Bogor, 21-22 October 2015
693 infiltration capacity of the soil. With the increased capacity of infiltration into the soil, the
water storage in the soil increased so that can be streamed over time, especially during the dry season.
Sustainable forest management is a necessity. One aspect to note is the preservation of the environmental aspects, in particular water system. Water system include water flood response,
the base flow the dry season, and water quality. Aspects of the water system are an indicator of sustainable forest management.
Low flows variability depend on drainage density, areal coverage of colluviums, slope, and percent of the channel network as first order stream Price et al.2011. Price 2011 mentioned
that geomorphology, soil, land use, and climate change controls base flow. Many studies concluded that lower base flow associated with higher watershed forest cover, forests have
high evapotranspiration rate. While other studies said that increased base flow with higher watershed forest cover due to higher infiltration rate Price, 2011. Beck et al. 2013
concluded that Base Flow Index BFI and Base Flow Recession Constant k were related to several climatic and physiographic characteristics, notably mean annual potential evaporation,
mean snow water equivalent depth, and abundance of surface water bodies. Mazvimavi et al. 2004 mentioned that Base Flow Index has positive correlation with annual rainfall r=0.7.
Stream water flows increase when transpiration decreases Schafer, 2014. Trees can reduce runoff at small catchment scale-at large scale, trees are more clearly linked to increased
precipitation and water availability Ellison et al., 2011. The aim of the research is to investigate water yield of the forests during dry season.
2. METHOD 2.1 Location
The study areas are located in Gombong for pine forest watershed and in Cepu for teak forest watershed. Administratively, Gombong is located in Kebumen district and Cepu area is
located in Blora district. Both areas are located in Central Java Province. The parent material in pine forest is volvanic while in teak forest is limestone sedimen.
2.2 Method There are 8 eight sub watersheds with different area of pine forests in Gombong. The pine
forest areas vary between 13 to 52 of sub watershed area. While in Cepu, there are 7 seven sub watersheds with different area of teak forest. The teak forest areas vary between 23 to 94
of sub watershed area. Each watershed was installed hydrological equipment for low flow measurements.
2.3 Data analysis Low flow data were measured during dry season, usually from July to September. Low flows
were analyzed at every measurement. In order to make comparison with different watershed forest area, the low flows were measured at sub watershed area. The unit measurement of low
flow is litresecondkm2 area of sub watershed. The low flows, both on pine and teak forest, were analyzed using trend line analysis with different forest cover area in sub
watersheds.
3. RESULT AND DISCUSSION
3.1 Land cover of pine forest watershed
The area of pine forested watershed in Gombong is 3,795 ha consisting of 8 sub-watersheds, namely Silengkong, Watujali, Tapak Gajah, Pasuruan, Kedung Pane, Lowereng, Kali Kemit,
Bogor, 21-22 October 2015
694 and Kedung Bulus sub watershed. Details of each sub-watershed and the land cover can be
seen in Table 1, while their distribution in the watershed can be seen in Figure 1 below. Figure 1 shows that the pine forests in Kedung Bulus sub watershed not clustered but spread
from the upstream, midstream, and downstream. Pine forests in the Kedung Bulus vary from the age of 5 five years to 30 thirty years. Dry land areas are located in the upper, middle,
lower of the watershed.
Table 1: The area and land cover of pine forest watershed
Sub Watershed
Area Forest
Garden Dryland
Paddy Settlement
km2
Silengkong 1.17
52 23
25 Watujali
1.03 49
28 23
Tapakgajah 0.55
13 25
58 1
3 Pasuruan
0.80 20
32 44
2 2
Kedungpane 3.11
31 29
37 1
2 Lowereng
11.61 33
23 33
10 1
Kali Kemit 22.75
37 24
19 8
12 Kd.Bulus
37.95 35
28 24
5 8
Figure 1: Land Cover Map of Kedungbulus sub watershed
3.2 Land cover of teak forest watershed
Land cover in teak forest watershed consists of a teak plantation forests, shrubs, paddy fields and settlements as shown in Table 3. The teak plantation forest in the watershed is divided
by Age Group KU from KU I to VII. To obtain varying forest area in a watershed then installed 7 seven hydrological stations namely Modang, Cemoro, Kejalen, Sambong,
Kendilan, Gagakan, and Ngroto sub watershed. From Table 3 it appears that the percentage of forest area varies from 23 to 94.3 of the sub-watershed. The spread of land cover can
be seen in Figure 2. Figure 2 appears that the teak forests clustered in the upper and middle sub-watershed. Paddy filed mostly located in the lower of the watershed. Shrubs are located in
the middle and upper of the watershed.
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695 Table 3: Land cover type of each sub watershed in teak forest watershed
Sub watershed
Area Forest
Shrubs Paddy field
Settlement Km2
Modang 3.38
94.3 2.3
0.19 3.21
Cemoro 13.47
91.1 6.6
0.2 2.1
Kejalen 20.14
80.9 7.9
1.3 9.9
Sambong 27.79
74.8 12
3.2 10
Kendilan 48.86
23 46.67
17.73 12.6
Gagakan 64.8
47.5 30.7
9.9 11.9
Ngroto 69.8
44.9 30.9
14.3 9.8
Figure 2: Land cover map of teak plantation forest sub watershed
3.3 Physical condition of pine forest watershed
One of the factors that determine the low flow is physical condition or watershed characteristics. The physical condition of the watershed, drainage density, slope,
geomorphology, and geology are also affect the low flow.. Watershed physical conditions of each sub-watershed in Pine forest can be seen in Table 5.
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696 Table 5: Physical condition of pine forest watershed
Sub Watershed
Area Dd
Slope Topography
km2 kmkm2
Silengkong 1.17
3.01 61
Mountainous Watujali
1.03 3.34
53 Mountainous
Tapakgajah 0.55
2.81 46
Mountainous Pasuruan
0.80 3.21
34 Hilly
Kedungpane 3.11
2.96 26
Hilly Lowereng
11.61 3.45
32 Hilly
Kali Kemit 22.75
3.18 43
Mountainous Kd.Bulus
37.95 3.27
36 Hilly
3.4 Physical condition of teak forest watershed
Physical condition of teak plantation forest watershed consist of drainage density Dd, slope, and topography. Most of the watershed shape in the sub watersheds are elongated, the Dd
vary from 1.62 to 2.18 kmkm
2,
slope vary from 15 to 32 . The detail physical condition of teak forest can be seen in Table 6.
Table 6: Physical condition of teak forest watershed
Sub watershed Area
Dd Slope
Topography Km2
kmkm2
Modang 3.38
1.62 20
Hilly Cemoro
13.47 2.07
18 Hilly
Kejalen 20.14
2.18 26
Hilly Sambong
27.79 1.98
32 Hilly
Kendilan 48.86
1.65 15
Plain Gagakan
64.8 1.86
22 Hilly
Ngroto 69.8
1.92 18
Plain
3.5 Monthly rainfall
Rainfall is a sources of water flows to river and other storages not only surface water but also ground water. Variation of the streamflow discharge depend on rainfall amount, ranfall
intensity, rainfall duration Hadisusanto, 2010. Dry season is took place when rainfall is zero or almost zero during the month. In the case of
pine forest the dry season started from June to September as can be seen in Figure 3. In case of teak forest the dry season started from July to September as seen in Figure 4.
Figure 3: Monthly rainfall at Silengkong Pine forest in 2012.
200 400
600 800
1000
1 2
3 4
5 6
7 8
9 10 11 12
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697 Figure 4: Monthly Rainfall at Ngroto Watershed Teak forest in 2012, 2013, and 2014
Based on minthly rainfall data, low flow in pine occured during July to September and low flow in teak forest occured during August to September.
3.6 Low Flow in pine forest
In order to compare with different sub watershed area, the low flow discharge has been divide by the area of the sub watershed. Therefore the unit of low flow will be Litresecondsquare
km. Low flows measurement in pine forest have been done in July and August. During September,
the low flows have not measured due to some sub watershed has no stream flow. The relationship between low flow and pine forest area can be seen from determinat
coeffisient R
2
. Based on the low flow measurement in July, R
2
various from 0.092 to 0.153 as seen in Figure 5. There is a trend that low flow increase when the forest area in the
watershed increase. The relationship between low flow and forest area on August shows the same trend as on July
but with determination coeffisien R
2
higher i.e 0.379 to 0.406 as seen in Figure 6. It means that forest cover more influence on low flow or the influence of forest cover increase
following dry season. Based on Figure 5 and 6, the influence of pine forest on low flow was 13 in July and 40
in August. It means that there are others factor influenced low flow. It could be geological layer, soil depth, slope steepness in each sub watershed. Lacey and Grayson 1998 mentioned
that there are direct and indirect influence of geological factor on base flow. The direct inluence on base flow includes groundwater flow will be stored in the rock. While indirect
influence on base flow includes soil formation. The different rock will product different soil types and soil depth influencing their water balance. Delin et al. 2007 concluded that soil
characteristic and climate are main factor to predict base flow in Minnesota. Furtheremore, Stukey 2006 made a regression between base flow and forest area, he also added others
variable such as annual rainfall, percentage of basin and percentage of settlement in the city.
3.7 Low flow on teak forest
The relationships between forest area and low flow on teak forest were similar pattern with those on pine forest. In the beginning of the dry season, the determinan coefficient R² was
lower than in the following months. During July 2012 the relationships between forest area and low flow have no pattern. The magnitude of low flow was almost same at different forest
area as seen in Figure 7. During September 2013, there is a trend that the low flow increase when forest area increase but the determinan coefficient is low, only 0.133 and 0.155.
Bogor, 21-22 October 2015
698 Figure 5: The relationship between forest area of pine and low flow a 17 July 2012, b 18 July
2012, c 19 July 2012, d 20 July 2012,
c Figure 6: The relationship between low flow and forest area of pine a 7 August 2012,
b 8 August 2012, and c 9 August 2012. There is a trend that more dry season period, the realtionship between forest area and low
flow has higher in determinat coefficient R². In July 2012, R² for pine forest is 0.092 while in August 2012, R² increase become 0.406. For teak forest, R² is 0.397 to 0.539 for July 2012 and
0.133 to 0.155 for September 2013 and 0.639 for Oct 2014.
. a
b
c d
a b
Bogor, 21-22 October 2015
699 Figure 7: The relationship between low flow and forest area of teak a 17 July 2012,
b 18 July 2012, c 19 July 2012, and d 20 July 2012
Figure 8: The relationship between low flow and forest area of teak a 11 September 2013, and b 12 September 2013
Figure 9: The relationship between low flow and forest area of teak a 27 Augst 2014, b 16 Oct 2014.
a b
a b
a b
c d
Bogor, 21-22 October 2015
700 Based on Figure 5 to 9 show that the more forest area in the watershed, the low flow will
increase both for pine and teak forest. The water produce in pine forest is higher than in teak forest. The magnitute of increasing low flow on that forests are depend on their geologic
structure, soil depth, slope steepness, litter thickness and annual rainfall. Low flow in pine forest vary from 0.5 litreseckm2 20 of forest area in the watershed to 1.5 litreseckm2
95 of forest area, while low flow in teak forest vary from 0.2 litreseckm2 20 forest area to 1 litreseckm2 90 forest area. There are some reason that pine forest produce
more water than teak forest. First, annual rainfall in pine forest was higher than those in teak forest 2747 mm and 1309 mm. Second, parent materail in pine forest is volkanic while in
teak forest is limestone sedimen. Third, the slope is more stepness in pine forest than those in teak forest.
Johnson 1998 in his review of low flows mentioned that the low flow are sustained in very dry summer mainly by supplies from the drift and solid geology so are only affected by land
use if the forest prevent sufficient winter recharge to there sources. Furtheremore, Robinson et al.2003 said that soil water under forest is drier than under grass. This reduce the soil
moisture reserves to sustain base flows in dry weather periods. Forest area still influence the magnitute of low flow. Furthermore, Krakauer Temimi 2011 predict low flow by using
stepwise multiple regression analysis. There are six predicted variable of low flow i.e.: 1 longitude, 2 soil infiltration capacity, 3 latitude, 4 channel length, 5 forest cover, and 6
precipitation. Low flow generation processes are complex and vary naturally in time and space. Forest management is only one of a number human activities that can potentially affect
a regime of watershed hydrology Pike Scherer, 2003. For future research, factors influencing low flow since the beginning dry season until the end of dry season should be
investigated in order to get the role of forest on generating low flow. Forests can have an important role in supplying fresh water, but their management must
complement water management. There are some potential ways which forest and water can be supportive.First, mountainous forested watersheds require special attention as the highest
fresh water producing area. Second, forest can be managed to enhance fresh water supllies. Third, the potential exist to mitigate the economic damage caused by flood through forest
management, Fourth, a watershed perspective should be incorporated into the planning and mangement of forests, water, urban, and agriculture landuse. Fifth, incentives must be
provided through forest and other land use management policies and institutions from local watershed to the river basin level Hofer, 2003
4. CONCLUSION AND RECOMMENDATION