Welcome to GroundwaterGo!
- Ground Surface Elevation
- Conductance Calculations
- Surface Water Characteristics
- Water Surface Elevation
- Hydraulic Conductivity
This article describes the data inputs that were used to construct the GroundwaterGo mapper program.
Ground surface elevations were downloaded from the Nation Elevation Dataset, a dataset produced by the USGS.1 This dataset stores elevation for the entire country in raster format. It is available in latitude/longitude coordinate at several resolution. The resolution used by the GroundwaterGo mapper is 1/3-arc-second by 1/3-arc-second, which is approximately equal to 10 m by 10 m. It has been reprojected into UTM 15 coordinates at a resolution of exactly 10 m by 10 m.
River conductance in MODFLOW is governed by the equation2:
C = (KA)/D
- C = Conductance
- K = River bed conductivity.
- A = Cross-sectional area of river bed
- D = The thickness of the river bed.
River conductance is difficult to estimate in the absence of field measurements. Some simplifying assumptions have been made in this application for ease of calculation.
- The river bed is assumed to be 1 m thick.
- The river is assumed to be 10 m in width.
- K is assumed to be 1/10 of the cell conductivity as the river bed is assumed to be made up of fines.
The equation therefore simplifies as follows:
C = [(K of cell/10) * length * 10]/1 = K of cell * length.
Length is calculated as the average of the column height and the row width.
The conductance of the general head boundary is calculated in a similar manner.
C = (KA)/L3.
- C = Conductance
- K = Hydraulic conductivity of the prism of porous material.
- A = Cross-sectional area of the prism of porous material.
- L = The length of prism of the porous material.
A simplifying assumption has been made that the cells are roughly square. Therefore the equations simplifies to:
C = K of cell * [Top of cell – Bottom of cell ]
A recharge map was generated for the entire nation by subtracting national evaporation and runoff data4 from national precipitation data.5 The evaporation and runoff data was developed by the Nation Weather Service and the precipitation data was developed by the Natural Resources Conservation Service.
The location of rivers and lakes is based on a USGS National Atlas shapefile, Streams and Waterbodies of the United States6 and the size of contributing watersheds is calculated by this program upon query based on the ground surface elevation data. This spatial operation is conducted using GRASS GIS, an open source geographic information system software.7 For each grid cell inside the GIS domain, the function r.watershed calculates how many up-gradient cells contribute flow. Essentially, the watershed is calculated for each cell. This allows the position of primary flow paths (used for for assigning drains) and upstream watershed areas to be determined.
In order to obtain stream flow, a regression analysis was conducted on stream flow vs watershed area. Data for this regression was collected from the Hydro-Climatic Data Network Streamflow Data Set.8 The Data Set divides the United States into 21 water-resources regions. For each of these regions a regression was conducted on watershed area and average stream flow, creating a unique relationship between watershed and average stream flow. This relationship was used to estimate the average flow in streams based on their known watershed.
Once the approximate flow had been established, it was used to estimate the stream depth using a relationship developed by the USGS:9
D = 0.2612S0.3966
D is average stream depth in meters and S is average stream flow in cubic meters per second with an R-squared of 0.83.
The location of drains is calculated by this program upon query based on the ground surface elevation data. In this program cells were labeled as drain cells if they had an upstream watershed larger than 80 square kilometers. The elevation of drains was set to one meter below the ground surface.
An approximation of the water table was created based on geomorphology of the landscape. The location of drains was developed as described above. A simplifying assumption were made that the elevation of drains approximately matched the elevation of the water table (streams generally intercept the water table in non-arid areas). The water elevation within the drains was set to one meter below the ground surface. Based on the water elevation in these locations, the water table elevation was interpolated for an area 20 latitude minutes by 20 longitude minutes surrounding the model domain.
The bedrock elevation was developed from four vector maps from the state geological surveys of Iowa, Indiana, Illinois, New Jersey and Ohio.10,11,12,13,14 For Maine and Minnesota, the bedrock surface was developed by subtracting from the surface elevation depth-to-bedrock maps developed by the state geological surveys.15,16 For Kansas, Kentucky, the depth the bedrock point data were downloaded from the water well databases.17,18,19,20 These databases are maintained by either each state’s geological survey or by each state’s department of environmental protection.
Two general layers were established for the models. The first corresponds to surficial deposits and the second corresponds to the principal bedrock aquifer. The top of the first layer corresponds to the elevation of the ground surface. The top of the second layer corresponds to the elevation of the bedrock surface.
The hydraulic conductivity of the surficial deposits layer is based on a USGS shapefile: Surficial Deposits and Materials in the Eastern and Central United.21 This shapefile give the horizontal position of various surficial deposits in the eastern untied states with a description of material type. Based on the material types, generic hydraulic conductivity values were assigned to the shapefile. A USGS shapefile was also used for assigning hydraulic conductivity values to the second layer. This shapefile, Principal Aquifers of the 48 Conterminous United States, gives the horizontal position and rock type for the principal aquifers. The rock types were used to assign hydraulic conductivity values to this shapefile. Hydraulic conductivity values for both layers were drawn from the literature sources.23,24,25
1. United States Geological Survey (USGS). 1999. National Elevation Dataset. Available at http://ned.usgs.gov/.
2. USGS. 2000. MODFLOW-2000, The U.S. Geological Survey Modular Ground-Water Model- UserGuide to the Observation, Sensitivity, and Parameter-Estimation Processes and Three Post-Processing Programs. Open-File Report 00-184.
3. USGS. 2000. MODFLOW-2000, The U.S. Geological Survey Modular Ground-Water Model-User Guide to Modularization Concepts and the Ground-Water Flow Process. Open-File Report 00-92. pg 22
4. National Weather Service. 2010. Land Surface Monitoring and Prediction CPC Leaky Bucket Model. Runoff and Evaporation Data 1931-2008. Available at: http://www.cpc.ncep.noaa.gov/products/Soilmst_Monitoring/index.shtml.
5. Daly, C., and Taylor, G. 2009. United States Average Annual Precipitation, 1961-1990. Corvallis, OR: Spatial Climate Analysis Service, Natural Resources Conservation Service: National Water and Climate Center. NRCS National Cartography and Geospatial Center. Available at: http://nationalatlas.gov/atlasftp.html
6. USGS. 2005. Streams and Waterbodies of the United States. Reston, VA: National Atlas of the United States. Available at: http://nationalatlas.gov/atlasftp.html
7. Open Source Geospatial Foundation. Accessed July 28, 2010. Geographic Resources Analysis Support System: GRASS GIS. Available at: http://grass.osgeo.org/
8. Slack, J.R. and Landwehr, J. M. 1988. Hydro-Climatic Data Network (HCDN): Streamflow Data Set, 1874 – 1988. USGS Water-Resources Investigations Report 93-4076
9. Leopold, L. B. and Maddock Jr., T. 1953. The Hydraulic Geometry of Stream Channels and Some Physiographic Implications. Washington D.C: United States Department of the Interior, Geological Survey Professional Paper 252.
10. Witzke, B. J., Anderson, R. R., and Pope, J. P. 2010. Iowa Bedrock Topography. Iowa City, Iowa: Iowa Geological and Water Survey, DNR. Available at: ftp://ftp.igsb.uiowa.edu/gis_library/ia_state/geologic/bedrock/bedrock_topography.zip
11. Herzog, B.L., Stiff, B.J., Chenoweth, C.A., Warner, K.L., Sievering, J.B., and Avery, C. 1994. Buried Bedrock Surface of Illinois. Champaign, Illinois: Illinois State Geological Survey. Available at: http://www.isgs.illinois.edu/
12. Gray, H. H., Held, D. A., Dintaman, C., and Sowder, K. H. 2003. Bedrock Topography Contours, Indiana. Bloomington, Indiana: Indiana Geological Survey. Avaialbe at:http://igs.indiana.edu/arcims/statewide/download.html
13. New Jersey Department of Environmental Protection/New Jersey Geological Survey. 2006. Bedrock-Surface Topography of New Jersey (1:100,000-scale). Trenton, New Jersey. Meta data available at: http://www.state.nj.us/dep/njgs/geodata/njbedtop.htm
14. Ohio Division of Geological Survey. 2003. 1:24,000-scale bedrock-topography contours for Ohio. Columbus, OH: Ohio Division of Geological Survey.
15. Maine Department of Conservation, Maine Geological Survey. 2010. Surficial Materials Points. Surficial Materials Maps. Augusta, Maine: Department of Conservation, Maine Geological Survey and Maine Office of Geographic Information Systems.
16. Lively, R.S., Bauer, E.J., and Chandler, V.M. 2006. Maps of Gridded Bedrock Elevation and Depth to Bedrock in Minnesota. Minnesota Geological Survey. Available at: ftp://mgssun6.mngs.umn.edu/pub5/ofr06_02/
17. Kansas Geological Survey. Accessed July 2011. Water Well Database. Available at: www.kgs.ku.edu/Magellan/WaterWell/
18. Kentucky Geological Survey. Accessed October 2010. Water Well & Spring Records Database. Available at: kgs.uky.edu/kgsweb/datasearching/water/waterwellsearch.asp
19. New York Department of Environmental Conservation. Water Well Program Information Search Wizard. Accessed May 2011. Available at: www.dec.ny.gov/cfmx/extapps/WaterWell/index.cfm?view=searchByCounty
20. Vermont Department of Environmental Conservation. Well Completion Report Searchable Database. Accessed April 2011. Available at: www.anr.state.vt.us/DEC/watersup/cfm/WellReportSearchForm.cfm
21. Fullerton, D. S., Bush, C. A., and Pennell, J. N. 2004. Surficial Deposits and Materials in the Eastern and Central United. Denver, CO: U.S. Geological Survey. Available at:http://pubs.usgs.gov/imap/i-2789
22. USGS. 2003. Principal Aquifers of the 48 Conterminous United States, Hawaii, Puerto Rico, and the U.S. Virgin Islands. Madison, WI, USA. Availalbe at: http://nationalatlas.gov/atlasftp.html
23. United States Environmental Protection Agency. 1986. Saturated Hydraulic Conductivity, Saturated Leachate Conductivity and Intrinsic Permeability. EPA Method 9100 /Cincinnati, OH.
24. Bear, J. 1972. Dynamics of Fluids in Porous Media. Dover Publications. ISBN 0-486-65675-6.
25. Domenico, P.A. and Schwartz, F.W. 1990. Physical and Chemical Hydrogeology, John Wiley & Sons, New York, 824 p.