DISTRIBUTED ROUTING TECHNIQUES DEVELOPED ON GIS FOR GENERATING SYNTHETIC UNIT HYDROGRAPHS

Abstract

The increasing interest in small irrigation reservoirs in rural areas of Mediterranean countries requires detailed hydrologic analyses in order to assess water resources and design runoff discharges. Emerging technologies, such as Geographic Information Systems (GIS) and remote sensing, enhance the possibilities of distributed modeling in hydrology. This article presents several distributed approaches, based on precipitation and basin physical properties, which allow generation of synthetic unit hydrographs for routing excess rainfall. The routing models were implemented in compliance with the GRID module of ArcInfo GIS on a 10–m grid cell representation of the watershed. The generated excess rainfall was routed from each cell of the grid to the outlet of the watershed following the steepest slope path. Several approaches considering different runoff rates, distinction between overland and channeled flows, and different excess rainfall intensities were analyzed and used for predicting friction surfaces. A time–area method was finally applied to obtain surface runoff unit hydrographs. The proposed approaches were applied to two small rural watersheds near Madrid, Spain. The estimated unit hydrographs clearly show the need to discriminate between overland and channeled flows. Increase in excess rainfall intensities significantly affects the unit hydrograph by gradually increasing hydrograph peak and decreasing base times. It appears that there is a significant influence of excess rainfall intensity in the resulting hydrographs for small basins, where channel storage is of little importance. The results were compared to those obtained by means of two lumped unit hydrograph techniques for ungaged watersheds (USDA–SCS dimensionless and triangular unit hydrographs). The lumped models give similar results to those based on a variable flow approach with excess rainfall intensity of 3 mm/h, which corresponds to a 2–year return period for the studied watersheds.