Climate Change Canada

 

6.4: Soil moisture and drought

Periodic droughts have occurred across much of Canada, but no long-term changes are evident. Future droughts and soil moisture deficits are projected to be more frequent and intense across the southern Canadian Prairies and interior British Columbia during summer, and to be more prominent at the end of the century under a high emission scenario (medium confidence).

 

Soil moisture directly influences runoff and flooding, since it strongly affects the amount of precipitation/ snowmelt that makes its way into surface water bodies. It also determines the exchange of water and heat energy between the land surface and the atmosphere through evaporation and plant transpiration, and influences occurrence of precipitation through the recycling of moisture (see Seneviratne et al., 2010 for a detailed explanation of soil moisture–climate interactions). There are few direct measurements of soil moisture in Canada, and amounts are therefore estimated through remote sensing (e.g., with satellites) and/or modelling. The lack of an extensive monitoring network makes it difficult to make large-scale assessments of past trends (e.g., Mortsch et al., 2015). Future changes in soil moisture are primarily assessed using direct soil moisture output from GCMs. These changes are influenced by future precipitation and evaporation (the latter of which may be affected by changes in vegetation). However, modelled soil moisture is associated with large uncertainties, due to complexities in the representation of actual evapotranspiration, vegetation growth, and water use efficiency under enhanced atmospheric carbon dioxide concentrations (e.g., Seneviratne et al., 2010; Wehner et al., 2017). Longer-term climate variability, including droughts and excessive wet periods, are often directly related to soil moisture (and other aspects of freshwater availability). As a result, this section also assesses past and future changes to relevant indicators of drought.

 

6.4.1: Soil moisture

Quantifying soil moisture over large domains is challenging, as a result of the variability of soil moisture over time and among regions (Famiglietti et al., 2008). Several national-scale soil moisture networks exist globally (Doringo et al., 2011), including two in the United States (Schaefer et al., 2007; Bell et al., 2013). While there is no national network across Canada, there are some regional/provincial sites. For example, Alberta has monitored drought for the past 15 years, including soil moisture conditions, over a large network within the province, while Saskatchewan, Manitoba, and Ontario have established soil moisture and weather monitoring stations for selected regions. These networks have been used for validation of remote sensing data (described below) (e.g., Adams et al., 2015; Pacheco et al., 2015; Champagne et al., 2016) and for calibration and validation of hydrological models (Hayashi et al., 2010). Due to the difficulties (including high costs) of direct soil moisture monitoring, numerous remote sensing approaches have been used (Chan et al., 2016; Colliander et al., 2017). At present, continuous estimates of soil moisture for Canada as a whole are available from the Soil Moisture and Ocean Salinity (SMOS) satellite mission (2010–present) and more recently as part of the Soil Moisture Active Passive (SMAP) Mission (2015–present) (e.g., Champagne et al., 2011, 2012). SMOS data are distributed by Agriculture and Agri Food Canada.

A limitation to estimates of soil moisture from remote sensing is the relatively shallow observation depth, which is generally limited to the top few centimetres from the surface. Deeper values within the root zone (i.e., the top metre) are often determined using data assimilation systems, in which soil moisture data from satellite sensors are merged with estimates from a hydrological model (e.g., Reichle et al., 2017). In Canada, this is done operationally and nationally as part of the Canadian Land Data Assimilation System (Carrera et al., 2015). Due to the relatively short record, no studies have examined trends in these data. Daily soil moisture values in the Canadian prairie provinces for three soil layer depths (0–20 cm, 20–100 cm and 0–100 cm) were, however, reconstructed from 1950 to 2009 using the Variable Infiltration Capacity (VIC) land-surface hydrology model. The reconstructed soil moisture matched past observations across the prairies, but no trends were reported (Wen et al., 2011).

There have been a few global studies of future soil moisture using GCM output. An ensemble of 15 CMIP3 GCMs projected a decrease in June–August soil moisture for most of Canada for the late century under a medium-high emission scenario (SRES 1Ab) (Wang, 2005). Projected late-century changes in surface, total, and layer-by-layer soil moisture from 25 GCMs included in CMIP5 under a high emission scenario (RCP8.5) indicated that, in most mid-latitudes of the Northern Hemisphere, including southern Canada, the top 10 cm of soil will become drier during the summer, but the remainder of the soil, down to 3 m, will stay wet (Berg et al., 2016; Wehner et al., 2017).

 

6.4.2: Drought

Drought is often defined as a period of abnormally dry weather long enough to cause a serious hydrological imbalance (e.g., Seneviratne et al., 2012) and therefore impacts on several components of the water cycle. These impacts can also be exacerbated by increases in evapotranspiration associated with high temperatures. Drought impacts differ, however, depending on their timing. In general, warm-season droughts affect not only agricultural production (usually due to soil moisture deficits) but also surface and subsurface water levels. Precipitation deficits associated with the runoff season (including winter snow accumulation) primarily affect the replenishment of freshwater systems.

Numerous indices of drought (which also identify moisture surplus) have been used to characterize their occurrence and intensity. The indices incorporate various hydroclimatic inputs (e.g., precipitation, temperature, streamflow, groundwater, and snowpack), and each index has its own strengths and weaknesses (see WMO, 2016 for a comprehensive list). Some indices are based on precipitation alone (e.g., the Standardized Precipitation Index [SPI] (McKee et al., 1993)) and do not take into account that higher temperatures are often associated with below-normal precipitation. As a result, enhanced evapotranspiration is not considered. A few indices incorporate precipitation and estimates of potential evapotranspiration (based on air temperature) — for example, the Palmer Drought Severity Index (PDSI) (Palmer, 1965) and the Standardized Precipitation Evapotranspiration Index (SPEI) (Vicente-Serrano et al., 2010). A shortcoming of these indices is that they use potential evapotranspiration as a proxy for actual evapotranspiration and, thus, do not consider how soil moisture and vegetation may limit evapotranspiration and subsequent drought development. This can lead to overestimation of drought intensity, particularly for climate change projections (e.g., Donohue et al., 2010; Milly and Dunne, 2011, 2016; Shaw and Riha, 2011). The vast majority of global-scale and Canadian analyses of historical trends and projected future changes to drought have used indices based on precipitation alone or on the combined effects of temperature and precipitation (e.g., Bonsal et al., 2011), and these are the focus of this assessment.

A few global studies have highlighted past trends in specific regions, including, for example, drying over mid-latitude regions of Canada from 1950 to 2008 (Dai, 2011 using PDSI). However, since the beginning of the 20th century, the frequency of global drought remains generally unchanged; it appears that, over this longer period, increases in global temperature and potential evapotranspiration have been offset by increases in annual precipitation (e.g., Sheffield et al., 2012; McCabe and Wolock, 2015). Trend analyses in Canadian drought are fragmented, with no comprehensive country-wide analyses to date. The majority have focused on the Prairie region, because of the greater frequency of drought in this region (e.g., Mortsch et al., 2015). A Canadian drought review (Bonsal et al., 2011) provided examples of 20th-century changes in PDSI for individual stations in various regions of the country (1900 to 2007) (see Figure 6.14). Considerable multi-year variability is evident, with no discernible long-term trends. This variability was also apparent in regional studies of SPEI (1900–2011) in summer (June–August) and over the “water year” (October–September) in southeastern Alberta and southwestern Saskatchewan (Bonsal et al., 2017) and the Athabasca River Basin (Bonsal and Cuell, 2017). Other Canadian Prairie region drought studies have highlighted periodic droughts during the 1890s, 1910s, 1930s, 1980s, and early 2000s (e.g., Chipanshi et al., 2006; Bonsal and Regier, 2007; Bonsal et al., 2013). From the mid-to-late 2000s to approximately 2014, the Prairie region has experienced exceptionally wet conditions, highlighting the high variability in this region (e.g., Bonsal et al., 2017).

In other areas of the country, the Canadian Drought Code (based on maximum temperature and precipitation) showed that drought severity over the southern boreal forest regions of Canada was variable, with no longterm trend from 1913 to 1998 (Girardin et al., 2004). A more recent analysis using PDSI and the Climate Moisture Index (difference between annual precipitation and annual potential evapotranspiration) indicated that, for the Canadian boreal zone as a whole, several regions experienced significant drying between 1951 and 2010, but there were also some areas with significant wetting (Wang et al., 2014). An analysis of 20th century (1920–1999) drought events in southern Ontario revealed occurrences in 1930, 1933, 1934, 1936, 1963, 1998, and 1999, with no long-term trend (Klaassen, 2002). Canada-wide trends in actual evapotranspiration from 1960 to 2000 showed significant increasing values at 35% of the station locations, mainly on the Pacific and Atlantic coasts and in the Laurentian Great Lakes/St. Lawrence zones (Fernandes et al., 2007). Other studies found that annual actual evapotranspiration trends in the Prairie region were mixed (e.g., Gan, 1998). Observed pan evaporation and estimated potential evapotranspiration for 11 Prairie region sites from the 1960s to early 2000s showed significant decreasing and increasing trends at different sites. Overall, more locations had decreases in potential evapotranspiration, and these were concentrated during June and July (Burn and Hesch, 2006).

No Canadian studies have attempted to directly attribute past trends in drought to anthropogenic climate change, although there has been some research on the 2015 extreme drought event in western Canada. Anthropogenic climate change increased the likelihood of the extremely warm spring, but no human influence was detected on the persistent drought-producing weather pattern (Szeto et al., 2016).

To date, no Canada-wide studies of future drought projections have been carried out. There are, however, several regional-scale analyses, with the majority focusing on the Prairie region and incorporating one or more drought indices. For example, output from three CMIP3 GCMs incorporating high (A2), medium-high (A1B), and medium (B2) emission scenarios were used to project future (2011–2100) summer PDSI over the southern Canadian prairies. More persistent droughts are projected, particularly after 2040, and multi-year droughts of 10 or more years are projected to become more probable (Bonsal et al., 2013). Similarly, the Canadian Regional Climate Model, under a high emission (A2) scenario, projected that long droughts of six to 10 months will increase and become more severe by mid-century across southern Manitoba and Saskatchewan and the eastern slopes of the Rocky Mountains. However, in the northern Prairie region, long drought events will be less severe and less frequent (PaiMazumder et al., 2012). A number of other studies of the Prairie region have examined drought changes for the mid-century period using several climate models that are part of the North American Regional Climate Change Assessment Program (Mearns et al., 2009). For the southern Prairie region, results under a high emission scenario (A2) indicated an overall increased drought risk for both summer and winter. There were considerable differences among models, with projections ranging from a substantial increase in drought with a higher degree of year-to-year variability, to relatively no change from current conditions (Jeong et al., 2014; Masud et al., 2017; Bonsal et al., 2017). Further north, in the Athabasca River Basin, projections revealed an average change toward more summer drought, but, again, there was a substantial range among the climate models (Bonsal and Cuell, 2017). Future annual and summer SPEI changes over all western Canadian river basins were assessed with six CMIP5 GCMs for the periods 2041–2070 and 2071–2100 (relative to 1971–2000) using medium emission (RCP4.5) and high emission (RCP8.5) scenarios. Southern watersheds showed a gradual increase in annual water deficit throughout the 21st century, while the opposite was true for northern basins. For summer, however, all river basins except those in the extreme north are expected to experience decreasing water availability (see Figure 6.15) (Dibike et al., 2017). Twelve CMIP3 GCMs incorporating medium (B1), medium-high (A1B), and high (A2) emission scenarios showed that, by the end of the 21st century, the combined changes in precipitation and temperature will lead to generally drier conditions in much of the boreal forest region of western Canada and to a higher likelihood of drought. However, some regions in the east may become slightly wetter (Wang et al., 2014).

These future projections are consistent with other North American and global-scale studies using similar drought indices. For instance, drought projections using numerous CMIP5 GCMs (medium emission (RCP4.5) scenario) showed that the frequency of severe-to-extreme drought conditions is expected to increase by the late 21st century for much of southern Canada, including southeast British Columbia, the prairies and Ontario (as measured by PDSI and soil moisture) (Dai, 2012; Zhao and Dai, 2015, 2016). Similar results have been projected using PDSI and SPEI under a high emission (RCP 8.5) scenario (Cook et al., 2014; Touma et al., 2015). This included increases in drought magnitude and frequency over western, central, and eastern North America, with the greatest change over western and central regions. Year-to-year variability in SPI was projected to increase by the end of century (2080–2099) in various regions of North America, suggesting more extremes; however, there was considerable uncertainty in these results, due to large differences among regions and among the 21 CMIP5 GCMs (Swain and Hayhoe, 2015). Although there is overall consistency regarding the increased likelihood of future drought over southern interior continental regions of Canada, there is uncertainty concerning the magnitude of these changes. This is primarily due to shortcomings of the indices that estimate potential evapotranspiration, which may lead to an overestimation of drought intensity (e.g., Sheffield et al., 2012; Trenberth et al., 2014; Milly and Dunne, 2016)