New Attitude in the Frequency Analysis of 7-day,s Minimum Flows (Case study: Lake Urmia Basin)

The 7-day،s minimum flow، which is estimated using frequency analysis for a given return period، is one of the main parameters for the quantitative and qualitative management of a river and plays an important role in determining flow rates of the environmental sector. Due to the complex nature of low flow events and limited studies on the low flow probability distribution، yet the appropriate probability distribution for the frequency analysis of such events is unclear. In this study، the performance of conventional probability distributions in low flow event analysis was evaluated and also a new probability distribution called Extreme Value Type III was introduced to describe the low flow indices. The proposed probability distribution was obtained from lognormal distribution for the 7-day،s average flows. The investigation of the 7-day،s average flows in the Lake Urmia Basin showed that performance of the proposed probability distribution using Extreme Value Type III was much better than the other traditional probability distributions to model the 7-day،s average minimum flows e min