Improving water quality in the nile delta irrigation network by regulating reuse of agricultural drainage water

In Egypt, the reuse of agricultural drainage water provides an integral supplement to the fresh water supply. Government pumping stations (official) and farmers’ small diesel pumps (unofficial) lift water up from drainage canals and direct it back into the irrigation canals for reuse in agriculture, thereby increasing the country’s available water resources by 12.6%. However, as water passes through the soil and drainage network, it sorbs salts, agricultural chemicals and other pollutants, leading to differences in the quality of drainage and irrigation water. Therefore, mixing the two water types deteriorates the overall quality. The common practice in Egypt is to mix drainage into fresh water up to the point, where the salinity of the mixed water approach 1000 mg L-1. Some canals in the Kafr El-Sheikh Governorate have deficits of fresh water in some months. Therefore, agricultural drainage channeled back in culverts connect the canal ends with the main drain (Bhr Nashrat) to provide supplemental water. However, this return is not controlled and the flows are based only on differences in the hydraulic head. Current study evaluated the efficiency of using backflow to supplement the fresh water. Water supply ratio (WSR) indicator was employed in the analysis within two water supply conditions: (1) Fresh water supply only and (2) Fresh water supply plus backflow. During the summer 2008, WSR showed an average value of 0.93 and adding the backflow increased it to 1.27. During the following winter, WSR showed an average value of 1.50 and adding the backflow increased it to 1.82. Salinity measurements of water were taken during the study period at four locations - head, middle, tail and drain. Salinity significantly increased toward the end of the canals. Excessive backflow is the most serious constraint. The effect of backflow on the salinity was calculated and improved salinity values were obtained by regulating the backflow. During the summer, backflow significantly deteriorated water quality, but only part of this backflow was actually required in June and July, just to make up the shortage and not to exceed the requirements. During the following winter, fresh water availability was generally sufficient. However, backflow still occurred, leading to unnecessary deterioration of the water quality. If backflow is controlled according to the actual requirements, the water quality would be improved. An improvement in water salinity of over 30% was realised in June and July and by 100% in May, August, September and all winter months.