بررسي شدت و تداوم خشكسالي هيدرولوژيك در حوزه هاي آبخيز استان تهران

Investigation of hydrologic drought severity and duration at Tehran Fars province watersheds

Noticing to correct management in order to reduce drought effects which are necessary in development of regions as well as for planning and application of preventing measurements to combating with drought phenomenon. Drought forecasting needs an efficient knowledge in meteorology and hydrology. In the present study, low flows with 30 duration days were calculated in Tehran watersheds. 13 gauges with optimum condition were selected for this research. Log Pearson type 3 distributions were fitted for data and the amount of flows were determined for different return periods. Drought duration and its corresponding severity were derived based on truncation level of weibul method, the values of flows for considered truncation levels as 10 to 95 percent were derived. Monthly flows were deducted from truncation values and the negative values selected as drought identifications. Then, drought severities were obtained for different durations. The results show that the flows of 100 years return period, which were identified based on the log Pearson type 3 distribution, are practically nearly nil on the other hand, flows in drought periods using the truncation method are gradually decreasing with an increase in truncation level. The flows of 90 percentages are approximately equal to those of 2 years return period of droughts defined by the log Pearson type 3 distribution. It is also shown that there is a strong correlation between drought duration and severity. As the increasing truncation level, the drought severity will rise. Thus, the water management operations can be started from lower levels of drought severity. This can prepare enough opportunity to control of flow as well as to adjust it.