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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 2 of 34 <br /> <br />NOMENCLATURE & ABBREVIATIONS <br /> <br />ARS Agricultural Research Service <br />ASCE American Society of Civil Engineers <br />CoAgMET Colorado Agricultural Meteorological nETwork <br />DWR Colorado Division of Water Resources <br />ET Evapotranspiration <br />ETrs Evapotranspiration calculated with the ASCE Standardized Reference ET <br />Equation using a tall (alfalfa) reference crop <br />f Monthly consumptive use factor <br />K Seasonal crop consumptive use coefficient <br />k Monthly crop consumptive use coefficient <br />kc Crop growth stage coefficient <br />kt Climatic coefficient <br />KY Kentucky <br />IDSCU Integrated Decision Support Group’s Consumptive Use Model <br />NOAA National Oceanic and Atmospheric Administration <br />NEH National Engineering Handbook <br />NRCS Natural Resources Conservation Service <br />p Percent daylight hours <br />R2 Correlation coefficient <br />SCS Soil Conservation Service <br />SPDSS South Platte Decision Support System <br />t Mean monthly temperature, in Farenheight <br />TM59.1 SPDSS Task Memo 59.1 (Wilson, et al. 2005) <br />TR-21 Technical Release No.21 (USDA-SCS, 1967) <br />USDA U.S. Department of Agriculture <br />USGS U.S. Geological Survey <br />WMU Water Management Unit - Ft. Collins, CO <br /> <br />PART I: TECHNICAL REVIEW OF POCHOP <br />In Colorado, an elevation adjustment of 10 percent per 1,000m has been frequently applied in change of <br />use cases and Pochop is most often referenced as the basis of such an adjustment. Pochop published a <br />study on elevation adjustment based upon claims by researchers who noted that approximately 10 <br />percent per 1,000m above sea level was warranted when using the SCS modified Blaney-Criddle <br />equation. Potential consumptive use with the Blaney-Criddle equation has been described7 as being <br />biased by elevation due to solar radiation effects, and the method of calculation, which relies upon mean <br />monthly temperature. At higher elevations in arid climates, the difference in daily maximum and minimum <br />temperatures can be greater than at lower elevations. An average of the two neglects to consider that <br />most of the potential consumptive use occurred when there was significant solar radiation affects. A data <br />set for Kentucky bluegrass and alfalfa were analyzed and compared to lysimeter data in Pochop where it <br />was concluded that an elevation adjustment “will improve estimates of ET when using the Blaney Criddle <br />method” and that “the adjustment should increase the ET estimates for sites with elevations greater than <br />the location at which the formula is calibrated and decrease estimates for sites with lower elevations.” <br />Pochop developed coefficients for Kentucky bluegrass that represent an average elevation of 1,350m. <br />The appropriate calibration for these coefficients would be 9.4 percent for every 1,000m from 1,350m <br />elevation during April through October except during June through August, when the calibration would be <br />7.6 percent for every 1,000m. Similarly, Pochop developed coefficients for alfalfa, for which a 6.4 percent <br />adjustment for April through October is appropriate, except for June through August, which 9.1 percent is <br />more appropriate for the summer months. The table below summarizes the elevation adjustments <br />mentioned in this study. <br /> <br />7 See Pochop, et al. (1984) and Jensen, et al. (1990) pp.104-107