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Chapter XVII. Off-Site Yield of Ecosystems

Chapter XVII. Off-Site Yield of Ecosystems

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Plate 3'.

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-




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).



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.




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


Ninth century

End of nineteenth century


Early 1960s

Near future

Distant future







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"

Stream group

Monthly flow exceeded 0.9 of the time (mm


Flow over 36-months duration at frequency 0.02

yr-' (mm)

Catchment deficiency (month)

Source: McMahon (1968).













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.






Area of




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

the Root.

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



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

(Hutchinson, 1970).

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



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).




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

Mississippi River.

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



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




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-



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.


On-site transpiration goes with on-site photosynthetic production.

This relation is not causal, but expresses the fact that stomata that are

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Chapter XVII. Off-Site Yield of Ecosystems

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