Impacts of Land Use Changes on Hydrological Regime - A Case Study of Randenigala and Kotmale Catchments in Sri Lanka
Ranjith Premalal De Silva
Department of Agricultural Engineering
University of Peradeniya, Sri Lanka
Madusha Chandrasekera
Mahaweli Authority of Sri Lanka
Damsite, Polgolla, Sri Lanka
Introduction
The perennial water of river Mahaweli that flows from the central hills in the wet zone to the plains of the dry zone has a potential of over thousand megawatt of hydropower from its annual discharge of 53200 MCM. In 1968, with the help of UNDP and FAO funding, a major, multi-purpose development plan named as Mahaweli Development Programme (MDP) was initiated and expected to cover a period of 30 years (Mahaweli, 1986). Subsequently, this programme was accelerated as Accelerated Mahaweli Development Programme (AMDP) in order to complete within 6 years targeting hydropower generation of 605 MW and irrigation of 167,000 ha of agricultural land in the development process. Under this programme, seven reservoirs across the Mahaweli river system were constructed. The investment on the AMDP was Rs. 5200 million. The success of this massive investment depends on the sustainability of the water resources in the catchment areas of these reservoirs. The catchment areas for these reservoirs are collectively called as Upper Mahaweli Catchment (UMC) based on the elevation profile of the catchment.
As a result of the development programme, considerable changes in the structure and composition of the land use and land cover in the UMCA have been very obvious during the last three to four decades. This was further enhanced through the revised government policies moving towards a free market economy (De Silva, 1993). According to Tschakrt & Decurtins (1989), pressure on vacant and less suitable areas, lack of appropriate new settlement alternatives, vulnerability of ecosystem due to high intensity of rainfall and high share of steep areas, constant changes of economic and market environment, lack of integrated and coordinated land use planning are some reasons of rapid depletion of catchment resource base. Critics now argue that these changes have adversely affected the hydrological regime of these catchments resulting diminishing river flows. Further, it is also pointed out that the productivity of the agricultural land has been diminishing due to severe land degradation. Frequent landslides and earthslips damage the infrastructure and threaten the human lives. In this situation, in order to resolve present problems and avoid a future crisis, a comprehensive assessment of land use changes, its spatial distribution and its impact of hydrological regime is required and accordingly, suitable remedial methodologies should be employed for the sustainable utilization of the land and water resources of the catchment.
Objectives
The basic objective of the study was to investigate the spatio-temporal information on the status of land use/ cover changes and corresponding hydrological variability at the sub catchment level and evaluate the impacts of these changes on hydrological regime of the catchments.
Study Area
Two sub catchments of the UMCA namely Randenigala and Kotmale catchments were selected for this study as these two sub catchments represent the diverse hydrological environments and considerable land use changes during the AMDP period. . The location of these two sub catchments within UMC is shown in Figure 01. The Randenigala sub catchment mainly belongs to intermediate zone according to the classification based on rainfall while Kotmale sub catchment belongs to wet zone.
Randenigala sub catchment lies on parts of Kandy and N-Eliya districts and covers 447.84 sq. km area, which is about 14.8% of the total extent of UMC. The average annual rainfall ranges from 1250 mm to 3000 mm and the elevation ranges from 160 m to 2000 m. The capacity of the Randenigala reservoir is 860 MCM at full supply level and installed hydropower generation capacity is 122 MW.
Kotmale sub catchment is located in N-Eliya district and covers 571.20 sq. km, which is about 18.8% of the total extent of UMC. The average annual rainfall ranges from 2000 mm to above 4500 mm and the elevation ranges from 600 m to 2524 at Pidurutalagala. The capacity of the Kotmale reservoir is 174 MCM and hydropower generation capacity is 134 MW.