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round dots along the streams in Figure 1 represent stream gages. The following criteria were <br />used by HRS in defining URF zones: <br />• Where practical, stream gage sites were used to define edges of URF zones. <br />• Since stream effects decrease with increasing distance from the stream, zones near <br />streams are thinner than those farther from the stream. <br />• The river zones in layer 1 extend 0.25 miles on each side of the streams. <br />• The zones immediately beyond the river zones were set approximately 1.25 miles <br />wide, the next zones were approximately 3 miles wide. Zones beyond this were <br />approximately 6 miles wide. <br />• No zones were defined immediately adjacent to the south boundary of the model. <br />This area is likely to be strongly affected by the general head boundary at the state <br />line and URFs defined in this area would not be reliable. <br />• Near streams along the western edge of the modeled area, the location of the blue <br />clay was considered in drawing URF zones. <br />Groundwater Model Simulations <br />Transient simulations were performed by HRS with the groundwater model to produce monthly <br />response functions for stresses over a 20-year period. The transient simulations were based on <br />the 1991 SLV model (with modifications to represent additional surface water streams), <br />calibrated to steady state conditions. This calibrated model includes the estimated stresses on the <br />ground water system such as precipitation recharge, ET, pumping, springs, flowing wells, inflows <br />and outflows across boundaries, and surface water interactions. The URFs were defined with all <br />of these stresses active. One simulation was run for each zone in each layer although river zones <br />were not defined for near stream areas in layer two. The results of the simulation were used to <br />define URFs for each zone in the layer of interest. <br />Incorporation of URFs into SurFace Water Model <br />In order to assign return flow locations and unit response functions to individual ditches using a <br />data centered approach a new CDSS StateMod utility, makertn, was developed that automates the <br />task of creating the RGDSS StateMod return flow file, rg.rtn. makertn combines the locations of <br />irrigated parcels, the URF zone, river reach locations, the StateMod River Network file, rg.rin, <br />and a predefined set of return flow nodes within each river reach to create the return flow file, <br />rg. rtn. Following is a brief description of each of the input files used in makertn (Figure 2). For <br />more detailed information, see Attachment 1 (makertn documentation). <br />Irrigated Lands and Diversion Structures <br />A GIS map (coverage) of irrigated parcels within the Rio Grande basin was used to develop a <br />space delimited ASCII database file containing the diversion structure to irrigated parcel <br />relationships, rg. n2p. This file also includes the percentage of each parcel that the diversion <br />structure irrigates. Every diversion structure that is associated with an irrigated parcel is in this <br />file. For more detailed information about this file, see Attachment 1. <br />C:Acdss\Task6-4.doc Return Flow Characteristics February 24, 2000 -Page 2 of 5 <br />