Incorporation of SWAT & WEAP models for analysis of water demand deficits in the Kala Oya River Basin in Sri Lanka: perspective for climate and land change

Kala Oya is one of the drier river basins in Sri Lanka that is affected by droughts for certain time periods. Water shortages are visible in crop yields and public water supply due to climate change. Consequently, the Soil and Water Assessment Tool (SWAT) was used to develop hydrological models, and Water Evaluation and Planning (WEAP) software was used to analyze the water allocation for the basin. The software's evaluation and assessing capability of allocation of water, transmission, and diversion links for demands are some reasons to use WEAP as a separate water allocation model. The future land use and climate aspect (2040) has also been included in these models to enable the generation of "scenarios" that can be used to test the demand deficits for irrigation and public water supply. Three climatic conditions such as optimistic, pessimistic, and average for 2040 were considered for modeling. Our major findings include: 1. The pessimistic climate change scenario exhibits the highest rise in drought metrics while the optimistic represents the lowest. Under current land use conditions, annual Long-Term Average (LTA) public water supply deficits are 1.0 million cubic meters (MCM) (3.2%), and for future land use, in a pessimistic climate change scenario, annual LTA deficits are 4.7 MCM (4.4%). 2. For medium/major irrigated agriculture, annual LTA deficits for current conditions are 42.8 MCM (9.0%), and for future land use, pessimistic climate change scenarios are 56.1 MCM (12.7%). For minor irrigation, annual LTA deficits for current conditions are 20.8 MCM (17.9%) and future pessimistic climate change scenarios are 24.2 MCM (20.2%). 3. This study concludes that the public water supply demand deficits are considerably greater in the middle and lower catchments of Kala Oya basin for future land use (with basin developments) model simulations. This may create water scarcity and social stress for people who require immediate mitigation measures. 4. Overall, it was revealed that the agriculture-oriented drought losses (major/medium irrigation) are significant (around 66–67% of total demand deficits) in the Kala Oya basin, and they may create adverse impacts on the country's economy due to crop yield losses.