Losses That Affect Runoff
Precipitation. Precipitation or rainfall is considered the main moisture delivery mechanism. This is a primary
concern to most military drainage designers. The Rational Method will be used to estimate the proportion of
rainfall that actually becomes runoff. The total amount of rainfall is not considered as a whole, because one of the
six natural losses, or a combination of losses, would subtract a portion of the total amount of rainfall. This
amount would be considerably less than the total amount of precipitation. The losses of the hydrologic cycle are
explained below:
Interception. Interception is the proportion of water that is caught by the leaves or the canopy of the trees. This
amount of water would have no effect on the runoff estimate because it would not contribute to your project. The
water that is caught by vegetation would stay in place until it is evaporated back into the air.
Infiltration. Infiltration is the proportion of water that sinks into the ground. This water may become part of the
underground water found in aquifers, or a small part may be evaporated back into the hydrologic cycle. In many
cases the losses from infiltration will be the largest.
Infiltration depends upon the following factors:
The type and condition of the soil.
The type of cover (grass, bushes, and trees) or bare soil.
The general slope in the area.
Evapotranspiration. Evapotranspiration is the combined loss of water from a given area for a time by
evaporation from the soil surface and transpiration from plants. Evaporation is the portion of rainfall that is lost
back into the atmosphere as a result of water changing into vapor. This vapor rises because it is lighter than air
and becomes another phase in the hydrologic cycle.
Detention. Detention is the accumulation of water in low spots. An example of this would be a lake or a pond.
From the detention area, the water either infiltrates into the ground, or it evaporates back into the atmosphere.
Transpiration. Transpiration is the process whereby plants extract moisture from the soil by their roots.
Eventually the plants will transpire part of the moisture back into the atmosphere as water vapor.
Runoff. The amount of water that is left after these losses extract a portion of the total amount of rainfall is called
runoff. Water left over after these losses will flow across the surface. Runoff is the amount of water that you
would have to estimate in order to design drainage structures.
INTERPRETATION OF THE HYDROGRAPHY
Precipitation includes all forms of water which fall to the earth's surface; however, the rainstorm is the primary
source of precipitation which concerns the engineer on drainage problems. Rainfall data for the locality being
considered is important in designing a drainage system that will ensure removal of surface water and control of
ground water. Most military
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EN5465
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