WSP08301 - Laserfiche WebLink

Home Browse Search

28 Estimating Economic Impacts of Salinity... N -.J o w level. These data problems were reduced by selectively dropping certain aberrant data sets from the regression analysis. In deciding wbich data to retain and wbicb to drop, primary em- pbasis was given to data from Imperial County, California, because it represents a very large share of all salinity damages (93 percent of all damages at 800 mgiL according to Kleinman and Brown),11 A computer program was created specifi- cally to calculate a series of regression formulas from experimental data on agricultural crop yields against salinity level. The computer pro- gram aDows the user to enter up to 39 data sets, i.e., coordinates, and then determine a series of polynomial regression formulas, ranging through polynomial 1 (linear), polynomial 2 (quadratic), polynomial 3 (cubic), polynomial 4 and polyno- mial 5. In each case, the program calculates the regression curve of best fit, then calculates a Goodness of Fit Criterion to permit the user to compare tbe various regression curves to deter- mine whicb polynomial curve best fits the data sets. The program displays a plot of eacb curve and, witb an on-line printer, provides a printout of the regression formula, tbe Goodness of Fit Criterion, and the plot of tbe regression curve. An example of a regression curve is shown in Figure 6. The plotted curves extended only through the range of TDS levels in the experimental data, i.e., 611-892 mgiL. There is no reason to assume that the curve formulas could be extrapolated at either end of this range, so a judgmental extrapo- lation was used to extend tbe computer-plotted curve through the entire range of 200-1200 mgiL TDS. The curves tended to be flat in the range from 200 mg/L to the approximate thresbold of salinity damage determined by E.V. Maas, then dipped in the range from the damage threshold to 1200 mgiL. This nonlinear nature of the salinity function bad been expected, based on the earlier work of Kleinman and Brown as weD as the work of Gardner, who noted that Klein- man and Brown found average salinity damages per mgiL in the 900-1400 mgiL range to be ORANGES VIELD 3 COUNTIES 18 i'.'-. : " ~ ,.'" .: 145 '"'... : .......- . ~ . ..~ . I ...... . : ........ . ~ 19~ . .............. I . ..,~ . ...... . : . -.......-..- I -----.. . . --- ---- 14 --- . . . . : .. : . : . . 38---------+---------+---.-~--+----- 611.9 691.3 111.6 851.9 TDS ~ OJ I- U '" ...... ..... ~ u -c ~ OJ .~ >- . . . . Figure 6. - Regres.lon curve of yield of oranga./tangerlne. v. TDS level. l1Colorado River Salinity, p. 8.