河套灌区典型灌排单元农田耗水机制研究

由于耕荒地交错分布、作物插花种植、地下水埋深浅等特点,河套灌区灌溉水利用情况极为复杂。该研究以灌区典型灌排单元(农渠尺度)为研究对象,基于2 a野外试验观测数据,对整个观测区及其内部的不同作物田块分别建立水平衡方程,推求研究区平均给水度和不同作物田块腾发量,继而对研究区灌溉水利用状况及盐分归趋进行了评价分析。结果表明,研究区给水度为0.062,而仅考虑地下水位变动带的给水度为0.037;该研究提出的经验方法"上升下降法"可以较好地估算不同作物田块的腾发量;2 a中农毛渠系统输水损失水量(包括渗漏和蒸发)约占引水量的18%,灌到田间的水量占76%,直接退走的水量占6%;通过不同土地类型间地下水的横向交换,农田不但全部利用了田间灌溉水量,还通过地下水侧向流入的方式利用了约12%的渠道渗漏量,荒地利用了约65%的渠道渗漏量,排沟排走了23%;最终研究区农田腾发消耗了总引水量的78%,积累了总引入盐分的39%,荒地腾发消耗了总引水量的11%,承纳了总引入盐分的40%。研究结果可为灌区水盐管理提供依据。

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.