Low Flow Analysis in Arid and Semi-Arid Areas of Northeastern of Khorasan Razavi Province

Knowledge of frequency ratings of low flows is important to understand thephenomenon due to damage producing effects of drought. Low flow Analysis is a methodof hydrologic estimation of drought. One of the methods in the regions without adequatedata is the regional analysis of low flows, based on real data and its results have the higherreliability coefficients. In this study, the suitable stations for analysis of these flowswere selected, low annual flow series with duration of 7, 15 and 30 days werecalculated, 8 statistical distributions functions were fitted with the data and the Log-Pearson Type II distribution was selected as the suitable regional distribution. Also 16physiographical, climatic and geological parameters were determined for the selectedwatersheds. To choose the independent variables, factor analysis was used. The resultsshowed that among 16 input parameters, the area, mean annual rainfall and percentage ofpermeable formation were the most important factors, which explain 79% of variance.Two homogeneous regions were recognized in the study area with cluster analysis andthe two regions were completely separated by distinguishing analysis. The regionalanalysis of low flows was carried out using step-wise regression and multiple regressionmethods for each homogeneous region and the whole study area, considering r factorsof the area, mean annual rainfall and percentage of permeable formation. The low flowrates with duration of 7, 15 and 30 days and return periods of 2, 5, 10, 20 and 50 yearswere finally estimated. The study of the models showed that the models used forhomogeneous regions have higher correlation coefficients and lower standard errors withrespect to the models used for the whole area.