Assessing water- use efficiency in the agricultural sector in the Malwathu Oya river basin under changing climate scenarios

Abstract

Climate change, conventional agricultural techniques and malfunctioning irrigation systems pose significant challenges to efficient water management in the dry zone of Sri Lanka. Agricultural activities in the dry zone mainly depend upon irrigation reservoirs attached to the hydrodynamics of dry zone basins. Assessing the water use efficiency (WUE) of an irrigation scheme in a water-scarce region is paramount for developing a robust water management strategy considering the escalating impacts of climate change. Nachchaduwa Irrigation Scheme, situated in the Malwathu Oya basin, Sri Lanka, constitutes the required characteristics of a dry zone irrigation system for this research study about climate change impacts on irrigation supply efficiency. The developed methodology includes two primary components: crop water requirement estimation and irrigation supply estimation. The analysis of crop water requirement estimates revealed distinct seasonal variations. During the Maha season, the crop water demand decreased due to increased rainfall events, with the estimated mean crop water requirement dropping from 1,166.87 mm/season in the base period to 1,076.15 mm/season under SSP 1-2.6 and 1,053.17 mm/season under SSP 5-8.5. Conversely, the Yala season experienced higher water demand under the SSP 5-8.5 scenario with 774.41 mm/season, where the base period requirement is 767.18 mm/season, indicating a 0.9% rise. The HEC-HMS model has been used to simulate river inflow to the Nachchaduwa Reservoir prior to obtaining the irrigation supply through reservoir operation simulations. Results indicated significant variability in future water availability in the upstream catchment, with a projected mean annual inflow decrease of 8.14% under the SSP 5-8.5 scenario and an increase of 43.16% under the SSP 1-2.6 scenario. The increased variability in inflow highlighted the potential for frequent and severe extreme rainfall events under considered climate scenarios. The two-point hedging rule theory was adopted in reservoir operation simulations for irrigation supply estimation. The annual mean irrigation supply during the base period ranged from 866.56 mm/season to 1,148 mm/season, which shows a decline under the SSP 1-2.6 scenario but increases the variability under the SSP 5-8.5 scenario. These findings manifest the need for resilient irrigation management practices to address future climatic challenges in dry zone irrigation schemes in Sri Lanka.