Geoscience Reference
In-Depth Information
10
GROUNDWATER OUTFLOW
AND BASE FLOW
A major portion of the precipitation that percolates into the soil profile eventually finds
its way into creeks, rivers, lakes and other open water bodies. After the precipitation or
other input has ceased for some time, the entire streamflow can be assumed to consist
of the cumulative outflow from all upstream phreatic aquifers. The prediction of base
flow is of some practical importance because it is the rate of flow, that a given river
basin can sustain in the absence of precipitation and in the absence of artificial storage
works. Accordingly, this type of flow is variously known as
base flow
,
drought flow
,
low
flow
and
sustained
or
fair-weather runoff
. In engineering such flows have been studied
in connection with problems of water supply and water quality in rivers during drought
periods, and general basin and agricultural drainage.
During, and in response to, precipitation or snowmelt, the different pathways and the
detailed mechanisms, by which water reaches the stream, are more complex than during
drought flow episodes. Still, as will be seen in Chapter 11, it is generally agreed that also
storm runoff from natural basins with vegetation is largely supplied into the streams by
subsurface transport. Thus, subsurface drainage from the aquifers along the banks is one
of the key elements in catchment hydrology, not only under drought conditions but also in
response to precipitation. In this chapter, the subsurface outflow is first considered locally
at the point where it enters the stream, by analysis of the groundwater flow process in
the riparian unconfined aquifer; the first five sections describe the different formulations
that are available for this purpose. In the last section of the chapter, the phenomenon is
treated at the basin scale, by integration of these local groundwater outflows along the
streams and channels in the basin.
10.1
FLOW IN AN UNCONFINED RIPARIAN AQUIFER
10.1.1
General formulation
A typical cross section of an unconfined aquifer, whose water flows into the adjoining
stream, is sketched in Figure 10.1. As this flow system is usually relatively shallow
and exposed to the atmosphere through a partly saturated soil moisture zone, the water
pressures and effective stresses are rarely very large, so that the water and the solid
matrix can be assumed to be incompressible (see Brutsaert and El-Kadi, 1984, 1986).
Therefore, if the material can be assumed to be effectively isotropic, the flow, which
involves combined saturated and partly saturated conditions, is in principle governed by
