The Evaluation of the Impact of Development on The Quantity and Quality of Runoffs Using SWAT (Case study: The Langat urban watershed, Malaysia)

In recent decades the Hulu Langat basin that is a strategic watershed located in Malaysia، has been exposed to extensive changes in land use pattern and consequently hydrological condition. The impact of Land Use and Cover Change (LUCC) on hydrological conditions (water discharge and sediment load) of the basin are investigated using the Soil and Water Assessment Tool (SWAT) in this study. Four land use scenarios were defined for land use change impact analysis، i. e. past، present (baseline)، future and water conservation plan. The land use maps dated 1984، 1990، 1997 and 2002 were defined as the past scenarios for LUCC impact analysis. The present scenario was defined based on the 2006 land use map. The 2020 land use map was simulated using cellular automata-Markov model and defined as the future scenario. Water conservation scenarios were produced based on guidelines published by the Malaysia’s Department of Town and Country Planning and the Department of Environment. Both model calibration and uncertainty analysis were performed using the Sequential Uncertainty Fitting (SUFI-2) algorithm. The model robustness for water discharge simulation during the period 1997-2008 was good. However، due to uncertainties that mainly result from intense urban development in the basin، its robustness for sediment load simulation was only acceptable for the calibration period of 1997-2004. The optimized model was run using different land use maps over the periods 1997-2008 and 1997-2004 for water discharge and sediment load estimation، respectively. In comparison to the baseline scenario، SWAT simulation using the past and conservative scenarios showed significant reduction in monthly direct runoff and monthly sediment load، while SWAT simulation based on the future scenario showed a significant increase in the monthly direct runoff، monthly sediment load and ground water recharge.