Submitted to: Irrigation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2009
Publication Date: 8/28/2009
Citation: Chavez Eguez, J.L., Gowda, P., Howell, T.A., Neale, C., Copeland, K.S. 2009. Estimating hourly crop ET using a two-source energy balance model and multispectral airborne imagery. Irrigation Science. 28(1):79-91. [doi:10.1007/s00271-009-0177-9] Interpretive Summary: Remote sensing of land surface energy balance provides instantaneous estimates of latent heat flux or evapotranspiration (ET), which are used in the prediction and monitoring of spatially distributed daily (24 h) crop water use or evapotranspiration, irrigation scheduling, and in general hydrologic modeling. One of the widely used energy balance methods in estimating ETi is the so called two-source method, which differentiates contributions from the vegetation and the soil, to the surface energy budget, separately. This method was evaluated in terms of its accuracy in estimating hourly (ETi) values, using remote sensing imagery and weather station data, with large weighing lysimeters measured ETi values. Results indicated that the two-source model performs better when the surface aerodynamic resistance term is eliminated from the sensible heat flux contributed from the ground (when the crop is sparse and short), and when the contribution of sensible heat flux from the ground is eliminated from the composite sensible heat flux for tall crops that have reached canopy full cover.
Technical Abstract: Efficient water use through improved irrigation scheduling is expected to moderate fast declining groundwater levels and improve sustainability of the Ogallala Aquifer. Thus, an accurate estimation of spatial actual evapotranspiration (ET) is needed for this purpose. Therefore, during 2007, the Bushland Evapotranspiration and Agricultural Remote Sensing Experiment (BEAREX07) was conducted at the USDA-ARS Conservation and Production Research Laboratory (CPRL), Bushland, Tex. During BEAREX07, very high resolution aircraft images were acquired during the crop growing season using the Utah State University airborne multispectral system. Actual instantaneous ET was estimated using a two-source energy balance model. Data from four weighing lysimeters, planted to sorghum and corn fields, were used for evaluating ET predictions. Instantaneous ET was predicted with mean bias error and root mean square error of 0.03 and 0.07 mm h-1 (4.3 and 11.7%), respectively, after modifications were made to the sensible heat flux (H) equation. Results obtained from the modifications performed and from a sensitivity analysis indicated that the crop height, roughness length for momentum transfer, clumping factor and soil resistance sub-models need to be refined. Nevertheless, the application of the TSM using high resolution remote sensing imagery in the Southern High Plains is promising.