HYDROLOGICAL PROCESSES
Hydrol. Process. 27, 687–699 (2013)
Published online 20 December 2012 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/hyp.9626
Will catchment characteristics moderate the projected effects of
climate change on flow regimes in the Scottish Highlands?
R. Capell,* D. Tetzlaff and C. Soulsby
School of Geosciences, University of Aberdeen, Aberdeen, AB24 3UF, UK
Abstract:
Regional climate models were used with the UKCP09 weather generator to downscale outputs from the HadCM3 General Circulation
Model, to project climate change by 2050 in the Scottish Highlands. The resulting hydroclimatic data were used to drive a tracer-aided
hydrological model to assess likely changes in flow regimes in three experimental catchments. These are located along a hydroclimatic
transect from the wet, mild western Highlands (Strontian), through the colder, more continental central Highlands (Allt a’ Mharcaidh),
to the drier eastern Highlands (Girnock). At all sites, temperatures are projected to increase by around 2 C, with associated increases in
potential evapotranspiration. Precipitation is expected to increase by around 10–15% at Strontian but remains slightly changed at the
Allt a’ Mharcaidh and Girnock. However, the seasonal distribution of precipitation is projected to change, increasing in winter and
decreasing in summer. Cautious interpretation of model outputs indicates that flows are likely to change accordingly at all sites, though
the characteristics of each catchment result in some subtle differences. At Strontian, marked increases in winter high flows are
expected, at the Allt a’ Mharcaidh, reduction in winter snowfall and reduced snowpack will increase winter high flows and moderate
the influence of spring melts. In the Girnock, decreasing summer low flows are the most notable element of change. These subtle
differences in response to climatic drivers are consistent with the distinct storage characteristics of the catchments which in turn reflect
their landscape evolution histories. Copyright © 2012 John Wiley & Sons, Ltd.
KEY WORDS
climate change; inter-site comparison; modelling
Received 17 May 2012; Accepted 15 October 2012
INTRODUCTION
Anthropogenic emissions of greenhouse gases (GHGs)
have altered the composition of the atmosphere, thereby
changing the global climate (IPCC, 2007). The resulting
regional impacts are likely to increase in the coming
decades, but there remains much debate on the precise
nature of these changes and how well they can be
predicted by climate models (Beven and Alcock, 2012).
Consequently, whilst the potential effects on water
resources are likely to be significant (Gleick, 2010), they
are also highly uncertain (Beven and Alcock, 2012).
Heterogeneity in hydroclimate and catchment characteristics in contrasting geographical regions also means that
the hydrological functioning of catchments can be
variable (Buttle, 2006; Tetzlaff et al., 2009; Carey et al.,
2010), resulting in different likely changes to external
hydroclimatic drivers and different intrinsic sensitivity of
catchments to regional climate change (e.g. Capell et al.,
2012b). Projecting the likely effects of climate change on
the water balance and flow regimes in different catchments is a major challenge, yet it is a prerequisite for
assessing the implications for water resources, stream
chemistry (e.g. Laudon et al., 2011) and in-stream ecology
(Kruitbos et al., 2012). Critical to such projections is the
*Correspondence to: Rene Capell, School of Geosciences, University of
Aberdeen, Aberdeen, AB24 3UF, UK
E-mail: rene.capell@gmail.com
Copyright © 2012 John Wiley & Sons, Ltd.
derivation of reasonable forecast data from climate models
and the appropriate use of rainfall-runoff models to assess
hydrological change. The cascade of uncertainty here is
potentially great, and several authors have urged extreme
caution in interpretation of such studies (Hall, 2007).
Changes in drivers such as precipitation and evapotranspiration are the primary factors that will determine
hydrological responses to climate change at the catchment
scale. However, the physical form and structure
of catchments will determine their sensitivity to change
(e.g. Tetzlaff et al., 2011). The way that catchments
partition, store and then release water controls their flow
regime. This in turn determines the resistance of the
catchments flow regime to change induced by external
forcing factors and its resilience or rate recovery following
major perturbations (Carey et al., 2010). For example,
region-specific partitioning of precipitation into different
flow paths and subsurface storages and the intra-annual
effects on the flow regime has been shown to mediate
climate change impacts, resulting in regionally variable
hydrological changes (Tague and Grant, 2009; McNamara
et al., 2011). However, many climate change impact studies
have been restricted to individual catchments in single
geomorphic provinces (e.g. van Roosmalen et al., 2009;
Poyck et al., 2011). Comparison across climate gradients
and in different geomorphic regions has potential in
differentiating climatically driven impacts and the degree
to which these can be moderated by catchment structure and
landscape organisation (Tetzlaff et al., 2008; Carey et al.,
2010; Jones et al., 2012).
688
R. CAPELL, D. TETZLAFF AND C. SOULSBY
Upland landscapes in the northern mid- and highlatitudes are sensitive to climate change (Viviroli et al.,
2011). Small differences in temperature determine the
frequency with which the 0 C temperature threshold is
crossed and the longevity of periods above and below this
critical value. This controls precipitation type, the
magnitude and timing of snow accumulation and
subsequent melt (Shinohara et al., 2009). Being able to
project the likely consequences in northern headwaters is
of critical societal relevance (Smith, 2011). Downstream
flows from upland areas provide a suite of critical
ecosystem services, including the provision of water
supplies, maintenance of water quality and sustaining
in-stream ecology (Viviroli et al., 2011). Climate warming
may also interact with other agents of environmental
change in ways that may compromise these ecosystem
services. For example, it may have indirect effects by
promoting land use changes, due to natural ecohydrological responses or adaptive management to
changing temperatures and/or water availability. These
land use changes themselves can modify the catchment
water balance in ways that may mitigate or enhance
climate change impacts, but such feedbacks are extremely
difficult to assess (e.g. Brown et al., 2005; Quilbe et al.,
2008). Nevertheless, they can have implications for water
resource management with increased flood regimes and
reduced dry weather flows being two potential results of
climate non-stationarity (e.g. Tague and Grant, 2009;
Milly et al., 2008).
Using climate projections from General Circulation
Model (GCM) to drive hydrological models is the most
common method of assessing the likely consequences of
climate change on catchment hydrology (e.g. Christensen
et al., 2004; Fowler and Kilsby, 2007; van Roosmalen
et al., 2009). Here, we examine three Scottish sites from the
North-Watch (Northern Watershed Ecosystem Response to
Climate Change: http://www.abdn.ac.uk/northwatch/)
international inter-comparison project that explores the
inter-linkages between climate, hydrology and ecology. We
examine a west-east climatic transect in Scotland where the
three upland catchments give a regional perspective on the
responses to climatic change. Climate change projections
are based on the UK Climate Projections 2009 (UKCP09;
Murphy et al., 2009), which provide probabilistic climate
change functions derived from perturbed physics regional
climate model (RCM) ensembles. Synthetic time series
derived from UKCP09 projections were used to force a
tracer-aided conceptual hydrological model that was developed specifically for Scottish Highland landscapes. The
model structure and parameter ranges were constrained to
those identified as behavioural on the basis of their ability to
simulate stream flow dynamics as well as both geochemical
source area tracers and isotopic time domain tracers in
upland catchments (Capell et al., 2012a). This intercomparison tests the hypothesis that catchment landscape
properties can moderate climate change impacts. The
specific objectives are (1) To assess the projected climate
change impacts on precipitation and temperatures in three
Scottish upland catchments along a west-east transect, (2) to
Copyright © 2012 John Wiley & Sons, Ltd.
use projected synthetic climate forcing (2050s time slice) in
a hydrological model to examine the impact of climate
change on the water balance and hydrological dynamics of
these catchments at annual, seasonal and monthly scales and
(3) to assess how the differences in hydrologic response of
the catchments might reflect catchment characteristics that
influence their resistance (i.e. the degree to which runoff is
coupled with precipitation, relating to drainage efficiency
and storage capacity), and resilience (i.e. a measure of the
degree to which a catchment can return to normal
functioning following perturbations from extreme hydrological events (Carey et al., 2010)).
STUDY SITES
Three upland catchments situated along a west-east climate
transect across Scotland were compared to assess the likely
impact of projected climate changes on flow regimes
(Figure 1). The Strontian (STR, 8 km2) is located near the
Atlantic coast, the Allt a’ Mharcaidh (MHA, 10 km2) in the
more ‘continental’central highlands of the Cairngorm
mountains, and the Girnock (GIR, 30 km2) in the eastern
rain shadow. All three catchments are montane upland sites,
characterised by broadly similar steep terrain, with >600 m
of relief and low permeability metamorphic or igneous
bedrock geology. However, they differ substantially in
climate conditions and landscape evolution history. The
latter factor results in differences the distribution and
thickness of unconfined quaternary sediments and soils in
the catchments (Figure 1, Table I).
The westerly STR catchment rises from near sea level
to over 700 m. Annual precipitation is around 2700 mm
mainly brought by Atlantic frontal systems; the ocean
moderates temperatures which have an annual mean of 8 C (Table II). Winter temperatures generally remain above
freezing, and snow is typically <5% of inputs and
restricted to short periods. Bedrock geology is comprised
of schists and gneiss. Drift cover is largely restricted to
low permeability sediments in lower slope areas; soil
cover here is dominated by peaty gleys (histosols) with
rankers (regosols) in elevated areas (Ferrier and Harriman,
1990). The Hydrology Of Soil Types (HOST) classification
scheme (Boorman et al., 1995) describes these soils as being
dominated by lateral flow paths, and thus hydrologically
responsive with little dynamic storage change (Table I). The
vegetation is characterised by coniferous forest plantations
in the lower catchments and moorland grasses at high
altitudes. The combination of intense precipitation and
catchment topography, soils and geology results in a very
flashy hydrological regime (Table II), with groundwater
accounting for less than 20% of annual runoff (Soulsby
et al., 2010).
The MHA catchment is colder due to its location and
higher altitude. Mean annual temperatures are 5 C, and
annual precipitation is ~1100 mm with up to 30% falling
as snow (Soulsby et al., 1997), the climate is sub-arctic in
above 800 m. The bedrock is granite but largely covered
by drift. At higher altitudes, periglacial drifts and bedrock
Hydrol. Process. 27, 687–699 (2013)
689
EFFECTS OF CLIMATE CHANGE IN CATCHMENTS ALONG A HYDROCLIMATE TRANSECT
Figure 1. Catchment topography, stream network and location of the three sample catchments on the Scottish mainland. Scotland map shows average
annual precipitation from 1971 to 2000 (UK Met Office)
Table I. Physical catchment characteristics of the three studied catchments
Topography
Geology (%)
HOST soil classes (%)
Land Cover (%)
Strontian
Allt a’ Mharcaidh
Girnock
Location
West Coast
Western Cairngorms
Eastern Cairngorms
Area (km2)
Mean elevation (m)
Relief (m)
Median slope (degrees)
Igneous
Metamorphic
4,5,7: Alluvial soils
15: Peaty podzols and peaty gleys
17: Humus iron podzols
19, 22, 27: Rankers
29: Blanket peats
Forest
Grassland
Wetlands
Heather Moorland
Montane/rocky
8
340
737
14
10
704
779
16
99.8
0.2
0
35
30
7
28
5
30
405
619
9.9
52
48
2
60
25
9
4
8
5
18
63
fractures form groundwater stores (Soulsby et al., 1999).
In lower areas, glacial drift exceeds 10 m in depth and has
relatively high porosity and permeability in places. At
higher altitudes, freely draining alpine soils have formed
beneath montane heath. In steeper areas, podzols support
heather (Calluna) dominated moorland, grading to
regenerating Scots Pine (Pinus sylvestris) forest on lower
Copyright © 2012 John Wiley & Sons, Ltd.
100
0
71
0
29
0
48
52
59
36
slopes. On gentle slopes, deep peats support boreal
blanket bog. Storm runoff is generated by overland flow
from the peats, fed by shallow lateral flow from the
podzols (Wheater et al., 1993; Jenkins et al., 1994).
Drainage in the alpine soils and podzols recharges
significant groundwater stores (Soulsby et al., 2009).
Geochemical hydrograph separations showed that
Hydrol. Process. 27, 687–699 (2013)
690
R. CAPELL, D. TETZLAFF AND C. SOULSBY
Table II. Summary of water balance variables during the
observation periods in the three study catchments
Observation
period
Annual Q (mm)
Annual P (mm)
Annual ET (mm)
Mean daily flow
(mm d 1)
Q5 (mm d 1)
Q95 (mm d 1)
Strontian
Allt a’
Mharcaidh
Girnock
1998/99 to
2001/02
2493
2751
258
6.8
1990/91 to
1992/93
824
1163
339
2.3
2005/06 to
2007/08
666
1138
472
1.82
25.2
0.33
5.9
0.79
5.1
0.17
groundwater provides over 50% of annual runoff with
high baseflow (Table II) (Soulsby et al., 1998).
The GIR catchment is in the eastern rain shadow; it is
the driest sited with a lower mean elevation and high
mean annual temperature of around 6 C and <10% of
precipitation falls as snow. As with the other sites, the
landscape history is characterised by past glaciations;
here, the wide valley bottoms (Table I and II) are largely
covered by low-permeability glacial drift. The underlying
bedrock comprises low permeability metamorphic rocks
with granite intrusions (Soulsby et al., 2007). Soils are
mainly peats and gleys in the upper catchment, and
humus-iron podzols in lower areas (Tetzlaff et al., 2007).
Unlike the MHA, the vegetation is more managed mainly
to maintain habitat for animals and birds that are quarry in
field sports involving shooting. Thus, grazing by Red deer
(Cervus elaphus) is intense in the lower catchment, and
burning to create habitat for moorland birds (e.g. grouse
(Lagopus lagopus)) occurs in the upper catchment.
Forestry is restricted to small enclosed areas on the lower
slopes. Overland flow and shallow sub-surface storm flow
from the peaty soils dominate the storm hydrograph
(Tetzlaff et al., 2007). Groundwater recharge is restricted
to more freely draining soils (Soulsby et al., 2007) and
geochemically based hydrograph separations suggest that
groundwater accounts for ca. 30% of annual runoff with
low base flow (Table II) (Soulsby et al., 2005).
DATA AND METHODS
Measured and projected data
Precipitation, runoff and air temperature data were
available for each catchment at daily resolution for threeyear or four-year observation periods between 1990 and
2008, each covering drier and wetter years (Table II).
Monthly potential evapotranspiration (PET) was estimated using the Thornthwaite equation, then distributed
to daily amounts using air temperature as weighting factor
with PET equal to zero under freezing conditions, and a
simple volumetric correction to actual evapotranspiration
volumes from the average water balance (Thornthwaite
and Mather, 1955). Precipitation and discharge data were
used to calibrate conceptual rainfall-runoff models for the
Copyright © 2012 John Wiley & Sons, Ltd.
study catchments which were then forced with probabilistic climate change scenario data (see below).
Climate change projections provided by the UK
Climate Projection 2009 (UKCP09) network were used
(Murphy et al., 2009). These provide probabilistic
projections at a range of IPCC GHG emission scenarios
and future time slices for the British Isles. Within the
scope of this study, an average GHG emission scenario
(IPCC scenario A1B) for the 2050s decade was selected.
Previous modelling studies in the Scottish Highlands have
already contextualised this relative to other ‘worse case’
(A1FI) and ‘best case’ (B1) scenarios (Capell et al.,
2012b). The UKCP09 climate projections are downscaled
projections from HadCM3 GCM projections and a 1960
to 1991 baseline period. The approach addressed
uncertainty in the HadCM3 model due to imperfect
process representation within a Bayesian framework and
an ensemble of HadCM3 parameterisations. These were
downscaled to a 25 25 km resolution using a perturbed
physics ensemble of 11 HadRM3 RCMs, generating
probability density functions of target climate variables at
a monthly time scale (Murphy et al., 2009). A stochastic
weather generator (WG) is accessible through UKCP09,
which allows generation of further downscaled stochastic
daily time series projections of baseline and future
precipitation, PET and air temperature at a 5 5 km
resolution. UKCP09 WG simulations are statistically
downscaled RCM simulations which use monthly rainfall
statistics for the 1960 to 1991 baseline period (Jones
et al., 2009). The WG produces stationary synthetic time
series. Here, 100 synthetic time series of 55 km grid
cells covering each study catchments were generated,
using randomly sampled RCM variants (encompassing
uncertainty from RCM and GCM variants), thus giving a
range of projections addressing the model uncertainty
inherent in the climate model and down-scaling chain.
Each of the 100 time series span a 30-year sequence
(assuming stationary climate conditions) to sample
extreme events with large return periods. The synthetic
baseline data are also generated by the WG, reproducing
the statistical properties of the 1960 to 1991 baseline data
(further details on the WG in Jones et al., 2009). The
synthetic time series reflect anticipated changes in GHG
emissions under the chosen scenario and future time slice
and also give an indication of the uncertainty in these
projections.
Hydrological impact assessment using tracer-aided models
Changes in runoff were projected using hydrological
models with a common structure that was calibrated for
each catchment. The calibration period encompassed
short periods (i.e. up to 3 years) within the record that
included both wetter and drier years than average
(Table II). The model had a priori constrained structures
developed from application to other catchments in the
Scottish Highlands where a range of catchment types can
be modelled with a broadly similar structure (see
Hrachowitz et al., 2011). This was based on the use of
Hydrol. Process. 27, 687–699 (2013)
EFFECTS OF CLIMATE CHANGE IN CATCHMENTS ALONG A HYDROCLIMATE TRANSECT
geochemical and isotopic tracers as additional constraints
in an iterative process that produced models with
structures and parameter ranges that could simulate the
catchment runoff and tracer response. Details are given by
Capell et al. (2012b); briefly, this resulted in a model that
could account for the different geographical sources of
runoff (i.e. shallow and deeper flow paths) and their
temporal dynamics (i.e. fast and slow flow paths) as
indicated by the tracer responses, providing a firmer
process-basis for impact assessment. In summary, the
model comprises two linear stores and a saturation
overland flow (SOF) conceptualisation (Figure 2). These
conceptualise baseflows from a deeper groundwater zone
and a more dynamic storage volume (e.g. in the upper
soils and lower hillslopes) and SOF or similar saturationdependent lateral runoff generation related to responsive
soil cover (Capell et al., 2012a). The model partitions
precipitation between recharge into the soil store and
direct SOF runoff depending on a non-linear function of
the soil storage volume. Evaporation is drawn from and
limited by the soil storage. The baseflow storage receives
a constant recharge from the overlying soil. A degree-day
snow module, driven by daily air temperatures, similar to
the HBV model (Bergström, 1975) is used to estimate
snow accumulation and melt. The model requires five
691
calibrated parameters in the storage, and two in the snow
routines.
To project runoff responses in the three catchments,
behavioural parameter sets were selected by calibration to
measured hydrometric data. The synthetic time series
derived from UKCP09 projections were then used to
force the models with parameterisations identified as
behavioural to predict the runoff response. Parameter sets
were generated using a Monte Carlo approach sampling
wide initial parameter ranges; 5105 parameter sets were
generated. Model performances were evaluated by three
objective functions; the Nash-Sutcliffe efficiency (NSE),
the NSE of log-transformed runoff (logNSE) and the
coefficient of determination (R2) (cf Gupta et al., 1998).
Parameter ranges were evaluated against performance
peaks and threshold performances determined (Table III),
above which, parameter sets were accepted as behavioural.
From the accepted parameter sets, 100 were randomly
chosen and used to generate runoff simulations with each
synthetic projection and baseline time series, deriving a
range of rainfall-runoff model uncertainty. For assessing the
long-term changes, the projected daily time series of runoff
from the hydrological model were then aggregated to
monthly, seasonal and annual statistics, along with the
characteristics and magnitudes of projected climate change
for comparison. Additionally, the projected number in snow
days, i.e. days during which the models projected a resident
snow layer, were compared.
RESULTS
Projected climate change impacts on precipitation and
temperatures
Figure 2. Hydrological model structure showing water fluxes (dotted
lines) and storage components of the lumped model structure. Precipitation (P) is routed to a day-degree snow routine on a temperature threshold
condition. A linear shallow storage (S1) generates fast runoff responses
(Q1). An additional saturation excess overland flow runoff (QSOF) is
generated as a function of storage in S1. Baseflow (Q2) is generated from a
linear storage S2
The projected changes in climate for the 2050s are
visualised as annual summaries in Figure 3 and
summarised in Table IV. The projected increase in annual
average temperatures of about 2 C above the present is
consistent for all sites (Figure 4). Inter-annual variation
(shown as whiskers in the boxplots), however, increases
in the projected average temperatures indicating the
uncertainty (Figure 4a). However, in addition to the
inherent projection uncertainty, this may also indicate an
increase in extreme temperature patterns, which can in
turn mean a greater impact on catchment hydrology.
Annual precipitation in the STR catchment, already the
highest in the baseline figures (2400 mm), is projected to
increase by approximately 12% in the 2050s with
increased inter-annual variability (Figure 4b). Such
increase is not apparent in the more central and easterly
Table III. Model performance for all three catchments, showing maximum performances and performance thresholds for behavioural
parameter sets
Maximum model Performance
(performance threshold)
Copyright © 2012 John Wiley & Sons, Ltd.
NSE
logNSE
R2
Strontian
Allt a’ Mharcaidh
Girnock
0.83 (0.6)
0.85 (0.7)
0.83 (0.6)
0.77 (0.6)
0.75 (0.6)
0.78 (0.6)
0.71 (0.6)
0.85 (0.6)
0.78 (0.6)
Hydrol. Process. 27, 687–699 (2013)
692
R. CAPELL, D. TETZLAFF AND C. SOULSBY
Figure 3. Comparison of annual air temperature and water balance terms along the west-east transect as pie charts. Segment sizes of precipitation,
evapotranspiration and runoff scaled individually, allowing inter-catchment comparison of each term (segment sizes not to scale for within-catchment
water balances)
Table IV. Summary of annual change statistics relative to baseline (1960–91) for projected climate and hydrological impact in the study
catchments. Long-term annual averages and standard deviations (in parentheses) of air temperature, water balance terms and number of
snow days
Time period
Temperature
Precipitation
Runoff
Evapo-transpiration
No. of snow days
baseline
2050s
baseline
2050s
baseline
2050s
baseline
2050s
baseline
2050s
Strontian
8.4
10.7
2421
2711
2070
2327
602
688
1
0
sites of the MHA and GIR where precipitation changes
are within +2%. The inter-annual variability also seems
more stable at these sites, especially compared to
temperatures. PET largely follows the temperature trend,
with projected increases of 86 mm, 69 mm and 70 mm for
STR, MHA and GIR, respectively (Figure 4c).
The increased temperatures result in decreases in
predicted number of snow days (Table IV). These
Copyright © 2012 John Wiley & Sons, Ltd.
(0.3)
(0.9)
(262)
(357)
(251)
(346)
(19)
(41)
(1)
(0)
Allt a’ Mharcaidh
4.4
6.7
1296
1310
1043
1075
420
489
119
51
(0.4)
(0.9)
(174)
(183)
(163)
(167)
(20)
(35)
(23)
(5)
Girnock
5.6
7.7
1025
1022
826
805
479
549
6
2
(0.3)
(0.9)
(149)
(162)
(134)
(148)
(19)
(34)
(1)
(2)
numbers can only be considered as indicative, given the
simplicity of the snow model used, but the greatest impact
will be in the MHA. Snow influence under baseline
conditions is highest (119 days) here, with only 6 days in
the GIR and one or two days at STR. Snow days is
projected to decrease to ~40% of the baseline in the
MHA, so the potential hydrological influence is significant. In the other catchments, snow has a limited baseline
Hydrol. Process. 27, 687–699 (2013)
EFFECTS OF CLIMATE CHANGE IN CATCHMENTS ALONG A HYDROCLIMATE TRANSECT
693
Figure 4. Variation and projected change of annual air temperature and water balance components in the study catchments, derived from synthetic
weather generator time series for a baseline period (1961 to 1990; baseline) and projections for the 2050s decade under IPCC scenario A1B (2050s).
(a) Air temperature, (b) precipitation, (c) potential evapotranspiration and (d) runoff. Box plots showing median and inter-quartile range, whiskers
extending to data extremes
water balance influence. Thus, the hydrological impact is
much less important.
Seasonality of the long-term projections for hydroclimatic
drivers is shown in Figure 5. Air temperatures increase at
all sites in all seasons (Figure 5a). From a hydrological
perspective, the changes in autumn (OND) and winter
(JFM) in the sub-alpine MHA catchment are particularly
notable as raising temperatures corresponds to decreasing
winter snow. Changes in seasonal precipitation distributions
also project increases during the autumn/winter half year
(OND, JFM) at all sites, most notably at STR (Figure 5b).
All sites show precipitation decreases during summer (JAS),
but only the GIR shows a notable decline in spring (AMJ)
(Figure 5b). All sites show increases in PET mainly in the
spring (AMJ) and summer (JAS). The magnitude of
projected increases is consistent with current differences
with STR > GIR > MHA.
Changes to long-term monthly temperature and
precipitation regimes (Figures 6, 7, and 8, respectively)
provide more detailed resolution. The long-term precipitation averages are expected to increase most notably on
the west coast (STR) from September to May, and
slightly decrease in July to August, though monthly totals
remain high throughout the year. The projected changes
are comparatively subtle in MHA and GIR, and rather a
shift and the annual totals remain almost unchanged.
Nevertheless, winter increases at MHA are evident from
Copyright © 2012 John Wiley & Sons, Ltd.
October to May. Decreases are apparent in July and
August, consistent with STR. In contrast, GIR projects
decrease between May and September. Monthly PET
increases greatest in summer for all sites, being highest at
STR (Figures 6c, 7c and 8c).
Current hydrological dynamics and projected future impacts
Annually, the runoff from the catchments reflects
precipitation, with yields under baseline and projected
conditions approximately twice as high in the STR
catchment compared to MHA and GIR (Table IV,
Figure 4d). Reflecting precipitation inputs, annual runoff
at STR is projected to increase by over 10%, whilst slight
increases (+1%) are projected for MHA, and small
decreases in the GIR catchment ( 3%) indicate overall
limited change. This is despite likely increases in PET,
which probably reflects the greater proportion of annual
precipitation in winter when PET is lowest.
Seasonally (Figure 5d), spring and summer flows are
projected to decrease, while autumn and winter flows
increase (MHA, STR) or remain stable (GIR). However,
the inter-annual variation remains fairly stable in the
projected annual and seasonal values, with the exception
of the winter flows in the STR, where variation increases
similar to the precipitation pattern, Figure 5b.
The monthly flow regimes reveal greater resolution
(Figures 6d, 7d and 8d). Most dramatic changes are in
Hydrol. Process. 27, 687–699 (2013)
694
R. CAPELL, D. TETZLAFF AND C. SOULSBY
Figure 5. Variation and projected change of seasonal air temperature and water balance components in the study catchments, derived from synthetic
weather generator time series for a baseline period (1961 to 1990; baseline) and projections for the 2050s decade under IPCC scenario A1B (2050s).
(a) Air temperature, (b) precipitation, (c) potential evapotranspiration and (d) runoff. Box plots showing median and inter-quartile range, whiskers
extending to data extremes
Figure 6. Projected change in average monthly air temperature (a) and water balance components (b–d) in the Girnock catchment, derived from synthetic
weather generator time series for a baseline period (1961 to 1990; baseline) and projections for the 2050s decade under IPCC scenario A1B (2050s).
Error bars show standard deviations of the monthly aggregates of all projection time series
Copyright © 2012 John Wiley & Sons, Ltd.
Hydrol. Process. 27, 687–699 (2013)
EFFECTS OF CLIMATE CHANGE IN CATCHMENTS ALONG A HYDROCLIMATE TRANSECT
695
Figure 7. Projected change in average monthly air temperature (a) and water balance components (b–d) in the Mharcaidh catchment, derived from
synthetic weather generator time series for a baseline period (1961 to 1990; baseline) and projections for the 2050s decade under IPCC scenario A1B
(2050s). Error bars show standard deviations of the monthly aggregates of all projection time series
Figure 8. Projected change in average monthly air temperature (a) and water balance components (b–d) in the Strontian catchment, derived from
synthetic weather generator time series for a baseline period (1961 to 1990; baseline) and projections for the 2050s decade under IPCC scenario A1B
(2050s). Error bars show standard deviations of the monthly aggregates of all projection time series
Copyright © 2012 John Wiley & Sons, Ltd.
Hydrol. Process. 27, 687–699 (2013)
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R. CAPELL, D. TETZLAFF AND C. SOULSBY
winter at STR, where flows increase up to 25%
(December). A general increase in autumn and winter
flows at STR is apparent, with only a slight decline in
July and August. In the MHA, winter flows increase
between December and February. Decreases during the
baseline snowmelt period (March–April) are consistent
with loss of winter snow and the consequent runoff
increases in the preceding winter. Only drier months from
July to August result in decreases in projected runoff. In
GIR, decreased precipitation and increasing evaporation
lead to more consistent decrease in runoff from April to
October, and the projections indicate new low-flow
extremes in dry summers.
DISCUSSION AND CONCLUSIONS
Projected changes in climate
We examined three upland sites across a climatic transect
for a regional perspective on the hydrological impact of
climatic change in the Scottish North-Watch catchments.
The patterns of change projected are largely consistent with
other studies in other UK uplands (Fowler and Kilsby, 2007;
Capell et al., 2012b), with increasing temperatures throughout the year, increased winter and decreased summer
precipitation. The three sites also highlight the influence of
regional climate patterns and their relevance for the
hydrological change. The visualisation in Figure 3 shows
that the hydroclimate of STR is dominated by the large
precipitation inputs, a feature which is likely to be
accentuated in future with around 10% more precipitation
by 2050. The contrast with unchanged or drier conditions in
central (MHA) and eastern (GIR) Scotland suggests the
continuation of a trend over the past decades (Werritty,
2002). Despite the lack of input change at MHA and GIR,
subtle changes in precipitation distribution and type are
likely. At MHA, increased winter temperatures are likely to
decrease the size and longevity of snow accumulation, with
more winter precipitation falling as rain, and more rapid
melts (cf Soulsby et al., 1997). The main change expected at
GIR is decreased summer precipitation, which, with likely
increases in temperature and potential PET, may have
implications for summer flows.
Projected changes to the hydrological regime
The likely changes in runoff regimes mainly reflect the
influence of the changing climatic drivers. However, it is
also to an extent mediated by the characteristics of the
catchments themselves. At STR, the increased precipitation drives increased runoff which is likely to be much
greater than losses from increased evapotranspiration.
Higher winter flows may increase the frequency and
magnitude of storm flows. Although reduced summer
precipitation and increased temperatures are likely to
cause small declines in summer flows, these will remain
relatively high.
For MHA, annual runoff changes little, though
temperature increases will affect snowfall, an important
factor in the hydrology of this area. This increases runoff
Copyright © 2012 John Wiley & Sons, Ltd.
in November, December and January as more precipitation falls as rain and snowpack accumulation is reduced.
Although a March snowmelt peak is predicted to remain,
it is less marked than previously, and snowmelt
continuation into April and May is likely reduced though
flows are little changed. Flows in July and August are
projected to decline by ~10–15% as rainfall declines and
evaporation increases. At GIR, the changes in interannual precipitation distribution, with reduced summer
inputs and increased ET, are more likely caused by drier
conditions, and flows decreased by ~ 20% between June
and September.
Catchment characteristics and sensitivity to change – the
resistance and resilience of catchments
Previous studies have shown how the form and
structure of montane catchments which determine the
magnitude of different water storages and the partitioning
of precipitation along different flow paths can influence
sensitivity to climate change (Tague et al., 2008). In the
case of STR, the thin soils, impermeable geology and
steep slopes result in limited catchment storage, so there
is very little resistance – in terms of coupling between
precipitation-runoff – to precipitation changes (Carey
et al., 2010). Modelling studies indicate that dynamic
storage in the catchment is typically in the order of
<20 mm, thus precipitation increases directly increases
runoff (Soulsby et al., 2011). Tracer studies have shown
very short mean residence times in the catchment,
typically in the order of a few months (Hrachowitz
et al., 2010a), which infers total passive storage in the
catchment of at least 300 mm (Soulsby et al., 2009). The
projected climate changes with increasing precipitation
are likely to change these storages little with little
capacity to buffer effects on increased streamflows.
The MHA, with deeper soils and more permeable drift
deposits, has relatively larger groundwater stores which
contribute around half of the annual discharge. Thus, the
hydrological regime is highly attenuated compared to
STR. Mean residence times in the catchment are in the
order of 3 years (Hrachowitz et al., 2010a), inferring
passive storage probably >2000 mm (Soulsby et al.,
2009). In the drier climate of the MHA, summer soil
moisture deficits may exceed 50 mm giving some
dynamic storage (Soulsby et al., 2011). Increased winter
precipitation results in moderate increases in winter
runoff indicating little resistance to increased high flows.
In summer, however, the sub-surface storages allow a
certain resistance to reduced precipitation and increased
PET because winter recharge into the deeper subsurface is
able to sustain low flows (cf Table II).
This resistance of the GIR in summer is lower,
probably because the drift deposits here are less
permeable. This results in a catchment with a high
coverage of responsive peat and gleyed soils with only
around 30% of annual flow being derived from
groundwater. Consequently, the catchment already
experiences low summer flows as groundwater inputs are
lower than at MHA (Tetzlaff et al., 2007). Tracer-based
Hydrol. Process. 27, 687–699 (2013)
EFFECTS OF CLIMATE CHANGE IN CATCHMENTS ALONG A HYDROCLIMATE TRANSECT
studies indicate mean residence times of about 18 months
(Hrachowitz et al., 2010a), inferring a passive storage of
around 1000 mm, though dynamic storage may vary by
around 70 mm. Thus, the catchment storages have potentially less resistance to changing seasonal precipitation, and
flows during periods of low summer precipitation are likely
to decrease accordingly.
These differences in the likely potential for catchments to
buffer or mediate the impacts of change in climatic drivers
are quite subtle. This contrasts with the large-scale
differences that geology has been shown to have on
sensitivity to environmental change in other montane
regions (cf Tague et al., 2008; Tague and Grant, 2009).
Nevertheless, it highlights likely intra-regional subtleties
that will play out in the coming decades and may have
management implications. Thus, although the general
patterns of warming, reduced snow, increased winter runoff
and reduced summer flows are similar to the recent trends
and projections for many other northern temperate/subboreal regions (see Smith, 2011 for a review), the exact
impacts may be catchment specific (cf Beven, 2001).
Potential for improving projections with tracer-aided models
Results showed that the ensemble projections and
downscaling chain result in wide uncertainty ranges for
the projected climate variables. This precludes the
estimation of absolute values and makes evaluation of
impact scenarios necessarily cautious. However, the longterm annual, seasonal and monthly projections were
considered instructive for comparing the three catchment
hydroclimates. The large sample size of RCM realisations
provides a more stable basis for the climate change
projections over the coming decades, even though only
one GCM provided the global change conditioning, this
shortcoming is likely to be offset by using several GCM
in an ensemble (Teutschbein et al., 2011). However, this
could increase uncertainty if different outcomes result.
Additional uncertainties are associated with the hydrological modelling. The use of bespoke tracer-aided
models for Scottish upland catchments helped constrain
some of the structural and parameter uncertainty (Capell
et al., 2012a). However, through calibration, the modelling the approach assumes stationarity in the way the
catchment generates runoff (Wagener, 2007), which is a
limitation considering that the projected climate changes
might cause non-linear responses in the longer term.
The probabilistic nature of the projection time series, the
inclusion of several model parameterisations and the
relatively short projection timescale somewhat offset this
risk, but at the cost of wide projection ranges and
sometimes ambiguous results.
Evolving catchment hydrology – the greatest uncertainty?
Perhaps the largest unknown in terms of nonstationarity in the catchment response is how the
vegetation cover changes, which, in turn, is likely to
affect the catchment soils. This will either result from
natural vegetation adaptation to the changing climate, or,
more likely, be dependent on how land management
Copyright © 2012 John Wiley & Sons, Ltd.
697
strategies evolve in response. At all three sites, land
management is deliberate. At STR, commercial forestry
in the lower catchment and rough grazing for sheep in
the upper catchment are the current characteristics.
However, even with forest cover on thin, peaty soils in
a wet climate increased evapotranspiration from forest
cover probably has limited potential in increasing
catchment scale water storage capacity (Tetzlaff et al.,
2007). At MHA, nature conservation is the main aim of
land management, and natural forest regeneration is
active with Scots Pine (Pinus sylvestris), and given the
soil cover and altitudinal limits of the tree line, perhaps
as much as half of the catchment could be afforested. In
rapidly growing, dense stands of Scots Pine, this has the
potential to substantially increase evapotranspiration,
mainly through interceptions losses which could reduce
groundwater recharge and summer low flows, though
this will be offset by an increased proportion of winter
precipitation (Haria and Price, 2000). There will also be
effects on snowmelt processes, though most snow in the
catchment falls above the tree line. Although GIR
currently has only small areas of forest cover, like the
MHA, it lays within the Cairngorm National Park, where
restoration of natural forests is a management objective
(Hall, 2007). Expansion of forest cover could similarly
exacerbate summer low flows decreases projected by
climate change and the GIR could be less resistant to
these than MHA.
Interestingly, some of the potential conflicts and
paradoxes may result from adaptive management of land
and water resources to climate change. For example, in all
three streams, salmonids (especially Atlantic salmon (Salmo
salar) are important components of the freshwater ecosystem. Concerns have been expressed over likely increases in
stream temperatures projected under climate change
scenarios, as salmonids are cold water species that can
suffer both lethal and sub-lethal physiological effects of
elevated temperatures; reduced summer flows will reduce
flow volumes and increase the risk of high temperatures
(Malmqvist and Rundle, 2002; Hrachowitz et al., 2010b). A
potential solution is increased riparian tree cover in
catchments, which has a moderating effect on temperatures
by reducing short wave inputs to the water column (Imholt
et al., 2012). Clearly, extension of forest cover beyond
the riparian zones to cover larger parts of catchments has
potential to induce hydrological change.
CONCLUSIONS
Downscaled GCM climate projections were used to drive
a tracer-informed hydrological model to assess the effects
on stream flow in three long-term experimental sites in
the Scottish Highlands across a west-east hydroclimatic
gradient. Mean annual temperature increases of around
2 C are expected at all three sites, as is a shift to
increased winter precipitation and reduced snow influence. All sites are expected to see reduced summer
precipitation and increased PET, and only the most
Hydrol. Process. 27, 687–699 (2013)
698
R. CAPELL, D. TETZLAFF AND C. SOULSBY
westerly site is expected to see a change (increase) in
precipitation totals. Projected hydroclimatic change will
cause responses in stream flow regimes, with all three
catchments showing relatively little resistance. Subtle
differences in drift geology in the Allt a’ Mharcaidh will
mean summer low flows will be more resistant to change
than at the other sites, though in a climate that is expected
to remain wet and cool, winter flows are likely to be
resilient. Adaptive management of land use has the
potential to affect flows as much as projected climate
change; this is an area where future research will be
directed (Viviroli et al., 2011).
ACKNOWLEDGEMENT
We thank the Leverhulme Trust for funding the NorthWatch project (http://www.abdn.ac.uk/North-Watch/; F/00
152/AG).
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