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

Home Browse Search

al, 1991 i). In addition, differences in soil water profiles and drainage properties of lysimeters vs. their surroundings can have impact on ET measurements. One should be aware that soil- induced moisture stress can reduce ET from lysimeters with no visible indication of the stress when at mild stress levels. This possibility is alluded to somewhat in the top paragraph of page 4. Quality control and correction of weather data may not have been done very well in the earlier studies and should probably be implemented. 2) The recommended high altitude crop coefficients were developed by Walter for the South Park area. As described on page 4 and 5 of the Task 59.1 Memorandum, the Walter study appears to have made a reasonable effort to identify and exclude lysimeter data that was not reflective of irrigated meadows, and adjust data for consistency. The Walter crop coefficient curves represent an average of lysimeter results and associated crop coefficients from several different sites representing soil types and climatic conditions throughout the South Park area. The Walter study was one of the most comprehensive and complete lysimeter studies reviewed and we recommended the South Park -Walter coefficients be used for SPDSS in part because of the level of documentation provided with the Walter Study. It would be useful if the report, for example, more clearly explained the physical or physiological cause for the reduction in Kc in August and September. Is this solely due to cutting effects or are there other factors, including shifts in the Blaney-Griddle basis during these months or impacts of low nighttime temperature? If the reductions are due solely to cutting effects, then one might expect to see some rebound in the curves in mid to late August. It is somewhat reassuring that both the Walter data and other data follow similar trends. However, it would be useful to fully explain the reasons for these trends. 3) Walter found that in South Park, meadows are typically mowed in mid-August and harvesting leads to a decrease in evapotranspiration of about 47 percent when compared to the pre-harvest period. Walter investigated harvesting effects by comparing sites that were harvested to sites that were not, and adjusted the crop coefficients for sites that were not harvested to reflect harvesting effects. Therefore, the recommended South Park -Walter coefficients include harvesting effects (as do the standard TR-21 crop coefficients). A detailed description of these investigations and adjustments is included in the Walter study and briefly referenced in the last paragraph of page 4 of the Task 59.1 memorandum. I note that ASCE Manual 70 (pages 207-209), compared some of the South Park data from Kruse and Haise (1974) with various reference methods including TR-21 (converted to a `reference' (see p. 181-184)). The lysimeter ET for South Park did tend to substantially exceed the TR-21 based reference values during May and June (figure 7.49), with less exceedence during July and August. Thus, this does support (empirically, but not explanatorily) the higher Kc values reported for May and June. It is worthy to note that the combination methods (figures 7.46 and 7.47 of Manua170) did tend to follow the lysimeter measurements more closely over all four months reported. Thus, the use of a combination based method such as the PM may tend to produce more consistent and physically explainable values for Kc. 1 Allen, R.G., W.O. Pruitt, and M.E. Jensen. 1991. Environmental Requirements of Lysimeters. In: R.G. Allen et al. (ed) Lysimeters for Evapotranspiration and Environmental Measurements, ASCE, NY, NY, ISBN 0-87262- 813-2, pp. 170-181. available at: http://www.kimberly.uidaho.edu/water/papers/index.html Page B2 of B9