Thursday, 13 July 2006

Estimating Soil Evaporation with Reflectance and Radiometric Temperature Measurements.

Dong Wang and Jindong Wu. Univ. of Minnesota-Twin Cities Campus, 1991 Upper Buford Circle, St. Paul, MN 55108

Evaporation from soil surfaces is a major component of the surface water and energy budgets. The potential ability of estimating soil evaporation with remote sensing measurements has great significance in modeling land surface processes and assessing potential environmental stresses in natural ecosystems. The objective of this study was to explore possible algorithms of applying hyper-spectral and multi-temporal reflectance measurements for evaporation estimation. A 16-band hyper-spectral radiometer was used for the reflectance measurements. The radiometer covered a band range of 400 to 1720 nm with band widths ranged from 7 to 13 nm. An infrared thermometer was also used for temperature measurement. To create simultaneous measurements of soil reflectance and the rate of evaporation, a large steel frame or balance was fabricated which contained a 0.5-m wide by 0.5-m long by 0.07-m deep soil tray placed directly on top of an electronic balance. The radiometer was mounted at a height of 0.5 to 1.0 m above the tray during each measurement. The balance frame was needed to counter the excess or stationary weight from the tray and dry soils in the tray, yet sensitive enough to detect 0.01-g change of the tray. Micro-meteorological variables, including air temperature at different heights from the tray, solar radiation and net radiation, wind speed, and relative humidity, were also measured at the same time in order for an independent evaporation estimation. During each measurement, the tray was filled with one type of soil, saturated with water, then started the reflectance and weight-change measurements while evaporation was occurring. The experiments were carried out in clear, calm, sunny days, and repeated for different soil types. Each experimental run was maintained until at least 1-hr after the transition from atmosphere-limited to soil-limited evaporation (or the two-stage soil evaporation). To determine the most useful band combination, a spectral dependent soil line, instead of the traditional soil lines, was created based on the band index and maximum changes of soil surface spectral properties associated with water content decrease during evaporation. A spectral simple ratio was formulated, and in combination with the surface temperature measurement, to calculate the actual evaporation rate. The preliminary results served as a proof of concept, and it appeared to be promising. The simple statistical fit could be useful for other situations as well. However, more testing or validation is needed before using the technique in actual field applications with aircraft or satellite images.

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