Impact of Increasing Rainfall and Rain-on-Snow on Flood Generation in a Canadian Prairie Catchment

Abstract:

Snowmelt over frozen soils has been observed to account for over 80% of the annual runoff and almost all streamflow in the Canadian Prairies. It is also an important source for recharging soil moisture and filling wetlands. Typical streamflow hydrographs peak in April from snowmelt and cease in May from lack of sub-surface inputs. This study evaluated the detailed hydrological impact of regional changes to climate on an instrumented research catchment, Smith Creek Research Basin; a wetland dominated instrumented catchment in southeastern Saskatchewan, Canada. The region has recently experienced increased streamflow and repeated flooding. Precipitation, temperature, and streamflow records were analyzed for trends and change points and the source of streamflow was analyzed by examination of meteorological and hydrological records. The massive spring flood of 2011 in Smith Creek resulted from heavy rainfall onto a record deep melting snowpack. For the first time in any record, large summer runoff events occurred in 2012 and were repeated in 2014, breaking record peak flows. These unique summer floods resulted from rainfall runoff in mid-summer, well after spring snowmelt filled much of the available surface and sub-surface storage. Local meteorological observations from 1942 to present show substantial climate change: mean annual temperatures have increased by 1.1 °C, triggering a 9% increase in the rainfall fraction of annual precipitation. Despite the lack of trends in annual precipitation, annual streamflow volume has increased four-fold since 1994. Spring rainfall onto snowpacks overlying frozen soils tends to create and ice layer on top of the soil that restricts infiltration during snowmelt and can increase snowmelt rates. Streamflow observations since 1975 show a 10-fold increase in the Smith Creek streamflow volume derived from combined snowmelt and rainfall, whereas streamflow derived solely from snowmelt has declined. Storage of spring meltwater runoff in the prairie ponds creates antecedent conditions that promote summer streamflow generation if large rainfall volumes follow as occurred in 2012 and 2014. This research highlights the growing need to understand how increasing rainfall contributes to dramatically changing streamflow regimes on the Canadian Prairies.