Hydrologic Modeling of a Groundwater Dominated Watershed Using a Loosely Coupled Modeling Approach 1Barth, C. , 1P. Krause, 2D.P. Boyle and 3S.L. Markstrom 1Friedrich Schiller University Jena, Department of Geoinformatics, Hydrology,and Modeling, Loebdergraben 32, 07743 Jena, Germany, 2 Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, USA, . 3U.S. Geological Survey, Box 25046, Mail Stop 412, Denver Federal Center, Denver, CO 80225, USA E-Mail: [email protected] Keywords: surface-groundwater interaction; loosely coupled model; MODFLOW; PRMS. basin features a strong surface-groundwater EXTENDED ABSTRACT interaction of unknown degree. Since the late 1960s, hydrologic modeling focused This research paper describes the concept of an more and more on interactions between surface inexpensive and simple method of combining the and groundwater flow systems, leading to the advantages of two established hydrologic development of integrated modeling approaches modeling approaches by using output information simulating flow in these two systems and the of the surface water model as input for the simulation of interactions between them. These groundwater model. The basic assumption is made interactions are of variable character as well as that groundwater recharge rates provided by spatial and temporal heterogeneity. PRMS are reasonable because the estimation procedure benefits from the fairly high resolution The primary objective of this study is the of meteorological, land use and soil parameters of application and investigation of a loosely coupled the surface water model. At present, the approach modeling approach, combining two well-known presented in this paper is limited to a models of the United States Geological Survey, the unidirectional coupling of the surface and Precipitation Runoff Modeling System (PRMS) groundwater model, which is only accounting for and the Modular Finite-Difference Groundwater groundwater estimates provided by PRMS as input Flow Model (MODFLOW), in order to simulate for MODFLOW; whereas possible upward complex hydrological processes in a mesoscale movement of water, i.e. groundwater seepage to watershed. Within the scope of the hydrologic the surface is neglected. Performance and modeling groundwater recharge, the parameter efficiency of this approach are assessed in the unidirectionally linking the surface water and scope of applying the model in steady-state as well groundwater system is used to couple these two as transient mode to the Esperstedter Ried basin. models loosely. Considering factors that have a great influence on groundwater recharge (e.g. The results of this case study, which produced a climate, topography, soil, and land use), the number of reasonable as well as some unsatisfying potential of modeling the hydrologic system with a simulations, demonstrate the potential of coupling distributed surface water model is explored. In a surface water model and a groundwater model to addition, the option of constraining the PRMS obtain more complex and accurate analyses and parameter determining groundwater recharge, and simulations of hydrologic systems. However, the therefore the loosely coupled surface-groundwater study also shows the need for further research in model, with observed head levels only, is regard to constraining the parameter linking both investigated. models, i.e. groundwater recharge. Furthermore, the necessity for a more dynamic and complex This modeling approach is applied to the method of integrating surface and subsurface Esperstedter Ried basin, an ungauged, mesoscale, interactions is revealed, which accounts for groundwater-dominated catchment in central downward as well as upward movement of water Germany. The study represents a good example of in the subsurface system. problems frequently occurring in hydrologic modeling. Due to the lack of observed streamflow and surface runoff data, the calibration and validation of any hydrologic model for this watershed proves to be difficult. Furthermore, the 601 Model (MODFLOW), which are both available 1. INTRODUCTION free of charge on the internet. The factor unidirectionally linking both systems (i.e. the Traditional hydrologic modeling approaches parameter coupling the surface and the generally concentrate on either the surface or the groundwater model), namely groundwater subsurface system. Surface water models are recharge, is investigated regarding the performance designed and calibrated with emphasis on surface of this loosely coupled approach. Furthermore, the water movement, evapotranspiration and possibility of constraining the PRMS parameter infiltration; whereas the subsurface is commonly determining the amount of recharge into the viewed as a simple, lumped system and subsurface reservoir by merely using observed groundwater processes are not physically based. groundwater head levels is assessed. Groundwater models, on the other hand, focus on obtaining exact groundwater head levels in respect 2. MODEL DESCRIPTION to occurring stresses while surface water processes are oversimplified. 2.1. The groundwater model MODFLOW For the last three decades, a major objective of The Modular Three-Dimensional Finite-Difference hydrologists has been the coupling of both Groundwater Flow Model (MODFLOW) first approaches in order to model the entire hydrologic released by the U.S. Geological Survey in 1984, system and, furthermore, to investigate the has become the most widely used groundwater interaction between surface and subsurface model (McDonald & Harbaugh 1984) processes. Particularly basic methods of modeling MODFLOW is a mathematical model simulating surface-groundwater interactions based on the groundwater flow through heterogeneous, numerical solutions of differential equations and a porous media. In MODFOW three-dimensional variety of integrated models have been developed groundwater flow is described by the partial- since the late 1960s. Some of the first studies of differential equation surface-subsurface interactions were conducted by ∂ ∂ ∂ ∂ ∂ ∂ ∂ Smith & Woolhiser (1971), who coupled a one- ⎛ h ⎞ + ⎛ h ⎞ + ⎛ h ⎞ − = h (1) ⎜ K xx ⎟ ⎜ K yy ⎟ ⎜ K zz ⎟ W SS dimensional flow vertical infiltration model to an ∂x ⎝ ∂x ⎠ ∂y ⎝ ∂y ⎠ ∂z ⎝ ∂z ⎠ ∂t overland flow model, Pinder & Sauer (1971) linked a one-dimensional streamflow model to a where Kxx, Kyy, and Kzz = the hydraulic conductivity two-dimensional aquifer, and Freeze (1972) along the x, y, and z axes that are assumed to be designed a model simulating three-dimensional parallel to the major axes of hydraulic conductivity transient subsurface flow as well as one- (L/T), h = the potentiometric head (L), dimensional streamflow. In recent years, a number W = volumetric flux per unit value representing of integrated models has been developed, sources and/or sinks of water ( W < 0.0 for outflow including SHE, MIKE-SHE, and FHM. SHE of the groundwater system, W > 0.0 for inflow (T-1), SS = specific storage of porous material (L- (SYSTÈME HYDROLOGIQUE EUROPÈEN) is a physically based modeling system, simulating 1), and T = time (McDonald & Harbaugh 1984). various components of water movement (such as evapotranspiration, overland and channel flow, Spatial discretization in MODFLOW is achieved flow in the unsaturated and saturated zones, and by considering real-world aquifers a system of grid many more) based on finite-difference or empirical cells which are characterized in terms of rows (i), equations. In addition to representing the entire columns (j), and layers (k). Every cell represents a land phase of the hydrologic cycle (interception, unit of homogenous properties. In the current evapotranspiration, surface runoff, river routing version MODFLOW-2000, the finite-difference etc.), MIKE-SHE also describes water movement grid is assumed to be of rectangular shape in the unsaturated soil zone and groundwater flow. horizontally, while the vertical dimension is The coupled surface and groundwater model FHM, distorted. Temporal discretization is based on time which incorporates components of the surface steps, which are grouped into stress periods. The water model HSPF and the groundwater model length of particular time steps is user-defined MODFLOW, is another example for an integrated during the model setup. model. In this study MODFLOW-2000 is used, which is This paper describes an alternative approach to operated in the comprehensive graphical user modeling a surface-groundwater system by loosely environment GMS (Department of Defense coupling two well-established models of the Groundwater Modeling System). United States Geological Survey, the Precipitation Runoff Modeling System (PRMS) and the Modular Finite-Difference Groundwater Flow 602 2.2. The surface water model PRMS recharge rates for each grid cell, the proportion of area of each HRU per cell is computed; hence, the The U.S. Geological Survey’s Precipitation Runoff amount of recharge can be determined for every Modeling System (PRMS), a deterministic, active cell in the MODFLOW grid model based on physical-process based watershed model was this proportion. The conversion of temporal scales developed by Leavesley et al. in 1983 (Leavesley needed for the transient MODFLOW simulation is et al. 1983). PRMS which operates either as performed by calculating the mean monthly lumped- or distributed parameter model, is amount of recharge for each HRU based on daily designed to analyze the dynamics and responses of output of ssr2gwrate values. The monthly recharge a hydrologic system regarding the