|Evett, Steven - Steve|
Submitted to: Proceedings ASCE North American Water and Environment Congress
Publication Type: Proceedings
Publication Acceptance Date: 1/30/2010
Publication Date: 5/16/2010
Citation: Chavez, J., Howell, T.A., Straw, D., Gowda, P., Garcia, L., Evett, S.R., Ley, T., Simmons, L., Bartolo, M., Colaizzi, P.D., Andales, A. 2010. Surface Aerodynamic Temperature Derived from Wind/Termperature Profile Measurements Over Cotton and Alfalfa in a Semi-Arid Environment. In: Proceedings ASCE North American Water and Environment Congress, May 16-20, 2010, Providence, Rhode Island. 2010 CDROM. Interpretive Summary: It is important to know the crop water use from larger spatial scales for modeling and regional water planning. This research measured energy balance parameters over dryland cotton in Texas and irrigated alfalfa in Colorado to estimate aerodynamic surface temperature needed to accurately determine the water use rates from crops. An aerodynamic profile method was used to assess the aerodynamic surface temperature from the vertical air temperature and other crop and air properties. The modeled aerodynamic surface temperature was with 2 degrees C to measured vales and accurate for spatial energy balance uses. This information will permit the spatial crop water use to be determined to better estimate irrigation requirements and improve water planning estimates.
Technical Abstract: Assessing the project efficiency of irrigation systems and water use efficiency of crops over large irrigated areas requires daily or seasonal evapotranspiration (ET) maps. Mapping ET or latent heat flux (LE) can be achieved spatially for land surfaces using remote sensing inputs such as surface reflectance and radiometric surface temperature (Ts). The energy balance (EB) equation requires net radiation (Rn), soil heat flux (G), and sensible heat flux (H) to derive LE as a residual. Although Rn and G can be estimated with acceptable accuracy, H may be under estimated when Ts is used rather than the surface aerodynamic temperature (To) in the aerodynamic resistance equation. The value of To cannot be measured directly because it varies with atmospheric forcing resulting from radiation, wind speed, and air temperature, and with variable surface conditions. In this study, To for dryland cotton in the Texas High Plains and irrigated alfalfa in Colorado was determined using Ts, air temperature, leaf area index, and surface aerodynamic resistance, and ET measurements obtained with large weighing lysimeters. The specific performance of the modeled To for cotton and alfalfa was assessed using an aerodynamic profile method. Results indicated that the To model based on aerodynamic resistance better agreed (0.2 plus or minus 1 deg C) with measured To values using the aerodynamic profile method.