Presentations 2016

Watershed modeling to evaluate the impact of irrigated agriculture on surface water – groundwater interactions

U.S. Geological Survey

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We examine the impacts of irrigation and associated surface water (SW) diversions and groundwater (GW) pumping on streamflow, GW recharge and SW-GW interactions using a watershed-scale coupled SW-GW flow model. The U.S. Geological Survey (USGS) model GSFLOW (Markstrom et al., 2008), an integration of the USGS Precipitation-Runoff Modeling System (PRMS) and the Modular Ground-Water Flow Model (MODFLOW), is being utilized for this effort. Processes represented in the model include daily rain, snowfall, snowmelt, streamflow, surface runoff, interflow, infiltration, soil-zone evapotranspiration (ET), and subsurface unsaturated and saturated GW flow and ET. We use the upper Smith River watershed, an important agricultural and recreational area in west-central Montana, as the basis for watershed climate, topography, hydrography, vegetation, and soil properties as well as for scenarios of irrigation and associated practices. The 640 square kilometer watershed area has been discretized into coincident 200 m by 200 m hydrologic response units (for climate and soil zone flow processes) and grid blocks (for unsaturated zone and GW flow processes). The subsurface GW system is discretized into 6 layers representing Quaternary alluvium, Tertiary sediments and bedrock. The model is used to compare streamflow, GW recharge and SW-GW interactions in the watershed under natural, pre-irrigation conditions; current irrigation conditions; and a scenario of future increased irrigation. Model results reproduce observed hydrologic responses to both natural climate variability and irrigation practices. Current irrigation practices have decreased streamflow out of the watershed relative to pre-irrigation conditions as result of SW diversion. Irrigation has increased GW recharge below irrigated areas. Despite these local increases in GW recharge, more widespread lowering of the water table by GW pumping for irrigation decreases GW ET in lowlands with shallow water tables, decreases GW discharge to streams, and induces SW infiltration from streams. Irrigation practices cause SW-GW interactions to become more temporally and spatially variable. Flood irrigation in riparian zones increases GW flow to the stream, whereas, GW pumping for irrigation can cause naturally gaining stream reaches to become losing stream reaches. These changes in SW-GW interactions could influence stream ecology.

Potatoes and Trout: Groundwater Model Optimization to Balance Agricultural and Ecosystem Stakeholder Needs in the Little Plover River Basin, Wisconsin

USGS Wisconsin Water Science Center

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The well-drained sandy soil in the Wisconsin Central Sands is ideal for growing potatoes, corn, and other vegetables. A shallow sand and gravel aquifer provides abundant water for agricultural irrigation but also supplies critical base flow to cold-water trout streams. These needs compete with one another, and stakeholders from various perspectives are collaborating to seek solutions. Stakeholders were engaged in providing and verifying data to guide construction of a groundwater flow model which was used with linear and sequential linear programming to evaluate optimal tradeoffs between agricultural pumping and ecologically based minimum base flow values. The connection between individual irrigation wells as well as industrial and municipal supply and streamflow depletion can be evaluated using the model. Rather than addressing 1000s of wells individually, a variety of well management groups were established through k-means clustering. These groups are based on location, potential impact, water-use categories, depletion potential, and other factors. Through optimization, pumping rates were reduced to attain mandated minimum base flows. This formalization enables exploration of possible solutions for the stakeholders, and provides a tool which is transparent and forms a basis for discussion and negotiation.

Managing the Groundwater-Surface Water Interface under California’s new GroundwaterLaw

University of California, Davis

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The California 2014 Sustainable Groundwater Management Act (SGMA), for the first time in the state's history, protects beneficial uses of surface water from significant and undesirable impacts due to groundwater pumping. The law also explicitly protects groundwater dependent ecosystems. Under SGMA, local groundwater sustainability agencies (GSAs) must define monitoring networks, minimum thresholds, and measurable objectives to sustain the groundwater-surface water connection and groundwater-dependent ecosystems. Regulations spell out some minimum monitoring requirements, but provide flexibility in how to plan and implement sustainable groundwater-surface water connections. Among the groundwater sustainability objectives prescribed by SGMA, achieving sustainable groundwater-surface water and groundwater-dependent ecosystem objectives may be among the most challenging: California groundwater basins with some of the least prior groundwater management activities are most affected; the dynamics of the interface may cause long and hidden delays in impacts; and management of groundwater-surface water connectivity is uncommon, hence there are no ready-made toolboxes to look for. Instruments available to GSAs to assess groundwater-surface water connections and the potential impact from groundwater use and management activities can be broadly categorized into: water level data, water budget information, streamflow data, analytical modeling tools, numerical modeling tools, and statistical tools.

Incorporating a dynamic irrigation demand module into an integrated groundwater/surface water model to assess drought sustainability

Earthfx Inc.

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Understanding the cumulative effects of pumping, irrigation, drought and groundwater/surface water interaction is central to the agricultural water management problem. Assessing all of these complex processes within a single modelling framework is optimal, but difficult because of the dynamic interaction between climate, soil moisture and irrigation patterns. The USGS has developed, in parallel, two important model codes to study these processes. The USGS GSFLOW code combines the PRMS hydrologic model with the MODFLOW-NWT groundwater model into a fully integrated model that is ideal for studying groundwater and surface water interaction under drought and climate change conditions. Meanwhile, the USGS MODFLOW-OWHM model includes the Farm Process module that represents dynamic farm pumping, irrigation and return flows as a function of soil water demand. Earthfx has created a unified tool to address this challenge by modifying the GSFLOW code to incorporate the approaches and features of the Farm Process module. This evolution of the integrated GSFLOW model can simulate daily irrigation water use based on dynamic soil moisture conditions. Water is applied to the farms by adding the pumped volume to either daily precipitation or throughflow (after interception) based on irrigation method. Return flows are estimated directly by PRMS as overland runoff or interflow and routed to streams. Additional modifications were added to PRMS to account for discharge to tile-drains under saturated soil conditions. Thresholds and other irrigation model parameters are dynamically applied to match water takings for each crop type and subwatershed under average and dry year conditions. While the primary benefit is a fully integrated approach, components of the Farm Package are also improved through integration with the more complete PRMS soil zone hydrology code. This extended version of GSFLOW has been tested in the Whitemans Creek watershed in southwestern Ontario, Canada. Whitemans Creek drains an area of approximately 400 km² (150 square miles), with actively-cultivated agricultural fields comprising 60% of the watershed. The upper portion of the watershed is extensively tile drained, while the central and lower portions of the watershed contain significant wetlands and groundwater supported fish habitat. Total permitted takings, primarily from drought sensitive shallow sand aquifers in the lower watershed, exceed 80,000 m3/d. The model simulates groundwater heads, streamflow, and wetland stage on a daily basis under current conditions, along with future climate, drought and development scenarios. The effect of agricultural demand-driven pumping on specific sensitive stream reaches is fully represented. Detailed breakdowns of water budget components, aggregated on a monthly and annual basis, provide insight into future development scenarios. Results of the model will be utilized in the future to better manage water allocation under drought conditions.

Anthropogenic depletion of water resources in the TG Halli catchment near Bangalore, India

University of California, Berkeley

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The TG Halli watershed outside of Bangalore, India, faces a multitude of challenges due to urbanization and intensification of agriculture. Groundwater irrigation became popular in the 1970s, followed by shifts from traditional crops to eucalyptus plantations and in-stream check dams for aquifer recharge. The river, which supplied Bangalore with all of its water through the early 1970s, now yields only a small fraction of historical flows. Bangalore and its rapidly growing population have increasingly relied on imported water.We seek to understand the links between reductions in streamflow and local anthropogenic activities (e.g., groundwater pumping, land-use change, and watershed management practices such as managed aquifer recharge). Using a series of Landsat images from the 1970s to present, we classified over 500 surface water bodies in the catchment in each image and found that streamflow depletion is present throughout the catchment, except in the vicinity and downstream of urban areas. We conducted a suite of field research analyses, which demonstrate that most present-day streamflow is generated as infiltration excess runoff. Observed land-use changes within the catchment are unlikely to cause a reduction in infiltration excess runoff. We conclude that reductions in streamflow are caused by the loss of the shallow groundwater table due to groundwater pumping, and in-stream check dams which impound streamflow for groundwater recharge.While local water managers are aware of the changes within the catchment, the details of the local water balance are not well understood. Sustainable solutions are often overlooked as the incentives favor addressing short-term water needs, both for farmers and urban water managers. Farmers continue pumping as much as possible to sustain irrigated agriculture and the city continues to look further away for its water supply, with major projects for inter-basin transfers underway. Our work is a step towards understanding the local water balance and sustainable use of water resources in the Bangalore area.

Ecosystem-based Groundwater Recharge to Help Farmers and Fish: Why California Needs 10,000 More Dams

National Oceanic and Atmopsheric Administration

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Instream structures such as wood jams, living vegetation, beaver dams, certain geomorphic features and other obstacles that slow the downstream movement of surface water and sediment are essential to the restoration of streams and the recharge of alluvial aquifers. In particular, such ecologically functional dams help restore complex fluvial ecosystems with high groundwater recharge potential and multiple and synergistic ecosystem goods and services. We provide an overview of how ecologically functional dams can be used to increase groundwater recharge of alluvial aquifers and provide examples from pilot studies in California and Oregon where such ecologically functional dams have been used to increase groundwater levels in agricultural landscapes, providing water that benefits both farmers and the taxa that utilize fluvial ecosystems. Despite the promise of such ecosystem-based groundwater recharge efforts, numerous regulatory and policy obstacles cast doubt as to whether such an approach can be used on a widespread basis, despite the fact that many of the technological challenges have been addressed. Many of the regulatory and policy obstacles derive from the fact that surface water management has historically been focusing on mitigating flood damage rather than sustaining natural groundwater recharge rates. Thus streams and rivers have been viewed as drainage networks rather than groundwater recharge networks, and managed accordingly, such that much of the surface water that historically would have contributed to recharging alluvial aquifers has instead been managed for rapid transport out to sea. We conclude that new approaches to stream restoration and surface water management are needed that take into account society’s economic and ecological imperatives to create resilient, structurally complex and dynamic fluvial systems that can substantially increase groundwater recharge rates in agricultural landscapes.

Global Scale Study for Determining Groundwater Contribution to Environmental Flows and Sustainable Groundwater Abstraction Limits for SDGs

IWMI

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Goal 6 of the Sustainable Development Goals (SDGs) has at least three targets that explicitly or implicitly cover issues of sustainability of water resources development and freshwater ecosystem maintenance. To incorporate sustainability, environmental flows (EFs) and sustainable groundwater abstraction have to be an integral part of the SDG discourse, but there is a lack of awareness and application of EFs at multiple levels. If countries are to accept and implement EFs over the next 15 years in the context of achieving the SDG targets, some initial EF information is in high demand. In this work, the first global EF assessment carried out by International Water Management Institute from 2004 is modified, specifically to provide initial EF information useful for the calculation of SDG target indicators. The spatial resolution of the analysis has been improved from 0.5 to 0.1 degrees and enables assessment at any larger aggregated scale. Importantly, the EFs are also split into the surface runoff contributed EFs and groundwater contributed EF. The groundwater-contributed EFs help in defining sustainable groundwater abstraction from renewable sources. The desired conditions of rivers are defined by four environmental flow management classes (EMCs). The EF (as percentage of flow required relative to the pristine conditions) and volume of groundwater abstractable without impacting the EFs are calculated for each EMC. The EFs are determined based on modifying synthetic pre-development natural flows (derived from the global hydrological model PCR-GLOBWB). In reality, the current flow and environmental condition of the rivers vary globally. A modified Index for Global Threat to River Biodiversity determined globally by other researchers were used to link the current condition of the rivers to the desired EMC. The results suggest that total accumulated global EFs vary from 80% to 42% of natural flows, from the highest to lowest EMC, and the groundwater contribution ranges from 77% to 39% of natural base flows. Globally, 149 to 376 km3 yr-1 of groundwater can be abstracted sustainably, depending upon the EMC. An online interactive tool was developed, where the user, based on selected area and EMC, can determine the acceptable EF requirements, base flow contribution and sustainable groundwater abstraction. This can then be compared either directly with the information on water withdrawals in the selected area or can be fed into the SDG target equations to determine the sustainability indicators.

Informing Restoration Practice Through Estimation of Groundwater-Surface Water Time Lags With Windowed Cross-Correlation

University of California, Merced

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California’s floodplain ecosystems and their riparian wetlands provide habitat for wildlife and essential ecosystem services such as water quality regulation and flood protection, but have seen major declines due to land use changes and development of water management infrastructure. Resource managers are increasingly recognizing the benefit of restoring floodplains for the multiple benefits they provide. For example, floodplain restoration provides opportunities for aquifer recharge, a strategy that can boost flexibility in water management portfolios in terms of ameliorating water scarcity challenges due to drought, groundwater overdraft, and projected climate-driven precipitation shifts from snow to rain. Planning of multi-benefit floodplain restoration projects will need tools to support project siting and understanding of groundwater-surface water interactions while informing the regulation of stream flow regimes. Our research addresses this need through the application of standard time-series analysis of groundwater and surface water data in the lower San Joaquin River in the California Central Valley. Windowed-cross correlations were applied to times series data to estimate time lags between stage and groundwater levels, providing insight to the strength of these flow connections. This method allows for quick assessment of groundwater-surface water interactions that can be a preliminary step in developing monitoring action to help identify suitable floodplain restoration sites.

Predicted Impacts of Conjunctive Water Management on Late Summer Streamflow in an Agricultural Groundwater Basin with Limited Storage, Scott Valley, CA

UC Davis

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Late summer streamflow for the Scott River in northern California has decreased approximately 50% since the mid 1960’s, resulting in increased water temperatures and disconnection of the stream. This negatively impacts aquatic habitat of fish species such as coho and fall-run Chinook salmon. In collaboration with local stakeholders, the Scott Valley Integrated Hydrologic Model has been developed, which combines a water budget model and a groundwater-surface water model (MODFLOW) of the 200 km2 basin. The goal of the integrated model is to better understand the hydrologic system of the valley and explore effects of different conjunctive management scenarios on streamflow during the critically dry months (Aug-Oct). The groundwater model has a 100 m lateral resolution with aggregated monthly stresses over a 21 year simulation period (1990-2011). The Scott River and tributaries are represented using the streamflow routing (SFR) package. Sensitivity analysis and calibration were performed using 812 head observations from 50 wells in the basin and average daily streamflow observations from a USGS stream gauge during the simulation period. The calibrated model was used to evaluate two different management scenarios: 1) in-lieu recharge where surface-water instead of groundwater is used to irrigate fields near the river while streamflow is sufficiently high, and 2) managed aquifer recharge during the winter months (Jan-Mar) on agricultural fields located in gulches on the eastern side of the valley using existing infrastructure. Preliminary results indicate that managed aquifer recharge may increase streamflow during the critically dry months at the basin outlet by 1-2 cubic feet per second (cfs), while in-lieu recharge may increase flows during the same period by 10-30 cfs. The greater increase in flow from the in-lieu recharge scenario is largely due to reductions of groundwater pumping by 10-25% from the base case scenario, with greater pumping reductions during wetter years. This increase in flow during the critically dry months decreases the length of dry reaches both spatially and temporally, allowing for earlier reconnection of the Scott River and decreased stress on fish.

Addressing Model Uncertainty in Groundwater-Management Modeling: A Case Study from the Upper Klamath Basin, Oregon and California.

U.S. Geological Survey

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Resource-management agencies in the upper Klamath Basin (located in southern Oregon and northern California) require methods to determine groundwater-management strategies that augment surface-water irrigation supplies while simultaneously avoiding adverse effects of pumping on groundwater and surface-water resources. In an earlier study (Wagner and Gannett, 2014, U.S. Geological Survey Scientific Investigations Report 2014-5054, http://dx.doi.org/10.3133/sir20145054 ), we linked deterministic groundwater simulation with optimization to create a decision model that identifies practices that best meet the goals and constraints associated with groundwater development in the basin. The decision model was developed within the framework of the Klamath Basin Restoration Agreement (KBRA), which would establish a permanent limit on the amount of surface water that can be diverted for irrigation in the Bureau of Reclamation Klamath Project. The model evaluates groundwater-management alternatives to: (1) identify groundwater-pumping patterns that, to the extent possible, meet supplemental irrigation demand expected under the KBRA; (2) limit the effects of groundwater withdrawal on groundwater discharge to streams and springs that support aquatic habitat, as defined in the KBRA; (3) ensure that drawdown caused by managed pumping does not exceed limits allowed by water law; and (4) ensure that groundwater pumping does not adversely affect agricultural drain flows that supply downstream irrigators and wildlife refuges. The results indicate that groundwater pumping is limited primarily by drawdown restrictions defined by Oregon water law, and that the effects of managed pumping on streams and springs that support aquatic habitat are substantially less than limits defined in the KBRA. In this study, we extend the work of Wagner and Gannet (2014) to evaluate the effect of model uncertainty on groundwater-development planning in the upper Klamath Basin.We developed a stochastic management model that combines optimization with calibration-constrained uncertainty analysis to identify risk-averse groundwater-management plans. The calibration-constrained uncertainty analysis first generates a suite of 1000 parameter realizations, with each realization reproducing the groundwater-model calibration data within a specified tolerance. This set of parameter realizations forms the basis of model-prediction uncertainties obtained using Monte Carlo methods. The model-prediction uncertainties are then included in the optimization model using the chance-constrained method that defines the probability that the groundwater-management constraints will be satisfied. The resulting optimization model allows us to identify robust groundwater-management strategies that account for uncertainties associated with simulated drawdown and groundwater discharge, and to evaluate the trade-offs between pumpage and system reliability (as specified in the chance-constrained model). Preliminary results indicate that a robust groundwater-management strategy would reduce and redistribute managed pumping when compared to the deterministic results, and that the changes in pumpage result primarily from uncertainties associated with the calibrated values of hydraulic conductivity and specific storage for the model zones where managed wells are located. The results also indicate that, as with the original management-modeling effort, the pumpage associated with robust groundwater management is limited by drawdown restrictions and impacts to agricultural drain flows; the uncertainty-based constraints that limit the impact of pumping on groundwater discharge to aquatic habitat do not approach the limits defined in the KBRA.