Interactive comment on “Minimum forest cover for sustainable water flow regulation in a watershed under rapid expansion of oil palm and rubber plantations” by Suria Tarigan et al

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.

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Mitigation options for improving the ecosystem function of water flow regulation in a watershed with rapid expansion of oil palm plantations

Sustainability of Water Quality and Ecology ◽

10.1016/j.swaqe.2016.05.001 ◽

2016 ◽

Vol 8 ◽

pp. 4-13 ◽

Cited By ~ 9

Author(s):

Suria Darma Tarigan ◽

Sunarti ◽

Kerstin Wiegand ◽

Claudia Dislich ◽

Bejo Slamet ◽

...

Keyword(s):

Water Flow ◽

Ecosystem Function ◽

Oil Palm ◽

Flow Regulation ◽

Rapid Expansion ◽

Mitigation Options ◽

Water Flow Regulation

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Minimum forest cover required for sustainable water flow regulation of a watershed: a case study in Jambi Province, Indonesia

Hydrology and Earth System Sciences ◽

10.5194/hess-22-581-2018 ◽

2018 ◽

Vol 22 (1) ◽

pp. 581-594 ◽

Cited By ~ 17

Author(s):

Suria Tarigan ◽

Kerstin Wiegand ◽

Bejo Slamet ◽

Keyword(s):

Ecosystem Services ◽

Water Flow ◽

Oil Palm ◽

Forest Cover ◽

Assessment Tool ◽

Elaeis Guineensis ◽

Swat Model ◽

Rapid Expansion ◽

Control Surface ◽

Direct Runoff

Abstract. In many tropical regions, the rapid expansion of monoculture plantations has led to a sharp decline in forest cover, potentially degrading the ability of watersheds to regulate water flow. Therefore, regional planners need to determine the minimum proportion of forest cover that is required to support adequate ecosystem services in these watersheds. However, to date, there has been little research on this issue, particularly in tropical areas where monoculture plantations are expanding at an alarming rate. Therefore, in this study, we investigated the influence of forest cover and oil palm (Elaeis guineensis) and rubber (Hevea brasiliensis) plantations on the partitioning of rainfall into direct runoff and subsurface flow in a humid, tropical watershed in Jambi Province, Indonesia. To do this, we simulated streamflow with a calibrated Soil and Water Assessment Tool (SWAT) model and observed several watersheds to derive the direct runoff coefficient (C) and baseflow index (BFI). The model had a strong performance, with Nash–Sutcliffe efficiency values of 0.80–0.88 (calibration) and 0.80–0.85 (validation) and percent bias values of −2.9–1.2 (calibration) and 7.0–11.9 (validation). We found that the percentage of forest cover in a watershed was significantly negatively correlated with C and significantly positively correlated with BFI, whereas the rubber and oil palm plantation cover showed the opposite pattern. Our findings also suggested that at least 30 % of the forest cover was required in the study area for sustainable ecosystem services. This study provides new adjusted crop parameter values for monoculture plantations, particularly those that control surface runoff and baseflow processes, and it also describes the quantitative association between forest cover and flow indicators in a watershed, which will help regional planners in determining the minimum proportion of forest and the maximum proportion of plantation to ensure that a watershed can provide adequate ecosystem services.

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