SPDSS_Task81-2_Consumptive Use and Water Budget Technical Peer Review Meeting Follow-Up

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Rick Allen's original comments are shown in black text. LRE's response to comments are shown in blue text. Review Comments on SPDSS Memorandum entitled "Task 59.1 Develop Locally Calibrated Blaney-Griddle Crop Coefficients" Richard G. Allen March 10, 2007 This report summarizes work on an important topic for water rights management in Colorado, namely potential evapotranspiration (consumptive use) of agricultural crops. The report has produced a useful summary and comparison of the more widely-known lysimeter-measured ET for high-altitude grass forage common to Colorado and has provided recommendations on values for crop coefficient (Kc) and overall methods to use to produce ET for both high-elevation and low-elevation regions of NE Colorado. While the report and study follow a generally approved and defensible procedure, overall, in determining proper Kc values to use with the Blaney- Criddle (BC) method and for calibrating Kc values using more modern methods, there are some items in the report that may be of some concern. The report hints that the Kc's in Table 1 represent conditions of full water availability (except for Division VI lysimeters). This should be noted more clearly, as well as the recommendation that the derived Kc values be used to calculate the PCU that represents ET under conditions of full water supply, but that these values then be adjusted year to year, in practice, using asoil-water balance and ET stress coefficients to account for impacts of less-than full supplies that may occur in some drainages. If the former paragraph is not correct (that the Kc values in Table 1, with the exception of Division VI lysimeters, represent PCU under full water supply), then this needs to be more clearly articulated. In addition, if the Kc values in general, including the final curves derived for high altitude grasses, do not represent PCU under full water supply, then future application of these curves may provide questionable results, since the effect of year to year and location to location variation in water supply will not be taken into account for any specific year. 1) Additional language has been added to the last paragraph on the first page and as a footnote to Tables 1 and 5 through 10 of the Task 59.1 memorandum clarifying that the calibrated crop coefficients and lysimeter-derived crop coefficients are used to estimate potential consumptive use under full water supply conditions. The report expresses some familiarity with background and procedures used in the data collection, analysis and interpretation that supports the various reports and summaries that have been assembled for the high altitude ET analysis. However, it is not clear that the writers have scrutinized these procedures to determine any potential causes for biases in the data. For example, impacts of maintaining vegetation height, cover and leaf-area inside the lysimeter that nearly exactly replicates the surrounding field conditions and impacts of lack of similar vegetation immediately outside the lysimeter and its impact on expanded vegetation area, increased solar radiation capture and positive ET measurement bias (see, for example, Allen et Page B 1 of B 9