Snow drought

By Anne Van Loon
Lately we have seen some stories in the news and in scientific media about snow drought. Low snowpacks are worrying, both for the tourist industry and for water managers and water users who depend on the seasonal water storage in snow. To effectively manage (the consequences of) snow droughts, we need to understand the hydroclimatological processes underlying these events. Here, I give an overview of the drivers and impacts of different types of snow droughts, with a hydrological perspective.

Figure 1. Below-normal reservoir levels in Norway (photo by Anne Van Loon).

Recently various studies have looked at long-term changes in average snow cover due to climate change (e.g. Marty et al., 2017; Sproles et al., 2017) and in the past weeks, both the European Geosciences Union and the American Geophysical Union have highlighted the effects of diminished snow cover in mountain regions in Europe and the USA (http://www.egu.eu/news/323/less-snow-and-a-shorter-ski-season-in-the-alps/ & https://eos.org/opinions/defining-snow-drought-and-why-it-matters). But snow drought is not a new phenomenon, with the winters of 1976-77 and 1980-1981 being described as snow drought winters in the USA (Schneider & Matson, 1977; Wiesnet, 1981) and water resource managers being well aware of variations in snow pack that influence water resource availability.

Research has shown that the drivers of snow drought vary considerably per region and per event. For example, Harpold et al. (2012), Van Loon et al. (2015) and Cooper et al. (2016) found that in some regions in the USA and Europe variation in temperature is determining Snow Water Equivalent (SWE) and in others anomalies in precipitation are more important. Harpold et al. (2017) therefore call for a distinction to be made between “dry snow drought” for a precipitation-driven snow deficit and “warm snow drought” for a temperature-driven snow deficit.

The distinction between dry and warm snow drought has a meteorological focus. If we look at drought from a hydrological perspective we can distinguish more drought types that depend on anomalies in precipitation and temperature in different seasons and on the climatology of the region. The latter is important because drought is defined as less water than normal and because the seasonality in climate plays an important role in the development and persistence of hydrological droughts.

In regions with winter temperatures normally far below zero for more than 6 months of the year, a winter temperature anomaly of a few degrees above normal can only slightly influence the start and end of the snow accumulation period and will therefore have a negligible effect on SWE and streamflow. In these climates, streamflow droughts develop as a consequence of winter precipitation being considerably below normal. This lack of winter precipitation leads to below-normal snow accumulation and consequently to lower spring melt discharges. Van Loon et al. (2015) termed this a “snowmelt drought” and it can have large impacts on sectors that depend on spring inflow, such as hydropower or irrigated agriculture. These impacts will mainly occur in spring and summer.

Even more severe hydrological droughts in these climates are droughts that develop over the summer and do not recover in winter. These “rain-to-snow season droughts” are caused by a rainfall deficit in the summer leading to low groundwater levels, reservoir levels and river flows. In contrast to the meteorological drought the hydrological drought does not recover because precipitation falls as snow in winter and hydrological stores do not get replenished. Rain-to-snow season droughts impact electricity production and drinking water supply in Scandinavia (e.g. Cattiaux et al., 2010) and induce livestock mortality and economic loss in regions like Mongolia (e.g. Sternberg, 2010), where these droughts are called ‘Dzud’ (http://reliefweb.int/disaster/cw-2017-000001-mng). In this case impacts occur mainly during the winter itself.

In climates with shorter winters and winter temperatures around zero, temperature plays a more important role. Normally in these regions, winter river flows are high due to occasional melt of the snow pack. However, if winter temperatures are much below normal, more snow accumulates and less water infiltrates to replenish the groundwater or runs off to become discharge. Such a “cold snow season drought” will only have a temporary effect on water resources during the winter and in many regions it will not cause problems, especially because spring melt will be higher than normal.

Contrastingly, if winter temperatures are higher than normal in these climates, no snowpack will develop. This will also not be a problem for water resources if winter rainfall is above normal. But if the high winter temperatures coincide with below-normal winter precipitation, a “warm snow season drought” develops and water resources are impacted, both in winter itself and in the subsequent spring and summer. Historical analysis identified many warm snow season droughts in Western Europe with severe impacts on agriculture (Van Loon et al., 2015).

In intermediate climates both precipitation and temperature in winter are crucial; it is an intricate balance. “Snowmelt droughts” are common in these regions. Like in the very cold climates they are caused by a below-normal snowpack, but in intermediate winter climates low SWE can be due to above-normal temperatures or below-normal precipitation anomalies in winter or by a combination of both. The drought drivers can differ per event as do the impacts (Van Loon et al., 2015).

Interestingly, these climate regions with their different hydrological drought types are not necessarily separated in space, with drought types occurring in isolation. Mountainous regions have different climates within the same catchment, varying with altitude, which means that different drought types might occur in different parts of the catchment. The downstream water availability is then a mix of these different upstream drought types. The hydrological drought types are not only variable in space but also in time. In a changing climate, some drought types are expected to occur less often and others more frequently, shifting along with changes in the climatology (Barnett et al., 2005; Van Loon and Van Lanen, 2012).

For robust water management, now and in the future, it is important to understand these snow-related processes causing hydrological drought and to use the right tools to measure them (i.e. by using a drought index that includes snow; Staudinger et al., 2011). This is important because each drought type has different impacts, either during the winter and/or in the subsequent spring and summer. I therefore urge all water managers and hydrologists in areas with seasonal snow storage to examine which hydrological drought types occur in their region and to quantify the drivers of these drought types by looking at anomalies in temperature and precipitation (Mote, 2003). It would be great if we can map and monitor these processes in space and in time in order to prevent impacts of these events both in the short and in the long term.

Figure 2. Schematic diagram of snow-related hydrological drought types (adapted from Van Loon et al. (2015).

References:
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Cooper, M. G., Nolin, A. W., & Safeeq, M. (2016). Testing the recent snow drought as an analog for climate warming sensitivity of Cascades snowpacks. Environmental Research Letters, 11(8), 084009.
Harpold, A., P.Brooks, S.Rajagopal, I.Heidbuchel, A.Jardine, and C.Stielstra (2012), Changes in snowpack accumulation and ablation in the intermountain west, Water Resour. Res., 48, W11501, doi:10.1029/2012WR011949.
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