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ŠUMARSKI LIST 13/2005 str. 202     <-- 202 -->        PDF

PRESENTATION AT THE INTERNATIONAL SYMPOSIUM
Šumarski list - SUPLEMENT (2005), 195-201


APPLICATION OF THE METHOD OF RUNOFF CURVE NUMBERS
TO EVALUATE THE EFFECT OF FOREST ON SURFACE RUNOFF


Miloslav JANEČEK*


SUMMARY: The forest, in general, can be regarded as a very efficient vegetation
cover. It enhances the water retention capacity of a catchment and
protects the soil against erosion. However, because of the extreme variability
of hydrological properties of underlying soils, especially their uppermost layers
(the forest litter), the forest may sometimes generate a considerable amount
of surface runoff causing, among other impacts, soil erosion.


The CN´-method and, in particular, the recently developed HydroCAD model
(http://www.hydrocad.net), which is based on this method, can be recommended
as tools for estimation of probable runoff from the forest catchments
up to 10 km .


Key words: forest, retention capacity, surface runoff, hydrological con


ditions


In general, forest stands are considered as the best
soil cover markedly contributing to a reduction of surface
runoff and erosion. These effects are generally based
on:


protection of soil from destructive effects of incident
raindrops


support of water infiltration into the soil


improvement of soil consistence


reduction of the transporting power of water and
surface runoff accumulation


retardation of surface runoff and washed soil.
It is to note that not all forest stands fully fulfil this
function as indicated by the evaluation of forest stands
by the method of curve numbers (CN) (Kent - 1971


et al.).


Hydrological conditions of forest stands are mainly
related with the density of the cover - vegetation, litter,
etc. Naturally, leaves, needles, twigs, bark and other
residues of vegetation on the forest soil are decomposed
to form litter from which the layer of forest floor
originates. Evaluation of the forest floor by the CN
method is as follows:


*
Miloslav Janeček, Research Institute for Soil and Water Conservation
Praha, Žabovreska 250, 156 27 Praha 5 - Zbraslav,
Czech Republic, phone: 00420 257 921 497
e-mail: janecek@vumop.cz
If appropriate cultural practices are applied in the
forest, the forest floor is porous and its infiltration and
accumulation capacity is high. To determine hydrological
conditions of any forest it is necessary to know
the hydrological group of soil, litter layer, humus layer
and its type.


According to the CN method there exist four hydrological
groups of soils on the basis of minimum rate of
infiltration into the soil without cover after long-term
saturation.


The class of hydrological conditions is determined
according to the layer of litter or forest floor and its
compaction.


If the litter layer is lower than 1 cm, the forest floor
is not considered as protected and the class of hydrological
conditions is reduced by the coefficient 0.5.


Differences in hydrological conditions are generally
expressed for forests by the CN method - see Table 3.


The above values of runoff curve numbers describe
the potential of runoff from the watershed - see the
graph in Fig. 2.


The above relationships are based on an assumption
that the ratio of runoff volume to the amount
of torrential rain equals the ratio of the volume of water
intercepted during runoff to potential volume that
may be intercepted by the soil and cover.