Reliability of Land Capability Map in Watershed Hydrological Simulation using SWAT Model

Soil and Water Assessment Tool model, SWAT, uses different layers of watershed data which out of soil layer is the important one. Soil layer data should be extracted from detailed soil maps. In most developing countries including Iran, these detailed maps are not available except for irrigation districts and rarely for dry land farming areas and also for some strategic places of development purposes. For larger plains, the available soil maps information are reduced to semi-detailed scale which adds uncertainty in data required for catchments and hydrologic modeling. This research studied the relations between land use (LU) maps, land suitability for agricultural activities (LS) and physiographic soil unit (PU) on one hand and detailed soil maps and their embedded data on the other hand. The research has focused on the replacement of data derived from the detailed soil maps (from large scale soil map information) with those of readily available LS, PU and LU maps. Using these two sets of data for modeling of hydrologic system with SWAT on a 5793 Km2-watershed in west of Iran showed no significant difference between the simulated discharges at the watershed outlet. So the available LS map for the whole country may be used for both plains and catchments in SWAT simulation.