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IV. Phosphorus Loads in Runoff and Streams
PHOSPHORUS IN RUNOFF AND STREAMS
of chemical interactions or the energy of the water itself. Several factors
affect the amount and energy of surface runoff water at any particular location and, therefore, the amount of additional P entering and carried by
it. These include nature of land use, extent of vegetative cover, slope, intensity of rainfall, and permeability of the land surface.
The quantity of precipitation entering subsurface and groundwater
runoff is inversely related to that disposed of in surface runoff and evapotranspiration. It is consequently affected by the factors listed above for
surface runoff. The major portion of P in subsurface and groundwater runoff is expected to be in dissolved forms. If subsurface runoff is accelerated
by artificial drainage systems, however, soil colloids, with associated P,
may appear in the water as it enters streams.
The P load carried by a stream under given flow conditions will represent the relative contribution of P loads in each of the runoff components,
as well as the influence of any point source of P.
INFLUENCE OF POINT SOURCESON PHOSPHORUS
Estimates of the contribution of P to surface waters from domestic
wastes in the United States range from 91 x loo to 227 x l o G kg
per year with total P concentrations ranging from 3.5 to 9.0 pg/ml
(McCarty, 1967; Ferguson, 1968). Weibel et al. ( 1964) estimated that
P discharged as raw sewage from combined storm sewers in Cincinnati,
Ohio, amounted to 3.4 kg/ha per year as total dissolved P. In the area
of Madison, Wisconsin, the per capita contribution of P to surface waters
from treated domestic waste was estimated to be 0.544 kg/capita per year
(Sawyer, 1947), whereas an estimate of 1 kg/capita per year was given
by Metzler et al. (1958) for Chanute, Kansas. The difference between
the estimates of Sawyer (1947) and Metzler et al. (1958) may reflect
the increased use of P in domestic products, particularly detergents.
The impact of sewage outfall on the dissolved inorganic P concentration
of streams and rivers was studied by Brink and Gustafsson (1970). Their
results are summarized in Table 11. Obviously the impact of the outfall
is dependent on factors which include flow rate of the receiving stream
and the P content of the effluent.
Under certain agricultural management conditions animal excrement
may constitute a point source of P to streams. Excrement may enter
streams during surface runoff from feedlot operations or by the cleaning
of milking sheds into open drains. The magnitude of these sources of P
will be discussed later.
McCarty (1967) was unable to estimate the magnitude of contributions
of P made to streams from industrial wastes. The amounts of P discharged
J. C. RYDEN, J. K. SYERS, AND R. F . HARRIS
Effect of Sewage Outfall on tllc Dissolved Inorganic Phosphorus
Concentration of the Receiving Water"
IXssolved inorganic P concentration
Data from Brink and Gustafsson (1970).
to streams will depend on the industrial process concerned and local legislation covering the discharge of industrial effluent. Mackenthun et al.
(1968), for example, estimated that a potato canning factory and a
woollen mill contributed 3446 and 835 kg of P per year, respectively, to
the East Branch of the Sebasticook River, Maine.
Domestic and many industrial wastes not only supply large amounts of
total P to streams but also have a pronounced effect on the concentrations
of dissolved forms of P in the receiving stream. Because domestic and industrial wastes are point sources, they are easily recognized within a watershed and are amenable to direct manipulation.
B. RUNOFFFROM FORESTWATERSHEDS
A compilation of data from several studies of the quantities of P lost
in streanis from stable forest and woodland watersheds is presented in
Table 111. Exports of P in streams from long-established and stable forest
watersheds provide a useful datum line against which losses of P from
other land-use areas may be compared. The data in Table I11 show a considerable degree of uniformity. Total P losses range from 0.68 to 0.02
kg/ha per year with three out of the four values being less than or equal
to 0.1 kg/ha per year.
Only a few measurements have been made of the dissolved inorganic
P concentration of stream water in forested watersheds. The values reported by Brink and Gustafson (1970) in Sweden show a mean of 0.015
pg/ml, with this fraction amounting to 33% of the total annual loss of
P. The data suggest that total P and dissolved inorganic P concentrations
rarely exceed 0.115 and 0.025 pg/ml, respectively. Two interesting points
arise from the data in Table 111. From the study of a stream draining a
Losses of P in Streams Draining Forest Watersheds
in streamwater (pg P/ml)
Bormann et al. (1968)
Brink and Gustafsson (1970)
Jaworslii and Hetting (1970)
Potomac River Basin
Taylor et al. (1971)
Dissolved inorganic P
Dissolved inorganic P
Total soluble P
J . C. RYDEN, J . K. SYERS, A N D R. F. HARRIS
woodland area at Coshocton, Ohio, to which no fertilizer P had been applied for over 30 years (Taylor et al., 1971), it would appear that the
woodland is conservative of P. The average total soluble P content of rainfall was 0.020 pg/ml, whereas that in the stream draining the watershed
was 0.015 pg/ml. The extent of addition of total dissolved P to the woodland can be calculated from precipitation data given by Taylor et al.
(1971 ) ; a value of 0.17 kg/ha per year is obtained. This value is more
than three times greater than the annual P loss in the stream. The conservative nature of forests for P is further borne out by the fact that the
annual contributions of P to the land surface in precipitation, quoted previously, are in most cases considerably greater than annual exports of P in
streams from forest watersheds. In many cases there is an order of magnitude difference. This hypothesis assumes that data covering the P content
of precipitation are correct.
The second point of interest relates to the “background” P concentration
in forest streams. The data suggest only minor seasonal fluctuations in P
concentrations, particularly that of dissolved inorganic P. As a major portion of streamflow is considered to have a groundwater origin (Biggar and
Corey, 1969; Johnson and Moldenhauer, 1970), it is conceivable that the
dissolved inorganic P load in streams of forested areas is primarily due
to that in groundwater runoff. If the reported mean P concentrations of
forest streams are compared to those for groundwaters, a marked similarity
is observed. Juday and Birge ( 193 1) found that the total dissolved P concentrations of 19 wells in northern Wisconsin, an extensively forested area,
ranged from 0.002 to 0.197 pg/ml, with an average of 0.018 pg/ml when
the highest value is omitted. This mean value is, if anything, slightly higher
than the mean concentrations for dissolved fractions of P reported in Table
111. The higher mean concentrations of total P probably arise from suspended inorganic and organic solids that enter streamflow due to turbulence, especially during high flow.
The minor fluctuations in P concentrations reported for forest streams
suggest that P export is minimally affected by P input from surface runoff.
Amounts of surface runoff in forest watersheds will be low owing to the
protection afforded by canopy cover and/or forest floor vegetation. The
“background” P export in forest streams is a direct reflection of the chemical and physical factors that affect P concentrations in groundwater and
subsurface runoff. Because larger amounts of stream flow from forest
watersheds will arise from groundwater and subsurface runoff, the “chemical sieving” action of the soil plays a major role in maintaining the consistently low dissolved inorganic P concentrations in forest streams and
may also account in part for the apparent conservative nature of forest
watersheds for P.
PHOSPHORUS IN RUNOFF AND STREAMS
C. RUNOFFFROM AGRICULTURAL
The loss of P in streams draining agricultural (in most cases arable)
watersheds is far less well defined than that for forest streams. This is probably due to the fact that in studies designed to estimate this loss, little
differentiation has been made with respect to the forms of runoff. Consequently, there are major problems in estimating P loss from agricultural
watersheds using many of the data presented in the literature. Losses of
P from agricultural land have not only been based on analyses of streams
draining a specific watershed (Campbell and Webber, 1969; Taylor et al.,
1971 ), but have also been estimated from data obtained in surface runoff
studies (Timmons et al., 1968). Many previous reviews of this subject
have relied on such data (Taylor, 1967).
Losses of P in streams draining various agricultural watersheds are summarized in Table IV. The lowest loss of total P is from rangeland in Ontario, Canada (Campbell and Webber, 1969) which had received no P
fertilizer in living memory. This loss is very similar to losses of total P
from forest watersheds, suggesting a minimal contribution if P from surface runoff. Similarly, the total P carried in the base flow, primarily attributable to groundwater runoff, of several streams draining arable agricultural watersheds in S.W. Wisconsin (Minshall et al., 1969) is also little
different from total P loads in streams draining forest watersheds. Minshall
et al. (1969) reported the total P loss in base flow to be less than 0.12
kg/ha per year. If stream flow during periods of surface and subsurface
runoff is included, however, the estimated annual loss of total P increases
by one order of magnitude, as indicated by the data of Witzel et al. (1 969)
for the same area of S.W. Wisconsin (Table I V ) .
These studies suggest that the groundwater runoff or base-flow component of streams draining agricultural watersheds is little different from the
total P load of forest streams. It is therefore necessary to estimate the extent to which the P load of streams draining agricultural watersheds may
be augmented by P loads of surface and subsurface runoff.
The major factors affecting the loads of P in surface runoff from agricultural land include time, amount, and intensity of rainfall, rates of infiltration and percolation, slope, soil texture, nature and distribution of native
soil P, P fertilization history, cropping practice, crop type, and crop cover
A selection of reported losses of P in surface runoff from arable land
of various slopes and cropping practices is summarized in Table V. Losses
range from the extremely high values of 67 kg/ha per year to almost zero.
Losses of P in all studies listed in Table V have been based on the collection of surface runoff (water and particulates) from small experimental
TABLE I V
Losses of P in Streams Draining Agricultural Watersheds
Taylor d al.
Witzel d al.
January through September 1967.
Form of P
PHOSPHORUS IN RUNOFF AND STREAMS
plots frequently no larger than 30 x 6 m, with subsequent analysis for one
or more forms of P. Although this approach was originally developed to
investigate soil fertility losses due to soil erosion, it is still used to estimate
P loads in surface runoff as it relates to the fertility of surface waters (Timmons et al.,1968; Nelson and Romkens, 1969).
It is difficult to make any generalizations regarding the P loads carried
in surface runoff or to draw conclusions from them in terms of how agricultural practices and natural variables affect P loads in streams draining agricultural watersheds. This is due to the differences in forms of P measured
and the lack of comparative studies with respect to slope, soil texture,
cropping, and climatic variables.
One of the few studies from which meaningful interpretations of P loss
in surface runoff can be made in relation to degree of slope and cropping
practice is that by Massey et al. (1953) in Wisconsin (Table V ) . As expected, greater “available” (water-soluble plus pH 3 extractable) P losses
to surface runoff occurred on the steeper slopes when cropping practice
was kept constant. The introduction of two years hay into the rotation
reduced the P loss by a factor of approximately four. The value of “improved” or “conservative” agricultural practices in reducing the magnitude
of P losses is illustrated in the studies at Coshocton, Ohio (Weidner et
al., 1969) and at Lafayette, Indiana (Stoltenberg and White, 1953). It
should be noted, however, that although the “improved” practice reduced
the total amounts of acid-hydrolyzable P lost in surface runoff at Coshocton, the concentration of this fraction during surface runoff increased from
0.43 to 0.59 pg/ml.
Attempts have been made to measure the relative contributions of the
aqueous and particulate fractions of surface runoff to the total loss of a
measured form of P. In a plot study using simulated rainfall, Nelson and
Romkens ( 1969) obtained dissolved inorganic P concentrations of 0.05,
0.30, and 0.50 pg of P per milliliter in the aqueous phase of surface runoff
from fallow plots 12 days after 0, 56, and 1 1 2 kg of P per hectare, respectively, had been disked into the soil, with only slight decreases in concentrations up to 75 days after fertilizer application. Although very high artificial rainfall rates were employed (up to 73.5 mm/hr), indications are
that high concentrations of dissolved inorganic P may be maintained in
surface runoff water. Timmons et al. (1968) determined the distribution
of total P loss in surface runoff between the aqueous and particulate phases
from plots under natural precipitation. Although these workers did not
report P concentrations, losses of total P in the aqueous phase of surface
runoff arising from snowmelt far o.utweighed those in the particulate phase.
In contrast, total P loss in the aqueous phase varied in most cases between
5 and 40% of the loss in particulates in surface runoff arising from rainfall.
Losses of Phosphorus in Surface Runoff from Field Plots
Knoblauch et al.
Massey et al.
Form of P
Soluble pH 3
0.5 M N H 4 F
0.1 N HCI
Thomas et al.
0.05 N HCI
0.025 N H ~ S O I
(i) No manure
(iii) Cover crop
(iv) Cover crop
Corn-oats2 yr hay
Oats-5 yr hay
Timmons et al.
Weinder et al.
Total acidhydrolyzable P
Prevailing practice: moderate fertilizer levels; liming t o p H 6.0; straight row planting and cultivation.
* Conservation practice: higher fertilizer levels; liming t o p H 6.5; manure before corn; contour planting and cultivation.
Prevailing practice: straight row tillage across slope; low P fertilizer level; alsike-red clover-timothy meadow mixture; liming t o p H 5.4.
Improved practice: contour tillage; high P fertilizer level; clover-alfalfa-timothy meadow mixture; liming t o p H 6.8.
J. C. RYDEN, J. K. SYERS, AND R. F. HARRIS
These observations are not unexpected because rainfall tends to loosen
soil particles by drop impact, facilitating their entry into surface runoff
waters. It is apparent that an appreciable dilution of dissolved P may occur
when surface runoff augments base flow in streams. Taylor et al. (1971)
reported a mean total dissolved P concentration of 0.022 pg/ml in a stream
draining an agricultural watershed at Coshocton, Ohio; concentrations
never exceeded 0.100 ,g/ml even under conditions of high stream flow
when surface runoff was occurring.
It is generally considered that P is retained sufficiently strongly by soil
particulates that movement out of the soil profile in percolating waters is
minimal (Way, 1850; Black, 1970). Subsurface runoff from agricultural
land, however, may contain significant concentrations of dissolved inorganic P in relation to those present in surface waters (Table VI) . It should
be noted, however, that the data in Table VI represent losses of dissolved
inorganic P in tile and irrigation return flow drains. Artificial drainage systems increase the rates of infiltration and percolation, reducing contact
times between the soil solution and soil components capable of sorbing
inorganic P from solution. Furthermore, tile drains will remove water from
surface horizons of the soil profile, diminishing the possibility for contact
between percolating waters and more P-deficient subsoil material.
Not all the data in Table VI, however, indicate a net loss of P from
the soil profile to subsurface runoff. In the Snake River Valley, Idaho,
Carter et al. (1971) found that only 30% of the dissolved inorganic P
in irrigation water left an irrigation tract by return flow. When the dissolved inorganic P concentration in irrigation water exceeded 0.010-0.020
pg/ml, irrigation decreased the downstream P load, a useful field example
of the chemical sieving action of soils. Johnston et al. (1965), however,
reported a net loss of 3% at an applied P fertilizer rate of 51.9 kg/ha
on irrigated land in the San Joaquin Valley, California.
The data in Table VI indicate that a reasonable proportion of P loss
to streams draining arable watersheds can be due to subsurface runoff.
Although no data are available to compare P loads due to surface and
subsurface runoff, Sylvester (1961) reported that total P loss by irrigation
return flow in the Yakima Valley, Washington, ranged from 3.8 to 14.3
kg/ha per year, values higher than many reported for surface runoff losses.
Under a nonirrigated farming system, Bolton et a!. (1970) observed losses
of dissolved inorganic P in tile drain effluent ranging from 0.13 to 0.29
kg/ha per year at a fertilization rate of 28.9 kg of P per hectare per year.
It would appear, therefore, that losses of P in subsurface runoff can be
similar or even greater than those in surface runoff. Furthermore, subsurface runoff will occur not only during periods of surface runoff, but also
when evapotranspiration is less than infiltration.
Losses of Dissolved Inorganic Phosphorus in Subsurface Runoff
Dissolved inorganic P
Bolton et al. (1970)
Brink and Gustafsson
Carter et al. (1971)
Cooke and Wlliams
San Joaquin Valley, California
No P fertilizer applied.
28.9 kg P applied per hectare per year.
Clay to sandy
Heavy silty clay
0 . 210b
Arable and grassland
0 . 0uo
Erickson and Ellis
Johnston et al.
Tile drains and