Introduction:
From different studies and witnessed abnormalities around the globe, it
is now clear that climate change has brought and will bring
vulnerabilities. CO2 in the atmosphere has set the
record in 2018 since preindustrial era (1850-1900) (Poloczanska,
Mintenbeck et al. 2018). Some individual sites indicate that the level
has been increasing for the year of 2019. Consequently, global mean
temperature has been rising 1.5℃ above preindustrial (1850-1900) era
(Poloczanska, Mintenbeck et al. 2018). Other key indicators of the
critical situation are Sea Level Rise (SLR) and Sea Ice Extent (SIC);
both are the direct consequence of greenhouse gas (GHG) increase in the
atmosphere. SLR has hit the record in 2019 of 3.2 mm/year during the
1993-2019 period and also Arctic extent has been decreasing
(Poloczanska, Mintenbeck et al. 2018). On the local scale, the warming
has consistent with the weather and climate variabilities related to
climate change; North America has been unusually cold due to the crisis
(Poloczanska, Mintenbeck et al. 2018). These changes have changed and
will continue to change the climate, making the weather harsh. Extreme
precipitations, more frequent hurricanes, intense tropical cyclones,
unexpected thunderstorms and tornados, sever cold breaks, prolonged
droughts, and seasonal timing shifts are expected to be common
(Groisman, Knight et al. 2005; Diffenbaugh, Scherer et al. 2013; Emanuel
2013; Diffenbaugh, Swain et al. 2015; Gao, Leung et al. 2015; Sobel,
Camargo et al. 2016). These threats not only target human communities
but also do threaten ecosystems functionality.
Climate models have proven to be reliable enough to take actions against
climate change reverse impacts. A projected hydrological model has the
ability to plan for management practices in a given watershed.
Hydrological projections have been studied by researchers around the
globe at local and global scales. Significant changes in hydrological
regimes in most part of the land surface of the planet will be likely to
occur by midcentury (2050) or they are sensitive to global mean
temperature below 2℃ (Arnell and Gosling 2013). AR4 (Field, Barros et
al. 2012) of IPCC reported low confidence on the anthropogenic climate
change footprints in flood records, which was due to limited
instrumental records in terms of space and time. However, regarding
impact on hydrological cycle like precipitation and snowmelt, and
earlier spring peak flows, the report showed medium to high confidence
which the latter was very likely (Field, Barros et al. 2012;
Georgakakos, Fleming et al. 2014). Hydrological projections indicate
sensitivity of flood frequency and climate change will be the partial
reason for increasing flood likelihood on the global scale (Cisneros, BE
et al. 2014). Climate change will adversely change streamflow and water
quality and consequently will jeopardize freshwater ecosystems
(Cisneros, BE et al. 2014). The projections also reveal medium to high
level of confidence in posing risk to potable water (Cisneros, BE et al.
2014).
In Southeast of the US, seasonal drying has been observed for spring,
fall , and winter and in summer the soil moisture has increase during
1988-2010 (Georgakakos, Fleming et al. 2014). Although potential
evapotranspiration (PET) has projected to increase, evapotranspiration
(ET) as the 2nd largest component of the hydrological
cycle requires further studies to see possible implication of the causes
like soil moisture in future ET trends (Georgakakos, Fleming et al.
2014). Projections for annual runoff and consequent stream flow in the
Southeast indicate declines, which is consistent with long-term
(multi-seasonal) droughts that are projected for the Southeast
(Georgakakos, Fleming et al. 2014).
River floods are more complex to be considered as the direct result of
the heavy precipitation and topography, soil moisture, channel
condition, and anthropogenic influences are thought to play the key
roles (Georgakakos, Fleming et al. 2014). River floods have been
decreasing in most part of the Southeast at least 6% percent per decade
(Villarini, Serinaldi et al. 2009; Georgakakos, Fleming et al. 2014).
Areas close to the Golf Coast have been hit by hurricanes several times
in recent years. Catchment specific characteristics like seasonality and
storm frequencies have implications in the flood peaks (Villarini and
Smith 2010). Since it is difficult to carry out a reliable projection
for very heavy rainfall, it is necessary to study hydrological feedback
of the area to the storms. Few studies like Ge Sun et.al., (2013) have
studied the impact of climate change on the entire southeast and
demonstrated the importance of water supply stress using projections by
2050. They showed increase in runoff and sediment yield due to increase
in erosivity and/or vegetation cover loss. They also stated that climate
change and possible future stressor like population growth, land use
change, energy security, and policy shift would interact with surface
and groundwater availability (Sun 2013).
The Southeast in past has experienced political tensions over water
resources (Manuel 2008). Projected warm weather will increase ET,
leading to reduced water availability and ground water recharge (Ingram,
Dow et al. 2013; Sun 2013; Walsh, Wuebbles et al. 2014). Uptake of soil
water by forests is expected to increase, leading to decline in water
yield under increased temperature and decreased precipitation
projections (Ingram, Dow et al. 2013). Projected population growth and
land use change will worsen the situation and pose threat on the economy
and unique ecosystems; and land use change in Southeast which ultimately
exacerbates the water scarcity, is faster than any other areas in US
(Carter, Jones et al. 2014). Future trends derived from projections for
2010-2060, show 5 to 6.5 % decrease in the net water yield for the
western part of the Southeast region including Alabama (Sun 2013).
Projected temperature extremes are noticeable in the Southeast region.
Projections show 4.3℃ and 7.72℃ rise by mid-century (2036-2065) and
late-century (2071-2100) under RCP 8.5 respectively (Vose, Easterling et
al. 2017). The historical changes for temperature extremes, however,
showed insignificant warming of 0.6℃ (difference between mean of
,1986-2016 and 1901-1960) for the Southeast, which was the smallest
increase over the continental US (Meehl, Arblaster et al. 2012; Vose,
Easterling et al. 2017). Projections under RCP 8.5 by mid-century also
reveal 40-50 days per year with temperature greater than 32℃ as a key
temperature threshold (Vose, Easterling et al. 2017). For changes in
numbers of nights below 0℃, projections (2041-2070 compared to
1971-2000) show increase of 10-15 days for most the region and more than
20 for the northern part of the Southeast (Katz, Parlange et al. 2003).
Southeast is categorized as the second vulnerable to weather and climate
disasters in the US for the past three decades (1980-2012); Hurricanes
can be considered as disasters for the coastal area and tornados and
storms are disasters for inland regions where they are close to the Golf
and Atlantic coasts (Carter, Jones et al. 2014). Many factors contribute
to the climate of the Southeast region including closeness to the
Atlantic Ocean and the Gulf of Mexico and El Nino-Southern Oscillation
(ENSO), and land falling tropical weather systems (Katz, Parlange et al.
2003). There are few studies that specifically addressed the effects of
climate change on the southeast. On the other hand, the scope of studies
was the entire Southeast of US and land use change had not been
considered to quantify the rapid land use change in the area (Trail,
Tsimpidi et al. 2013). Natural hazards like drought, flood, and in
general vulnerabilities produced by climate, are results of regional
behavior not global (Mahmood, Pielke Sr et al. 2010). Also, it is
essential to improve regional projections to determine the mechanisms of
the regional forcings and related climate impacts clearly (NRC and CRC
2005). Although, most studies on hydrological future projections have
come to the conclusion that water balance components including water
yield, surface runoff, ET, baseflow will be affected under in the future
under climate scenarios, the hydrological response itself varies
depending on region-specific characteristics, topography geography
location, and precipitation regimes (Sunde, He et al. 2017). Previous
studies have revealed the general impacts of the climate change for the
entire Southeast of the US. But they have not used the Coupled Model
Intercomparison Projects -Phase 5 (CMIP5) downscaled data with increased
robustness and detailed outcome than the Coupled Model Intercomparison
Projects -Phase 3 (CMIP3) combined with land use projections. Therefore,
in this study, we carried out the investigation on the subbasin scale
and with new developed techniques for data preparation. This study
provides greater details for a better understanding of the hydrological
process, leading in sustainable climate change adaptation. We use SWAT
for hydrological modeling and couple it with three representative GCM
models in which the data are downscaled using a new developed method
called Localized Constructed Analogs (LOCA). In the recent past,
researcher have used the same methodology, but with different
downscaling method and with predecessors of CMIP5 (Sunde, He et al.
2017; Chen, Marek et al. 2019; Hoyos, Correa-Metrio et al. 2019; Pandey,
Khare et al. 2019). Pandey et.al. (2019) have found decline in blue and
green water under both RCP4.5 and RCP8.5 pathways for their study area.
Comparing different GCMs coupled by SWAT, Sunde et al. (2017) showed
decrease in stream flow and increase in ET for their case study. Chen et
al. (2017) have investigated land use change and projected 12% to 20 %
decline in crop ET by mid and end of 21st century
respectively (Chen, Ale et al. 2017). Impacts of afforestation and
deforestation on hydrological response have also been studied (McNulty,
Caldwell et al. 2013; Sunde, He et al. 2017; Cecílio, Pimentel et al.
2019). Consistent results have not been an outcome of the impact of
forest on water yield (Cecílio, Pimentel et al. 2019). Geographical
location of the afforestation in Brazilian Atlantic Rainforest showed
small significance regarding impact on the average stream flow; however,
the minimum flow was reduced (Cecílio, Pimentel et al. 2019). Longer
growing season and increased wildfire likelihood and reduction in stream
flow were projected for southeast US forests (McNulty, Caldwell et al.
2013). Hoyos et al. (2019) have investigated the hydrology response to
drought and reported that the watershed feedback relies on climatic
mechanisms and catchment characteristics. Since climate variables should
be calculated to investigate the climate change impact on hydrology of a
given watershed, it is necessary to couple improved downscaled GCMs with
SWAT (Pandey, Khare et al. 2019). On the other hand, hydrological cycle
response is unique for each watershed due to the different factors
engaged in the process. Thus, it is important to study hydrologic
behavior in a smaller scale with greater details. Therefore, herein, we
study the hydrological response of Upper Choctawhatchee Subbasin (UCS)
to address the possible hydrological response under different climate
and land use scenarios. The goals of the study are i) to establish a
robust hydrological model for the UCS, ii) to couple the detailed
projections with a new downscaling method and iii) to analyze the
response of the UCS regarding future stressors.