Comparison of the Glover-Balmer solution with a calibrated groundwater model to estimate aquifer-stream interactions in an irrigated alluvial valley

Abstract

In many alluvial valleys wherein streams are hydraulically connected to the aquifer system, understanding and quantifying the impact of aquifer stresses (e.g. pumping, injection, recharge) on streamflow is of primary importance. Due to their relative simplicity and straightforward application, analytical models such as the Glover-Balmer solution often are employed to quantify these impacts. However, the predictive capacity of such models in intensively-irrigated systems, wherein canals, spatially-varying irrigation application patterns, and spatially-variable aquifer characteristics are often present, is not well known. In this study, the Glover-Balmer solution is compared to a calibrated MODFLOW-UZF numerical model for a study area within the Lower Arkansas River Valley in southeastern Colorado, USA. Comparison is made by simulating field-scale water extraction, addition, and fallowing scenarios, and comparing the predictions by both models of stream depletion or accretion. To create an ideal comparison, inputs to the Glover-Balmer model (stress, aquifer parameters) are obtained from the calibrated numerical model. Results for a few fallowing scenarios and from 52 extraction and addition scenarios from a variety of distances from the Arkansas River show that, under certain circumstances, the two models have good agreement in results, particularly in regions close (< about 0.5 to 1 km) to the river. However, due to aquifer heterogeneity and the overall hydrologic complexity in the natural system, results of the two models often diverge, with the Glover-Balmer model typically estimating greater impacts on the stream than the MODFLOW-UZF model. Suggested considerations are given for applying the Glover-Balmer solution, including the consideration of hydrologic components that may intercept or contribute to groundwater flow (such as irrigation canals, upflux to ET, groundwater storage, and tributaries), the potential influence of unsaturated zone processes, and changes in depletion/accretion locations and timing due to aquifer heterogeneity.