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<br />. <br /> <br />. <br /> <br />C-l <br />.l <br />I.D <br />,-1 <br />'. ~) <br />c::;, <br /> <br />An attempt was made to analyze furrow irrigation data presented by ARS in <br />their October 1976 report "Irrigation Return Flow Water Quality as effected by <br />Irrigation Water Management in the Grand Valley of Colorado", to provide <br />information as to possible rates of deep percolation. Specifically, data from <br />Tables 10 and 11 were used. <br /> <br />On page 5 of Table 10 there is a footnote which states, *Computed from <br />"Depth Applied" assuming 30 in. row spacing. This in reference to two stream <br />sizes which were identified by astericks. This footnote was assumed to <br />indicate that all row spacing would be 30 inches. Further analyses indicated <br />that row spacings were not always 30 inches. Actually" the first asterick <br />indicated stream size must have been calculated using 24 inch row spacing. <br />From the data provided: gross application depth, furrow stream size, appli- <br />cation time and run lengths; row spacings were found to vary from 20.6 inches <br />to 60 inches. <br /> <br />Table 11 presents H draulic characteristics of furrow flow at selected <br />sites, 1975 average. While the itle indicates 1975 average, the furrow <br />flows must be from individual irrigations rather than the average of all <br />irrigations in 1975. The flows identified with sites STS, FKW and LS are much <br />too large to correspond to any 1975 irrigation data as presented in Table 10. <br /> <br />A tabulation of assumed hydraulic characteristics was developed <br />attempting to be compatible with data presented in Table 11. <br /> <br />The application period was segmented into an advance period and a runoff <br />period. Based on furrow flow, the advance period application and the runoff <br />period application are computed. Based on the measured runoff, the average <br />outflow for the runoff period is developed. With two discharges, the average <br />inflow and the average outflow, two wetted perimeters were developed. One-half <br />the wetted perimeter at the inflow discharge is considered applicable during <br />the advance period. The average of the two wetted perimeters is assumed to be <br />the average wetted perimeter for the duration of the runoff period. Furrow <br />storage is calculated as the average of a full furrow at the upper or inflow <br />point and an essentially empty furrow at the lower or outflow point at the end <br />of the advance period. The water which enters the soil profile during the <br />advance period is the advance application less the furrow storage. The water <br />which enters the soil profile during the runoff period is the runoff appli- <br />cation plus the furrow storage minus the measured runoff. These values are <br />presented as soil moisture replenishment is inches per hour. Furrow seepage <br />rate is then calculated based on the inch per hour rate, the row spacing and <br />the average wetted perimeter of the furrow flow and is presented in units of <br />ft3/ft2d or ft/d. <br /> <br />Every Irrigation identified 1n Table 10 which had a given stream size was <br />analyzed in the above manner. <br /> <br />Table 2 of the ARS study lists the Approximate acreage and proportionate <br />,extent of major soil series in the Grand Valley. By adjusting these acreages <br />to represent 100 percent of the valley acreage which the Bureau assumes to be <br />123,100 acres and by assUming irrigated land at the same ratio as total area, <br />the following distribution by soil series was established. <br /> <br />1 <br />