Monocacy River Basin Ground Water Flow Model

Basin Outline:

The Monocacy River Basin has a total area of approximately 970 square miles, and includes most of Frederick, about half of Carroll, and a small part of Montgomery counties in Maryland and about half of Adams and a small part of Franklin counties in Pennsylvania. The basin is bounded on the west by Catoctin and South mountains and the Potomac River on the south. The eastern boundary is in the Piedmont upland in Carroll County and the northern boundary is in the Mesozoic lowlands north of Gettysburg in Adams County.

Model Grid:

This dataset contains a polygon shapefile of the finite-difference grid used in the MODFLOW-2000 ground water flow model of the Monocacy River Basin. The model grid consists of 83 rows and 55 columns of cells, each 1000 m by 1000 m in three layers. The grid is rotated by 22 degrees to the east to align the principle directions of the flow model with the general orientation of the primary structural features in the Mesozoic lowland part of the model area.

Land surface elevation:

This dataset contains the elevation of the land surface as defined by a digital elevation model (DEM) created by the USGS. The horizontal resolution of the DEM is 10 m. This was averaged using a ‘nearest neighbor’ method in the process of applying the data to the 1000 m by 1000 m model grid. The elevation data is in meters. This dataset is used to define the elevation of the top of the upper most layer of the flow model.

Regolith thickness:

The thickness of the regolith (colluvium, alluvium, saprolite, and weathered bedrock) overlying the bedrock in the area is estimated from the length of well casings as reported in well databases from Maryland and Pennsylvania. Some smoothing and averaging of the data has been performed to account for the lack of even spatial distribution and to deal with extremes of well lengths in the record.

Bedrock geology:

The USGS National Water-Quality Assessment (NAWQA) program's Potomac River Basin study is an integrated water-quality assessment designed to address a broad array of water-quality properties in several environmental media at various spatial and temporal scales. This investigation was conducted as part of a nation-wide study to help assess the nation's water-quality conditions.

Physiography and geology, the two most influential natural factors responsible for most of the water-quality variability within the Potomac River basin, were used to develop a framework within which to assess the water quality of the basin. Eight major subunits were created based on the geographic distribution of physiographic and geologic characteristics within the basin. The resulting distribution of subunits resembles the distribution of physiographic provinces in most cases. Differences in rock type were considered important enough in the Valley and Ridge ( Great Valley section) to further subdivide this province into Great Valley carbonate and siliciclastic subunits.

The Monocacy River basin lies in the northeast part of the Potomac River basin and contains a subset of the physiographic and geologic subunits as defined above. This dataset was clipped from the original dataset produced by the USGS for the NAWQA program's Potomac River Basin study. This dataset is used to define the distribution of hydraulic conductivity within the flow model (see the Hydraulic conductivity dataset).

Hydraulic conductivity:

Hydraulic conductivity values are assigned for each cell in the finite-difference model grid. These values have been grouped into zones based on the underlying lithology as defined in the revised NAWQA geology layer and estimated conductivity of the regolith overlying the bedrock. The hydraulic conductivity values assigned to each zone were based on data from a review of available literature for the area. The distribution of the zones is kept constant in the underlying layers with only the hydraulic conductivity values changing for each layer.

Recharge:

This raster (grid) dataset is an index of mean annual natural ground-water recharge. The dataset was created by multiplying a grid of base-flow index (BFI) values by a grid of mean annual runoff values derived from a 1951-80 mean annual runoff contour map. Mean annual runoff is long-term average streamflow expressed on a per-unit-area basis.

The concept used to construct the dataset is based on two assumptions: (1) long-term average natural ground-water recharge is equal to long-term average natural ground-water discharge to streams, and (2) the base-flow index reasonably represents, over the long term, the percentage of natural ground-water discharge in streamflow.

This dataset has been clipped from the dataset distributed by the USGS to include only the area of the Monocacy River Basin. The values of recharge within the Monocacy River Basin were grouped into ten categories. The geographic distribution of these values was used to create the recharge zone definition within the ground water flow model.

Evapotranspiration:

Estimates of evapotranspiration from the ground water aquifer was estimated for the Monocacy River basin ground water flow model from the seasonal water budget analysis study by ICPRB. Estimates were made of annual recharge, based on annual baseflow, for a set of gauged sub-basins within or near the Monocacy River and Catoctin Creek watersheds. These ET estimates were extrapolated to the ungauged basins within the Monocacy River basin using basin area and hydrogeology. Evapotranspiration data for the fifteen sub-basins within the Monocacy River basin are used as input to the initial version of the ground water flow model.

Monocacy River main stem:

This dataset represents the main stem of the Monocacy River as defined in the USGS NHD 2003 stream dataset with the NAME field containing 'Monocacy R'. The extracted polyline shapefile produced was intersected with polygon shapefiles of the flow model grid and bedrock geology to give segment lengths for the stream in each model cell and hydraulic conductivity of the underlying geology and the corresponding model zone. This data was used to determine streambed vertical conductivity. This dataset was used to create the MODFLOW RIV package, which is an input to the ground water flow model. This dataset is an interim river definition until more complete stream routing input is available.

Extraction wells:

This dataset contains extraction well data for input into a USGS MODFLOW-2000 flow model of the Monocacy River Basin. The data was compiled from Maryland Department of the Environment water use data and Pennsylvania PaGWSI data. This database includes only wells that were in use in 1980, as the initial version of the flow model is a steady state model using the conditions existing around 1980 to approximate "pre-development" conditions.

Water level targets:

This dataset contains records of ground water levels as recorded in wells contained in the USGS Ground Water Site Inventory for the Monocacy River basin. These wells were selected because they contain water level data from around theearly 1980s, the period being used to represent "initial conditions" for the ground water flow model. The water level data in this dataset are annual average water levels. The data in this dataset will be used as targets for evaluating the quality of the simulation in the initial steady state version of the flow model.