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Water - its quality often depends the forester

Samuel H. Kunkle

Samuel H. Kunkle is the staff hydrologist for forestry at FAO headquarters. Before that he was with the U.S. Forest Service, as chief of the Water Quality Branch covering 20 eastern states. He has also worked with the forest services of Switzerland and Denmark.

Man's fresh water flows initially from forests, and water quality begins there. The author surveys the influences of forest activities on ground water how they can be designed to protect quality.

The greatest depth of precipitation usually falls on forested land because forests are often located at higher elevations, initially capturing and gradually releasing water to vast areas below. Also, a forest cover is almost always the best and most natural protection for streams because it maintains good water quality and stabilizes flow. Forests are thus the main source areas for man's supply of fresh water.

From this it should be clear that forest management in all its aspects constitutes a major and continual influence on the hydrological regime of whole drainage basins, and the forester should be conscious of this all the time. But often he is not. Involved as he is with the many economic and administrative sides of daily management, he tends to forget that he is in fact the primary manager of water supplies throughout his area and for great distances downstream, places that he may never visit or that may even be beyond the political frontiers of his country.

Man's use of forest lands, then, is the major influence on streamflow over much of the surface of the earth; that is, on stream behaviour, water losses from a basin, floods, low flows and water quality. In this context we shall consider water quality and touch on water pollution, including sedimentation from logging and forest roads, the effects of clear-cutting on water chemistry and stream temperatures, the problematical effects and proper uses of pesticides and other toxic chemicals, waste disposal in forests, and some of the effects of roads and highways on forested basin streams.

In urban areas pollution problems usually pour from obvious sources in measurable quantities, and these obvious problems are amenable to well-defined (albeit expensive) solutions. In rural and wildland areas, however, water quality is a complicated function of hydrological processes, climatic conditions, biological reactions and other interacting factors. To comprehend wildland streams we must therefore understand some of the hydrological processes.

About ten years ago researchers came to a new and most important conception of forest land runoff-the concept of the variable source area. This concept is now recognized as the essence of runoff in many forest lands and is certainly the basis for most interpretations of influences on water quality.

The variable source area concept recognizes that for many forest lands having a good vegetation cover-especially in humid areas-the following runoff process occurs. During a typical rain storm only a small part of the basin around the channel actually yields surface runoff, whereas on the upper reaches of the basin the rain infiltrates, becomes subsurface flow and does not appear in the streams until long after the "storm hydrograph" is finished. Therefore, the high flows observed during storm runoff often come from only a small part of the catchment and the storm hydrograph contains only a very small part of the total rain falling on the basin (Betson and Marins, 1969; Dunne and Black, 1970; Hewlett and Nutter, 1970; Whipkey, 1965). For example, in research on rural and forested basins, the writer has observed that typically only 5-10 percent of a storm's precipitation appears under the storm's hydrograph - the graphic presentation of streamflow during and following a storm. Most important, field measurements indicated that this fraction of runoff came principally only from around the channel areas. The runoff areas, or variable source areas, expand during a storm, as illustrated in Figure 1, which is based on a series of field measurements of groundwater, overland and channel flows (Kunkle, 1971).

What do these hydrological concepts mean in terms of water pollution on forest lands ? The following general statements are meaningful:

- Because in forest lands with a good vegetative cover surface runoff may come largely from around channels, the likelihood of pollutants being washed into streams by storms is usually a direct function of the distance of pollutants from a channel (e.g., even at only 50 metres from channels surface runoff per se may be rather rare in a forest). The implications for the need of protective strips around streams are obvious.

- Surface runoff-hence pollutant surface transport-is also common on frozen ground, on marshy sites, in areas where shallow soils or rock outcrops prevail, and sites where protective vegetation and soils have been ruined by deforestation, overgrazing, fires or man-induced erosion.

- Surface pollutants (sediment, bacterial organic particles and easily dissolved substances) will be flushed from surfaces in proportion to the volumes of surface runoff. The transport rate of surface pollutants (kg/minute) is quite high during storms, since pollutant concentrations and streamflow both increase.

- Generally, the longer a storm continues the larger is the surface area supplying runoff, hence the more the basin can contribute surface pollutants. Also, the wetter the soil conditions prior to a storm, the faster the runoff contributing area grows in size. The importance of climatic considerations prior to, for example, herbicide spraying, is obvious.

- The dissolved substances which infiltrate into the upper reaches of a basin are subject to soil absorption (e.g., PO4 - P is often absorbed) but these substances may also move into the groundwater, to appear later in stream baseflows (as in the highway salt example discussed later). There is an inverse relationship of dissolved substances to streamflow in many instances. (Note: natural chemicals in streams, e.g. bicarbonates, also typically show this inverse relationship.) On the other hand, particles like bacteria are trapped quickly in soils (Romero, 1970, Salvato et al., 1971).

- With increasing industrialization, the precipitation falling on forest lands is not necessarily "pure" rainwater. For example, in the downwind areas of industries using mercury, contaminants may be much higher than normal in rain and snow and in the vegetation (Holder, 1972).

- Because of the variable source area process of runoff - where the greater area of a basin is not usually washed by overland flow - it is meaningless to attempt to develop budgets of pollution based on kg/ha, cows/ha or other area-wise approaches. For example, the kg/ha applications of fertilizers on a basin are not nearly as important as specifically where on the basin the fertilizers are applied. Upper basin soils are often highly effective filters.


Observations on the process of surface runoff, showing how the runoff area grows during a storm, with runoff coming from around the channel area

There are, of course, exceptions to the above rules, as with any generalizations. Many of the storm runoff concepts also apply to melting snow, especially when a snow cover protects the ground from deep freezing.

Sediment is the most familiar form of water pollution. Soil erosion often results in serious and costly damage downstream in terms of siltation of reservoirs (Fig. 2); heavy loads which harm fish (Tebo, 1968) and added treatment costs for municipal water supplies; damage to irrigation canals, bridges and other structures; siltation of river channels which raises flood levels; aesthetic and biological damage to lakes; and a tendency for eroded lands to yield higher runoff and lower dry-season flows, which means that pollution will be more severe during low flows because of less dilution.

Careless timber harvesting practices can lead to severe sediment problems, but the forester is in a position to control most of these. For example, in one logging study, turbidity in the stream below a clear-cut area reached 56 000 parts per million, compared to only 5 ppm in adjacent unlogged streams (Hornbeck, 1968). Turbidity is an optical index of suspended sediment, used as a simple measure of solid material concentration in water (Kunkle and Comer, 1971)

On the other hand, this and other studies have also demonstrated that if logging is done in conjunction with suitable conservation measures, turbidity will remain almost as low as in streams in undisturbed areas, and for the most part will stay below the 10 mg/litre standard for drinking water (U.S. Environmental Protection Agency, 1972). The necessary conservation measures include:

- Leaving uncut strips along streams.
- Selecting logging machinery which is less damaging to the soil.
- Moderation of road gradients, usually to less than 10 percent.
- Design of road layouts and drainage installations to avoid the channelling of overland flow (Aubertin and Patric, 1972; Hornbeck, 1968).

Of course there are limits, and certain forest areas are simply too steep and erosive for such measures to be effective or economical, in which case logging is not advisable.

The harvesting of forests can also increase stream temperatures, if trees are cut along their banks. For example, in one research project logging was observed to raise stream temperatures more than 6°C above normal, which exceeded acceptable levels for trout. However, when the stream-bank vegetation was left intact stream thermal pollution did not occur (Swift and Messer, 1971).

2. A reservoir in Africa, four years old but already half full of sediment because of land erosion upstream


The problem of eutrophication is not new, and was under study in Swiss lakes by limnologists before the turn of the century. However, there is more awareness today of the damage caused in lakes by algal blooms and other plants. Decomposing masses of algae can be detrimental to fish life and to municipal water supplies, as they cause depletion of oxygen and problems of taste, odour and toxicity. These problems are especially critical when they occur in lakes in forested areas frequented by large numbers of people from crowded cities seeking recreation.

There are several theories on the exact cause of eutrophication. Most limnologists agree that nutrients such as phosphorus and nitrogen, plus organic matter, are the principal causal agents (Canada Centre for Inland Waters, 1970; Sawyer, 1965; Shapiro, 1970). In sensitive, oligotrophic (rather sterile) lakes in some forested areas, even small additions of nutrients and organic matter can convert a pristine lake into an unaesthetic mass of algae (Fig. 3).

3. Algal masses floating on the surface of a lake suffering from eutrophication

Forest fertilization and other activities can result in nutrients and organic matter being added to-streams and lakes, thereby increasing the level of eutrophication. In coming years, fertilization of woodlands is likely to increase, for instance in Sweden, where trials are under way, and in the northwestern United States, where fertilization is expected to exceed 40 000 hectares annually during the next decade (Kunkle, 1973; Bullard, 1966). We shall need to be increasingly alert to the eutrophication problems which may follow.

Recreational developments in forests - ski resorts,: vacation houses and camping sites - are sometimes sources of pollution, especially as they are often on critical sites near lakes or streams and, in the experience of the writer, may have inadequate provisions for waste disposal, causing eutrophication as well as pollution.

Other potential sources of nutrients and organic matter in forestry include logging debris (logs, leaves, etc.) and eroded soil from logging, forest fires and new roads, where the sediment carries nutrients into streams.

Long-term studies at the Boundary Waters Canoe Area, on the Canadian-United States border (U. S. Forest Service, 1972) found the following factors especially indicative of a lake's eutrophic level and its degree of sensitivity to eutrophication:

- The chemical parameters P, N, Fe and HCO3.
- Plankton primary productivity evaluations.
- Lake shoreline to area ratio.
- Colour.
- Lake volume to drainage basin ratio (an index of water exchange).
- Lake bottom geological constitution.

After clear-cutting

Clear-cutting of forests in some cases may add nutrients to streams, lakes and groundwater by disrupting the nutrient cycle in a forest ecosystem or by increasing surface runoff and sediment inputs into streams. Research in northwestern North America showed an increase in stream-dissolved chemicals following experiments in clear-cutting and slash-burning of Douglas fir forests (Fredriksen, 1970). During the 12 days following the slash-burning, a stream below one experimental clear-cut showeda distinct increase in nutrient content, while an adjacent control stream was unaffected (Table 1).

For about two years following the cutting and slash-burning, while re-growth occurred, the differences in nutrient content between the two streams decreased. It is important to note that cutting alone - without burning - added less nutrients; for example, the phosphorus in the stream did not increase. Also, much of the nutrient input was attached to sediment particles from erosion. Another experimental study in New England, United States, where all the vegetation on a catchment was cut, also showed nutrient releases into streams (Bormann et al., 1968). It must be noted that these were experimental cuttings.

What occurs during a more typical cutting operation? A study in West Virginia, which took careful conservation measures to protect the streams, found that clear-cutting did not cause significant change in nutrients during the first year after clear-cutting, "when maximum nutrient outflow has been expected" (Aubertin and Patric, 1972).

There are lessons to be learned from these and similar studies: with careless forest-cutting procedures, water pollution by nutrients may occur; some basins, because of inherent soil properties and other characteristics, are more likely to release nutrients after cutting; and suitable stream protection measures during cutting, including uncut strips along channels, may avoid any significant nutrient problem in many, perhaps most, cases. These lessons also clearly show that more research information is needed. Whether or to what degree the problem of nutrients is significant, especially in reference to eutrophication, is still not clear and remains an open question. Guidelines are especially needed to show exactly what protective measures should be taken during logging.

Biological magnification

Much popular writing has been devoted to the detrimental effects of the use of poisonous chemicals in natural ecosystems, and there are numerous documented case studies of fish kills and damage to wildlife or the human environment, such as the example in Canada where DDT in a large-scale forestry project resulted in heavy fish die-off (Kunkle, 1973). The process of biological magnification, whereby chemicals are concentrated in living food chains, is known to be a hazard with DDT, endrin, dieldrin and other chlorinated hydrocarbons which are used in forestry. It must therefore be recognized that the benefits need to outweigh the hazards before the use of pesticides can be considered justifiable. We still have much to learn about the long-term, low-concentration hazards of these chemicals, which according to some scientists are underrated.



Clear-cut catchment

Uncut control catchment








(not detectable)













SOURCE: U.S. Forest Service.

¹A comparison of maximum concentrations of chemical nutrients in two streams following clear-cutting and slash-burning. In one stream below the clear-cut a distinct increase in nutrients was noted (left). In the control stream nearby nutrient content was unaffected (right). The high levels in the first stream persisted for 12 days after slash-burning. During the following two years the levels slowly decreased at varying rates for different nutrients.
² Values in mg/litre; other parameters were also measured.

Using herbicides

On the positive side, we now recognize that if certain precautions are taken it may be possible to safely utilize certain forest management chemicals, especially herbicides. For example, Tarrant and Norris, in reviewing 30 field studies of herbicides, concluded: A summary of evidence from research indicates that many herbicides and their carriers, when used in a responsible manner [our italics], can be employed in forest vegetation control with a minimum impact on water quality (Kunkle, 1973).

We therefore know that the forester must respect certain safety guidelines in the use of herbicides. The following, based on various field studies and personal observations, are suggested by the writer:

1. The rate of aerial application should generally be less than about 6 kg/ha active ingredient, and only tested and approved substances should be used. Recently developed, Unfamiliar chemicals should be avoided. Various governments issue lists of approved herbicides (Kunkle, 1973).

2. Marshy areas and areas near streams and lakes should be avoided, since surface runoff is common in these sites.; a no-spray of 50 metres or more around wet areas and streams should therefore be observed.

3. The spray droplet size from aircraft should be greater than 200 microns. A 100-micron droplet, for example, will generally blow about 10 times the distance of a 200-micron droplet in the wind. (Note: There are spray nozzle arrangements for aircraft [Kunkle and Law, 1972] and thickening agents [Kunkle, 1973] which can be added to the liquid to ensure larger droplets.)

4. The wind during spraying should be less than 8 kph, and spraying should be at as low an altitude as safety permits(e.g., a 200-micron droplet drifts 15-20 metres if released at a height of 5 metres, but from a height of 10 metres it will drift over 50 metres, in a wind at 8 kph).

5. Possible stormy days must be avoided, since storm runoff provides the greatest possibility of stream pollution.

6. Water monitoring or surveillance is especially desirable, to detect the effects of excessive pollution and to give the forester a check on the adequacy of his guidelines, so that he may improve them if necessary.

7. Other types of monitoring may be desirable; for example, the biological analysis of aquatic organisms.

8. If at all possible, it is always safer to avoid chemical use altogether, and we need much research to develop safer control methods. Most of the above principles also apply to chemicals other than herbicides.

In the light of present concern for the environment, waste disposal may become one of the most valuable uses of forest lands. There has been a flurry of interest in recent years in the utilization of forests for sewage disposal sites. The recycling of wastes on to forest lands offers certain advantages:

- Stream pollution from effluent dumping can be avoided.

- The nutrients and organic matter in wastes are useful for forest fertilization, especially for poor soils or degraded sites, such as strip-mined areas.

- The disposal of secondary effluents in forests can be more economical than building and operating expensive tertiary engineering treatment facilities.

- The hydrological regime may be improved by returning surface water to the ground where, after purification through the soil, it is once more available as a groundwater supply.

- Many soils are excellent water purifiers, and effectively remove nutrients and bacteria by serving as a "living filter." However, it is necessary to be alert to certain disadvantages, such as soil contamination by metals in the waste.

The best-known research in this subject is at Pennsylvania State University, where since 1963 the application of sewage effluent in forests (and on agricultural crops) has demonstrated that the advantages listed above are indeed attainable (Evans and Sopper, 1972). Many investigations and practical applications are under way.

The use of wastes to improve degraded soils may well offer an exceptional opportunity to control erosion, to return such sites to productive use and to prevent water pollution. For example, recent preliminary investigations have shown that the use of sewage sludge to treat severely strip-mined areas may not only allow revegetation - which otherwise can be difficult or impossible - but may quite possibly reduce the acid and other water pollutants which flow from such an area (Lejcher and Kunkle, 1972).

Garbage in the forest

The disposal of solid wastes - from plastic bottles to every imaginable sort of city refuse - often takes place on open patches of forest land. These wastes may pollute streams or lakes via groundwater. The concentrations of pollutants may be high in groundwater flowing from disposal areas. For example, the chemical oxygen demand (COD), a common index of organic matter content, in water leaching from a landfill may be 8 000 to 10 000 mg/litre according to some studies, and levels of organic pollution, cations and nitrogen may be 1 000 mg/litre or more. However, soil filtration is usually effective in taking out most of the organic pollutants (for example, 4 metres of soil removed 95 percent of COD in one study).

Chlorides, nitrates, certain metals and other inorganic substances which are not readily adsorbed on to soils may move great distances in the ground, especially in sandy soils, and eventually appear in lakes or streams.

It is therefore important to locate solid waste disposal sites in suitably drained areas and to avoid sites near streams, lakes or marshes (where groundwater is likely to emerge at the surface) or near wells or springs. Disposal sites may also be designed to reduce leaching.

Salt is commonly used in the colder climates of Europe and North America for the removal of ice and snow from highways, especially roads traversing shady forested basins. A clean stream may become contaminated with NaCl and other salts. These salt applications not only appear as pollutants in meltwater runoff, but may also seep into the groundwater, where much later in the summer they trickle into streams. For example, it has been demonstrated through research that during summer a small stream in a forested basin traversed by a highway contained five times as much salt below the highway as upstream or compared to a nearby control stream. In summer, springs below the highway were polluted with salt (Kunkle, 1972). A large-scale study in Massachusetts, United States, also showed that the groundwater salt levels over hundreds of square kilometres have increased from four to ten times since 1890, causing increasing pollution of water supplies (Motts and Saines, 1969).

The water quality of streams in forested areas is very much affected by the activities of forest management and land use. In the case of careless logging, indiscriminate chemical use or other ill-planned projects, stream pollution may result. This pollution, while usually not as concentrated as that further downstream, is nonetheless especially significant, since forested basins often represent the last areas where man enjoys clean water. Furthermore, these areas are a major source of increasingly scarce water supplies.

It is clear that forest activities can often be conducted without polluting waters, if only operations are designed to protect water bodies. The forester's role is crucial in protecting the aquatic environment and water supplies.


ANDERSON, D.A., KUNKLE, S.H. & HEDRICH, D.R. 1972. Affluence, effluence and new roles for forest hydrology in the East. National Symposium on Watersheds in Transition, p. 59-62. Urbana, Illinois, American Water Resources Association.

AUBERTIN, G.M. & J.H. 1972. Quality water from clear-cut forest land? Northern Logger and Timber Processer, 20(8): 14-15, 22-23.

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BORMANN, F.H. et al. 1968. Nutrient loss accelerated by clear-cutting of a forest ecosystem. Science, 159: 882-884.

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DUNNE, T. & BLACK, R.D. 1970. Partial area contributions to storm runoff in a small New England watershed. Water Resources Research, 6(5): 1296-1311.

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EVANS, J.O. & SOPPER, W.E. 1972. Forest areas for disposal of municipal, agricultural and industrial wastes. Paper presented at the seventh World. Forestry Congress, Argentina, 1972. (In press)

FREDRIKSEN, R.L. 1970. Comparative chemical water quality-natural and disturbed streams following logging and slash burning. Proceedings of a Symposium of Forest Land Uses and Stream Environment, School of Forestry Oregon State University, Corvallis, p. 125-137.

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HOLDEN, A.V. 1972. Present levels of mercury in man and his environment. In International Atomic Energy Agency. Mercury contamination in man and his environment, p. 133-168. Vienna. Technical Reports Series No. 137.

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HUTCHINSON, F.E. 1970. Environmental pollution from highway de-icing compounds. Journal of Soil and Water Conservation, 25: 144-146.

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KUNKLE, S.H. 1971. Sources and transport of bacterial indicators in rural streams. Document, Symposium on Interdisciplinary Aspects of Watershed Management, Montana State University, August 3-6, 1970.

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Approximate conversion factors between metric and English systems


1 centimetre = 0.3937 inch

1 inch = 2.54 centimetres

1 metre = 3.2808 feet

1 foot = 0.3048 metre

1 metre = 1.0936 yards

1 yard = 0.9144 metre

1 kilometre = 0.6214 mile

1 mile = 1.6093 kilometre


1 square centimetre = 0.155 square inch

1 square inch = 6.4516 square centimetres

1 square metre = 10.764 square feet

1 square foot = 0.0929 square metre

1 square metre = 1.196 square yards

1 square yard = 0.8361 square metre

1 square kilometre = 0.3861 square mile

1 square mile = 2.59 square kilometres

1 hectare = 2.471 acres

1 acre = 0.4047 hectare


1 cubic centimetre = 0.061 cubic inch

1 cubic inch = 16.3871 cubic centimetres

1 cubic metre = 35.315 cubic feet

1 cubic foot = 0.02832 cubic metre


1 kilogram = 2.205 pounds

1 pound = 0.4536 kilogram

1 metric ton = 1.102 short tons

1 short ton = 0.9072 metric ton

1 metric ton = 0.9842 long ton

1 long ton = 1.016 metric ton

1 metric ton = 19.684 hundredweight (of 112 pounds)

1 hundredweight (of 112 pounds) = 0.0508 metric ton

1 metric ton = 22.046 hundredweight (of 100 pounds)

1 hundredweight (of 100 pounds) = 0.04536 metric ton


1 kilogram cubic metre per cubic foot = 0.06243 pound

1 pound per cubic foot = 16.018 kilograms per cubic metre


1 square metre per hectare = 4.356 square feet per acre

1 square foot per acre = 0.2296 square metre per hectare

1 cubic metre per hectare = 14.291 cubic feet per acre

1 cubic foot per acre = 0.07 cubic metre per hectare

Forest products

Product and unit

Cubic metres

Cubic feet

1 000 board feet

Standards (Petrograd)


1 hoppus cubic foot



1 ton of 50 cubic feet (hoppus)



1 cunit

stacked volume



1 cord

stacked volume



1 stere

stacked volume



1 fathom




1 standard (Petrograd)





1 000 board feet





1 000 super feet




0 505

1 ton of 50 cubic feet






1 000 square metres (1 mm thickness)




1 000 square feet (1/8 in thickness)




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