Laserfiche WebLink
<br />. <br /> <br />content 'of the water, The per acre water use was a weighted aver- <br />age based on the relative amounts of land above and below Palo Verde <br />Dam. Water quality was based on the relative amounts of vegetation <br />supplied with river water and with a combination of agricultural <br />drainage and ground water, Average salt content estimates varied <br />from 1. 41 tons per acre-foot in 1965 to 1. 62 tons per acre-foot in <br />1976, The resulting salt retention by natural vegetation is shown <br />graphically by Plot A on Figure 5, <br />Flushing of Ground Water, Subsurface irrigation drainage sets <br />in motion a pattern of subsurface flow that tends to flush out ground <br />water from beneath irrigated lands, It is assumed that the zone of <br />active ground-water flow in response to irrigation occurs to a depth <br />of about 130 feet below the flood plain in the Parker Valley, because <br />the top of the Bouse formation, an extensive layer of clay, lies at <br />that depth. <br />The ground-water flushing effect of deep percolation from irriga- <br />tion and canal seepage was estimated by comparing the salinity of the <br />leaching fraction plus seepage with the quality of the underlying <br />ground water, Based on apparent relationships in the Palo Verde <br />Irrigation District, it was assumed that the drainage outflow was half <br />ground water, The salt discharge from flushing estimated in this way <br />was minor--about 20,000 tons per year between 1965 and 1975, The <br />quality of the deep percolation was estimated to be slightly better <br />than the quality of the underlying ground water, The effect of <br />ground-water flushing is shown graphically by Plot B on Figure 5. <br />Leaching of New Lands, The use of ground water by natural <br />vegetation and possibly other factors cause an accumulation of soluble <br />salts in the unsaturated soil. When natural areas are cleared and <br />irrigated, most of the accumulated salts are flushed from the soil into <br />the ground water and subsequently into the drainage system. The <br />tonnage of salt involved was estimated as follows: 002250 <br />The amount of salt per acre of new land has been influenced by <br />flooding of the Colorado River, Records of riverflow indicate that <br />much of the central and southern part of the Reservation was flooded <br />regularly prior to Hoover Dam, and again in 1941, High water in the <br />river prior to Hoover Dam was more likely to cause overbank flows <br />17 <br />