Assessing causes and identifying solutions for high groundwater levels in a highly managed irrigated region

Abstract

High groundwater levels in urban and irrigated areas around the world can lead to infrastructure damage, land degradation, and crop yield reduction. Causes can include groundwater flooding due to fluvial processes, excess rainfall and irrigation, inadequate subsurface drainage, and additional sources such as injection and seepage from earthen canals and recharge ponds. The principal causes of shallow water tables, however, are difficult to quantify due to the inter-connectedness of all possible causes. This paper presents a method to analyze and quantify the cause of high groundwater levels in highly managed, irrigated stream-aquifer systems, using a combination of numerical groundwater flow modeling and global sensitivity analysis (GSA) tools. A tested MODFLOW groundwater model and Sobol GSA methods are used to simulate and then quantify the influence of all major groundwater stresses on water table elevation for a region in northern Colorado, USA experiencing high groundwater levels, with results showing that recharge from surface water irrigation, canal seepage, and groundwater pumping have the strongest influence on water table elevation, whereas precipitation recharge and recharge from groundwater irrigation have small influences. Time series sensitivity plots quantify the seasonality of these influences over a decadal period, and spatial sensitivity plots indicate regions that are strongly influenced by individual stresses. Results from best management practice (BMP) implementation indicate that limiting canal seepage and transitioning >50 % of cultivated fields from surface water irrigation to groundwater irrigation can decrease water table elevation by 1.5 m–3 m over a 5-year period, leading to beneficial conditions for crop growth in the root zone and dewatering of subsurface infrastructure. These methods can be applied to any waterlogged region worldwide. However, proposed management practices to lower water table may be constrained by local, state, or national water law.