Changes of Temporal Trends and Spatial Patterns of Nitrate-Nitrogen in the Farmland Groundwater Resulting from Water-Saving Irrigation in the Hetao Irrigation District of China

Nitrogen leaching is a serious water quality concern for irrigated farmlands. This concern is likely to be alleviated by reconstructing water-saving projects as well as implementing water-saving irrigation practices. The objective of this study was to evaluate effects of water-saving irrigation practices on nitrogen leaching pollution and water quality in regional farmland groundwater. The evaluation was conducted in the Jiefangzha Irrigation Scheme of Hetao Irrigation District (HID) in the Inner Mongolia Autonomous Region of China. For the growing-season months of March, May, July, and September in the years 2003 (before the water-saving irrigation practices being implemented) and 2008 (after the water-saving irrigation practices being implemented), the nitrate-nitrogen contents and other water chemistry parameters (e.g., pH, TDS) were determined by taking and analyzing groundwater samples fetched from a number of wells across the scheme. The analysis compared changes between these two experiment years of the irrigation volumes, the depths to water table, and the groundwater nitrate-nitrogen contents and water quality. Using the inverse-distance weighted (IDW) interpolation method and the measured data in 27 wells, this study generated maps illustrating changes of the temporal trends and spatial patterns as a result of implementing the water-saving irrigation practices. The results indicated that prior to the first irrigations in March, the areas with the nitrate-nitrogen concentrations higher than 10 mg/L (i.e., the national drinking water standard ) accounted for 9.95% and 5.22% in size of the irrigation scheme in 2003 and 2008, respectively, while the annual average nitrate-nitrogen concentrations higher than 10 mg/L accounted for 9.02 and 5.20%. These area reductions can be attributed to the water-saving irrigation practices implemented between these two years. At the same time, the groundwater quality also has been improved because the TSD was lowered by 21.4% from 2003 to 2008. The noticeable effects include the: 1) lowered nitrate-nitrogen leaching by reducing irrigation volume; 2) reduced irrigation quote in fall; 3) decreased water table; and 4) improved groundwater quality. A reasonable generalization of these results is that the water-saving irrigation practices will be beneficial to the ongoing efforts to control non-point source pollution and improve water quality across the HID.