Laserfiche WebLink
3) Estimate Precipitation to ET (PE) and Soil Storage (PS) on Non-Irrigated Lands <br />During the Growing Season (April through October) <br />The surface water runoff was subtracted from the total precipitation (PT_P~. The <br />remaining precipitation was estimated to go to the following uses: (a) satisfy native <br />vegetation ET, (b) soil moisture reservoir storage, and (c) deep percolation: <br />a. The modified Blaney-Griddle method was used with SPDSS calibrated <br />coefficients for grass pasture to estimate the component of precipitation that <br />directly satisfies native vegetation ET (PE) during the growing season. This <br />method is appropriate as an upper limit because with the exception of ground <br />water, where available, precipitation is the only water supply for non-irrigated <br />lands. Sensitivity analyses show that less than half of the potential consumptive <br />use estimated with this approach can be met by precipitation. Additional <br />precipitation would likely lead to a denser crop and still be consumed. <br />b. The potential amount of precipitation available for soil moisture storage was <br />calculated as (PT -PQ-PE). A soil moisture reservoir balance was used to <br />estimate the actual soil moisture storage (PS). <br />c. Equation 1 was used to calculate deep percolation (P~). <br />A root zone depth of 3.3 feet was used to represent non-irrigated lands (corresponding to <br />the root zone depth recommended for pasture grass in ASCE Manual 70). <br />4) Estimate Soil Storage on Irrigated and Non-Irrigated Lands During the Non- <br />Growing Season (November through March) <br />STATSGO data show that the available water capacity (AWC) of the soils throughout the <br />ground water model area varies from approximately 0.1 to 0.13 inches of water per inch <br />of soil, with an average of 0.1 inch per inch. An AWC of 0.1 was used to provide an <br />initial estimate of the soil moisture reservoir capacity and resulting deep percolation. <br />Because precipitation recharge increases as the soil moisture reservoir capacity decreases, <br />this represents an upper limit to deep percolation. <br />a. Based on the Akron study, 38% of the total monthly precipitation during the non- <br />irrigation season was made available to recharge the soil moisture reservoir; the <br />remainder was considered to be losses to runoff, evaporation, or sublimation. <br />b. A soil moisture reservoir balance was used to estimate soil moisture storage (PS), <br />starting with a 50 percent full soil reservoir. If the addition of precipitation <br />caused to soil moisture reservoir to fill, excess recharge amounts were passed <br />through as deep percolation to the underlying aquifer (P~ + PS = .38 x PT). Deep <br />percolation was calculated as the remainder (P~ = 0.38 x PT - PS). <br />A root zone depth of 3.3 feet was used to represent non-irrigated lands and root zone <br />depths of 3.3 feet and 4.9 feet were used to represent irrigated row-crop and irrigated <br />alfalfa (corresponding to root zone depth recommendations in ASCE Manual 70). <br />Task 64.doc 10 of 13 <br />