Changes to flood peaks of a mountain river: implications for analysis of the 2013 flood in the Upper Bow River,Canada

The mountain headwater Bow River at Banff, Alberta, Canada, was subject to a large flood in June 2013, over which considerable debate has ensued regarding its probability of occurrence. It is therefore instructive to consider what information long‐term streamflow discharge records provide about environmental change in the Upper Bow River basin above Banff. Though protected as part of Banff National Park, since 1885, the basin has experienced considerable climate and land cover changes, each of which has the potential to impact observations, and hence the interpretations of flood probability. The Bow River at Banff hydrometric station is one of Canada's longest‐operating reference hydrological basin network stations and so has great value for assessing changes in flow regime over time. Furthermore, the station measures a river that provides an extremely important water supply for Calgary and irrigation district downstream and so is of great interest for assessing regional water security. These records were examined for changes in several flood attributes and to determine whether flow changes may have been related to landscape change within the basin as caused by forest fires, conversion from grasslands to forest with fire suppression, and regional climate variations and/or trends. Floods in the Upper Bow River are generated by both snowmelt and rain‐on‐snow (ROS) events, the latter type which include flood events generated by spatially and temporally large storms such as occurred in 2013. The two types of floods also have different frequency characteristics. Snowmelt and ROS flood attributes were not correlated significantly with any climate index or with burned area except that snowmelt event duration correlated negatively to the Pacific Decadal Oscillation. While there is a significant negative trend in all floods over the past 100 years, when separated based on generating process, neither snowmelt floods nor large ROS floods associated with mesoscale storms show any trends over time. Despite extensive changes to the landscape of the basin and in within the climate system, the flood regime remains unchanged, something identified at smaller scales in the region but never at larger scales. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatio‐temporal controls of snowmelt and runoff generation during rain‐on‐snow events in a mid‐latitude mountain catchment

A network of 30 standalone snow monitoring stations was used to investigate the snow cover distribution, snowmelt dynamics, and runoff generation during two rain‐on‐snow (ROS) events in a 40 km² montane catchment in the Black Forest region of southwestern Germany. A multiple linear regression analysis using elevation, aspect, and land cover as predictors for the snow water equivalent (SWE) distribution within the catchment was applied on an hourly basis for two significant ROS flood events that occurred in December 2012. The available snowmelt water, liquid precipitation, as well as the total retention storage of the snow cover were considered in order to estimate the amount of water potentially available for the runoff generation. The study provides a spatially and temporally distributed picture of how the two observed ROS floods developed in the catchment. It became evident that the retention capacity of the snow cover is a crucial mechanism during ROS. It took several hours before water was released from the snowpack during the first ROS event, while retention storage was exceeded within 1 h from the start of the second event. Elevation was the most important terrain feature. South‐facing terrain contributed more water for runoff than north‐facing slopes, and only slightly more runoff was generated at open compared to forested areas. The results highlight the importance of snowmelt together with liquid precipitation for the generation of flood runoff during ROS and the large temporal and spatial variability of the relevant processes. Copyright © 2015 John Wiley & Sons, Ltd.     

The cold rain‐on‐snow event of June 2013 in the Canadian Rockies — characteristics and diagnosis

The June 2013 flood in the Canadian Rockies featured rain‐on‐snow (ROS) runoff generation at alpine elevations that contributed to the high streamflows observed during the event. Such a mid‐summer ROS event has not been diagnosed in detail, and a diagnosis may help to understand future high discharge‐producing hydrometeorological events in mountainous cold regions. The alpine hydrology of the flood was simulated using a physically based model created with the modular cold regions hydrological modelling platform. The event was distinctive in that, although at first, relatively warm rain fell onto existing snowdrifts inducing ROS melt; the rainfall turned to snowfall as the air mass cooled and so increased snowcover and snowpacks in alpine regions, which then melted rapidly from ground heat fluxes in the latter part of the event. Melt rates of existing snowpacks were substantially lower during the ROS than during the relatively sunny periods preceding and following the event as a result of low wind speeds, cloud cover and cool temperatures. However, at the basin scale, melt volumes increased during the event as a result of increased snowcover from the fresh snowfall and consequent large ground heat contributions to melt energy, causing snowmelt to enhance rainfall–runoff by one fifth. Flow pathways also shifted during the event from relatively slow sub‐surface flow prior to the flood to an even contribution from sub‐surface and fast overland flow during and immediately after the event. This early summer, high precipitation ROS event was distinctive for the impact of decreased solar irradiance in suppressing melt rates, the contribution of ground heat flux to basin scale snowmelt after precipitation turned to snowfall, the transition from slow sub‐surface to fast overland flow runoff as the sub‐surface storage saturated and streamflow volumes that exceeded precipitation. These distinctions show that summer, mountain ROS events should be considered quite distinct from winter ROS and can be important contributors to catastrophic events. Copyright © 2016 John Wiley & Sons, Ltd.     

Quantifying the role of the snowpack in generating water available for run‐off during rain‐on‐snow events from snow pillow records

Rain‐on‐snow events have generated major floods around the world, particularly in coastal, mountainous regions. Most previous studies focused on a limited number of major rain‐on‐snow events or were based primarily on model results, largely due to a lack of long‐term records from lysimeters or other instrumentation for quantifying event water balances. In this analysis, we used records from five automated snow pillow sites in south coastal British Columbia, Canada, to reconstruct event water balances for 286 rain‐on‐snow events over a 10‐year period. For large rain‐on‐snow events (event rainfall >40 mm), snowmelt enhanced the production of water available for run‐off (WAR) by approximately 25% over rainfall alone. For smaller events, a range of antecedent and meteorological factors influenced WAR generation, particularly the antecedent liquid water content of the snowpack. Most large events were associated with atmospheric rivers. Rainfall dominated WAR generation during autumn and winter events, whereas snowmelt dominated during spring and summer events. In the majority of events, the sensible heat of rain contributed less than 10% of the total energy consumed by snowmelt. This analysis illustrated the importance of understanding the amount of rainfall occurring at high elevations during rain‐on‐snow events in mountainous regions.     

Hydrological functioning of a beaver dam sequence and regional dam persistence during an extreme rainstorm

It is becoming increasingly popular to reintroduce beaver to streams with the hopes of restoring riparian ecosystem function or reducing some of the hydrological impacts of climate change. One of the risks of relying on beaver to enhance ecosystem water storage is that their dams are reportedly more apt to fail during floods which can exacerbate flood severity. Missing are observations of beaver dam persistence and water storage capacity during floods, information needed to evaluate the risk of relying on beaver as a nature-based flood solution. A June rainstorm in 2013 triggered the largest recorded flood in the Canadian Rocky Mountains west of Calgary, Alberta. We opportunistically recorded hydrometric data during the rainfall event at a beaver-occupied peatland that has been studied for more than a decade. We supplemented these observations with a post-event regional analysis of beaver dam persistence. Results do not support two long-held hypotheses—that beaver ponds have limited flood attenuation capacity and commonly fail during large flood events. Instead we found that 68% of the beaver dam cascade systems across the region were intact or partially intact after the event. Pond fullness, in addition to the magnitude of the water-sediment surge, emerged as important factors in determining the structural fate of dam cascade sequences. Beaver ponds at the instrumented site quickly filled in the first few hours of the rain event and levels were dynamic during the event. Water storage offered by the beaver ponds, even ones that failed, delayed downstream floodwater transmission. Study findings have important implications for reintroducing beaver as part of nature-based restoration and climate change adaptation strategies.     

A comparison of surface and subsurface controls on summer temperature in a headwater stream

This study compared summer stream temperature between two years in the Star Creek catchment, Alberta, a headwater basin on the eastern slopes of the Canadian Rocky Mountains. Star Creek is a subsurface water dominated stream, which represents important habitat for native salmonid species. Hydrometeorological data from May to September of 2010 and 2011 accompanied by stream energy budget calculations were used to describe the drivers of stream temperature in this small forested stream. Mean, maximum, and minimum weekly stream temperatures were lower from May to August and higher in September 2011 compared to 2010. Weekly range in stream temperature was also different between years with a higher range in 2010. Inter‐annual stream temperature variation was attributed discharge differences between years, shown to be primarily governed by catchment‐scale moisture conditions. This study demonstrates that both meteorological and hydrological processes must be considered in order to understand stream temperature response to changing environmental conditions in mountainous regions. Copyright © 2013 John Wiley & Sons, Ltd.     

One hundred years of mountain hydrology in Switzerland by the WSL

In 1903 the Swiss Federal Research Institute WSL started its first forest hydrology measurements with the aim to deliver a sound scientific basis for the implementation of new forest legislation introduced in Switzerland in 1876. This legislation was triggered by several large floods that occurred in Switzerland, for which a major cause was widely seen as the poor condition of forests at that time. Consequently, hydrologic research at WSL first focused on the influence of forests on floods. In the second half of the 20th century, other hydrological issues such as water quality, snow hydrology and sediment transport complemented the hydrologic research at WSL. Some recent results of this work are presented in three papers joining this introductory paper to mark the 100th anniversary of hydrologic research at WSL. Copyright © 2006 John Wiley & Sons, Ltd.     

Hydrochemical behaviour of dissolved nitrogen and carbon in a headwater stream of the Canadian Shield: relevance of antecedent soil moisture conditions

This paper examines the impact of contrasting antecedent soil moisture conditions on the hydrochemical response, here the changes in dissolved nitrogen (NO₃^?, NH₄⁺ and dissolved organic nitrogen (DON)) and dissolved organic carbon (DOC) concentrations, of a first‐order stream during hydrological events. The study was performed in the Hermine, a 5 ha forested watershed of the Canadian Shield. It focused on a series of eight precipitation events (spring, summer and fall) sampled every 2 or 3 h and showing contrasted antecedent moisture conditions. The partition of the eight events between two groups (dry or wet) of antecedent moisture conditions was conducted using a principal component analysis (PCA). The partition was controlled (first axis explained 86% of the variability) by the antecedent streamflow, the streamflow to precipitation ratio Q/P and by the antecedent groundwater depth. The mean H⁺, NO₃^?, NH₄⁺, total dissolved nitrogen and DOC concentrations and electrical conductivity values in the stream were significantly higher following dry antecedent conditions than after wetter conditions had prevailed in the Hermine, although the temporal variability was high (17 to 138%). At the event scale, a significantly higher proportion of the changes in DON, NO₃^?, and DOC concentrations in the stream was explained by temporal variations in discharge compared with the seasonal and annual scales. Two of the key hydrochemical features of the dry events were the synchronous changes in DOC and flow and the frequent negative relationships between discharge and NO₃^?. The DON concentrations were much less responsive than DOC to changes in discharge, whereas NH was not in phase with streamflow. During wet events, the synchronicity between streamflow and DON or NO₃^? was higher than during dry events and discharge and NO₃^? were generally positively linked. Based on these observations, the hydrological behaviour of the Hermine is conceptually compatible with a two‐component model of shallow (DON and DOC rich; variable NO₃^?) and deep (DON and DOC poor; variable NO₃^?) subsurface flow. The high NO₃^? and DOC levels measured at the early stages of dry events reflected the contribution from NO₃^?‐rich groundwaters. The contribution of rapid surface flow on water‐repellent soil materials located close to the stream channel is hypothesized to explain the DOC levels. An understanding of the complex interactions between antecedent soil moisture conditions, the presence of soil nutrients available for leaching and the dynamics of soil water flow paths during storms is essential to explain the fluxes of dissolved nitrogen and carbon in streams of forested watersheds. Copyright © 2007 John Wiley & Sons, Ltd.     

Throughflow variability during snowmelt in a forested mountain catchment,coastal British Columbia,Canada

This study documented the spatial and temporal variability of outflow from a forested hillslope segment during snowmelt at a small mountain catchment in south coastal British Columbia, Canada. A pit 5 m wide was established just upslope from the stream channel. Outflow from the organic horizon was intercepted and measured by a single trough, and outflow from the mineral horizons was measured separately for three adjacent sections. Throughflow exhibited non‐steady‐state behaviour involving shifting allocations of flow amongst different sections of the outflow pit, as well as threshold effects and hysteresis in the relationship between pit outflow and water table elevation. Most of the pit outflow originated from the mineral horizons, indicating that throughflow was the dominant pathway by which water was delivered to the stream channel. Direct precipitation and snowmelt onto near‐stream saturated areas can account for less than 20% of the total outflow from the hillslope segment. Throughflow from the mineral sections consistently peaked either at the same time as or earlier than stream flow from the 150‐ha catchment during diurnal snowmelt cycles, indicating that throughflow appears to respond rapidly enough to contribute to snowmelt‐induced peak stream flow. Pit outflow cannot be extrapolated reliably to the catchment scale on the basis of simple length‐ or area‐based ratios. Copyright © 2004 John Wiley & Sons, Ltd.     

Insights on stream temperature processes through development of a coupled hydrologic and stream temperature model for forested coastal headwater catchments

Stream temperature controls a number of biological, chemical, and physical processes occurring in aquatic environments. Transient snow cover and advection associated with lateral throughflow inputs can have a dominant influence on stream thermal regimes for headwater catchments in the rain‐on‐snow zone. Most existing stream temperature models lack the ability to properly simulate these processes. We developed and evaluated a conceptual‐parametric catchment‐scale stream temperature model that includes the role of transient snow cover and lateral advection associated with throughflow. The model consists of routines for simulating canopy interception, snow accumulation and melt, hillslope throughflow runoff and temperature, and stream channel energy exchange processes. The model was used to predict discharge and stream temperature for a small forested headwater catchment near Vancouver, Canada, using long‐term (1963–2013) weather data to compute model forcing variables. The model was evaluated against 4 years of observed stream temperature. The model generally predicted daily mean stream temperature accurately (annual RMSE between 0.57 and 1.24 °C) although it overpredicted daily summer stream temperatures by up to 3 °C during extended low streamflow conditions. Model development and testing provided insights on the roles of advection associated with lateral throughflow, channel interception of snow, and surface–subsurface water interactions on stream thermal regimes. This study shows that a relatively simple but process‐based model can provide reasonable stream temperature predictions for forested headwater catchments located in the rain‐on‐snow zone.     

Seasonal snowpack dynamics and runoff in a cool temperate forest: lysimeter experiment in Niigata,Japan

Seasonal snowpack dynamics are described through field measurements under contrasting canopy conditions for a mountainous catchment in the Japan Sea region. Microclimatic data, snow accumulation, albedo and lysimeter runoff are given through the complete winter season 2002–03 in (1) a mature cedar stand, (2) a larch stand, and (3) a regenerating cedar stand or opening. The accumulation and melt of seasonal snowpack strongly influences streamflow runoff during December to May, including winter baseflow, mid‐winter melt, rain on snow, and diurnal peaks driven by radiation melt in spring. Lysimeter runoff at all sites is characterized by constant ground melt of 0·8–1·0 mm day⁻¹. Rapid response to mid‐winter melt or rainfall shows that the snowpack remains in a ripe or near‐ripe condition throughout the snow‐cover season. Hourly and daily lysimeter discharge was greatest during rain on snow (e.g. 7 mm h⁻¹ and 53 mm day⁻¹ on 17 December) with the majority of runoff due to rainfall passing through the snowpack as opposed to snowmelt. For both rain‐on‐snow and radiation melt events lysimeter discharge was generally greatest at the open site, although there were exceptions such as during interception melt events. During radiation melt instantaneous discharge was up to 4·0 times greater in the opening compared with the mature cedar, and 48 h discharge was up to 2·5 times greater. Perhaps characteristic of maritime climates, forest interception melt is shown to be important in addition to sublimation in reducing snow accumulation beneath dense canopies. While sublimation represents a loss from the catchment water balance, interception melt percolates through the snowpack and contributes to soil moisture during the winter season. Strong differences in microclimate and snowpack albedo persisted between cedar, larch and open sites, and it is suggested further work is needed to account for this in hydrological simulation models. Copyright © 2005 John Wiley & Sons, Ltd.     

On the estimation of antecedent wetness conditions in rainfall–runoff modelling

The analysis of the physical processes involved in a conceptual model of soil water content balance is addressed with the objective of its application as a component of rainfall–runoff modelling. The model uses routinely measured meteorological variables (rainfall and air temperature) and incorporates a limited number of significant parameters. Its performance in estimating the soil moisture temporal pattern was tested through local measurements of volumetric water content carried out continuously on an experimental plot located in central Italy. The analysis was carried out for different periods in order to test both the representation of infiltration at the short time‐scale and drainage and evapotranspiration processes at the long time‐scale. A robust conceptual model was identified that incorporated the Green–Ampt approach for infiltration and a gravity‐driven approximation for drainage. A sensitivity analysis was performed for the selected model to assess the model robustness and to identify the more significant parameters involved in the principal processes that control the soil moisture temporal pattern. The usefulness of the selected model was tested for the estimation of the initial wetness conditions for rainfall–runoff modelling at the catchment scale. Specifically, the runoff characteristics (runoff depth and peak discharge) were found to be dependent on the pre‐event surface soil moisture. Both observed values and those estimated by the model gave good results. On the contrary, with the antecedent wetness conditions furnished by two versions of the antecedent precipitation index (API), large errors were obtained. Copyright © 2007 John Wiley & Sons, Ltd.     

Development of a rain‐on‐snow detection algorithm using passive microwave radiometry

Currently observed climate warming in the Arctic has numerous consequences. Of particular relevance, the precipitation regime is modified where mixed and liquid precipitation can occur during the winter season leading to rain‐on‐snow (ROS) events. This phenomenon is responsible for ice crust formation, which has a significant impact on ecosystems (such as biological, hydrological, ecological and physical processes). The spatially and temporally sporadic nature of ROS events makes the phenomenon difficult to monitor using meteorological observations. This paper focuses on the detection of ROS events using passive microwave (PMW) data from a modified brightness temperature (T_B) gradient approach at 19 and 37 GHz. The approach presented here was developed empirically for observed ROS events with coincident ground‐based PMW measurements in Sherbrooke, Quebec, Canada. It was then tested in Nunavik, Quebec, with the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR‐E). We obtained a detection accuracy of 57, 71 and 89% for ROS detection for three AMSR‐E grid cells with a maximum error of 7% when considering all omissions and commissions with regard to the total number of AMSR‐E passes throughout the winter period. Copyright © 2016 John Wiley & Sons, Ltd.     

Hydrological response to timber harvest in northern Idaho: implications for channel scour and persistence of salmonids

The potential for forest harvest to increase snowmelt rates in maritime snow climates is well recognized. However, questions still exist about the magnitude of peak flow increases in basins larger than 10 km² and the geomorphic and biological consequences of these changes. In this study, we used observations from two nearly adjacent small basins (13 and 30 km²) in the Coeur d'Alene River basin, one with recent, relatively extensive, timber harvest, and the other with little disturbance in the last 50 years to explore changes in peak flows due to timber harvest and their potential effects on fish. Peak discharge was computed for a specific rain‐on‐snow event using a series of physical models that linked predicted values of snowmelt input to a runoff‐routing model. Predictions indicate that timber harvest caused a 25% increase in the peak flow of the modelled event and increased the frequency of events of this magnitude from a 9‐year recurrence interval to a 3·6‐year event. These changes in hydrologic regime, with larger discharges at shorter recurrence intervals, are predicted to increase the depth and frequency of streambed scour, causing up to 15% added mortality of bull trout (Salvelinus confluentus) embryos. Mortality from increased scour, although not catastrophic, may have contributed to the extirpation of this species from the Coeur d'Alene basin, given the widespread timber harvest that occurred in this region. Copyright © 2008 John Wiley & Sons, Ltd.     

Impact of climate change on runoff from a mid‐latitude mountainous catchment in central Japan

Hydrologic balance in high‐altitude, mid‐latitude mountain areas is important in terms of the water resources available to associated lowlands. This study examined how current and historical shifts in precipitation (P) patterns and concurrent increases in temperature (T) affected runoff (Q) and other hydrologic components in a mid‐latitude mountain catchment of central Japan, using a combination of long‐term data and a simplified hydrologic model, along with their stochastic treatment. The availability of intensive meteorological and hydrological data from the period 1997–2001 allowed the derivation of key relationships for the current climate that tie the forcing term to the parameters or state variables. By using the data recorded in the period 1965–2001, the force for driving the historical simulation was generated. Based on this model and historical shifts in P and T, the probability density functions of Q (pdf(Q)) was computed. A main novelty in this study is that such a stochastic representation, which is useful for considering the influence of projected shifts in environmental factors on the hydrologic budget, was provided. Despite the large increase in the rate of T in winter and spring, pdf(Q) in spring and summer varied appreciably during the time studied mainly because of an increase in snowmelt. An interannual change in whole‐year Q was robust to shifts in T because while Q in spring increased, in summer it decreased, implying a crucial effect of global warming on mountain hydrologic regimes is change in the timing of Q. Copyright © 2009 John Wiley & Sons, Ltd.     

Reconstruction of a flash flood with large wood transport and its influence on hazard patterns in an ungauged mountain basin

The reconstruction of past flash floods in ungauged basins leads to a high level of uncertainty, which increases if other processes are involved such as the transport of large wood material. An important flash flood occurred in 1997 in Venero Claro (Central Spain), causing significant economic losses. The wood material clogged bridge sections, raising the water level upstream. The aim of this study was to reconstruct this event, analysing the influence of woody debris transport on the flood hazard pattern. Because the reach in question was affected by backwater effects due to bridge clogging, using only high water mark or palaeostage indicators may overestimate discharges, and so other methods are required to estimate peak flows. Therefore, the peak discharge was estimated (123 ± 18 m³ s^–1) using indirect methods, but one‐dimensional hydraulic simulation was also used to validate these indirect estimates through an iterative process (127 ± 33 m³ s^–1) and reconstruct the bridge obstruction to obtain the blockage ratio during the 1997 event (~48%) and the bridge clogging curves. Rainfall–Runoff modelling with stochastic simulation of different rainfall field configurations also helped to confirm that a peak discharge greater than 150 m³ s^–1 is very unlikely to occur and that the estimated discharge range is consistent with the estimated rainfall amount (233 ± 27 mm). It was observed that the backwater effect due to the obstruction (water level ~7 m) made the 1997 flood (~35‐year return period) equivalent to the 50‐year flood. This allowed the equivalent return period to be defined as the recurrence interval of an event of specified magnitude, which, where large woody debris is present, is equivalent in water depth and extent of flooded area to a more extreme event of greater magnitude. These results highlight the need to include obstruction phenomena in flood hazard analysis. Copyright © 2012 John Wiley & Sons, Ltd.     

Impact of road‐generated storm runoff on a small catchment response

The impact of road‐generated runoff on the hydrological response of a zero‐order basin was monitored for a sequence of 24 storm events. The study was conducted in a zero‐order basin (C1; 0·5ha) with an unpaved mountain road; an adjacent unroaded zero‐order basin (C2; 0·2 ha) with similar topography and lithology was used to evaluate the hydrological behaviour of the affected zero‐order basin prior to construction of the road. The impact of the road at the zero‐order basin scale was highly dependent on the antecedent soil‐moisture conditions, total storm precipitation, and to some extent rainfall intensity. At the beginning of the monitoring period, during dry antecedent conditions, road runoff contributed 50% of the total runoff and 70% of the peak flow from the affected catchment (C1). The response from the unroaded catchment was almost insignificant during dry antecedent conditions. As soil moisture increased, the road exerted less influence on the total runoff from the roaded catchment. For very wet conditions, the influence of road‐generated runoff on total outflow from the roaded catchment diminished to only 5·4%. Both catchments, roaded and unroaded, produced equivalent amount of outflow during very wet antecedent conditions on a unit area basis. The lag time between the rainfall and runoff peaks observed in the unroaded catchment during the monitoring period ranged from 0 to 4 h depending on the amount of precipitation and antecedent conditions, owing mainly to much slower subsurface flow pathways in the unroaded zero‐order basin. In contrast, the lag time in the roaded zero‐order basin was virtually nil during all storms. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance of flood frequency pooling analysis in a low CV context

Abstract

Pooling of flood data is widely used to provide a framework to estimate design floods by the Index Flood method. Design flood estimation with this approach involves derivation of a growth curve which shows the relationship between X_T and the return period T, where X_T ?=?Q_T /Q_I and Q_I is the index flood at the site of interest. An implicit assumption with the Index Flood procedure of pooling analysis is that the X_T –T relationship is the same at all sites in a homogeneous pooling group, although this assumption would generally be violated to some extent in practical cases, i.e. some degree of heterogeneity exists. In fact, in only some cases is the homogeneity criterion effectively satisfied for Irish conditions. In this paper, the performance of the index-flood pooling analysis is assessed in the Irish low CV (coefficient of variation) hydrology context considering that heterogeneity is taken into account. It is found that the performance of the pooling method is satisfactory provided there are at least 350 station years of data included. Also it is found that, in a highly heterogeneous group, it is more desirable to have many sites with short record lengths than a smaller number of sites with long record lengths. Increased heterogeneity decreases the advantage of pooling group-based estimation over at-site estimation. Only a heterogeneity measure (H1) less than 4.0 can render the pooled estimation preferable to that obtained for at-site estimation for the estimation of 100-year flood. In moderately to highly heterogeneous regions it is preferable to conduct at-site analysis for the estimation of 100-year flood if the record length at the site concerned exceeds 50.

Editor Z.W. Kundzewicz; Associate editor A. Carsteanu

Citation Das, S. and Cunnane, C., 2012. Performance of flood frequency pooling analysis in a low CV context. Hydrological Sciences Journal, 57 (3), 433–444.     

Climate variability,snow, and physiographic controls on storm hydrographs in small forested basins,western Cascades,Oregon

Large floods are often attributed to the melting of snow during a rain event. This study tested how climate variability, snowpack presence, and basin physiography were related to storm hydrograph shape in three small (<1 km²) basins with old‐growth forest in western Oregon. Relationships between hydrograph characteristics and precipitation were tested for approximately 800 storms over a nearly 30‐year period. Analyses controlled for (1) snowpack presence/absence, (2) antecedent soil moisture, and (3) hillslope length and gradient. For small storms (<150 mm precipitation), controlling for precipitation, the presence of a snowpack on near‐saturated soil increased the threshold of precipitation before hydrograph rise, extended the start lag, centroid lag, and duration of storm hydrographs, and increased the peak discharge. The presence of a snowpack on near‐saturated soil sped up and steepened storm hydrographs in a basin with short steep slopes, but delayed storm hydrographs in basins with longer or more gentle slopes. Hydrographs of the largest events, which were extreme regional rain and rain‐on‐snow floods, were not sensitive to landform characteristics or snowpack presence/absence. Although the presence of a snowpack did not increase peak discharge in small, forested basins during large storms, it had contrasting effects on storm timing in small basins, potentially synchronizing small basin contributions to the larger basin hydrograph during large rain‐on‐snow events. By altering the relative timing of hydrographs, snowpack melting could produce extreme floods from precipitation events whose size is not extreme. Further work is needed to examine effects of canopy openings, snowpack, and climate warming on extreme rain‐on‐snow floods at the large basin scale. Copyright © 2008 John Wiley & Sons, Ltd.     

Scaling and regionalization of flood flows in British Columbia,Canada

A regionalization of flood data in British Columbia reveals a common scaling with drainage area over the range 0·5×10²<A_d<10⁴ km². This scaling is not a function of flood return period, which implies that simple scaling—consistent with a snowmelt‐dominated flow regime—applies to the province. The observed scale relation takes the form , similar to values reported in previous studies. The scaling relation identified was used to define the regional pattern of hydroclimatic variability for flood flows in British Columbia after discounting the effect of drainage area. The pattern was determined by kriging a scale‐independent runoff factor k for the mean annual flood, 5 year flood and 20 year flood. The analysis permits quantification of uncertainty of the estimates, which can be used in conjunction with the mapped k‐fields to calculate a mean and range for floods with the identified return period for ungauged basins. Owing to the sparsity of data, the precision is relatively poor. The standard error is generally less than 75% of the estimate in the southern half of the province, whereas in the northern half it is often between 75 and 100%. Examination of the relative increase in flood magnitude with increasing return period reveals spatially consistent but statistically insignificant differences. Flood magnitude tends to increase more rapidly in the western regions, where rain events may contribute to flood generation. The relative increase in flood magnitude with return period is consistently lower in the eastern mountain ranges, where snowmelt dominates the flood flow regime. Copyright © 2002 John Wiley & Sons, Ltd.