RGDSS_Task6-4_HistoricMonthly Model_UnitReturnFlows

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RGDSS Memorandum Final TO: Ray Bennett FROM: Jim Brannon, Ed Armbruster, Hydrosphere SUBJECT: RGDSS Surface Water, Task 6.4 -Determine Return Flow Characteristics DATE: February 24, 2000 CC: Randy Seaholm Introduction This memorandum represents delivery associated with Task 6.4, Determine Return Flow Characteristics /Memoranda. The objective of this task is as follows: Develop return flow characteristics for use during Phase 1 a1 c of model development, including surface/groundwater split, unit response functions, and location of returns. The determination of return flow characteristics for a surface water model such as StateMod requires every structure be assigned one or more return flow location using one or more unit response functions. The development of the unit response functions was largely accomplished by the groundwater contractor on the RGDSS project (HRS Water Consultants). This effort is documented in a memorandum entitled Develop Unit Response Functions (URFs) (Nov.l 1, 1999). A summary of this effort is provided below under the section titled URF Background. The assignment of return flow locations and Unit Response Functions to individual diversion structures was performed by Hydrosphere and is presented below in the section titled Incorporation of URFs into Surface Water Model. URF Background A Unit Response Function is a series of values which defines the response of a river to stress over time. URFs can be used to calculate depletion or accretion of a surface water system to pumping or recharge. Once a response function has been determined, it can be applied to a large set of stresses as long as the user understands and agrees with the assumptions used to develop them. The methodology to apply URFs has been in use for many years and can be derived analytically or with a numerical model. For RGDSS, the existing, calibrated numerical model of the San Luis Valley was used by HRS Water Consultants to calculate URFs. These URFs were then incorporated into the RGDSS surface water model by Hydrosphere to approximate the interactions between surface and groundwater. URF Zones HRS divided the groundwater model area into 75 zones for definition of URFs. In addition, 18 river zones were defined as characteristic of the stream response to stresses within about''/4 mile of the surface water features. URF zones are shown in Figure 1. Zones 1 through 75 are to be used for the general aquifer area. Zones 76 through 93 represent river zones in layer 1. The large C:Acdss\Task6-4.doc Return Flow Characteristics February 24, 2000 -Page 1 of 5