Interpreting L-THIA's Results: Runoff Basics
Runoff
is that portion of precipitation that flows over
land surfaces toward larger bodies of water. Before
runoff can occur, rainfall must satisfy the immediate
demands of infiltration, evaporation, interception,
surface storage, surface detention and/or channel
detention. Some are very minor losses, (e.g.,
interception by a corn crop is only about 0.02
inches). However, in a forested area interception
may account for up to 25 percent
of the rainfall. For short time periods (storms)
on agricultural lands:
- rainfall
- runoff = infiltration
This
can be illustrated by a hydrograph with a steady
rainfall input:
Notice that runoff
is an approximate mirror image of infiltration
(with some additional time-lag for overland flow
travel lag).
Factors Affecting
Runoff:
There
are two broad categories of factors that control
runoff: rainfall (storm) characteristics and watershed
physical conditions. Important rainfall characteristics
include duration, amount, intensity and distribution.
Key watershed factors are:
Size
- For a fixed
return interval, as watershed size increases,
the runoff per unit area decreases. This occurs
primarily because average
rainfall amount decreases with increasing area;
secondarily, increased travel time for runoff
allows more infiltration and other losses.
Shape
- For equal-sized
watersheds, runoff decreases as overland flow
length increases. This results from the increased
time of concentration. Longer duration storms,
needed to produce runoff from all points in
watershed, have lower average intensities.
-
Topography
- Surface slopes
and roughness greatly influence runoff. Steep
slopes reduce time of concentration and detention
volume. Roughness increases surface storage
and promotes greater infiltration, both of which
decrease runoff.
-
Soils
- Watershed soils
influence infiltration and deep seepage rates.
Infiltration must be satisfied before runoff
begins.
-
Surface culture
- Modern agricultural
practices promote infiltration, slow runoff
and reduce the antecedent water content of soils
prior to a storm event.
Runoff hydrograph:
A
graph of runoff rate vs. time is called a runoff
hydrograph. The shape of a hydrograph depends
on the time distribution of rainfall and upon
watershed flow characteristics. However, most
hydrographs bear some resemblance to the "typical
shape" shown below:
The
receding limb of a hydrograph usually extends
over a longer period of time than the rising limb.
The area under the curve gives the volume runoff
(volume/time x time = volume). In this course,
we will primarily use the peak runoff rate in
our problems. Since hydrographs of previous storm
events are seldom available for small watersheds,
estimates of peak rates and/or volume must be
made using computational models rather than from
statistical analyses of past records.
|