Grid based rainfall-runoff GIS modelling to study the anthropogenic effect on the hydrology of a small watershed

In the Po basin urban development, land use change and variations in rain intensity have influenced watershed runoff and increased floods. In order to better study the anthropogenic effect on the basin hydrology the Olona river watershed proved to be an interesting small catchment to test. It went through a rapid change from agricultural land to urbanized and partly forest areas over the period 1954 – 1994. The Olona River is known for frequent flooding along its course and in some districts of Milan city. Using SCS-CN methodology, a rainflow-runoff model was calibrated with the observed outflow data in the Ponte Gurone station in the upper Olona basin (≈110 km2 ). The extreme rainfall data series of three meteorological stations near and inside the watershed are statistically elaborated to obtain the depth duration frequency curves, used as the model input data. Basin concentration time and SCS parameter Ia (Initial abstraction) values are chosen to fit the model results and the observed discharges in the two rainfall intensity classes tested. Within the upper Olona basin, waterproof surfaces increase, leading to less infiltration and a quicker runoff, and produce higher outflow peaks. Model results give an estimate of land use change effect on the basin rainfall-runoff process change.