top and bottom soil layers, respectively. STOR_EVAP, TOP_LYR_EVAP, and BTM_LYR_EVAP
are set to zero. "F" (which decrements STOR) is equal to zero because no water is located in depres-
sion storage (STOR = 0). "E" (which decrements TP_LYR) is greater than zero because there is
water stored in the top layer of soil (TP_LYR is greater than zero). "P" (which decrements
BTM_LYR) is greater than zero, because water is stored in the bottom soil layer (BTM_LYR is
greater than zero). RI_2 is zero because it is the sum of RAINs and LIQ_WTR_REL, which are both
equal to zero. SURF is zero because STOR is zero. RAINs is equal to the initial condition shown on
p. 4-26 of the GAWSER manual. TINF was set to 0.48 (the initial moisture content). Please note that
in the original GAWSER model, TINF is set to 0.00001, but 0.48 yielded more accurate approxima-
tions of F and SURF in Object-GAWSER (Schroeter 1989).
Discussion of equations
The following equations are embedded in the objects shown in Figure 18:
STOR t1 = STOR t 0 + dt(RI_2 ∆t - F∆t - STOR_EVAP∆t - SURF2∆t )
(22)
TP_LYRt1 = TP_LYRt 0 + dt(F∆t - E∆t - TP_LYR_EVAP∆t )
(23)
BTM_LYRt1 = BTM_LYRt 0 + dt(E∆t - P∆t - BTM_LYR_EVAP∆t ) .
(24)
As shown in Figure 18, STOR is incremented by RI_2 and decremented by F, STOR_EVAP, and
SURF2 (eq 22). TP_LYR is incremented by F and decremented by E and TP_LYR_EVAP (eq 23).
BTM_LYR is incremented by E and decremented by P and BTM_LYR_EVAP (eq 24). RAIN,
LIQ_WTR_REL, BARE are part of an equation within RI_2 which calculates the amount by which
RI_2 increments STOR (see eq 25). DSa2 is part of an equation in SURF2 which calculates the
amount by which SURF2 decrements STOR (see eq 26).
Equation 25 calculates the amount of rainfall or snowmelt that will enter depression storage
(STOR). If the subwatershed is completely snowcovered, BARE equals zero and only snowmelt
will enter STOR. If no snow cover is present, BARE equals one and only rainfall will enter STOR.
Finally, if the subwatershed is partially snow covered, BARE will be between zero and one and a
weighted amount of rainfall and snowmelt will enter STOR:
If
BARE = 0
then RI_2 = LIQ_WTR_REL
then RI_2 = [(RAINa BARE) + (1-BARE) LIQ_WTR_REL] (25)
If
BARE > 0
Equation 26 calculates the runoff fin GROFF2. This equation decrements STOR by the difference
between STOR and DSa when STOR is greater than DSa2:
If
STOR>DSa2 then SURF = STOR-DSa2
If
STOR<=DSa2 then SURF = 0
(26)
The available storage in the top and bottom soil layers is calculated with the equations embed-
ded in the objects shown in Figure 19. In this figure, SA_I is incremented by "SA_I_in" and dec-
remented by "SA_I_out" (eq 27). "SA_I_in" contains the value of "E" and "SA_I_out" contains
the value of "F" (eq 29 AND 30). SA_II is incremented by "SA_II_in" and decremented by
SA_II_out (eq 28). "SA_II_in" contains the value of P and "SA_II_out" contains the value of E
(eq 31 and 32.):
SA_It1 = SA_It 0 + dt(SA_I_in ∆t - SA_I_out ∆t )
(27)
SA_IIt1 = SA_IIt 0 + dt(SA_II_in ∆t - SA_II_out ∆t )
(28)
28
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