西藏高海拔地区气象数据缺失条件下的ET_0计算研究

Simplified limited data ET_0 equation adapted for high-elevation locations in Tibet

西藏高海拔地区低氧低压(平均不足海平面的2/3)、太阳辐射强(年太阳辐射6 000~8 000 MJ/m2)、近地层空气湿度变化大,加之西藏地区气象资料系列短、站点少,该地区ET_0计算具有特殊性及不便性。本研究基于西藏地区9个典型站点20年逐日气象资料,通过引入海拔因子与修正温度常数对Hargreaves(HS)模型进行改进,旨在得到一种少参、准确的高海拔地区ET_0简易计算方法。结果表明,海拔2 000 m以上地区Hargreaves-Elevation(HS-E)改进模型在不同时间尺度条件下的修正结果均明显优于HS模型且避免了原HS模型在高海拔地区ET_0计算出现负值的情况,提升了ET_0计算值的实用性与精度。以PM模型ET_0计算值为标准进行误差分析,HS-E模型逐日ET_0计算的纳什效率系数(NSE)、均方根误差(RMSE)和平均相对误差(MRE)分别为0.8、0.53mm/d和13.80%,逐月ET_0计算的NSE、RMSE和MRE分别为0.84、11.90 mm/month和12.50%;对比不同时间尺度条件下(日、月)误差分析结果可知,计算时间尺度越大HS-E模型结果越优。HS-E改进模型在高海拔地区适应性较强,具有较高的计算精度,可作为西藏海拔2000 m以上地区气象数据缺失条件下ET_0计算的推荐模型。

The Food and Agriculture Organization of the United Nations （FAO） has recommended the Pen- man-Monteith （FAO56 P-M） method as a standard method for estimating reference evapotranspiration （ETO） and for evaluating other methods. But the FAO56 P-M method requires many parameters that are not available in many developing regions of high elevation in Tibet. Meanwhile, the low pressure, strong so- lar radiation, intensive evaporation, and frequent heat transfer are special meteorological phenomena in high-elevation areas. Accordingly, the basic objective of this study is to develop a new equation requiring fewer parameters for simulating the ETo at high-elevation stations. When solar radiation, relative humidity and/or wind speed data are missing, a possible exception is the Hargreaves-Samani （HS） method which has shown reasonable ETo results with a global validity according to the FAO＇s Irrigation and Drainage Paper No. 56. Therefore, the new equation （HS-E） based on HS equation and elevation was developed using the meteorological data of nine stations in the Tibet Plateau from 1981 to 1990. Then the HS-E and HS equation, which requires less meteorological data to calculate ETo, were evaluated as compared to the FAO56 P-M method. Results indicate that the computations of the improved HS-E model are obviously bet- ter than the HS model for the areas of higher than 2000m under the conditions of different time scale. The new mode, which enhanced the practical applications and computational accuracy of ETo, can make up the shortage of HS model that the ETo would have negative values in arctic-alpine region of Tibet. Compared with the ETo calculated by FAO56 P-M equation, the daily ETo and monthly ETo calculated by HS-E mod- el were analyzed. The Nash-Sutcliffe efficiency coefficient （NSE）, root-mean-square error （RMSE） and Mean Relative Error（MRE）for the daily ETo calculated by HS-E model are 0.8, 0.53 mm/d and 13.80% and for the monthly ETo are 0.84, 11.90 mm/month and 12.50 %, respectively, which indicated that the HS-E model is high-quality and can calculate the ETo more exactly. Considering the error resuh of differ- ent time scales, the larger the time scale, the better the results can be obtained by the HS-E model. In conclusion, the HS-E model is suitable and accurate in the high-altitude district, which can be recom- mended as a simple method for calculating ETo in the area higher than 2000m.