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Hydrological impacts of thinning in a deciduous forest ecosystem PRESENTED BY HZAL SERENGIL, Y., G Ö KBULAK, F., Ö ZHAN, S., HZAL , A., SENG Ö N Ü L, K., BALCI, A.N., Ö ZYUVACI, N ISTANBUL UNIVERSITY FACULTY OF FORESTRY DEPT. OF WATERSHED MANAGEMENT
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PRESENTED BY HZAL
SERENGIL, Y., GÖKBULAK, F., ÖZHAN, S., HZAL, A., SENGÖNÜL, K., BALCI, A.N., ÖZYUVACI, N
FACULTY OF FORESTRY
DEPT. OF WATERSHED MANAGEMENT
NEED FOR THE STUDY
Turkey is located on a geography that
includes diverse ecological conditions.
The 72 millions of population is concentrated in larger cities which led to the implementation of huge water resources development projects.
Istanbul, the economic capital is one of the largest cities in Europe with a population of over eleven million
More than 15 water reservoirs are in operation around the city and new pipeline projects to carry all the water within a circle of 300 km in diameter are on the way. The annual supply of water resources doubled from 1994 to 2002 with the accelerated investments (ISKI, 2002).
The results of most paired studies have been summarized and discussed in a number of reviews starting with Hibbert (1967) and followed by Bosch and Hewlett (1982), Hornbeck et al. (1993), Stednick (1996), Sahin and Hall (1996), Vertessy (1999, 2000) and finally by Brown et al. (2005).
The common knowledge that suggests a 20 percent lower limit for cutting treatments to be able to cause any detectible change on the streamflow is valid for Belgrad Forest ecological conditions. Therefore an 11 percent thinning is not expected to cause an alteration in water yield and regime.
EXPERIMENTAL WATERSHEDS IN BELGRAD FOREST
The experimental watersheds are located in the Belgrad Forest (41° N, 28° E) which has been preserved as the only old-growth oak-beech natural forest near Istanbul (Figure 1). The climate of the watersheds and surrounding area according to Thornthwaite classification system is humid, mesothermal oceanic with a moderate soil-water deficit in summer. Long term mean annual precipitation is 1050 mm and mainly fall from October to March.
Figure 1. Location of experimental watersheds.
Table 1. Mean annual precipitation, temperature and Thorntwaite potential evapotranspiration (PotET) during the calibration period (1979-1985).
Parent materials mainly consist of carboniferous clay schists and neogene loamy, gravelly deposits. The soils are usually shallow to deep, gravelly, loamy clay in texture, rich in organic matter with medium to good permeability rates and high erodibility potentials without carbonate reaction. The mull type forest floor has an average depth of 5 cm (Ozhan, 1977). Subsurface flow is the main mechanism to feed the streams in the watersheds (Balci et al., 1986).
Topography is generally gentle, and mean elevation is around 140 m. Both watersheds are on a southern aspect adjacent to the divide which is about 3-4 km from the Black Sea, under the influence of prevailing northern maritime winds during the rainy period.
Dominant vegetation includes oak (Q. frainetto Ten., Q. cerris L.) and beech (F. orientalis L.) tree species mixed with varying amounts of Carpinus betulus L., Castanea sativa Mill., Populus tremula L., Alnus glutinosa L., Acer trautvetteri Med., Acer campestre, Ulmus campestris L., and Sorbus torminalis Crantz. (Yaltirik, 1966) with a normal crown closure.
Table 2. Some properties of experimental watersheds.
Table 3. Forest stand properties in the watersheds before treatment
(derived from Belgrad Forest; forest management plan).
Species; O:Oak, H:Hornbeam, B:Beech, M:Minor broadleaved (<10%), Overstorey/Understorey
Diameter classes (cm); a: <7.9, b: 8-19.9, c: 20-35.9, d: >36
Crown closure (0 - 1.0); 1: 0.11-0.40, 2: 0.41-0.70, 3: >0.71
2.2. Field methods
Both streams draining Watershed-I (control) and Watershed-II (treatment) were instrumented with 90° and
120° concrete sharp-crested V-notch weirs
and automatic water level recorders.
Data collection has been started in 1979 in an
attempt to study the effects of timber
harvesting upon water quality and yield.
The paired watersheds were calibrated from
1979 to 1985. In February 1986 11 percent
of the standing timber volume was removed
from the treatment watershed (Watershed-II)
by employing a standard individual selective
cutting technique. The felled timber was taken
out of the watershed through horse dragging on forest roads.
2.3. Rainfall-Runoff Process
Mean annual precipitation was 1090.5 mm during the calibration period (1979-1985), while the following year (March 1986-February 1987) after treatment it was 1124.9 mm, little more than average. Runoff coefficient of W-II was mostly higher than the W-I in the long term (0.27>0.21) (Figure 2), both highly variable ranging from 0.02 to 0.63. Coefficients were higher in the rainy years and after rainy periods of 2-3 subsequent years. The precipitation in the previous 3 hydrologic years before 1985 was lower than average.
Figure 2. Runoff
of the experimental
Figure 3. Annual hydrographs of the experimental watersheds during the calibration period (1979-1985).
The high flow period was from October to May (Table 1). The monthly precipitation was over 100 mm in the first 6 months of this period, including a nonsignificant amount of snowfall. The hydrographs reached to the peak in January, while the rainiest month was December. The general shape of the hydrographs reflected the soil water deficit in summer months, and the soil water replenishment, continued throughout the autumn.
The history of the rainfall-runoff process can be determined by correlograms (Muftuoglu, 1991; Salas, 1993). The monthly correlogram shows at least 2 months of strong, 2 months of weak linear history (r0=0.648, r1=0.644, r2=0.533, r3:=0.234, r4=0.058) which means that the runoff amount of the current month is significantly affected not only with the precipitation of this month but also the previous 4 months.
Table 4. The 0-8 lag cross correlations between monthly precipitation and streamflow.
Figure 4. The monthly variation of precipitation, streamflow (W1) and the change
in the correlation coefficient between the two experimental watersheds.
2 REGRESSION APPROACHES WERE APPLIED. DAILY AND MONTHLY.
We considered the monthly flows that run over the prediction interval as statistically significant. Because the frequency distribution of the daily flows, as mentioned in the flow regime section below, was far from being Gaussian, application of a t test procedure would not be adequate.
It is a representation of the range of values that an individual y might take on for a given xi. It incorporates the parameter uncertainty as well as the unexplained variability of y. The (1-) 100% prediction interval for a single response, given xi, is
yi t s [1 +1/n + (xi-χ)2 /SSx]1/2
s = [(SSy-b1Sxy) / n-2]1/2
yi is the best estimation of Y according to X=xi, while t is the t value for n-2 degree of freedom and exceedence probability of 0.05. χ is the mean value of X, s is the standard error of the regression (Bayazit, 1996) and Hirsch et al., 1993).
The prediction interval bands are further from the best-fit line than the confidence bands, a lot further if you have many data points. The 95% prediction interval is the area in which you expect 95% of all data points to fall (Motulsky and Christopoulos, 2003).
Figure 5. The predictions of both regression approaches for the post treatment year.
The post treatment year water surplus calculated by regression equations (observed-predicted) based on monthly and daily data were 79.28 mm and 106.11 mm, respectively.
Figure 6. The observed and predicted monthly streamflow of W-II with prediction intervals (p= 0.05).
January was the only month that observed flow exceeded the prediction interval (p<0.05).
The difference was 7.76 mm which means that at least this amount of water was
achieved by thinning, statistically. It is 2.92 percent of the annual post treatment flow.
TREATMENT NOT EFFECTIVE ON FLOW REGIME
FLOW DURATION CURVES
Figure 8. FDC for the dry months
(April-August) after the treatment.
Figure 9. FDC for the rainy months
(October-February) after the treatment.
DISCUSSION and CONCLUSIONS
The climatic condition of a region is a significant factor to determine the results of a forestry treatment.
In this study the hydrologic response of the treatment watershed to the thinning was mostly concealed in the following summer months because of the high potential ET rate in the region and also possibly stimulated growth of the thinned stands.
In published studies based on the works in USA, the largest relative changes in streamflow were observed in summer months after removal of eastern deciduous forest and western conifer forest. However, the largest absolute streamflow increases occurred during wet winter months (Jones and Post, 2004).
In this study the situation was quite different, as the summer flows were very low due to limited summer precipitation in the region. The largest absolute streamflow increases were observed in the winter months.
The 11 percent thinning was not effective enough to change the flow regime of the streams draining watersheds covered with deciduous forests, but caused a statistically significant increase on the streamflow of January in the following year.
The mechanism to explain the water surplus in the winter months in this situation might be the quicker replenishment of soil water beneath the thinned stands in fall. Owing to the 4 months long rainfall runoff history of the watersheds, the difference in the recharging time of the soil moisture might have caused the difference on the water yield in the dormant winter months.
The additional water gained by the thinning treatment was around 3 percent of the annual flow in the first year while the annual precipitation was over the average. Consequently, it is not possible to suggest such a slight treatment as a tool to be used in water resources development works. However, it can be considered as a baseline which suggests that less than 20 percent thinning might also cause a statistically detectible change in water yield.