Forest Watershed Hydrology
Forests affect the hydrology of watersheds in various and complex ways, by increasing evapotranspiration, increasing soil infiltration, intercepting cloud moisture, reducing the nutrient load of runoff, and more. Harvesting, road building, shifiting agriculture, grazing, and invasion by alien plant species may all have profound effects on the functioning of watersheds. Forests protect our watersheds in many ways, as discussed in an article from the 2003 Hawaiii Forestry News, below.
A presentation by Dr. Travis Idol reviews our current knowledge of how vegetation affects watershed function in Hawaii (originally presented at the 2006 Hawaii Association of Watershed Partnerships symposium).
Another presentation by Dr. Idol on Hydrologic Effects and of Changes in Forest Structure and Species Composition examines the science behind watershed management.
For more information on forests and watersheds, see the Hawaii Division of Forestry and Wildlife's Watershed Management Program website.
For information on coursework on forest hydrology, see the UH Manoa CTAHR Hydology Lab website.
For information on what you can do to help protect our watersheds and preserve water quality, see the CTAHR Hawaii Water Quality website.
The Hawaii Association of Watershed Partnerships brings public and private landowners together to address issues affecting the entire watershed, such as invasive species and feral ungulates. HAWP has several video slide shows on why watersheds are important and what can be done to protect them.
One of the great success stories of Hawaiian forestry is the reforestation some of the state’s critical watersheds. With today’s demands on the forest, from commercial timber to native species protection, it’s worthwhile to take a look at the role Hawaii’s forests play in protecting our watersheds.
The upper slopes above the towns of Honolulu and Lahaina were cleared of forest by the late 1800s. The trees had been cut for timber or for fuelwood for the whaling industry and local manufacturing. While the forest might have been able to recover from logging alone, it was not able to regenerate because feral animals devoured any regrowth. Overgrazing by cattle, sheep, and goats led to soil compaction and increased runoff and erosion.
Both public and private landowners worked together to restore the watersheds, as water supply was critical for both agriculture and the towns. Foresters first fenced the forest reserves, then eliminated feral animals. Plantation workers, territorial foresters, workers from the CCC, and volunteers worked to plant trees. Most of the forest visible above Honolulu consists of planted trees or their wild descendants.
Trees help prevent erosion not so much by intercepting rainfall directly as by laying down a thick mat of litter that protects the soil surface. In a mature forest, tree crowns are high enough above the soil surface that drops falling from the leaves can actually cause as much erosion as rainfall in the open. (You can see this sometimes in teak plantations where the leaf litter has been burned away and gullies have formed between the trees.) The forest leaf litter and understory plants actually do the job of breaking the force of the falling rain and protecting the soil. Undisturbed forest plantations typically show low rates of erosion. Other types of vegetation, such as orchards with groundcovers, agroforests, and well-established turf, can protect the soil as well as forests.
Over time, forests can also improve soil conditions, especially in overworked pasture or agricultural soils. Increased organic matter in the soil improves soil structure, which increases infiltration and decreases the amount of water running over the surface after heavy rainfall. Infiltration may also be improved by increased earthworm activity and the development of root channels as trees grow. Overgrazing with the concurrent soil compaction and row crops where the soil is frequently bare are more likely to lead to serious erosion, especially in susceptible soils. A plow pan of compacted soil below the plow layer may form in clay or loamy soils after years of mechanized agriculture. Plow pans significantly reduce infiltration and increase surface runoff, thereby increasing the potential for erosion and nutrient loss. In cases where site preparation for forestry breaks up plow pans, such as along the Hamakua coast and along the north shore of Kauai, infiltration increases and runoff and erosion decrease following the establishment of forest plantations. In some parts of Hawaii, of course, the hydrology is determined by the underlying rock. On the Big Island where the underlying substrate is pahoehoe lava, infiltration will be low and runoff will be high no matter what the vegetation. Conversely, soils on aa lava rock will not be susceptible to compaction even if they are heavily used. On most of Mauna Loa, there are no perennial streams because all precipitation seeps into the underlying lava rock and into the aquifer directly.
What effects does harvesting have on forested watersheds? Trees, especially fast-growing ones, use a lot of water. Evaporation and transpiration from a tropical wet forest is typically 50 to 60 inches per year. Stream flow usually increases after a forested watershed is cut because the water the trees were using now seeps into the soil and eventually into the stream. Harvesting machinery may also compact the soil in places and increase surface runoff. As the trees regrow or are replanted, stream flow returns to pre-harvest levels in a few years. Of course, if only a small area of a watershed is cut at any one time, downstream effects will be minimal. One instance where trees increase rather than decrease stream flow is in high elevation cloud forests. Here trees intercept clouds directly and moisture condensed on the leaves and branches drips onto the soil. One study on Lanai showed 60% greater precipitation under a Norfolk Island pine at the edge of a stand than out in the open.
Harvesting both removes forest cover and disturbs the soil. Most erosion during logging occurs along roads, skid trails, and landings. Here the soil is compacted and the rainfall is unable to sink in. Loggers and foresters minimize erosion during harvests by constructing culverts and drainage ditches to disperse water before it erodes the soil, locating roads where they will not become streambeds during heavy rainfall, leaving buffer strips along streams, and using other management practices to protect the land. The state Division of Forestry and Wildlife has developed a set of Best Management Practices for forestry which all forest landowners are encouraged to follow and which are required for forestry projects on state lands or for projects which receive cost-share assistance.
Alien invasive species in some cases threaten our watersheds. Miconia shades out understory plants and leaves the soil under its canopy exposed to erosion. Fountain grass encourages wildfires which kill the native vegetation protecting dryland soils.
Forests, whether planted or natural, can protect watersheds, but they need to be carefully managed. Careful management does not exclude harvesting in all cases, and does not mean that we can only plant native species. Foresters and managers must understand how forests work to protect the watersheds so that they can protect the forest.
In a word, in Hawaii the most valuable product of the forest is water, rather than wood. It follows that the conservation of its watersheds by keeping them permanently clothed with protection forests, is there the chief duty of the forester.
- Ralph Hosmer, first Territorial forester of Hawaii