High-magnitude Precipitation Events Research Articles

Use of Ensemble-Based Gridded Precipitation Products for Assessing Input Data Uncertainty Prior to Hydrologic Modeling

The spatial and temporal performance of an ensemble of five gridded climate datasets (precipitation) (North American Regional Reanalysis, European Centre for Medium-Range Weather Forecasts interim reanalysis, European Union Water and Global Change (WATCH) Watch Forcing data ERA-Interim, Global Forcing Data-Hydro, and The Australian National University spline interpolation) was evaluated towards quantifying gridded precipitation data ensemble uncertainty for hydrologic model input. Performance was evaluated over the Nelson–Churchill Watershed via comparison to two ground-based climate station datasets for year-to-year and season-to-season periods (1981–2010) at three spatial discretizations: distributed, sub-basin aggregation, and full watershed aggregation. All gridded datasets showed spatial performance variations, most notably in year-to-year total precipitation bias. Absolute minimum and maximum realizations were generated and assumed to represent total possible uncertainty bounds of the ensemble. Analyses showed that high magnitude precipitation events were often outside the uncertainty envelope; some increase in spatial aggregation, however, enveloped more observations. Results suggest that hydrologic models can reduce input uncertainty with some spatial aggregation, but begin to lose information as aggregation increases. Uncertainty bounds also revealed periods of elevated uncertainty. Assessing input ensemble bounds can be used to include high and low uncertainty periods in hydrologic model calibration and validation.

Shifts in air temperature and high-magnitude winter precipitation events in coastal North America: Implications for Environmental Assessment and Management.

This commentary examines recent shifts in air temperature data coinciding with high-magnitude precipitation events at climate stations spanning an elevational and longitudinal gradient on the south coast of British Columbia, Canada. Results presented show that high-magnitude winter precipitation events are occurring on British Columbia's south coast under progressively warmer conditions. In the future, proportionally more winter precipitation is anticipated to report as rainfall versus snow, and over time these changes will have a marked impact on the snowmelt-dominated hydrographs that characterize local watersheds. Robust preparedness strategies will be needed to balance competing interests such as the security of domestic water supplies, the permitting and operation of major projects (e.g., mines, hydrodevelopments), and the achievement of broader ecosystem health goals under these changing hydroclimatic conditions. Integr Environ Assess Manag 2018;14:185-188. © 2018 SETAC.

Modelling surface geomorphic processes using the RUSLE and specific stream power in a GIS framework, NE Peloponnese, Greece

Mediterranean regions, with climate variability and long histories of human disturbance, are particularly vulnerable to soil erosion and sediment redistribution. This study examines surface soil stability and stream energy of the 243 km2 Inachos River watershed in the northeast Peloponnese, Greece. This mountainous, semi-arid Mediterranean region has an extensive history of human activity. Soil loss and stream energy are each quantified by applying the Revised Universal Soil Loss Equation (RUSLE) using the Unit Stream Power Erosion Deposition (USPED) method and the specific stream power approach to the main river channels. These models are used to indicate the spatial variability in geomorphic activity. Results show an average soil loss for the Inachos River catchment of 15.0 t ha−1 a−1, exceeding the rate of soil formation. Values range from nil in low gradient environments to 4287 t ha−1 a−1 in steep, mountainous regions. Gradient and rainfall erosivity are the primary factors. High specific stream power in the upper watershed exceeds 17,100 W m−2, resulting in the mobilization of sediment into channelized debris flows that transport sediment from the steep hillslopes. Episodic high-magnitude precipitation events promote the longitudinal connectivity of the catchment. The long occupation and agricultural history, extending as far back as Neolithic time, has accelerated downslope sediment transport.

Precipitation, landsliding, and erosion across the Olympic Mountains, Washington State, USA

In the Olympic Mountains of Washington State, landsliding is the primary surface process by which bedrock and hillslope regolith are delivered to river networks. However, the relative importance of large earthquakes versus high magnitude precipitation events to the total volume of landslide material transported to valley bottoms remains unknown in part due to the absence of large historical earthquakes. To test the hypothesis that erosion is linked to precipitation, approximately 1000 landslides were mapped from Google Earth imagery between 1990 and 2015 along a ~15km-wide×~85km-long (1250km2) swath across the range. The volume of hillslope material moved by each slide was calculated using previously published area–volume scaling relationships, and the spatial distribution of landslide volume was compared to mean annual precipitation data acquired from the PRISM climate group for the period 1981–2010. Statistical analysis reveals a significant correlation (r=0.55; p<0.001) between total landslide volume and mean annual precipitation, with 98% of landslide volume occurring along the windward, high-precipitation side of the range during the 25-year interval. Normalized to area, this volume yields a basin-wide erosion rate of 0.28±0.11mmyr−1, which is similar to previous time-variable estimates of erosion throughout the Olympic Mountains, including those from river sediment yield, cosmogenic 10Be, fluvial terrace incision, and thermochronometry. The lack of large historic earthquakes makes it difficult to assess the relative contributions of precipitation and seismic shaking to total erosion, but our results suggest that climate, and more specifically a sharp precipitation gradient, plays an important role in controlling erosion and landscape evolution over both short and long timescales across the Olympic Mountains.

Chenier-type ridges in Giralia Bay (Exmouth Gulf, Western Australia) - Processes, chronostratigraphy, and significance for recording past tropical cyclones

Past coastal flooding events may be inferred from geomorphic and sedimentary archives, including particular landforms (e.g., beach ridges, washover fans), deposits (e.g., washover sediments in lagoons) or erosional features (e.g., erosional scarps within strandplains). In Giralia Bay, southern Exmouth Gulf (Western Australia), sandy ridge sequences in supratidal elevations form the landward margin of extensive mudflats. The formation of these ridges is assumed to be mainly driven by tropical cyclones (TCs), although their depositional processes need to be clarified. We investigated the supratidal sandy ridge sequence in Giralia Bay by carrying out process monitoring, geomorphological mapping by means of an unmanned aerial vehicle survey, as well as sedimentological and geochronological investigations and multivariate statistics. Based on the resulting data, this study aims at (i) identifying the most important driving processes to form the sandy ridges; (ii) establishing their chronostratigraphy; and (iii) understanding their significance for recording past TC activity. Trench excavations revealed sandy units that are interbedded with mud layers at the base, similar to the present distal mudflat sediments. On top, mud intercalations recede, and sand layers of varying grain size distribution dominate. In the upper part of the trenches, younger sediment layers onlap older ones documenting the stepwise seaward accretion of the ridges onto the mudflat. While our data suggests that tidal processes have only limited effects on ridge activity, sediment transport, erosion and deposition seems to be driven by both TC-induced storm surges and high magnitude precipitation events causing surface discharge. Most accretionary sand units are thus assumed to represent events of morphodynamic activity during TC-induced flooding since the mid-Holocene. Ridge activity is recorded in a roughly decadal resolution and over historical as well as prehistorical/Holocene time scales. While the ridges do not represent beach or chenier ridges sensu stricto, they may be described as chenier-type ridges due to their stratigraphical architecture. Ridge evolution, however, over a millennial time scale seems to be indicated by the landward rise of the sequence possibly corresponding to the mid-Holocene sea-level highstand of Western Australia of at least 1–2m above present mean sea level.

Lateglacial to Late Holocene landscape history derived from floodplain sediments in context to prehistoric settlement sites of the southern foreland of the Harz Mountains, Germany

The Harz Mountains are Germany's northernmost low mountain range. The southern foreland is characterised by a fragmentary distribution of loess and loess-like deposits. During prehistoric times this region was sparsely populated, in contrast to the adjoining southern floodplain of the Helme River, which has been a favoured settlement area since the Neolithic period. This study focusses on the onset of human impact on the landscape and aims to reconstruct the landscape development of this peripheral prehistoric settlement area under a long term perspective (Lateglacial to Late Holocene). The study is based on the sedimentological analysis of six sediment profiles from the floodplain of two small valleys. The chronology of the sedimentological records is provided by 24 new AMS radiocarbon ages that range from 21.7 ka BP to 0.1 ka BP. Three major changes in the depositional environment are observed in the sediment records representing the landscape evolution: a) During the Lateglacial, periglacial conditions prevailed and deposition and reworking of loess occurred on the slopes, while in the floodplains periglacial cover beds and alternating partly gravelly layers of silts and clays were deposited. b) From Preboreal to Atlantic periods a marked rise in the groundwater table led to authigenic precipitation of carbonates in the floodplains under ameliorated climatic conditions. During this phase the valleys were progressively filled by intercalated organogenic and calcareous sediments. c) Human influence on the landscape is reflected in colluvial and alluvial deposits indicating the occurrence of human-induced soil erosion and is dated prior to the beginning of the Bronze Age (here corresponding to Subboreal period). During the period of the Little Ice Age gully incision occurred, probably as a result of an intensified land use in upslope areas and of high-magnitude precipitation events during this period.

Recharge estimation and soil moisture dynamics in a Mediterranean, semi-arid karst region

Abstract. Knowledge of soil moisture dynamics in the unsaturated soil zone provides valuable information on the temporal and spatial variability of groundwater recharge. This is especially true for the Mediterranean region, where a substantial fraction of long-term groundwater recharge is expected to occur during high magnitude precipitation events of above-average wet winters. To elucidate process understanding of infiltration processes during these extreme events, a monitoring network of precipitation gauges, meteorological stations, and soil moisture plots was installed in an area with a steep climatic gradient in the Jordan Valley region. In three soil moisture plots, Hydrus-1D was used to simulate water movement in the unsaturated soil zone with soil hydraulic parameters estimated by the Shuffled Complex Evolution Metropolis algorithm. To generalize our results, we modified soil depth and rainfall input to simulate the effect of the pronounced climatic gradient and soil depth variability on percolation fluxes and applied the calibrated model to a time series with 62 years of meteorological data. Soil moisture measurements showed a pronounced seasonality and suggested rapid infiltration during heavy rainstorms. Hydrus-1D successfully simulated short and long-term soil moisture patterns, with the majority of simulated deep percolation occurring during a few intensive rainfall events. Temperature drops in a nearby groundwater well were observed synchronously with simulated percolation pulses, indicating rapid groundwater recharge mechanisms. The 62-year model run yielded annual percolation fluxes of up to 66% of precipitation depths during wet years and of 0% during dry years. Furthermore, a dependence of recharge on the temporal rainfall distribution could be shown. Strong correlations between depth of recharge and soil depth were also observed.

Moisture-advection feedback supports strong early-to-mid Holocene monsoon climate on the eastern Tibetan Plateau as inferred from a pollen-based reconstruction

(Paleo-)climatologists are challenged to identify mechanisms that cause the observed abrupt Holocene monsoon events despite the fact that monsoonal circulation is assumed to be driven by gradual insolation changes. Here we provide proxy and model evidence to show that moisture-advection feedback can lead to a non-linear relationship between sea-surface and continental temperatures and monsoonal precipitation. A pollen record from Lake Ximencuo (Nianbaoyeze Mountains) indicates that vegetation from the eastern margin of the Tibetan Plateau was characterized by alpine deserts and glacial flora after the Last Glacial Maximum (LGM) (21–15.5calkyr BP), by alpine meadows during the Late Glacial (15.5–10.4calkyr BP) and second half of the Holocene (5.0cal kyr BP to present) and by mixed forests during the first half of the Holocene (10.4–5.0calkyr BP). The application of pollen-based transfer functions yields an abrupt temperature increase at 10.4calkyr BP and a decrease at 5.0calkyr BP of about 3°C. By applying endmember modeling to grain-size data from the same sediment core we infer that frequent fluvial events (probably originating from high-magnitude precipitation events) were more common in the early and mid Holocene. We assign the inferred exceptional strong monsoonal circulation to the initiation of moisture-advection feedback, a result supported by a simple model that reproduces this feedback pattern over the same time period.

Loess and floods: late Pleistocene fine-grained valley-fill deposits in the Flinders Ranges, South Australia

Terrace remnants of late Pleistocene fine-grained valley-fills, at present eroded by ephemeral traction-load streams, are reported from many semi-arid and arid parts of the world. While they present promising palaeo-environmental archives for recent geological times such as the Last Glacial Maximum (LGM) for which few other terrestrial depositional records exist, their poorly understood nature has limited their significance. This study examines the fine-grained valley-fill deposits from the Flinders Ranges in South Australia, here called Flinders Silts. It establishes the timing, mode and environmental controls of deposition as opposed to their advancing erosion under the current climate. A regional chronostratigraphy based on 124 numerical dates is discussed, of which 43 radiocarbon and 22 luminescence ages were obtained from 12 sections across three major catchments within the scope of this thesis. Regionally significant intervals of rapid aggradation, relative surface stability and erosion are established. Regional climatic controls are differentiated from intrinsic catchment- and site-specific effects on the system. Further, individual age proxies and age models are critically assessed in how far they reflect depositional events. The final aggradational interval bracketing the extended LGM is discussed in detail on a continuous layered to laminated stratigraphic sequence. The provenance question of the fine-grained sediments and the depositional environment of the Flinders Silts are further addressed by high-resolution particle-size analysis. In order to study the subtle variations within the fine-grained partially-aggregated material, an original parametric sediment-sizing approach is employed. Finally, a range of traditional and emerging analytical techniques are applied to improve our understanding of palaeo-environments promoting aggradation. In conclusion, arid intervals throughout the last glacial cycle resulted in significant quantities of proximal dust being deposited as loess mantles within the catchments of the Flinders Ranges, acting as a near-longitudinal dust trap in the centre of the late Pleistocene “dust bowl”. The fine-grained aeolian accessions were repeatedly eroded by low-frequency high-magnitude precipitation events and redistributed as loess-derived alluvium, congesting narrow gorges and raising the base level for tributaries. Locally, backflooding resulted in the aggradation of layered to laminated slackwater deposits, the most continuous recording at least 12 large and numerous smaller flood events between 24 ka and 18 ka. The synchronous termination of the Flinders Silts coincides with early Deglacial climatic amelioration. The re-establishment of a perennial plant cover stabilising both dune fields and slope mantes is discussed as a potential scenario that would have discontinued dust supply to the fluvial system, in turn promoting incision and erosion. The studied aeolian-fluvial interplay of loess and floods has large implications for our understanding of landscape evolution in…

El Niño forcing on 10Be-based surface denudation rates in the northwestern Peruvian Andes?

High magnitude precipitation events provide large contributions to landscape formation and surface denudation in arid environments. Here, we quantify the precipitation-dependent geomorphic processes within the Rio Piura drainage basin located on the Western Escarpment of the northern Peruvian Andes at 5°S latitude. In this region, monsoonal easterly winds bring precipitation to the > 3000 m asl high headwaters, from where the annual amount of precipitation decreases downstream toward the Pacific coast. Denudation rates are highest in the knickzones near the headwaters, ~ 200–300 mm ky − 1 , and sediment discharge is limited by the transport capacity of the channel network. Every few years, this situation is perturbed by westerly, wind-driven heavy precipitation during El Niño events and results in supply-limited sediment discharge as indicated by bedrock channels. The detailed analysis of the stream-long profiles of two river basins within the Rio Piura catchment reveals a distinct knickzone in the transition zone between the easterly and westerly climatic influences, suggesting an En Niño forcing on the longitudinal channel profiles over at least Holocene timescales. Measured trunk stream catchment-wide denudation rates are up to ca. 300 mm ky − 1 and decrease successively downstream along the river profiles. Denudation rates of tributary rivers are ca. 200 mm ky − 1 near the plateau and show a stronger downstream decreasing trend than trunk stream rates. This suggests that the landscape is in a transient stage of local relief growth, which is driven by fluvial incision. This corroborates the results of paleoclimate studies that point towards higher El Nino frequencies during the past ca. 3000 years, leading to higher runoff and more erosion in the trunk channel compared to the hillslopes and thus growth of local relief. Downstream increases in channel gradient spatially coincide with the reaches of highest precipitation rates during El Niño events, we therefore interpret that Holocene landscape evolution has largely been controlled by climate. The ky-timescale of the 10Be data together with the transience of the landscape implies that El Niño events in northwestern Peru have occurred since at least the Holocene, and that adjustment to channel incision is still taking place.

Stream temperature dynamics within a New Zealand glacierized river basin

AbstractKnowledge of river thermal dynamics for glacierized basins is limited, despite the high sensitivity of these systems to climatic change/variability. This study examined spatio‐temporal water column and streambed temperature dynamics within a New Zealand glacierized river basin over two melt seasons. Water column temperature was recorded at three sites along the mainstem channel and four hillslope/groundwater‐fed tributaries. Air temperature, precipitation and stream discharge were monitored to characterize hydroclimatological conditions. Streambed temperature was monitored at the upper and lower main river sites at 0.05, 0.2 and 0.4 m depth. Water column temperature rose on average 0.6°C km−1 along the glacier‐fed mainstem. Temperature was elevated during warmer periods but the downstream increase was reduced due to greater meltwater production (consequently a larger total stream flow volume for atmospheric heating) plus a proportional reduction in warmer groundwater contributions. Hillslope/groundwater‐fed tributaries yielded a range of temperature patterns, indicating variable sourcing (meltwater or rainfall) and residence times. In the upper basin, streambed temperature was warmer than the water column, suggesting groundwater upwelling; however, during high runoff events, water column and streambed temperature converged, indicating downwelling/heat advection by channel water. At the lower site, streambed temperature mirrored the water column, suggesting greater surface water/atmospheric influences. Key drivers of stream thermal regime were: (1) relative water source contributions, (2) prevailing hydroclimatological conditions, (3) distance from source, (4) total stream flow volume and (5) basin factors (specifically, valley/channel geomorphology and riparian forest). High magnitude precipitation events produced a contrasting stream thermal response to that reported elsewhere. In contrast to European alpine research, streams showed a reduced thermal range owing to the relatively mild, wet melt season climate. This New Zealand case study highlights the importance of understanding basin‐specific modification of energy and hydrological fluxes for accurate prediction of stream thermal dynamics/habitat and ecological response to climatic variability and change. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Modelling low-frequency rainfall events using airflow indices, weather patterns and frontal frequencies

Low-frequency, high-magnitude daily rainfall amounts recorded at several sites in central and southern England were related to the prevailing Lamb Weather Types (LWTs), daily weather front and airflow data. Three statistically distinct weather-type clusters were indentified and used to construct a simplified frontal model of daily precipitation occurrence/amount. The model was calibrated against station data for the period 1970–1990 and used to reconstruct observed daily precipitation between 1875 and 1969 given the historic sequence of LWTs. Although the model reproduced the incidence of low-frequency, high-magnitude events, it failed to capture variations in mean wet day probabilities and wet/dry spell persistence. This inability was attributed to the general limitations of the weather classification methodology, which did not capture all aspects of the precipitation regime with equal levels of proficiency. Therefore, the prospects for downscaling high-resolution precipitation series directly from indices of mean sea-level pressure rather than via weather patterns was discussed. Preliminary results indicate that relationships can be established between mean daily precipitation occurrence and airflow indices such as vorticity and strength of air flow. However, further research is required to establish the value of such indices for modelling low-frequency, high-magnitude precipitation events.