Abstract:
Abstract: In the irrigated agricultural areas, especially in the downstream regions, the poor drainage condition or blocked drainage outlet often result in high water table during the irrigation season. Such condition may be properly used to encourage crops to use in-situ shallow groundwater in order to reduce irrigation application and the subsequent drainage discharge. This will help improve the irrigation water use efficiency and mitigate the negative impact of agricultural drainage. To estimate the amount of shallow groundwater use in field condition, however, is often difficult due to the complex relationships among soils, crops and the field hydrology. In this paper, we presented a method for calculating shallow groundwater use by field crops based an assumption that crop uptake of groundwater caused the timely difference in water table depth between the fields with and without crops, providing that the weather condition and the irrigation and drainage practices were similar. With a case study in a semi-arid irrigation area Shaanxi, China, we demonstrated the application of the proposed method in a cotton and wheat rotational field; the daily water table depths under the two different surface cover conditions were predicted with the field hydrology model - DRAINMOD, which has been tested at the same site previously; the salt accumulation in crop root zone under shallow groundwater use was predicted with a simple salt balance model. The difference in potential evapotranspirations (PET) in the fields with and without crops were calculated separately and fed into the model as inputs in order to represent the different effect of PET on field hydrology in DRAINMOD. The sub-irrigation module in the DRAINMOD was used to simulate the effect of high water level in drainage ditches due to the occasional block up of the drainage outlet. The calculation results showed that the approach used in this paper produced reasonable estimate of water balance in comparison with the actual field conditions; the DRAINMOD predicted soil moisture content was in close agreement with the field measurements since the average deviation was 0.475%, the average absolute deviation was 0.356%, and the correlation coefficient was 0.86. The DRAINMOD predicted that the average water table in the crop field was about 50 cm lower than that in no crop fields due to the consumptive use of the shallow groundwater by crops; the predicted water table responded more rapidly to rainfall and irrigation events in no crop fields due to the shallower unsaturated zone. The calculated total crop use of shallow ground water was 305.8 mm per unit area in the study period, in which 160 mm occurred in the cotton growth period, accounting for 24% of the annual water requirement of cotton in the study area. The salt balance analysis showed that crop use of the shallow ground water led to salinity buildup in the soil profile, but the average salinity within 1 m depth of soil profile was below the crop salt tolerance level. Based on existing studies on the salt leaching effect of rainfall and irrigation in the study area, the salinity buildup under shallow groundwater use condition only has a limited impact on crop production. Therefore, encouraging shallow groundwater use through engineering measures such as the controlled drainage may be a viable option for more efficient water use in irrigated areas where shallow water table exists. Findings from this research may provide technical reference for irrigation scheduling in similar regions.