29 Nov 2022
29 Nov 2022
Status: a revised version of this preprint is currently under review for the journal HESS.

Controls on managed aquifer recharge through a heterogeneous vadose zone: hydrologic modeling at a site characterized with surface geophysics

Zach Perzan1, Gordon Osterman2,3, and Kate Maher1 Zach Perzan et al.
  • 1Department of Earth System Science, Stanford University, Stanford, California, USA
  • 2Agricultural Research Service, U.S. Department of Agriculture, Davis, California, USA
  • 3Department of Geophysics, Stanford University, Stanford, California, USA

Abstract. In water-stressed regions of the world, managed aquifer recharge (MAR), the process of intentionally recharging depleted aquifers, is an essential tool for combating groundwater depletion. Many groundwater-dependant regions, including the Central Valley in California, USA, are underlain by deep vadose zones (ca. 10 to 40 meters thick), nested within complex valley-fill deposits that can hinder or facilitate recharge. Within the saturated zone, interconnected deposits of coarse-grained material (sands and gravel) can act as preferential recharge pathways, while fine-textured facies (silts and clays) accommodate the majority of the long-term increase in aquifer storage. However, this relationship is more complex within the vadose zone. Coarse facies can act as capillary barriers that restrict flow and contrasts in matric potential draw water from coarse-grained flowpaths into fine-grained, low permeability zones.

To determine the impact of unsaturated zone stratigraphic heterogeneity on MAR effectiveness, we simulate recharge at a Central Valley almond orchard surveyed with a towed transient electromagnetic system. First, we identified three outcomes of interest for MAR sites: infiltration rate at the surface, residence time of water in the root zone and saturated zone recharge efficiency, which is defined as the increase in saturated zone storage induced by MAR. Next, we developed a geostatistical approach for parameterizing a 3D variably saturated groundwater flow model using geophysical data. We use the resulting workflow to evaluate the three outcomes of interest and perform Monte Carlo simulations to quantify their uncertainty as a function of model input parameters and spatial uncertainty. Model results show that coarse-grained facies accommodate rapid infiltration rates and that contiguous blocks of fine-grained sediments within the root zone are >20 % likely to remain saturated longer than almond trees can tolerate. Simulations also reveal that capillary-driven flow draws recharge water into unsaturated, fine-grained sediments, limiting saturated zone recharge efficiency. Two years after inundation, fine-grained facies within the vadose zone retain an average of 37 % of recharge water across all simulations, where it is inaccessible to either plants or pumping wells. Global sensitivity analyses demonstrate that each outcome of interest is most sensitive to parameters that describe the fine facies, implying that future work to reduce MAR uncertainty should focus on characterizing fine-grained sediments.

Zach Perzan et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2022-369', Anonymous Referee #1, 22 Dec 2022
    • AC1: 'Reply to Reviewer 1', Zach Perzan, 25 Jan 2023
      • AC3: 'Revised manuscript addressing Reviewer 1's Comments', Zach Perzan, 25 Jan 2023
  • RC2: 'Comment on hess-2022-369', Anonymous Referee #2, 06 Jan 2023
    • AC2: 'Reply to Reviewer 2', Zach Perzan, 25 Jan 2023
      • AC4: 'Revised manuscript addressing Reviewer 2's comments', Zach Perzan, 25 Jan 2023

Zach Perzan et al.

Model code and software

ParFlow-CLM v3.10.0 Steven Smith; reedmaxwell; i-ferguson; Nick Engdahl; FabianGasper; Patrick Avery; Calla Chennault; Sebastien Jourdain; grapp1; Laura Condon; Ketan Kulkarni; Vineet Bansal; xy124; Andrew Bennett; basileh; David Thompson; DrewLazzeriKitware; Jackson Swilley; Joe Beisman; alanquits; Ethan Coon; Ian Bertolacci M.S.; Sebastian Lührs; arezaii; aureliayang; cswoodward

Zach Perzan et al.


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Short summary
In this study, we simulate flood managed aquifer recharge – the process of intentionally inundating land to replenish depleted aquifers – at a site imaged with geophysical equipment. Results show that layers of clay and silt trap recharge water above the water table, where it is inaccessible to both plants and groundwater wells. Sensitivity analyses also identify the main sources of uncertainty when simulating managed aquifer recharge, helping to improve future forecasts of site performance.