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<br />3~,68
<br />
<br />physical soil chamcteristics of the irrigated lands within 110: Jensen/Walter 1997; NRCE
<br />2002; V-iAC 2003; MET: 5cott2003, Grismer 2003; and Rhoades, Dee, 2002 (1998
<br />Review), Appendix A. I, p, 2
<br />
<br />The topography of lID is well suited for surface (gravity) irrigation with the majority of
<br />the area nearly level with land slopes generally ranging from 0,1 to 0,3% (SCS, USDA,
<br />1 'is I ), Soils across the lID ilTigated lands were identified based upon the SCS mapping
<br />of soil types, i,e, fine (clay), medium (silts) and coarse (sand) texture, Each category of
<br />soil has different c1nracteristics affecting the rate and amount of mfiltration (or
<br />penneability) and surface inigation perfonnance, which likewise affects the ilTigation
<br />management practices of applying water to the cropped lands, The fine textured clay
<br />soils (characterized as "poorly drained soils" by SCS) show extensive surface cracking
<br />when dry and swell shut with the application of irrigation water (WAC: Gilley et aI., p,
<br />54), Conversely, coarse textured sandy soil (characterized as "well drained soils" by
<br />SCS) absorbs water at a relatively constant rate,
<br />
<br />Many studies quantify the types of soil found in 110, Bali et aI., Dee, 1999 quantified the
<br />heavy clay soils as representing more than 60 percent of the irrigated land in the Imperial
<br />Valley, NRCE 2002, p, IV-20 through IV-23 and Table IV-8, identified 62,1 percent of
<br />surface soils within lID irrigated boundary as being of low penneability and the
<br />remaining areas as medium penneability (7 percent) and high pemleability (30,9
<br />percent), MET: Scott 2003, p, 6 references a USDA soil survey geographic database of
<br />1995, providing data suggesting that 69.3 percent of the 110 service area is underlain by
<br />silty clay and silty clay loam soils with high ,hrink-swell potential. Further this same
<br />USDA source identified 57,8 percent of the 110 service area as having soils with very
<br />slow infiltration rates,
<br />
<br />Importantly, infiltration characteristics relate to the salinity levels contained in the
<br />background soil profile, which requires some degree of leaching to minimize salt buildup
<br />resulting from irrigation over time, In addition to the natural existence of salts in the
<br />soils, the salinity of the Colorado River water being applied influences the amount of
<br />leaching that may be required as a part of the irrigation practices within liD (NRCE 2002,
<br />p, IV-23; JensenlWaIter 2002, p, 12; and Rhoades 2002), The amount of water required
<br />to provide adequate leaching of salts is discussed in Operating Efficiencies - Factor 12 of
<br />this technical analysis,
<br />
<br />SCS soil data were used by all of the technical experts retained by 110, MWD, CVWD
<br />and Reclamation, The key consideration for land classification is that estimates for the
<br />acreage of fine textured, low penneability soils is 69% of the total irrigated area in 110
<br />(62% low pemleability; 7% medium penneability) with the remaining 31 % of land bell1g
<br />in coarse texture categories, Reclamatron relied upon the land classification or soil type
<br />information collected and compiled by SCS, as the best available data for land
<br />classification for this detennmatioll
<br />
<br />lull' 2, 2003
<br />
<br />17
<br />
<br />Detenninations and Recoll1ll1endatiorE
<br />F'or liD - Calendar Year 2003
<br />
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