Adjusting Irrigation Frequency to Ameliorate Winter Wheat Yield Reduction Due to Restricted Groundwater Extraction for Irrigation: A Case Study

Abstract

【Objective】 Over-extraction of groundwater over the past decades has resulted in shallow groundwater table dropping at an alarming rate in most regions in Hebei province. Reducing groundwater extraction for wheat winter irrigation has been proposed as a remedy to reverse this trend. Quantitative understanding of wheat yield reduction due to the restricted groundwater extraction and its consequence for groundwater dynamics is essential. Taking a piedmont plain of Taihang Mountain as an example, this paper investigates how to minimize the wheat yield loss by optimizing irrigation frequency when the irrigation amount is the same. 【Method】 The study was based on numerical simulation. We designed three scenarios with the irrigation amount kept the same at 90 mm, but the crop was irrigated at different frequencies. For each frequency, we simulated the winter wheat yield, variation in shallow groundwater table and soil water balance in the 0~200 cm of soil using the modified SWAT model. We divided the simulation area into 22 sub-basins, and the optimal irrigation frequency for each subbasin was calculated based on the index of crop-groundwater irrigation productivity. 【Result】 ① Irrigating the crop twice, one at jointing and one at the heading stages, at equal irrigation amount (45 mm) gave the highest winter wheat yield, with the average yield reduced approximately by 20% compared to business-as-usual irrigation method. ② Irrigating all 90mm of water at the jointing stage led to largest irrigation water leaching. ③ Increasing the irrigation frequency from two to three with the same irrigation amount in each (30 mm) did not noticeably change water leaching and evapotranspiration, but increased soil evaporation at the expense of transpiration. ④ To minimize yield reduction, 13 subbasins (approximately 54% of the studied area) should irrigate once only, 8 subbasins (approximately 40% of the studied area) should irrigate twice each with 45 mm of water, and one subbasin (approximately 6% of the studied area) should irrigate three times each with 30 mm of water. 【Conclusion】 Reducing groundwater extraction for winter wheat irrigation to 90 mm could mitigate shallow groundwater table falling by 60%~75% at the price of yield reduction of 20%~25%. To minimize the yield reduction, different regions should adjust the irrigation frequencies based on its heterogeneous aquifers. Optimizing irrigation frequency should consider the trade-off effect of irrigation on groundwater conservation and crop production, as well as irrigation water use efficiency.

Keywords

• limited irrigation scheme
• irrigation frequency
• winter wheat
• groundwater irrigation productivity
• swat model