Abstract:
In most parts of Sri Lanka, water is the critical factor for cultivation. Using the appropriate amount of water is the key factor and therefore efficient water management is very important to increase food production. Common practice of Irrigation water distribution is with the help of Irrigation Department Guidelines. Present work is a study of irrigation water issue practice in Rajangana Irrigation Scheme situated at Anuradhapura which is a district the North Central Province of Sri Lanka. The present study using field data from 2008-2013, computed the theoretical irrigation water requirements as recommended by the Guideline using 75% probable rainfall values and this was named as " Recommended Irrigation Plan", Then this plan was modified with the consideration of actual rainfall that had been experienced during operations. This modification represented the actual water issues that need to be anticipated during operation and hence was named as "Anticipated water use". At the Rajangana Irrigation scheme there is also the actual plan developed for each year along with the water issue at the sluice gate corresponding to the 2008-2013 period. The present research compared the case of Left Bank gravity fed irrigation area which covers an approximate 2500 Ha area with 39 Km tertiary canal network. This area is cultivated mainly with paddy for two main rainy seasons namely "Maha" and "Yala". Water issue model for the study was developed at a weekly time resolution. Comparison of actual water use with the quantities are computed by following Irrigation Department Guidelines disclosed a significant over issue in Maha and Yala seasons amounting to 63% and 52% respectively. In the case of making adjustment to the plan with the receipt of actual rainfall, then a further reduction of water issue by 35% and 8% in Maha and Yala respectively could have been possible. Evaluation revealed the need of gauge network, a spatially distributed performance monitoring system and a critical evaluation on the base of present Guideline in order to suitably manage the water utilization in the Rajangana Left bank irrigation scheme.
