A new method for quantifying threshold water tables in a phreatic aquifer feeding an irrigation district in northwestern China

In semi-arid northwestern China, the groundwater level continues to decline with the development of the irrigated agriculture, resulting in grassland ecosystem degradation. Threshold water tables play an important role in sustaining both grassland ecology and irrigated agriculture. This paper proposes a new method to calculate the threshold water tables based on the quantification of the transport rate of wetting front, by which the optimal layout of well groups in an irrigation district will be discussed; furthermore, this paper will discuss the advantages of using the threshold water tables as the management target to regulate groundwater pumping and ensure an appropriate water table for the ecological system in the irrigation district. The proposed method is applied to an irrigation district in the West Liaohe River Plain, which is a typical area with mixed agricultural and rural lands in northwestern China. The threshold water table in the case study irrigation district is estimated as 10 m; the interference radii of the pumping wells are 340 m, 270 m and 150 m in three different sub-districts, where the well depth is greater than 100 m, 60–80 m and less than 60 m, and the interference radius values are 225–310 m, 165–225 m and 150 m, respectively.