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in which a ~ Outflow at subscripted time C = Routing coefficient I = Inflow at subscripted time (\.') o The surface runoff is routed to the c.c upper reaches of a microchannel. The micro- 0> channel flow is routed to the top of a reach in the next order channel. In the Price River Basin, lateral channels on the vslley floor sre normslly ephemeral. In the model, these channels are assumed to have infil- tration characteristics similar to those of the overland soil surface. For rerennial streams, such 8S Coal Creek, channe seepage and groundwater inflow rates are estimated from field observations. The channel routing subroutine described above is listed in Appendix E, Table E.2. Salinity Component (SALIN) The hydrographs of precipitation excess produced by HYDRGY, as routed and combined 'downstream, are used 8S inputs to the salt loading functions in SALIN. Overland flow salt loading The overland flow salt loading function was taken from Ponce (1975) to be of the form: TDS = BO + BIXl + B2x2 . . . . . . (5.6) in which TDS ~ Concentration of total dissolved solids in mg/l Xl = Precipitation intensity in depth per unit time x2 = Rate of precipitation excess in depth per unit time Ponce's calibtations for various Mancos Shale members are shown in Table 5.2. His low mean r2 value of 0.46 suggests that additional independent variables should also be explored. His large values for BO .~ compared to Bl snd B2 suggest the same need. In attempting to add one more variable, Ponce was not able to detect any effects on salinity concentration of distance traveled by overland flow. Channel salt loading The accumulated salt load from surface channels was estimated by Equation 4.5, using the average salt loading rate of 2.51 gms/minO.5 for all locations and stream orders. The salinity uptake with respect to time was estimated by forward finite differencing. In order to estimate the channel length parameter required by Equation 4.5, Horton's Law of Streams (Chow 1964) was applied to the area being modeled. Coal Creek was identified as a fourth order stream (Strahler 1957), and its tributary channels were ordered 8S on Figure 5,8. Data on drainage areas and channel lengths were obtained from topo- graphic maps and aerisl photographs and plotted for the Coal Creek drainage. These lines were extrapolated to stream order 1. From Figure 5.8, estimates were made of the length of channels of a given order. Mean channel cross-sections with respect to order were estimated from field observs- tions, and the mean wetted perimeters were estimated by Dixon (1977). 1 WP = a Qb in which . . . . . . . . (5.7) WP Q a,b = Mean wetted perimeter Mean flow Constants Flows in the tributaries (orders 3, 2, and 1) are routed to Coal Creek by assuming that the source areas ate uniformly distri- buted throughout the tributary area during the previous time step. The time-dependent salt release is initiated at the beginning of overland flow and cantin'led until re- Table 5.2, Coefficients of overland flow load function for the various members of the Mancos Shale (Ponce 1975). BO B1 B B2 m:h Mancos Member (mg/l)' ppm mgh ppm (hr/in) (hr/mm) (hr/in) (hr/mm) Masuk 30,70 0 0 - 0.01 -0.0003 Upper Blue Gate 274.64 11. 77 0.4633 - 3.66 -0.1441 Middle Blue Gate 52.44 0.92 0.0364 - 1.09 -0.0429 Lower Blue Gate 324.18 cO.36 -0.0143 0.22 0.0087 Tununk 119.14 -0.09 -0.0035 - 0.08 -0.0031 Mancos Undivided 366,68 60.97 2,4004 -72,76 -2.8644 . mg/l 1s equal to.ppm at TOS values below 7000 ppm. 54