Mapping Drought and Soil Moisture with a Thermal Two-Source Surface Flux Model

The Two-Source (Soil+Canopy) Surface Energy Balance (TSEB) model of Norman et al. (1995) has proven to be a versatile and robust framework for mapping surface fluxes at multiple spatial scales using thermal remote sensing data. The TSEB has been coupled with an atmospheric boundary layer model in a time-differencing mode to routinely map fluxes across the U.S. continent at 5-10km resolution using thermal band imagery from the Geostationary Operational Environmental Satellites (GOES). In a disaggregation mode (Norman et al., 2003), the TSEB can also generate high-resolution flux maps at 10^0-10^3 meter resolution over watersheds and flux validation sites using images from platforms like Landsat, ASTER and MODIS. Two recently developed applications for the Norman TSEB model are the ability to map drought conditions and soil moisture content at local to regional scales. The TSEB demonstrates good sensitivity to depleted moisture availability in the soil surface layer and in the plant root-zone, reflected in the elevated soil and canopy temperatures diagnosed by the model. This sensitivity facilitates remote sensing of moisture contents down to 1-2m depth – even under dense vegetation, where microwave retrieval algorithms typically fail. A TSEB-derived Evaporative Stress Index (ESI), related to the ratio of actual to potential evapotranspiration, shows good spatiotemporal correspondence with standard drought indices and with antecedent precipitation patterns but at significantly higher spatial resolution. Work is underway to develop an operational thermal-band drought monitoring product for the contiguous U.S. based on the TSEB land-surface representation, with potential for integration into the newly developed U.S. Drought Portal. Through the past decade, John Norman has been a firm believer and staunch supporter of the value of thermal remote sensing at sub-field (Landsat) scales to continental (GOES) scales, arguing that early skepticism regarding thermal utility was based on results from overly simplistic modeling approaches. The TSEB modeling scheme exemplifies John's career-long commitment to creating biophysical models that are simple but not too simple, striving to balance operational utility with physical robustness.