Journal cover Journal topic
Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/hess-2017-116
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
29 May 2017
Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Hydrology and Earth System Sciences (HESS).
Minimum forest cover for sustainable water flow regulation in a watershed under rapid expansion of oil palm and rubber plantations
Suria Tarigan1, Kerstin Wiegand2, and Sunarti3 1Department of Soil Sciences and Natural Resource Management, Bogor Agricultural University, Indonesia
2Department of Ecosystem Modeling, University of Göttingen, Büsgenweg 4, 37077 Göttingen, Germany
3Faculty of Agriculture, University of Jambi, Jambi, Indonesia
Abstract. In many tropical regions, rapid expansion of monoculture plantations has led to a sharp decline of forest cover, which potentially degraded the water flow regulation function of watersheds. The flow regulation function of a watershed is defined as the ability of the watershed to store the rain water, therefore reducing the direct runoff and sustaining the baseflow during dry season. In the tropical region where rainfall is highly seasonal, water flow regulation is an important ecosystem function of a watershed. It determines the proportion of direct runoff of the rainfall and the proportion of the baseflow in the streamflow. The higher the proportion of the direct runoff of the rainfall the higher the probability that water resources problems occur such as flooding in the wet season and drought in the dry season. Therefore proper water flow regulation function of a watershed is a key factor for water resources management. It is generally known that forest land use improves the water flow regulation function of a watershed. The contribution of forest land use on water flow regulation function of a watershed depends primarily on its proportion in the entire watershed. In a watershed where expansion of agricultural plantations occurs rapidly, the spatial planner needs to know the minimum proportion of forest cover required to maintain proper water flow regulation function of a watershed. Research dealing with this issue is still rare, especially in the tropical area where oil palm expansion occurs at alarming rate. We employed the SWAT hydrological model to calculate two indicators of water regulation function of a watershed: the proportion of the direct runoff to the rainfall (C) and the proportion of the baseflow in the total streamflow (BFI). Using regression analysis, we show a strong correlation between indicators of water flow regulation (C and BFI values) with the proportion of forest cover and agricultural plantation cover in a watershed. To achieve the required C value of less than 0.35, the proportion of forest cover in the entire watershed should be greater than 30 % and the proportion of plantation cover should be less than 40 %. The results of this study are very useful as a guide for spatial planners to determine the minimum proportion of forest conservation area to maintain a sustainable ecosystem service of water flow regulation in a watershed.

Citation: Tarigan, S., Wiegand, K., and Sunarti: Minimum forest cover for sustainable water flow regulation in a watershed under rapid expansion of oil palm and rubber plantations, Hydrol. Earth Syst. Sci. Discuss., https://doi.org/10.5194/hess-2017-116, in review, 2017.
Suria Tarigan et al.
Suria Tarigan et al.
Suria Tarigan et al.

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Short summary
In respect of the rapid expansion of the oil palm plantation and its negative impact on water flow regulation, the government in Indonesia is questioning the minimum proportion of the forest cover and the maximum area of the oil palm in a watershed for acceptable water flow regulation of the watershed. This paper will be the first publication to answer this very crucial question and to help spatial planner to manage scientifically based land cover in a watershed.
In respect of the rapid expansion of the oil palm plantation and its negative impact on water...
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