The Importance of Simulating Unsaturated Zone Flow in the Humid Climate of the Trout Lake Basin, Northern Wisconsin
Abstract
Recharge is the most important source of water to groundwater systems, but is often not well quantified. Uncertainty is especially acute with respect to transient recharge. Recently, USGS researchers have been working toward adding unsaturated-zone processes to MODFLOW. A one-dimensional, kinematic wave approach is used that maintains conservation of mass in a homogeneous unsaturated zone but does not consider flow due to capillary forces (UZF package - Niswonger et al. 2006). This code was applied to the sandy sediments of the Trout Lake watershed in northern Wisconsin, where an existing transient simulation was used to evaluate the importance of unsaturated-zone flow. The model has 30 lakes simulated with the lake (LAK) and stream flow routing (SFR) packages, and consists of four layers with a total of 220800 cells. Time was discretized into monthly stress periods, and total length of the simulation was ten years. The evaluation consisted of a comparison of two cases: 1) the standard approach of adding infiltration directly to the water table using the recharge package (RCH) for MODFLOW; and 2) adding the same infiltration below the root zone and subsequently routing it through the unsaturated zone using the UZF package for MODFLOW. Results show that in areas with thin (<2 m) unsaturated zones, little difference in timing of recharge was observed between methods but saturation excess did occur with the UZF package; thus, it appears that in this basin, a standard RCH package approach was adding more water to the water table than would be expected given the soils present in the watershed. In areas with thicker unsaturated zones (>10 m), the recharge pulse was more diffuse when routed through the unsaturated zone, and coalescing wetting fronts occurred that represented spring snowmelt infiltration overtaking and combining with infiltration from the previous fall. Thus, in thicker unsaturated zones, the volume of water infiltrated was properly simulated using the standard RCH approach, but the timing was less representative of field conditions. In addition, accounting for unsaturated zone flow provided a more representative simulation of the watershed-scale groundwater system, both in a better fit to transient head data as well as stream flows and lake stages. These results demonstrate that even transient flow models in humid climates with relatively shallow water tables may benefit from a more realistic representation of unsaturated zone flow. Niswonger, R.G., D.E. Prudic, and R.S. Regan, 2006, Documentation of the Unsaturated-Zone Flow (UZF1) Package for Modeling Unsaturated Flow Between the Land Surface and the Water Table with MODFLOW- 2005. Techniques and Methods 6-A19, Chapter 19 of Section A, Ground Water, of Book 6, Modeling Techniques, 62 p.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.H32B..02H
- Keywords:
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- 1830 Groundwater/surface water interaction;
- 1838 Infiltration;
- 1847 Modeling;
- 1875 Vadose zone