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  • View in gallery

    Map of BCSAK showing the location of the hydrometric stations used in this analysis. The station numbers on the map refer to the station IDs listed in Table S1. The inset map shows the extent of the main figure and the elevation distribution across BCSAK. The histogram above the inset map shows the distribution of average watershed elevation in bins of 200 m.

  • View in gallery

    Climatology and hydrological regimes of the watersheds in BCSAK. (a) Mean and (b) maximum daily runoff (mm day−1) of the selected watersheds across BCSAK, WYs 1979–2016. (c) A polar plot showing the location of the average maximum runoff day in seasonality space. Dispersion of the points from the center indicates regularity of runoff timing; 1 being the most regular. Consult Burn et al. (2010) for the details on directional statistics of maximum runoff day. (d) Average hydrological regimes of each of the watersheds following the distribution of monthly average discharge, WYs 1979–2016.

  • View in gallery

    The contribution of ARs to total annual runoff (in %) across BCSAK, WYs 1979–2012.

  • View in gallery

    (a) AR-related annual maximum runoff (%) and (b) the number of AR-related AM runoff out of the top 10 AM runoff in each watershed, WYs 1979–2016.

  • View in gallery

    (a) Number of AM runoff and (b) the percentage of AR-related AM runoff in each season across BCSAK, WYs 1979–2016.

  • View in gallery

    Distribution of AR-related and non-AR-related daily maximum runoff across BCSAK, WYs 1979–2016. The black vertical lines indicate median daily maximum runoff, red dots indicate mean runoff of all the watersheds, and horizontal lines show the range of daily maximum runoff.

  • View in gallery

    Average daily maximum runoff, WYs 1979–2016 vs (a) basin area (on a base 10 logarithmic scale), (b) distance from the nearest coastline to the centroid of the watershed, and (c) average slope for 168 watersheds across BCSAK. (d) Relation between watershed aspect and average AR-related AM runoff percentage. The color gradient indicates average AR-related annual maximum runoff association percentage. The red, black, and green diamonds in (a) indicate the Homathko, Iskut, and Stikine River basins, respectively.

  • View in gallery

    (a) Mean daily air temperatures at different elevation grids during AR and non-AR winter maximum runoff days for three selected watersheds representing different hydrological regimes (Fig. 1). (b) Cumulative fraction of the watershed area for three selected watersheds representing different hydrological regimes (Fig. 1). The horizontal dashed lines show the percentage of the watershed area contributing to the surface runoff as a function of elevation, the red and blue solid lines indicate the average elevation with mean daily air temperature of 0°C during AR and non-AR days winter maximum runoff series.

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Linking Atmospheric Rivers to Annual and Extreme River Runoff in British Columbia and Southeastern Alaska

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  • 1 Natural Resources and Environmental Studies Program, University of Northern British Columbia, Prince George, British Columbia, Canada
  • | 2 Environmental Science and Engineering Program, University of Northern British Columbia, Prince George, British Columbia, Canada
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Abstract

This study quantifies the contribution of atmospheric rivers (ARs) to annual and extreme river runoff and evaluates the relationships between watershed characteristics and AR-related maximum river runoff across British Columbia and southeastern Alaska (BCSAK). Datasets used include gauged runoff from 168 unregulated watersheds, topographic characteristics of those watersheds, a regional AR catalog, and integrated vapor transport fields for water years (WYs) 1979–2016. ARs contribute ~22% of annual river runoff along the Coast and Insular Mountains watersheds, which decreases inland to ~11% in the watersheds of the Interior Mountains and Plateau. Average association between ARs and annual maximum river runoff attains >80%, >50%, and <50% along the watersheds of the western flanks of the Coast Mountains, the Interior Mountains, and Interior Plateau, respectively. There is no significant change in AR-related extreme annual maximum runoff across BCSAK during 1979–2016. AR conditions occur during 25 out of 32 of the flood-related natural disasters in British Columbia during WYs 1979–2016. AR-related annual maximum runoff magnitude is significantly higher than non-AR-related annual maximum runoff for 30% of the watersheds studied. Smaller and steeper watersheds closer to the coast are more susceptible to AR-related annual maximum runoff than their inland counterparts. These results illustrate the importance of AR activity as a major control for the distribution of peak runoff in BCSAK. This work provides insights on the hydrological response of watersheds of northwestern North America to landfalling ARs that may improve flood risk assessment and disaster management in this region.

Supplemental information related to this paper is available at the Journals Online website: https://doi.org/10.1175/JHM-D-19-0281.s1.

© 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Aseem R. Sharma, aseem.sharma@alumni.unbc.ca

Abstract

This study quantifies the contribution of atmospheric rivers (ARs) to annual and extreme river runoff and evaluates the relationships between watershed characteristics and AR-related maximum river runoff across British Columbia and southeastern Alaska (BCSAK). Datasets used include gauged runoff from 168 unregulated watersheds, topographic characteristics of those watersheds, a regional AR catalog, and integrated vapor transport fields for water years (WYs) 1979–2016. ARs contribute ~22% of annual river runoff along the Coast and Insular Mountains watersheds, which decreases inland to ~11% in the watersheds of the Interior Mountains and Plateau. Average association between ARs and annual maximum river runoff attains >80%, >50%, and <50% along the watersheds of the western flanks of the Coast Mountains, the Interior Mountains, and Interior Plateau, respectively. There is no significant change in AR-related extreme annual maximum runoff across BCSAK during 1979–2016. AR conditions occur during 25 out of 32 of the flood-related natural disasters in British Columbia during WYs 1979–2016. AR-related annual maximum runoff magnitude is significantly higher than non-AR-related annual maximum runoff for 30% of the watersheds studied. Smaller and steeper watersheds closer to the coast are more susceptible to AR-related annual maximum runoff than their inland counterparts. These results illustrate the importance of AR activity as a major control for the distribution of peak runoff in BCSAK. This work provides insights on the hydrological response of watersheds of northwestern North America to landfalling ARs that may improve flood risk assessment and disaster management in this region.

Supplemental information related to this paper is available at the Journals Online website: https://doi.org/10.1175/JHM-D-19-0281.s1.

© 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Aseem R. Sharma, aseem.sharma@alumni.unbc.ca

1. Introduction

The area of British Columbia and southeastern Alaska (BCSAK) is affected by the prevailing westerlies commonly observed in the northern midlatitudes (Hare 1998; Stahl et al. 2006). BCSAK experiences extreme runoff through different flood-generating mechanisms such as rapid spring snowmelt, rainstorms, rain-on-snow (ROS) events, and occasional ice jams (Melone 1985; Buttle et al. 2016; Zahmatkesh et al. 2019). Moreover, the complex topography and sharp elevational gradients within the watersheds influence local precipitation and runoff processes across this region (Jarosch et al. 2012; O’Neel et al. 2015; Reynoldson et al. 2005; Richardson and Milner 2005).

The warm, moist air masses that result in heavy precipitation and floods in BCSAK are often associated with midlatitude horizontal water vapor transport through elongated moisture plumes emanating from the North Pacific Ocean called atmospheric rivers (ARs), also dubbed Pineapple Expresses (Ralph et al. 2006; Zhu and Newell 1994; Lavers et al. 2011; Roberge et al. 2009; Sharma and Déry 2020a; Lackmann and Gyakum 1999; Mo et al. 2019). Generally located within the warm sector of extratropical cyclones and concentrated in the lower troposphere, ARs form long and narrow bands of moisture inducing abundant precipitation in coastal basins (Bao et al. 2006; Dettinger et al. 2011; Ralph et al. 2017). ARs act as a key synoptic-scale mechanism that replenishes water resources while occasionally causing extreme runoff along western North America (e.g., Barth et al. 2017; Konrad and Dettinger 2017; Neiman et al. 2011; Ralph et al. 2006). When a moisture-laden AR intersects with the mountainous terrain of BCSAK, it produces heavy orographic precipitation resulting from forced uplift (Jarosch et al. 2012; Sharma and Déry 2020b; Smith et al. 2010). The orographic precipitation often occurs as rain at lower elevations and snow at higher elevations. On occasion, this precipitation can induce peak flows even greater than during the spring snowmelt-driven freshet (ECCC 2019).

Previous studies along the Pacific Northwest and lower latitudes of coastal British Columbia (BC) have observed AR attributes in atmospheric circulations such as narrow plumes of vertically integrated water vapor and warm and moist southwesterly airflow during or before flooding events (e.g., Dettinger et al. 2011; Lackmann and Gyakum 1999; Melone 1985; Spry et al. 2014). For example, an intense AR impacted the Bella Coola region of BC on 25 September 2010 causing catastrophic flooding (PSC 2019; PCIC 2013). Similarly, an intense AR event along the Sea-to-Sky Highway of BC in October 2003 triggered major flooding throughout the Squamish and Pemberton regions of BC, claiming the lives of two people and causing approximately $90 million (Canadian dollars) in damages (PSC 2019). As such, it is crucial to understand the linkages between ARs and extreme runoff, historical changes in AR-related flooding, and anticipate changes in future water availability and hydrological extremes associated with ARs in BCSAK.

While there is an emerging literature investigating the linkages between ARs and peak runoff (Curry et al. 2019; Demaria et al. 2017; Dettinger et al. 2011; Kingston et al. 2016; Konrad and Dettinger 2017; Lavers et al. 2011; Lavers and Villarini 2013; Ralph et al. 2006), there has not been any targeted research on linkages between ARs, runoff, and their extremes across BCSAK except Curry et al. (2019). Curry et al. (2019) investigated the future flood risk associated with ARs in the Fraser River basin, whose many subbasins are analyzed in this study, under the representative concentration pathway (RCP) 8.5 scenario. BCSAK experiences a range of different hydrological regimes (e.g., pluvial, nival, glacial, and their hybrids; Burn et al. 2010; Déry et al. 2009) and provides an opportunity to better understand responses to ARs and extreme runoff across these regimes. In this context, this work investigates linkages between ARs and the observed annual extreme runoff across BCSAK. Specifically, this research addresses the following questions:

  1. How much do landfalling ARs contribute to water year (WY) runoff across BCSAK?

  2. What is the association between landfalling ARs and maximum runoff across BCSAK for WYs 1979–2016?

  3. How do AR-related maximum runoff magnitudes differ from non-AR-related maximum runoff magnitudes across BCSAK?

  4. How does the extreme hydrological response of a watershed to ARs differ with watershed characteristics in BCSAK?

2. Study area

For this study, we consider 168 watersheds in BCSAK along the Pacific coast of northwestern North America (Fig. 1). The study area comprises coastal regions, mountainous terrain (e.g., the Rocky Mountains in the east, the Columbia Mountains in the interior, the Coast and Insular Mountains in the west, and the Saint Elias Mountains in the north), the Interior Plateau, and the BC’s northeastern Plains (Hernández-Henríquez et al. 2017; Holland 1976; Richardson and Milner 2005). The major watershed characteristics include glacierized mountains, transitional tundra landscapes, and decreasing vegetation coverage with increasing elevations (Demarchi 2011; Holland 1976). Influenced by the maritime and midlatitude westerlies and enhanced through localized orography, the western arc of the coastal mountains of BCSAK receives more precipitation than the drier interior plateau region (Moore et al. 2010). The mountainous regions of BCSAK form headwaters for many of the major river basins of western Canada that drain to the Pacific Ocean (Columbia, Fraser, Nass, Skeena, and Stikine Rivers), Hudson Bay (North and South Saskatchewan Rivers), Bering Strait (Yukon River), and Arctic Ocean (Mackenzie River) (Richardson and Milner 2005; Benke and Cushing 2005). Runoff characteristics vary with location and season; coastal regions experience mostly rain-fed runoff whereas watersheds in the Interior Plateau and the Rocky and Columbia Mountains (hereafter Interior Mountains) are snowfall and/or glacial dominated (Benke and Cushing 2005).

Fig. 1.
Fig. 1.

Map of BCSAK showing the location of the hydrometric stations used in this analysis. The station numbers on the map refer to the station IDs listed in Table S1. The inset map shows the extent of the main figure and the elevation distribution across BCSAK. The histogram above the inset map shows the distribution of average watershed elevation in bins of 200 m.

Citation: Journal of Hydrometeorology 21, 11; 10.1175/JHM-D-19-0281.1

3. Data and methods

a. Data

1) Streamflow and watershed characteristics

We use observed daily streamflow data from 168 hydrometric stations selected across BCSAK sourced from the Hydrometric Database (HYDAT) of the Water Survey of Canada (WSC) for BC, and the U.S. Geological Survey (USGS) database for southeastern Alaska. Stations are chosen following three criteria: (i) the station represents a river/stream with unregulated flow, (ii) data are available for WYs 1979–2016, and (iii) the station represents an upstream gauged area ≥ 10 km2. The mean daily flow over the period of record fills in any data gaps at a given hydrometric station. Some of the daily discharge data may be subject to errors associated with collection issues, as discussed in-depth in Déry et al. (2016). Seventy-two out of 168 watersheds are nested within the larger watersheds and represent 22% of the total watershed area analyzed in this study (Table S1 in the online supplemental material). A period from 1 October to 30 September of the following calendar year defines a WY; we perform analyses for WYs 1979–2016, i.e., from 1 October 1978 to 30 September 2016. We choose the period of WYs 1979–2016 since it has a maximum number of hydrometric stations with daily data. Similarly, a high-quality AR chronology and integrated vapor transport (IVT) fields exist starting in 1979 due to the ingestion of modern satellite weather data in the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis datasets (Kalnay et al. 1996). Of note, AR-related runoff analysis associated with precipitation covers only 1979–2012 due to the availability of the precipitation dataset only up to 2012 [see section 3b(2)].

For each watershed, the hydrometric station metadata from the WSC and USGS provide the station’s coordinates, basin area, mean basin elevation, and percentage of glacierized area within the watershed. Similarly, the WSC geodatabase provides shapefiles of the upstream gauged area; however, the shapefiles are available for only 142 watersheds. For the remaining 26 watersheds, we delineate the watershed boundaries and develop the upstream gauged shapefile using the Shuttle Radar Topography Mission (SRTM) 90-m digital elevation model (DEM) (Jarvis et al. 2008). Additionally, we calculate the slope and aspect of the watersheds and fill the missing gaps on average basin elevation and area using the SRTM DEM. Equations (1a) and (1b) yield the slope and aspect of a grid cell where dz/dx and dz/dy measure the rate of change of the surface in the horizontal and vertical directions, respectively, from that grid cell (Burrough and McDonnell 1998). The aspect ranges from 0° to 360° and calculated with the convention that 0°, 90°, 180°, and 270° faces north, east, south, and west, respectively:
slope(in degrees)=[tan1(dzdx)2+(dzdy)2]×180°π,
aspect(in degrees)=270°360°2πtan1(dzdx,dzdy).
Averaging of the slope and circular averaging of the aspect of all grid cells within a given watershed provide the watershed slope and aspect, respectively. The slope of a watershed plays a key role in determining the variations in surface hydrology, particularly in the pluvial watersheds, by impacting the time between precipitation and maximum discharge within the watershed (Viessman and Lewis 2003). For those selected watersheds, the upstream gauged area ranges from 16 to 1.04 × 105 km2, average watershed elevation varies from 24 to 2124 m, and the slope ranges from 1° to 30° (Table S1).

2) Atmospheric rivers data

We use the AR chronology and 6-hourly IVT fields for WYs 1979–2016 from a regional AR catalog published by the Scripps Institution of Oceanography (SIO-R1-AR Catalog), available through http://cw3e.ucsd.edu/Publications/SIO-R1-Catalog/ (accessed 13 February 2018) (Gershunov et al. 2017). The SIO-R1-AR Catalog derives the AR chronology and the IVT fields using 6-hourly specific humidity and wind fields at 2.5° × 2.5° spatial resolution from the NCEP–NCAR reanalysis dataset (Kalnay et al. 1996). In this AR catalog, Gershunov et al. (2017) identify an AR as a contiguous region of moisture flux that crosses the coastline and extends >1500 km in length with an IVT threshold > 250 kg m−1 s−1. The SIO-R1-AR Catalog has a higher temporal resolution (6-hourly) and is developed explicitly for western North America (20°–60°N); therefore, it is appropriate for this type of regional analysis compared to other AR databases (Sharma and Déry 2020a).

b. Methods

1) Annual and extreme runoff

A normalization of the daily discharge value (m3 s−1) by the watershed area gives daily areal runoff (mm day−1); daily runoff values are in a similar range between watersheds of different sizes. Summation of the daily runoff values during each WY provides annual runoff for each watershed. Implementation of the block maxima approach of the extreme value theory (Coles 2001) for WYs 1979–2016 provides the annual and seasonal maxima runoff of each watershed. The block is defined by dividing the observation period, i.e., WYs 1979–2016, into nonoverlapping periods of equal size (years and seasons). Then the maximum value in each block generates a series of annual and seasonal maximum runoff. This approach minimizes the serial dependence, covers a longer temporal range, and is widely used for extreme runoff analysis (Coles 2001; Katz et al. 2002; Lavers and Villarini 2013; Rood et al. 2016). Implementation of the block maxima procedure in this analysis generates a series of annual maxima (AM) and four seasonal (winter: December–February; spring: March–May; summer: June–August; and autumn: September–November) maxima runoff for each watershed (e.g., Fig. S1 in the online supplemental material).

A computation of daily mean runoff, maximum runoff, and average total annual runoff gives the general climatology of the watersheds. The month and day of maximum runoff averaged over the study period provide a typical hydrological regime (Déry et al. 2009; Burn et al. 2016). Additionally, detailed investigations on three watersheds representing different hydrological regimes, namely, pluvial (San Juan River, Station ID: 08HA010), nival (Lardeau River, Station ID: 08NH007), and glacial (Lillooet River, Station ID: 08MG005; Fig. 1) (Déry et al. 2009), help to further explore the physical processes associated with ARs and extreme runoff.

2) AR contribution to annual runoff

To calculate AR-related WY runoff in each watershed, we implement an empirical equation that is based on each WY’s water budget within a watershed. As a first-order estimate, we quantified the contribution of ARs to total WY runoff as equating the contribution of ARs to total WY precipitation. Use of the recently developed, high-resolution Pacific Climate Impacts Consortium meteorology for northwestern North America (PNWNAmet) gridded precipitation data available up to 31 December 2012 (Werner et al. 2019) provides the proportion of AR-related and non-AR-related precipitation in each watershed for WYs 1979–2012 as in Eqs. (2a) and (2b). A division of the total AR-day’s precipitation in a WY by the total precipitation within that WY provides the fraction of AR-related precipitation in each grid cell across BCSAK [see Sharma and Déry (2020b) for details on the contribution of ARs to precipitation]:
PAR+PNAR=PTOT,
a+b=1,
where PTOT, PAR, and PNAR are the total, total AR-related, and total non-AR-related precipitation averaged from all the grid cells within the watershed, respectively. Coefficient a = PAR/PTOT denotes the fraction of WY precipitation associated with ARs and b = PNAR/PTOT the remainder of the precipitation. Using the same approach for runoff with coefficients a = RAR/RTOT and b = RNAR/RTOT in Eq. (3) with the total WY runoff RTOT gives the proportion of AR-related annual runoff for each watershed in BCSAK during WYs 1979–2012:
RAR+RNAR=RTOT,
where RAR and RNAR are the AR-related and non-AR-related total WY runoff, respectively.

Of note, this method provides only a first-order estimate of the contribution of ARs to annual WY runoff across the watersheds of BCSAK with different hydrological regimes. It assumes that the ARs may bring precipitation as snow and/or rain or a mixture of both; however, the contribution of ARs to runoff would be proportional to the total precipitation over the period of each WY, thus yielding the coefficients a and b. These coefficients, in turn, are used to calculate AR-related runoff for each WY with assumptions that no changes occur in the total water storage within the watershed and the ratio of AR-related to total WY runoff is equal to that of precipitation i.e., RAR/RTOT = PAR/PTOT. Moreover, further analysis on how AR contribution to runoff using Eq. (3) compares with a directly calculated (percentage of total AR-days runoff divided by total WY’s runoff) AR contribution percentage for the pluvial San Juan River shows a good agreement between the first-order estimate (mean 26% ± 6%) and a direct calculation (mean 25% ± 8%) (Fig. S2a). Similarly, the difference in AR-related runoff and precipitation ratio i.e., RAR/RTOTPAR/PTOT is minimal (average 0.04) for the pluvial San Juan River with slightly higher RAR/RTOT for most of the 1979–2012 WYs. The slightly higher ratio is likely due to relatively more runoff induced from intense rainfall associated with ARs than smaller-intensity rainfall events (Fig. S2b).

3) AR association to extreme runoff

To associate an AR with maximum runoff, we first identify the dates of extreme annual (or seasonal) runoff occurrence for each watershed then cross-reference whether the IVT field in that watershed exceeds the threshold of 250 kg m−1 s−1 within the maximum runoff day or 6 days prior to that day. The maximum runoff is AR-related if there is presence of a landfalling AR within at least 1 of those 7 days and the IVT field within a watershed exceeds the threshold value. The maximum runoff duration of a watershed depends on precipitation characteristics and basin properties (Viessman and Lewis 2003). The use of maximum runoff day and previous 6 days in this study accounts for the maximum duration of the runoff even in the larger watersheds of BC (e.g., Fraser River Basin) (Curry et al. 2019; Lavers and Villarini 2013). The percentage of number of AR-related maximum runoff days out of the total number of maximum runoff days in each watershed provides the linkages between ARs and maximum runoff. The landfall of an AR indicates the occurrence of AR-related precipitation in the region whereas the IVT > 250 kg m−1 s−1 within a watershed implies the occurrence of AR-related precipitation in that specific watershed. The IVT of 250 kg m−1 s−1 is a widely used threshold to separate the AR attributes from contiguous grid cells (Gershunov et al. 2017; Rutz et al. 2014; Lamjiri et al. 2017).

Of note, the AR-related precipitation may occur as snow leading to a lag between the precipitation and the extreme runoff. Therefore, we further establish direct linkages (in percentages) between AR-related extreme precipitation and runoff for WYs 1979–2012. To establish such links, we followed the steps mentioned above but only for the maximum precipitation days in each watershed as described in Sharma and Déry (2020b). The Spearman’s rank correlation coefficient (r) [statistically significant when probability value (p) < 0.05] shows a relation between AR-related maximum runoff and watershed characteristics. Finally, a division of BCSAK into four main hydrozones: Coastal (including southeastern Alaska), Southern Interior, the Interior Mountains, and Northern Interior and Plains (Fig. 1 and Table S1) yields regional patterns of the role of major topographic features on AR-related maximum runoff.

4) AR-related versus non-AR-related extreme runoff

Distribution plots and the Mann–Whitney–Wilcoxon (MWW) test (Hollander et al. 2014) compare the AR-related extreme runoff with non-AR-related extreme runoff during WYs 1979–2016. The MWW is a nonparametric statistical test to analyze whether the number and magnitude of AR-related maximum runoff are significantly greater than non-AR-related maximum runoff.

4. Results

a. Runoff climatology

Rivers from the coastal region (Vancouver Island, Haida Gwaii, and southeastern Alaska), those draining westward from the Coast Mountains, and the Interior Mountains exhibit higher daily mean (>5 mm day−1) and maximum (~50 mm day−1) runoff compared to the rivers of the Interior Plateau and northeastern Plains (mean < 2 mm day−1, maximum < 20 mm day−1) of BCSAK (Figs. 2a,b). This variation suggests, in part, the important role of orography and proximity to the Pacific Ocean in determining mean and maximum runoff across this region. Areas of higher daily mean and maximum runoff correspond to the abundant precipitation that occurs in those regions of BCSAK (Melone 1985). For example, some of the highest daily runoff values occur along the panhandle region of Alaska and Vancouver Island, receiving on average ≥10 mm day−1 (maximum ≥ 200 mm day−1) of precipitation. Daily mean precipitation along the Interior Mountains reaches ~7 mm day−1 (maximum ~100 mm day−1) whereas that in the Interior Plateau and across the northeastern Plains of BC is ~1 mm day−1 (maximum ~10 mm day−1), with correspondingly lower daily runoff values (Figs. 2a,b). The peak runoff occurs during autumn and/or winter months along the coastal watersheds; however, there exists high variability on the day of maximum runoff (Fig. 2c). The majority of watersheds in the Interior Plateau and Mountains exhibit maximum runoff during late spring and summer, mainly due to snow and glacier melting (Figs. 2c,d).

Fig. 2.
Fig. 2.

Climatology and hydrological regimes of the watersheds in BCSAK. (a) Mean and (b) maximum daily runoff (mm day−1) of the selected watersheds across BCSAK, WYs 1979–2016. (c) A polar plot showing the location of the average maximum runoff day in seasonality space. Dispersion of the points from the center indicates regularity of runoff timing; 1 being the most regular. Consult Burn et al. (2010) for the details on directional statistics of maximum runoff day. (d) Average hydrological regimes of each of the watersheds following the distribution of monthly average discharge, WYs 1979–2016.

Citation: Journal of Hydrometeorology 21, 11; 10.1175/JHM-D-19-0281.1

Rivers located along the coastal regions and on the western flanks of the Coast Mountains exhibit higher mean annual runoff (mean 1778 mm yr−1, maximum > 5000 mm yr−1) compared to the Interior Mountains (mean 926 mm yr−1, maximum > 2100 mm yr−1) and the Interior Plateau (mean 594 mm yr−1, maximum > 1700 mm yr−1) (Fig. S3).

b. ARs and annual and extreme runoff

1) AR contribution to annual runoff

A distinct spatial variation in the percentage of the contribution of ARs to annual runoff during WYs 1979–2012 arises across BCSAK (Fig. 3 and Fig. S4). ARs contribute, on average, 14% ± 6% (spatial range: 2%–29%) of the total annual runoff across BCSAK with a higher contribution along the Coast and Interior Mountains watersheds compared to the watersheds of the Interior Plateau and northeastern Plains of BC. The percentage of the total annual runoff associated with ARs reaches 22% ± 4% (spatial range: 10%–29%), 11% ± 1% (spatial range: 8%–13%), and 11% ± 6% (spatial range: 2%–21%) along the Coast, Interior Mountains, and Interior Plateau watersheds, respectively, during WYs 1979–2012. Moreover, substantial interannual variability exists in the contribution of ARs to annual runoff. For example, the contribution of ARs to annual runoff in some of the coastal watersheds reaches 44% during WY 2005 whereas it reaches only ~20% during WY 1995 in the same watersheds.

Fig. 3.
Fig. 3.

The contribution of ARs to total annual runoff (in %) across BCSAK, WYs 1979–2012.

Citation: Journal of Hydrometeorology 21, 11; 10.1175/JHM-D-19-0281.1

2) ARs and AM runoff across BCSAK

A total of 1384 ARs [most in autumn (522) and least in spring (207)] made landfall in BCSAK, with an average duration of 2 ± 1.8 days, during WYs 1979–2016; 726 (>52%) of these ARs were linked to extreme runoff. The variations in the percentage of AR-related AM runoff across BCSAK reveal the influence of the proximity of the watersheds to the Pacific coast and time of year. The average percentage of AR-related AM runoff from all the stations is 48% ± 24% (median 42%) and spatially ranges from ~10%–100%. Out of 168 watersheds, 25 show ≥80% AR-related AM, 38 report ≥50% AR-related AM whereas only 13 show ≤20% AR-related AM (Fig. 4a). Higher AR-related AM runoff on the western side of the Coast Mountains exists compared to runoff from the eastern side. Watersheds draining the Coast and Insular Mountains show >80% linkages between ARs and AM runoff (Fig. 4a) suggesting that ARs act as the primary synoptic-scale mechanism leading to AM runoff. Further inland from the Pacific coast, AR-related AM runoff decreases to only ~25% in the Interior Plateau; however, this rises again to >60% in the watersheds of the Interior Mountains.

Fig. 4.
Fig. 4.

(a) AR-related annual maximum runoff (%) and (b) the number of AR-related AM runoff out of the top 10 AM runoff in each watershed, WYs 1979–2016.

Citation: Journal of Hydrometeorology 21, 11; 10.1175/JHM-D-19-0281.1

On average, ARs that produce extreme runoff in BCSAK are 14 h longer (average duration 48 ± 5 h) than those not related to extreme runoff (average duration 34 ± 4 h). ARs that lead to extreme runoff in BCSAK have higher IVT (average 423 ± 133 kg m−1 s−1, maximum 1260 kg m−1 s−1) than those ARs that do not initiate extreme runoff (average 387 ± 101 kg m−1 s−1, maximum 898 kg m−1 s−1).

A strong association (≥6 AM runoff) between ARs and AM runoff events arises for the majority (26 out of 46) of coastal and westward draining watersheds from the Coast Mountains (Fig. 4b). Similarly, 22 out of 83 watersheds in the Southern Interior and the Interior Mountains show ≥4 (out of the top 10) AM runoff associated with ARs. Along the coastal watersheds, the AR-related AM runoff is particularly strong (>80%), but ARs contribute to ≤6 of the top 10 AM runoff (Fig. 4). In contrast, for some watersheds along the Interior Mountains, the AR-related AM runoff reaches <50%; however, ARs contribute to more than half of its top 10 AM runoff. In BCSAK, 48 out of 168 watersheds (33 out of 46 in Coastal, 8 out of 49 in the Interior Mountains, and 7 out of 67 in the Interior Plateau and northeastern Plains) have ≥5 of their top 10 AM runoff associated with ARs. Thus, ARs act as an important phenomenon controlling the distribution of extreme runoff, especially along the coastal and mountainous regions of BCSAK.

3) Seasonal distribution of AR-related annual maxima runoff

The highest number of AM runoff events (>20 out of 38) occurs during autumn and winter in the pluvial watersheds along the western and southern flanks of the Coast Mountains of BCSAK (Fig. 5a). Similarly, the majority of AM runoff events (>20 out of 38) occurs in spring and/or summer in the nival watersheds of the Interior Plateau and Mountains. There are scant AM runoff events along the watersheds of the Interior Mountains and Plateau (up to 5 out of 38) of BCSAK during winter likely due to precipitation falling as snow and the presence of frozen rivers. A high proportion of AM runoff occurs during spring and/or summer in these regions perhaps causing a temporal disconnect between precipitation (as snow) and extreme runoff. A seasonal breakdown of the number of AR-related AM runoff events reveals that ARs initiate almost all of AM runoff events (>90%) during autumn and winter (Fig. 5b). The spring and summer AM runoff events along the coastal watersheds also relate to ARs for ≥80% of watersheds; however, the association reaches no more than 30% along the Interior Mountains and Plateau watersheds. The spring AM runoff may be dominantly associated with snowmelt whereas the summer AM runoff may be due to snow and/or glacier melt in combination with summer convective precipitation (Déry et al. 2009).

Fig. 5.
Fig. 5.

(a) Number of AM runoff and (b) the percentage of AR-related AM runoff in each season across BCSAK, WYs 1979–2016.

Citation: Journal of Hydrometeorology 21, 11; 10.1175/JHM-D-19-0281.1

Further exploration on the linkages between AR-related extreme runoff and precipitation in each watershed shows a >50% association along the coastal and westward draining watersheds from the Coast Mountains (Fig. S5) during WYs 1979–2012. The association lies between 20% and 40% in the Interior Mountains watersheds and <10% in most of the Interior Plateau watersheds. The lower percentage of day-by-day linkages between AR-related extreme precipitation and runoff, particularly in the Interior Plateau, is likely due to lower amounts of orographic precipitation and the less conducive (pre)existing conditions (e.g., precipitation occurring as snow, the level of soil saturation, etc.) in the watersheds.

The seasonal maxima runoff and their linkages to ARs reflect a similar pattern to that of the aforementioned seasonal distribution of AM runoff (Table S1 and Fig. S6). The AR-related autumn maximum runoff is highest across BCSAK for the majority of the watersheds (>90% coastal watersheds and >70% for coastal watersheds and >70% for Interior Mountains and Plateau) indicating ARs are a major, if not the only, causative factor for the maximum runoff during this season. The AR-related winter maximum runoff remains dominant (>80%) for coastal watersheds and the lower latitudinal regions of BCSAK; however, it decreases in the northern Interior Plateau and northeastern Plains, likely due to precipitation falling as snow. The AR association to spring and summer maximum runoff are lower (<50%) for most of the watersheds, again, except the coastal region (Fig. S5).

c. Change in AR-related maximum runoff percentage over time

An exploration on changes in AR-related annual and seasonal maximum runoff over time using the nonparametric Mann–Kendall trend test (Kendall 1975; Mann 1945) indicates no-change (p < 0.05) for the majority of the watersheds during WYs 1979–2016 across BCSAK (Figs. S7 and S8). Only six (one Coastal and five Interior Plateau stations) out of the 168 watersheds show a significant (p < 0.05) increase in the percentage of AR-related annual maximum runoff over time while two watersheds show a significant decrease. Moreover, none of the stations exhibits significant changes when tested for field significance following the procedure from Wilks (2011, 2016) as explained in Sharma and Déry (2020b).

d. AR-related versus non-AR-related maximum runoff

There is more AR-related runoff compared to non-AR-related runoff for 102 out of 168 watersheds of BCSAK; 48 watersheds exhibit the same average maximum runoff magnitude (Fig. 6 and Table S2). The results of the MWW test show that for 30% of 168 watersheds (51% coastal, 21% northern Interior Plateau and northeastern Plains, 25% Interior Mountains, and 12% southern Interior Plateau) AR-related AM runoff magnitude is significantly (p < 0.05) higher than non-AR-related AM runoff magnitude. However, the field significance test shows only 17% of the stations have significantly higher AR-related AM runoff than non-AR-related AM runoff in BCSAK. These results indicate that, in general, ARs increase the magnitude of floods along the coastal regions of BCSAK, but not necessarily in other areas. Other flood generating mechanisms such as spring snowmelt, summertime convection, and glacial lake outbursts are likely to produce higher magnitude runoff than ARs in the Interior Plateau and Mountains watersheds (Buttle et al. 2016; Zahmatkesh et al. 2019; Eaton and Moore 2010; Melone 1985).

Fig. 6.
Fig. 6.

Distribution of AR-related and non-AR-related daily maximum runoff across BCSAK, WYs 1979–2016. The black vertical lines indicate median daily maximum runoff, red dots indicate mean runoff of all the watersheds, and horizontal lines show the range of daily maximum runoff.

Citation: Journal of Hydrometeorology 21, 11; 10.1175/JHM-D-19-0281.1

e. Watershed characteristics and AR-related extreme runoff

The watershed area and the distance from the coast influence the AR-related AM runoff in BCSAK (Figs. 7a,b). For example, the expansive Stikine River basin (area 51 600 km2) shows only 50% AR-related AM runoff whereas, one of its subwatersheds, the Iskut River (area 9350 km2), shows 70% AR-related AM runoff. For all watersheds with an area > 15 000 km2, the AR–AM association is <50% (Fig. 7a). The watershed area response to AR-related AM runoff varies likely due to the relatively small but intense precipitation areas associated with an AR that induce higher AR–AM links in smaller watersheds. Besides, the transmountain watersheds, especially those draining to the Pacific coast (e.g., Homathko River, Fig. 7a and ID 62 in Fig. 1), act as channels for farther inland AR moisture flux and lead to an increase in the AR-related runoff contribution area. Finally, an AR could straddle the boundary of two or more watersheds leading to a muted response or there could be a lagged response and compensation effects in the larger watersheds. As expected, watersheds near to the coast experience more AR-related AM runoff than farther inland; the percentage of AR-related AM is >60% within 300 km inland from the Coast that decreases to <50% after 400 km (Fig. 7b).

Fig. 7.
Fig. 7.

Average daily maximum runoff, WYs 1979–2016 vs (a) basin area (on a base 10 logarithmic scale), (b) distance from the nearest coastline to the centroid of the watershed, and (c) average slope for 168 watersheds across BCSAK. (d) Relation between watershed aspect and average AR-related AM runoff percentage. The color gradient indicates average AR-related annual maximum runoff association percentage. The red, black, and green diamonds in (a) indicate the Homathko, Iskut, and Stikine River basins, respectively.

Citation: Journal of Hydrometeorology 21, 11; 10.1175/JHM-D-19-0281.1

There exists a significant positive rank correlation between AR-related AM runoff and the percentage of the glacierized area (r = 0.55, p < 0.05, n = 90) within the watershed. Of note, the correlation between AM runoff and the percentage of the glacierized area is also significantly positive (r = 0.46, p < 0.05, n = 90). The slope of the watersheds shows a significant positive correlation with AR-related AM runoff percentage (r = 0.45, p < 0.05, n = 168) across BCSAK during WYs 1979–2016. Similarly, the AR-related seasonal maximum runoff and the percentage of the glacierized area as well as the slope of the watersheds exhibit a significant positive correlation (Table 1). Additionally, watersheds with greater slopes may be more susceptible to AR-related AM runoff compared to those with gentler slopes signifying strong AR-related orographic precipitation linkages to runoff (Fig. 7c). Besides, a strong relationship between average watershed orientation and AR-related maximum runoff does not exist likely due to varying trajectories of landfalling ARs (Fig. 7d).

Table 1.

Spearman’s rank correlation coefficient (r) between watershed characteristics and annual and seasonal AR-related maximum runoff across BCSAK, WYs 1979–2016. Bold values indicate statistically significant (p < 0.05) correlation.

Table 1.

f. Watershed response to ARs

The distribution of the percentage of watershed area within certain elevation ranges imparts an important hydrological response because it dictates the amount of rain and/or snow a watershed receives that in turn directly influences the runoff magnitude and timing. Therefore, we investigated the changes in freezing level during or one day before the winter maximum runoff for three selected watersheds of BCSAK representing different hydrological regimes (Fig. 1 and Table S1). The mean daily air temperatures at all elevation grid cells of the three watersheds are higher during (or one day before) the AR-related maximum runoff day in winter (Fig. 8) and spring (Fig. S9). Moreover, the number of grid cells with air temperature > 0°C increases for both nival and glacial watersheds during (or one day before) the AR-related maximum runoff. This suggests an increase in the melting elevation and thus the runoff contributing area during winter (Fig. 8). A variation in the percentage of runoff contributing areas associated with ARs also depends on the proximity of the watershed from the coast of the Pacific Ocean. For example, the melting elevation in the Lillooet River increases more than in the Lardeau River, which is farther inland. Moreover, a seasonal maximum runoff may occur in winter not only along the coastal regions of BCSAK, which may be more susceptible to ARs but also further inland (e.g., Lardeau River in the Interior Mountains) because of the transported warm, moist air associated with ARs (Figs. 5 and 8).

Fig. 8.
Fig. 8.

(a) Mean daily air temperatures at different elevation grids during AR and non-AR winter maximum runoff days for three selected watersheds representing different hydrological regimes (Fig. 1). (b) Cumulative fraction of the watershed area for three selected watersheds representing different hydrological regimes (Fig. 1). The horizontal dashed lines show the percentage of the watershed area contributing to the surface runoff as a function of elevation, the red and blue solid lines indicate the average elevation with mean daily air temperature of 0°C during AR and non-AR days winter maximum runoff series.

Citation: Journal of Hydrometeorology 21, 11; 10.1175/JHM-D-19-0281.1

5. Discussion

While the presence of ARs during runoff and peak flow events in the coastal watersheds have been studied previously along the western United States and Europe (Ralph et al. 2006; Lavers and Villarini 2013; Neiman et al. 2011; Konrad and Dettinger 2017; Ralph et al. 2013; Dettinger et al. 2011), this study expands similar work to northern latitudes of North America, i.e., BCSAK, and further analyzes relationships between AR-related runoff and watershed characteristics in this region. This is the first comprehensive study to explore the association between ARs, annual runoff, and annual/seasonal maximum runoff across BCSAK. It implements an empirical equation to quantify the contribution of ARs to annual runoff, but does not consider uncertainties associated with AR detection algorithms (Shields et al. 2018; Sharma and Déry 2020b). The use of the upstream gauged area shapefile of a watershed to clip the IVT field allows for the annual and maximum runoff to be based on the IVT threshold within each watershed. This approach is especially important for mountainous watersheds like that of BCSAK because of the high spatial variability in precipitation due to the orography.

Results in this study highlight the importance of AR activity on the hydrological processes of northwestern North American watersheds where substantial precipitation occurs as snow except at the lower elevations (Hare 1998). Moreover, the results herein provide a spatially consistent baseline assessment of the historical linkages between ARs, annual runoff, and extreme runoff across BCSAK. The contribution of ARs to the annual runoff in BCSAK is similar to the coastal region of western Washington, where Dettinger et al. (2011) report 25%–30% of annual streamflow associated with ARs. The AR-related AM runoff in BCSAK compares well with that from watersheds in western Washington (e.g., Neiman et al. 2011; Konrad and Dettinger 2017), but less than observed in California (Ralph et al. 2006). Ralph et al. (2006) report that 100% of the flooding events in the Russian River, California, between 1997 and February 2006 corresponded to ARs. The difference in AR-related extreme runoff between BCSAK and California is likely due to a greater number of landfalling ARs in BCSAK (Sharma and Déry 2020a; Dettinger et al. 2011). Globally, AR association to extreme runoff varies based on the region of study and local topography. For example, Lavers et al. (2012) find ~50% of winter extreme runoff associated with ARs in nine major rivers across the British Isles. Demaria et al. (2017) observe 43% of AM runoff linked to ARs in the semiarid Salt and Verde River basins of Arizona. Lavers and Villarini (2013) find 10% to >70% of AM runoff associated with ARs in the central United States. These studies highlight the importance of ARs on water resources and flood-related natural disasters. Similarly, results herein indicate ARs act as a key contributor to regional extreme hydrological events across BCSAK. ARs explain the majority of extreme daily flow characteristics of BCSAK, especially along the coastal and mountainous watersheds.

As observed here, ARs control the distribution of the majority of annual and seasonal maxima runoff, and thus the flooding events across BCSAK, however, with distinct spatial and seasonal variability. Therefore, we further discuss the physical processes that control AR-related maximum runoff at the watershed level, the role of ARs on flood-related natural disasters in BCSAK, and the response of extreme runoff to ARs in the future warmer climate scenarios. Of note, ARs across BCSAK produce some of the largest AM runoff. Yet in mountainous and high latitude watersheds of BCSAK, the ARs contribute to heavy snow accumulation (Sharma and Déry 2020b) that represents a potential flood peak for the subsequent melt season. This may potentially result in a lower estimate of AR-related AM runoff percentage in these regions.

a. Physical processes associated with maximum runoff response to ARs

The pattern observed in maximum runoff response to ARs reflects the distribution of landfalling ARs and their contribution to precipitation (e.g., more intense precipitation in elevated terrain) across BCSAK (Sharma and Déry 2020a,b). While many of the watersheds in the Interior Plateau do not exhibit a higher percentage of AM runoff directly associated with ARs (Fig. 4 and Fig. S3), autumn and winter maximum runoff is largely still a response to ARs. The variations observed in the AR-related annual and seasonal maximum runoff are likely due to a combination of (i) variability and characteristics of landfalling ARs in BCSAK, (ii) hydrological regime and proximity to the Pacific Ocean coast, (iii) watershed orography, and (iv) change in snow melting elevation within the watersheds.

One of the primary mechanisms leading to AR-related extreme runoff in BCSAK is the orographically enhanced precipitation, especially along the windward slopes of mountains. The synoptic conditions during an AR landfall are conducive to heavy orographic precipitation such as moist low-level jet, steep mountains, and weak static stability (Lin et al. 2001; Ralph et al. 2003). For example, Neiman et al. (2011) and Ralph et al. (2006) also observed conditions favorable for orographic precipitation during landfalling AR events that lead to peak flows in the watersheds of western Washington and the Russian River, California, respectively.

The hydrological regime of a watershed influences the phase of precipitation and thus, for example, creates a temporal disconnect between AR-related precipitation and extreme runoff in nival watersheds (Neiman et al. 2011; Lundquist et al. 2008) (Fig. S5). The relation between elevation distribution and fractional area of a watershed impacts the phase of precipitation as rain versus snow for a given melting level altitude (Fig. 8 and Neiman et al. 2011); this, in turn, substantially influences the runoff magnitudes. ARs typically transport relatively warm, moist air that initiates snow melting or change in the melting elevation of a watershed (Fig. 8) (Lundquist et al. 2008; Neiman et al. 2011). Moreover, ARs often lead to ROS events that increase extreme runoff through the combined effects of rainfall and snowmelt (Guan et al. 2016). Indeed, Guan et al. (2016) observe a substantial increase in ROS events from 6% of precipitation days to 15% during AR days in the Sierra Nevada, California. Results here from the Lillooet and Lardeau Rivers (Fig. S1 and Table S1) also indicate, on average, 94% (39%) of winter (spring) maximum runoff events associated with landfalling ARs are likely due to ROS in these glacial and nival watersheds of BCSAK. Of note, although the distribution and intensity of ARs along with watershed characteristics explain most of the extreme runoff across BCSAK, these are not the only contributors to maximum runoff. The amount of snow accumulation, the timing of the spring freshet, the level of soil saturation preceding the landfalling ARs, extreme precipitation associated with convective activity, and slow-moving low pressure systems may also lead to maximum runoff in this region (Loukas et al. 2000; Melone 1985; Wang et al. 2016).

b. ARs and flood disasters

An investigation on the linkages between ARs and the flood-related natural disasters across BC listed in the Public Safety Canada’s Canada Disaster Database (CDD) during WYs 1979–2016 suggests an important role of ARs. There was a presence of an AR within 7 days on or before the start of each flood-related natural disaster for 25 out of 32 (78%) events, costing >$0.5 billion (Canadian dollars) in damages (PSC 2019). Given that storms associated with ARs contribute >90% of extreme precipitation in some parts of BCSAK (Sharma and Déry 2020b), produce twice as much precipitation (Neiman et al. 2008), and 4 times more snow water equivalent (SWE) than non-AR storms (Guan et al. 2010), ARs lead to significantly higher runoff and flooding hazards predominantly during autumn and winter. The AR-related flooding may cause considerable infrastructure damages and economic losses [e.g., October 2003 flooding event along the Sea-to-Sky Highway and Bella Coola flooding event in September 2010 (PSC 2019)] and decrease water quality because of slope failure and channel instability (Spry et al. 2014).

c. Extreme runoff response to ARs in a future, warmer climate

Sharma and Déry (2020a) find a significantly increasing trend in the number of landfalling ARs in BCSAK during 1979–2016, likely due to increases in moisture availability in response to an increase in the global surface air temperature (Lavers et al. 2015; Tsonis 2013) and possibly changes in storm tracks driven by a wavier jet stream due to arctic amplification (Francis and Vavrus 2015). As observed in this and previous studies (Konrad and Dettinger 2017; Lavers et al. 2011), the intensity and duration of a landfalling AR affect extreme runoff within a watershed. The response of maximum runoff to the strength of ARs has implications on future extreme runoff events associated with ARs across BCSAK. Future projections report an increase in the intensity of ARs in warmer climate scenarios across western North America (Dettinger 2011; Payne and Magnusdottir 2015; Radić et al. 2015; Warner et al. 2015; Espinoza et al. 2018). Curry et al. (2019) project an increase in AR-related daily peak flow in the Fraser River basin because of its nival to pluvial regime shift and an increase in the extreme rainfall associated with ARs by the end of the twenty-first century under the RCP8.5 climate change scenario. However, we did not observe significant changes (increase and/or decrease) over time in the historical AR-related maximum runoff percentage during WYs 1979–2016 across BCSAK. This static pattern agrees with Sharma and Déry (2020a), who did not observe changes in the AR contribution amount to precipitation over time during 1979–2012, likely due to an increase in overall precipitation associated with convective activity along with ARs (Trenberth et al. 2003; Berg et al. 2013).

The actual changes in AR-related extreme runoff and flooding for specific watersheds depend on the variations in the IVT [section 4b(2) and Konrad and Dettinger 2017], AR trajectories and the extent of their penetration further inland (Radić et al. 2015; Payne and Magnusdottir 2015; Rutz et al. 2014; Sharma and Déry 2020b), and antecedent conditions (e.g., soil saturation, freezing elevation) (Ralph et al. 2013). The projected increase in IVT, northward shift of AR trajectories (Warner et al. 2015; Lavers et al. 2015; Radić et al. 2015), and transition in hydrological regimes from nival to pluvial (Curry et al. 2019; Islam et al. 2017) in future, warmer climate scenarios across BCSAK may lead to an increase in AR-related extreme runoff. Although it remains uncertain to what extent the AR-related maximum runoff percentage will change across BCSAK in the future, this study provides characteristics and quantification of historical AR-related maximum runoff.

6. Conclusions

This study investigates the nexus between landfalling ARs, annual runoff, annual and seasonal extreme runoff, watershed characteristics, and the changing pattern of AR-related flooding over time in BCSAK during WYs 1979–2016. In exploring these linkages, this study utilizes nearly four decades of daily maximum streamflow data for 168 unregulated watersheds of BCSAK, topographic characteristics of the watersheds, a regional AR catalog, and IVT fields. ARs contribute 22%, 11%, and 11% of the total annual runoff in the watersheds of the Coast Mountains, Interior Mountains, and Interior Plateau regions of BCSAK, respectively, during WYs 1979–2012. ARs control the distribution of peak runoff for most of BCSAK with 51% of 168 watersheds analyzed having more than half of their annual maxima runoff linked to ARs during WYs 1979–2016. Similarly, more than half of the top 10 annual maxima runoff events are associated with ARs for >60% of the watersheds, predominantly along the coastal and mountainous regions of BCSAK. ARs act as the precursor of flooding-related natural disasters in BCSAK; indeed, 78% of such disasters occurred in BC between WYs 1979–2016 were linked to ARs. AR-related maximum runoff magnitude is significantly higher than non-AR-related runoff magnitudes for the coastal watersheds of BCSAK. ARs are the major, if not only, atmospheric phenomena during autumn and winter that cause seasonal maximum runoff in BCSAK. ARs, with transported warm, moist air, increase air temperatures and melting elevations that lead to an increase in runoff even in the nival and glacial watersheds of BCSAK during winter months. Watershed characteristics also control the AR-related maximum runoff with steeper watersheds close to the coast experiencing higher AR contributions.

This study provides baseline information on the historical AR association to total annual and annual maximum runoff and illustrates the importance of AR activity as a major control for the distribution of peak runoff in BCSAK. This work extends previous similar studies from the west coast of the United States to BCSAK and provides broader perspectives on spatial variations in AR-related flooding events along western North America. Moreover, the information obtained here provides a basis for further research on ROS events in BCSAK that would be useful for flood forecasting and flood risk assessment. Similarly, knowledge from this study improves our understanding on hydrological extremes in the complex topographic areas of BCSAK and adds value to the studies on ARs and flooding, especially future projections and model validation. The knowledge obtained from this study may be useful for water resources managers, hydrologists, and flood forecasters. For example, flood forecasters and watershed managers can recognize conditions that may lead to AR-related flooding, identify watersheds susceptible to AR-related flooding, and perform flood risk mapping to prevent/mitigate damages, personal injuries, and loss of life. With strong relationships between ARs and extreme runoff observed here based on historical data, it is important to assess how extreme runoff characteristics will change in the future in BCSAK along with anticipated changes in AR frequency and intensity. Further research on AR-induced ROS events and return period analysis of AR-related flooding events across BCSAK will reduce/mitigate flood risk associated with extreme hydrological events in BCSAK.

Acknowledgments

Thanks to the creators and contributors of the SIO-R1-Catalog, WSC, and USGS. Thanks to Drs. Brian Menounos (UNBC), Ellen Petticrew (UNBC), Alex Cannon (ECCC), John Gyakum (McGill University), the Editor Dr. L. Ruby Leung, and two anonymous referees who provided constructive comments on this work that led to a much improved paper. Thanks to Jeremy Morris (UNBC) for proofreading an earlier draft of the manuscript. Funding provided by UNBC and the Mountain Water Futures project of the Global Water Futures (GWF) programme.

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