Environmental Engineering Reference
In-Depth Information
and rate followed by a base flow that frequently approaches zero, altering and
frequently destroying the aquatic habitat.
Reduced Stream Base Flow
Because the mechanism of infiltration is not observable on the land surface, we
can only measure it indirectly. The same observations of stream and river flow
that we use to quantify the discharge in a surface channel serve to measure the
groundwater as a part of this flow. When it has not rained in a watershed for
several days, all of the water observed (and measured) flowing in the stream
(Figure 1-20) or surface channel is coming from the subsurface storage system.
During an extended period of stream flow measurement, the flow observed can
be distinguished between that which occurs immediately following rainfall as
direct surface runoff, and that discharge which comprises the stream flow during
the balance of the year. This base flow usually represents the major portion of
surface stream flow during most of the year, usually on the order of 60% of the
annual volume and comprising stream flow about 90% of the year.
Within a given physiographic region, where the annual rainfall is relatively
uniform, the base flow can vary significantly. This variation is generally attributed
to the surface vegetative system, which can utilize more or less of the infiltrating
rainfall. For example, the water balance can vary greatly in different watersheds
across the country. Climate also plays a major role, with longer summer seasons
and higher average temperatures producing greater evapotranspiration.
Countless small (and large) perennial streams in urban portions of the country
have undergone a slow transformation over the past century to a channel that is
frequently dry or experiences a very low base flow most of the year, and then
is flooded during relatively minor rainfalls as the rapid runoff from extensive
Figure 1-20 Base flow in a perennial stream occurs 90% of the time.
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