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PAPER C8 - Hydrological Function of Teak Forest in Cepu, Central Java
Irfan Budi Pramono
1
, Nining Wahyuningrum
1 1
Forestry Technology Research Institute for Watershed Management Jalan A. YaniPabelanP.O.Box 205 Surakarta
Corresponding Email: ibpramonoyahoo.com
ABSTRACT
Most plantation forest in Java Island is teak forest Tectonagrandis. The teak forest is planted in limestone area where water is a problem during dry season in the area. The aim of the research
is investigate the function of teak forest on reducing peak flow, increasing low flow, maintaining water quality, and continuing water flow. Sub watershed is used as a unit analysis.
Each watershed has different area of teak forest. Peak flow, low flow, and water quality, have been measured in each sub watershed. The results show that peak flow is almost constant
where the percentage of teak forest in the sub watershed is more than 53. The wider thepercentage ofteakforestsin a watershed, the greater the base flow. The highestbase
flowdoes notoccur inthe largest forest area, butoccurs in80 offorest area. The higher percentage forest area in a watershed, the water quality is better. When the forested area
reached 60 of the watershed, the parameters of water quality such as Sulphat, Chlorine, Total Dissolve Solution TDS, and turbidity become almost constant. For Nitrite and
Nitrate, it will decrease when the forested area reached 40 . In addition, the quality of water is suitable for drinking water when the forested area reached 90 of the watershed area.
When it reaches the area of 50to 60, teak forests can function optimally in regulating water system.
Keywords: teak forest, hydrological function, peak flow, low flow, water quality
1. INTRODUCTION
River flow is the result ofcomplex natural processes at the watershed scale. Conceptually watershed can be regarded as a catchment area that has three components, absorption, storage
and flow. At the damaged watershed, rainwater that falls on the surface ofthe soilis no longerable to fill water reserves reservoir in the soil, rivers overflowed during hard storm,
and water shortagedrought in the dry season. Availability of water, especially on dry land, often depends on rainfall.
Water results mainly influenced by three main factors, namely climatic conditions, soil moisture and vegetation structure. The structure of forest vegetation also affects directly or
indirectly in the water cycle, the influential indicators areas species composition, plant density, leaf area, spacious and biomass Wang et al., 2008. In addition forests also affect fluctuations
inflow and peak flow Liu et al., 2011; Stähli et al., 2011 and lowering the annual flow Stehr et al., 2010; Wang, et al., 2008. However, forest land is also capable of supplying 53 of
water supplies in the United States frontier Brown, Hobbins, Ramirez, 2008. In addition, forested area is a water filter for a better water quality in the way it protects soil
from erosion and run off, moreover in supplying ground water discharge which is provide oxygen to the stream Hairiah et al., 2012.
Bogor, 21-22 October 2015
287 Because of the importance of the role of forests mentioned above, this study aims to obtain
extensive information about the optimal teak forests in the watershed made from limestone parent material who is able to regulate the water system, particularly in affecting peak flow,
low flow, and water quality.
2. METHOD 2.1 Location and the duration of the study
During 2007 to 2014 the study was conducted in teak forests managed by Perum Perhutani Unit I Central Java which includes Forest Management Unit KPH Cepu, administratively
located in Blora, Central Java Province. Geographicaly, it is located 55600m E-563500m E and 9214000m N-9222000m N. Research area is divided into sevent sub watersheds namely
Modang, Cemoro, Kejalen, Sambong, Kendilan, Gagakan and Ngroto Sub Watershed. Ngroto sub watershed covers all of the others sub watershed.
2.2 Land cover classification
Land cover map was generated from SPOT 5of 2007.Erdas 8.5 was used in image processing, such as geo referencing and unsupervisedsupervised classification. Geo referencing process
utilizes coordinate system of WGS 84 based on ground control points. Unsupervised classification was done initially prior to field survey. Land cover type was classified into four
land cover classes, i.e. forest, shrub, paddy and settlement. Based on the unsupervised classification map and RBI map 1:25.000, field survey was conducted in 2010 to re-check the
actual land cover on the field. The checking points then were used to conduct supervised classification. To update and elaborate the landcover data World view image of 2011 was used.
Further analises was coducted using ArcMap 10.2.
2.3 Measurement ofpeak flowandlow flow
Peak flow and lowflow measurements carried out directly on each sub watershed which has different precentage on forested cover. Flow measurement method was performed by
measuring the cross-sectional area of the river and flow rate that occurred with a float method.
To generate peak flow and low flow data, measurements were taken atthe relatively the same time in the 7 watershed. This is done to obtain data on the effect of any rain-flow events to
find out the response of the existing land cover to the occurrence of rain and flow. Observations were made on any events that have the potential to produce peak flow and low
flow. Peak flowusually occurs during therainy season while low flow in dry season. Measurements carried out 2 times a day, in the morning and in the afternoon to determine the
effect of the evapotranspiration on the flow. Each measurement is done 3 to 6 repetitions. The results of measurement were divided by the sub watershed area so that the original
discharge unit litersecond becomes litersecondkm
2
. Water quality data were obtain through water samples that were collected from each sub
watershed in 2010 during rainy season simultaneously in the 7 sub watersheds. 2.4 The parameters observed
2.4.1 Forested area in each sub-watershed 2.4.2 Daily rainfall in a year
2.4.3 Flow velocity at the time of peak flow and low flow 2.4.4 Water quality parameter: water color, turbidity, TDS, COD, BOD, Fluorine, Sulphat,
Chlorine, Nitrite and Nitrate.
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2.5 Data analysis 2.5.1 Peak flow
Every pair data rainfall-peak flow were analysed by creating a graph illustrating the relationship between forest cover and peak flow. The forest area which were showing the
most minor changes in peak flow represented the almost flat line was the most optimal forested area.
2.5.2 Low flow
Low flow is calculated based ont he dataflow that occurs at the start of the dry season, which has no rain on the previous month. Further testing whether there is a difference between the
low flow rate in the morning with the low flow rate in the afternoon was coducted using mean different test analysis Independent Sample TTest. Calculations were performed using
SPSS11 module.
2.5.3 Water quality
Each sample consists of 1500 ml water then was analyzed in Laboratory of Indonesia Ministry of Health’s Laboratory, Yogyakarta. Parameters analyzed includes: water colour, turbidity,
TDS, COD, BOD, Fluorine, Sulphate, Chlorine, Nitrite and Nitrate.
3. RESULT AND DISCUSSION 3.1.1
