径流量Z指数与Palmer指数对河西干旱的监测

The Drought Monitoring in Hexi Area Using the Runoff-denoted Z Index and the Palmer Index

利用河西内陆河水文代表站1959—2004年逐月径流量资料、内陆河流域灌溉区1949—2001年耕地面积及代表站1961—2005年的气象资料,通过对径流量进行正态化处理来确定径流量Z指数,并以径流量Z指数作为径流干旱指数,对旱涝等级进行划分;考虑径流量Z指数的旱涝等级与农业灌溉用水实际情况之间的关系,给出了径流量Z指数的灌溉指标。将径流量转化为降水量,改进Palmer旱度模式,且在作改进后,又将潜在蒸散量的计算法由利用桑斯威特公式改为利用彭曼公式。结果表明:将径流量考虑到Palmer干旱指数中并改变蒸散量的算法,使得该指数对河西灌溉区干旱情况的监测均有所改善。对照河西地区的干旱事件,径流量Z指数监测到的干旱情况,比Palmer干旱指数改进前、后监测到的干旱情况效果更佳。径流量Z指数能更真实地反映河西灌溉区干旱状况。

Hexi is a typical irrigation agricultural area, it is impossible to grow crops based on the natural precipitation, which is only 100--150 mm every year on average. The runoff of inland river is the main water resource for agriculture. That means the high/low runoff is the main determination of dry/wet condition in the target area. Therefore, runoff is considered as the variable to define a Z-index in order to monitor the drought in Hexi irrigation area. Being the Palmer drought severity index--PDSI has been widely applied in the world since the 1960s. In addition, the PDSI is also widely used in drought assessment, comparison and its spatial/temporal distributions analysis. So PDSI is selected to compare with the runoff-denoted Z index in the drought monitoring in Hexi area. The monthly runoff data from hydrological representative station, the tilth area and the meteorological data from representative station of inland river in Hexi area are utilized. A runoff-denoted drought index, the runoff-denoted Z index is established by processing runoff normally. Dry and wet grades are also divided using this index. Considering the relationship between the dry/wet grades and the actual situation of agricultural irrigation, the irrigation grades of Z index are obtained. The precipitation is derived from runoff to revise the Palmer drought severity index. In addition, Penman method is adopted replacing the Thornthwaite method to improve the potential evaporation calculating scheme. Results show that drought monitoring is improved when the runoff is calculated in Palmer drought severity index and the calculation method of potential evaporation is changed. Based on the drought events in Hexi area, the monitoring effect of the Z index is better than the Palmer drought severity index. So it can be concluded that the drought monitored by the runoff-denoted Z index is closer to the actual status in Hexi irrigation area.