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)
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