Investigation of Regional Equations of Annual Water Yield Estimation in Different Return Periods over Eight regions of the Country

Watershed management planning and prioritizing need the awareness and analysis of some factors such water yield and its fluctuation. This matter is highly important in tributaries. Most designs of hydraulic structures have forced engineers to have catchment water yield at different levels of probability. Water resources plan, small and large dams need the amount of mean annual water yield with various return periods. Besides, in order to utilize agricultural lands, annual water yield is very essential; nevertheless, there has not been a comprehensive study on this component based on available information so far. For this objective, a research project entitled "Investigation of Regional Equations of Annual Water Yield Estimation in Different Return Periods over Eight regions of the Country" has been approved and conducted by Soil Conservation and Watershed Management Research Institute. The country has been divided into eight regions and discharge data have been collected and analyzed in each hydrometery station. Some stations, having sufficient qualitative and quantitative data, have been chosen and mean annual discharge has been calculated for these stations. Hydrometry data, physiographic parameters and climatic factors have been used to analyze hydrologic homogeneity in order to determine regional equations of annual water yield. Some of the observed hydrometry station data have been selected for extracting regional equations in each region and other stations have been used for evaluation of obtained equations. Also estimation and probabilistic analysis of mean annual water yield have been carried out by corresponding data to probabilistic distribution functions in order to obtain the best fit function; then probable mean annual water yield with 2-100 year return periods has been determined based on the best statistic distribution. Using hydrologic and physiographic parameters such as area, perimeter, mean elevation, length of main stream, equivalent rectangular length, basin slope, slope of the main stream, drainage density, Gravilius coefficient and mean annual precipitation have been used to obtain probable regional discharge equations with different return periods. These equations have been evaluated based on observed stations to choose the most suitable equations. After evaluation of obtained equations, more comprehensive equations have been chosen. The results show that in all eight regions, among different factors, area and annual precipitation have the most crucial role in estimating annual water yield. In addition mean annual depth of discharge over the basin has a better correlation with precipitation and area of the basin compared to discharge itself. Finally achieved regional equations with various return periods can be directly applied for the design of watershed management and water resource projects. Likewise, the results can be used to determine the amount of discharge in ungauged basins.