Location: Southeast Watershed ResearchTitle: Effect of rainfall events on SMAP radiometer-based soil moisture accuracy using core validation sites
|COLLIANDER, ANDREAS - California Institute Of Technology|
|JACKSON, THOMAS - Retired ARS Employee|
|BERG, AARON - University Of Guelph|
|Bosch, David - Dave|
|CALDWELL, TODD - University Of Texas|
|CHAN, STEVEN - California Institute Of Technology|
|Holifield Collins, Chandra|
|MARTINEZ-FERNANDEZ, JOSE - University Of Salamanca|
|MCNAIRN, HEATHER - Agriculture And Agri-Food Canada|
|Starks, Patrick - Pat|
|WALKER, JEFFREY - Monash University|
|YUEH, SIMON - California Institute Of Technology|
Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2019
Publication Date: 2/10/2020
Citation: Colliander, A., Jackson, T., Berg, A., Bosch, D.D., Caldwell, T., Chan, S., Cosh, M.H., Holifield Collins, C.D., Martinez-Fernandez, J., McNairn, H., Prueger, J.H., Starks, P.J., Walker, J., Yueh, S. 2020. Effect of rainfall events on SMAP radiometer-based soil moisture accuracy using core validation sites. Journal of Hydrometeorology. 21:255-265.
Interpretive Summary: Estimates of soil moisture across the globe are critical with regard to prediction of climate, water balance, and crop production. Estimation is particularly challenging during and immediately after precipitation events because of the rapid changes taking place in soil moisture during that period. Dense networks of soil moisture and precipitation measurements were used to assess the estimation accuracy of soil moisture derived from Soil Moisture Active Passive (SMAP) satellite data collected during and immediately after rain events. It was concluded that SMAP maintains its sensitivity to soil moisture even during rain events and, depending on the application, screening of rain events may not be necessary to ensure the relative quality of the soil moisture retrievals.
Technical Abstract: Soil moisture retrieval is particularly challenging during and immediately after precipitation events because of the transient movement of water in the shallow subsurface. Conventional L-band microwave radiometer based soil moisture products use algorithms that assume a static state and even a constant vertical soil moisture distribution. In this study, dense in situ measurement networks equipped with both soil moisture and precipitation sensors were used to assess the retrieval performance of a SMAP radiometer-based soil moisture product during and immediately after rain events. The removal of the rain event samples systematically improved the unbiased root mean square error (ubRMSE) from 0.037 to 0.028 m3/m3, while the magnitude of the bias became larger (from -0.005 to -0.014 m3/m3); RMSE improved from 0.047 to 0.042 m3/m3. The results indicate that removing samples during the transitional period causes the comparison to improve, but also suggests that the true bias may be larger than the one estimated using all the samples. Furthermore, the results revealed that the effect was stronger for areas with high clay content. An assessment of the performance of the product during the actual rain events (overpass within 3 hours from the start of the rain) showed that the ubRMSE degraded from the benchmarked 0.036 to 0.043 m3/m3, while bias became wetter, as expected because SMAP sensed the water on the surface before propagating to the sensors. The study showed that SMAP maintains its sensitivity to soil moisture even during rain events and screening of rain events may not be necessary to ensure sufficient quality of the soil moisture retrievals.