Abstract
Abstract. The Tropical Rainfall Measuring Mission (TRMM) carries aboard the Precipitation Radar (TRMMPR) that measures the backscatter (σ°) of the surface. σ° is sensitive to surface soil moisture and vegetation conditions. Due to sparse vegetation in arid and semi-arid regions, TRMMPR σ° primarily depends on the soil water content. In this study we relate TRMMPR σ° measurements to soil water content (ms) in the Lower Colorado River Basin (LCRB). σ° dependence on ms is studied for different vegetation greenness values determined through Normalized Difference Vegetation Index (NDVI). A new model of σ° that couples incidence angle, ms, and NDVI is used to derive parameters and retrieve soil water content. The calibration and validation of this model are performed using simulated and measured ms data. Simulated ms is estimated using the Variable Infiltration Capacity (VIC) model and measured ms is acquired from ground measuring stations in Walnut Gulch Experimental Watershed (WGEW). σ° model is calibrated using VIC and WGEW ms data during 1998 and the calibrated model is used to derive ms during later years. The temporal trends of derived ms are consistent with VIC and WGEW ms data with a correlation coefficient (R) of 0.89 and 0.74, respectively. Derived ms is also consistent with the measured precipitation data with R=0.76. The gridded VIC data is used to calibrate the model at each grid point in LCRB and spatial maps of the model parameters are prepared. The model parameters are spatially coherent with the general regional topography in LCRB. TRMMPR σ° derived soil moisture maps during May (dry) and August (wet) 1999 are spatially similar to VIC estimates with correlation 0.67 and 0.76, respectively. This research provides new insights into Ku-band σ° dependence on soil water content in the arid regions.
Highlights
The ongoing drought of the Colorado River Basin in the southwestern United States started in 2000 and has become the longest drought in the recorded history of the basin (Timilsena et al, 2007)
The computed model parameters from the 1998 data listed in Table 1 are used to derive soil water content from TRMMPR σ ◦ data during the later years by reordering of Eq (2), i.e., ms
Radar backscatter over land depends upon the soil moisture and vegetation characteristics of the land surface
Summary
The ongoing drought of the Colorado River Basin in the southwestern United States started in 2000 and has become the longest drought in the recorded history of the basin (Timilsena et al, 2007). This is evident from the historic low levels in most of the storage lakes that has resulted in severe water shortage in some user states. Due to the regional importance of this basin and agricultural and social impacts of water scarcity, it is important to understand the factors related to droughts. Soil water content reflects the recent precipitation, agricultural potential, and water storage and can serve as a good index of drought (Sheffield et al, 2004; Cosh et al, 2008)
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