Monitoring irrigation performance in Sri Lanka with high-frequency satellite measurements during the dry season

Information on crop-water demand, water use, soil moisture and plant growth at different stages of the cultivation can nowadays be retrieved from satellite data. In this research study, NOAA satellite data were used to assess the performance of three large irrigation systems in Sri Lanka during the 1999 yala (dry season from April to July): Polonnaruwa, Kirindi Oya and Gal Oya. The Polonnaruwa system has a total water supply that is greater than the demand; the relative water supply (RWS) is 1.88. The cropping system has used only 47.5% of the total irrigation supply. In the Gal Oya system, the overall water supply was 1.27 of the crop demand whereas the crop has utilized 71.1% of the total water supply indicating high irrigation efficiency. This shows that, in certain circumstances, under-irrigation can make water management more effective. The reasons may be the reuse of irrigation water through the drainage network and net groundwater draft. For both systems, productivity of water is high: Polonnaruwa 0.29 kg m3 and Gal Oya 0.40 kg m3 per unit of water inflow (precipitation plus irrigation). In Kirindi Oya, the RWS is higher (2.71) than in the other two systems and irrigation efficiency is considerably lower (32.4%). This paper shows that remote sensing measurements can help objectively analyze irrigation processes throughout the country and at a monthly time step. These quantitative irrigation data and its near-real time availability provides the Irrigation Department with information not previously available.