[1] Typhoon rainfall characteristics over a mesoscale mountainous watershed (drainage 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.
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.
As a unique biomarker of terrigenous organic matter (OM), lignin has provided valuable information for tracing the sources of OM in land to ocean transfer. Oceanian small mountainous rivers (SMRs) are characterized by extremely high erosional rate and quick change in microclimate within watershed, which may potentially affect the distribution of soil OC and lignin concentrations and compositions. Bulk OC% and lignin were determined on surface soils and soil profiles from a Taiwanese SMR (Jhuoshuei River) and nearby region along a large altitudinal gradient (3–3176 m) to investigate the influence of microclimate on soil OC and lignin. Both surface soils OC% and lignin increased in higher altitude, suggesting higher preservation of OM in the cold region. Variations in lignin vegetation indices (S/V and C/V) in surface soils generally reflect the vegetation change in this river basin, and were more affected by precipitation seasonality than mean annual precipitation. Lignin concentration decreased with depth, along with a decrease in S/V and C/V and an increase in degradation indices ((Ad/Al)v and DHBA/V), reflecting a decreased input and/or biodegradation of lignin in subsoils. Our survey on soil lignin in Taiwan SMR provided the basis for utilizing lignin to trace the source of OC in land to ocean transfer as well as paleo-climate and paleo-vegetation reconstruction study in Taiwan SMRs.
The nutrient budget, the difference between the nutrient output via stream and input via precipitation, can provide insights into how environmental processes affect forested ecosystem biogeochemistry. In this study, field measurements of the nutrient budgets—including Na+, Cl−, K+, Mg2+, Ca2+, NO3−, and SO42−—of 19 sites were conducted in Feitsui Reservoir Watershed (FRW) of northeastern Taiwan. A series of power-law regressions were developed to establish the relationship of the nutrient budget to the discharge, nutrient input, agricultural land cover, and slope. The result show that the weekly nutrient budget is significantly affected by agricultural land and input via precipitation (R2 of regression models ≥ 0.90), yet the relationship varies among different nutrient elements. The agricultural land cover is the major factor, while the input via precipitation plays a relatively minor role in the budget of Cl−, Mg2+, Ca2+, and SO42−. These nutrients could be provisioned abundantly from the system, and thus the input via precipitation is not the predominant controlling factor. By contrast, the Na+ and K+ inputs via precipitation are indispensable for accurately estimating the riverine exports. Because weathering is a limited source of K+, the roles of agricultural activities and input via precipitation are likely decisive for transport. Besides, the NO3− budget reveals a strong interplay between the atmospheric input and agricultural land, as expected. Because the nutrient budget model of NO3− is strongly improved, the R2 changes from 0.34 to 0.99 when a larger coefficient in exponent term (10.2) for agricultural land cover (showing that NO3− export is strongly hydrologically controlled) and precipitation input are included. Our analysis is based on one year of data, so extrapolating the result to a long-term period should be done with caution, as there could be substantial inter-annual variation. The nutrient budget approach provides a preliminary assessment to evaluate the impacts of agriculture and atmospheric deposition on nutrient export, which can provide a precursory reference for watershed management for improving water quality and mitigating eutrophication.
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