Geoscience Reference
In-Depth Information
Figure 2.2
Creating a time-area histogram by dividing a catchment into zones at different travel times from
the outlet (from Imbeaux, 1892).
Note that these early studies made an assumption of
linearity
in routing the runoff. Linearity means
that the routing times for the different zones are always the same, regardless of the amount of runoff
being routed: the routing process is then mathematically a linear operation (see Box 2.1). This was an
approximation. It has been known for some centuries that flow velocities change in a nonlinear way with
flow rate or flow depth. The assumption of linearity, however, makes the computations very much easier.
It also works surprisingly well, as will be shown in Chapter 4. Any inaccuracies due to the linearity
assumption for routing the runoff are generally less than the inaccuracies associated with deciding how
much of the rainfall to route, i.e. the problem of estimating the
effective rainfalls
or
runoff coefficient
for
an event. Effective rainfall is that part of a rainfall event that is equal in volume to the runoff generated
by the event. The runoff coefficient is that proportion of the total rainfall in an event that becomes runoff.
