Understanding Your Watershed
A good basis for managing natural resources is to understand the water environment. Water constitutes 70 percent of our environment. Some important elements include the hydrologic cycle, the watershed, and the factors that influence watershed health.
A watershed is area above and below the earth's surface that drains to an associated water resource such as a wetland, river, lake, or aquifer. Watersheds can contain numerous tributaries and ponding areas. A practical approach to water management is based on the watershed as a basic organizing unit.
The Hydrologic Cycle
Water moves through the environment in a pattern known as the hydrologic cycle. Through a process including evaporation, transpiration, condensation, surface waters become clouds, which in turn release their contents as precipitation (rain or snow).
Click to enlarge Water Cycle diagram.
Precipitation will dissipate through several routes once it reaches the earths surface. Some will be absorbed into the ground. Once the topsoil has reached its saturation point the excess may infiltrate into the underlying groundwater. Excess water may begin to run off the land as surface flow, following the force of gravity. Runoff will eventually find its way into surface waters.
Factors Influencing Watershed Health
There are four primary factors that affect the quality and function of resources in a watershed:
- water quality
- flow regime
- habitat (structure and function)
- energy source.
The term water quality refers to the physical, chemical, and biological characteristics of the water. It includes; temperature, pH, turbidity, dissolved oxygen, nutrients, presence of organic and inorganic chemicals, heavy metals, and toxic substances. In addition, macroinvertebrates, fish and plant life are considered in assessing water quality.
Water quality is impaired by land uses that contribute pollutants to groundwater or runoff. The following conditions adversely affect water quality:
- elevated nutrient inputs,
- too many solids, such as sediment,
- toxic substances, such as heavy metals, pesticides, oil, and road salt,
- high levels or organic matter,
- litter and trash along streambanks,
- low levels of dissolved oxygen, and
- alterations of stream temperature.
"Flow regime" includes water volume, floods and low flows, and water velocity.
This is determined by the watershed's physical characteristics, such as the amount of vegetative cover, impervious surface, and slope characteristics. In addition, structures like gutters, pipes, and ditches can affect flow.
Changes in any of these factors can have the following results:
- an increase in velocity may also increase the frequency and severity of flooding, accelerate channel erosion both locally and downstream, contribute to sedimentation, and alter streambed composition;
- an increase in volume when combined with higher velocity can accelerate channel erosion rates and change streambed composition; destroy habitat and disrupt stream ecology;
- a reduced infiltration into the ground, commonly called a decrease in base flow, will decrease groundwater recharge, which in turn can lower the level of surface water in surrounding lakes, streams, and wetlands.
Aquatic and nearshore habitat structure includes substrate, water depth, current velocity, spawning and nursery places, riparian stability, and habitat diversity. Changes in habitat structure and function can be caused by modifications in the shape of an area (morphology), condition of banks and upland areas, presence or absence of vegetation, and quality of substrates. Changes in habitat structure affect
- the diversity and amount of plant and animal species (species composition and abundance);
- the nesting and resting areas for aquatic and water dependent species; and
- the stream's flow or meander - from relatively curvy to straight channels.
Sources of energy include the organic material entering a water resource from the banks and upland areas, referred to as the riparian zone; the process of photosynthesis; and the seasonal pattern of available energy from sunlight.
The energy sources within a watershed depend on two processes: photosynthesis and metabolism. Photosynthesis (the conversion of light energy to chemical energy) takes place in plants. Metabolism (the use of chemical energy to sustain life forms) occurs in all living things. These processes are disrupted when light energy is not effectively transmitted, or when chemical energy in the form of organic materials and nutrients is not present in large enough quantities to sustain the food web.
The food web depends on maintaining a sufficient supply of energy in the system. Changes to the energy source within a system may have serious consequences:
- food scarcity,
- genetic deterioration of fish and animal species,
- invertebrate deficits, and
- plant and animal species redistribution.
As you consider land use changes within the watershed remember that each modification can influence one or more of these factors.
Phillips, Nancy. 1996. A Watershed Approach to Urban Runoff: Handbook for Decisionmakers, Terrene Institute in coop. with USEPA, Chicago IL. Horner, R.R., J.J. Skupien, E.H. Livingston, and H.E.
Shaver. 1994. Fundamentals of Urban Runoff Management: Technical and Institutional Issues, Terrene Institute in coop. with USEPA, Washington DC.