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SPDSS_Task42-3_Phase3_LowerSPlatteAlluviumRegionAquiferConfiguration
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SPDSS_Task42-3_Phase3_LowerSPlatteAlluviumRegionAquiferConfiguration
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Last modified
4/17/2013 9:20:41 AM
Creation date
6/11/2008 10:36:35 AM
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Decision Support Systems
Title
SPDSS Task 42.3 - Lower South Platte Alluvium Region Aquifer Configuration - Phase 3
Description
This Technical Memorandum was undertaken under Task 42.3, and summarizes the compilation, analysis and mapping of existing published aquifer configuration data for the Lower South Platte Alluvium Region.
Decision Support - Doc Type
Task Memorandum
Date
11/30/2006
DSS Category
Groundwater
DSS
South Platte
Basin
South Platte
Contract/PO #
C153953
Grant Type
Non-Reimbursable
Bill Number
SB01-157, HB02-1152, SB03-110, HB04-1221, SB05-084, HB06-1313, SB07-122
Prepared By
CDM
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1.2.2 Development of Contours in Areas not Covered by Historical Sources <br />Contoured data of the bottom of alluvium and the groundwater level were not available from <br />historic reports for the alluvium of Crow Creek, Upper Box Elder Creek, Upper Beaver Creek, <br />and the South Platte River just below Chatfield Reservoir. In these areas, picks from DWR well <br />permit images and other sources were used in this task to manually draw contours of the <br />bottom of the alluvium and groundwater level surfaces. Additionally, the contours from <br />adjacent historic reports were used to guide the geometry of the contouring. These areas are <br />identified as SPDSS Task 42 in Figure 2. <br />1.2.3 Merging of Contours <br />The contours obtained from historical reports and the contours created in this TM were merged <br />together into one dataset. Contours from one report that overlapped or were adjacent to the <br />contours of another report were evaluated to determine how well the contours agreed between <br />the reports. Incases where there was not good agreement, the electronic contours were <br />modified to seamlessly merge the historic report contours. If picks were present in the area of <br />interest they were used to guide the merging, and where no picks existed then engineering <br />judgment based on the trend of the contours was used. Contours from more recent reports were <br />given preference over contours from more historic reports. The areas where contours were <br />modified to merge the datasets are shown in Figure 3. <br />1.2.4 Gridding of Bottom of Alluvium and Groundwater Level Contours <br />Gridding is the process by which randomly scattered data are projected onto a uniformly <br />spaced grid using an algorithm of choice. For the South Platte Alluvium Region sufficient data <br />picks were not available to create an accurate grid for the entire study area. However, sufficient <br />contour data from the historical reports and those contours created for this TM in the areas of <br />Crow Creek, Upper Box Elder Creek, Upper Beaver Creek, and the South Platte River just below <br />Chatfield Reservoir were available to accurately define the region. <br />Grids were created by first converting the contours into regularly spaced points along the <br />contour line using GIS. This allowed the development of a horizontal coordinate (X,Y) and <br />elevation value (Z) data set. The XYZ data set was used to grid a comprehensive bottom of the <br />alluvium and groundwater level surface in Surfer® (version 8; Golden Software, Inc.) a software <br />Bidding package. A statistical analysis using kriging techniques was used to develop <br />variograms for each surface. Variograms define the variation in data related to distance between <br />pairs of data points. A function was then fit to each variogram, which was used to interpolate <br />estimated elevation values onto a uniform grid. <br />For the bottom of alluvium, an exponential variogram model was applied with a sill of 17,800 <br />and a range of 1,900. For the groundwater level data, the gridding was conducted with the <br />kriging method using a linear variogram model with a slope of 1.40 and no nugget effect. The <br />sill, range, and nugget are terms that describe the kriging nonlinear curve-fitting parameters <br />and are defined in geostatistical textbooks and in the Surfer software manual. Both sets of data <br />were gridded on a 100-meter spacing, which is sufficient to capture the detail of the narrowly <br />incised paleochannels previous studies have shown to exist in the region. <br />After the grids were developed, contours lines were created by using Surfer® to fit lines of equal <br />value to the grid cells values. These contour lines were imported into ArcGIS and compared to <br />SPDSS Phase 3 Task 42.3 TM -Final <br />11/30/2006 <br />
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