A Conceptual Framework for Integration Development of GSFLOW Model: Concerns and Issues Identified and Addressed for Model Development Efficiency

In Coupled Groundwater and Surface-Water Flow (GSFLOW) model, the three-dimensional finite-difference groundwater model (MODFLOW) plays a critical role of groundwater flow simulation, together with which the Precipitation-Runoff Modeling System (PRMS) simulates the surface hydrologic processes. While the model development of each individual PRMS and MODFLOW model requires tremendous time and efforts, further integration development of these two models exerts additional concerns and issues due to different simulation realm, data communication, and computation algorithms. To address these concerns and issues in GSFLOW, the present paper proposes a conceptual framework from perspectives of: Model Conceptualization, Data Linkages and Transference, Model Calibration, and Sensitivity Analysis. As a demonstration, a MODFLOW groundwater flow system was developed and coupled with the PRMS model in the Lehman Creek watershed, eastern Nevada, resulting in a smooth and efficient integration as the hydrogeologic features were well captured and represented. The proposed conceptual integration framework with techniques and concerns identified substantially improves GSFLOW model development efficiency and help better model result interpretations. This may also find applications in other integrated hydrologic modelings

APEX-MODFLOW: A New integrated model to simulate hydrological processes in watershed systems

APEX (Agricultural Policy/Environmental eXtender) is an oft-used agroecosystem model but has limited use in groundwater-driven watersheds due to a simplistic representation of groundwater processes. This paper presents the linkage of APEX and the groundwater flow model MODFLOW into a single modeling code. The mapping of recharge, groundwater head, and groundwater-surface water interactions are handled internally via subroutines. The APEX-MODFLOW model is applied to three watersheds in the United States for testing code accuracy and hydrologic state variables and fluxes: the Animas River Watershed, Colorado and New Mexico (3543 km2); the Price River Watershed, Utah (4886 km2); and the Middle Bosque River Watershed, Texas (470 km2). Whereas the hydrology of the Animas River and Price River watersheds is driven by snowmelt and spring runoff, the hydrology of the Middle Bosque River Watershed is driven by summer thunderstorms. The model can be used for scenario analysis in groundwater-driven watersheds

Earth Systems Modeling in the Brazos River Alluvium Aquifer: Improvement of Computational Methods and Development of Conceptual Model

Traditional hydrologic modeling has compartmentalized the water cycle into distinct components (e.g. Traditional hydrologic modeling has compartmentalized the water cycle into distinct components (e.g. rainfall-runoff, river routing, or groundwater flow models). In river valley alluvium aquifers, these processes are too interconnected to be represented accurately by separate models. An integrated modeling framework assesses two or more of these components simultaneously, reducing the error associated with approximated boundary conditions. One integrated model, ParFlow.CLM, offers the advantage of parallel computing, but it lacks any mechanism for incorporating time-varying streamflow as an upstream boundary condition. Previous studies have been limited to headwater catchments. Here, a generalized method is developed for applying transient streamflow at an upstream boundary in ParFlow.CLM. The upstream inflow method was successfully tested on two domains – one idealized domain with a straight channel, and one small stream catchment in the Brazos River Basin. The stream in the second domain is gaged at the upstream and downstream boundaries. Both tests assumed a homogeneous subsurface, so that the efficacy of the transient streamflow method could be evaluated with minimal complications by groundwater interactions. Additionally, an integrated conceptual model is presented for the Brazos River Alluvium Aquifer (BRAA), the Brazos River, and the overlying terrain. The BRAA is a floodplain aquifer in central to southeast Texas. This aquifer is highly connected to the Brazos River and experiences localized semi-confined conditions beneath thick surface clay layers. The conceptual model is designed to be implemented in an Earth system modeling framework and is limited to the central portion of the aquifer in Brazos and Burleson Counties, Texas. Unlike previous models in ParFlow.CLM, this is a high-order subbasin with large inflows from upstream. Additionally, the model incorporates no-flow, transient head, and free drainage boundaries. Preliminary tests suggest the need for a long spin-up period. Long-term simulations will require calibration of surface and subsurface parameters before using the model to assess system behavior

Using an Integrated Model to Assess Groundwater Recharge in Martis Valley, CA

Groundwater contributes an essential water supply to several communities and ecosystems in the Truckee River Basin. Water resource investigations were conducted through numerical modeling and comparisons to previous work to assess groundwater recharge in the Martis Valley watershed, which is an essential component to the Truckee River hydrographic region. A baseflow analysis was performed to relate annual baseflow to streamflow and precipitation. Results show that changes in groundwater fluctuations are driven by changes in precipitation, and baseflow response is affected by previous precipitation trends. It was estimated that baseflow is roughly one-sixth of mean annual precipitation. A novel method for constructing a hydrogeologic framework model was developed and applied to an integrated surface water-groundwater hydrologic model, GSFLOW, from which groundwater recharge locations and magnitudes were extracted. Model results supplemented previous work and provided enhanced conceptualizations of surface and groundwater interactions, as well as spatial and temporal recharge trends. Results show that the most significant recharge zones are low to mid-elevation stream channel and alluvial areas. During peak snowmelt periods, upper elevation alluvial areas also contribute significant recharge. The findings herein promote a more detailed understanding of groundwater recharge characteristics in high elevation, snow dependent, alpine catchments

Using MODFLOW to Predict Impacts of Groundwater Pumpage to Instream Flow: Upper Kittitas County, Washington

Surface waters in the Yakima River Basin in central Washington are considered over allocated. Since 1960, new water demands have been met through groundwater withdrawals, with most groundwater users holding a later priority date than senior and junior surface water users. As a result of the discussions surrounding this issue, the Upper Kittitas Groundwater Rule has been in effect since 2010. Pumping from new domestic (i.e., permit-exempt or “exempt”) groundwater wells in Upper Kittitas County is not allowed unless mitigation is used to offset the groundwater use. The United States Geological Survey (USGS) has already created a basin-wide model for the Yakima River Basin for the period October 1959 through September 2001; however, the hydrogeology of Upper Kittitas County is coarsely represented in the USGS model because individual bedrock units are not delineated. Based on the USGS Yakima River Basin groundwater flow model (hereafter the YRB-GFM), an Upper Kittitas County groundwater flow model (hereafter the UKC-GFM) was extrapolated to refine the Upper Kittitas County modeled region. This new model constitutes an M.S. thesis, done in collaboration with the USGS. The UKC-GFM contains 246 columns and 195 rows, with 1,000 foot grid cells, and five layers representing three basin fill units, basalt, and bedrock; it is populated with model information for the period October 1991 through September 2001. Refinements to the UKC-GFM include: (1) using a newer version of MODFLOW (MODFLOW-NWT) with the new Newton Solver and the Upstream Weighting (UPW) package. The YRB-GFM used MODFLOW-2005, the PCG2 Solver, and the Hydrogeologic-Unit Flow (HUF) Package; (2) incorporating zone arrays with multiple hydraulic properties into model bedrock layers; (3) extending streamflow-routing cells into smaller headland creeks; (4) changing simulated monthly reservoir stages from steady state to time variant; and (5) estimating new parameter values. The UKC-GFM was calibrated using trial-and-error methods and automated parameter estimation with the software PEST. Groundwater model calibration involves comparing measured water levels and streamflow observations with simulated water levels and streamflow values. At 116 well observation points, the calibrated model produces a root-mean-square (RMS) error divided by the total difference in water levels of 1.5 percent, an acceptable error. Annual differences for measured and simulated streamflow ranged from 7 to 11 percent (percent difference) along the Yakima River, and ranged from 19 to 49 percent along tributaries. Once calibrated, the UKC-GFM was run as three scenarios to assess responses of the flow system to potential changes in stresses. These scenarios are: (1) Existing Conditions without All Pumping, (2) Decrease Recharge by Fifteen Percent, and (3) Increase Pumpage by Fifteen Percent. The scenario with the greatest impacts to stream leakage is Scenario 2, where the annual difference in streamflow for the most downstream gage in 2001, the end of the model simulation period, is approximately 80 ft3/sec. This is a 4.7 percent decrease in streamflow, versus Scenario 1 (all pumping removed), which produces a 0.17 percent increase in streamflow. A comparison of the applied scenarios suggests that potential climate changes that decrease recharge have more impacts on streamflow than groundwater pumping

Finding water management practices to reduce selenium and nitrate concentrations in the irrigated stream-aquifer system along the lower reach of Colorado's Arkansas River Valley

2018 Summer.Includes bibliographical references.Agricultural productivity in the Lower Arkansas River Valley (LARV) in southeastern Colorado has been high over the last 100 years due to extensive irrigation practices. In the face of this high productivity, however, the LARV currently face many issues as a result of the long period of irrigation, including waterlogging and soil salinization, leading to a decline in crops yields and high concentrations of nutrients and trace elements. In particular, irrigation practices have led to high concentrations of selenium (Se) and nitrate (NO3) in groundwater, surface water, and soils, similar to other semi-arid irrigated watersheds worldwide. Environmental concerns due to these high concentrations include human health, health of fish and waterfowl, and eutrophication of surface water bodies. The objective of this thesis is to identify water management strategies that can lead to a decrease in the concentrations of Se and NO3 in groundwater and surface water in the LARV by evaluating the three-water management BMPs which is reduced irrigation (RI), lease fallowing of irrigated land (LF), and canal sealing (CS). This is accomplished by constructing and testing a computational model that simulates the fate and transport of Se and NO3 in a coupled irrigated stream-aquifer system, and then applying the model to evaluate selected best management practices (BMPs) to decrease the concentration of Se and NO3 to comply with Colorado water quality regulations. The modeling system consists of MODFLOW, which simulates groundwater and stream flow, and RT3D-OTIS, which simulates the reactive transport of the principal Se and nitrogen (N) species in groundwater and a connected stream network. RT3D-OTIS uses simulated flows from MODFLOW to exchange Se and N species' mass between streams and the aquifer on a daily time step. The coupled flow and reactive transport model is applied to an approximately 552 km² study region in the LARV between Lamar, Colorado and the Colorado-Kansas border. The model is tested against Se and NO3 concentrations measured in a network of groundwater monitoring wells and stream sampling site, and against return flows and mass loads to the river estimated from the mass balance. Model calibration was performed manually and by using PEST software tool, and the effects BMPs on Se and NO3 concentrations in groundwater, streams, and groundwater mass loadings to the Arkansas River within the stream-aquifer system are quantified. Three BMPs are considered RI, LF, and CS, which are simulated for a 40-year period and then compared to a baseline ("do nothing") scenario. The results indicate that implementation of the CS scenario might lead to lower groundwater concentrations of Se and NO3 by 40% and 38%, respectively, a reduction in groundwater mass loading to the Arkansas River by 100% and 60% for Se and NO3, and a reduction in stream concentrations of Se and NO3 by 30% and 40%, respectively. In contrast, the RI and LF scenario, while lowering the water table and in consequence the rate of groundwater return flow to the Arkansas River, leads to elevated groundwater concentrations of both Se and NO3 in the riparian areas, resulting in an overall increase in groundwater mass loading to the river. This may be due to changes in the rate of groundwater flow due to lower hydraulic gradients leading to longer residence times of NO3 in the aquifer, increasing the potential for the release of Se from the bedrock shale through oxidation processes. Also, lowering the water table due to reduced recharge from irrigation reduces the size of the saturated zone, perhaps contributing to a higher concentration of Se and NO3. Moreover, changes in water and mass flux between the saturated and unsaturated zone occur under RI and LF scenarios. As a consequence of these altered processes, the RI and LF scenarios do not decrease the in-stream concentrations of Se and NO3 in the Arkansas River, with values for Se and NO3 increasing by 15% and 8%, respectively under the RI scenario, and by 10% and 10.5% for the LF scenario. Further, the results are compared with results obtained from a modeling study in the Upstream Study Region of the Lower Arkansas River Valley, to determine the similarity and differences of BMP implementation in the two regions. Further assessment of localized BMPs should be performed to determine key regions where they should be implemented for the largest impact on Se and NO3. Combined water management BMPs and land management BMPs, like reduced fertilizer application and enhanced riparian buffers, should also be evaluated