Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2006
Publication Date: 11/1/2006
Citation: Colaizzi, P.D., Evett, S.R., Howell, T.A., Tolk, J.A. 2006. Comparison of five models to scale daily evapotranspiration from one-time-of-day measurements. Transactions of the ASABE. 49(5):1409-1417. Interpretive Summary: Estimating water use (evapotranspiration, ET) of agricultural crops and other vegetation has numerous applications in meteorology, ground and surface water hydrology, crop science, and on-farm irrigation management. The ET of vegetated surfaces can be estimated using surface temperature and standard meteorological data. Surface temperature can be remotely sensed by thermal infrared thermometers from the ground or aboard aircraft or satellites. The ET estimated by remotely sensed surface temperature is instantaneous in nature, but ET on a daily basis is of primary interest. We tested five models that estimate daily ET based on instantaneous measurements of ET. Performance of each model was slightly different but they generally agreed well for cropped surfaces but poorly for bare soil. The poor agreement for bare soil was mainly due to the small ET values involved compared to crops, which can have relatively large ET. The models all performed best using one-time-of-day ET measurements around solar noon, when incoming solar energy was at the daily maxima, but not as well using one-time-of-day ET measurements taken during the morning or late afternoon. These are important findings because 1) there are limitations in estimating ET for surfaces with relatively low ET (dry bare soil or vegetation, and fields with crop residue); and 2) the one-time-of-day ET measurement should occur as close to solar noon as possible.
Technical Abstract: Calculation of regional, spatially distributed evapotranspiration (ET) is possible using remotely sensed surface temperatures from sensors aboard air or space platforms. These platforms provide instantaneous data at frequencies of days to weeks, so that instantaneous latent heat flux can be computed from energy balance algorithms. However, instantaneous latent heat flux must be converted to ET and then scaled to daily (24-h) totals for most practical applications. We compared five scaling models where a single measurement of 0.5-h ET was used to estimate the daily total. Each model takes advantage of the quasi-sinusoidal nature of daytime ET and other daytime parameters including solar radiation, available energy, or reference ET. The surfaces were fully irrigated alfalfa, partially irrigated cotton, dryland grain sorghum, and bare soil (tilled fallow sorghum). Actual ET was measured by precision weighing lysimeters. For cropped surfaces, the models based on grass reference ET resulted in the best agreement between observed and predicted daily ET totals. For bare soil, the model based on available energy (i.e., evaporative fraction) resulted in the best agreement. Relative error between observed and predicted daily ET increased as daily ET decreased. Observed and predicted daily ET agreed well for the transpiring crops (RMSE of 0.33 to 0.46 mm d-1 for mean daily ET of 3.9 to 5.8 mm d-1) but poorly for bare soil (RMSE of 0.47 mm d-1 for mean daily ET of 1.4 mm d-1).