same way in situ observations do, and (2) at the given 1-3 day observation frequency of SMAP, the satellite and in situ observations reflect the same timescale of drying.

Next, we calculate linear drying rates between pairs of consecutive SMAP overpasses. In the first 5 days after a rain event, during which the majority of drying occurs, soil moisture as measured by SMAP dries at twice the rate measured by corresponding in situ observations. We attribute this difference to a shortening of microwave penetration depth over wet soil: when only the top couple of centimeters are wetted, SMAP’s nominal 0-5 cm sensing depth is biased high.