Fundamentals of Engineering Design
! Environmental Protection Agency
! U.S. Census Bureau
- TIGER files
- TIGER Map Browser
! U.S. Fish and Wildlife Service
- National Wetlands Inventory
! NASA
! U.S. Dept. of Agriculture (USDA) - Natural Resources Conservation Service
5.1.5.1.7 Land Use
Land use may have a profound effect on hydrologic processes including runoff quantity, quality,
and timing, infiltration, and sediment transport and delivery. For this reason, the effects of land use must
be accounted for in any comprehensive watershed analysis. In many watersheds, land use changes have
directly resulted in accelerated geomorphic activity and excessive sedimentation. The effect of land use
alterations may be generalized as a change in the natural storage in a watershed. This alteration may lead
to an increase in both runoff volume and rate with an associated increase in erosion and sedimentation
potential. Even where land use changes do not result in a significant increase in upslope erosion, altered
runoff delivery may increase channel erosion and downstream sediment deposition. Changes in the
magnitude, relative proportions, and timing of sediment and water delivery result in loss of water quality via
a wide variety of mechanisms. These mechanisms include changes in channel bed material, increased
suspended sediment loads, loss of riparian habitat due to stream bank erosion, and changes in the
predictability and variability of flow and sediment transport characteristics relative to aquatic life cycles
(Waters, 1995).
There is an increasing variety of sources of land use / land cover data. The most common forms
are aerial photography and digital satellite imagery. Important considerations in selecting land use / land
cover information include scale, resolution, and applicability to current conditions (vintage). As an example,
consider Landsat Thematic Mapper satellite imagery (30 m pixels) from this decade that is available for
many parts of the country. These data are usually classified into several types of land cover including
various hardwood and evergreen forest types, agricultural land cover types, and differing densities of urban
and suburban development. For practical purposes, especially in predominately forested areas, Landsat
data may be used to differentiate evergreen from deciduous forest, grassland from most cropland, and to
identify urban centers without substantial forest canopy cover. Attempts to "push" the data beyond these
limits will often result in erroneous results. It is also important to remember that remotely-sensed land use
/ land cover information may not necessarily be representative of below-canopy processes. For example,
forested residential areas or severe disturbance to streambanks and riparian areas may be indistinguishable
from relatively pristine locations if covered by a forest canopy. Furthermore, changes in land use that may
have occurred since the imagery was taken will not be reflected in the data. Pixel size and cell averaging
techniques also limit data applicability in identifying features such as narrow riparian zones. Field
reconnaissance and recognition of the limitations of remotely sensed data through the examination of error
analyses and metadata are critical in estimating model parameters and determining appropriate action.
119
Previous Page
