Abstract
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.
Highlights
In recent years, monoculture plantations have rapidly expanded in Southeast Asia, and the areas under oil palm (Elaeis guineensis) and rubber (Hevea brasiliensis) plantations are expected to increase further (Fox et al, 2012; Van der Laan et al, 2016)
We found that the percentage of forest cover in a watershed was significantly negatively correlated with C and significantly positively correlated with baseflow index (BFI), whereas the rubber and oil palm plantation cover showed the opposite pattern
Some of these parameters play an important role in controlling the initial abstraction of rainfall (e.g., CANMX), rainfall partitioning into surface runoff (e.g., CN2 and overland flow (OV_N)), and vertical movement of water through the soil (e.g., SOL_BD, SOL_K, and SOL_AWC)
Summary
Monoculture plantations have rapidly expanded in Southeast Asia, and the areas under oil palm (Elaeis guineensis) and rubber (Hevea brasiliensis) plantations are expected to increase further (Fox et al, 2012; Van der Laan et al, 2016). The conversion of tropical rainforest into oil palm and rubber plantations affects the local hydrological cycle by increasing transpiration (Ziegler et al, 2009; Sterling et al, 2012; Röll et al, 2015; Hardanto et al, 2017), increasing evapotranspiration (ET) (Meijide et al, 2017), decreasing infiltration (Banabas et al, 2008; Tarigan et al, 2016b), increasing the flooding frequency (Tarigan, 2016a), and decreasing low flow levels (Yusop et al, 2007; Adnan and Atkinson, 2011; Comte et al, 2012; Merten et al, 2016) These climatic impacts that occur due to land use change are expected to be stronger under maritime conditions, such as those in Indonesia, than under continental conditions because 40 % of the global tropical latent heating of the upper troposphere occurs over the maritime continent (Van der Molen et al, 2006)
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