Estimation of Runoff from Precipitation is a base for most the different development and utilization projects of Water Resources. So due to environmental constraints, its measurements and computations always face a lot of problems.
The study of most of the hydrologic models and methods indicates that due to the lack of enough and accurate statistical data on the effectiveness of temporal patterns and some other environmental problems. Sometimes, their coefficients show errors. Nowadays, through combining the empirical models and methods with GIS techniques, We can determine their unknown coefficients in order to obtain correct and precise runof1. In this research, Watershed of Kashkan River with the area of 9275.66 Km2 in the Southwest of IRAN located in Lorestan Province, is chosen as a case study.
To approach the desired goals of the project, annual Precipitation data, within the years 1964-1996 (32 years) were deducted. Then, in order to analyze the obtained data, Isohyetal Maps (Interpolation methods) were prepared using GIS algorithms. The main goal has been to test a combination of empirical models with GIS techniques to prepare lsohyetal maps (models) in order to measure the local rainfall anti effective Precipitation to he used in Rainfall-Runoff Simulations.
Results (using linear equations) show that correlation coefficients between the observed and computed data (Rainfall-Runoft) fbr the subbasin of this region are between 97.5 and 99.99 percents, In conclusion preparing Isohyertal models using GIS is recommended as the most certain criteria (standard) method of deter-mining (measuring) the local rainfall in studies related to Water Resources.
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