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ŠUMARSKI LIST 3-4/2009 str. 60     <-- 60 -->        PDF

L. Butorac, V.Topić, A. Jazbec, G. Jelić: POVRŠINSKO OTJECANJE OBORINAI GUBICI TLA...Šumarski list br. 3–4, CXXXIII (2009), 165-174
seasons. The cold part of the year is responsible for 63 % of the mean annual precipitation
quantity.


A stationary forest experiment, set up in August 2002, involved a burnt area of
Aleppo pine growing on colluvial soil with predominant stone detritus at an altitude
of 83 m and slope of 30° (GPS coordinates: N 43° 31’, E 16° 22’). Research encompassed
a period spanning from August 2002 to the end of December 2005.


Research was centered round a 20 m x 5 m experimental plot set up parallel with
the slope. The plot was furnished with an erosion collection tank and fenced off with
a tin fence. The positioning of the fence prevented the reception of lateral water
and/or soil suspension, as well as uncontrolled water or soil suspension loss. Precipitation
quantity, precipitation intensity and surface runoff were measured in the
plot, and so was the quantity of erosion sediment as the basic indicator of the effect
of forest vegetation and vegetation in general. A pedological profile opened in the
plot provided soil samples that were analyzed in the laboratory.


The measure of the association between surface runoff, soil loss and precipitation
characteristics was estimated with Pearson’s correlation coefficient (Sokal, 1995). A
significance level of 5 % was considered statistically significant for all statistical
analyses. All statistical analyses were made using the STATISTICA 7.1 software
(StatSoft, Inc. (2006).


During the study period (2002 – 2005) there were 265 rainy days with total precipitation
of 3,244.1 mm. Mean annual precipitation reached 973.2 mm. Surface runoff
and soil losses were recorded in 63 rainy days. They were caused by 24
low-intensity precipitation events, 20 moderate intensity events, 9 high intensity
events and 2 very high intensity precipitation events. No data on precipitation intensity
for 8 days with runoff are available because there was no ombrograph in the plot
at the very beginning of monitoring.


Surface runoff was caused by precipitation between 8.2 mm (high intensity) and


133.7 mm (very high intensity). Precipitation less than 10 mm caused surface runoff
in only two cases. In almost 50 % of the cases the quantity of surface runoff was
equal to or less than 0.1 mm/m2.
Total surface runoff in this plot for the study period (2002–2005) was 14.6 mm/m2;


(4.71mm/m2in 2002, 2.08 mm/m2in 2003, 4.40 mm/m2in 2004 and 3.41 mm/m2in
2005). Monthly values of surface runoff ranged from zero to maximal 3.03 mm/m2in
September 2002. The mean surface runoff coefficient was 0.0042. It is evident that the
surface runoff percentage in the burnt area on skeletal colluvial soil was low and that interception,
evaporation and infiltration of water in the soil reached as much as 99.6 %.
The total erosion sediment quantity in the study area in the period 2002 to 2005
was 11.22 g/m2; (10.83 g/m2in 2002, 0.016 g/m2in 2003, 0.123 g/m2in 2004, and


0.246g/m2in 2005). Monthly soil loss values ranged from zero in summer months to
maximal 9.50 g/m2in September 2002. The surface runoff coefficient ranged from
0.0020 to maximal 0.1165 in erodible days. The highest surface runoff and soil loss
was concentrated in autumn and winter months, which coincides with the seasonal
distribution of precipitation in the study area.
The burnt area is covered with natural grass vegetation. In contrast, Aleppo pine
regenerates poorly and occurs only in traces. It is the grass cover, occurring as early
as the first year after the fire, and the features of the soil in which the plot was set up
that are responsible for such low surface runoff and soil loss values. The grass cover
protects the soil from raindrops hitting the ground, while its dense roots guard it
against erosion. Such low soil losses can also be attributed to the geological and pedological
characteristics of the plot, which was set up in distinctly skeletal colluvial
soil that is exceptionally water permeable and acts as a “sieve”, thus making the losses
negligible or almost non-existent. Surface runoff and soil erosion in such soil and
vegetation are not distinct.


Key words:soil erosion, surface runoff, precipitation, colluvial soil, karst,
Aleppo pine, burnt area