Geoscience Reference
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the nonuniform distribution of the physical characteristics within a catchment. Equation
(10.167) indicates that the steeper parts of a basin, where Hi is larger, must have faster
depletion rates, and therefore larger values of
a
; such areas are usually located near the
headwater sections of a basin. In contrast the downstream regions of a basin have smaller
inclinations, hence relatively lower rates of depletion.
All this shows that, although the characterization of a basin as a single lumped unit
with basin scale parameters is a useful paradigm, it has definite limitations. The total
outflow rate is the sum of flow contributions from aquifer sections with unequal response
characteristics. This total flow is initially dominated by the discharges of channel storage
and of the steeper aquifers, which contribute a large fraction of the total flow during the
first few hours or days of a recession period. As the recession progresses, however, these
storage elements become rapidly depleted and the gentler parts of the aquifer, now being
the main contributors, determine the outflow. This also means that the determination of
the basin scale aquifer properties by means of an analysis of the lower envelopes is valid
mainly in relatively flat and even terrain. In more rugged catchments the lower envelope
tends to reflect the properties of the broader valley sections in the lower parts. Therefore, in
practical applications for design purposes in hilly watersheds, it may be advisable to adopt
an average value of
a
and
b
in Equation (10.157), say by regression through all the data
points (rather than from the lower envelope alone), to describe representative basin scale
parameters. These issues will require further study.
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