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Chapter XVII. Off-Site Yield of Ecosystems
OUTFLOWS FROM GROUNDWATER STORAGE
Watercress farm iocated in the overflowing groundwatei of the basal
aquifer of Oahu, near Pearl Haroo; This crop add5 much to the pleasures of Hawmi 5
oriental cuisine (April 1970)
must be steeper, a n d hence more easily measured, than the gradient
in a strearx channel. i t can be mapped from well or piezometer
readings, a n d such maps show the direction of flow. With auxiliary
data on aquifer permeabilit), the maps can be used to calculate the
rate of fiow; 2 different method tnan that needed to keep track of the
episodic pulses of water that trave! over and near the ground surface.
If water comes out of the rock in a concentrated flow and reaches the
channels of a drainage network, much of it escapes being evaporated
Iocally as described in an earlier chapter, but rather is conveyed
entirely out of the locality, entitling us to include it in the idea of
"yield." In the basin of the Central Sierra Snow Laboratory, for
example, much groundwater from the springs along the contact
between the volcanic and granitic rocks comes out while the landscape
is snow covered or still wet with meltwater, and reaches one of the 60
km of channels that carry water in this season.
Mingled with the direct meltwater flow, this flow is conveyed out of
the basin into the South Yuba River. Even in early summer, springs
support a large enough flow in the larger channels to keep a continu-
O F F - S I T E Y I E L D OF EC O S Y S TEM S
ous thread of water moving out of the basin. Only in August do these
larger channels go dry so that further outflows from groundwater
bodies cannot escape the basin. The total volume of groundwater that
flows out of the basin is about 40 mm.
Base Flow in Networks of Natural Channels
The largest outflow from groundwater in humid lands occurs where
fluvial erosion has incised valleys down into the volume of rock
occupied by a groundwater body. Water reaches these streams
through headwater springs and undefined seepages through channel
banks along their courses, and, by reason of its virtually continuous
nature, is called ”base flow.”
In small channels in the headwaters of a drainage basin there is
some indication that €low is not entirely drawn from the zone of
saturation, i.e., the groundwater body itself, but that some comes
from the layer of unsaturated soil. This contribution becomes minor as
one goes downstream to channels of higher order; the more deeply
incised channels tap strata that may contain large volumes of groundwater. Large channels therefore can tap more groundwater outflow
than small ones, and carry base flow more continuously and in larger
volume. Their regime is less flashy than that of small streams; they
carry less sediment and more dissolved matter.
The increased base flow in large channels represents water that
bypassed the small headwater tributaries; thus drainage-basin area
becomes an important parameter in base-flow determination. This
relation is shown in Fig. XVII-1, in which the open circles depict
Fig. XVII-1. Yield of water in base flow plus storm flow in streams near Coshocton,
Ohio in relation to area of their drainage basins (from McGuinness et a/., 1962).
OUTFLOWS FROM GROUNDWATER STORAGE
annual flow volume in streams draining areas of different size.
Streams that drain basins of up to about 30 ha in area carry about 200
mm of runoff, most of which would be off-site flow moving at or near
the ground surface. Streams that drain basins of 500 ha or more area
carry about 300 mm of runoff. The excess over the 200 mm of surface
flow represents additional base flow.
The same relation is found in the wettest and the driest of the years,
as shown by the solid lines, Streams that are shallowly incised into the
rock layers of the Appalachian plateau are bypassed by a third of the
off-site movement of water.
The excess 100 mm or more of water has percolated, fed the
groundwater, and issued into the deeper stream valleys, where i t
helps them cut their channels still deeper. Also, by favoring erosional
sapping of the toe of the side slopes, base flow helps keep the slopes
steep. The association of high infiltration and percolation with steep
slopes, which was noted earlier, is a part of the same relationship.
A similar increase in base flow with area is found in drainage basins
ir, Michigan (Hudson and Hazen, 1964). The 7-day low-flow rates,
which represent pure groundwater contributions to the streams, when
converted to a unit-area basis of millimeter outflow, increase with
one-fourth power of the increase in basin area.
Volume of Base Flow The variation with time displayed by base
flow fits the general theory of an outflow rate dependent on the
volume of water remaining in storage. As storage diminishes, so does
the rate; the decline is logarithmic, i.e., proportional to the amount in
storage at any given time.
In this respect it probably does not differ from other outflows from
storages in ecosystems, such as the outflow from storage of intercepted
water on foliage, or of detention water at the ground surface. However, concentrated in stream channels, its systematic changes are
visible and are in contrast with water fluxes like rainfall that seldom
display a pattern of steady change.
The base flow fraction of total yield, which varies with the geological environment of the underground water and the volume of percolation, averages 0.23 over the Soviet Union. In different economic
regions the fraction varies as low as 0.12 in Moldavia to as much as
0.43 in White Russia (Dreyer, 1969). Insofar as excessive surface flow
suggests that an ecosystem is not functioning well, is out of equilibrium with its environment, and may be generating mass fluxes that
can cause damage downslope and downstream, a higher fraction of
base flow in total yield indicates a healthier long-term situation,
geological factors being equal.
O F F - S I T E YIELD OF ECOSYSTEMS
In the historical sequence of land-use changes in the central chernozem region of European Russia, discussed earlier with respect to
infiltration, the following long-term changes (from Grin, 1965) are
estimated to have occurred in the fraction of yield contributed by
End of nineteenth century
This valuable fraction of the total yield of water has slowly recovered.
Over the whole world, the average voiume of base flow is 90 mm
yr-l (L'vovich, 1972). This amounts to 0.31 of the total volume of river
flow, and is augmented by artificial storage In reservoirs, which, at
great expense, adds about 14 mm. The large investments in such
reservoirs indicate the value we attach to stability of river flow.
Base flow is less than 5 min yr-' in semiarid lands like the Great
Plains of North America. It increases eastward to about 50 m m in
Wisconsin. in the Milwaukee River i t is 0.26 of 195 mm, or about 50
mm again. Still larger amounts of base flow are found In the rivers of
the southeastern United States.
Geological Factovs Specific characteristics of rock texture and structure that affect the yield of base flow can be illustrated from the
Hunter Valley in Australia. A set of streams that head in tertiary basalt
capping uplands north of the Valley behaves quite differently in lowflow periods than streams that head 10-20 km downslope from the
Low-Flow Characteristics of Two Groups of Streams in the H u n t e r Valley, Australla"
Monthly flow exceeded 0.9 of the time (mm
Flow over 36-months duration at frequency 0.02
Catchment deficiency (month)
Source: McMahon (1968).
OUTFLOWS FROM GROUNDWATER STORAGE
edge of the basalt cap (Table I). Low flow at the 0.9 level in the
western Barrington group of streams is 1.2 mm month-', which is
much greater than low flow of the same frequency in the central
Barrington group, which lacks the benefit of the basalt storage.
Catchment deficiency, defined to index the "inability of a catchment
to yield water during low rainfall periods" (McMahon, 1968), is shown
in the table, indicating the amount of storage capacity that must be
supplied by dam construction in order to attain a desired level of
reliable yield. This index is much larger for the streams lacking the
basalt headwater geology.
Large-scale differences in the geology of shields, platforms, and
folded mountains produce differences in groundwater yield, as shown
by Popov (1968, p. 201) in the accompanying tabulation. Much of the
greater turnover of groundwater in the Alpine structure zones results
from their greater degree of dissection, as well as high precipitation.
Mean annual amount of
groundwater yield (kg m-')
Geologic conditions are naturally important in determining low
flow. They determine the amount of water in underground storage,
and its rate of discharge into stream channels. Figure XVII-2 (Hutchinson, 1970, p. 32) presents flow-duration curves for the Root River,
draining a rather tight basin, and the Fox, draining an area with much
coarse glacial outwash, just to the west of the Root basin. At the 0.5
frequency, the flow rates in both streams differ in proportion to the
difference in their drainage areas, but at the 0.9 frequency the Fox
draws proportionally much more base flow from its sandy basin than
Figure XVII-2 is compiled from all days of record, thrown into the
pot without regard to possible clustering in time of high or low values.
Low values, however, do tend to cluster, a reflection of the large-scale
and hence long-period phenomenon of the depletion of water in a
large aquifer.* Clustering is particularly important because it strains
* Such depletion is related to the typical seasonality of recharge in low-energy periods
(described in Chapter XIV) and also to atmospheric circulation patterns that bring few
rain-generating systems (Chapter V).
O F F - S I T E Y I E L D OF E C O S Y S T E M S
% OF TIME INDICATED DISCHARGE WAS
EQUALED OR EXCEEDED
Fig. XVII-2. Flow-duration curves of two rivers in southeastern Wisconsin, the Fox
River at Wilmot (drainage area of sandy soil, 2200 km2) (solid line), and the Root River at
Racine (470 km2 clay and silt) (dashed line). At the 50% ordinate, the flow in the Fox
River, which comes from five times as large a drainage basin as that in the Root, is five
times as large as flow in the Root. At the 90% ordinate, however, the Fox River, which
drains a more permeable terrain, carries 14 times the flow in the Root. The bottom
dashed line portrays flow in the Root River Canal, draining 145 km2 of tight silt and clay
the capacity of storage reservoirs in small city water-supply systems. It
is customary, therefore, to calculate low-flow probabilities in terms of
the lowest 30 consecutive days, the lowest 7 consecutive days, and so
on (Fig. XVII-3). The 7-day flow is often used to index the base-flow
behavior of a river; for the Fox River it is 2.4 m3 sec-' at an everyother-year probability, but only 1.6 at a frequency of one in ten. This
last number, the so-called ?-day Qlo, is taken as the critical value,
below which the Wisconsin standards for water quality do not apply
(Wisconsin Statutes, 1973).
Urbanization The process of urbanization tends to choke off the
volume of water supporting base flow. Streets and roofs, as noted in
the preceding chapter, shift the partition of water at the earth's surface
toward fast overland runoff and away from the downward stream of
OUTFLOWS FROM GROUNDWATER STORAGE
infiltration and percolation. The resulting reduction in groundwater
recharge is seen in the reduction of base flow in the local stream
Many urban areas, however, make a compensating contribution to
low flow in the form of effluent from sewage-treatment plants. Since
the throuzhput of domestic water is uniform throughout the year,
effluent holds up well during the dry season, when it might contribute
half or even more of the total volume of low flow.
Significant Characteristics of Base Flow Off-site movement of water
from deep bodies of groundwater is characterized by steadiness. The
individual inputs of percolate from separate storms slow down and are
smoothed out as they travel through the deep storage-outflow sequence. The longer the underground period, the steadier the outflow,
as was described earlier. As it feeds into stream channels i t sustains
them long after the wave of storm flow has passed, hence its name.
This sustained thread of water in a stream channel provides a stable
biological habitat that is quite different from all other habitats in the
Unfortunately our inadequate knowledge of the groundwater environment sometimes limits our understanding of base flow. For i n stance, it is hard to separate the rapid outflow from shallow groundwater from the slow response of deeper groundwater. One reviewer
(Hall, 1968) concludes that basic concepts and mathematical expressions of the recession of base flow have long been at hand, yet the
practical art is not well advanced. This gap seems to be the consequence of barriers between scientists in different countries (and of
different languages), and in different disciplines (Hall, 1968). The
same barriers of monolingual, monodisciplinary training that hindered the study of snowfall interception and wasted much research
effort (Chapter VI) appear to be a problem in the study of base-flow
Fig. XVII-3. Mean rate of low flow over 1, 7, 30, and 90 consecutive days, at
frequencies from 0.95 to 0.04 in the Fox River at Wilmot, Wisconsin (drainage area 2200
km’) (Hutchinson, 1970).
O F F - S I T E YIELD OF ECOSYSTEMS
We can think of base flow as somewhat analogous to memory of
past rainstorms. One model defines base flow as "propagating antecedent or historical effects, carry-over signals from the past, now stored
and in transit. . ." (Appleby, 1970). Depending on local geologic
conditions that affect size of the groundwater body and the speed of
movement of water through it, base flow contributes varying amounts
of water to the total amount of off-site yield.
Steadiness of flow is one of the desirable characteristics of base flow
in a river. Another is its counterseasonal temperature contrast with the
surface environment. Cold-water fish like trout usually are found only
in streams that carry a large fraction of groundwater outflow, as in the
sand plain of central Wisconsin and certain dissected regions near the
The relative warmth of groundwater outflow in the spring is utilized
in the Schwarzwald of Germany to speed up the warming of the soil
(Plate 35). Irrigating mountain pastures by groundwater outflow, then
substantially warmer than the air or top soil, raises the soil temperature and improves the climate of plant roots.
A third valuable characteristic of base flow is its relative freedom
from turbidity and suspended sediment.
Distant Outflows from Groundwater
Water travels slowly through the rocks of deep aquifers but occupies
a very large cross-sectional area so that the total rate of transport can
be significantly large. Some of the sandstone formations that crop out
at the edge of the central Rocky Mountains in an area favoring high
percolation convey water far out into the Great Plains, where it is
tapped by deep wells. Such groundwater bodies function as pipelines,
although movement is so slow that changes in the conditions in the
intake areas might not register in their outflows for a long time.
Water in a pervious layer cut off from the earth's surface by upper
impervious rocks travels downslope and comes under increasing
pressure. Artesian flow may result when this pressurized water body
is tapped by wells drilled through the overlying cap rock. This
happens in the Great Artesian basin of eastern Australia, which slopes
inland from the wet mountains near the Queensland coast. It is drilled
into and water rises in the bores to the surface a thousand kilometers
inland. Although warm and saline it can be utilized for watering
After a few decades of excessive release of the pressurized water and
much wastage of both the water and the pressure resource, the basin
OUTFLOWS FROM GROUNDWATER STORAGE
Plate 38. Water-management ditches in the German Schwarzwald late1 will convey
relativelv warm groundwater to speed up the spring warm up of the soil (December
has now more or less been brought under control (Costin, 1971). The
regulated outflow averages, on an areal basis, about 3 mm yr-‘, small
in volume but vital for the population of animals that is the best
means of harvesting the biological production that utilizes local soil
moisture. We obtain ”protein from the wasteland” (Macfarlane, 1968).
Mass Fluxes Associated with Base Flow
While little suspended sediment is carried by the outflow from
bodies of groundwater, dissolved material is ubiquitous and organic
pollutants also are being found. The long time of intimate contact
between water and underground rocks permits solution to do its
work, and most outflows from groundwater are mineralized.
In limestone country like southeastern Wisconsin, both the groundwater and the base flow in the Milwaukee River is hard, with high
concentrations of calcium, magnesium, and bicarbonate ions (Southeastern Wisconsin Regional Planning Commission, 1970, p. 280). All
three aquifers contain hard water; the content of dissolved solids
OFF-SITE YIELD OF ECOSYSTEMS
being 380 in the uppermost one, the sand and gravel aquifer, which
probably supplies most of the base flow in the river, 470 in the middle
or dolomite aquifer, and 900 in the deep sandstone aquifer (Southeastern Wisconsin Regional Planning Commission, 1970, p. 270).
Base flow in the Colorado River as it leaves its upper basin, which
amounts to about 650,000 tons day-’, carries 100 tons of salts (Iorns et
al., 1965, p . 31), some of which represent outflow from deep groundwater bodies, some deriving from accelerated solution as irrigation
water percolated through the soil, into shallow groundwater, and into
the upper river. This salt burden presents a problem to farmers in the
lower basin, e.g., in the Imperial and Coachella Valleys, and to city
users. It is indeed possible to remove this salt loading from the water,
just as any load can be removed from a vehicle; the question,
however, is whether this should be done at the expense of the
downstream users or the upstream polluters.
Although, as we saw in Chapter XIV, percolation fluctuates from
year to year, the beginning of irrigation produces a still more radical
change, in that the volume of percolate is much increased and is held
at this high level year after year. The water-budget consequences may
appear in stirred-up saline ground water; the mass-budget consequences appear in accelerated weathering of soil minerals (Rhoades et
al., 1974). Over-irrigation “can result in the discharge of more salt
from the soil than was applied in the irrigation water because of
mineral weathering and dissolution phenomena.” This finding applies
to the western United States, but we noted earlier the effect of cane
irrigation on water quality in the basal aquifer of Oahu.
The enrichment of groundwater by fertilizer nitrates in many basins
is evident in the base flow in streams draining this farm area. Such
off-site transport of nitrates from the San Joaquin Valley of California
has reached such a size as to occasion proposals to construct a
drainage network that would completely bypass the San Joaquin River
Mass fluxes from agricultural ecosystems are carried in different
proportions by surface runoff and percolate; a study in New England
found that ”nitrates but not phosphates will move both vertically and
laterally through the soil to subsurface drains, [and] that surface runoff
contains few nitrates but significant concentrations of phosphates”
(Benoit, 1973). Since much off-site transport of nutrients is associated
with the transport of sediment particles, percolate and base flow are a
desirable form of water yield for their relative cleanness as well as
their stable regimes. In a study of more intensive farming in Iowa,
base flow carried only 0.04 of the nitrogen and 0.05 of the phosphorus
from a contour-planted corn-cropped basin, which shed great quan-
WATER YIELD A S A S S O C I A T E D W I T H B I O L O G I C A L YIELD
tities of both nutrients in association with sediment (Burwell et a l . ,
1974). Reducing sediment transport by instituting level terracing
correspondingly reduced transport of sediment-associated nutrients;
the level-terraced basin also produced more base flow (96 versus 63
mm), which moreover carried off less phosphorus (4 mg m-’ versus 5
from the contour corn basin) and less nitrogen (100 mg m-’ versus
Studies of land cleared for wheat cultivation in Western Australia
(Peck and Hurle, 1973) indicate that when percolation of water under
cleared land increased by 30-60 mm yr-’ and under entire drainage
basins by 20 mm yr-l, the outflow from groundwater into the streams
increased. Unfortunately, this augmented base flow is of high salinity.
The salt outflow from forested basins in the region is about the same
as the input of salt in rains and by dry fall; from basins even partly
cleared the salt flow is far greater, being increased by about 30 g yr-’
m-’ of land area. This degradation of base flow was set in motion by
the increased input of percolate water to the groundwater body, as we
saw earlier in the swamping process in the Belka case study.
One of the costs associated with mining is the loading of groundwater outflows with acids or other pollutants. When the pyrites commonly associated with coal beds are exposed to the air, they oxidize
and the sulfur is mobilized. Acidity depends on the underground
environment, limestone counteracting it, but often is a damaging
characteristic of mine-drainage water. The Water Quality Act of 1972
(U.S. Congress, 1972), in Section 107(a) provides for demonstration
projects on ”approaches to the elimination or control of acid or other
mine water pollution resulting from active or abandoned mining
operations.” Many of these are found in the Appalachians, often
representing a legacy from mining companies long gone out of
Base flow has value as a reliable source for withdrawals of water for
domestic, industrial, or irrigation needs, and also for the in-channel
uses of water: as habitat, for navigation, and as amenity. Aquatic life
can survive dry seasons if base flow does not drop too low or get too
warm or become too loaded with oxygen-demanding organic pollutants. The stream is most vulnerable to man’s actions at these periods
of low flow, and is most needed then.
WATER YIELD AS ASSOCIATED WITH BIOLOGICAL YIELD
On-site transpiration goes with on-site photosynthetic production.
This relation is not causal, but expresses the fact that stomata that are