FLOOD00425

• County: Statewide
• Title: HYDPAR- users manual
• Date: 11/1/1978
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

excess rainfall (direct runoff) to discharge as a function of time (U.S.

Soil Conservation Service, 1972). The shape of the SCS unit hydrograph is

based on a single parameter, basin time lag (time response of runoff to

rainfall), which ia normal practise is estimated from one of several empir-

ical equations, from travel time studies, or from observed flood hydrograph

reconstitutions. The HYDPAR program computes basin lag from an tmpirical

equation recommended by the SCS.

This particular lag equation was adop~ed because its structure permits easy

modification of the untt hydrograph based on changes in land use. The lag

equation used is

Lag (hours) =

(L).8
(1900)

*

(8+1)'7
(y).5

(3-2)

~

where

L

=

hydraulic length of subbasin (that watercourse
subbasin outlet to the upstream boundary which
longest time of travel) in feet,
average subbasin land slope (determined from the HYDPAR program
based on slope data for each grid cell) in percent
(lOOO!CN) - 10, and

length
yields

from

the

y

=

8 =

eN - arithmetic average curve number

NOTE: L is input by the user in miles and converted to feet within the pro-
Rram for calculation purposes.

The subbasin or watershed CN used in the lag computation is the sase value

previously computed with the aid of the HYDPAR program; and the land slope,

y, is computed as an arithmetic average of the land slope of each cell within

the subbasin under investigation.

=

Figure 3-1, conceptually displays the data that must be available in the grid

cell data bank and the computational procedures used to determine the hydro-

logic parameters, curve number and subbasin lag time.

3-5