Climate variability and the population boom during the past century have disturbed the long-established regimes of the hydrological cycle and have limited the availability of water resources on Earth. Prolonged droughts have increased water shortage in many parts of the world and stressed ecosystem services that rely on water. Rivers and lakes are drying up at accelerating rates and ecosystem degradation is becoming the ‘new normal’. Therefore, understanding components of the hydrological cycle is more important than ever, especially under extreme drought conditions. In this regard, snow is a crucial element of Earth’s hydrology, climate, and biology, feeding streamflow during the summer when demand is the highest. In a recent study, Huning and AghaKouchak (2020) studied global snow patterns to characterize the intensity (or severity) and duration of “snow droughts” and to identify snow drought hotspots worldwide.
Snow drought is defined as a deficit in snow water equivalent (SWE; the amount of water obtained if the snowpack melted instantaneously) at a particular location and time of year. Snow governs the biogeochemical processes on the Earth’s surface and below and provides water supply for agriculture, industrial, hydropower, and domestic uses. Approximately 1 billion people around the world depend on snow for water. On average, ~25 million km2 of the Northern Hemisphere is covered by snow annually.
Huning and AghaKouchak used two satellite-based products from NASA during the period of 1980-2018 to develop the Standardized Snow Water Equivalent Index (SWEI): 1) Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) snow information, and 2) level-3 Moderate Resolution Imaging Spectroradiometer (MODIS)/Terra monthly snow cover product. SWEI provides a global assessment of snow drought and could be used to monitor SWE on the ground.
Detailed investigation of snow drought using SWEI across seven selected regions around the world (Figure 1) showed significant changes in duration of snow droughts over the last 40 years. While snow drought duration increased 28, 16, and 2% in the western US, Europe, and eastern Russia, respectively, from 2000 to 2018, average snow duration declined −16, −8, −7, and −4% in Patagonia, the Himalayas, extratropical Andes, and Hindu Kush. Analysis of SWEI across these seven regions also suggests that weaker-intensity droughts are more likely after the year 2000 in Hindu Kush, Himalayas, extratropical Andes, and Patagonia.
Figure 1. (Top) Relative change in snow drought characteristics around the world from 1980 through 2018 (i.e., change in the respective variable over the second half of years relative to the first half). The exceedance probability refers to the chance that the drought intensity (I) is greater than the average intensity from the first half of years (Iave). Changes in probabilities are displayed as differences. (Bottom) The sample global SWEI classification map shows the seven study regions (regions A through G). [Source: Figure 1 from Huning and AghaKouchak (2020)].
An interesting observation in the global snow patterns is that Eurasia (the region between Siberia and the Himalayas) has experienced more snow during the fall and winter seasons of 1980-2015 period. One possible explanation for such variability in snow accumulation is the Arctic Amplification (rapid warming of the Arctic and melting its sea ice). However, decline in the Arctic sea ice has resulted in drier conditions in California and reduced snowfall in the western US.
Change in snow and precipitation patterns, in particular increased drought events, has had socio-economic impacts on the lives of many people around the world. For example, during the 2014-2015 drought event in the western US, California experienced increased temperatures and reduced precipitation which resulted in reduced snow as evident by the SWEI index. Under these conditions, the state of California endured an economic loss of US$2.74 billion and nearly 21,000 jobs were lost in 2015. Fortunately, a wet winter in 2016-2017 brought sufficient snow and precipitation such that the emergency state was lifted in many areas across California. In another instance, Afghanistan was affected by a snow drought event in 2017-2018 (Figure 2) and the food availability for about 11 million people was put at risk due to the occurrence of several droughts followed by SWE deficits. Afghans rely on snow as a major source of water supply and water shortage amid ongoing political conflicts and socioeconomic challenges could cause crop failure, livestock losses, dry wells and rivers, and consequently, food shortages in the country.
Figure 2. Three-month SWEI classification during the December 2017 to March 2018 snow drought in Afghanistan: (Left) Northern Hemisphere and (Right) Afghanistan and surrounding area. Warmer colors indicate dry and cooler colors indicate wet conditions. [Source: Figure 3 from Huning and AghaKouchak (2020)].
Although dire consequences of snow drought are not limited to impacts on the economy and behavior of human systems, these few examples show how our life could be stressed by the occurrence of droughts and snow deficits. To prepare for snow droughts and mitigate their impacts in the future, we need tools that help us understand the behavior of the Earth’s system dynamics. In this regard, SWEI can be incorporated into drought monitoring platforms and offer opportunities to better understand physical drivers of snow drought and its role in the hydrological cycle, especially in regions with insufficient observational data. SWEI and similar indices can be used as complementary information in the development of management plans and adaptation strategies for future population projections and climate scenarios.
Bio
Dr. Laurie Huning is an assistant professor in the Department of Civil Engineering and Construction Engineering Management at the California State University-Long Beach. Her research is focused on characterization of hydrologic processes and their interaction with natural and human systems. Her research addresses global water resources problems by linking the fields of hydrometeorology, climatology, remote sensing, and modeling together.
References:
Huning, L. S., & AghaKouchak, A. (2020). Global snow drought hot spots and characteristics. Proceedings of the National Academy of Sciences, 117 (33): 19753-19759. doi: 10.1073/pnas.1915921117
By: Dr. Bahram Khazaei
PostDoctoral Research Fellow
National Center for Atmospheric Research