Agriculture Reference
In-Depth Information
is channel flow. Again, these distinctions are somewhat arbitrary and difficult to
describe quantitatively, but they are useful, conceptually and mathematically.
Soil particles are detached when the impact of raindrops or the erosive force
of flowing water is in excess of the ability of the soil to resist erosion. Sediment
particles are transported by raindrop splash and by overland flow. Deposition of soil
particles occurs when the weight of the particle exceeds the forces tending to move
it. This condition is often expressed as sediment load exceeding sediment transport
capacity.
Particles detached in the interrill areas move to the rills by splash mechanisms
and as a result of suspension and siltation in overland flow. Thus, their detachment
and movement is independent (except for morphological features of rill and channel
systems controlling length and slope of interrill areas) of processes in rill and stream
channels. The converse, however, is definitely not true; the amount and rate of water
and sediment delivered to the rills determine rill erosion rates, sediment transport
capacity in rills, and rate of sediment deposition.
6.5.2 Factors Affecting Runoff
A range of factors like climate (especially rainfall), soil, topography, and land-use
determine rates of runoff and the total volume.
6.5.2.1 Soil Type/Infiltration Rate
The soil type and the infiltration rate influence surface runoff. If the infiltration rate
is rapid, there will be little surface runoff.
6.5.2.2 Rainfall
The rainfall intensity and duration primarily contribute to peak runoff rate and total
volume. High intensity rainfall for a short duration (but higher than the time of
concentration) will result in a higher peak runoff rate. On the other hand, uniform
and moderate intensity rain for prolonged period will yield higher runoff volume.
If the climate is dry, more water will evaporate, hence less will be available for
contributing to runoff.
6.5.2.3 Vegetation
Vegetation retards the velocity of flow, thus increases infiltration opportunity. As a
result the runoff and sediment transport will be decreased.
6.5.2.4 Topography
Slope length, steepness, and shape are the topographic characteristics that most
influence peak runoff rate and total runoff volume. Sloppy lands facilitate quick
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