Asian Journal of Research in Agriculture and Forestry

The allocation of water from irrigation reservoirs to urban drinking supplies directly reduces the quantity available for agricultural use. Understanding this impact is crucial for evaluating changes in agricultural performance and crop yields. This study aimed to assess the effects on agricultural performance of Unnichchai irrigation scheme by assessing water availability and comparing key performance indicators before and after the implementation of the drinking water initiative. Results indicate that average output per crop area, output per unit command area, output per unit irrigation supply, and output per unit water consumed increased after the introduction of the drinking water scheme. During Maha season, these values improved from Rs.107,340/ha, Rs.92,674/ha, Rs.80.05/m³ and Rs.11.60/m³ before implementation, to Rs.239,081/ha, Rs.248,930/ha, Rs.117.17/m³ and Rs.25.84/m³, respectively after implementation. Similarly, for Yala season, values increased from Rs.108,457/ha, Rs.66,195/ha, Rs.7.04/m³ and Rs.7.57/m³,to Rs.358,558/ha, Rs.330,580/ha, Rs.29.45/m³ and Rs.25.02/m³, respectively. Water performance indicators also showed variation. Average relative water supply for the Maha season increased from 0.54 (2000–2011) to 1.36 (2012–2019), while for the Yala season, it improved slightly from 0.72 to 0.74. However, the relative irrigation supply decreased for the Maha season from 0.31 to 0.25 during the same periods, whereas it increased in Yala from 0.72 to 0.78. Average cropping intensity improved from 1.83 to 1.91 between the two periods. The average irrigation ratio during the Maha season was 0.78 for the period 2000–2011, but it increased to 1.0 for the period 2012–2019. For Yala season it increased from 0.47 (2000–2011) to 0.92 (2012–2019). Despite concerns, the study concludes that the overall performance of the Unnichchai irrigation scheme improved even after the implementation of the drinking water supply project, suggesting effective management of competing water demands. However, the system’s performance remains lower compared to other major irrigation schemes in Sri Lanka, necessitating targeted interventions to achieve further improvements.