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MEMORANDUM December 13, 2013 Issues with Selecting an Appropriate Crop Growth Stage Coefficient for the SCS Mod. Blaney-Criddle Eqn. Page 3 of 34 Crop Reference April May June July Aug. Sept. Oct. Kentucky Bluegrass Pochop 9.4 9.4 7.6 7.6 7.6 9.4 9.4 Alfalfa Pochop 6.4 6.4 9.1 9.1 9.1 6.4 6.4 All “other” crops none 10 10 10 10 10 10 10 Table 1 – Elevation adjustment expressed as percent per 1,000m in elevation In Pochop elevation corrections are only provided for months during the growing season. Neither Pochop nor the cited research suggests an elevation adjustment outside of the growing season. Based upon my personal communication with Dr. Larry Pochop8 the purpose of the paper was to verify if the trend of 10 percent per 1,000m was true or not. The paper was not intended to be used as an application-type paper or for anything more than for what the data indicated. Dr. Pochop is not sure how one would apply this methodology to crops other than bluegrass and alfalfa or to the SCS TR-21 coefficients since he is not aware of where the coefficients were developed, or at what elevation. Dr. Pochop’s research showed that an elevation adjustment of around 10 percent is reasonable. In order to apply an elevation adjustment to the SCS TR-21 coefficients, the elevation at which the coefficients were developed is needed. Dr. Pochop pointed out that if elevation was known, temperature effects would need to be considered so that any adjustment is purely a function of the calculation method. In Part II of this memorandum, the data source of the SCS TR-21 coefficients was researched and an analysis was performed to identify the elevation that the coefficients represent. Side Note: It is interesting to note that in Pochop, the adjustment for bluegrass is less during the months of June through August than for the rest of the growing season, which suggests a crop curve that does not follow the typical bell shape that peaks during the mid summer. Rather, the crop curve shows two peaks, one in the spring and one in the fall. This is consistent with cool-season grasses commonly found in temperate climates, where high temperatures can cause these types of grasses to go into dormancy. Examples of cool-season grasses include Kentucky Bluegrass, wheatgrasses (Crested Wheat), Orchardgrass, Brome (smooth), Ryegrasses, and Fescues, to name a few. Pochop’s trend for Kentucky Bluegrass may not apply to warm-season grasses, which may be referred to as “drought resistant,” and include Blue Grama, bluestems, and Sideoats Grama, to name a few. Alfalfa can be considered a warm-season forage, which would explain why Pochop’s findings show that the elevation adjustment for alfalfa follows the traditional bell shape curve that peaks during the summer. A comparison of typical crop curves for cool and warm season grasses is shown below in Figure 1. Historically in Division One, many irrigated pastures have cool-season grasses, especially Smooth Brome. These species are well suited for our climate and make good graze and good hay. Common dryland mixes in Colorado may include alfalfa and Crested Wheat, or include various mixtures of warm and cool-season grasses. Cook9 provides recommendations of grass mixes for irrigated, non-irrigated, and sub-irrigated sites that may reflect common species of “pasture grasses” found in Colorado. Knowing what plant species are well adapted to certain soil textures under varied water supplies can be useful to help estimate the consumptive use of historically irrigated parcels in a change of use proceeding. Figure 1 – Example of Crop Curves from Cook (2012) p.3 - fig. 3 8 Personal Communication on September 25, 2013 9 Cook, et al. (2012)