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Watershed modeling is at the heart of modern hydrology, supplying rich information that is vital to addressing resource planning, environmental, and social problems. Even in light of this important role, many books relegate the subject to a single chapter while books devoted to modeling focus only on a specific area of application. Recognizing the
This second edition is extensively revised throughout with expanded discussion of modeling fundamentals and coverage of advances in model calibration and uncertainty analysis that are revolutionizing the science of groundwater modeling. The text is intended for undergraduate and graduate level courses in applied groundwater modeling and as a comprehensive reference for environmental consultants and scientists/engineers in industry and governmental agencies. Explains how to formulate a conceptual model of a groundwater system and translate it into a numerical model Demonstrates how modeling concepts, including boundary conditions, are implemented in two groundwater flow codes-- MODFLOW (for finite differences) and FEFLOW (for finite elements) Discusses particle tracking methods and codes for flowpath analysis and advective transport of contaminants Summarizes parameter estimation and uncertainty analysis approaches using the code PEST to illustrate how concepts are implemented Discusses modeling ethics and preparation of the modeling report Includes Boxes that amplify and supplement topics covered in the text Each chapter presents lists of common modeling errors and problem sets that illustrate concepts
Riparian buffers have been introduced as a means of decreasing nitrate concentrations and other nutrients in surface and subsurface runoff from agricultural activities. The goal of this study was to use groundwater flow models to determine possible hydrogeologic controls on the effectiveness of riparian buffers with different thickness and extents of alluvial and glacial units. A steady-state, finite-difference, groundwater flow model was constructed to simulate groundwater flow in profile at the Risdal North site in the Bear Creek watershed. The simulated profile was 155 m long and 1 row wide, and discretized into 62 columns and 14 layers (646 active cells). Four generalized model zones representing unoxidized till, oxidized till, loam and sand were assigned hydraulic conductivity values obtained from previous studies. Recharge rate was assumed to be 10 percent of mean annual precipitation of 82.9 cm and applied on the top layer. The USGS finite-difference model, MODFLOW was used to simulate hydraulic head and the water table. The model was calibrated by trial-and-error, UCODE, and PEST simulations. Calibration results showed good match between observed heads and simulated heads and the mass balance difference between recharge and discharge of the model was 0.03 percent. The USGS particle-tracking code, MODPATH was used to track particles in the groundwater flow system. The results showed that the average residence time was 94 days in the buffer and the source of groundwater contributing to Bear Creek ranged from distances of 8 to 137.5 m from the creek. Eleven generic models were constructed to assess the effects of geology on groundwater flow and residence times. The models used parameters of the calibrated model and hypothetical geologic conditions based on previous studies at the buffers. Results indicated that loam beneath a buffer had long residence time and supported the shallowest water table. Till beneath a buffer showed the shortest residence time. Sand or limestone beneath a buffer produced a deep water table. Based on these results, if long residence times and shallow water table are favorable for denitrification, loam beneath a buffer might provide the best hydrogeologic setting for removal of nitrate from groundwater.
A groundwater-flow model was developed to contribute to an improved understanding of water resources in the Chambers–Clover Creek Watershed. The model covers an area of about 491 square miles in western Pierce County, Washington, and is bounded to the northeast by the Puyallup River valley, to the southwest by the Nisqually River valley, and extends northwest to Puget Sound, and southeast to Tanwax Creek. The Puyallup and Nisqually Rivers occupy large, relatively flat alluvial valleys that are separated by a broad, poorly drained, upland area that covers most of the model area. Chambers and Clover Creeks drain much of the central uplands and flow westward to Puget Sound. The model area is underlain by a northwest-thickening sequence of unconsolidated glacial (till and outwash) and interglacial (fluvial and lacustrine) deposits. Ten unconsolidated hydrogeologic units in the model area form the basis of the groundwater-flow model.