Click here to download presentation from September 30th "Water Resources in Adams County's Monocacy Basin - Charting a Course Toward Sustainability"

Background

In rapidly growing regions like the upper Monocacy River basin (see Figure 1), more and more water is being diverted for human use. Increased withdrawal of water from wells and streams results in decreases in stream flow which may degrade aquatic habitat. During summertime low flow periods, the availability of pools, riffles, and other habitats may shrink dramatically, and in extreme cases, a stream may dry up completely due to water withdrawals from the stream itself, or due to withdrawals from the ground water aquifers that feed the stream.

Project Description

The goal of this project is to investigate the potential impact of future ground water withdrawals on stream flows. A ground water/stream flow model has been constructed to simulate aquifer levels and stream flows in the upper Monocacy basin, using the USGS’s MODFLOW-2000 finite difference ground water modeling software. The upper Monocacy model has the following features:

• A grid with horizontal grid spacing of 500 meters by 500 meters
• A new refined representation of upper Monocacy streams (see Figure 2), implemented with MODFLOW’s STR stream package
• A steady-state calibration to summertime base flows under "average" hydrologic conditions, and verification runs demonstrating the model's capabilities to simulate aquifer levels under "dry", "average-dry","average-wet", and "wet" conditions
• Predictions of increases in dry and losing stream reaches for future scenarios in which ground water withdrawals within the Marsh/Rock/Alloway Creek drainage area increase by 0.5, 1.0, and 1.5 million gallons per day

Project Partners

• ICPRB
• Adams County Conservation District
• Watershed Alliance of Adams County
• Adams County Office of Planning and Development
• Strawberry Hill Nature Preserve
• USGS – PA District
• with funding from the National Fish and Wildlife Foundation and ICPRB

Project Outcomes

Details on model construction, calibration and verification can be found in the report, Seasonal Steady-State Ground Water/Stream Flow Model of the Upper Monocacy River Basin, ICPRB, C. Schultz and J. Palmer, 2008.

Model predictions indicate that during “dry” summers, which represent approximately the driest 20% of summers in the study period, a significant percentage of represented stream reaches are dry or losing, that is, that the underlying aquifer has dropped to a level below the bottom of the stream bed. The model also predicts that simulated increases in ground water withdrawals would significantly increase the percentage of dry or losing reaches during dry summers. For stream reaches represented in the model, the basin-wide percentage of dry or losing stream miles during dry summers is predicted to increase from approximately 40% to approximately 50% for the scenario in which withdrawals increase by 1.5 mgd. Click here to see maps with model predictions.

The accuracy of the upper Monocacy ground water/stream flow model is expected to be limited, especially in its predictions of local conditions. These limitations in accuracy stem from the regional scale of the model and the lack of detailed information on geologic structure. In addition, there is a lack of water level observation data and little information or data to verify the stream flow predictions of the model calibration and verification runs. However, the predicted increases in total miles of dry and losing reaches in the upper Monocacy basin may be indicative of the impact that future increases in ground water withdrawals will have on basin streams.