LYSIMETER WATER BALANCE AND CALCULATED KCR VALUES
Lysimeter water balance and calculated crop coefficients were determined for each year at each site for the period of calculation as listed in Tables 4 through 8. An estimate of actual (measured) ETa was made from the lysimeter water balance. For the 2007 annual report seasonal turfgrass ET crop coefficients (alfalfa reference, ETr) were estimated using daily and hourly calculation time steps for ETr from the ASCE standardized Penman Monteith (ASCEst PM) and the 1982 Kimberly Penman Combination (1982 Kim Pen) Equations. The following discussion and figures were adapted from that annual report. Comparisons of cumulative calculated ETr and the season progression of accumulated lysimeter water use and corresponding Kc values throughout the seasons are illustrated in Figures 7, 8, and 9, respectively, for Logan, Murray, and Southgate. The difference in ET between the two old lysimeters (East and West) and the new lysimeter at Logan is particularly evident in Figure 7a. There was more difference between the estimated ET for the two lysimeters at Southgate (Figure 9) than at Murray (Figure 8). The sum of calculated hourly ETr was lower than the daily time step ETr sum, except at Southgate (Figure 9a). The most dramatic difference is at Logan (Figure 7a) where the hourly ASCEst PM ETr was 24% less than the daily calculated value. This is attributed to the high nighttime canyon winds at Logan. Generally, the ASCEst PM ETr values were higher than the ETr of the 1982 Kim Pen. Early in the study, attempts were made to calculate weekly crop coefficient values with the assumption that the Kc value may be a bit lower in the early spring than during the summer. The results were extremely erratic with calculated Kc varying from over 2 to negative values in successive weeks. This effect is somewhat illustrated in Figure 9b where the cumulative lysimeter crop coefficient fluctuates from 0.85 down to about 0.6 and then increases again in successive calculation periods in the early season. This may be the result of a timing mismatch between weekly measurements and the occurrence of rain, irrigation and subsequent drainage. It is also an artifact of the calculation procedure which does not account for soil water depletion and subsequent refill, because soil water content was not measured. Similarly, the addition of extra water to the Murray lysimeter caused a “jump” in the calculated ET and in the crop coefficient (Figure 8).
23