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Salton Sea 1995 Hydrographic GPS Survey

Reservoir Area and Capacity

Topography Development

The topography of the Salton Sea was developed from collected underwater data and from USGS 7.5 Minute Quadrangle Maps (USGS quad). The upper contours of Salton Sea were developed by digitizing the elevation -220 feet and elevation -230 feet contour lines from the seventeen USGS quad maps that cover the Salton Sea area. These USGS quad maps were dated 1956; many of them were photorevised in the 1970s. ARC/INFO V7.0.2 Geographic Information System software was used to digitize the USGS quad contours. The digitized contours were transformed to State Plane Coordinates, NAD 1927 (North American Datum of 1927), using the ARC/INFO PROJECT command.

Contours for elevations lower than -230 feet were computed from collected underwater data using the TIN (triangular irregular network) surface modeling package within ARC/INFO. The underwater survey data were collected in the California Zone 6 State Plane Coordinates in NAD 1927. The collected underwater data ranged in elevation from -278.4 feet to -230.9 feet. A TIN is a set of adjacent, non-overlapping triangles computed from irregularly spaced points with x,y coordinates and z values. TIN was designed to deal with continuous data such as elevations.

The TIN software uses a method know as Delaunay's criteria for triangulation. Triangles are formed between all data points including all boundary points. This method preserves all collected survey points. The method requires that a circle drawn through the three nodes of a triangle will contain no other point. This requirement means that sample points are connected to their nearest neighbors to form triangles. Elevation contours are then interpolated along the triangle elements. The TIN method is discussed in great detail in the ARC/INFO V7.0.2 Users Documentation.

The elevation -230-foot contour digitized from USGS quad maps was used to clip the Salton Sea TIN such that interpolation was not allowed outside of the -230-foot contour. The clip also incorporated the vertices of the -220-foot contour as additional points to be considered in the development of the Salton Sea TIN. This clip was performed using the hardclip option of the ARC/INFO CREATETIN command. In creating the TIN, points that fell within a set distance of each other were weeded out to eliminate flat triangular elements. Flat triangles occur where all three points making up a triangle have the same elevation. Elimination of redundant points helped to improve the performance of the contouring process as well as helped to create more continuous contours in the lower elevations of the reservoir.

The linear interpolation option of the ARC/INFO TINCONTOUR command was used to interpolate contours from the Salton Sea TIN. In addition, contours were generalized by weeding out vertices along the contours. This process improved the presentability of the resulting contours by removing small contour line variations. This generalization had no bearing on surface area and volume computations for the Salton Sea. The contour topography at 5-foot intervals is presented on figures 2 and 3.

Development of 1995 Contour Areas

The 1995 contour surface areas for the Salton Sea were computed in 1-foot intervals from elevation -278 to -230 using the Salton Sea TIN discussed above. These calculations were performed using the ARC/INFO VOLUME command. This command computes areas at user specified elevations directly from the TIN and takes into consideration all regions of equal elevation. This method was also used to compute the -220 contour that was digitized from the USGS quad sheets.

1995 Storage Capacity

The storage-elevation relationships based on the measured surface areas were developed using the area-capacity computer program ACAP85 (Reclamation, 1985). Surface areas at 1-foot contour intervals from elevation -278 to -230 and contour elevation -220, computed from the digitized and underwater survey data, were used as the control parameters for computing the Salton Sea capacity. The program can compute an area and capacity at elevation increments of 0.01 to 1.0 foot by linear interpolation between the given contour surface areas. The program begins by testing the initial capacity equation over successive intervals to ensure that the equation fits within an allowable error limit, which was set at 0.000001 for the Salton Sea. This capacity equation is then used over the full range of intervals fitting within this allowable error limit. For the first interval at which the initial allowable error limit is exceeded, a new capacity equation (integrated from the basic area curve over that interval) tests the fit until it also exceeds the error limit. Thus, the capacity curve is defined by a series of curves, each fitting a certain region of data. Final area equations are derived by differentiating the capacity equations, which are of second order polynomial form:

y = a + a2x + a3x2

where:

Results of the 1995 Salton Sea area and capacity computations are listed in tables 1 and 2. Table 2 is a separate set of the 1995 area and capacity table at 0.1-foot elevation increments. A description of the computations and coefficients output from the ACAP85 program is included. The 1995 area-capacity curves are plotted on figure 4. As of February 1995, at elevation 220.0 feet below sea level, the surface area was 262,517 acres with a total capacity of 9,420,566 acre-feet.

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