Modelling the hydrological impacts of land cover change in the Siran Basin, Pakistan.

Many forested catchments in northern Pakistan have undergone land cover change during the last few decades. Extreme floods and extended droughts observed in these areas have lead to the question: How do human influences affect the water balance of a montane catchment. The underlying socio-political factors that have lead to the changes in forest cover and catchment hydrology are well documented, but there have been very few efforts to spatially correlate the cover changes with the catchment water balance. A deterministic model based on high resolution spatial and temporal data offers the ability to simulate the hydrological impacts of changes in land cover in a spatial context. In an attempt to assess the impacts of changing forest covers on individual hydrological processes, a GIS-based model Siran_HYDMAPS has been developed for the Siran Basin, Pakistan. This model integrates the spatial databases with the well-known hydrological process algorithms (e.g. Penman-Monteith evpotranspiration and Green-Ampt infiltration models). Spatially distributed static (topographic and soil) parameters for this model are extracted from a regional GIS developed specifically for the project. The dynamic (vegetation-related) parameters are estimated from the land cover maps, derived by digital processing of multi-resolution, multi-temporal Landsat MSS (5.3.1979) and TM (10.7.1989). Relative relief and shadowing in rugged terrain of the Himalayan foothills, that cause major problems in image processing, have been given particular attention. A rule-based approach was adopted to refine land cover maps with the integration of GIS for mapping the level II forest classes. Mapping of forest cover changes was carried out by post-classification change detection techniques. The Siran_HYDMAPS predicts a decrease in radiation balance and interception capacity, and an increase in evapotranspiration and catchment response of the Siran Basin, as a result of land cover changes. It was concluded that the water imbalances in this catchment, observed during the last two decades, were caused by the integrated effects of land cover changes and climatic factors.