反渗条件下排水沟与农田水盐交换关系

因干旱和半干旱下游灌区地势较低,排水出路不畅,排水系统往往成为承泄区外来水(上游灌溉退水和排水)的蓄水场所,使排水沟水位高于农田地下水位,反渗补给农田地下水,作物利用部分排水以后,如何维持农田良性的水盐平衡成为下游灌区一个迫切需要解决的科学问题。该文基于农田水盐平衡原理,以陕西一半干旱区下游灌区为例,在实测资料的基础上,首先利用田间水文模型DRAINMOD模拟了排水沟蓄水条件下,农田水位变化情况,然后计算分析了农田与排水沟的水盐交换关系。结果表明:在一个完整的种植年内单位长度排水沟上累计承接区外来水量为9.3 m3,减去流出水量,累计蓄积区外来水量为5.5 m3,农田单位面积上反渗累计补给田间地下水量为49.2 mm;累计农田排水量仅为2.3 mm。与作物蒸散发相比,现状条件下补给量虽然较小,但对维持和补给农田地下水起到了一定的作用。所产生的补给作用虽然增加了排水沟内盐分向田间地下水中的运动,但作物利用地下水过程中根区没有出现严重的盐分累积,对田间地下水盐分浓度影响也不大。所以,通过合理调控措施,充分利用区外来水,可以提高水资源利用效率。但排水系统长期运行条件下,高水位对农田水盐平衡的影响尚需进一步研究。

Salt and water exchange between drainage ditches and farmland under sub-irrigation condition

Abstract: Many downstream irrigation areas are constructed with drainage ditches to prevent the potential threat of soil salinization; but the irrigation water supply to these areas can rarely be guaranteed due to their disadvantage of being located far away from the water source. It is critical for local agricultural development and environmental protection to find practical water management practice for crop production in these downstream irrigation areas. In this paper, we present an analytical study on salt and water exchange between drainage ditches and the farmlands in a downstream irrigation area, where the drainage ditches were periodically filled with a large amount of inflow (irrigation return flow and drainage water) from the upstream irrigation area. Due to its low elevation and poor drainage outlet of the study area, the drainage ditches originally built for salinity control now capacitate water storage for crop fields in the growing season; the high water level in the drainage ditches produced sub-irrigation effect on crop fields to meet some crop water requirement. To investigate the effect of reduced drainage intensity on salt and water balance in the crop fields, we employed the field hydrology model - DRAINMOD to predict the field water table fluctuations under subirrigation condition based on observed data from summer 2009 to fall 2010. Salt and water exchange between crop fields and drainage ditches was then calculated based on the predicted water table variations. The results showed that the water table depth in the study area was generally below 2 m, while the water level in the drainage ditches was generally higher; the salinity level in drainage ditches was much lower than the groundwater in the crop fields. The water table depth predicted by the DRAINMOD model agreed with the field measurements reasonably well; the average deviation was 3.29 cm, the mean absolute error was 8.6 cm, and the correlation coefficient was 0.97. With DRAINMOD simulations of water table depths in the crop fields and the measured water level changes in the drainage ditches, we calculated salt and water exchange between drainage ditches and the subirrigatd fields; the results showed that in one planting year, the drainage ditches received a total inflow water volume of 9.3 m3 per unit length; while considering the outflow from the drainage ditches, the net ditch storage was 5.5 m3 per unit length per annum. The calculated subsurface drainage was only 2.3 mm while the cumulative subirrigation water depth was 49 mm during the study period; subsurface drainage occurred only occasionally during some heavy rain and irrigation time period, the drainage ditches provided subirrigation to crop fields during the rest of the time due to the periodic inflow from the upstream irrigation area. Comparing with the crop evapotranspiration, the subirrigation water amount was relatively small, but the heightened water level in the drainage ditches reduced drainage intensity, resulting in less subsurface drainage in the growing season. The high water level in drainage ditches had positive effect on recharging the water table of the farm fields. The calculated salt load moved into fields with the subirrigation water was about 20 times of that lost with the subsurface drainage discharge, but the actual salinity in the field was much lower than the salt tolerance level of cotton, no significant salinity buildup was observed in the soil profile. Currently, the inflow from the upstream irrigation area had no negative impact on salt and water balance in the study area; instead, it alleviates the draught stress in the downstream irrigation area. The long-term effect of maintaining high water level in the drainage ditches on salt balance in the crop fields, however, needs further investigation.