Many studies have indicated that discharge is the key factor affecting the amount of sediment load in the rivers [44
]. In order to compare the characteristics of sediment transport in different watersheds, we first calculated the annual streamflow which is calculated as the average of the daily streamflow in a year, and the annual sediment load which is annual-averaged daily sediment load which is the average of daily sediment load in a year calculated as the daily measured streamflow multiplied by the daily sediment concentration estimated by the best fitted regression model (Table 6
). Then, the annual unit-area sediment load is calculated by dividing the annual sediment loads at each gauging station by its drainage area, and also the annual unit-discharge sediment load by dividing the annual sediment loads by its annual discharge.
shows the temporal variation in annual streamflow, annual sediment load, unit-area sediment load and unit-discharge sediment load in the five river basins from 2000 to 2017. The length of the box represents the variation of streamflow and sediment load in Taiwan of the specific year. Due to fewer typhoons during 2002–2003, 2010–2011 and 2014–2015, the annual streamflow was lower than other years (Figure 6
a). Consequently, the annual sediment load, unit-area sediment load and unit-discharge sediment load (Figure 6
b,d) were also low in those years. In contrast, the high values of streamflow and sediment load were found in years that had significant typhoon events. For example, the top three highest annual streamflows were found in 2001, 2007 and 2008 (Figure 6
a), while the top three highest sediment loads and unit-area sediment loads (Figure 6
b,c) were found in 2008, 2009 and 2017. For unit-discharge sediment load, the top three highest values were found in 2008, 2009 and 2017. The significant typhoon events that brought high values of streamflow or/and sediment loads include typhoon Toraji and Nari in 2000, Sepat and Krosa in 2007, Fungwong, Sinlaku and Jangmi in 2008, Morakot in 2009, Saola in 2012, Soulik and Usagi in 2013, and Haitang in 2017. The average unit-area sediment load in Taiwan during 2000–2017 ranged between 1.80 and 61.11 ton/km2
with the maximum value of 313.78 ton/km2
at STN_3 in the Zhuoshui River basin in 2017, while the average unit-discharge sediment load ranged between 0.99 and 13.58 kg/m3
with the maximum value of 60.26 kg/m3
at STN_3 in the Zhuoshui River basin in 2009 due to the invasion of typhoon Morakot during which heavy rainfalls brought abundant sediments. The time lag between the peak flow and peak sediment shown in Figure 6
a,b may be linked with the occurrence of hysteresis effect, when the suspended sediment concentration values were different during the rising and falling limb for the same discharge [45
]. Therefore, if the instant discharge and suspended sediment concentration were measured during the rising limb of water discharge, the resultant sediment load may be overestimated. In contrast, the sediment load may be underestimated if the sediment rating curve was constructed using the instant discharge and suspended sediment concentration measured during the falling limb.
In Figure 7
, the spatial variation of sediment transport capacity in the five river basins is illustrated by the annual streamflow, annual sediment load, unit-area sediment load, and unit-discharge sediment load of the nine gauging stations. The downstream (STN_7) of the Gaoping River basin received the highest annual streamflow, while the Zhuoshui River basin received greater sediment loads than the Gaoping River basin (Figure 7
a,b). It shows that even with large hydrological force the soil in the Gaoping River basin is much less erodible than that in the Zhuoshui River basin. It was found that the Zhuoshui River basin (STN_3 and STN_4) has greater annual sediment load, unit-area sediment load and unit-discharge sediment load than other basins, mainly due to the geological characteristics of the Zhuoshui River and its location to receive the impacts of landslides, typhoons, and extreme rainfalls that bring large amounts of sediments to the river.
Sediment transport capacity can spatially vary in a basin. In the Zhuoshui River basin, the sediment loads from the upstream (STN_3) was greater than the downstream (STN_4), in terms of the maximum and average values. Moreover, the high values of unit-area and unit-discharge sediment load at STN_3 and STN_4 also indicate a larger sediment transport capacity in per unit area and per unit flow of the Zhuoshui River basin than the capacity of other river basins. Thus, soil conservation should be effectively implemented in the upstream area. Typhoons are the main triggers to the significant abrupt sediment loads. The maximum unit-discharge sediment loads for each gauging station were 2.00 kg/m3 in 2001 for STN_1, 1.79 kg/m3 in 2008 for STN_2, 60.26 kg/m3 in 2009 for STN_3, 32.21 kg/m3 in 2009 for STN_4, 13.45 kg/m3 in 2000 for STN_5, 7.53 kg/m3 in 2009 for STN_6, 8.28 kg/m3 in 2009 for STN_7, 9.28 kg/m3 in 2014 for STN_8 and 8.97 kg/m3 in 2013 for STN_9. Typhoon Nari in 2001 and Typhoon Sinlaku and Jangmi in 2008 hit northern Taiwan and the flushed sediments led to the highest sediment load and unit-discharge sediment loads in STN_1 and STN_2 located in the Tamshui River basin, respectively. Moreover, STN_3 and 4 located in the Zhuoshui River basin, STN_6 in the downstream Zengwen River basin, and STN_7 located in the Gaoping River basin had the highest unit-discharge sediment load among all the stations in 2009 due to the huge amounts of sediments brought by typhoon Morakot.