Tsung-Yu Lee

Linking typhoon tracks and spatial rainfall patterns for improving flood lead time predictions over a mesoscale mountainous watershed

area of 620 km 2 ) located in eastern Taiwan were analyzed to fill the gaps in our knowledge concerning the linkage between typhoon track, rainfall patterns, and flood peak time. This study used spatially high-resolution radar-derived rainfall estimates from 38 storm events (� 2800 h) to investigate this linkage. The effect of spatial rainfall patterns on the timing of flood peak for the selected events was examined with the aid of a diffusive wave model. The results show that the typhoon rainfall was spatially aggregated and that the relative variations in the rainfall became smaller at higher rainfall rates. The maximum hourly rainfall was approximately twice the areal mean rainfall. Three major rainfall types were identified statistically, and different typhoon tracks appeared to have preferable rainfall types. This finding is presumably due to the interaction of the typhoon circulation and precipitation with the mountainous landscape. Flood lead times were derived for the different rainfall types, and it was found that differences in their lead times could be as large as � 3 h over the studied mesoscale watershed. It is recommended that this empirical approach be incorporated into flood forecasting and warning systems.

Observed magnified runoff response to rainfall intensification under global warming

Runoff response to rainfall intensification under global warming is crucial, but is poorly discussed due to the limited data length and human alteration. Historical rainfall and runoff records in pristine catchments in Taiwan were investigated through trend analysis and cross temperature difference analysis. Trend analysis showed that both rainfall and runoff in the 99.9-percentile have been significantly increasing in terms of frequency and intensity over the past four decades. Cross temperature difference analysis quantified that the rainfall and runoff extremes (including the 99.0?99.9-percentiles) may increase by 69.5% and 99.8%, respectively, under a future scenario of 1??C increase in temperature. This increase in intensity resembles the increase in intensity observed between 1971?1990 and 1991?2010. The amplified runoff response can be related to the limited catchment storage capacity being preoccupied by rainfall extremes. The quantified temperature effect on rainfall and runoff intensification can be a strong basis for designing scenarios, confirming and fusing GCMs? results. In addition, the runoff amplification should be a warning for other regions with significant rainfall intensification. Appropriate strategies are indispensable and urgently needed to maintain and protect the development of societies.

Magnified Sediment Export of Small Mountainous Rivers in Taiwan: Chain Reactions from Increased Rainfall Intensity under Global Warming.

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.

Importance of Oceanian small mountainous rivers (SMRs) in global land-to-ocean output of lignin and modern biospheric carbon

The land-to-ocean export of particulate organic carbon (POC) connects carbon flow from the atmosphere through land to the ocean, of which the contemporary fraction that reaches the deep sea for burial may effectively affect atmospheric CO2. In this regard, small mountainous rivers (SMRs) in Oceania, a global erosion hotspot driven by torrential typhoon rain and active earthquakes are potentially important. Here we measured typhoon lignin discharges for Taiwan SMRs. We found that the particulate lignin export in 96 hours by a single SMR amounting to ~20% of the annual export by Mississippi River. The yearly particulate lignin discharge from Taiwan Island (35,980 km2) is governed by the frequency and magnitude of typhoon; thus, the historical lignin export ranged widely from 1.5 to 99.7 Gg yr−1, which resulted in a 10–100 times higher areal yield relative to non-Oceanian rivers. The lignin-derived modern POC output from Oceania region is 37 ± 21 Tg C yr−1, account for approximately 20% of the annual modern POC export from global rivers. Coupled with the hyperpycnal pathway, the forested watersheds of SMRs in Oceania may serve as a giant factory to rapidly produce and efficiently convey modern POC into deep sea for sequestration.

Stream discharge characteristics through urbanization gradient in Danshui River, Taiwan: perspectives from observation and simulation

Urbanization and the subsequent changes in land use/cover inevitably influence the quality and even the quantity of stream water. This issue is widely studied through evaluations on land-use change scenarios or comparisons among historical patterns at the same watershed. However, observational stream discharge changes through urbanization gradient have rarely been discussed. In this study, we analyzed 5-year discharge data from 13 gauges in the Danshui River network with a wide range of urbanization gradient to explore the impacts on observational hydrological characteristics in individual catchments. The results reveal that stream discharge in pristine watersheds is characterized by a larger proportion of baseflow and is less fluctuating. When the forest coverage is <90%, the discharge fluctuation almost doubles. Meanwhile, the baseflow fraction decreases gradually with the increase of paddy area, which may concomitantly result from the increasing irrigation. Such a drop in baseflow may threaten the maintenance of the minimum flow required for the stream aquatic ecosystem. Furthermore, we simulated the stream discharges by TOPMODEL with blind land-use-independent parameters. The results show that the simulated discharges are satisfactory, particularly for the pristine catchments, but not as fitting for the paddy-intensive watersheds perhaps due to the unexpected irrigation. On the whole, the calibrated parameters are dependent with the landscape characteristics. The landscape-based parameter estimations can be applied to simulate discharge well, meaning the potential to assess the ungauged watersheds.

Apportioning riverine DIN load to export coefficients of land uses in an urbanized watershed.

The apportionment of riverine dissolved inorganic nitrogen (DIN) load to individual land use on a watershed scale demands the support of accurate DIN load estimation and differentiation of point and non-point sources, but both of them are rarely quantitatively determined in small montane watersheds. We introduced the Danshui River watershed of Taiwan, a mountainous urbanized watershed, to determine the export coefficients via a reverse Monte Carlo approach from riverine DIN load. The results showed that the dynamics of N fluctuation determines the load estimation method and sampling frequency. On a monthly sampling frequency basis, the average load estimation of the methods (GM, FW, and LI) outperformed that of individual method. Export coefficient analysis showed that the forest DIN yield of 521.5kg-Nkm(-2)yr(-1) was ~2.7-fold higher than the global riverine DIN yield (mainly from temperate large rivers with various land use compositions). Such a high yield was attributable to high rainfall and atmospheric N deposition. The export coefficient of agriculture was disproportionately larger than forest suggesting that a small replacement of forest to agriculture could lead to considerable change of DIN load. The analysis of differentiation between point and non-point sources showed that the untreated wastewater (non-point source), accounting for ~93% of the total human-associated wastewater, resulted in a high export coefficient of urban. The inclusion of the treated and untreated wastewater completes the N budget of wastewater. The export coefficient approach serves well to assess the riverine DIN load and to improve the understanding of N cascade.

The sources of streamwater to small mountainous rivers in Taiwan during typhoon and non-typhoon seasons.

The dynamics and behaviors of streamwater chemistry are rarely documented for subtropical small mountainous rivers. A 1-year detailed time series of streamwater chemistry, using non-typhoon and typhoon samples, was monitored in two watersheds, with and without cultivation, in central Taiwan. Rainwater, soil leachate, and well water were supplemented to explain the streamwater chemistry. The concentrations of fluoride, chloride, sulfate, magnesium, potassium, calcium, strontium, silicon, and barium of all the water samples were measured. Principal component analysis and residual analysis were applied to examine the mechanisms of solute transport and investigate possible sources contributing to the streamwater chemistry. In addition to the influence of well water and soil leachate on streamwater chemistry during non-typhoon period, overland flow and surface erosion affect streamwater chemistry during the typhoon period. The latter has not been discussed in previous studies. Surface erosion is likely to be an end member and non-conservatively mixed with other end members, resulting in a previously unobserved blank zone in the mixing space. This has a particularly great impact on small mountainous watersheds, which suffer from rapid erosion. Moreover, fertilizer contaminates agricultural soil, making soil water end members more identifiable. To our knowledge, this study is the first to clearly illustrate the dynamics and sources of streamwater chemistry of small mountainous rivers that are analogous to rivers in Oceania.

Validating the Operational Bias and Hypothesis of Universal Exponent in Landslide Frequency-Area Distribution

The exponent decay in landslide frequency-area distribution is widely used for assessing the consequences of landslides and with some studies arguing that the slope of the exponent decay is universal and independent of mechanisms and environmental settings. However, the documented exponent slopes are diverse and hence data processing is hypothesized for this inconsistency. An elaborated statistical experiment and two actual landslide inventories were used here to demonstrate the influences of the data processing on the determination of the exponent. Seven categories with different landslide numbers were generated from the predefined inverse-gamma distribution and then analyzed by three data processing procedures (logarithmic binning, LB, normalized logarithmic binning, NLB and cumulative distribution function, CDF). Five different bin widths were also considered while applying LB and NLB. Following that, the maximum likelihood estimation was used to estimate the exponent slopes. The results showed that the exponents estimated by CDF were unbiased while LB and NLB performed poorly. Two binning-based methods led to considerable biases that increased with the increase of landslide number and bin width. The standard deviations of the estimated exponents were dependent not just on the landslide number but also on binning method and bin width. Both extremely few and plentiful landslide numbers reduced the confidence of the estimated exponents, which could be attributed to limited landslide numbers and considerable operational bias, respectively. The diverse documented exponents in literature should therefore be adjusted accordingly. Our study strongly suggests that the considerable bias due to data processing and the data quality should be constrained in order to advance the understanding of landslide processes.