Spatially Distributed Hydrological Modelling considering Land-use changes using Remote Sensing and GIS
Warwick and Haness (1994) used Arc/Info to determine hydrologic parameters directly for HEC-1 hydrologic model while separate line coverage defining the runoff routing are created manually. Suwanwelarkamtorm (1994) derived semi-distributed hydrologic modeling using GIS for the management of watersheds and assessed the effect of land-use change using integrated approach with HEC-1 and ILWIS. The ability of the model to simulate future and past flood hydrographs based on hypothetical future and past land-use conditions were demonstrated. The results of the simulation runs show that when the forest area is reduced, more runoff will occur in every sub-catchment and also at the outlet.
In this study, the computational elements of ‘Hydrological Similar Units’ (Ott et al, 1991) are considered in distributed hydrological modeling with accurate mapping of land-use at micro level using remote sensing and GIS. In this paper, the focus is on spatially distributed modelling using remote sensing and GIS, considering physiographic heterogeneity in terms of topography, soil, and land-use. The model will assess the hydrological change due to land-use change, which is most essential study for 21st century with the problem of rapidly growing urbanization, or land-use changes.
System for Study
The system considered for the study is Kathmandu Valley basin. The valley is a roughly circular bowl shaped intramontane basin, of 651 km2 and lies between 27° 32' N to 27° 49' N and 85° 11' E to 85° 32' E. Bagmati river is the main river originates from north hill and flows towards south-west and forms a typical centripetal drainage system. It passes through Chovar gorge, which is the only outlet of the basin. The maximum and minimum temperatures are 35°C and -2.5°C respectively. The rainfall occurs about 80% of the total annual rainfall during the months of June to September. The average annual rainfall in the basin is 1600 mm.
The land use map for the year 1978 is derived from topomaps using Arc/Info. Digital images for 1984 (Landsat TM), 1990 (Landsat TM) and 1996 (Landsat TM) are used to derive the land use maps by digital image process. Maps are projected in Modified Universal Transverse Macerator (UTM) with spheroid system of Everest 1830 on scale factor of 0.999 at Central Meridian. Visual image interpretation of satellite data is carried out using an interpretation key generated through field survey and verifications. The ground checks are made for confirming the land use units. The spatial database containing information on land use, soil type, topography, hydraulic characteristics and meteorological information is created. The hydrological soil group (HSG) map is derived from the soil map whereas subabsin boundary map is derived from the drainage map. There are 14 subbasins delineated. The Thiessen Polygon map is derived using available rain gauge stations. In the study area, the forest (mountainous) area is about 30% of the total basin area having slop range from 20 to 30%, and remaining area (70%) is having average slope of 0 to 4%. The daily and monthly rainfall record of 9 raingauge stations for period 1965 to 1996 are used. The daily data for five stream gauging stations, namely Chovar, Gaurighat, Buddhanilkantha, Sundarijal and Tika Bhairab are collected. Some missing records are filled in considering the correlation structure with other stations. The correlation coefficients are found in between 0.87 and 0.97.