Laserfiche WebLink
SPDSS Task 58 Memo <br />August 5, 2004 <br />Page 17 of 22 <br />Release 21 with some examples of exceptions listed below. In the plains area of the <br />South Platte Basin, consulting engineers generally believe that application of the <br />modified Blaney-Criddle method as outlined in Technical Release 21 provides <br />reasonable results. Comparisons in the South Platte Basin (i.e. at Ft. Morgan) <br />presented at the April 8th seminar by Ivan Walter show reasonable agreement between <br />annual values of PCU calculated with the modified Blaney-Criddle method and PCU <br />values calculated with the Penman-Monteith method. However, Mr. Walter did <br />indicate that the modified Blaney-Criddle method did not give a very good monthly <br />representation of PCU when compared to aPenman-Monteith method. A similar <br />conclusion about the monthly pattern of PCU from an uncalibrated modified Blaney- <br />Criddle was derived in studies for the Rio Grande DSS. <br />a. A consultant in the San Luis Valley frequently applies a Modified Hargreaves <br />method (with wind and radiation inputs) that was developed by his company. <br />b. Often representatives of the Division of Water Resources and some consultants, <br />inside and outside the South Platte Basin, apply an elevation adjustment factor to <br />the results of the modified Blaney-Criddle method per a NRCS suggested <br />practice. <br />c. It was found that consultants would very infrequently apply methods such as <br />Hargreaves or a Penman based method. <br />d. Calculations of PCU for irrigated pasture grasses at higher elevations in the South <br />Park area of the South Platte Basin typically use calibrated crop coefficients based <br />on lysimeter studies. <br />e. A number of the consulting engineers in the South Platte Basin indicate they are <br />considering refining their modified Blaney-Criddle estimates through a form of <br />"local calibration" by comparing to results from a more detailed consumptive use <br />method in recent years. This would be similar to the CDSS application in the Rio <br />Grande Basin or the State's recent application in the Republican River Basin. <br />f. The calculation of lawn grass consumptive use (both at the State and by <br />consultants) is typically made with the modified Blaney-Criddle method but with <br />crop coefficients, a revised temperature coefficient equation and an elevation <br />adjustment suggested by Pochop (1984). <br />g. There has been recent publicity about a simplified form of the Penman-Monteith <br />equation called the ASCE Standardized Penman-Monteith equation. Many of the <br />Division of Water Resources representatives and consultants indicate that they are <br />considering this method for future use. Even the representative of the company <br />that developed the Modified Hargreaves method indicated he thought he would be <br />adopting the ASCE Standardized P-M equation within several years, which <br />method he believes as accurate as his Modified Hargreaves method in the San <br />Luis Valley. This equation has been adopted for calculations on the NCWCD <br />web site as well as adopted for future Compact compliance calculations in the <br />Arkansas River Basin. This equation (described further in the Appendix) is <br />recommended in this memo for use in the calibration of modified Blaney-Criddle <br />crop coefficients for the South Platte DSS study area (similar in concept to the <br />RGDSS calibration of crop coefficients). <br />