Investigation and determination of factors affecting the shape of the flow duration curve in different climates of Iran

Background and objectives: Improvement and development of forecasts in ungaged catchments needs to understand the interaction between the catchments parameters and hydrological response of catchments in different climates. Flow duration curve is one of the best methods for showing the hydrological response of basins and have different application in the fields of hydrology and related sciences. The shape of flow duration curve also reflected the impact of climatic, geologic and physiographic parameters on river flow and is hydrological response of catchment. In the past decade, several studies have been done on the impact of geometric and hydrological parameters of basins on the shape of flow duration curve. Most of these studies are empirical and classified in graphical and statistical methods. Graphical method, focus on o the effect of physiographic and climatic characteristics on the shape of flow duration curve. Statistical methods on statistical distribution and correlation of the physical characteristics of the catchments are concentrated. Factor analysis of parameters affecting the flow duration curve lead to accurate modeling and interpretation of catchments hydrology. The aim of this study was to determine the most important hydro-climatic and geometric factors affecting on the shape of flow duration curve and investigation of relation between them in the different climates.
Materials and methods: In this study, for factor analysis of effective parameter on the shape of flow duration curve, climate maps were prepared and intersect with the layer of fourth-order watersheds, and watersheds located in every climate were selected. Then at least 30 hydrometric stations with appropriate data and common period (1976-2001) in each climate zone were selected. Then 10 geometric and hydrological parameters affecting the flow duration curve including: average height, area of the watersheds, Gravelius coefficient, slope, main river length and hydro climatologically parameters including: annual rain fall, base flow index, curve number, permeability and the number of rainy days, were calculated for each basin. Flow duration curves plotted using daily flow data and the slope between Q33 to Q66 was computed as an indicator of the shape of flow duration curve, using coding in MATLAB programming environment. Factor analysis was performed and effective factors on the shape of curve were identified. The regression between the flow duration curve index and selected factors in different climate zones were extracted and analyzed.
Results: The results showed that the parameters of first factor including: curve number, base flow index, recession index and the number of rainy days in all climatic zones were common except dry climate zone. Curve Number in all the climate zones has the highest weight of influence. The Weight influence of geometric parameters in all areas was higher than hydrological parameter, except in very humid zone. Parameters selected for factor analysis in the humid zone were found the highest variance explained with 88 percent and the lowest in the Mediterranean zone with 72 percent. Normally distributed errors, the coefficient of determination more than 0.90 and the coefficient of Durbin Watson between (1.5-2.5) reflects the confidence on the regression equations to estimate the slope of flow duration curve in untagged catchments in different climatic zones.
Conclusion: Overall Conclusions of factor analysis in different climatic zones became clear that some of the parameters classified in the first class including: curve number, Base flow index, and number of rainy days were common in all climatic zones. With the one exception that in dry zone the number of rainy days placed in a second class. The factor of curve number in all climate zones has more effective than other parameters.