Parametrization of reference crop evapotranspiration model in Anhui Province

Abstract: Model parameter optimization is one of key issues to accurately estimate reference crop evapotranspiration (ET0). This study evaluated feasibility of different methods for estimation of net long-wave radiation, air pressure and vapor pressure and established optimal parameters for ET0 estimation in Anhui Province. The data collected included daily observations of 81 meteorological stations in Anhui Province from 1961 to 2011 and daily observed radiation data of Hefei, Wuhan, Nanjing, Hangzhou and Nanchang radiation stations from 1993 to 2011. Seven different net long-wave radiation methods were Brunt method, Penman method, Bepлянд method, FAO24 and FAO56 PM method, Deng Genyun method and Tong Hongliang method. The net long-wave radiation calculated based on radiation balance equation was considered as standard value. Vapor pressure was estimated based on 2 methods: 1) daily maximum and minimum temperature; and 2) mean air temperature. They were compared with measured vapor pressure. Air pressure was estimated by FAO56 PM method, which was compared with measured air pressure. Furthermore, the optimal parameterization scheme of ET0 for Anhui Province was established by combining the results and the previous optimal daily solar radiation parameterized model. Finally, the effect of parameter optimization on the ET0 estimation was analyzed. Results showed that the net long-wave radiation over Anhui Province was underestimated by all these 7 methods with the mean absolute relative errors almost larger than 30%. The mean absolute relative error and root mean square error of the Deng Genyun method was 24.8%-31.3% and 1.44-1.93 MJ/(m2·d) for the 5 stations, which were less than those of the other 6 models. In addition, the Tong Hongliang method was also better than the others. Both were good for Anhui Province. Due to a large value of air pressure, estimators of air pressure based on FAO56 PM model had smaller errors in most areas of Anhui. The vapor pressure calculated based on mean air temperature and relative humidity was better than that based on maximum and minimum temperature. The former had mean absolute relative error of 2.0% and root mean square error of 0.04 kPa. The errors in plain stations were smaller than at stations located at mountain. Though the empirical air pressure and calculated vapor pressure estimation models were suitable in Anhui Province except 3 mountain stations, the measured values were still recommended if possible, especially for high latitude. The mean absolute relative errors between the standard values of ET0 values calculated by measured values and that by the optimal model were 15.5%, 9.05% and 6.12% at the daily, monthly and annual scale, respectively, indicating that the optimal model was applicable in Anhui Province. The FAO56 PM model overestimated the solar radiations and underestimated the net long-wave radiations in Anhui Province, resulting in that an overestimation of ET0 up to 40.0% at all daily, monthly and annual scales. Thus, a recommended scheme for ET0 estimation in Anhui Province was Caowen modified method for solar radiation, Deng Genyun method for net long-wave radiation, and measured values for air and vapor pressure.