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ŠUMARSKI LIST 13/2005 str. 60     <-- 60 -->        PDF

V. Tlapak. .1. Caska. J. Herynek: THE 1NFLUF.NCF OF FORESTS AND VEGETATION ON EROSION Šumarski list SUI´LEMENT (2005). 51-60
volumes of infiltration corresponding to them. Rainfalls
can be retained by forest vegetation in quantity
3-7 mm. This quantity depended on species of woods,
their age, canopy and stocking. Forest floor is able to
retain 1.5-10 mm by its thickness.


Infiltration is critical element in forest stand. Relations
inter infiltration, surface retention and interception
is represented by ratio 12:6: 1. If interception retained
3-7 mm of rainfalls, surface retention is 18—42 mm and
infiltration volume could be 36-84 mm. But there are
only theoretical values. Values are significantly affected
by actual condition of forest biocenosis and soils. Infiltration
is essential process from point of view of surface
runoff forming. Forest stands and especially forest soils
rising standard of retention capacity of small rivers and
torrents watersheds. Their retention capacity is limited
but evidentially positively affect lag times and culminations
levels. Speeds of accumulated surface run off are
0.1-3.0 m.s"1 but speeds of run off in forest floor are
usually 0.01-1.0 m.s"1.


Possibilities of forestry to intervene effectively in
runoff control are:


water retention which is achieved by rainfall interception
by vegetation cover, by rise of water infiltration
and accumulation in soil, by utilisation and
construction of flood water retention areas


rainfall runoff retardation by reduce of longitudinal
slope of watershed surface and watercourses, by rise
of soil surface roughness and cross-section areas
of flow, by reducing of conditions for surface runoff
creation
Above mentioned testify importance of infiltration.
Its quantity is affected by thickness, form, compactness
(compaction) and integrity of humus level and kind of


soil and by their water saturation value. Raised volume
of infiltration is stronger in the first 30 minutes when infiltration
intensity is 0.5-2.0 mm.rnin" and more. Volume
of precipitation can be 60 mm. Infiltration decrease
at volume from 0.3 to 1.0 mm.min´´ with longer rainfall
time. Volume of infiltration could be from 60 to 120 mm
of rainfall in interval 120 min.


We can talk about five classes of hydrological quality
of soils. This classification system is based on
thickness of humus (horizon H or A,) and litter (horizons
L and F) and on degree of erosion value. Soft degree
has volume from 1 to 5, medium from 6 to 8, hard
9 and more (see table 1).


At the first class are soils with the worst and in the
fifth class are soils with the best hydrological quality. By
the soil permeability authors define four groups of soils.


Relationship between infiltration quality of soil surface
and soil permeability is demonstrated on figure 1.


Infiltrated water is not permanently eliminated from
runoff process. Infiltrated water creates subsurface runoff
with significantly lower speeds. By this way forests
and forest soils extended times of runoff concentration
and raise probability than times of runoff concentration
will be longer than times or rain. This is premise then
peak flow is not formed by rainfalls from complete area
of watershed. Therefore forest stands and soils are important
factors in process of runoff retardation.


Negative effect at retention capacity of forest soils
has humus erosion and soil compaction by wood harvesting
and transport. Forest transport network has negative
effect too because reduce roughness coefficient,
affected surface accumulation and raise volume of runoff
coefficients.


PLUVIOLOGICAL INFILTRATION CURVES


For determination of soil hydrological characteristics
is used method of pluviological infiltration curves.
Forms of curves depend only at kind and initial state of
soil not at rain intensity.


Rainfall has kinetic energy and by this energy can
change some volumes of soil attributes. This effect is
called pluviogenous transformation of soil structure
(Kasparzak, 1989) and deformation of soil infiltration.
Pluviological infiltration curve is function of relation
between rainfall infiltration speed (VJ) and rainfall
transformation factor (W^). It applies to (see figure 2):


v, = f(Wk) = f(wkt)
(1)


Vj = actual speed of infiltration (m .s"1)


Wk = cumulated areal density of rainfall kinetic
energy (J.m~~)


wk = specific areal density of rainfall kinetic energy
(J.s´.nT2)


t = time (s)
Infiltration, surface runoff and splash erosion were
measured in following variants:


-
at bare, dry, loose, loam soil in the course of simulated
rains with various intensities
at bare, wet, loam soil with compact, wet soil crust
(result of rain and 24 hours of soil desiccation in
sunny weather)


at bare, semi humid loam soil with cracked soil
crust (result of rain and 10 days of soil desiccation
in sunny weather)


at bare, dry, loose, loam soil with dry, mechanically
loosed soil surface


Results and evaluation of measuring are mentioned
at chosen representative graphs (figures 3 and 4). On
the basis of results we can charakterize relation bet