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<br />. <br /> <br />001H8 <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />~ <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />result of preCipitation during the non-growing season or winter precipitation. Greb) <br />measured over-winter soil water storage efficiencies (WSE) from snowmelt of 55% and - '- <br />38%, respectively, in undisturbed wheat stubble and ungrazed pasture at Akron, Colorado. <br />Wheeler analyzed Greb's data and determined the relationship WSMBU = -0.81 + 0.70 <br />(total precipitation), where WSMBU represents the winter soil moisture buildup or increase. <br />Hill et aI., 1898, used neutron probe soil moisture measurements and estimated the WSE <br />to be 0.67% in the Bear River basin. They then reduced the estimated depletion for a <br />given site or basin by the amount of WSMBU. Walter et al. found that WSE was a function <br />of depth to the groundwater table. Where the groundwater table ranged from 2 to 4 feet, <br />WSE was found to be 55%. At sites where the groundwater table was greater than 4 <br />feet, WSE was found to be 74%. <br /> <br />Using average monthly precipitation for the weather stations used in the June 1991 <br />report, the potential amount of WSMBU was calculated. SCS 1970 reports that a <br />threshold temperature of 45 degrees Fahrenheit can be used to determine the growing <br />season of pasture grass. Using 45 degrees Fahrenheit as a threshold temperature to define <br />the non-growing season, the non-growing season was defined at the June 1991 weather <br />stations; then using 'the relationship between total precipitation and WSE, Wheeler <br />determined for the Akron data at the lower elevations and a WSE of 55 % at the elevation <br />above 6,000 feet, the amount of soil moisture increase was estimated. The maximum <br />amount of soil moisture increase was'limited to 6 inFhes per ~cre. The result of this <br />analysis, which is summarized in Table 7, shows tha potentially 35,000 acre-feet of <br />consumptive use reported by the USBR may have been met by the winter precipitation. <br />\Y~ <br /> <br />ImDrove Procedure for Estimation of Effective PreciDitation. The USBR estimated <br />effective precipitation using the methodology described in SCS's TR-21. A required input <br />to the method is an estimate of the net depth of irrigation application. Net depth of <br />application is reflect of the soil moisture depletion (SMD) at the time of irrigation. The <br />USBR assumed a constant value of 3 inches. The TR-21 procedure is based upon a 3-inch <br />net depth of application. If the net depth is not equal to 3 inches, a factor is applied to <br />either increase or decrease the amount of effective precipitation. (For a net depth of 2 and <br /> <br />12 <br />