There are numerous factors that affect stream flow. These may be listed as
follows:
1. Precipitation, particularly the amount of water equivalent, its distribution across a
given watershed, and the extent and duration of storms.
2. Temperature is important particularly with regard to the accumulation of ice and
snow during the winter months in temperate and polar climates.
3. Topography, specifically the slope and character of the ground.
4. Geology with regard to the relative imperviousness of the soil and the bottom of a
stream. This represents the difference between surface water runoff and ground
water.
5. Ground surface covering, specifically the extent, character, and type of
vegetation.
6. The storage of water, whether it be naturally in lakes, ponds, swamps, or marshes,
or under artificial storage in dams, in reservoirs, or within dykes.
7. The watershed itself, particularly the size, the prevailing winds, and the orientation of the watershed in relation to storms and mountains.
8. Wind intensity which will carry precipitation into or out of a basin and which,
under unusual conditions, may even control the outflow of a river.
9. Erosion and silt, which tend to fill the stream bottom causing the channels to
change and to increase flooding potential with subsequent storms.
When studying a given watershed, items 3–9 are normally considered to be
constant. There can be some allowable changes in the character and type of vegetation, and artificial storage can be provided. Within limits, erosion and silt pollution
can be controlled. Also in studying runoff, conditions during the nonfreezing time of
the year are usually considered. However, it must be recalled that the worst potential
for flooding in a stream occurs when a heavy rainfall coincides with spring snowmelt. This leaves the amount and distribution of precipitation as the most important
factor to consider when evaluating storm runoff in a given drainage basin.
In evaluating storm runoff, the greatest concern is normally the peak or maximum
rate of discharge. Normally, the peak discharge occurs at the time of concentration,
which is the longest time for flow from the farthest point in time in any given
drainage basin to reach a specific location downstream. The development of the time
of concentration, t c , is shown in Fig. 6.8. An idealized drainage basin is depicted
with a stream flowing through it. The point of determination of flow is designated as
point A. The units of time are designated as the numbers 1–5. This indicates that in
the first unit of time all of the water falling within line B will reach point A. These
lines are not necessarily arcs of a circle, but may be contoured as a function of the
slope and the character of the ground. However, for convenience they are shown as
relatively smooth lines on the figure. By the end of the second time period, all areas
within line C will contribute flow to point A. Continuing this analogy, finally, after
somewhat more than five time periods, any rainfall that occurs within the given
drainage basin area will flow to point A during that given amount of time. The time
needed for the rainfall that comes down the farthest distance from point A in the
basin to reach that point A is considered t c . As the area contributing to point A
252
D. B. Aulenbach et al.
follows:
1. Precipitation, particularly the amount of water equivalent, its distribution across a
given watershed, and the extent and duration of storms.
2. Temperature is important particularly with regard to the accumulation of ice and
snow during the winter months in temperate and polar climates.
3. Topography, specifically the slope and character of the ground.
4. Geology with regard to the relative imperviousness of the soil and the bottom of a
stream. This represents the difference between surface water runoff and ground
water.
5. Ground surface covering, specifically the extent, character, and type of
vegetation.
6. The storage of water, whether it be naturally in lakes, ponds, swamps, or marshes,
or under artificial storage in dams, in reservoirs, or within dykes.
7. The watershed itself, particularly the size, the prevailing winds, and the orientation of the watershed in relation to storms and mountains.
8. Wind intensity which will carry precipitation into or out of a basin and which,
under unusual conditions, may even control the outflow of a river.
9. Erosion and silt, which tend to fill the stream bottom causing the channels to
change and to increase flooding potential with subsequent storms.
When studying a given watershed, items 3–9 are normally considered to be
constant. There can be some allowable changes in the character and type of vegetation, and artificial storage can be provided. Within limits, erosion and silt pollution
can be controlled. Also in studying runoff, conditions during the nonfreezing time of
the year are usually considered. However, it must be recalled that the worst potential
for flooding in a stream occurs when a heavy rainfall coincides with spring snowmelt. This leaves the amount and distribution of precipitation as the most important
factor to consider when evaluating storm runoff in a given drainage basin.
In evaluating storm runoff, the greatest concern is normally the peak or maximum
rate of discharge. Normally, the peak discharge occurs at the time of concentration,
which is the longest time for flow from the farthest point in time in any given
drainage basin to reach a specific location downstream. The development of the time
of concentration, t c , is shown in Fig. 6.8. An idealized drainage basin is depicted
with a stream flowing through it. The point of determination of flow is designated as
point A. The units of time are designated as the numbers 1–5. This indicates that in
the first unit of time all of the water falling within line B will reach point A. These
lines are not necessarily arcs of a circle, but may be contoured as a function of the
slope and the character of the ground. However, for convenience they are shown as
relatively smooth lines on the figure. By the end of the second time period, all areas
within line C will contribute flow to point A. Continuing this analogy, finally, after
somewhat more than five time periods, any rainfall that occurs within the given
drainage basin area will flow to point A during that given amount of time. The time
needed for the rainfall that comes down the farthest distance from point A in the
basin to reach that point A is considered t c . As the area contributing to point A
252
D. B. Aulenbach et al.