JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION Vol. 52, No. 3 AMERICAN WATER RESOURCES ASSOCIATION June 2016 EVALUATING HYDROLOGICALLY CONNECTED SURFACE WATER AND GROUNDWATER USING A GROUNDWATER MODEL1 Ruopu Li, Mahesh Pun, Jesse Bradley, Gengxin Ou, Jim Schneider, Brandi Flyr, Jessie Winter, and Sudhansh Chinta2 ABSTRACT: Determination of the nature and extent of the connection between groundwater and surface water is of paramount importance to managing water supplies. The development of analyses that detail the surface water-groundwater system may lead to more effective utilization of available water. A tool was developed to help determine the effects of groundwater and surface water interactions. The software tool includes two graphic user interfaces to allow full compatibility with numerical MODFLOW groundwater models. This case study shows the tool, in conjunction with MODFLOW groundwater models and carefully designed scenarios, can suc- cessfully calculate the rates of stream-groundwater interactions, thereby providing the basis for designating management areas with the most significant hydrologic impact. This tool can be applied in other regions with similar settings and needs for integrated water management. (KEY TERMS: integrated water management; hydrologically connected area; stream depletion; MODFLOW; Zone Budget; Nebraska.) Li, Ruopu, Mahesh Pun, Jesse Bradley, Gengxin Ou, Jim Schneider, Brandi Flyr, Jessie Winter, and Sudhansh Chinta, 2016. Evaluating Hydrologically Connected Surface Water and Groundwater Using a Groundwater Model. Journal of the American Water Resources Association (JAWRA) 52(3):799-805. DOI: 10.1111/1752- 1688.12420 INTRODUCTION streamflow. For example, over-appropriation of streams can reduce the groundwater levels, and con- versely extraction of groundwater often depletes Surface water and groundwater interactions that streamflow. However, statutory and legal frameworks result in a single integrated resource have long been often designated surface water and groundwater as acknowledged in the scientific community. The inter- separate water resources (Sophocleous et al., 1995; action between groundwater and surface water Winter et al., 1998; Hoffman and Zellmer, 2013). affects stream discharge, water quality, geomorphic Inability to integrate both water resources as a single evolution, and ecosystem services (Sophocleous, resource into one management framework often 2010). Anthropogenic influences on this interaction causes negative implications such as reduction in include land-use modifications that affect recharge streamflow, aquifer drawdown, water conflicts rates, groundwater pumping, and depletion to between users, and insufficient environmental flows 1Paper No. JAWRA-15-0170-N of the Journal of the American Water Resources Association (JAWRA). Received October 8, 2015; accepted February 10, 2016. © 2016 American Water Resources Association. Discussions are open until six months from issue publication. 2Assistant Professor (Li), Department of Geography and Environmental Resources, Southern Illinois University-Carbondale, 4543 Faner Hall, Carbondale, Illinois 62910; Integrated Water Management Analyst (Pun, Winter), Natural Resource Programs Director (Bradley), Inte- grated Water Management Specialist (Ou), Formerly Deputy Director (Schneider), Formerly Integrated Water Management Coordinator (Flyr), and IT Contractor (Chinta), The Nebraska Department of Natural Resources, Lincoln, Nebraska 68509 (E-Mail/Li: [email protected]). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 799 JAWRA LI,PUN,BRADLEY,OU,SCHNEIDER,FLYR,WINTER, AND CHINTA (Steward et al., 2013). Therefore, modern water agement actions will exert the greatest impacts. To resource management requires the coordinated plan- facilitate the determination as a policy-based deci- ning of groundwater use and surface water use, also sion, the Nebraska Department of Natural Resources known as Integrated Water Management (IWM). (NDNR) considers hydrologically connected area to be Other equivalent and frequently used alternative the area in which pumping of a well for 50 years terms include Integrated Water Resource Manage- would deplete streamflow by at least 10% of the ment (Biswas, 2004) or Conjunctive Water Manage- amount pumped. In Nebraska, the hydrologically con- ment (Blomquist et al., 2001). nected area is also known as the “10/50 Area” (hydro- Integrated Water Management is the most widely logically connected area and 10/50 area will be used used management strategy to tackle global water con- interchangeably hereafter). flicts where water demands often surpass supplies In this article, we outlined a computer program (Biswas, 2004; Rassam et al., 2013). The U.S. State of that can be used to assist in determining the geo- Nebraska, as the leading state in IWM (Viessman, graphic extents of the hydrologically connected area 2007), focuses on the broadly connected groundwater and the nature of surface water-groundwater rela- and surface water supplies (Nebraska Department of tionships by identifying the rate of water movement Natural Resources (NDNR), 2006). The majority of between the systems. In a policy context, this rate of Nebraska streams are base-flow dominant, and interaction helps define areas of management focus, groundwater contributions are the dominant source which are the hydrologically connected areas. This of water to the system that supports surface water tool is expected to fill the gaps between the develop- irrigation and many ecological and recreational ser- ment of groundwater models and application of the vices. Historically, the development of groundwater models for water resource management. in some basins (e.g., the Republican River Basin) had led to substantial river depletions and reduced out- flow to downstream users. Understanding the extent and magnitude of the METHOD connectivity between streams and adjacent aquifers is one of the key challenges for the effective manage- ment of integrated water resources (Winter et al., To delineate the scientifically defensible 10/50 1998; Sophocleous, 2002). The nature of these rela- area, NDNR reviewed available numerical models to tionships and varied temporal effects between the assess the validity and appropriateness for hydrologic fast-response surface water system and the slow- connectivity analyses. After selection of a numerical response groundwater system are dependent on a model, NDNR relied on a process known as “Cycle number of factors, such as the hydraulic properties of Well Analysis” (CWA) to conduct the analysis. The the aquifer/stream system, the density and distribu- results of the CWA are used to address the following tion of the stream network, the proximity of ground- questions: in which geographic area along the water pumping to streams, and the severity of streams are the groundwater and surface water anthropogenic modifications. Among numerous exist- regarded as hydrologically connected (extent); and ing approaches (e.g., hydrographs, hydrochemistry, what percentage of stream depletion can be caused analytical, and numerical flow modeling), numerical by newly installed pumping wells (magnitude). Since modeling has been adopted with exceptional frequency the above process is typically applied to the multi- as a decision support tool for planning and managing basin numerical models with tens of thousands of water resources (Bejranonda et al., 2013). Delineating model grid cells, it is expected that a manual step-by- hydrologically connected area is regarded as one of step implementation of CWA is not plausible. the most important steps in IWM of Nebraska. The basic design principle of the CWA tool is to The hydrologically connected area in the context of place a hypothetical well into each model grid cell, this article is defined as “the geographic area for the run the groundwater model developed using MOD- purposes of management, within which surface FLOW, and then the Zone Budget program in a cycli- water-groundwater interactions occur at specified cal manner. MODFLOW is a finite-differential rates, within a given time frame” (Groundwater Man- groundwater flow modeling program commonly used agement and Protection Act (GMPA), 2004). The to assess the effects of pumping on streamflow and delineation of this area utilizes the stream depletion aquifer levels (Harbaugh and McDonald, 1996). Zone factor (SDF), which calculates the potential of water Budget is a MODFLOW post-processing program that pumped from a groundwater well to deplete stream- computes subregional water budgets using results flow (Jenkins, 1968; Fredericks et al., 1998). The produced by MODFLOW (Harbaugh, 1990). It is potential for depletive effects is a concept frequently noted that the Zone Budget allows the stream deple- utilized by Nebraska to designate areas where man- tion to be summarized at the levels of subregions or JAWRA 800 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION EVALUATING HYDROLOGICALLY CONNECTED SURFACE WATER AND GROUNDWATER USING A GROUNDWATER MODEL subbasins, and the groundwater model should be tion of the long-term SDF and the determination of appropriate for a 50-year model simulation. Water the 10/50 area depend on the process of the post-pro- budget outputs from each cycle are compared with cessing using a separate program CWA Summary those from the baseline run (i.e., the original model Analysis Tool (Figure 2C). This software tool may help run without the hypothetical well). The stream deple- water resource managers and technical staff who are tion factor