Training Course in Watershed Management

4. Effects of Watershed Management Practices on Water Resources

Manipulation of vegetation that accompanies management of natural ecosystems can affect the long-term productivity of watershed lands. Of particular concern are impacts on quantities and qualities of water that originates from upland watersheds. It is important, therefore, to recognize the environmental effects that watershed management practices frequently have on the hydrology of watershed lands.

Environmental Effects

Many watershed lands are subjected to grazing by livestock and wild ungulates, harvesting of trees and shrubs for fuel and other wood products, agricultural cultivation, and other forms of human intervention. The harvesting of trees or shrubs or converting from one vegetative type to another can enhance water supplies if the watersheds are managed properly. However, when managed improperly, these manipulations can also lead to:

• Excessive surface runoff.
• Increased soil compaction and surface erosion.
• Increased gully erosion and mass soil movement.
• Increased sedimentation in downstream channels.
• Increased export of nutrients from upland watersheds.
• Increased temperature of stream waters.

If surface runoff and downstream sedimentation increase as a result of improper land practices, the prospects for downstream flooding are also increased. Soil erosion and the export of nutrients reduces the nutrient capital and, hence, the subsequent productivities of these watersheds. Stream water temperatures and increases in nitrate, phosphorus, and other nutrients adversely affect aquatic organisms. Introduction of residues from the harvesting of trees and shrubs into stream systems can lead to higher levels of biochemical oxygen demands and reduce dissolved oxygen, also adversely affecting aquatic ecosystems.

Detrimental impacts on watershed lands should be minimized with properly planned and implemented watershed management practices. Re-vegetation of deforested areas, for example, might return watershed lands to their former conditions, although the time for recovery is longer in dryland environments than in more humid ecosystems. "Best management practices" have been established in the United States and a number of other countries as guidelines for watershed management practices that help to avoid environmental problems.

Water Yield Increase

Water yields are often increased through manipulations of vegetative cover on watersheds when one or more of the following actions are taken:

• Plant species with high consumptive use are replaced with species with lower consumptive use.
• Forests or woodlands are clearcut or thinned.
• Species with high interception capacities are replaced with species with lower interception capacities.

Opportunities for water yield improvement from upland areas are best when deep-rooted plant species are replaced with shallow-rooted species. Clearcutting or thinning of trees and shrubs, often prescribed as part of wood harvesting operations, can also increase yields of water, although the effects of these silvicultural treatments diminishes as the trees and shrubs regenerate. Reductions in interceptive losses of vegetative cover alone normally do not result in significant changes in water yields unless these changes are accompanied by reductions in transpiration rates. These reductions are attributed to the fact that soil moisture tends to be at a level below field capacity of the soil much of the time. For the most part, differences in interception and, therefore, net rainfall simply are added to the soil and transpired at a later time.

Opportunities to increase water yield in upland areas through manipulation of vegetation are related directly to annual precipitation amounts. It has been suggested that the potential for increasing water yield in the southwestern United States is realized only on sites with annual precipitation in excess of 15 to 18 inches. The greatest increases are observed in regions of higher precipitation, and where precipitation is concentrated in the cool season of the year.

Increases in water yield on upland watersheds represent on-site effects. However, the net increases in water yield diminish as distance increases between upland areas and downstream reservoirs and other areas where the water is used. These reductions in streamflow result from transmission losses in channels, evaporation of water in route, and transpiration by vegetation along the stream bank. To illustrate the magnitude of these losses, increases in water yield attributed to vegetative changes on upland watersheds in the Verde River Basin of central Arizona are reduced to less than one-half by the time water has traveled 100 miles to downstream points of use.

Consumptive use of water by phreatophytes can be substantial in lowland areas. Opportunities for salvaging groundwater through eradication of phreatophytes has been attempted in parts of the western United States. One problem with such eradication practices is that these riparian systems also have values as wildlife habitats, which can be higher than the value of groundwater that is salvaged. However, where dense plant communities have developed, such as saltcedar stands in the southwestern United States, there can be opportunities to salvage groundwater to a limited extent and, at the same time, maintain a sufficient riparian ecosystem for wildlife habitats and other purposes.

Riparian Systems

Riparian systems are valuable components of dryland environments throughout the world. Riparian systems are found in the transition between aquatic and adjacent terrestrial ecosystems, and identified largely by soil characteristics and unique vegetative communities that require free or unbounded water. Standing water and running water habitats are found in riparian systems. As relatively small areas, riparian ecosystems are diverse and unique systems that are subjected to a number of uses and, as a frequent consequence, detrimental pressures.

Riparian systems are often the only sites on a landscape that have trees and shrubs. These systems represent areas of relatively high levels of forage production and, as a result, are attractive to livestock. Riparian systems are important wildlife habitats because of their abundance of food and cover, extensive edges between different forms of vegetation, and proximity to water. These ecosystems frequently are corridors of migration for animals. Riparian plant communities stabilize stream banks, and reduce soil erosion and delivery of sediments to aquatic ecosystems.

Special care is necessary to protect riparian systems from environmental degradation because of their frequent high use by people and their livestock. Cuttings of trees and shrubs, and grazing by livestock, must be controlled to maintain protective vegetative covers. Riparian sites can be fenced where excessive livestock grazing occurs. Water might be piped to tanks located outside of the enclosure to move livestock away from sites susceptible to compaction, erosion, or concentrations of animal wastes that can degrade water quality. Construction of water developments elsewhere, salting, and herding of livestock help in protection when fencing is not feasible. Activities such as road construction and intensive, unplanned outdoor recreational use should be minimized in riparian systems.

Degraded riparian systems can be returned to a more productive status by improving the conditions on upland watersheds. One objective of these improvements is to increase streamflow, although these flows should be more stable and less variable than when the systems are degraded. Increased streamflow is often accomplished by encouraging subsurface flow rather than overland flow. Measures that increase duration of flows in stream channels also promote re-establishment and maintenance of riparian vegetation.

Gully plugs, check dams, and other small engineering structures constructed in stream channels can be used to increase durations of flows and stabilize the channels to help in the re-establishment of riparian vegetation. These structures trap and store sediments, providing water retention systems. The stored sediments become saturated following storm events and, as drainage continues, the water is released more slowly and sustained for longer periods of time than in unobstructed channels.

Optimum management of riparian systems requires consideration of both the environmental factors and economic needs of the area. It is seldom that riparian areas are managed best for only a single use. A compromise form of management usually results in the greatest value to people. Riparian systems might have to be set aside as natural areas in some instances.

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