WSP07478 - Laserfiche WebLink

Home Browse Search

N o ()l o c .0 o+:; - 0 IL ~ ..- o c 0" ~ 0 ~ C " 0 (flU High Flow Low Flow Antecedent Flow Index Figure 1.6. Hypothetical antecedent flow in- dex (taken fro~ Lane 1975), (1970) modeled average monthly salinity mass flow on a major subbasin of the Upper Colo- rado River. A distributed parameter hydro- log!c. watershed model was coupled with a salInIty uptake model. Flow separation was lit il ized in the hydrologic model, and sepa- rate salt loads were associated with surface flow, groundwater flow, and interflow. Salt concentrations in groundwater and interflow wer.e assumed constant. The surface inflow c oocent 1'8 t ions for ungaged S Qurces were related to water flow rates by utilizing exponential regression equations. To incor- porate flash flows from small watersheds, the average monthly salt concentrations were increased. It ,was assumed initially tha-t salt load increases within the valley bottoms could be attributed entirely to agriculture. However, on the basis of this assumption, the initial simulated salinity concentrations associated with subbasin outflows were low by factors ranging from two to ten. To add to the salt loading, a channel salt uptake mechanism was assumed according to the following hypothesis: ~ .. .Much of the water which enters the alluvium as influent flow in the upstream portion of the basin returns again to the stream channel in the lower reaches, and that within a particular subbasin the rate of interchange between surface water and, groundwater may be influenced by water levels in the stream channels. Hence, during periods of high streamflow some increase in the interchange rate might he expected (Hyatt 1970, p, 34) . The following two empirical equations were used to account for this loading: Kp = n (Qr>m (J .15) in which Kp Qr m n .. Percentage of surface flow inter- changed or recirculated through the stream alluvium or groundwater Monthly surface flow rate in cfs Slope of the line of Kp plotted agaInst Qr on log-log paper Intercept on the Kp-axis of the log-log plor and SNS r . , , . ' (1,16) Kp Qr Cg in which SNS r Rate of salt flow contributed from natural sources within the basin Average water salinity level within the groundwater basin or stream alluvium. This quantity assumed to be constant throughout the simulation period, is esti- mated from either well samples or the average salinity level of the base flows of the streams within the subbas in. C g The water and salt budgets Hyatt derived by applying this model to the Price River Basin are tabulated in Table 1.2. These figures suggest that irrigation is a rela- tively minor salt contributor to the waters of the Price River. The report concluded that "... more research is needed to de- lineate between natural and man induced salt loading before stringent and perhaps unnecessary controls are placed on human activities" (Hyatt 1970, p. 9"1). , Thom~s, et a1. (1971) proposed a hydro- logIc-~al~nlty model that can be applied to bo~h. IrrIgated and nonirrigated areas and utIlIzed tQermodynamic ionic relationships for estimating salt uptake concentrations. The model was successfully applied to the Bear River, Utah, and simulated Ca, Mg, Na, 804, Cl, and HC03. The model however is unwieldy due to its extensiv~ data re~ quirements. ,Hill (1973) applied a hydro1ogic- salInIty model to the LIttle Bear River, Utah. Natural weathering was not considered, and salt uptake was assumed to be limited to agricultural and groundwater sources. Flow separation and average monthly salt loading factors were used. Narasimhan (1975) added a biochemical nitrogen subroutine for agricultural per- colated waters to the Thomas et al. (1971) model. The expanded model was successfully applied to the Twin Falls tract of the Snake River Basin in I daho. However, the amount and complexity of the required data are also a problem in applying this model. 7