Extreme Rain Events Research Articles

Different Responses of Dissolved Black Carbon and Dissolved Lignin to Seasonal Hydrological Changes and an Extreme Rain Event

AbstractHydrology, especially extreme hydrological events, has been recognized as an important driver of the land‐to‐ocean export of terrigenous dissolved organic matter (tDOM). Nevertheless, how various types of tDOM that differ in source and reactivity respond to changes in hydrology is not known. Seasonal and event exports of dissolved organic carbon (DOC), dissolved black carbon (DBC), and dissolved lignin were studied in a small subtropical river. We found that seasonal variations in DBC concentration were significantly related to hydrology, while DOC and dissolved lignin were not. In contrast, DOC, DBC, and dissolved lignin changed similarly during an extreme rain event. The variation magnitudes of DOC, DBC, and dissolved lignin concentrations were in the lower end compared to other rivers, which may be related to the limited coverage of wetlands and riparian vegetation and poor development of organic‐rich soil. Dilution effects were observed when the runoff exceeded 0.4 mm/hr, and the fluxes of both DBC and dissolved lignin decreased during the runoff peak, which was caused by surface flow and potentially by removal processes during peak discharge. Our results suggest that the influence of hydrology varies with tDOM source and reactivity and that high enough runoff (e.g., 0.7 mm/hr in the Jiulong River) may not enhance the export rate of tDOM. However, our study was carried out in a small watershed with limited wetlands and riparian vegetation, and more studies are needed to verify whether this trend is consistent among global rivers.

Metal Release under Anaerobic Conditions of Urban Soils of Four European Cities

Urban soil contamination may represent an environmental threat in view of their proximity to humans. The ecological homogenization of urban areas has been postulated, and as the sources of pollution are the same in most European cities, it is possible that soil contamination is another factor of convergence. The current climate change with consequent increase of extreme rain events may affect the mobility of potentially toxic elements (PTE) thus increasing the risks. If the soil is submerged, Eh decreases and causes the solubilization of Fe and Mn oxides, which are important carriers of PTE. We compared the release of Cu, Pb, and Zn from 48 soils of four cities (namely Glasgow, Ljubljana, Sevilla, and Torino) when submerged for up to 30 days. A decrease of the redox potential was observed in all soils after a few days and an increase of Mn and then Fe in solution. Cu, Pb, and Zn were consequently released to the solution according to the general soil contamination. Despite the marked differences in soil properties, the reaction to anaerobiosis appeared to be similar in all samples indicating that waterlogging of urban soil contaminated with PTE may pose a serious environmental risk and substantiating the hypothesis of ecological convergence.

New Cloud Detection Index (CDI) for Forecasting the Extreme Rain Events

The water vapor content and clouds play a very significant role in atmospheric conditions. This paper is derived from the new cloud detection index (CDI) which is useful to forecast extreme weather events like heavy or extreme rain. The CDI is retrieved using two crucial elements of cloud formation, the critical humidity and critical water vapor. The cloud thickness is determined by using CDI for a radiosonde site (VABB) located in Mumbai, the western part of India. The obtained results are compared with the cloud thickness required for extreme rain. The outcome of the comparison is discussed in this paper. The cloud detection index is also useful in establishing the atmospheric stability along with another four atmospheric stability indices.

Environmental interactions during the extreme rain event associated with ex-tropical cyclone Oswald (2013)

Tropical cyclone (TC) Oswald made landfall over north-east Australia as a minimal or Category 1 TC on the Australian scale on 21 January 2013. As it moved southward, it intensified over land and produced extreme rainfall for nearly 7 days. Tornadoes were reported and confirmed. Tragically, seven people died and insurance estimates were ~$1 billion. It is demonstrated that the event was associated with an interaction between the ex-Oswald circulation and an amplifying Rossby wave, which propagated north-eastward from high latitudes. Diagnoses showed that as the wave amplified and broke, a potential vorticity (PV) anomaly (PVA) extended to mid-levels, moved equatorward, merged with or axisymmetrised the ex-Oswald circulation through mid-levels. Backward trajectories from locations regularly scattered within the mid-level circulation illustrated that the storm transitioned from an isolated vortex into a circulation which was strongly influenced by its environment for at least 5 days. During this interaction, PV was advected from the environment towards the storm through mid-levels. The heavy rain coincided with the commencement and maintenance of this PV injection. The PV injection is quantified and shown to be consistent with PV advection by the mean radial flow. In addition, eddy angular momentum convergence in the mid- to upper levels coincided with an intensification of the circulation through this region. This was first related to outward transport of anticyclonic momentum by the asymmetric outflow at upper levels, followed by inward transport of cyclonic momentum by the asymmetric inflow. It is shown that the environmental interaction had an impact on vortex structure changes, rainfall and tornado development. We propose that the environmental processes influenced the ascent within the storm (1) via differential vorticity advection and baroclinic forcing, as the mid- to upper level PVA approached the circulation and (2) by low- to mid-level warm air advection.

Increased Frequency of Extreme Tropical Deep Convection: AIRS Observations and Climate Model Predictions

AbstractAtmospheric Infrared Sounder (AIRS) data from the tropical oceans (30°N to 30°S) are used to derive the probability of the process resulting in deep convective clouds (DCCs) as function of the sea surface temperature (SST). For DCC at or below the tropopause the onset temperature of this process shifts at the same rate as the increase in the mean SST. For tropopause overshooting DCC, which are associated with extreme rain events, the shift of the onset temperature is slower, causing their frequency to increase by about 21%/K of warming of the oceans. This sensitivity is not inconsistent with the sensitivity of the increase of extreme deep convective rain in the National Center for Atmospheric Research Community Atmosphere Model version 5 model for a warmer SST. The mean of the 36 fifth Phase of the Coupled Model Intercomparison Project models predicts a 2.7 K warmer tropical SST by the end of this century, resulting in a 60% increases in the frequency of tropopause overshooting DCC.

Earlier Seasonal Onset of Intense Mesoscale Convective Systems in the Congo Basin Since 1999

AbstractMesoscale convective systems (MCSs) produce some of the most intense rainfall on the planet, and their response to climate variability and change is rather uncertain. Under global warming, increased water vapor is expected to intensify the most extreme rain events and enhance flood frequency. However, MCS dynamics are also sensitive to other atmospheric variables, most notably, wind shear. Here we build on a recent study showing strong MCS intensification in the African Sahel, and examine evidence of similar trends elsewhere in tropical Africa. Using satellite data, we find a remarkable increase post‐1999 in intense MCS frequency over the Congo Basin during the month of February. This earlier onset of the spring rainy season has been accompanied by strong increases in the February meridional temperature gradient and associated wind shear. This supports the hypothesis that contrasts in warming across the continent can drive important decadal‐scale trends in storm intensity.

Evaluation of Landslide Susceptibility in Cau River Basin Using a Physical-Based Model under Impact of Climate Change

This paper evaluated the probability of landslide susceptibilities through the applica-tion of the Transient Rainfall Infiltration and Grid-Based Region Slope-Stability model in Cau river basin (Vietnam) using the scenarios-based approach under the influence of the warming climate. The tested cases were developed based on various options including rainfall amount and distribution, soil depth determination, and land-cover conditions. Input data for extreme rain events included historical rainstorm in 2013, the Probable Maximum Precipitation (PMP) with the durations of 24 hours and 48 hours. The results illustrated the reduction of slope stability when the land cover changed from land-use data in 2007 (Ha12) to land-use data in 2015 (Ha22). When the whole region was assumed to be replaced by soil (Ha02), the factor of safety (Fs) decreased to lower magnitude when compared to Fs value regarding to changes in land cover condition (Ha12 & Ha22) and changes in soil-depth (Ha33). The model simulations demonstrated the agreement with the slope-failure hazard association with the destabilizing factor such as slope-cutting activities at historical landslide events. Under the same land-cover and soil depth condition, the average value of factor of safety regarding to the historical rainstorm in 2013 (Ha32) declined by 0.069 and 0.189 when compared to Fs of the 24-hour PMP with the storm distribution type 3 (1332) and Fs of the 48-hour PMP with the storm distribution type 3 (2332), respectively. The results reveal that in a warming climate, changes in extreme precipitation in terms of rain-total, rain-duration, and rain-distribution would result in the expansion of slope instability in the hilly region. This application is considered as a prevailing method for landslide susceptibility analysis and would provide important information for authorities in developing adequate land-management in the river basin.

The Impact of Rainfall Space‐Time Structure in Flood Frequency Analysis

AbstractFlood hydrologic response is influenced by rainfall structure (i.e., variability in space and time). How this structure shapes flood frequency is unknown, and flood frequency analyses typically neglect or simplify potentially important aspects of rainfall variability. This study seeks to understand how rainfall structure impacts flood frequency. We use stochastic storm transposition combined with a 15‐year record of hourly, 4‐km2 radar rainfall to generate 10,000 synthetic extreme rain events. These events are resampled into four “scenarios” with differing spatial and temporal resolutions, which are used as input to a distributed hydrologic model. Analysis of variance is used to identify the proportions of total flood peak variability attributable to spatial and to temporal rainfall variability under two antecedent soil moisture conditions. We simulate peak discharges for recurrence intervals of 2 to 500 years for 1,343 subwatersheds ranging in size from 16 to 4,400 km2 in Turkey River in the Midwestern United States, which is situated in a typically humid continental climactic region. Antecedent soil moisture modulates the role of rainfall structure in simulated flood response, particularly for more frequent events and large watershed scales. Rainfall spatial structure is more important than temporal structure for drainage areas larger than approximately 2,000 km2 (approximately 200 km2) for wet (dry) initial soil conditions, especially when soils are dry, while the reverse is true for smaller subwatersheds. The results appear to be related to the differing propensities for surface and subsurface runoff production as a function of basin scale, event magnitude, and soil saturation. Our results suggest that hydrologic model‐based flood frequency analyses, and particularly efforts attempting to spanning a range of scales, must carefully consider rainfall structure.

Inter-annual rainfall variability may foster lake regime shifts: An example from Lake Bourget in France

The Intergovernmental Panel on Climate Change anticipates increasing extreme rain events in the 21st century, leading to more frequent floods and more phosphorus transported into lakes, enhancing the risk of their eutrophication. This paper reports research assessing this risk on Lake Bourget in France, by coupling a statistical estimation of annual phosphorus transportation from annual rainfall and a dynamical model estimating the evolution of the phosphorus concentration in the lake in the period 2000–2050 according to yearly phosphorus transportation. The study of the model on the past suggests that the relative drought of the 2000s has fostered the effect of management measures to reduce phosphorus loading, enabling compliance with the Organisation for Economic Co-operation and Development guidance target for 2020. Simulations on different scenarios for the future show that a 10% increase in the rainfall standard deviation nearly doubles the probability of eutrophication, from 2.40% to 4.26% between 2016 and 2050.

Analysis of Flood Inundation in Ungauged Mountainous River Basins: A Case Study of an Extreme Rain Event on 5–6 July 2017 in Northern Kyushu, Japan

The heavy rainfall event that occurred on 5–6 July 2017 in Northern Kyushu, Japan, caused extensive flooding across several mountainous river basins and resulted in fatalities and extensive damage to infrastructure along those rivers. For the periods before and during the extreme event, there are no hydrological observations for many of the flooded river basins, most of which are small and located in mountainous regions. We used the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model, a physically based model, to acquire more detailed information about the hydrological processes in the flood-affected ungauged mountain basins. We calibrated the GSSHA model using data from an adjacent gauged river basin, and then applied it to several small ungauged basins without changing the parameters of the model. We simulated the gridded flow and generated a map of the possible maximum flood depth across the basins. By comparing the extent of flood-affected areas from the model with data of the Japanese Geospatial Information Authority (GSI), we found that the maximum flood inundation areas of the river networks estimated by the GSSHA model are sometimes less than those estimated by the GSI, as the influence of landslides and erosion was not considered in the modeling. The model accuracy could be improved by taking these factors into account, although this task would be challenging. The results indicated that simulations of flood inundation in ungauged mountain river basins could contribute to disaster management during extreme rain events.

Fecal source tracking and eDNA profiling in an urban creek following an extreme rain event

Fecal contamination of recreational waters (i.e. lakes, rivers, beaches) poses an on-going problem for environmental and public health. Heavy rainfall can exacerbate existing problems with fecal contamination. As there could be variable sources of fecal contamination, identifying the source is critical for remediation efforts. This study utilized microbial source tracking (MST), chemical source tracking (CST) markers and environmental DNA (eDNA) metabarcoding to profile sampling areas and identify sources of fecal contamination in creek, stormwater outfall and beach sites in the Etobicoke Creek watershed (Toronto, ON). Water samples were collected before and immediately following an extreme rain event. MST and CST identified stormwater outfalls as an important source of human fecal contamination during dry and wet conditions. eDNA metabarcoding allowed for potential identification of additional sources of fecal contamination and provided additional evidence of human fecal contamination. The extreme rainfall event altered the eDNA profiles, causing creek and beach sites to reflect a greater diversity of mammal and bird eDNA sequences. The profiles provided by eDNA metabarcoding provide a proof of concept suggesting that eDNA metabarcoding can be a useful tool to complement MST and CST methods for profiling sources of fecal contamination and studying impacts of extreme rain events.

Clay dispersion: An important factor in channel runoff generation in a semi-arid, loess-covered area with very low rain intensities

Overland flow is usually regarded as an important contributor to storm channel flow. This observation is certainly applicable to dryland areas, where base flow is often irrelevant, particularly in small watersheds. This study examines channel runoff generation in the extensive loess-covered areas that characterize the mildly arid area of western Israel, where the average annual rainfall is 280 mm. Hydrological data point to a peculiar hydrological behavior of the ephemeral streams that experience a high frequency of sporadic channel flow events. Even in extreme rain events, peak discharges are exceptionally low, indicative of a limited contributing area. Hydrographs are characterized by very steep rising and falling limbs, usually representative of saturated areas, located in the vicinity of the runoff recording station. Based on this observation, we advanced the hypothesis that storm runoff originated in the limited area of the active channel, with negligible runoff from the adjoining hillslopes. We argue that a quasi-permanent surface seal, at the top of the alluvial deposit, drastically limits the hydraulic conductivity of the alluvial fill, allowing runoff generation at very low rain intensities. The occurrence of the surface seal is ascribed to the combination of two main factors. A high clay content (~40%), where the dominant clays are smectite and illite, characterized by a laminar structure and a high-water absorption capacity. The swelling of the clay particles considerably reduce the porosity of the alluvial material, allowing runoff generation at very low rain intensities while limiting the depth of water penetration in the channel itself. Data presented fit the concept of “Partial Area Contribution” identified in humid areas. However, the application of this concept to dryland areas is based on completely different reasons.

Origin and fate of nitrate runoff in an agricultural catchment: Haean, South Korea – Comparison of two extremely different monsoon seasons

The monsoon season in South Korea has great influences on biogeochemical and hydrological processes in the entire country, but is specifically of concern in the Soyang lake watershed, the main drinking water reservoir for the 20-million-people metropolis Seoul. Therefore, water quality and nitrate concentration control in Lake Soyang is of high public priority. The Haean catchment is the most prominent agriculture-dominated sub-catchment of the Soyang lake watershed. It is a complex terrain influenced by extreme rain events and non-point nitrate sources. In this investigation we used input-output calculations and a stable isotope approach to quantify and determinate the origin of nitrate inputs into the rivers that later flow into the lake. During pre-monsoon and monsoon seasons in 2013 and 2014 we measured daily rainfall and river water discharge within the Haean catchment and collected rain, river water and groundwater samples in order to analyze nitrate concentrations and nitrate nitrogen and oxygen isotope abundances. Furthermore, we collected nitrogen fertilizers as applied in the catchment. Heavy monsoon events, as in 2013, were the most pronounced drivers of nitrate leaching being responsible for >80% of the nitrate output in the river runoff. On the other hand, an almost missing summer monsoon in 2014 drove the nitrate runoff in a different manner, being responsible for only 0.4% of the total nitrate nitrogen river discharge in the previous year. Results of nitrate nitrogen and oxygen isotope abundance analyses suggest soil microbial nitrification as the most important contributor to the nitrate in the river runoff. In addition, nitrate from groundwater partially affected by microbial denitrification contributed to the nitrate in the runoff due to river-aquifer exchange fluxes during the monsoon season. Direct leaching of nitrate from mineral fertilizers and atmospheric nitrate deposition were obviously only minor contributors to the nitrate in the runoff.

Mineralogy and physicochemical features of Saharan dust wet deposited in the Iberian Peninsula during an extreme red rain event

Abstract. The mineralogy and physicochemical features of Saharan dust particles help to identify source areas and determine their biogeochemical, radiative, and health effects, but their characterization is challenging. Using a multianalytical approach, here we characterized with unprecedented level of detail the mineralogy and physicochemical properties of Saharan dust particles massively wet deposited ( ∼ 18 g m−2) following an extreme red rain event triggered by a northern African cyclone that affected the southern Iberian Peninsula during 21–23 February 2017. Abundant palygorskite and illite, and relatively high carbonate contents, well-known northern and north-western Saharan dust indicators, along with low chlorite content and significant amounts of smectites and kaolinite, whose abundance increases southwards in the western Sahara, complemented by satellite imagery and back/forward trajectories, show that the most probable dust source areas were (i) southern/central Algeria, northern Mali, and northwestern Niger, and (ii) northern Algeria, southern Tunisia, and northwestern Libya. Scanning and transmission electron microscopy analyses, including Z-contrast high angle annular dark field (HAADF) imaging and analytical electron microscopy (AEM), show that clay minerals include abundant structural Fe (55 % of the total Fe) and typically form nanogranular aggregates covered or interspersed with amorphous/poorly crystalline iron oxyhydroxide nanoparticles (ferrihydrite), which account for ∼ 18 % of the free Fe, the rest being goethite and hematite. These nanogranular aggregates tend to form rims lining large silicate and carbonate particles. Such internally mixed iron-containing phases are the main contributors to the observed absorption of solar and thermal radiation, and along with the abundant coarse/giant particles ( > 10 µm) strongly affect the dust direct radiative forcing. The lack of secondary sulfates in aggregates of unaltered calcite internally mixed with clays/iron-rich nanoparticles shows that iron-rich nanoparticles did not form via atmospheric (acid) processing but were already present in the dust source soils. Such iron-rich nanoparticles, in addition to iron-containing clay (nano)particles, are the source of the ∼ 20 % soluble (bioavailable) iron in the studied desert dust. The dust particles are a potential health hazard, specifically the abundant and potentially carcinogenic iron-containing palygorskite fibers. Ultimately, we show that different source areas are activated over large desert extensions, and large quantities of complex dust mixtures are transported thousands of kilometers and wet-deposited during such extreme events, which dwarf any other Saharan dust event affecting southwestern Europe. The past, present, and future trends, as well as impacts, of such extreme events must be taken into account when evaluating and modeling the manifold effects of the desert dust cycle.

Definitions of event magnitudes, spatial scales, and goals for climate change adaptation and their importance for innovation and implementation

We examine how core professional and institutional actors in the innovation system conceptualize climate change adaptation in regards to pluvial flooding—and how this influences innovation. We do this through a qualitative case study in Copenhagen with interconnected research rounds, including 32 semi-structured interviews, to strengthen the interpretation and analysis of qualitative data. We find that the term “climate change adaptation” currently has no clearly agreed definition in Copenhagen; instead, different actors use different conceptualizations of climate change adaptation according to the characteristics of their specific innovation and implementation projects. However, there is convergence among actors towards a new cognitive paradigm, whereby economic goals and multifunctionality are linked with cost-benefit analyses for adapting to extreme rain events on a surface water catchment scale. Differences in definitions can lead to both successful innovation and to conflict, and thus they affect the city's capacity for change. Our empirical work suggests that climate change adaptation can be characterized according to three attributes: event magnitudes (everyday, design, and extreme), spatial scales (small/local, medium/urban, and large/national-international), and (a wide range of) goals, thereby resulting in different technology choices.

Selective successional transport of bacterial populations from rooted agricultural topsoil to deeper layers upon extreme precipitation events

Substantial amounts of organic matter are mobilized from upper soil layers during extreme precipitation events. This results in considerable fluxes of carbon from plant-associated topsoil to deeper mineral soil and to groundwater. Microbes constitute an important part of this mobile organic matter (MOM) pool. Previous work has shown that specific bacteria associated with the rhizosphere of decaying maize roots were selectively transported with seepage water upon snowmelt in winter. However, effective mechanisms of mobilization and also possible distinctions to microbial transport for living root systems remain poorly understood. In the present study, bacteria in seepage water were sampled from lysimeters at an experimental maize field after extreme rain events in summer. We show that a distinctive subset of rhizoplane-associated bacterial populations was mobilized after summer rain, especially including abundant members of the Bacteroidetes, representing a microbial conduit for fresh plant-derived carbon inputs into deeper soil layers. Marked distinctions of seepage communities were not observed between lysimeters with a different relative contribution of preferential vs. matrix flow. Time-resolved analyses of seepage water during an artificial rain event revealed temporal patterns in the mobilization of certain lineages, with members of the Chitinophagaceae, Sphingomonadaceae, and Bradyrhizobiaceae preferentially mobilized in early and late seepage fractions, and members of the candidate phyla Parcubacteria and Microgenomates mobilized mostly in intermediate fractions. While average bacterial cell counts were at ∼107 ml−1 in seepage water, the recovery of amended fluorescently labeled cells of Arthrobacter globiformis was low (0.2–0.6%) over seepage events. Still, mobilized bacteria clearly have the potential to influence bacterial activities and communities in subsoils. These findings demonstrate that dynamic hydraulic events must be considered for a better understanding of the connectivities between microbial populations and communities in soil, as well as of the links between distinct carbon pools over depth.

Scales for rating heavy rainfall events in the West African Sahel

Abstract In the field of climate services, characterization of rainfall extremes is useful to identify and quantify rain rates that can trigger floods, on-farm water stagnation, excess run-off causing arable soil depletion and other natural hazards. Delving through multiple sources of observational uncertainties, we define extreme rain events (EREs) using the 99th percentile thresholds of daily accumulated rainfall, extracted from historical data records (1960–2016) of manual and tipping bucket gauges. The results of the analysis show that the average amplitude of these threshold values has been increasing in the recent years. Meanwhile, the three categories of heavy rains exhibit an intra-seasonal timing that follows different phases of the West African monsoon. Category 1 & 2 occur mostly in the northern Sahel, between weeks 27 and 35 of the year with an accumulated daily amount varying in the 37–65 mm range and less than or equal to 85 mm/day respectively. In category 3, rain rates are greater than 85 mm/day, observed between 28th and 38th week-of-the-year predominantly in the southern and western of the Sahelsub-region. For each category of ERE, high risk areas are mapped using the relative probability of occurrence at local scale. This classification can be exploited for forecasts verification, climate model evaluations and operational early warning services against high impact rainfall events.

Tale of Two Storms: Impact of Extreme Rain Events on the Biogeochemistry of Lake Superior.

Climate change is expected to profoundly affect the Great Lakes region of North America. An increase in intensity and frequency of rain events is anticipated to deliver more runoff and to increase riverine inputs to Lake Superior's ecosystem. The effects of these changes on key biogeochemical parameters were analyzed by coupling satellite data, water column sensor profiles, and discrete surface-water sampling after two "500-year" flood events in the Lake Superior basin. This study provides both a spatial and a temporal sense of how plumes interacted within the ecosystem. We also determined the significant differences in water quality parameters for plume versus non-plume waters. These two plumes were important for delivery of nutrients, with variable transport of sediments and colored dissolved organic matter (CDOM). Data from the 2012 storm event showed a significant input of total nitrogen (TN), total phosphorous (TP) and CDOM to the system. In the 2016 storm event, carbon cycling parameters (acidity, total inorganic carbon (TIC), and dissolved organic carbon (DOC), and ammonia levels were elevated within the plume. In neither storm event was there a significant difference in chlorophyll a between plume and non-plume waters during our sampling cruises. These two plume events were similar in amount of precipitation, but their effect on the biogeochemistry of Lake Superior varied due to differences in the watersheds where the rain fell. The studied plume events were dynamic, changing with currents, winds and the settling of suspended sediments.

Wetland hydroperiod classification in the western prairies using multitemporal synthetic aperture radar

AbstractWetlands represent one of the world's most biodiverse and threatened ecosystem types and were diminished globally by about two‐thirds in the 20th century. There is continuing decline in wetland quantity and function due to infilling and other human activities. In addition, with climate change, warmer temperatures and changes in precipitation and evapotranspiration are reducing wetland surface and groundwater supplies, further altering wetland hydrology and vegetation. There is a need to automate inventory and monitoring of wetlands, and as a study system, we investigated the Shepard Slough wetlands complex, which includes numerous wetlands in urban, suburban, and agricultural zones in the prairie pothole region of southern Alberta, Canada. Here, wetlands are generally confined to depressions in the undulating terrain, challenging wetlands inventory and monitoring. This study applied threshold and frequency analysis routines for high‐resolution, single‐polarization (HH) RADARSAT‐2, synthetic aperture radar mapping. This enabled a growing season surface water extent hyroperiod‐based wetland classification, which can support water and wetland resource monitoring. This 3‐year study demonstrated synthetic aperture radar‐derived multitemporal open‐water masks provided an effective index of wetland permanence class, with overall accuracies of 89% to 95% compared with optical validation data, and RMSE between 0.2 and 0.7 m between model and field validation data. This allowed for characterizing the distribution and dynamics of 4 marsh wetlands hydroperiod classes, temporary, seasonal, semipermanent, and permanent, and mapping of the sequential vegetation bands that included emergent, obligate wetland, facultative wetland, and upland plant communities. Hydroperiod variation and surface water extent were found to be influenced by short‐term rainfall events in both wet and dry years. Seasonal hydroperiods in wetlands were particularly variable if there was a decrease in the temporary or semipermanent hydroperiod classes. In years with extreme rain events, the temporary wetlands especially increased relative to longer lasting wetlands (84% in 2015 with significant rainfall events, compared with 42% otherwise).

Hydrological Simulation of Small River Basins in Northern Kyushu, Japan, During the Extreme Rainfall Event of July 5–6, 2017

Extreme rainfall and associated flooding are common during the summer in Japan. Heavy rain caused extensive damage in many parts of Kyushu, Japan, on July 5–6, 2017. Many small mountainous river basins were subject to the core of this heavy rainfall event and were flooded, but no hydrological measurements were taken in most of these flooded basins during the event. There are few gauging stations in this mountainous region, and most that do exist are designed to monitor the larger watersheds. Consequently, it is difficult to determine the hydrological properties of the small subbasins within these larger watersheds. Therefore, to improve our understanding of the basic hydrological processes that affect small ungauged mountain river basins during periods of intense rainfall, a quasi-distributed model (i.e. the Hydrologic Engineering Center-Hydrologic Modeling System, HEC-HMS) was used in this study. The Hikosan (area: 65 km2) and Akatani (area: 21 km2) mountainous river basins were selected for the hydrological simulations. The model was validated using the Hikosan River basin because observational data are available from the outlet of this basin. However, there is no record of any hydrological observations for the Akatani River basin. Therefore, reference parameters from the Hikosan River basin were used for hydrological analysis of the Akatani River basin. This was possible because the basins are close to one another and have similar physiographic and topographic properties. The simulations of both basins, and the associated uncertainties, are discussed in detail in this paper. Based on the hydrological simulations, an attempt was made to analyze the maximum flood discharge caused by the event. The results generated using this approach to hydrological simulations in small ungauged basins could contribute to the management of water resources in these and other river basins during future extreme rain events.