OCR Text |
Show probably is not great. Hence, the average annual change in storage can be used as the budget estimate for average annual recharge. The average annual change in storage in the water- table aquifer is equal to the product of the annual change in saturated thickness, the specific yield of the aquifer material, and the area of the aquifer. Water levels in about 25 wells in all parts of Heber Valley were measured by various agencies, and were reported by the Provo River Commissioner, during the period 1945- 60. The Commissioners' reports distinguish four subareas or divisions of the valley. The four divisions, their approximate areas, and the average annual change of saturated thickness in each division for the period 1945- 60 ( from the Provo River Commissioners' Annual Reports) are tabulated below: Average annual change Area in saturated thickness Division ( acres) ( feet) Above irrigation 3,000 4.97 Midvalley 21,000 25.58 Lower valley 3,200 13.52 River bottom lands 800 7.58 The estimated average specific yield of the upper 30 feet of the aquifer materials is 14 percent; if that estimate and the tabulated figures are used in the equation, the computed average annual change in storage in the unconsolidated deposits in Heber Valley is 86,000 acre- feet. The principal sources of recharge to the valley fill, as stated earlier, are infiltration of irrigation water and subsurface inflow from the consolidated rocks. Neglecting minor sources of recharge, the approximate contribution from each of the principal sources can be calculated from the following data: The total amount of water diverted for irrigation in Heber Valley each year is reported by the Provo River Commissioner; the average for the period 1945- 60 was 87,000 acre- feet per year. The average amount of water required by crops in the valley during the irrigation season ( May- September) can be calculated by the Blaney- Criddle method ( Blaney and Criddle, 1962). Using data published by the Utah State Engineer's office ( Criddle and others, 1962) for hay and mixed pastures in Heber Valley, the crop water requirement is calculated as 43,000 acre- feet per irrigation season. Part of the water required by the crops will be furnished by precipitation during the growing season. Using data from the May- September precipitation map of Utah ( U. S. Weather Bur., 1963), the precipitation on the valley floor during the irrigation season is calculated as 12,000 acre- feet. 28 |