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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 5 of 34 <br /> <br />what elevation. However, after reviewing the data in ARS 1275, it is still not entirely clear what some of <br />the SCS TR-21 curves truly represent. For example, the crop curve for “Pasture Grasses” is ambiguous <br />and can be interpreted to mean entirely different plant species across the Western United States. <br />Pasture grasses in South Texas is commonly irrigated coastal Bermuda grass, or on non-irrigated lands, <br />Buffel grass. In Colorado, cool-season grasses like smooth brome, timothy, or orchard grasses are more <br />common. Neither the SCS TR-21, Woodward, nor The Supplement specifies what the curve for “Pasture <br />Grasses” truly represents. This information is important, especially when calculating historical <br />consumptive use in a change of use case. If the SCS TR-21 curve for “Pasture Grasses” represents <br />something more similar to a coastal Bermuda grass, then the curve may not be appropriate for evaluating <br />Bromegrass in Colorado. Furthermore, it is not clear if the SCS TR-21 curve represents data from a <br />particular location or an average of the entire data set. <br /> <br />Background: <br />A forensic analysis was performed where SCS TR-21 crop growth stage coefficients (herein referred to as <br />“SCS TR-21 coefficients” or “kc”) were compared to ARS 1275 in an attempt to determine what plant <br />species represented by the SCS TR-21 crop curves and the location and elevation of the data that the <br />crop curves represent. <br /> <br />As mentioned in Woodward, there was some qualitative and visual interpretation in the development of <br />the SCS TR-21 curves, which may cause confusion in determining what data set they originated from. As <br />such, the findings and observations from this study rely on some visual interpretation and a simple <br />regression analysis22. The ARS 1275 data were evaluated by plotting them against monthly crop <br />coefficients from the SCS TR-21 curves. In theory, if the data in ARS 1275 were the sole source of the <br />SCS TR-21 coefficients, then a correlation coefficient of close to one should be achieved. Taking into <br />account the fact that the curves were smoothed between data points, it is reasonable to expect that some <br />correlation coefficients may be less than one. The data were analyzed for each crop to explain and <br />identify a data set that achieved a correlation coefficient of 0.90 or higher. In some case, this included the <br />average of multiple data points. In other instances, a single data set was used to achieve the optimal <br />correlation coefficient. <br /> <br />The crop growth stage coefficients are provided in the Appendix of SCS TR-21 and can be interpolated <br />for monthly or daily values (i.e., alfalfa and pasture grass). However, ARS 1275 data used to develop the <br />SCS TR-21 crop growth stage curves were developed from monthly data points. When provided, the <br />monthly data were used directly (i.e., SCS TR-21 Curve No. 2) or calculated as a monthly average when <br />plotted out against percent of growing season (i.e., SCS TR-21 Curve No. 1). <br /> <br />Temperature Correction. During our conversation, Dr. Pochop pointed out that the effects of <br />temperature need to be corrected for a proper elevation adjustment. The coefficients provided in ARS <br />1275 are not independent of meteorological effects and represent monthly empirical consumptive-use <br />crop coefficients (k) for use with the Blaney-Criddle equation. The SCS attempted to modify the original <br />Blaney-Criddle equation to isolate the meteorological effects from kc by adjusting k as a function of a <br />climatic coefficient (kt), that is dependent of mean monthly temperature (t), and an independent growth <br />stage coefficient (kc) that reflects the influence of the crop growth stage on consumptive use rates. <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />If the mean monthly temperature for a particular site is known, then kc can be determined. Mean monthly <br />temperatures used in this study were calculated or obtained from referenced sources. Since the SCS <br />TR-21 curves were developed some time in or prior to 1967, the mean monthly temperature data would <br />reflect some period of record that did not extend past 1967. When available, the mean monthly <br /> <br />22 Results from the regression analysis are shown in Charts 1 through 15 as found in Appendix B, pp.31-35