A Quantitative Analysis of Surface water in the Uruboku Oya basin Demonstrating the Application Potential of IWRM Principles to Complex Irrigation Systems

Abstract

Utilizing IWRM principals to manage water resources is often limited to policy and institutional options which are qualitative in nature (Mehta et al., 2016). Though application friendly modelling examples which satisfactorily incorporate both water quantity and thresholds of quality are essential for watershed managers to ensure educated participatory management the lack of detailed case studies has been noted as a gap that needs to be filled. Muruthawela irrigation scheme of Uruboku Oya basin in Hambantota district of Sri Lanka having a medium scale reservoir of 47.8 MCM capacity, a source area of 4400 ha, and a command area of 1710 ha was taken as a case study. Irrigation, water supply & sanitation, hydro power, inland fishery and environment are the competing water use sectors associated with the system. This system which frequently experiences water conflict situations has limited data to evaluate sectoral water uses. A water balance model for this system was developed to assess multiple water uses by incorporating both water quantity and quality. A situation analysis was carried out with available measurements, guidelines and rational approximations using field observations. This study with an order of magnitude water balance study demonstrated the capability to evaluate the present water conflict scenario and then propose a solution to manage the water quantity and water quality of the system to satisfy all stakeholders. The study concluded that the alternative of IWRM can increase the cropping intensity of Muruthewela scheme by 35% (up to 100%) in maha with introduction of cowpea and allocation of 55.6 MCM for total Irrigation demand while allocating 0.9 MCM annually for water supply and sanitation sector. Pollution status in downstream of Muruthewela tank were evaluated at three locations, Node A –Udukiriwila, Node B-Wakamulla & Node C-Andupelena in order to identify the most vulnerable section for pollution due to agricultural & domestic return flows. The threshold value of dilution was taken as 8 as recommended by Central Environmental Authority (Central Environmental Authority, 1980). The study found that Node C-Andepelena is the most vulnerable section for pollution and the priority area which needs attention by all stakeholders. Pollution level at Node B-Wakamulla can be managed to a certain extent by releasing an environmental flow of 2.4 MCM (4.3% of Irrigation demand) annually