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.