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<br />Mean Mean Averaie
<br />annual annual frost
<br />prec:1pi. tempera. free
<br />tatlon, ture, period,
<br />em .c days
<br />25.4 4,0 113
<br />20.5 0.. 133
<br />36.3 8,0 140
<br />31.2 10.1 ,..
<br />32.8 17.2 316
<br />
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<br />GO,s
<br />
<br />TABLE 1. SITE DESCRIPTIONS FOR KENTUCKY BLUEGRASS DATA
<br />
<br />Location wtitude Elevation,
<br />Longitude m
<br />Laramie, 41019'N 10S041'W 2,195
<br />Wy
<br />Whelltland, 42005'N 104057'W 1,433
<br />WY
<br />Fort Cullins, 40035'N 10So OS'W 1,525
<br />CO
<br />North~emn. 39OS4'N 104059'W 1,665
<br />CO
<br />Santa Ana, 330 45'N 117042'W 3.
<br />CA
<br />
<br />y,,,,,
<br />uf dllta
<br />
<br />.
<br />
<br />1976-19;&
<br />
<br />1976-1971
<br />
<br />1977-1971
<br />
<br />1977.19U:
<br />1970.19H I
<br />
<br />~stimate ET using the SCS Blaney Criddle are of special
<br />lOterest.
<br />
<br />APPROACH AND RESULTS
<br />
<br />Kentucky Bluegrass
<br />Borrelli, et a!. (! 981) recently reported measured
<br />c~lDsu.mptive use values for Kentucky bluegrass at five
<br />sltes 10 the western United States. The sites range in
<br />elevation from 3S m to 2195 m above sea level (Table 1).
<br />Measurements in Wyoming and Colorado were taken
<br />using small weighable Iysimeters in which available
<br />moisture depletion was never allowed to exceed 35%.
<br />Measu!eme,nts in California were taken at the University
<br />of Cahforma South Coast Field Station in Santa Ana
<br />(Youngn,er: 1?79~. Measurement~ were from plots having
<br />automatIc IrrIgation when a tenSiometer at either the 15
<br />. or 30 cm depths reached 40 cb. These were defined as
<br />medium tensiometer treatments and probably do not
<br />totally meet the criteria of unlimited water availability.
<br />The consumptive use measurements for Kentucky
<br />bluegrass (Borrelli et aI., 1981) wece used to calibrate the
<br />SCS (1967) version of the Blaney Criddle formula for
<br />monthly estimates of E,,, where
<br />
<br />U :>:: 25.4 ktkcf. . . . . . . . . . . . _ . . . . . . . . . . . . . . (1]
<br />
<br />in which u = the consumptive use (which may be
<br />assumed to be E,p in this study) for the month in
<br />millimeters; kl' = a coefficient related to temperature;
<br />k, = a crop growth stage coefficient for the month; and
<br />f = a consumptive use factor. The SCS coefficient k,
<br />was related to temperature by
<br />
<br />kt '" O.O~ll~.... o.~~o -" . : . . . . . . . . . . . . . . . . . . . . [2]
<br />
<br />in. ,which t = the mean temperature for the month, in
<br />:.degrees centigrade. The consumptive use factor f is
<br />defined as
<br />
<br />(
<br />
<br />(1.8t + 32)
<br />1= 100 p.......",......,......... t3]
<br />
<br />. in which p = the monthly percentage of daytime hours
<br />of the year. Calibration of the formula by Borrelli et aI.
<br />(1981) consisted of calculating k, values for each month
<br />using the averages of the measured consumptive use and
<br />temperature data over ~o or more seasons,.Two factors
<br />indi~ate that the general k, relationship of equation [2],
<br />"as given by the SCS, does not appropriately account for
<br />the temperature effect of an, individual ,crop, such as
<br />Kentucky bluegrass. First, the seasonal variations of the
<br />calibrated crop coefficients formed U-shaped curves.
<br />
<br />126
<br />
<br />This is not consisterit with the shape of the curves for th,
<br />SCS ~oefficients, which are bell. shaped. Secondly,
<br />followlOg the release of the publication by Borrelli et ai, i
<br />(1981) which presented the calibrated k, values,
<br />numerous inquiries were received requesting information!
<br />on coefficients at sites not included in the original dal> i
<br />set. In response to these inquiries, linear regressions were I
<br />performed for the monthly crop coefficients verSIl1 i
<br />temperatures. As a first approximation, regressions w,,, f
<br />performed for each month. The results indicated I t
<br />strong relation between temperature and tliek. values. i
<br />Linear regressions were then performed by grouping the :
<br />data for the four cooler months - April, May, Sept. and i
<br />Oct. - and for the three wl!rmer months - June, lul) ,
<br />and Aug. Again, .a goqd relation bel",een temperature '
<br />and the k. values was found. Part of thids probablr !
<br />unexplained tel1]perature effect and part an elevation :
<br />effect. ,. ~. - l
<br />Jensen (1966) has suggested that the use of monthlr i
<br />crop growth stage coeffiCients in the Blaney Criddl, t
<br />formula is an indirect adjustmen~.for variatioQ in averagf J
<br />solar radialion during the season. This suggestion;
<br />apparently was based on the assumption that lawn gr3Sl !
<br />does not have a dormant period during the growing;
<br />season and thus should not exhibit stage of grow1h i
<br />characteristics for individual months. Borrelli et.al. Ii
<br />(1981) have clearly shown that, at high temperatu..... ,
<br />cool season grasses such as Kentucky. bluegrass will i
<br />exhibit heat stress that reduces their evapotranspiration :
<br />rates below what would be expected with the warm i
<br />temperatures. Thus the, k. values derived herein.!'
<br />although called c~op .gro~ih stage coeffiCients, probably I
<br />reflect the cO,mblned., #fects of crop growlll
<br />characteristics ana 'seasoniHiadiation variation(" . I
<br />The SCS tel1]perature',coeflicient k,was"recompuleC!
<br />based on the original ~entucky bluegrass conslimp!i" :
<br />use data set (Borrelli eCal. 1981), giving.. .
<br />." . " t
<br />kt:: O.~~~9t + 0.71)1) . . . . . . . . . . . . :. . . . . . . . . . 14) :
<br />
<br />TABLE 2. CALlBRATED CROP GROWTH
<br />STAGE COEFFicIEN-TS FOR KENTUCKY
<br />BLUEGRASS AVERAGED FOR'-
<br />THE ~ SITES OF TrA:BLE I.
<br />
<br />Munth
<br />
<br />Cu~ffic[~nt
<br />
<br />April
<br />M.y
<br />June
<br />July
<br />AUjWit
<br />Septl:m ber
<br />October
<br />
<br />. 0:97'
<br />}I.OO
<br />1.10
<br />1.06
<br />. .().98
<br />"-0.97
<br />0.80
<br />
<br />-
<br />
<br />1964-TRANSACTtONS of Ihe ASAE
<br />
<br />~:~~-~~~~ __~:~~~ ___~ -.:~:~_- ~, _.h . e.' ..~..- - < . . _ ___.n_ ~_.." ':;:Ktl"'L.~ _,
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