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CRDSS <br />TASK MEMORANDUM 1.14-17 <br />Consumptive Use Model <br /> Soil Characteristics for the Consumptive Use Model <br />1.0 ISSUE <br />This document describes the methodology used to convert spatial soil texture data to an available water <br />holding capacity and soil surface texture. These data are used for estimating soil moisture characteristics <br />for irrigated agricultural lands. <br />2.0 DISCUSSION/ANALYSIS <br />As part of the consumptive use modeling for this phase of the CRDSS project, two major soil <br />characteristics were estimated for local areas within the Gunnison River basin. First, the available water <br />capacity (AWC) of modeled areas was evaluated. In consumptive use modeling, AWC is used as an input <br />to determine irrigation water requirements. In addition, soil textures (for example ?sandy loam?) were <br />identified for the surface layer in the modeled areas. In consumptive use modeling, this characteristic is <br />primarily used in evaluating evaporative losses from soil surfaces. Estimates of both variables were <br />determined from soil maps and associated information available from the Soil Conservation Service <br />(SCS). Delineations on general soil maps were used as the basic level of resolution for both the AWC <br />and surface texture estimates. <br />AWC <br />AWC may be generally defined as the amount of water a soil type can retain for growing plants. In more <br />technical terms, AWC may be defined as the amount of water held by a soil between field capacity <br />(moisture tension of one-third atmosphere) and the wilting point (moisture tension of 15 atmospheres). <br />AWC is generally expressed on a length-per-length basis, such as inches of water per inch of soil depth. <br />The SCS identifies AWC for soil types mapped in the field as part of the National Cooperative Soil <br />Survey. These data and additional information about soil characteristics, location, and extent, are <br />compiled and stored by the SCS in the computerized State Soil Geographic Database (STATSGO). <br />STATSGO information is available for many of the areas being included for estimating consumptive use <br />as part of the CRDSS project. <br />To estimate AWC for areas within the Gunnison River basin, a data aggregation scheme was developed <br />using the SCS Colorado STATSGO database and ARC/INFO data manipulation routines. ARC/INFO is <br />a computerized geographic information system (GIS) used for the CRDSS project. Data were accessed <br />and the aggregation methods were applied using a computer workstation. The following paragraphs <br />describe the steps and data types employed. Soil characteristics stored as data in the STATSGO system <br />are shown in a different font than the rest of the text. <br />A soil map consists of delineations (or map polygons) of soil types as they occur on the landscape. <br />Associated with each delineation is a symbol or mapping unit identifier that correlates soil descriptions <br />to the area drawn on the map. The ARC/INFO geographic data coverage <br />/crdss_arc/scs/soil <br />was used to identify the location of the soil mapping units and their alphanumeric identifiers (the item <br />1 <br />A275 01.09.95 1.14-17 CSU IDS <br />