Impacts of eucalyptus (Eucalyptus exserta) plantation on sediment yield in Guangdong Province, Southern China—a kinetic energy approach
Introduction
Control of soil erosion in Southern China has long been an important resource management objective for sustainability of long-term site productivity and protection of aquatic ecosystems. In Guangdong Province, Southern China, recent attention is being focused on the selection of appropriate plantation species and structures for rehabilitation of highly eroded ecosystems. The provincial government is also seeking to develop prototypes on soil rehabilitation for large-scale demonstration in the province.
Impacts of forest plantations on soil erosion vary in Southern China. Studies from Guangdong Academy of Sciences (1991) and Zhang et al. (1994) showed that the presence of forest canopy and understory vegetation can significantly decrease the amount of water-contacting surface soil through canopy and understory vegetation interception, and thus, decrease the surface soil erosion. Studies from Zhou, 1997, Zhou, 1999, however, indicated that the single-layer plantation has a limited role in soil protection during early forest development, and some plantations may even accelerate soil erosion. Therefore, there is clearly a need to undertake investigations to identify the key mechanisms or factors contributing to soil erosion in plantation forests in Guangdong Province, Southern China.
Forest canopy can greatly influence soil surface erosion due to its rainfall interception. It alters both the rainfall magnitude and the kinetic energy reaching soil surface. Because of the presence of a canopy, the relationship between rainfall and surface soil erosion is different between forested and unvegetated lands. Comparable amounts of rainfall can cause significant differences in erosion between forested and unvegetated surfaces Hoover and Hursh, 1943, Fletcher, 1952, Wischmeier and Smith, 1958, Lee, 1980, Ferreira, 1985, Zhang et al., 1994, Zhou, 1995a. This difference is more directly related to the difference in the kinetic energies of throughfall and atmospheric rainfall Zhou, 1997, Zhou, 1999. This clearly indicates that the impacts of plantation forests on soil erosion might be best evaluated through a kinetic energy approach.
Studies on the determination and application of appropriate rain erosivity indices for the evaluation of soil detachment or soil splash erosion are numerous. Quansah (1981) and Morgan et al. (1998) linked soil detachment with the kinetic energy of the rain. However, Park et al. (1983) showed that the rate of soil detachment per unit area depends more closely on the momentum than on kinetic energy. Govers (1991) obtained better estimates of splash loss with the product of kinetic energy and drop circumference. Salles et al. (2000) conducted laboratory experiments with the splash cup technique, and found that although all indices containing the mass of water (i.e., D3) predicted splash detachment rates relatively well, the product of momentum and drop diameter (D4V) was slightly superior in describing splash detachment. In this study, we consider the kinetic energy approach because our prime objective is to evaluate impacts of plantation on soil erosion.
Rainfall kinetic energy depends on rainfall physics such as diameters and velocities of raindrops. Recently, interest on the estimation of diameters of raindrops has been growing. Wischmeier and Smith (1958) developed the following equation to estimate the median size of raindrops (D50, in cm) for various rainfall intensities (in cm h−1): D50=0.188P0.182. Lee (1980) studied atmospheric raindrop energy and developed an experimental equation of raindrop diameter D50 (mm) with rainfall intensity P (mm h−1): D50=1.238P0.182. Zhou (1997) studied the rainfall and throughfall energies in eucalyptus forests, and constructed an empirical equation for the estimation of the median size of raindrops Dmean or 50 (mm): Dmean=1.33+0.32lnP (P—rainfall intensities, in mm h−1). The study by Zhou (1997) also indicates that the variation of raindrop diameters with rainfall intensities can be described by a normal distribution: N(1.33+0.32lnP, 0.76+0.0022P). However, studies of the rainfall kinetic energy in relation to soil erosion in a forest are rare in Southern China (Zhou, 1997), and such studies must be conducted in order to identify suitable forest resource management strategies for soil protection and rehabilitation.
In this study, two small-scale neighboring watersheds [one completely without vegetation and the other with eucalyptus plantation (Eucalyptus exserta)] were used. Detailed description of these two watersheds will be presented in Section 2. The objectives of this study were to: (1) test if kinetic energy is a suitable indicator of soil erosion; (2) assess relationships among rainfall intensity, kinetic energy and soil erosion; (3) develop an approach for estimation of the kinetic energy; and (4) identify the key factors causing soil erosion. The management implications of our results are also discussed.
Section snippets
Research sites
The two neighboring experimental watersheds studied belong to the Xiaoliang ecological experimental station located on the coastal highland of Dianbei County, Guangdong Province, China, northern border of the tropic zone (Fig. 1). Their geographic position is 110°54′18ʺE, 212°7′49ʺN. Mean annual rainfall was 1454.5 mm during 1981–1990. Dry (October to March) and wet (April to September) seasons are quite distinct. Total rainfall during the dry and wet seasons is 29% and 71% of the annual total,
Calculations of rainfall and throughfall kinetic energy
For convenience, throughfall is divided into two parts, the “free throughfall” (FT) which is not intercepted by canopy and the “dripping throughfall” (DT) which is intercepted by canopy, with part of their intercepted water dripping to the soil. The characteristics of FT are the same as the atmospheric rainfall while those of DT are affected by atmospheric rainfall as well as forest canopy.
By definition, the total kinetic energy (E) of a population of waterdrops in a forest can be described as
Factors affecting sediment yield
Fig. 3 shows that both the SS and BL per unit rainfall (kg mm−1) in the control watershed are significantly correlated with atmospheric rainfall intensity. The correlation coefficients are 0.833 and 0.779 for SS and BL, respectively, which indicate that splashing of raindrops is the main cause of soil erosion. Linsley et al. (1975) demonstrated theoretically that erosion may be viewed as starting with the detachment of soil particles by the impact of raindrops, and the kinetic energy of the
Conclusions
Erosion may be viewed as starting with the detachment of soil particles by the impact of raindrops (Ellison, 1944). The kinetic energy of the drops can splash soil particles into the air. Linsley et al. (1975) qualitatively confirmed that raindrop splashing is the original driving force of soil erosion. This study supports those findings.
Studies on how forest canopy affects DT diameters, kinetic energy and sediment yield are rare. From this study, we have the following conclusions. The sediment
Acknowledgements
Several projects have contributed to this paper. They include CAS (KZCX2-407), NSFC (39928007), Preliminary Project of the State Key Basic Research (2001CCB00600), CAS Oversea Talent Chinese Foundation, and NSFG (010567). The authors would also like to thank Dave Zirul from Ministry of Environment, British Columbia, Canada for his valuable comments.
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