WSP07478

• File Number: 8272.600.60
• Description: Colorado River Basin Salinity Control Program - Basin Member State Info - Utah
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 3/1/1982
• Title: Salt Uptake in Natural Channels Traversing Mancos Shales in the Price River Basin - Utah
• Water Supply Pro - Doc Type: Report/Study

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this purpose besides those used to develop
the model. As Table 5.1 shows, RAIN produced
standard deviations which are very close to
actual values and average monthly totals a
little but not significantly higher than
recorded values. A listing of the model is
cnntained in Appendix E (Table E.2).

Precipitation excess (HYDRGY)

Surface runoff (overland flow) picks
up salt and transports it to the channel.
The second subroutine was developed to
calculate surface runoff from the storm
hyetographs produced by RAIN. This subroutine
(HYDRGY) was modified from previous work
(Riley et al. 1974) to fit the needs of this
study.

The subroutine subtracts interception
and depression storages from the first part
of the rainfall hyetograph, Then infiltration
begins. The infiltration rate is assumed to
decline exponentially from a field measured
maximum rate when the soil is at the wilting
point to a field measured minimum rate when
the soil is at field capscity. Soil moisture
conditions at the beginning of a storm
dictate the initial point on the infiltration
curve. The precipitation excess is estimated
as the volume of the rainfall hyetograph
minus interception and depression storage and
minus an infiltration volume estimated from
the infiltration curve. Negative values are
taken to indicate no runoff.

The HYDRGY subroutine is initialized
with a beginning soil moisture. HYDRGY deter-
mines the soil moisture recharge during
storms. A subroutine (CONSUM) employs the
Jensen-Haise consumptive use equation (Jensen
1973) to determine soil moisture depletion
between storms. These two subroutines there-
fore maintain a running estimate of the ante-
cedent moisture level for use by HYDRGY in
computing the precipitation excess during
each storm. A listing of the two subroutines
HYDRGY and CONSUM is in Appendix E.

Surface runoff (SRO)

Th is component of the model routes the
precipitation excess genenated by HYDRGY
through the successive surface runoff stages
of overland flow, microchannel flow, and
primary channel flow. Three flow routing
techniques were considered. Two were the
Saint-Venant equations described by Jepfson
(1974) and the kinematic wave equat ons
described by Henderson (1971). However
neither of these techniques was adopted
because of extensive data requirements on
flow and channel characteristics. The
relatively simple Muskinghum routing equation
(Linsley and Franzini 1972) was considered
satisfactory for the small watersheds of this
study. Henderson (1971) noted that the
Mus~inghum technique provides a fair approxi-
matIon for natural floods in rivers whose
slopes exceed 0.002.

Given an estimated inflow volume to the
study area from upstream, a hydrograph was
formed by:

Lt - lbase + AO . [I - cos(a.t)]

. (5. I)

in which

Lt
lbase~
AO
a
T

Channel inflow at time t
Base channel inflow
One-half hydrograph peak inflow
Constant, 2~/T
Tributary basin time to peak

The inflow is then routed down successive
storage reaches by the Muskinghum method.
Lateral inflow, groundwater inflow, seepage,
and diversions are added at the top of a
reach.

The Muskinghum coefficients K and X were
adjusted to provide the best reproduction of
observed hydrographs following a method
described in Chow (1964). Once calibrated
the coefficient X was assumed constant and
the coefficient K was varied with the flow-
rate. Stability of the Muskinghum method is
generally insured when:

2 K X < ~t < K . .

. . (5.2)

in which

K Time routing constant
X Inflow effect routing constant
~t The time step

Failure to select a time step for routing
that meets these conditions may result in
oscillating flow values or other errors
(Linsley and Franzini 1972).

Overland flow and lateral channel storm
event flows are routed to the main channel by
assuming that the flows can be represented as
two linear reservoirs in series (Chow 1964).
Storage is assumed to be directly propor-
tional to outflow.

S = K2 a
in which

. (5.3)

S Storage
a Outflow
K2 Storage coefficient

The first order finite differencing of
Equation 5.3 with respect to time followed by
algebraic manipulation gives:

a2 - al + C (11 - al) + 1/2 C (12 - II)
. . . . . (5.4)

~t
C K2 + 1/2 ~t . . . . . . . . . (5.5)

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