| Irrigated lands in Colorado's Lower Arkansas River Valley (LARV), like many such lands worldwide, face an uncertain future as pressures on the region's limited water resources continue to increase. This work describes 1) the study locations, 2) data-collection efforts for system characterization and for support of model development, 3) development, calibration, and testing, of groundwater flow and salt transport models and 4) application of the models for describing baseline conditions and for exploring the trade-offs of potential solution strategies for two distinct regions in the LARV [referred to as the Upstream Study Region (USR) and the Downstream Study Region (DSR)].;Data indicate high soil water salinity that is exacerbated by shallow saline groundwater tables in the regions. Extensive soil water salinity surveys reveal average electrical conductivity of soil saturated extract, EC e, throughout the LARV that are near or exceeding crop salt tolerance thresholds in the 3-5 dS m-1 range. Soil water extracted from cultivated field soil samples averaged 4.1 and 6.2 dS m-1 between the USR and DSR, respectively. Of the more than 122,000 locations surveyed for soil water salinity in the LARV over a 7-year survey period, 42% exceeded the estimated crop salt tolerance threshold for the recorded crop type. These conditions correspond to estimated average crop yield reductions of 6% in the USR and 17% in the DSR.;Simulated output from model runs of several alternative management interventions (reduced irrigation applications, reduced canal seepage, lease-fallowing of cultivated land, and combinations thereof) is compared to baseline model results and relative improvements in water table depth and reduced non-beneficial consumptive use are quantified. Potential for lowering the water table by up to 1.1 and 0.7 m on average in the USR and DSR, respectively, is demonstrated. Under each of the investigated scenarios, patterns of groundwater return flows to the river varied, highlighting the need to augment river flows to avoid violations to the interstate river compact that governs river operations in the LARV. In addition to simulating conditions resulting from altered groundwater flow, the calibrated baseline groundwater flow models provide the simulated subsurface flow required for salt transport simulations, whether for the non-reactive solute transport models described herein or for multi-species reactive transport models being developed in parallel research.;Baseline salinity conditions are quantified using the modified subsurface variably saturated solute transport code UZF-MT3DMS. The modified code was developed and verified with 1-, 2-, and 3-dimensional simulations under a variety of conditions, including steady, unsteady, nonreactive and reactive conditions. The average simulated groundwater return salt loads to the Arkansas River are 5,200 and 5,800 MT (metric tons) wk-1, respectively. An evaluation of the baseline model output suggests that chemical reaction processes, such as precipitation and dissolution, should be incorporated into the salt transport models. By providing the simulations with a mechanism for adding and releasing salt to and from storage, model fits to observations of groundwater salt concentrations, soil water salt concentrations, and seasonal salt loading dynamics associated with groundwater return flows likely will be improved.;Without only a minimum level of intervention, the productivity of irrigated agriculture and the environmental quality of the stream-aquifer system in the LARV will remain at or below current sub-par levels. Due to the ever-increasing demand (and value) of water along Colorado's urbanized Front Range, the potential remains for permanent land dry-up as the water is reallocated to other uses. However, simulations of corrective actions that include mitigating inefficient application and delivery of irrigation water offer hope that the LARV's long and rich history of crop production will continue. Alternative management intervention scenarios highlight opportunity for lowering water tables for improving growing conditions, reducing non-beneficial consumptive use, and restoring river water quality. (Abstract shortened by UMI.). |
