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'" o CO ~ THE HYDROSALINITY MODEL CHAP TER V Introduction The stated study objectives included developing a hydrosalinity model of salt loading and transport, calibrating the model to Price River tributary conditions, and running the calibrated model to compare salt loadings from various sources quantitatively. This chapter presents the model development. Modeling strategy Numerous watershed hydrologic/salinity (hydrosalinity) models have been developed. They vary in resolution from Durumls (1953) hyperbolic relationship to Narasimhan's (1975) bio-chemical salinity model. The better models have successfully represented perennial streams with time-averaged results. The modeling of ephemeral streams with only short periods of flow has, however, had little success (e.g., Pionke and Nicks 1970). This study builds a first generation mathe- matical model to estimate salinity con- centration in an ephemeral stream traversing Mancos Shale wildlands. The procedure (Figure 5.1) for develop- ment and application of a simulation model, described by Riley et al. (1974), was at- tempted in this study. While data limitations prevented adequate model verification, the Coal Creek model is considered capable of providing a reasonable estimate of the relative salt levels in that stream from 1) overland flows, and 2) channel flows. System identification The model objective was better quantita- tive understanding of the salt loading of the Price River. The relevant system incorporates the processes which bring water and salt into the channel. These can be selected from the representation of the runoff phase of the hydrologi c cycle on Figure 5.2. The boxes represent catchment storages, and the solid lines represent physical processes whereby water moves from one storage to another. Salts are moved by water, and thus most of the soli d lines represent iog water move- ment are associated with salt movement represented by a dashed line. The exceptions are storages and movements in the atmosphere where salt contents are low enough to be neglected for accomplishing the objectives of this model. The conceptual hydrosalinity model developed by sdding the dashed lines to Figure 5.2 is expanded into a mathematical model by equations portraying the physical processes of water and salt movement from box to box and box storage capacities. Because this study focuses on salt pickup by surface runoff processes (overland and channel flows), the total system depicted by Figure 5.2 can be simplified to consider only flows overland and in surface channels. For application to the Coal Creek study unit, further simplifications were possible because salt transport occurs mainly during surface runoff events and little or no surface runoff occurs during the snowmelt period. Furthermore, because the major' salt loading is associated with surface runoff producing events of short duration, it was possible to simplify the system by consider- ing all long-term, time dependent processes to have negligible salt loading effects. The above focus and assumptions were used to simplify the hydrosalinity flow diagram to Figure 5.3. The remainder of this chapter explains the formulation of a hydro- salinity model covering the storages and processes shown in that flow diagram with equations developed from the data on salt pickup processes presented in the previous chapter. As a strategy for beginning, the model was constructed to replicate individual storm events between April I and October 31. Most natural salt movement occurs during isolated periods of storm runoff during the otherwise long dry summer. Cant inuous and winter modeling might enhance model performance in estimating antecedent moisture for predicting storm runoff or percolation through the ground seeping into the stream through its banks, but such refinements can be added once the basic structure of Figure 5.3 is implemented. Hydrolo~y Component Precipitation (RAIN) Summer storm events on the Price River Basin are few, short, and localized. Histori- cal precipitation series have not been measured in the watersheds of primary interest for this study, are generally measured on too coarse a time grid, and are too short to cover the range of storm pat- 45