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Nelson et al. (1997) suggest that although distributed models are the focus of current research, lumped models are still more common and preferred because

regulatory agencies have not accepted distributed models due to

involved

in

calibrating

and

verifying

them.

Models

known

as

data

the effort reduction

(DR)

models

are

one

way

of

converting

distributed

properties

of

an

area,

such

as

slope and properties

subcatchment length, into a representative

into lumped parameters by reducing distributed value for each subcatchment (Garbrecht et al.,

1999a). GIS is a tool that more spatially distributed files for lumped models (Nelson et al., 1997).

allows the user to jump from strictly lumped models to models, in that a GIS may be used to generate input based on a distributed interpretation of the terrain

If the rainfall-runoff response of a watershed is linear, a unit hydrograph can be used to relate rainfall to runoff. Most lumped models are based on either synthetic or derived unit hydrographs. Once a unit hydrograph is determined for a watershed, then one can determine the flood hydrograph resulting from any measured or design rainfall. For both traditional and automated processes, the unit hydrograph method is commonly used to model rainfall-runoff processes. Since the systems are linear, the overall response time can be calculated as the sum of the sub-area responses (Nelson et al., 1997).

2.5

SCALE DEPENDENCY OF HYDROLOGIC PARAMETERS

Although using DEMs provides for quick analysis, there are several

disadvantages to using DEMs, which include the effect of grid size on some

15

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