Assessing the factors governing the ability to predict late-spring flooding in cold-region mountain basins

Abstract

Abstract. From 19 to 22 June 2013, intense rainfall and concurrent snowmelt led to devastating floods in the Canadian Rockies, foothills and downstream areas of southern Alberta and southeastern British Columbia, Canada. Such an event is typical of late-spring floods in cold-region mountain headwater, combining intense precipitation with rapid melting of late-lying snowpack, and represents a challenge for hydrological forecasting systems. This study investigated the factors governing the ability to predict such an event. Three sources of uncertainty, other than the hydrological model processes and parameters, were considered: (i) the resolution of the atmospheric forcings, (ii) the snow and soil moisture initial conditions (ICs) and (iii) the representation of the soil texture. The Global Environmental Multiscale hydrological modeling platform (GEM-Hydro), running at a 1 km grid spacing, was used to simulate hydrometeorological conditions in the main headwater basins of southern Alberta during this event. The GEM atmospheric model and the Canadian Precipitation Analysis (CaPA) system were combined to generate atmospheric forcing at 10, 2.5 and 1 km over southern Alberta. Gridded estimates of snow water equivalent (SWE) from the Snow Data Assimilation System (SNODAS) were used to replace the model SWE at peak snow accumulation and generate alternative snow and soil moisture ICs before the event. Two global soil texture datasets were also used. Overall 12 simulations of the flooding event were carried out. Results show that the resolution of the atmospheric forcing affected primarily the flood volume and peak flow in all river basins due to a more accurate estimation of intensity and total amount of precipitation during the flooding event provided by CaPA analysis at convection-permitting scales (2.5 and 1 km). Basin-averaged snowmelt also changed with the resolution due to changes in near-surface wind and resulting turbulent fluxes contributing to snowmelt. Snow ICs were the main sources of uncertainty for half of the headwater basins. Finally, the soil texture had less impact and only affected peak flow magnitude and timing for some stations. These results highlight the need to combine atmospheric forcing at convection-permitting scales with high-quality snow ICs to provide accurate streamflow predictions during late-spring floods in cold-region mountain river basins. The predictive improvement by inclusion of high-elevation weather stations in the precipitation analysis and the need for accurate mountain snow information suggest the necessity of integrated observation and prediction systems for forecasting extreme events in mountain river basins.

Flexible vector-based spatial configurations in land models

“…The high Nash-Sutcliffe efficiency, E NS , is partly due to the fact that it is rather easy for the land model to capture the yearly cycle of the streamflow only with snow processes (see, e.g., Knoben et al, 2020, demonstrating this for the Kling-Gupta efficiency), while rapid subsurface water movement, such as macropore movement, is largely missing in the land models (Gharari et al, 2019). Therefore, more caution is needed for the calibration of land model parameters for flood forecasting (Vionnet et al, 2020) for the Bow region and all the nivo-glacial river systems in western Canada, McKenzie, Yukon, and Colombia River basins.…”

Review

“…I would suggest that is not correct. A few examples: Rasmussen, et al 2010;Ikeda, et al 2010;Bonekamp, et al, 2018;Vionnet, et al 2020 L93 L368 "Decreases in terrain resolution may lead. .…”

“…The authors note that as the computational grid becomes more coarse, fine scale topography smooths out and thus impacts the surface energetics and subsequent SWE distributions. Certainly, this was to have been expected and especially the case with snow cover simulation in mountain terrain where smaller computational cells can better represent the heterogeneities in mass and energy (Dornes, et al, 2008;Schlögl, et al 2016;DeBeer, et al 2017;Vionnet, et al 2020). However, there is seemingly no consideration of any of the small-to medium-scale (i.e., the 50 m to 250 m length scales considered) processes that might be affected by the scale perturbations, and no mechanistic discussion thereof to put the results into context.…”

Meteorological observations collected during the Storms and Precipitation Across the continental Divide Experiment (SPADE), April–June 2019

“…The elevation of the continental divide varies from 1130 to over 3600 m a.s.l. Meteorological data over the continental divide remain sparse and predominantly lie in the valleys where sites are accessible by road and AC power is more readily available (Vionnet et al, 2020). There are even fewer stations at higher elevations, hence the paucity of meteorological data over high-elevation regions (Pepin et al, 2015;Hernández-Henríquez et al, 2018).…”

“…Additionally, several significant flooding events have recently impacted the eastern slopes of the Canadian Rockies such as in June 2002 (Szeto et al, 2011), June 2005 (Ou, 2008;Shook, 2016), and the recent major flooding in June 2013 (Pomeroy et al, 2016;Liu et al, 2016;Kochtubajda et al, 2016). Whitfield and Pomeroy (2016) showed that flooding due to rain-on-snow events occurred more frequently in the late 19th and early 20th centuries than it does in the more recent period.…”