A Raster GIS Model for Water Supply Tower and Source Option Prioritisation in Community Based Water Supply Schemes at Attanagalla, Sri Lanka

Optimum location identification for the water tower and source is very important for any water supply scheme mainly due to storage capacity, elevation, landuse, yield of the source throughout the year, and the costs for transmission and distribution system. Towers need water from several alternative sources. Construction of distribution pipe lines is expensive due to physical features, terrain, water and urbanisation. Considering these factors design option prioritisation can be carried out by using Raster GIS. To demonstrate the potential of Raster GIS a case study was undertaken for the prioritisation of source locations for a Community Based Water Supply Scheme (CBWSS) to deliver safe and reliable drinking water for rural community living in approximately 64 km2 within Attanagalla of Gampaha District. A Raster GIS model was developed to prioritise the community based water supply scheme by using terrain features with the resolution of 10 m. Base layers for the key parameters of population, roads, elevation, land use, soil, rainfall and streams were prepared and analysed to obtain the final output. Four options of two tower and two source locations for CBWSS were evaluated and Ihalagama & Algama were selected for water tower & source respectively. This paper demonstrates the weighted overlay for the cost surface (60% Road +30% Slope+10% Soil) and the least cost path for transmission and distribution (5,486,173.50 in Cost units) and close proximity to the urban area was selected. Raster GIS can overlay the layers easily, has terrain modelling capability and incorporates cost functions. Therefore Raster GIS is a great facilitator for spatial modelling for the prioritisation of planning and management of water supply schemes