Nonlinear effects of locally heterogeneous hydraulic conductivity fields on regional stream–aquifer exchanges

Abstract

Abstract.Computational experiments are performed to evaluate the effects of locally heterogeneous conductivity fields on regional exchanges of water between stream and aquifer systems in the Middle Heihe River basin (MHRB) of northwestern China. The effects are found to be nonlinear in the sense that simulated discharges from aquifers to streams are systematically lower than discharges produced by a base model parameterized with relatively coarse effective conductivity. A similar, but weaker, effect is observed for stream leakage. The study is organized around three hypotheses: (H1) small-scale spatial variations of conductivity significantly affect regional exchanges of water between streams and aquifers in river basins, (H2) aggregating small-scale heterogeneities into regional effective parameters systematically biases estimates of stream-aquifer exchanges, and (H3) the biases result from slow paths in groundwater flow that emerge due to small-scale heterogeneities. The hypotheses are evaluated by comparing stream-aquifer fluxes produced by the base model to fluxes simulated using realizations of the MHRB characterized by local (grid-scale) heterogeneity. Levels of local heterogeneity are manipulated as control variables by adjusting coefficients of variation. All models are implemented using the MODFLOW (Modular Three-dimensional Finite-difference Groundwater Flow Model) simulation environment, and the PEST (parameter estimation) tool is used to calibrate effective conductivities defined over 16 zones within the MHRB. The effective parameters are also used as expected values to develop lognormally distributed conductivity (K) fields on local grid scales. Stream-aquifer exchanges are simulated with K fields at both scales and then compared. Results show that the effects of small-scale heterogeneities significantly influence exchanges with simulations based on local-scale heterogeneities always producing discharges that are less than those produced by the base model. Although aquifer heterogeneities are uncorrelated at local scales, they appear to induce coherent slow paths in groundwater fluxes that in turn reduce aquifer-stream exchanges. Since surface watergroundwater exchanges are critical hydrologic processes in basin-scale water budgets, these results also have implications for water resources management.

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“…1b). The region is comprised of four typical inland arid and semi-arid regions (Zhu et al, 2015): the Shiyang River basin (41 600 km 2 ), the Heihe River basin (143 000 km 2 ), the Shule River basin (157 000 km 2 ), and a desert region (152 445 km 2 ) (Geng and Wardlaw, 2013;Zhu et al, 2015). Located next to the Gobi Desert, most parts of the region have a cold desert climate (Peel et al, 2007), where precipitation is relatively low to sustain vegetation or crops.…”

“…1). A semi-arid region can be broadly classified as an area on the boundary of a larger desert, receiving just enough annual precipitation (300 mm or less) to sustain a limited amount of agriculture (Gong et al, 2004;Zhu et al, 2015). Inefficient use of the limited amount of surface water can often lead to overuse of groundwater resources and salinization of the soil (Cui and Shao, 2005).…”