In Situ Validation of Soil Water Content From Satellite Remote Sensing.

Surface soil moisture estimation impacts a wide range of concerns, including agricultural management, climate, and weather modeling.  Satellite technologies have been developed which allow for this estimation on a high temporal basis with moderate accuracy for the agricultural heartland of the U.S.  These remote sensing platforms have a scale of approximately 60 km (Advanced Microwave Scanning Radiometer-AMSR-E) currently, with future sensors extending to less than 10 km (Soil Moisture Ocean Salinity, and Soil Moisture Active Passive satellites).  The depth of the satellite measurements is on the order of several centimeters, which can be monitored with in situ sensors. There is a huge difference in the scales of the two types of measurements; therefore, it is necessary to develop methodologies for scaling from the ground measurement (in situ) scale to the remote sensing scale.  As part of the calibration and validation program for the AMSR-E instrument, four watersheds were designated for instrumentation and validation of the soil moisture product.  These watersheds varied in size from 150 km² (Walnut Gulch Experimental Watershed in Tombstone, AZ) to 625 km² (Little Washita River Experimental Watershed in Chickasha, OK) in a variety of hydrologic regions including semi-arid to sub-humid.  Operated since 2002, these watersheds have provided an hourly record of soil moisture data at the near surface (5 cm), revealing much about the temporal stability and spatial variability of soil moisture at the watershed scale, providing a basis for future in situ network deployment and operation with regards to remote sensing validation.  Individual sensors were identified and replaced or relocated depending on the character of the anomalous behavior.  Intensive field experiments also measured the ability of these networks to accurately represent the watershed average, including the Soil Moisture Experiments in 2003 and 2004 (SMEX03 and SMEX04 respectively).  Each watershed presents individual challenges in monitoring and adaptive strategies have been developed.