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Product Details
| ISBN-13: | 9781843391098 |
|---|---|
| Publisher: | IWA Publishing |
| Publication date: | 11/01/2006 |
| Pages: | 170 |
| Product dimensions: | 6.12(w) x 9.25(h) x 0.75(d) |
Read an Excerpt
CHAPTER 1
Introduction: Watershed Basics
1.1 WATERSHED: DEFINITION and DELINEATION
1.1.1 What are watersheds?
Watersheds represent a natural way of dividing the landscape for management and planning purposes. Watersheds are generally referred to as drainage areas, and the boundaries are delineated using changes in elevation of the landscape. Most land is a part of a watershed. The mountains, valleys, plains, and streams are natural features of a landscape that direct and distribute water and are components of the watershed ecosystem. Watershed boundaries do not generally coincide with administrative, property, and political boundaries. However, watershed boundaries should be considered in community based decision making (Figure 1.1). Rivers, lakes, estuaries, wetlands, streams and the coastal portions of oceans are water bodies that are of interest in natural resource management. The status of these water bodies is closely related to upstream land use within the watershed. The actions of people living within a watershed thus affect the quality and quantity of waters that drain from it, thereby having an impact on natural systems and regional economics.
John Wesley Powell, a scientist and geographer, defined the watershed best as "that area of land, a bounded hydrologic system, within which all living things are inextricably linked by their common water course and where, as humans settled, simple logic demanded that they become part of a community." (Powell 1890).
A watershed is commonly defined as the area of land that collects precipitation in the form of rain and snow and discharges or allows it to seep into a marsh, stream, river, lake or groundwater (Figure 1.2). A watershed is defined by the area that drains to an outlet point, often starting at a ridge line in the headwater areas, and including the land that drains into streams and tributaries, to higher order rivers, and into downstream water bodies.
The watershed is increasingly gaining recognition as a hydrologic and ecological unit of natural resource planning and management. This is because of the watershed's relatively stable boundaries, the increasing use of a systems approach in natural resource planning and management, grassroots support from communities, and because a watershed approach taps into peoples' awareness of landscape features. The application of a watershed-based strategy to land management problems can aid in the understanding of natural and human-induced stresses of natural resources. The watershed can also provide a useful basis for comprehensive assessment and planning for the sustainable use of natural resources. A systems approach in a watershed context can be highly effective in natural resource planning and management compared to methods that focus on single components of a watershed system, such as a stream, lake, or a single discharge point.
1.1.2 Watershed Delineation
Delineation is the process of identifying the boundaries of the watershed and is an important step in watershed management. Watershed boundaries are often identified on elevation maps by starting at the outlet point and marking ascending contours up to the ridge line. Black (1996) suggests three rules for delineation of watersheds using contour maps: 1) Water tends to flow perpendicularly across contour lines; 2) Ridges are indicated by contour V's pointing downhill; 3) Drainages are indicated by contour V's pointing upstream. Geographic Information Systems (GIS) can be used to derive accurate watershed boundaries.
A common procedure for the rapid delineation of a watershed is to use a contour map (Figure 1.3). Start by marking the outlet or downstream point of the water body. All high points are then identified along both sides of the watercourse. Often this mapping can be done by moving upstream toward the higher points in the watershed. The watershed boundary is identified by progressively connecting these high points, starting on one side of the outlet and ending on the other, making sure that the line is perpendicular to the contours. The watershed boundary starts at the outlet and connects the ridge lines on both sides of the stream and then continues back to the outlet. The accuracy of the delineated boundaries can be checked by paying close attention to the elevation values of the contours.
Watersheds are ecological units that are nested hierarchically; that is, watersheds can be subdivided (aggregated) to lower (higher) sized watersheds. At a global scale, there are 114 major watersheds (World Resources Institute 2003) draining major rivers of the world. These watersheds can be further delineated into sub-watersheds based on their major tributaries, and further subdivided to the first order streams that are in the headwaters of a watershed. Thus watersheds are nested at various scales. At a national scale, such hierarchical classification of the watersheds is common. For example, the United States Geological Survey (USGS) system divides the U.S. (Figure 1.4) into 21 regions, 222 sub-regions, 352 accounting units, and 2,262 cataloguing units. A hierarchical hydrologic unit code (HUC) consisting of 2 digits for each level in the hydrologic unit system is used to identify any hydrologic area. The 6-digit accounting units and the 8-digit cataloguing units generally refer to a basin and subbasin. This system is defined in the U.S. Federal Information Processing Standard (FIPS) and serves as the backbone for hydrologic delineation. Each watershed in the U.S. Environmental Protection Agency's (U.S. EPA's) Surf Your Watershed program is defined by the 8-digit cataloguing unit.
1.2 WATERSHED APPROACH
Why Watersheds?
A watershed approach can be defined as an integrated framework for environmental and natural resource management that coordinates public and private efforts for decision making and planning and which considers the hydrologic cycle, ecosystem dynamics, and socioeconomic and political characteristics. There are several advantages in using a watershed approach for planning and management:
• Natural management units Watersheds are based on natural hydrology and have relatively fixed boundaries. They are often recognized as practical units for understanding the interconnections among ecology, geography, geology, and cultural features that affect land and water. While defined through hydrological means, watersheds are increasingly being used as ecological and regional units to manage resources as a part of a system.
• Better results We can expect better results when the resource interactions and usage are understood well. Since watersheds are based on natural hydrological and ecological processes, resource managers can better understand and evaluate underlying problems and conditions and develop comprehensive solutions. Such results are often better for achieving sustainable solutions than fragmented and end-of-pipe methods.
• Ecosystem-based and comprehensive A watershed approach uses all components of the system and their interactions in evaluating and assessing natural resource problems. These components also include economic and human interactions and are thus more comprehensive than a non-system approach. The watershed-based system boundary allows a better understanding of the interaction among the abiotic (soil, water, and air), biotic (plants, animals, and human), and socioeconomic (markets, technology, and other institutions) elements.
• Economic efficiency A watershed approach encourages cooperation and collaboration among government, business and citizens, and thus aims at long term, sustainable solutions. These associations can result in substantial savings in time and resources. In addition, a watershed approach identifies potential impacts throughout the system and can be used to avoid mistakes. Watershed-scale markets can also be used to achieve cost-effective reductions in pollutants.
• Public support Community awareness and participation are emphasized in a watershed approach, which develop a sense of community to achieve environmental goals. When people are involved in decision making and management, the likelihood of conflicts is reduced. A good watershed plan increases the commitment of the public in protecting and restoring their natural resources.
• Grassroots planning A watershed approach encourages "bottom-up" planning with the recognition of all stakeholders and their interests at the grassroots level. This recognition encourages public participation in decision making. A participatory process should also incorporate maximum information on the societal issues of the region to avoid bias and the dominance of private interests.
• Watershed units are readily recognized by communities, and therefore people can relate to their landscapes much more easily. A sense of belonging to a landscape can encourage community-based planning and management and voluntary efforts.
• Encourages interdisciplinary and interagency cooperation A watershed approach can encourage researchers and agencies to cooperate by providing a common framework for their efforts. Most watershed research is multidisciplinary, and this leads to a high degree of interagency cooperation in watershed-based projects.
• Administrative streamlining Watershed-based projects allow better planning and streamlining of resources, a reduction in reporting needs, and targeted financial assistance.
• Demonstration of successes There are numerous examples of success in watershed-based planning in various parts of the U.S. and the world. The projects recognize the uniqueness and advantages of this approach. Comprehensive assessment and management encourage conservation and efficient resource use, and when well planned, can be used to achieve sustainable resource use.
1.3 HYDROLOGIC CYCLE
Water is the source of all life and is often distributed unevenly, which makes it important to watershed management. The circulation of water is called the hydrologic cycle, and this is the fundamental framework of watershed management (Figure 1.5).
The continuous transfer of water from the atmosphere to the earth's surface, into the groundwater and surface water systems, and its return to the atmosphere is a process collectively called the hydrologic cycle. This cycle occurs in the biosphere and collects, purifies, and distributes the earth's water supply. Change or disruption of the cycle often results in changes in the status of landscapes and ecosystems. The hydrologic cycle can be divided into two portions: the atmospheric branch in which movement of water is in a gaseous phase, and the terrestrial branch, where water flows mainly as a liquid.
Solar energy drives the hydrologic cycle. When heated by the sun, the surface molecules of water are energized and start the process of evaporation, rising as water vapor into the atmosphere. Approximately 80 percent of the evaporation is from oceans. The direct evaporation process accounts for 84 percent of atmospheric water vapor. Water vapor is also exchanged from plant leaves through a process called transpiration, which occurs through the stomatal openings (air sacs) on the surface of plant leaves. The water vapor then collects into the clouds. Surface circulation and jet streams can move these clouds over long distances on the earth's surface.
When the atmosphere is saturated with water vapor, water changes from a gaseous to a liquid phase. As water vapour moves across the earth, it rises and cools until it condenses around atmospheric dust to form liquid water droplets, which may fall as precipitation (rain, hail, snow, sleet, freezing rain). Processes that trigger precipitation include orographic (influence of mountain ranges), frontal systems (interface between high and low pressure systems), and convective processes (heating of the earth's surface). As precipitation reaches the earth's surface and flows down through a watershed, it moves down gradient following the quickest paths. The flow travels through the drainage system formed by the movement of water in the watershed, which starts in small streams, drains into larger rivers, and eventually discharges into the ocean.
Depending on the land cover in a watershed, some water infiltrates into the ground through the soil and reaches groundwater. The groundwater consists of water that lies below the land surface in openings between soil particles, fractures, and porous rock strata. Water-bearing porous rock and soil strata are called aquifers. Groundwater can be connected through channels that allow water to flow slowly through layers of sand, gravel, and porous rock. Groundwater forms the major source of freshwater in the world and is often tapped by pumping it from wells for human uses.
Both overland flow and groundwater eventually discharge into streams that empty into the ocean or other large water bodies. Plants can take up the water or it evaporates from the surface, and thus the cycle continues.
1.4 WATERSHED COMPONENTS
The watershed is an ecosystem where biotic (plants, animals), abiotic (soils, water, air), and socioeconomic components interact. Successful watershed planning must consider all of the socioeconomic, biophysical, ecological, regional, and political elements.
The physical components of a watershed include its biophysical parts and land uses through which water drains into a riverine system. The watershed begins at the headwaters, which are usually at the highest elevation in the watershed, such as the top of a mountain range or hill. The highest elevation areas (ridge lines) mark the physical boundaries between different watersheds. Water flowing down the opposite slopes of ridge lines usually feeds different drainage basins. As water flows down to lower elevations, it gains volume and velocity and erodes soil to form well-defined creeks, brooks, and rivers. The network of the drainage in a watershed varies in structure depending on the geological history and composition of the watershed. Since headwaters are usually fed by precipitation, the headwater flows often have good water quality compared to other places in a watershed with similar land uses. Because headwaters have very small streams (first order streams), they are also the most vulnerable to human disturbances. These small streams often respond rapidly to changes in land use and disturbance conditions.
Riverine systems consist of a network of water bodies (rivers, lakes, ponds, and streams) that receive and accumulate flows from smaller streams in a watershed. The riverine system is the main flow of water exiting the watershed into a larger river system or into the ocean.
Wetlands are defined as areas saturated with water that have unique plants adapted to waterlogged conditions and which have hydric soils (saturated and anaerobic). Wetlands are important components of a watershed ecosystem that perform unique hydrologic, biological, and economic functions.
The floodplain of a river system is the flat land area of the valley floor that is adjacent to a river and which is susceptible to being inundated by water. The floodplain often serves as a natural flood and erosion control structure during regular flooding. Riparian vegetation that occurs at the water's edge filters out sediments and other contaminants from runoff before they enter the river. The dense vegetation also provides a canopy over the river, lowering the water temperature in shallow areas of the stream. In addition, during flooding, the dense vegetation in a riparian zone can regulate flows by slowing the velocity, temporarily retaining, and slowly releasing water back into the river system.
Benchmark watersheds are areas (subwatersheds or patches) within the watershed that are relatively less disturbed by human activities (Figure 1.6).
These areas can form a reference point for the potentially attainable habitat status, especially during an assessment of habitat conditions elsewhere in the watershed. Benchmark watersheds also serve as a performance target for restoring disturbed watersheds.
Biotic refuges are pockets that maintain the biodiversity of the watershed because the habitat is still undisturbed and relatively healthy. Many endangered species depend on these biotic refuges when their habitat is almost completely destroyed elsewhere.
Biological hot spots are intact patches of riverine habitat that provide critical functions to the ecosystem. They contain a rich, but threatened diversity of plant and animal life. Urbanized areas have high levels of human-induced degradation in these watershed ecosystems. Benchmark watersheds could be used as references in the restoration of biological hot spots (Doppelt et al. 1993).
Managed rural components include agricultural lands and managed forests whose operations influence watershed soils and vegetation.
The socioeconomic components of the watershed ecosystem include human components that are represented by population density and flux, resource values, markets, belief systems, employment patterns, and products and services. Community interactions, institutions, organizations, rules and regulations, and societal values are all important parts of a watershed system.
(Continues…)
Excerpted from "Watershed Management"
by .
Copyright © 2007 IWA Publishing.
Excerpted by permission of IWA Publishing.
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Table of Contents
Preface, vii,
1. Introduction: Watershed Basics, 1,
2. Land Use and Water Quality Issues, 25,
3. Inland Water Bodies, 81,
4. Coastal Watersheds, 92,
5. Biodiversity and Ecosystem Health, 106,
6. Water Assessment: Quality and Quantity, 119,
References, 137,
Glossary, 144,
Subject Index, 146,