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The MRCS has developed conveyance efficiency curves that can be used to estimate ditch <br />efficiency as a function of soil type, which is based on the permeability of the soil, and <br />ditch length for ditches less than one mile in length. This information can be readily <br />developed from GIS coverages for the study area. Based on the apparent correlation of <br />ditch loss per mile and permeability in Figure B-4 and the data restrictions that preclude <br />the use of the other methodologies described previously, this method was investigated for <br />use in the South Platte basin. Note this method was previously used for the Rio Grande <br />Decision Support System (RGDSS) analysis. Figure B-5 shows the curves, published by <br />the MRCS in the Farm Irrigation Ratinglndex (FIRI) A method for planning, <br />evaluating, and improving irrigation management, June 1991. <br />Figure B-5 <br />l.oo <br />0.90 <br />o.so <br />U <br />4~`"-i-i <br />W <br />~ 0.70 <br />T <br />N <br />7 <br />~ j 0.60 <br />0.50 <br />0.40 <br />Ditch Length -Feet <br />~~r ~o~ <br />~o~ <br />Sand <br />uy <br />The MRCS STATSGO soil mapping was reviewed to determine the relationship between <br />permeability in the lower soil layer and soil description so the curves shown in Figure B- <br />5could beused to develop soil type-weighted conveyance loss estimates. Table B-3 <br />shows the general relationship for the South Platte basin between soil description and <br />permeability. Other soil types are not present in the irrigated portion of the basin. <br />Table B-3 <br />Relationship between Soil Classification and Permeability <br />Soil Descri tion Permeabilit (inches/hour) <br />Cla /Silt Loam 02 - 1.0 <br />Sand Loam 1.0 - 7.0 <br />Sand 7.0 - 15.0 <br />Gravell -Sand 15.0 - 20.0 <br />Figure B-6 shows the soil permeability and soil descriptions in the SPDSS study area. <br />As shown, most of the South Platte ditches flow through soil described either as sand or <br />sandy loam. <br />Page B7 of B9 <br />0 1000 2000 3000 4000 5000 <br />