The hierarchy of controls on snowmelt-runoff generation over seasonally-frozen hillslopes

Abstract

Abstract. Understanding and modeling snowmelt-runoff generation in seasonally-frozen regions is a major challenge in hydrology. Partly, this is because the controls on hillslope-scale snowmelt-runoff generation are potentially extensive and their hierarchy is poorly understood. Understanding the relative importance of controls (e.g. topography, vegetation, land use, soil characteristics, and precipitation dynamics) on runoff response is necessary for model development, spatial extrapolation, and runoff classification schemes. Multiple interacting process controls, the nonlinearities between them, and the resultant 15 threshold-like activation of runoff, typically are not observable in short-term experiments or single-season field studies.Therefore, long-term datasets and analyses are needed. Here, we use a 52-year dataset of runoff, precipitation, soil water content, snow cover, and meteorological data from three monitored c.5 ha hillslopes on the Canadian Prairies to determine the controls on snowmelt-runoff, their time-varying hierarchy, and the interactions between the controls. We use decision tree learning to extract information from the dataset on the controls on runoff ratio. Our analysis shows that there was a variable 20 relationship between total spring runoff amount and either winter snowfall amount or snow cover water equivalent. Other factors came into play to control the fraction of precipitated water that infiltrated into the frozen ground. In descending order of importance, these were: total snowfall, snow cover, fall soil surface water content, melt rate, melt season length, and fall soil profile water content. While mid-winter warm periods in some years likely increased soil water content and/or led to development of impermeable ice lenses that affected the runoff response, hillslope memory of fall soil moisture conditions 25 played a strong role in the spring runoff response. The hierarchy of these controls was condition-dependent, with the biggest differences between high and low snow cover seasons, and wet and dry fall soil moisture conditions. For example, when snow cover was high, the top three controls on runoff ratio matched the overall hierarchy of controls, with fall soil surface water content being the most important of these. By comparison, when snow cover was low, fall soil surface content was relatively unimportant and superseded by four other controls. Existing empirical methods for predicting infiltration into frozen ground 30 failed to adequately predict runoff response at our site. Our analysis of the hierarchy of controls on meltwater runoff will aid Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016Discuss., doi:10.5194/hess- -564, 2016 Manuscript under review for journal Hydrol. Earth Syst. Sci.

Response to Reviewer 3 comments

“…With respect to trends in the APF specifically, Cunderlik and Ouarda (2009) found no significant trend at most Reference Hydrometric Basin Network (RHBN) stations in the FRB over the 1974-2003 period, but noted a weak trend (∼ 1-5 %) toward decreasing magnitude and earlier APF occurrence at two stations. A Canada-wide nonstationary analysis by Tan and Gan (2015), suggested that APF increased over recent decades at two stations in the FRB: the Stellako R. at Glenannan and Chilliwack R. at Chilliwack Lake.…”

“…Acknowledgements. We thank Markus Schnorbus and Arelia (Werner) Schoeneberg for helpful discussions regarding the VIC model, Reza Najafi for providing observed SWE data for the Fraser basin, Faron Anslow for providing the regional temperature and precipitation time series data over BC, and Juraj Cunderlik for sharing station locations from Cunderlik and Ouarda (2009). The comments of Siraj Ul Islam and referees Stephen Déry, Michal Jenicek and an anonymous reviewer also led to substantial improvements to the final paper.…”

Examining controls on peak annual streamflow and floods in the Fraser River Basin of British Columbia

“…Prior studies that were particularly helpful in this regard were Gurrapu et al (2016) who examined the influence of 20 the PDO on streamflow in Western Canada, Jenicek et al (2016) who examined the influence of snow accumulation and other variables on summer low flows, Coles et al (2017) who studied snowmelt-runoff generation on Canadian prairie hillslopes, and Wever et al (2017) who conducted model simulations of the joint effect of snowmelt and soil moisture on streamflow in a Swiss alpine catchment. The predictor variables chosen are listed in Table 2.…”

Fifty years of recorded hillslope runoff on seasonally-frozen ground: The Swift Current, Saskatchewan, Canada dataset

“…The long-term data from the hillslopes have so far been used to investigate the effects of tillage practices on runoff water quality (Cessna et al, 2013), and the effects of climate change on snowmelt timing (Cutforth et al, 1999) and runoff amounts (Coles et al, 2017 the dataset to determine the magnitude of soil erosion (McConkey et al, 1997) and nutrient transport (Nicholaichuk and Read, 1978) from agricultural hillslopes. More recent study has used these data to understand the relative importance of hydrological controls on snowmelt-runoff generation (Coles et al, 2018a). Other recent work on the Swift Current hillslopes undertaking single melt season intensive data collection has quantified the spatial patterns of hydrological variables and the generation of snowmelt-runoff connectivity over low-angled terrain (Appels et al, 2017;Coles and McDonnell, 2018).…”