Methods to estimate daily evapotranspiration from hourly evapotranspiration

Remote sensing provides an effective way for the estimation of regional evapotranspiration, therefore, a reliable scaling-up method from instantaneous evapotranspiration (ET) to daily ET is of great importance. In this paper, five commonly used scaling-up methods estimating daily ET from hourly latent heat measurements were compared under different energy and crop growing conditions. Field data collected during the whole growing season at 1.0 h intervals by eddy covariance system in North China were used. Evaporative fraction (EF), reference evaporative fraction (K-c), crop coefficient (modified-K-c) and surface resistance (r(c)) varied slightly during the daytime, which gave a solid theoretical basis for the scaling-up methods based on the constant value of these parameters. In most cases, the EF and K-c methods performed better than the others, the relative bias from 9:00-16:00 for the two methods ranged from -11.2% to 5.9% and -6.6% to 4.8%, respectively. The EF method was applicable when surface energy was limited, when R-n was less than 50 W m(-2); the average modified coefficient of efficiency of EF method during the daytime from 9 am to 5 pm was 0.68, which is around twice that of the second best method. The K-c and the modified-K-c methods were applicable when surface energy was relatively large. The performance of the five methods didn't seem sensitive to variation of leaf area. Overall, the K-c and EF methods performed better, especially between 11:00 and 15:00. The results of this paper could be useful for optimisation of ET models. (c) 2016 IAgrE. Published by Elsevier Ltd. All rights reserved.