論文の公開元へASSESSMENT OF SOIL EROSION AND SEASONAL WATER QUALITY CHARACTERISTICS IN SOCIAL FOREST–DOMINATED WATERSHEDS, LAMPUNG, INDONESIA
概要
（Format No. 13）
SUMMARY OF DOCTORAL THESIS
Name: Rahmah Dewi Yustika
Title: ASSESSMENT OF SOIL EROSION AND SEASONAL WATER QUALITY
CHARACTERISTICS IN SOCIAL FOREST–DOMINATED WATERSHEDS,
LAMPUNG, INDONESIA
(インドネシア・ランプン州の社会林業が支配的な流域における土壌侵食と季節的な水質特
性の評価)
The growing population in Indonesia influences land demand for agriculture that
accelerates converting land uses from forest to agricultural land. This situation provides
issues in soil erosion and water quality degradation. It is important to protect forest from
deforestation and forest degradation, as forest has function supporting biodiversity (flora
and fauna), determining hydrological characteristics, and support life for communities.
Local communities surrounding forest area have strong bonding with forest as they depend
on forest resources for living. Utilization of forest area by local communities often rise
conflict of forest tenure. Local communities that live surrounding forest area have potential
on forest management to ensure forest functions. Therefore, forest management policy for
giving legal access to local communities is a solution to use forest resources in order to
support economic growth and reduce conflicts with consideration in maintaining forest
functions.
Social forestry is the system of forest management that empowers local
communities by giving legal access to forest resources. This policy gives equity to local
communities for increasing prosperity and economic development, for balancing the
relationship between environment and dynamic social culture, and for maintaining
sustainable forest function. Besides that, legal access to forest tenure can reduce
deforestation and improve rehabilitation. In Tanggamus Regency, Lampung, Indonesia,
coffee plantation is predominant land use and cultivated by smallholder in social forestry
area. Coffee cultivation becomes one of the important sources to support economic growth.
Local community around forest in Tanggamus Regency area has legal access to use forest in
order to increase their livelihoods and welfare through community forest and forestry
partnership programs, but in other side they have to maintain forest function.
The first study assessed the water quality of Sekampung Hulu and Sangharus
Rivers in Lampung, Indonesia, based on their total suspended solids (TSS) concentrations.
Subsequently, the extent of soil erosion in the two watersheds was determined and best
management practices (BMPs) were recommended for the study area. Water sampling was
conducted in 2016 to estimate TSS levels in the two watersheds. Additionally, the Universal
Soil Loss Equation (USLE) was integrated with an ArcGIS model to evaluate soil erosion in
the watersheds. The results indicate that TSS concentrations in the Sekampung Hulu and
Sangharus Rivers ranged from 36–813 mg/L and 16–146 mg/L, respectively. The mean and
standard error of TSS concentrations in Sekampung Hulu and Sangharus Rivers were 228
± 87.5 and 69.3 ± 15.2, respectively. Statistical analysis indicated that the mean TSS
concentration for the Sekampung Hulu River was significantly higher than that for the
Sangharus River. Further, the average soil erosion rates in the Sekampung Hulu and
Sangharus watersheds were 12.5 Mg/ha/year and 5.6 Mg/ha/year, respectively. Erosion
rates greater than 10 Mg/ha/year corresponded to 21.8% and 15.5% of the total area for
Sekampung Hulu and Sangharus watersheds, respectively. The results indicated that
young coffee trees increased soil erosion rates, especially in areas characterized by
vulnerable soil. The USLE results concurred with the TSS analysis and indicated higher
erosion rates for the Sekampung Hulu watershed than the Sangharus watershed. The
simulation scenarios of BMPs were developed in this study to know the potential reduction
of soil erosion. The simulation scenarios indicated that BMPs could effectively reduce soil
erosion in the following order (from highest to lowest reduction): agroforestry > cover crops
> contour systems. Adoption of agroforestry coffee is the most effective BMP for reducing
soil erosion. Furthermore, it is crucial to raise awareness regarding the importance of this
system, especially with respect to both income generation and environmental conservation,
to encourage the adoption of agroforestry among farmers.
The subsequent study identified seasonal water quality characteristics in two
adjacent mountainous rivers (Sangharus and Sekampung Hulu Rivers) in Lampung,
Indonesia, and determined the impacts of fertilizer application on river chemistry as a
result of social forestry management. Water chemistry was measured in 2016 covering 15
parameters, including calcium (Ca), potassium (K), magnesium (Mg), sodium (Na), chloride
(Cl), nitrate (NO3), phosphate (PO4), sulfate (SO4), aluminum (Al), iron (Fe), silicon (Si),
water temperature, electric conductivity (EC), dissolved oxygen (DO), and pH. A farmers’
questionnaire survey to obtain information on fertilizer application was conducted in study
area. The water quality results indicated that seasonal patterns of Ca, K, Mg, Na, Si, Cl,
and PO4 concentrations were significantly higher in the dry season for both rivers relative
to the rainy season due to lower water discharge and therefore lower dilution during the
dry season.. The seasonal patterns of Al and Fe concentrations in both rivers showed no
significant difference. EC and NO3 were higher in the rainy season, likely linked to the
dominant timing of urea fertilizer application during the rainy season. Moreover, geological
characteristics played an important role in the Sangharus River in determining its water
quality characteristics because the watershed consisted of higher basaltic andesite tuff
compared to the Sekampung Hulu watershed. Based on the survey, it was determined that
urea application was significantly higher in the Sangharus watershed (166.8 kg/ha) relative
to the Sekampung Hulu watershed (120.3 kg/ha). By contrast, the application of mutiara
and phonska fertilizers showed no significant difference in both watersheds. Despite the
application of fertilizers in the watersheds, NO 3 levels remained below the recommended
standard. However, Al and Fe concentrations were higher than the recommended level for
drinking water, which was likely due to elevated soil erosion from improper land
management. Therefore, we recommend that effective land management policies be
implemented through the adoption of soil conservation practices such as cover cropping,
contour cropping, terracing, and agroforestry for nutrient loss prevention.
Finally, based on the results of this study, the evaluation of soil erosion and
seasonal water quality characteristics in both watersheds could potentially aid sustainable
land management in the study area. Application of BMPs in social forestry is necessary to
decrease soil erosion and improve water quality in stream water. The Batutegi Dam
reservoir is a water supply source for irrigation, drinking water, and nearby power plants,
and is located downstream of the rivers in the study area. The adoption of BMPs such as
agroforestry, cover crops, and contour cropping could maintain hydrological characteristics
and prevent reservoir from decreasing functions. Policy makers could develop regulations
about future conservation plans of applying BMPs in order to prevent water quality
degradation in the Batutegi Dam, maintain forest function, implement water resources
management, and protect fresh water resources.
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