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Impacts of Land Use Changes on Hydrological Regime - A Case Study of Randenigala and Kotmale Catchments in Sri Lanka
Since 1956, tea, annual crops, forest scrublands and water bodies at Randenigala show considerable changes and only significant change in land use in Kotmale is tea. Land use change detection maps revealed that the tree cover has increased by 8.45% in Randenigala catchment due to the establishment of forest sanctuaries under the AMDP while 8.62% reduction of tree cover is found in Kotmale catchment as a result of the nationalization policy of the tea industry. The change of tree cover in opposite directions in these two catchments resulted an interesting approach to evaluate the impacts of tree cover change on hydrology of these catchments. In addition, 15.25% reduction in annual crop cultivation was observed at Randenigala catchment and an increase of 3.23% of annual crops was found at Kotmale. Further, a 4.92% decrease in grasslands was found at Randenigala while it was 1.97% increase at Kotmale. The direction of change of the other land use categories was the same for both catchments.
- Land Use changes and Hydrological Response
The hydrological responses to land use changes were investigated through the analysis of rainfall runoff relationships. There were no obvious impacts of the change of tree cover or any other land use changes on the river flow during rainy seasons. The analysis of rainfall runoff correlation coefficient and runoff to rainfall ration do not show the effects of changing tree cover and other marginal land use changes during the runoff generation process. However, there were obvious deviations identified in the dry weather flow during the last decade in both catchments. This indicates that there is an interaction of the land use and rainfall in determining the hydrological response. The increase of the dry weather flow in Randenigala could be related to the increase of the tree cover and the reduction in canopy cover could be attributed to the decrease in dry weather flow at Kotmale. Further studies revealed that in addition to the buffering action of forest to produce delayed flows, there is a significant contribution of water from the fog interception at high altitudes where dense tree cover is present. Fog interception at very high altitudes in Horton Planes was found to be reaching 48% of the total precipitation during the dry months. This study provides conclusive evidence that the increase in tree cover would positively contribute to the water yield in the catchments in addition to its protective role of the environment.
Discussion and Conclusions
Considerable land use changes were found in both catchments due to the AMDP, which was implemented within a relatively short time. However, hydrological responses have not been adversely affected due to the changes in the land use. It can also be argued that the time span of the data series is not sufficient to manifest any real changes of the hydrological regime. Rainfall runoff relationships do not display any deviations from the stationary process. However, this study needs to be continued to include more time series data and the next stage of development should focus on GIS modeling approach in order to simulate the catchment dynamics for different land use scenarios. A comprehensive spatial modeling environment for hydrological modeling would provide information for land use planning and water resources management to ensure the sustainability of the benefits of development programme and the catchment environment.
References
- Chandrasekera, C.M.M.M.K (2000). Investigation of Hydrological Responses to Land Use Changes in Two Sub Catchments of the Upper Mahaweli Catchment, Unpublished M.Phil Thesis, Postgraduate Institute of Agriculture, University of Peradeniya, Sri Lanka.
- De Silva, W.P.R.P. (1993). Determination of Water Resources Sustainability of the Upper Mahaweli Catchment by Time Series Analysis, Tropical Agriculture Research, Vol 5, pp 11-14.
- Mahaweli (1986). Projects and Programme,Ministry of Mahaweli Development, Colombo, Sri Lanka.
- Tschakert, H & Decurtins, S. (1989). The Project Concept of Watershed Management in the Upper Mahaweli Basin. Hydrology of the Natural and Man-made Forests. Sri Lankan- German Upper Mahaweli Watershed Management Project, Kindly, Sri Lanka.
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