Date Published: September 15, 2015
Publisher: Public Library of Science
Author(s): Tsung-Yu Lee, Jr-Chuan Huang, Jun-Yi Lee, Shih-Hao Jien, Franz Zehetner, Shuh-Ji Kao, Chon-Lin Lee.
Fluvial sediment export from small mountainous rivers in Oceania has global biogeochemical significance affecting the turnover rate and export of terrestrial carbon, which might be speeding up at the recognized conditions of increased rainfall intensity. In this study, the historical runoff and sediment export from 16 major rivers in Taiwan are investigated and separated into an early stage (1970–1989) and a recent stage (1990–2010) to illustrate the changes of both runoff and sediment export. The mean daily sediment export from Taiwan Island in the recent stage significantly increased by >80% with subtle increase in daily runoff, indicating more sediment being delivered to the ocean per unit of runoff in the recent stage. The medians of the runoff depth and sediment yield extremes (99.0–99.9 percentiles) among the 16 rivers increased by 6.5%-37% and 62%-94%, respectively, reflecting the disproportionately magnified response of sediment export to the increased runoff. Taiwan is facing increasing event rainfall intensity which has resulted in chain reactions on magnified runoff and sediment export responses. As the globe is warming, rainfall extremes, which are proved to be temperature-dependent, very likely intensify runoff and trigger more sediment associated hazards. Such impacts might occur globally because significant increases of high-intensity precipitation have been observed not only in Taiwan but over most land areas of the globe.
The small mountainous rivers (SMRs) in Oceania are one of the well-known hot spots of global sediment export from land to ocean; they drain only ~2.5% of the global land surface but collectively transport ~40% of the annual global land-to-ocean sediment export . Fluvial sediment export is tightly related to the evolution of geomorphology [2–3], off-shore aquatic ecosystems [4–5], physical and chemical weathering [6–7], and even the occurrence of earthquakes . Recently, the scientific community has paid much more attention to it because the fate of eroded sediment is involved in the global carbon cycle, e.g. through the exhumation of carbon stored in the bedrock [9–10], fate of terrestrial biogenic organic carbon [11–12] and sequestration of carbon from different sources [13–16]. Among them, the terrestrial biogenic organic carbon, i.e. the recently-fixed carbon dioxide from the atmosphere, is of particular interest . More than one third of the organic carbon delivered to the oceans via erosion and riverine transport comes from sediment-laden rivers that drain the mountains in the western Pacific region where tropical cyclones invade frequently [18–19]. A small change in SMRs might significantly alter the global sediment budget and associated consequences. Although fluvial sediment export has high global significance, the opportunities to analyze long-term sediment export data from SMRs are scarce owing to inadequate measurements. Taiwan, as one of the most representative islands in Oceania, has relatively good hydrometric records to examine the history of fluvial sediment export.
The time series of historical daily runoff and sediment export from 16 rivers (the summation of the 16 rivers) are illustrated in Fig 2A and 2B, respectively. The patterns of annual cycles are apparent for both runoff and sediment export, peaking in the middle of each year which is mostly associated with episodic events, i.e. typhoons. Intense typhoons, generally lasting for 3–5 days, are often responsible for much of the annual runoff discharge and sediment transport of many Taiwanese rivers . The mean daily runoff in the early stage (1970–1989) was ~0.090±0.024 (variance) km3 and then increased slightly to ~0.093±0.032 km3 in the recent stage (1990–2010). Although the increment was too insignificant (p = 0.149) to pass the Student’s t-test assuming unequal variances, previous studies have demonstrated that the extreme values of runoff are actually significantly increasing . This is due to the increasing rainfall intensity of extreme events  and the increasing rainfall amount from typhoons , which is not reflected by year-round data analysis. The sediment export is apparently responsive to episodic events as illustrated in Fig 2B . Noticeably, the mean daily sediment export significantly increased (p < 0.05) from 0.42±12.5 Mt in the early stage to 0.77±90.85 Mt in the recent stage, revealing a disproportionate increase in sediment export at relatively unchanged daily runoff. As a result, transport efficiency, defined as daily sediment export divided by daily runoff, significantly increased from 1.10±7.43 Mt km-3 to 1.69±20.79 Mt km-3 (p < 0.05), indicating more sediment being delivered to the ocean per unit of runoff in the recent stage compared to the early stage (Fig 2C). The significant increases of transport efficiency are found in all the flow regimes, i.e. low-flow months, high-flow months, extremes (defined as when daily runoff is larger than the 97th percentile in a year) and high-flow months excluding the extremes (not shown). Increasing supply of sediment may be anticipated as it is well known that landslide erosion is positively correlated to rainfall intensity and cumulative rainfall amount , both showing increasing trends in the past decades [20–21]. The relations between annual island-wide sediment export and runoff in Fig 2D apparently follow two rating curves for the early (grey curve) and recent stages (red), respectively. This shows again that at any given runoff (particularly evident at runoff >30 km3) more sediment was exported in the recent stage compared to the early stage.
Episodic events, i.e. typhoons, dominate annual sediment export as shown in Fig 3B. In Taiwan, most sediment erosion and delivery occur in response to typhoon-generated floods, as evidenced by the fact that >75% of the long-term flux occurs in <1% of time . This feature is also demonstrated by a fairly good relationship between the annual sediment export and the recorded annual maximum hourly discharge for each river (Fig 4A). With this relationship, the annual sediment export from a river could be roughly estimated from maximum hourly discharge data although each river follows different relationships depending mainly on the lithology in the watershed . The slope of the regression line for Erren River (blue dashed line in Fig 4A) is steeper than the one for Gaoping River (red dashed line), reflecting the fact that Erren River drains a highly erodible lithology, mostly mudstone. Erren River is the highest sediment-yielding river in Taiwan despite the fact that it drains a low-gradient watershed with relatively low runoff . Besides, erratic sediment supply like the massive landslides triggered by Typhoon Morakot in the Gaoping watershed may deviate the annual sediment export from the regression line (the red dot having the largest y value in Fig 4A). Fig 4A clearly shows that sediment export in Taiwan is basically dominated by event discharge. The estimation of fluvial sediment export can further benefit from good estimations in landslide inventory, together with hourly discharge data [24,39]. Taiwan is facing a changing rainfall pattern, a trend where lighter rain is descending and heavier rain is ascending [40,44], which is strongly related to global warming. The top 10% bin of rainfall intensity, mostly from typhoons, has increased by about 95% for each degree Kelvin increase in global mean temperature . Besides, the pattern of typhoons invading Taiwan has also been changing. An abrupt shift in the number of typhoons influencing Taiwan has been found from 3.3 typhoons per year (1970–1999) to 5.7 per year (2000–2006), resulting from the warm sea surface temperature anomalies over the equatorial western and central Pacific . Forceful typhoons (category 4 and 5) have tended to occur more frequently in May since the year 2000. Before 2000, intense typhoons occurred in May around once per decade, but now almost once per year . Moreover, it has also been found that typhoons tend to translate more slowly, which partly explains the increasing rainfall intensity of the extreme rainfall events . A previous study has calculated the relative changes of extreme rainfall events (99.0–99.9 percentiles) from 1971–1990 to 1991–2010 using data from island-wide rain gauges in Taiwan and found an average increase of 22.6% . The changing rainfall pattern is challenging Taiwan’s government in dealing with water resource management and disaster prevention. It is important to note that rainfall intensification is not only happening in Taiwan but generally in the ‘wet’ region (e.g., low latitude and the West Pacific region) . The 21st century began with numerous unprecedented climatic extremes worldwide. Among them, the signal of rainfall intensification is more and more evident. In this study, the chain reactions from rainfall extreme to fluvial sediment export were investigated in Taiwan, which is characterized by abundant precipitation and massive mass wasting. Our analysis reveals that the island-wide daily runoff remains statistically unchanged, whereas the average daily sediment discharge increases significantly from 0.42 Mt in the early stage (1970–1989) to 0.77 Mt in the recent stage (1990–2010). Hence, stream transport efficiency (represented by daily sediment/runoff) increased from 1.10 to 1.69 Mt km-3 for the two stages, respectively. This is likely due to the exceedance of triggering thresholds as well as carry-over effects from previous events. Our results also highlight an increase of sediment supply from watersheds. In landslide-dominated regions, sediment supply is usually not a limiting factor. A previous study demonstrated that the landslide-associated erosion depth increased by ~2-order in magnitude in three reservoir watersheds in Taiwan when the average rainfall intensity of typhoon increased by 2-fold , implying a dominant ruling of nature (weather and lithology) that overwhelms the impacts of human activities. In fact, Taiwan government has been spending huge money on maintaining hillslope stability. Landslides occur not only in human-altered areas but also (much more) in the pristine regions. This seems Taiwan’s destiny having such a high rainfall intensity and highly erodible lithology. Though there is a Chinese saying, “Man always conquers Nature”, we are afraid that even the state-of-the-art engineering is still no rival for nature. Source: http://doi.org/10.1371/journal.pone.0138283