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Colorado Water Dec 2003
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Colorado Water Dec 2003
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Publications
Year
2003
Title
Colorado Water
CWCB Section
Administration
Author
Water Center of Colorado State University
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December 2003 Issue
Publications - Doc Type
Newsletter
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River Valley; it gives them the ability to predict whether deple- <br />tions are likely over a 10 -year period; and, if there are, they can <br />take actions to replace those depletions, such as putting more <br />water into storage in an offset account in John Martin Reser- <br />voir. <br />Finally, I want to address a recommendation that I think is of <br />interest to water users in the South <br />Platte River Basin. The Special Master <br />accepted a new method to determine <br />potential evapotranspiration in the <br />model that is being used to determine <br />Compact compliance. In previous <br />versions of the model, the modified <br />Blaney - Criddle method developed <br />by the Soil Conservation Service <br />was used to determine the potential <br />evapotranspiration of crops in the <br />Arkansas River Valley. It is a widely <br />used method, and it has been used to <br />determine historical consumptive use <br />for water rights transfers in Colorado <br />and has been widely accepted as giving <br />reasonable results. <br />However, the scientific and engineer- <br />ing communities are committed to <br />using more physically -based equations <br />to determine potential evapotranspira- <br />tion, using what are called "reference <br />crop ET" methods, which use crop <br />coefficients that are multiplied times <br />a reference crop ET to determine the <br />potential evapotranspiration of crops. <br />Such methods are now widely used for <br />irrigation scheduling. The Special Master accepted the Kansas <br />position that the Penman- Monteith equation is the most accu- <br />rate method to determine reference crop ET. He also accepted <br />the Kansas position that crop coefficients developed by Dr. <br />James Wright of the Agricultural Research Service in Kimber- <br />ly, Idaho, were appropriate to determine the potential evapo- <br />transpiration of crops in the Arkansas River Valley. After <br />hearing the experts testify, I remain somewhat skeptical about <br />the use of the Penman- Monteith equation and, in particular, <br />the crop coefficients that Dr. Wright developed to determine <br />potential evapotranspiration. Those crop coefficients were <br />developed under what I would describe as ideal conditions. <br />Dr. Wright has published a number of papers that describe the <br />research he conducted to determine those crop coefficients. He <br />placed a lysimeter in an alfalfa field near the Agricultural Re- <br />search Station in Kimberly, Idaho, to determine the consump- <br />tive use of alfalfa. However, he noted that the alfalfa yield in <br />the lysimeter was slightly greater than in the surrounding field. <br />The alfalfa in the lysimeter was hand - harvested. The alfalfa <br />in the surrounding field was mechanically harvested, and he <br />thought that the mechanical harvesting equipment and lower <br />producing areas in the field resulted in slightly lower yields <br />than in the lysimeter. <br />He also noted that the alfalfa yields from the lysimeter were <br />40 percent higher than the Twin Falls county average, but <br />noted that the county crop averages included several large ar- <br />eas that were water -short in most years. So, some of that yield <br />In my opinion, there needs to be <br />additional research of the type <br />that Dr. Wright conducted at the <br />Agricultural Research Station <br />in Kimberly, Idaho, to determine <br />whether the Penman - Monteith <br />equation is, in fact, reasonable <br />for use in Colorado and whether <br />the crop coefficients developed by <br />Dr. Wright need to be adjusted <br />for conditions in Colorado, in <br />particular, for salinity ... I believe <br />that additional research is im- <br />portant for water users through- <br />out the state, including the South <br />Platte River Basin, and urge <br />your support for such research. <br />reduction was due to the fact that <br />there was a limited water sup- <br />ply, but he also noted that these <br />were areas where alfalfa was not <br />considered a high priority crop. <br />Those comments concern me in <br />applying those crop coefficients <br />in the Arkansas River or in the <br />South Platte River Basin to deter- <br />mine the potential consumptive <br />use of crops like alfalfa, which is <br />the largest crop in the Arkansas <br />Valley and a large crop in the <br />South Platte Basin. <br />In my opinion, there needs to be <br />additional research of the type <br />that Dr. Wright conducted at the <br />Agricultural Research Station in <br />Kimberly, Idaho, to determine <br />whether the Penman- Monteith <br />equation is, in fact, reasonable <br />for use in Colorado and whether <br />the crop coefficients developed <br />by Dr. Wright need to be adjusted <br />for conditions in Colorado, in <br />particular, for salinity. We have <br />higher salinity levels in the Ar- <br />kansas River Basin than in the Snake River near Twin Falls, <br />Idaho, which can reduce the amount of crop consumptive use. <br />So, I am here to make a pitch to those of you attending the <br />South Platte River Forum today to support research on the use <br />of the Penman- Monteith equation in Colorado. <br />The scientific and engineering communities are clearly mov- <br />ing in the direction of using reference crop ET methods to <br />determine crop consumptive use, and continued use of the <br />modified Blaney - Criddle method will be challenged. A U.S. <br />Supreme Court Special Master has accepted Penman- Monte- <br />ith equation and he has accepted crop coefficients developed <br />by Dr. Wright. The Penman- Monteith equation with the crop <br />coefficients developed by Dr. Wright will calculate higher <br />potential evapotranspiration for some crops, which in many <br />cases will result in higher transferable consumptive use and <br />a reduction in the amount of return flows that applicants are <br />obligated to replace in water rights transfers. For that reason, <br />I believe that additional research is important for water users <br />throughout the state, including the South Platte River Basin, <br />and urge your support for such research. <br />
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