EVAPORATION (WHEN SNOWPACK NOT PRESENT)
impenetrable to water such as
paved areas, ditches, and stream
RAIN SNOW SUBLIMATION
channels. Each of the two pervious
zones (zone 2 and zone 3) is a dis-
tinct soil group composed of a top
layer and a bottom layer. Subsur-
face storage is located beneath the
INFILTRATION (F)
bottom soil layer. Groundwater
SNOWPACK
storage is located beneath subsur-
RUNOFF (SURF)
face storage.
The control volume in Figure 1
SOIL SURFACE
SEEPAGE (E)
TOP SOIL LAYER (H1)
further shows how watersheds are
characterized in Object-GAWSER.
The control volume illustrates all
BOTTOM
the physical processes simulated
PERCOLATION (P)
by Object-GAWSER except water-
SOIL LAYER (H2)
shed discharge and channel flow.
SUBSURFACE
The sum of runoff, subsurface flow
SUBSURFACE FLOW
STORAGE
and baseflow is the discharge from
GROUNDWATER
BASEFLOW
the watershed outlet. Channel flow
STORAGE
in the watershed is considered
within the runoff routing equations Figure 1. The control volume. This figure shows the hydrologic
in Object-GAWSER.
processes (except discharge and channel flow) that occur in a
The inputs to Object-GAWSER watershed as characterized by Object-GAWSER. In Object-
are rain and snow and the outputs GAWSER, discharge from the base of the watershed is the sum of
runoff, subsurface flow, and baseflow. Channel flow within the
are evaporation (when the ground
watershed boundary is implicit within runoff routing equations.
is bare), sublimation (when the
ground is snow covered), runoff, subsurface flow, and baseflow. The soil surface is the interface
between the bottom of the snowpack and the top of the top layer of soil.
Within the control volume, rain and melted snow can follow one of two paths. Rain and melted
snow can either infiltrate from the top of the snowpack down into the top layer of soil or infiltrate
from the top of the snowpack to the soil surface and then move laterally over the soil surface as
runoff. The path taken by rain and melted snow is determined by the amount of depression storage at
the soil surface and the infiltrability of the soil. For example, once rain or melted snow infiltrates
from the top of the snowpack to the soil surface, it accumulates on the soil surface in depression
storage and then infiltrates into the top soil layer. When depression storage is filled, additional rain or
melted snow moves laterally over the soil surface as runoff. From the top layer of soil, all water seeps
down into the bottom soil layer. Finally, water percolates from the bottom soil layer into subsurface
storage. From subsurface storage, water percolates to groundwater storage. Water leaves subsurface
storage as subsurface flow and groundwater storage as baseflow. Subsurface flow is the lateral move-
ment of water near the soil surface, and baseflow is the lateral movement of water far beneath the soil
surface. Subsurface flow is quicker than baseflow and slower than runoff.
Preliminary description of STELLA II objects
An understanding of STELLA II objects is necessary to use Object-GAWSER. The objects are
shown in Figure 2. The following is from McKim et al. (1993):
Rectangles represent levels (integral equations). Levels accumulate or deplete depending on the X-
values that are connected to them (i.e. they are assigned an initial condition, and then allowed to integrate
the differential equations symbolized by the rates). The rectangles are referred to as the "state variables"
for the system since they have the capacity to change states through time and space. The term "steady
state" is used to describe a state variable invariant in time (and/or space).
2
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