Key points:

• SMAP and networks of in situ probes observe soil drying after rainfall
• SMAP observes soil drying to occur over a 44% shorter timescale than in situ
• SMAP observes soil drying to occur at twice the rate as in situ

We examine soil drying rates by comparing surface soil moisture observations from the NASA Soil Moisture Active Passive (SMAP) mission to those from networks of in situ probes upscaled to SMAP's sensing footprint. SMAP and upscaled in situ probes record different soil drying dynamics after rainfall. We modeled this process by fitting an exponential curve to 63 drydown events: the median SMAP drying timescale is 44% shorter and the magnitude of drying is 35% greater than in situ measurements. We also calculated drying rates between consecutive observations from 193 events. For 6 days after rainfall, soil moisture from SMAP dries at twice the rate of in situ measurements. Restricting in situ observations to times of SMAP observations does not change the drying timescale, magnitude, or rate. Therefore, observed differences are likely due to differences in sensing depths: SMAP measures shallower soil moisture than in situ probes, especially after rainfall.

• Mapped soil texture designations poorly characterize soil hydraulic properties (SHPs) at all 7 sites studied here.
• SHPs can be improved by calibrating to basinwide in situ surface soil moisture content and to unbiased SMOS (Soil Moisture Ocean Salinity) satellite observations.