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Summer Soil Moisture Spatiotemporal Variability in Southeastern Arizona

Susan Stillman, Jason Ninneman, Xubin Zeng, Trenton Franz, Russell Scott, William Shuttleworth, Ken Cummins
Journal of hydrometeorology 2014 v.15 no.4 pp. 1473-1485
hydrologic data, hydrologic models, rain, rain gauges, seasonal variation, soil water, soil water balance, spatial variation, summer, time series analysis, watersheds, Arizona
Soil moisture is important for many applications, but its measurements are lacking globally and even regionally. The Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona has measured near surface 5-cm soil moisture with 19 in situ probes since 2002 within its 150 km2 area. Using various criteria to identify erroneous data, it is found that in any given period from 1 July to 30 September from 2002 to 2011,13 17 of these probes were producing reasonable data, and this is sufficient to estimate area-averaged seasonal soil moisture. A soil water balance model is then developed using rainfall as its only input to spatially extrapolate soil moisture estimates to the 88 rain gauges located within the watershed and to extend the measurement period to 56 years. The model is calibrated from 2002 to 2011 so that the daily in situ and modeled soil moisture time series have a high average correlation of 0.89 and a root-mean-square deviation of 0.032 m3 rn3. By interpolating modeled soil moisture from the 88 rain gauges to a 100-rn gridded domain over WGEW, it is found that spatial variability often increases when 88 (rather than 13 17) estimates are taken. While no trend in the spatial average surface soil moisture is found, large variability in the spatial average soil moisture from 1 July to 30 September is observed from year to year, ranging from 0.05 to 0.09 m3 m3. In addition to spatiotemporal analysis of WGEW, this gridded soil moisture product from 1956 to 2011 can be used for validation of satellite-based and reanalysis products and land surface models.