Surface Water Input

addition of a small amount of water can saturate this soil and cause
the water table to rise quite rapidly, resulting in subsurface stormflow,
surface saturation and saturation excess overland flow. The moisture
content in the capillary fringe can also be affected by the history of
wetting and drying of the soil, a phenomenon known as hysteresis.
When soil has been draining the moisture content tends to remain
above what it would be if it were filling at the same pressure. The
addition of a small amount of water can switch the soil from draining
to filling mode, enhancing the effect of the capillary fringe on the rise
of the water table and subsurface stormflow response. The capillary
fringe and hysteresis are discussed in more detail in Chapter 4.
The discussion thus far has focused on the main processes involved
in runoff generation on a hillslope. To complete the discussion on
runoff generation processes it is necessary to mention briefly some
other processes and factors involved. Interception of precipitation
by vegetation can play a significant role in reducing runoff, especially
in forested environments. Much intercepted water is eventually
evaporated back to the atmosphere (Figure 1). In some hydrologic
models, interception is sometimes modeled as an initial abstraction that
is subtracted from precipitation inputs before they are used in
infiltration or runoff calculations. In other hydrologic models
detailed representations of the interception, storage of water in the
canopy, throughfall or stem flow are used (e.g. Rutter et al., 1972).
Direct precipitation onto a stream or water body also contributes to
runoff as indicated in Figure 6. This is important in areas where the
water surface is extensive, as with lakes, reservoirs and floodplains
that are flooded, because in these situations runoff generation is not
delayed by the usual hillslope processes.
The freezing state of the soil, in regions where freezing occurs, also
plays a role in runoff generation. Infiltration capacity is reduced due
to frozen ground, depending upon the soil moisture content at the
time of freezing.
Fire results in water repellency by soils which reduces infiltration
capacity. One cause for water repellency is chemicals released during
a fire that are absorbed in the soil, and can make it water repellent for
months to years following a fire. The heat from fire also removes the
thin films of irreducable water adhered to soil particles by capillary
forces, disconnecting potential flow paths. Penetration of water into
macropores following a fire is limited due to this effect. High
temperatures in deserts have the same effect, adding to the tendency
for infiltration capacities to be lower in arid regions making them
more subject to infiltration excess runoff generation processes. This
water repellency due to fire has been implicated in many floods
following severe bush or forest fires.
Many of the runoff generation processes described depend on the
soil moisture status of the soil. This is referred to as the antecedent
conditions. Between storms (surface water input events), processes of
evaporation, transpiration, percolation and drainage serve to set up
the soil moisture antecedent conditions. Runoff generation
mechanisms and processes therefore depend not only on conditions
during storms, but conditions in advance of storms and a complete
understanding or representation of all the land surface hydrologic
processes is required to quantify the generation of runoff.
Recognition of this has led to the development of continuous
simulation models, such as the National Weather Service Sacramento
soil moisture accounting model that keeps continuous track of the
state of different soil moisture components for the modeling of
runoff. Detailed presentation of these models is beyond the scope of
this module, although key ideas are reviewed at the end of this model.
The discussion above has reviewed, in a conceptual way many of the
processes and mechanisms involved in runoff generation. These can
be quite complex, and when efforts are made to perform quantitative
calculations the devil is in the details. Each watershed or hillslope is
different, with different topography, soils and physical properties.
The challenge for hydrologic modelers is to balance practical
simplifications with justifiable model complexity and the knowledge
that many specific physical properties required for detailed hydrologic
modeling are physically unknowable. Our understanding of runoff
generation involves the movement of water through soil pores and
macropores. These flows follow the physical laws governing fluid
flow (Navier Stokes equations) but we can never know in sufficient
detail the flow geometry to make use of fluid flow theory and
ultimately have to resort to simplifications or parameterizations of
the runoff generation processes. In the remainder of this module the
astute reader will note discrepancies between the physical
understanding given above and mathematical descriptions used to
perform practical calculations. The mathematical descriptions,
although frequently complex, incorporate significant simplifications
relative to the field based conceptual understanding of how runoff
processes work. This gap between field based and model based
representations makes the subject of rainfall – runoff processes a
fertile area for research to learn how to better model rainfall runoff
processes.

Figure 13 summarizes the main processes involved in runoff
generation, showing the interaction between infiltration excess,
saturation excess and groundwater flow pathways. Most rainfall
runoff models are organized around a representation similar to
Figure 13 involving partition of surface water input into infiltration
or overland flow, either due to infiltration excess or saturation excess.
Infiltrated water enters the soil regolith where it contributes to
interflow, percolates to deeper groundwater or is evaporated or
transpired back to the atmosphere. The quantity of water in the soil
affects the variable source area involved in the generation of
saturation overland flow. The deeper groundwater contributes to
baseflow and affects interflow through groundwater rise