Forest harvesting, or other reductions to vegetative cover, generally increases the average surface runoff volume and total water yield for a given area of land. These land cover alterations decrease time of concentration of flow, increase the intensity of peak flows for a given precipitation event, and incrase the frequency and intensity of extreme flow events, especially channel-forming flows. These alterations tend to deteriorate water quality by tranporting sediment and other pollutants from the landscape and increasing erosive forces within the stream channel.
Studies have shown that afforestation (re-establishment of forest cover) results in a decrease in water yield, though responses to treatment are highly variable and unpredictable. Studies also showed that afforestation produces a reduction in water yield proportionally to the growth rate of the stand.
Attempting to quantify the impact of forest harvesting on annual water yield is time consuming and expensive. It requires a long-term commitment from researchers, which necessitates sufficient funding. Research using (paired-catchment studies) since 1909 in the United States of America on the impact of forest harvesting on annual water yield has provided a reasonable understanding of the hydrologic effects of forest harvesting at the operation site or stand scale. Often, the difficulty is trying to scale up results to make quantitative predictions at the catchment scale.
Generalizing and expanding quantitative results of research from one particular watershed to other areas is difficult since hydrologic impacts depend on the types of forestry operations, catchment characteristics (e.g., vegetation, and soil types), climate, watershed size, topography, and other land-use practices. As a result, extrapolating results to other watersheds must be undertaken with care.
Paired-catchment experiments are the most appropriate, rigorous and common approach used to assess the impact of forestry operations on streamflow. The strength of this approach lies in the use of two or more catchments, one designated as the control and at least one other as the treatment. It is often difficult to measure hydrologic effects of land use practices over time because of the inherent variability of climate. This approach allows the separation of climatic effects from vegetative effects. Under this experimental design the first period of the study is called pre-treatment and is used to calibrate the watersheds and establish the relationship between the control and the treatment catchments. The second period is the treatment phase where, for example, forest cover is removed mechanically, by prescribed burning or chemical defoliants. Differences in streamflow are quantified and used to assess the impact of forest removal by comparing the observed flows in the treatment catchment and the predicted values calculated from the control catchment.
The loss of forest cover via forest harvest reduces interception of raindrops (increasing drop impact energy and soil detachment), reduces evapotranspiration, increasing the amount of water available for infiltration, soil storage, and runoff. Therefore, soil moisture capacity is reached with less rainfall, and any excess can produce surface runoff, or Hortonian flow, and increase peakflows and streamflow volumes.
In higher altitude watersheds where a much larger proportion of precipitation falls as snow, a reduction in forest cover will increase the size of the snowpack, and when temperature increases in the spring and summer, snowmelt will generate greater water yields. Additionally, larger deviations for a given year can be expected for exceptionally dry or wet years. Thus, while the hydrograph is highly dependent on climatic variations and precipitation, modifications in the area of a watershed occupied by tree cover will change the runoff hydrograph for that catchment or watershed.
Soil compaction and road construction also increase runoff. Commercial logging involving tractor skidding and high road density can increase mean peak flow rates by as much as 30% by increasing impervious area. Logging equipment will compact the soil surface and potentially cause Hortonian flow. Road networks can also intercept shallow subsurface flow and direct it more rapidly to stream channels (citations?).
In periods of low stream flow (i.e., base flow), the removal of forest cover in the riparian area can increase flows for smaller streams. Riparian transpiration can lower the water table and potentially dry up small streams. Therefore, harvesting in riparian zones can have a significant influence on the riparian zone hydrology and may increase low flows.
Approximately 20% of a forest must be harvested to have a significant measurable effect on water yield (citation?). The greatest increase in water yield generally occurs the first year after treatment. Streamflow variations are highly dependent on climate variations and antecedent moisture conditions. Under drier antecedent conditions, a forested catchment will generate much less runoff than a harvested one for a smaller rainfall. Conversely, heavy rainfalls over a longer period will generate negligible differences between the same two catchments.
In high rainfall regions changes in water yield are usually the highest but the effects of clearcutting are the shortest due to rapid afforestation. In drier regions changes in water yield are not as pronounced but are more persistent since the vegetative re-growth takes more time. Increases in water yield after forest harvesting are not equally distributed throughout the year. Precipitation tends to fall as rain in lower elevations and coastal regions, while snow is deposited in higher elevations or cold regions during the winter months. The greatest increases in water yield following logging are usually observed during the late spring to early autumn months. In general, winter months do not show major increases in stream flow after logging. Winter months are often characterized by high precipitation and snow accumulation. Therefore, either soil moisture is already fully recharged and excess rainfall is converted almost entirely to surface and subsurface runoff for both forested and harvested catchments, or snow accumulates and little runoff is generated until spring snow melt season.
There have been studies where forest harvesting resulted in decreased streamflow. In such cases, it is hypothesized that drip beneath the trees from fog and cloud interception or snowmelt adds to the total net precipitations. Under such circumstances, clearcutting could have a negative impact on annual water yield if the amount of water caught by trees is significant.
Forest harvesting can alter the timing of peak flows by de-synchronizing the snowmelt over a catchment and reducing the total peakflow. The resulting hydrograph usually shows two relatively low peaks instead of a bigger one. Such responses are attributed to early snowmelt in logged areas, followed later by snowmelt in forested areas. Similarly, it is possible to shift timing of water delivery to a channel from one period to another. For example, logging on a shaded slope could advance the timing of streamflow and resynchronize it with a fully sunlit slope. The timing of floods is also affected in harvested catchments during snowmelt, though this process is less well understood, and is often case (watershed) specific.
Full vegetation re-growth or afforestation, is defined as the return of annual water yields to pretreatment levels. This concept is very complex and tends to ignore streamflow generation and routing mechanisms on the watershed and the impact forestry operations have on those mechanisms. The response is unpredictable and in some sites, pre-treatment conditions have not returned 28 years later. Although the initial response to logging in a watershed that has been clearcut twice (with 23 years between each cut) is nearly identical, it changes rapidly in the early years. Second cuttings can recover earlier than first cuttings. Re-growth is much faster in high rainfall areas than in drier ones since post-treatment soil moisture is higher during the initial growing season. This highly varying process is not well understood, and current knowledge is often site specific.
- Bosch, J.M. and Hewlett, J.D., 1982. A review of catchment experiments to determine the effect of vegetation change on water yield and transpiration. Journal of Hydrology, 55:3-23.
- Harr, R. D., 1982. Fog drip in the Bull Run municipal watershed, Oregon. Water Resources Bulletin, 18(5):785-789.
- Harr, R. D., McCorison, F. M., 1979. Initial effect of clearcut logging on size and timing of peak flows in a small watershed in western Oregon. Water Resources Research, 15(1):90-94.
- Hibbert, A. R., 1967. Forest treatment effects on water yield. In: W. E. Sopper and H. W. Lull (Editors), International Symposium For Hydrology. Pergamon, Oxford, 813 pp.
- Horton, R. E., 1933. The role of infiltration in the hydrologic cycle. Transactions of the American Geophysical Union Fourteenth Annual Meeting. Washington, DC, pp. 446-460.
- Montheith, J. L. and Unsworth, M. H., 1990. Principles of Environmental Physics, 2nd ed., Edward Arnold by Routledge, Chapmen and Hall, New York.
- Moore, R. D. and Wondzell, S. M., 2005. Physical hydrology and the effects of forest harvesting in the Pacific Northwest: a review. Journal of the American Water Resources Association, 41:753-784.
- Stednick, J. D., 1996. Monitoring the effects of timber harvest on annual water yield. Journal of Hydrology, 176:79-95.
- Swank, W. T., Swift, L. W. and Douglass, J. E., 1988. Streamflow changes associated with forest cutting, species conversions, and natural disturbances. In W. T. Swank and D. A. Crossley (eds.) Forest Hydrology and Ecology at Coweeta. Springer Verlag, New York, pp. 297-312.