Investigation of land Use Effect on Nash Model Parameters

Flood forecasting is of great importance in hydrologic planning, hydraulic structure design, water resources management and flood control. Nash’s Instantaneous Unit Hydrograph (IUH) is frequently used for simulating hydrological response in natural watersheds. Rapid development of urban areas affect the hydrologic process of the watershed by decrease in soil permeability, flood base flow, lag time and increase in flood volume, peak of the hydrograph, flood frequency and surface runoff. In this study the influence of land use like urbanization on the significant parameters of the Nash model have been investigated. These parameters calculated using three popular methods i.e. moment, root mean square error and random sampling data generation, in two sub-watersheds with different land uses in the city of Sierra Vista, Arizona. The results showed that in all three methods the lag time of the watershed, which is product of K by n, in natural watershed is greater than urban one, which means more storage in natural watershed. The median of three mentioned method in calibration events were used in remain events for verification. The results showed that all the three method have acceptable accuracy in hydrograph simulation. Some specific upper and lower percentile values of the median of the generated parameters (i.e. 10, 20, 30 and 40 %) were analyzed to future investigates the derived parameters. There is a wider range in parameter values in urban sub-watershed than the natural one in which the efficiency has an acceptable value. It might be due to less uncertainty in urban watersheds where runoff to rainfall ratios are much larger than in the natural sub-watershed.