COMPARISON OF AERODYNAMIC AND RADIOMETRIC SURFACE TEMPERATURE USING PRECISION WEIGHING LYSIMETERS

Authors: Colaizzi, Paul; Evett, Steven - Steve; Howell, Terry; Tolk, Judy

Submitted to: Proceedings of the International Symposium on Optical Science and Technology
Publication Type: Proceedings
Publication Acceptance Date: 6/15/2004
Publication Date: 8/2/2004
Citation: Colaizzi, P.D., Evett, S.R., Howell, T.A., Tolk, J.A. 2004. Comparison of aerodynamic and radiometric surface temperature using precision weighing lysimeters. In: Proceedings of the International Symposium on Optical Science and Technology, The International Society for Optical Engineering 49th Annual Meeting, August 2-6, 2004, Denver, Colorado. 5544:215-229.

Interpretive Summary: Estimating water use of agricultural crops and other vegetation has numerous applications in meteorology, ground and surface water hydrology, crop science, on-farm irrigation management, etc. Water use of vegetated surfaces is termed evapotranspiration (ET) because it is the sum of evaporation from the soil/plant surfaces and transpiration from the plant. One of the most direct and accurate methods for measuring ET is by lysimeters, which are essentially large boxes equipped with highly accurate scales, buried in a field, filled with soil, with a crop or other vegetation planted on them. Lysimeters, however, are very expensive to install and maintain, so other methods are needed to estimate ET over a region. Presently, the most common alternative to estimate crop ET is by taking meteorological data (solar radiation, air temperature, humidity, and wind speed) measured near the surface (i.e., 2-10 m height) as input for crop-specific mathematical equations; however, the accuracy of this method is limited because it is very difficult to account for natural soil and plant variability across a field (spatial variability). It is sometimes possible to account for spatial variability by measuring the temperature of the crop using infrared thermometers (IRTs), but the response of the IRT to crop temperature can be confounded by numerous physical characteristics, such as the amount of crop covering the soil, crop roughness and structure, sun angle, IRT sensor angle, and IRT internal temperature. In this study, we measured ET of several crops from lysimeters, and compared these to ET derived from IRTs and meteorological data measured over the lysimeters. We found that accuracy of this method is more sensitive to the roughness of the crop surface than reported in previous studies, which often consider only one crop type. These results are part of continued research to evaluate and refine models that use temperature data from satellites orbiting the earth for routine ET estimation over a region.

Technical Abstract: Radiometric surface temperature (Ts) is commonly used as a surrogate for aerodynamic temperature (To) in computing the sensible heat flux term (H) in the energy balance. However, these temperatures may differ by several degrees, leading to possible errors (especially for large H) and their relationship is not well known. Previous researchers have established empirical and semi-empirical parameterizations of the radiometric roughness length (zor) or some related form (e.g., kBr-1 = ln[zom/zor], where zom is the momentum roughness length). In this paper, we estimated To – Ta (where Ta is air temperature at 2 m height) and zor using large, precision weighing lysimeters planted with irrigated alfalfa, irrigated and dryland cotton, and dryland grain sorghum. Ts was measured by infrared thermometers mounted over the lysimeters. No apparent relations were found between (To – Ta) and (Ts - Ta) or between zor (in the kBr-1 form) and meteorological variables or leaf area index (LAI). The kBr-1 parameter appeared to be most influenced by the different surface roughness of each crop type. Using constant kBr-1 values established for each type of surface, the energy balance model showed reasonable agreement with H and LE derived from lysimeter measurements.