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MEMORANDUM <br />December 13, 2013 <br />Issues with Selecting an Appropriate Crop Growth Stage Coefficient for the SCS Mod. Blaney-Criddle Eqn. <br />Page 6 of 34 <br /> <br />temperature was calculated from reported monthly consumptive use factors, f, in The Supplement and <br />percent daylight hours, p, from Table 1 in SCS TR-21. <br /> <br /> <br /> <br /> <br /> <br />Otherwise, mean monthly temperature data were taken from Woodward or, as a last resort, obtained from <br />TheWeatherChannel.com website23 and are shown in Table 2 (Appendix A). Mean monthly temperatures <br />calculated from TheWeatherChannel.com include data since 1967 and may yield different values from <br />ARS 1275 since 40 years of additional data are included. However, any discrepancy when calculating kc <br />is expected to be small, less than five degrees. For instance, if the mean monthly temperature varied by <br />five degrees, the resulting kc will be within ten percent24. <br /> <br />Corresponding values of kt for mean monthly temperatures between 36 and 100 degrees Farenheight, <br />were calculated for the locations and from the data in Table 2 and are shown in Table 3. Specific kt <br />values for various mean air temperatures are also shown in Table 4 of TR-21. To obtain a monthly kc <br />value, the k coefficients in ARS 1275 can be divided by the representative kt value. <br /> <br /> <br /> <br /> <br /> <br />Elevation Correction. In addition to a temperature correction, an adjustment for elevation based upon <br />Pochop was considered for some data. The Pochop correction for Kentucky Bluegrass was used when <br />evaluating pasture grasses and lawn grass. The Pochop correction for alfalfa applies only to alfalfa. Any <br />other adjustment of 10 percent per 1000m in elevation is not based upon Pochop. <br /> <br />Estimated elevations for the locations specified in ARS 1275 were obtained primarily from Wikipedia.com <br />and are presented in Table 4 (Appendix A). Some bias may exist due to the reported average elevation <br />of a particular location rather than using the exact elevation of the site where the ARS 1275 data were <br />developed. Any bias is expected to be very small since 100 feet in elevation will only affect the kc curve <br />by approximately 0.3 percent. <br /> <br />Growing Season Adjustment. In order to compare the SCS TR-21 coefficients to ARS 1275, the <br />growing season for SCS TR-21 coefficients were adjusted when necessary. The length of growing <br />season was taken from SCS TR-21 with the start dates adjusted to match the data provided by ARS <br />1275. <br /> <br />Observations, Data, Findings, & Results: <br />Pasture Grasses. As previously mentioned, interpretation of what “pasture grasses” represents is <br />subjective and not clearly stated in TR-21. The SCS TR-21, Woodward, nor The Supplement specifies <br />what the curve for “Pasture Grasses” really represents. However, Woodward does indicate that the curve <br />for alfalfa includes alfalfa-grass and legume-grass. As such, the data for clovers and alfalfa-grass were <br />not considered in the analysis of “pasture grasses.” <br /> <br />The SCS TR-21 coefficients for pasture grasses were compared to “Bromegrass” and “Irrigated Pastures” <br />as provided in ARS 1275.25 Lawn grass data from Pasadena, CA were not included since common turf <br />grasses can be distinguished from native grasses. Four data points from ARS 1275 were plotted, along <br /> <br />23 Due to the government shutdown at the time of this analysis, data from NOAA were not available. However, the source of data <br />presented by The Weather Channel.com is the National Climate Data Center, a part of NOAA <br />24 When evaluating at temperatures of 55 and 60 degrees <br />25 Data for clover from Prosser, WA in June was believed to be reported in error and the average value from the prior and post <br />month replaced the reported value.