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<br />000277-n <br /> <br />Crops grown in the valley tend to follow market trends and thus will vary accordingly. This <br />would account for the larger variance in evapotransportation in the valley. In the past decade the <br />value of vegetable crops has increased to the point where considerable acreage is planted in <br />vegetable crops each year. In general vegetable crops are planted in the fall and early spring, and <br />are then followed by wheat, cotton, com, melons, or sudan grass. Thus one acre of ground may <br />produce three crops in one year. Vegetable crops due tend to require more water application in <br />relation to their consumptive use than other crops, mostly due to their sensitivity to salt. More <br />water is applied to keep salts leached below the crops root zone. <br /> <br />Question 4: Why has most of the decrease in drainage pumping been attributed to a <br />reduction in the pumping capacity of the wells on the Yuma Mesa and Yuma Valley? <br /> <br />Answer: Many of the drainage wells were originally constructed in the 1970's. Over time the <br />capacity of these wells gradually degrades, mostly due to encrustation of calcium carbonate on <br />the well screen and to some extent in the aquifer immediately surrounding the well. We do have <br />a well replacement program in progress to upgrade those wells whose capacity has deteriorated <br />appreciably., However, in recent years we have been working under budget constraints that limit <br />our ability to replace only about one or two drainage wells a year. However, we plan to <br />upgrade the lower capacity wells that remain in the next few years. <br /> <br />We have tried various techniques to rehabilitate these wells. Most techniques tried in the past <br />have had only limited success, generally of a temporary nature. We are continuing to <br />experiment with different technology to see if we can rehabilitate our wells so that the increase in <br />pumping capacity will last for longer periods than we have been able to achieve in the past. <br /> <br />Question 5: Why is the water only accumulating in that one area (i.e. in the mound area)? <br />To what aquifers, if any, is the mound attached or adjacent? <br /> <br />Answer: The reasons that water accumulates in the mound on Yuma Mesa are as follows: <br />(1) The Mesa is topographically high (about 70-80 feet higher than the adjacent valley areas). <br />(2) Historically, quite large volumes of irrigation water have been applied in this area (Figure 2). <br />Large volumes have been required for crops because the soil is very sandy and does not retain <br />moisture well. <br />(3) The Algodones fault and possibly shallow bedrock lying south and southwest of the mound <br />appear to act as a partial barrier to flow and so may contribute to the maintenance of the mound. <br /> <br />The generally highly permeable river sediments underlying Yuma Mesa and Yuma Valley are all <br />hydraulically connected and part of the same aquifer system. Groundwater can flow generally <br />freely from the Mesa to the Valley, or vice versa. In other words a deep well near the <br />escsapment ofthe mesa can be pumping water deriving from the mesa or the valley. <br /> <br />Question 6: Please explain how the trifurcation structure works. What are the beginning <br />and resulting salinity levels? <br /> <br />Answer: The trifurcation structure will be constructed on the Yuma Mesa Conduit near the <br />