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A new method to estimate crop evapotranspiration from an empirical canopy temperature and the energy balance

Dalziza de Oliveira, University of Nebraska - Lincoln

Abstract

Crop coefficients have been widely used to estimate crop water requirements for irrigation scheduling. However, these coefficients are often management, site and weather specific and in some years they lead to large errors in estimates of crop evapotranspiration (ETc). A new method is presented to directly estimate ETc, without requiring crop coefficients. This method combines the resistance approach to estimate sensible heat flux with regression-fit functions to estimate canopy temperature (Tc) based on the theoretical relationships between Tc and air temperature (Ta) with the vapor pressure deficit (VPD). Half-hour estimates of ETc were summed for 24-hour periods and then compared to lysimeter measurements of ET (ETlys) for irrigated alfalfa, maize, sorghum, soybean, and winter wheat intended to have minimum soil water deficit, grown in Bushland, TX, USA. It was found that three baselines were required for each crop (early season, full cover, and late season), with individual fit for daytime and nighttime. Generally the root mean square errors in ETc were less than 1 mm/day for the full cover period and higher (up to 1.5 mm/day) for the whole season. The errors represent ±14% to ±24% of the average daily ETlys for the whole season, and ±7% and ±15% for the full cover period (when LAI ≥ 3). The corrections for atmospheric stability resulted in an improvement of 0.1 to 0.2 mm day −1. The canopy temperature-based energy balance (CTEB) method adjusted with the multiple baselines (CTEB-Baselines) was compared to two methods that require the use of crop coefficients: Penman, calculated on a daily basis, and FAO-Penman-Monteith (FAO-PM), calculated first on a half-hour basis, then summed to daily values. Statistical comparison of means showed no significant differences between methods for the same crop when using adjusted crop coefficients. The CTEB method resulted in errors comparable to those generated by the Penman and the FAO-PM methods. The CTEB method showed good results when applied to an independent data set for alfalfa, indicating that, once calibrated for a given species or hybrid, this method has a high potential for correctly estimate ETc in areas with high climatic variability ^

Subject Area

Agriculture, Agronomy|Engineering, Agricultural

Recommended Citation

Oliveira, Dalziza de, "A new method to estimate crop evapotranspiration from an empirical canopy temperature and the energy balance" (2002). ETD collection for University of Nebraska - Lincoln. AAI3059960.
http://digitalcommons.unl.edu/dissertations/AAI3059960

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