Irrigation water use in typical irrigation and drainage system of Hetao Irrigation District

Abstract: The Hetao Irrigation District (Hetao) located in the upper Yellow River basin represents a very typical arid agro-ecosystem in northern China. Due to complex cropping pattern and shallow groundwater table, the irrigation water use in Hetao is rather complicated. Clarifying the mechanism of irrigation water use and salt transportation in Hetao is of great significance to agricultural production and ecological sustainability in these areas. In this study, a typical irrigation and drainage system (Yangchang (a second-to-last order canal) canal area (YCA)) located in Hetao was selected as a case study area. Field experiments were conducted in it during the growing season (from April to September) in 2012 and 2013. Based on the 2-yr field observations, water balance model for the whole study area as well as the different crop fields within it were set up. For the field scale water balance model, to overcome the difficulties caused by lateral groundwater exchange among fields, the water balance calculations during groundwater level rising or declining period were conducted separately based on the water table fluctuation characteristics of Hetao. During the water level rising period, the FAO56 crop coefficient method was directly used to estimate actual evapotranspiration (ETa) as most of the fields were irrigated and the root zone had good salt and water conditions (almost no water/salt stress on vegetation). During the groundwater level declining period, a simple water balance equation was used to calculate ETa as the amount of lateral groundwater exchange could be neglected. This method for calculating field scale ETa was named "Up-Down Method". Thus the regional scale ET could be calculated by summing up all the field scale ETa with their area weight. Then the average specific yield of the study area could be calculated using the regional scale water balance equation during the groundwater level rising period. Based on this, the ETa of different crop fields and ET of the whole study area were calculated. At last, the irrigation water use, salt accumulation and redistribution were discussed through the water and salt balance calculation. Results showed that the specific yield for the groundwater level fluctuation zone was 0.037, while for the whole soil profile was 0.062. The ET of YCA from May to September was respectively 514 mm and 475 mm in 2012 and 2013. For the different crop fields in YCA, their evapotranspiration (ETa) was not so much different during the whole study period, however, that during the growing season of each crop varied a lot. The crops with longer growth period usually had a larger ETa during the growing season. The empirical method "Up-Down Method" can give a reasonable estimation of crop field ETa in Hetao comparing with model simulation. Analysis on the water and salt consumption and redistribution showed that during the growing season, the cropland ET consumed 78% of the total water diversion and accumulated 39% of the total salt introduced; while the natural patches consumed 11% of the total water diversion and accommodated 40% of the total salt introduced. This study provides a sound guidance to water and salt management in irrigation districts.