Modernization of an irrigation system : case study in the Heihe river basin, China

The Heihe River Basin is the second largest inland river basin in the arid area of China. Since the 1960s, water shortages have been growing in magnitude and frequency of occurrence due to the increasing industry, domestic and especially irrigated agriculture consumption in the middle catchment creating severe water conflicts and ecological degradation. One gravity irrigation district of about 15,000 ha in the middle catchment, the Yinke Irrigation District (YID) has been selected as a case study. The present overall water use efficiency is about 27 - 33% in the YID. Therefore, there is a huge potential to improve the water use efficiency by modernization ofthe present irrigation system. The main objective of this study is to analyze and to evaluate the existing irrigation system in the YID, and to develop methods or to give suggestions on how to modernize the present system. The study mainly focused on two technical and structural aspects of modernization, namely the on-farm practice and the water supply to the ~. A preliminary analysis shows that nowadays the water application efficiency is 56 - 63% for cereal crops, and 48 - 55% for cash crops. A detailed study of the relationships between the application efficiency, land shape, irrigation depth, land slope and the Manning coefficient for basin, border and fw""TOW was made. The study reveals that a high efficiency can be achieved for basin; a moderate to high efficiency can be achieved for border and furrow, if the inflow rate and duration are properly controlled. The potential application efficiency of border and furrow has been studied in more detail and the results can be used as a guideline for the selection of design application efficiency, irrigation depth, or for the modification of land shape or slope. The present irrigation schedule has been evaluated and an ideal irrigation schedule is suggested, which leaves as less water as possible in the soil after harvest, and which will refill the soil moisture reservoir just before the wintertime. This will prevent useless evapotranspiration and the soil moisture reservoir will be recharged when river water is relatively abundant and when there is no need by the corps. This schedule will avoid refilling of the root zone during the springtime when water in the river is ~carce. When the ideal irrigation schedule is adopted and the 70% application efficiency can be achieved, then the irrigation water supply will be reduced by 50% during the springtime, and about 20%, 23%, 34% and 240/0 of irrigation water will be saved from spring wheat, nonnal maize, potato and capsicum respectively, which are the main crops in the YID. Some suggestions are given on how to improve the present on-farm practice. An analysis of the present water supply to and distribution within the YlD shows that they are neither reliable nor equitable. The main reason is the unstable tail water of the hydropower stations, which is the only water resource of the YID. Evaluation of the water supply by Modernization of an irrigation system - Case study in the Heihe River Basin, China computer modelling shows that the present proportional water distribution from time to time and the operation of hydraulic engineering facilities upstream of the YID make the water supply even more unstable than expected. A new plan for proportional water distribution in about every 24 hours has been suggested. Alternatives of real time flow control by either manipulating the two existing gates or one gate together with a newly constructed Duckbill weir have been studied, which will result in a nearly constant water supply to the YID