Modeling the distribution of major salt ions in regional agricultural groundwater and surface water systems: model calibration and application
Date
2020
Authors
Javed, Abdullah B., author
Gates, Timothy K., advisor
Bailey, Ryan T., advisor
Ronayne, Michael J., committee member
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Abstract
Irrigated lands in Colorado's Lower Arkansas River Valley (LARV), like many irrigated agricultural areas worldwide, suffer from salinization of soil, groundwater, and adjacent river systems. Waterlogging and salinization are prevalent throughout the LARV, which have diminished the crop yields and threatened the long-term sustainability of irrigated agriculture. Increased salinity concentrations are primarily due to the presence of salt minerals and high rates of evapotranspiration in the LARV, coupled with inefficient irrigation practices. Shallow groundwater in the LARV drives saline groundwater back to the stream network, thereby degrading the surface water quality, which affects the downstream areas where it evapo-concentrates when saline water is diverted for additional use. The goal of the current study is to develop, calibrate and test a physically-based, spatially distributed numerical model to assess soil, groundwater and surface water salinity at a regional scale to better understand the baseline nature of the problem. Several salinity models have been developed in recent decades; however, no attempts thus far have been made at simulating the fate, storage, and transport of salt ions at a regional scale in both groundwater and streams within an irrigated stream-aquifer system. The model used in this thesis links MODFLOW-SFR2, which simulates the groundwater heads and stream flows, with RT3D/SEC-OTIS which addresses reactive solute transport in variably- saturated soil and stream-aquifer systems. Sources and sinks within an agricultural system such as canal seepage, infiltrated water from flood and sprinkler irrigation, groundwater pumping, evapotranspiration from both the unsaturated and shallow saturated zones; root zone processes such as cycling of salt ions, crop uptake, and leaching to the water table; addition of salt mass via fertilizer and irrigation water; chemical kinetics affecting salt ions such as influence of dissolved oxygen and nitrate; equilibrium chemistry processes such as precipitation-dissolution, complexation and cation exchange; and 1D transport of salt ions in the streams due to advection, dispersion and sorption are addressed. The coupled flow and reactive transport model is applied to an approximately 552 km2 salinity-affected irrigated stream-aquifer system of the LARV between Lamar, Colorado and the Colorado-Kansas border. The model is tested against an extensive set of field data (soil salinity data from field salinity surveys, groundwater salinity collected from a network of groundwater monitoring wells, salt loading from the aquifer to the Arkansas River, and salt concentrations measured from in-stream sampling). Model calibration and parameter estimation include manual and automated calibration using PEST. Runs were conducted to describe the current levels of root zone salinity which markedly exceeds threshold levels for crop yield reduction. Spatiotemporal distribution of groundwater levels and concentrations, mass and return flow rates to streams, and stream concentrations are also simulated for current baseline conditions. The calibrated and tested regional scale salinity model is in need of further refinement but shows promise for future implementation to explore potential solution strategies for the irrigated valley of the LARV, and similar salt-afflicted areas of the world, by applying different best management practices.
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Subject
salinization
salinity modeling
waterlogging