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spatial data and must be supplied by the user. The attributes are generated using the CRWR-PrePro command Calculate Attributes (Olivera et al., 1999).

3.4.2.2 Parameter Extraction using ArcGIS and CRWR-PrePro

CRWR-PrePro uses the SCS unit hydrograph method for sub-basin routing, and lag time may be calculated either with the SCS lagtime formula or length over velocity. The SCS equation is most frequently used owing to the fact that all the parameters for this equation can easily be extracted using GIS. The SCS Lag Time Equation (1972) is shown below in Equation 3.4. In Equation 3.4 TLAG is the lag time in hours, L is the length of the longest flow path in feet, S is the slope of the longest flow path, and CN is the average curve number in the sub- basin.

TLAG =

L0.8 [(1000 / CN )9]0.7 31.67 * S 0.5

Equation 3.4

In the case of the length over velocity equation, flow velocities must be known for each cell in the subwatershed, and generally this is not readily known. As shown in Equation 3.5, TLAG is the lag time in hours, L the length of the longest flow path in feet, and v (m/s) is the representative velocity in the longest flow path. As CRWR-PrePro only operates in metric units, all English units are converted into metric units.

TLAG = 0.6 *

0.3048 * L 60 * v

Equation 3.5

49

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