Dry drainage effect of Hetao irrigation district based on remote sensing evapotranspiration

Abstract: In arid region, a great amount of irrigation water diverted to irrigation districts leads to the rising of groundwater table in irrigated agricultural area. As a result, groundwater and salt flow from agricultural area to non-agricultural area, leading to the internal drainage for groundwater and dry drainage for salt. In order to analyze the water and salt transfer between agricultural and non-agricultural areas, water and salt balance models of agricultural and non-agricultural areas were proposed based on evapotranspiration estimated from hybrid dual-source scheme and trapezoid framework based evapotranspiration model (HTEM). Four counties (Linhe, Wuyuan, Dengkou and Hangjinhouqi) in Hetao irrigation district of Inner Mongolia Autonomous Region were selected as the study area, and the soil water balance and total water balance models were applied to estimate groundwater evaporation of agricultural and non-agricultural area, and groundwater and salt transfer from agricultural area to non-agricultural area on the annual time scale. Results showed that annual average evapotranspiration in the study region in 2006-2012 was 5.278 billion m3, among which the agricultural area occupied about 62%, and annual evapotranspiration rate of the agricultural area (679.1 mm) was higher than that of non-agricultural area (395.3 mm). Evapotranspiration during the vegetation growth period from April to October occupied about 90% of the total, while that during frost period from November to March was only about 10% of the total but should not be neglected. Annual average groundwater evaporation was 1.347 billion m3, where agricultural and non-agricultural areas accounted for approximately 50%, respectively. The annual average amount of groundwater transfer from agricultural area to non-agricultural area was 355 million m3, which accounted for 10% of annual average net water diversion and was close to the amount of drainage water into the Ulangsuhai Lake from the whole irrigation district. The annual average amount of salt transfer from agricultural area to non-agricultural area was 1.517 million t through groundwater. This resulted in an annual average desalinization of 0.4 t/hm2 in agricultural area and cumulative salt content of 2.7 t/hm2 in non-agricultural area. The results suggested that interior drainage and dry drainage play important roles in controlling groundwater table and soil salinization of agricultural area in Hetao irrigation district, and they should be integrated with the drainage projects in the planning of water and salt drainage.