managed on at least a decade scale. As we consider increasingly long time scales, we must account for increasing numbers of variables that change within those scales (Table 2.2). At the scale most often addressed in watershed management, decades, three vital components of physical watershed systems are likely to respond as dependent variables: the drainage network morphology, hillslope forms, and the discharges of water and sediment. These three variables are most susceptible to management.

From a management perspective, the role played by each watershed variable within the temporal scale of the management effort must influence measurements and their interpretation. Depending on the time span, a variable might not be relevant, might change under the control exerted by other variables, or might change independently of other variables. Watershed problems are often detected by measured changes in dependent variables, particularly changes in the channel discharge of water or sediment. Effective watershed managers identify the independent controlling variables and try to manipulate them while recognizing that some variables are beyond human control but must be accounted for. For instance, on a one-to-ten time scale (the one managers most often encounter), the manager might manipulate primarily the runoff and sediment yield per unit area and hillslope morphology (managed by land use controls), and the drainage network morphology (managed by sewers or drains and building-site regulations), but can not respond to other controlling variables such as climate or geology.

Human action can sometimes modify natural relationships between the magnitude and frequency of watershed events. Sometimes such modifications are

TABLE 2.2 The Status of Drainage Basin Variables During Various Time Spans.

 

Status of Variables During Designated Time Spans

Drainage Basin Variables

Greater Than a Millennium

Decade to Centuries

Year to Decade

Time

Controlling

Not Relevant

Not Relevant

Initial Relief

Controlling

Not Relevant

Not Relevant

Geology

Controlling

Controlling

Controlling

Climate

Controlling

Controlling

Controlling

Vegetation

Responding

Controlling

Controlling

Local Relief

Responding

Controlling

Controlling

Hydrology (runoff and sediment yield per unit area)

Responding

Controlling

Controlling

Drainage network morphology

Responding

Responding

Controlling

Hillslope morphology

Responding

Responding

Controlling

Hydrology (channel discharge of water and sediment)

Responding

Responding

Responding

 

SOURCE: Modified from Schumm and Lichty, 1969.



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