U3-L2 - PART B

Global Watersheds
& Hydrology

AIM: What are watersheds and how are they impacted by human behaviors?

I. What is a watershed?

It doesn't matter how far you live from a stream, river, or lake, you live in a watershed. Everyone resides in a watershed. A watershed is an area of land from which all water drains, running downhill, to a shared destination - a river, pond, stream, lake, or estuary. A watershed is a catchment basin that is bound by topographic features, such as ridge tops.

A watershed has three primary functions. First, it captures water from the atmosphere. Ideally, all moisture received from the atmosphere, whether in liquid or solid form, has the maximum opportunity to enter the ground where it falls. The water infiltrates the soil and percolates downward. Several factors affect the infiltration rate, including soil type, topography, climate, and vegetative cover. Percolation is also aided by the activity of burrowing animals, insects, and earthworms and plant roots..

Second, a watershed stores rainwater once it filters through the soil. Once the watershed's soils are saturated, water will either percolate deeper, or runoff the surface. This can result in freshwater aquifers and springs. The type and amount of vegetation, and the plant community structure, can greatly affect the storage capacity in any one watershed. The root mass associated with healthy vegetative cover keeps soil more permeable and allows the moisture to percolate deep into the soil for storage. Vegetation in the riparian zone affects (SEE VIDEO BELOW) both the quantity and quality of water moving through the soil.

Finally, water moves through the soil to seeps and springs, and is ultimately released into streams, rivers, and the ocean. Slow release rates are preferable to rapid release rates, which result in short and severe peaks instream flow. Storm events which generate large amounts of run-off can lead to flooding, soil erosion and siltation of streams.

Ultimately, the moisture will return to the atmosphere by way of evaporation. The hydrologic cycle (the capture, storage, release, and eventual evaporation of water) forms the basis of watershed function.

A watershed should be managed as a single unit. Each small piece of the landscape has an important role in the overall health of the watershed. Paying attention primarily to the riparian zone, an area critical to a watershed's release function, will not make up for lack of attention to the watershed's uplands. They play an equally important role in the watershed, the capture and storage of moisture. It is seamless management of the entire watershed, and an understanding of the hydrologic process, that ensures watershed health.

After reading the opening paragraphs:

Summarize the function of a watershed, and the factors that influence its health and structure.

Watershed: Area of land where all water drains into the same region, such as a lake, river, or reservoir.

II. Why does a healthy Riparian Zone help a watershed?

On your document, discuss with your partners and use these prompts to explain how a healthy Riparian Zone helps a watershed ecosystem?

A healthy riparian zone in a watershed helps Mitigate human disturbances ....(what can these be?)
Increases buffer resiliency🔗...(Protection from what?) .
Increases climate change resiliency... (what things are more protected with a stable Riparian Watershed zone?)
Increases opportunities for carbon sequestration🔗... (How does increased vegetation help the planet?)

III. Watersheds of the World

Natural Watersheds

Most natural watersheds around consist of mature forests, sometimes interspersed with open meadows. In these regions of abundant vegetation, evapotranspiration is a significant process: water evaporates from the soil and from water intercepted by the plant leaves, and transpires through pores in the leaves. Approximately 40% of precipitation in forested areas is returned to the atmosphere through evapotranspiration.

The soil in forests consists of a large amount of plants, fungi and decaying organic material that collectively acts as a sponge by soaking up moisture and encouraging infiltration. Approximately 50% of the precipitation in forests infiltrates the soil. Therefore only 10% of precipitation in natural watersheds flows across the surface into streams and other water bodies, and it is highly filtered on the way by plant roots and forest litter.

Urban Watersheds

In urban areas, on the other hand, large expanses of roads, parking lots, and roofs of buildings replace the forest and organic soils. These impervious surfaces do not allow water to soak into the ground. Consequently, infiltration in urban areas accounts for only 5 to 35% of rainfall. Evapotranspiration is also substantially reduced, to 20-35%, due to a lack of vegetation. Therefore, 30% to 70% of rainfall in urbanized watersheds runs off almost immediately into storm drains and subsequently into natural water bodies.

Since the 1950s, urban areas have increased by more than 400 percent, and are now home to 80 percent of Americans. Water resources in these areas are threatened, and understanding how urbanization affects water quantity and quality is increasingly important.

Adapted from Impacts of Urbanization on Stream Water Quantity and Quality

by Sarah Farmer, SRS Science Delivery • February 19, 2015

When forests are cleared for urban uses, the amount of water flowing through streams increases. The magnitude of the increase depends on how much of the area is covered by impervious surfaces such as roads and parking lots, how many trees remain, and other factors.
Some researchers have found that when approximately 40 percent of a watershed is impervious, the amount of runoff can double.
Urbanization affects water quality because more pollutants are produced in urban settings, and the watersheds lose the ability to hold and retain water because of the increase in impervious surfaces. Some studies suggest that water quality and stream integrity may be negatively affected when as little as 5 percent of surfaces are impervious to water.
Converting watersheds from forest to urban areas often elevates sediment and nutrient concentrations in surface waters by tens to hundreds of times.
Urban waters often contain pharmaceuticals such as antibiotics, analgesics, narcotics, and psychotherapeutics, as well as pesticides, heavy metals, pathogenic microbial populations, and organic pollutants such as PCBs.
The abundance, diversity, and health of fish, amphibians, reptiles, and invertebrates are declining due to water pollution. Aquatic animals are affected

IV. Conclusion summary

After reading this adapted summary, what connections can be made from the health and structure of an Urbanized watershed to the quality of coastal and oceanic environments? CITE EVIDENCE FROM THE WEBSITE RESOURCES

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