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ŠUMARSKI LIST 13/2005 str. 203     <-- 203 -->        PDF

M. Janeček: APPLICATION OF THE METHOD OF RUNOFF CURVE NUMBERS TO EVALUATE Šumarski list SUPLEMENT (2005), 195-201
Runoff usually originates after some accumulation
of rainfall, that means after an initial loss that is the
sum of interception, infiltration and surface accumulation
that was estimated to amount to 20 % of potential
retention (by experimental measurements).


This equation was derived from the above-mentioned
relationships:


(//„-0.2A)2
Hr = - [mm] Hp > 0.2A
(#,,+0.8/1)


where Hr= direct runoff (mm)
H = amount of design torrential rain,
usually maximum over 24 hours (mm)
A = potential retention (mm) expressed
by the curve number as


(CN-10)


Besides the hydrological properties of soils and
systems of land use runoff is particularly related with
soil moisture content. Three situations are applied in
the CN method: the soil is either quite dry or medium
saturated or oversaturated with water in relation with
the occurrence of preceding rainfall.


As documented by the graph of the relationship between
runoff curve numbers CN and hydrological


groups of soils, class of forest hydrological conditions
and saturation degree of soil the effects of the forest on
a reduction of surface runoff, and on erosion, may be
markedly different (Janeček - 2002).


The above graph illustrates the influence of saturation
on changes in the values of runoff curve numbers
CN, e.g. CN = 30 will decrease to CN = 15 on dry soils
and it will increase to CN = 50 on saturated soils.


Fig. 4 shows the influence of changes in runoff curve
numbers CN, describing hydrological characteristics
of forest lands, on the incipience of surface runoff.


The graph shows the precipitation amounts resulting
in the incipience of surface runoff. If e.g. CN = 30,
the precipitation amount must exceed 120 mm (to cause
surface runoff) while it is only 10 mm at CN = 80. It
also determines the water-retaining capacity of soil
and its cover for the given hydrological characteristics
described by runoff curve numbers CN.


Even though the forest can usually intercept a 4-5
times higher amount of water than the same area without
vegetation, it is necessary to take into account that
after preceding rains at full saturation of soil direct runoff
from the forest increases more times than from the
area without vegetation: the more times the more permeable
is the soil. This fact was surprising during the
last flood in Bohemia when unusually large amounts
of water ran off forest complexes because the soil was
saturated with preceding heavy rains.


CONCLUSION


In general, forests arc a very efficient soil cover
increasing the watershed retention and protecting the
soil from erosion. As the range of hydrological characteristics
of soils, and especially of their cover- litter is
large, it is necessary to be aware of the fact that large
amounts of water may leave the forest in the form of
surface runoff, causing also soil erosion.


To estimate potential runoffs from the forest in a
watershed maximally 10 km´ in size we can recommend
to use the CN method or a recently derived model
on the basis of this method HydroCAD
http://www.hydrocad.net.