SSP&A developed a suite of groundwater models for the shallow riparian groundwater environment along the Rio Grande to support analysis of restoration options and river conveyance efficiency. This project was conducted to support the Endangered Species Act Collaborative Program for the Middle Rio Grande, with goals related to fisheries, wetlands and interstate stream compact obligations. This multiple year project was completed in January 2008, with the development of eight fine-mesh, three-dimensional groundwater models from Cochiti Dam to Elephant Butte Reservoir. The shallow riparian groundwater models represent physical processes relevant to assessing shallow groundwater conditions and exchanges between surface water and shallow groundwater within the river floodplain. Using input from existing regional surface- and groundwater models, the models developed in this study can be used to analyze transient interactions between flow conditions in the river (and drains) and the shallow groundwater under various conditions that may occur in future years. Modeled interactions include seepage from the river, interception of shallow groundwater by drains, recharge to shallow groundwater from flooded overbank areas, and water depletions due to open water evaporation and riparian evapotranspiration. The models can be structured to provide surface water-groundwater interactions with time steps of hours or days, to track responses to flood waves and recession.
SSP&A has applied expertise in groundwater, surface water, and environmental science to improve understanding of processes at the interface between ecology and hydrology. SSP&A's contributions to this interdisciplinary field focus on groundwater conditions and groundwater-surface water exchanges in groundwater-dependent riverine and riparian environments.
Rio Grande High-Resolution Riparian Groundwater Models
Evaluation of Channel Re-Alignment Alternatives on Shallow Groundwater and Wetlands
As part of a feasibility study relating to channel alignment along the Rio Grande, SSP&A fine-tuned and applied a high-resolution riparian groundwater model to characterize alternative groundwater and wetland conditions under a range of seasonal and flow conditions. For this project, SSP&A drilled and monitored wells along river transects, monitored river and drain stage; then, incorporated the site-specific data into the model, including LiDAR land surface elevations and high-resolution vegetation coverages. SSP&A prepared a series of flow-dependent River Packages to specify a transient river boundary condition in simulation of the seasonal hydrograph, through wet spring and dry summer periods. For each of three channel re-alignment alternatives, SSP&A simulated groundwater conditions and surface water exchanges relevant to the objective of sustaining wetlands and of maintaining flows in the river channel for downstream water delivery and for aquatic species.
San Joaquin Near-River Groundwater Models
SSP&A has worked on the San Joaquin River to evaluate hydrologic conditions related to river restoration in a series of projects from 1999 to the present. In 1999-2000, SSP&A was retained by the U.S. Bureau of Reclamation to develop fine resolution groundwater models of the near-river zone, linked to existing HEC surface-water models along a 150-mile river reach, from Friant Dam to the Merced River. This groundwater model was developed to support evaluations of how much surface flow was needed to sustain desired hydrographs for repatriation of salmon.
SSP&A developed a technical approach that involved the use of very fine resolution groundwater models for specific river reaches, embedded within a structure incorporating regional groundwater conditions and with flow-dependent river stage and width identified with existing HEC surface-water models. The groundwater models simulate seasonal fluctuations of the water table in the riparian zone in response to user-specified river flow and regional groundwater conditions. A vadose zone package was developed and applied with MODFLOW as part of this effort. Example model simulations illustrated the sensitivity of groundwater levels and river gain/loss rates to antecedent river flow and regional groundwater conditions.
In 2005, SSP&A was retained on behalf of the Friant Water User’s Association to provide litigation support in evaluating losses from and gains to the San Joaquin River that would be associated with enhanced flows for re-establishment of an anadromous fishery on the San Joaquin River. This work involved evaluation of additional monitoring data, recalibration of models, and evaluation of various proposed restoration hydrographs to assess the feasibility of the delivery or maintenance of surface water flows at various flow levels. Time steps for the analyses ranged from hours to several days, with shorter time steps for ascending and descending hydrograph limbs to capture dynamics associated with significant changes in flow.
In 2008 and 2009, SSP&A applied the groundwater models to evaluate seepage losses associated with planned restoration hydrographs as part of a PEIS under NEPA/CEQA and to provide recommendations for monitoring locations and modeling tools for future evaluations of groundwater-surface water interactions.
Gila River Riparian Groundwater Model
SSP&A conducted a comprehensive study of hydrologic conditions in the riparian corridor of the Gila River to support water use and ecosystem decisions under the Arizona Water Settlement Act. This reach of the Gila River is unique in the West in that it remains largely unregulated, and while low flows are typical, flood flows ranging above 10,000 cfs are not uncommon. The river supports both a rich ecosystem and irrigated agriculture. Water uses in the basin are limited by interstate allocation under a Supreme Court decree, although Arizona Water Settlement Act offers some potential for flexibility in water management associated with the Central Arizona Project. SSP&A’s project included design of a riverine well monitoring network, river seepage runs, LiDAR acquisition and development of high-resolution groundwater models incorporating flow-dependent surface water boundary conditions based on companion HEC-RAS model simulations. Two groundwater models were constructed over approximately 12 river miles, with model cells 60 feet on a side. A framework for a regional model was developed to provide opportunity for exploring regional groundwater conditions relevant to water supply evaluations and to evaluate the sustainability of hydrologic conditions in the riparian zone under climate or water use scenarios. These models are among tools available to support Stakeholder analysis of water supply alternatives under the AWSA; and, to support evaluation of the sustainability of groundwater-dependent ecosystems under modified conditions.