Assessing water management alternatives in a multipurpose reservoir cascade system in Sri Lanka

Study region: The Mahaweli multipurpose water resources system of Sri Lanka, spread across 25,500 km(2), incorporates the Mahaweli, Kala Oya, Malwatu Oya, Kantale Oya and Maduru Oya river basins. Study focus: We developed a model that can be used to assess water resources management alternatives of reservoir cascade operation to fulfill diverse and often conflicting water demands. The Mahaweli project is mainly operated for hydropower generation and irrigated agriculture. This study quantifies performances of water management alternatives considering trade-offs between hydropower and agricultural yield. Reliability, resilience, and vulnerability are other considerations that we explore. New hydrological insights for the region: In the Mahaweli reservoir system, water is used primarily for paddy irrigation and hydropower generation. Increasing water diversions for paddy irrigation leads to decreases in hydropower so in times of limited water availability, decisions about tradeoffs are required. In addition to diversions, decisions about how much arable land to cultivate during times of water shortage affect measures of risk related to paddy yield. Our results show that existing infrastructure places a constraint on how much water diverted for irrigation can be used productively and also leads to spatial variability in improvements in risk measures at the expense of reductions in expected yield across the basin.