Measuring versus estimating net radiation and soil heat flux:: Impact on Penman-Monteith reference ET estimates in semiarid regions

The standardized ASCE Penman-Monteith and FAO-56 equations were used to estimate reference evapotranspiration (ET0) using estimated and measured net radiation (R-n) and soil heat flux (G), based on hourly and daily meteorological data. The estimates were evaluated against lysimeter measurements. The results indicate that using measured or estimated values of R-n and G can have significant effect on the accuracy of the ETO estimations, especially when calculations were made on an hourly basis. The FAO-56 version performed very well during the irrigation season on a daily basis. The use of measured R-n and G did not improve ETO estimation on a daily basis, therefore, the use of estimated R-n and G appears to be dependable when calculations are based on 24-h weather data. When daily ETO was calculated from hourly estimations, the results were different depending on the version used. The ASCE version was more accurate, especially when R-n and G were measured. Therefore, measurement of R-n and G may have potential to improve estimation only when daily ETO is calculated from hourly estimations. The PM FAO-56 version was always a little less accurate than the ASCE version. For hourly calculations, using a constant surface resistance (as in FAO-56 version), the PM method underpredicted for high evaporative demand and vice versa. The ASCE version performed better than PM FAO-56 version when R-n and G were measured and estimated. Therefore, ASCE version tended to provide quite accurate values of hourly ETO, even using estimated values of R-n and G. As conclusion, the methods proposed by FAO-56 for estimating R-n and G tended to produce accurate estimates for daily and hourly ETO under semiarid conditions and can be used with some degree of confidence for estimating ETO. In addition, results suggest that the ASCE standardized equation on an hourly basis improved the accuracy of ETO estimation with respect to the FAO-56 version. (C) 2007 Elsevier B.V. All rights reserved.