Introduction
Surface and subsurface water resources are among the world’s most
precious assets, which require highest priority especially in
arid/semiarid areas where the water supply is not enough to sustain
human water needs. Thus, it is essential to manage these resources to
guarantee the water sustainability. Terrestrial Water Storage [TWS]
is one of the components of the hydrological cycle with a pivotal role
in global and regional climate systems (Śliwińska, Birylo, Rzepecka &
Nastula, 2019). Precise knowledge about the variations of TWS is very
important for climatologists and hydrologists for assessing the
availability of water and understanding the changing climate patterns
over time (Tangdamrongsub et al. 2020). Ground Water Storage [GWS]
comprises the subsurface portion of TWS and stands for the water
available among the pore spaces beneath the ground (Rahaman, Thakur,
Kalra & Ahmad 2019). With an estimated storage of 96% of the world’s
readily available liquid fresh water, GWS is critical for supporting the
water requirements in domestic, agricultural and industrial sectors all
around the world (Feng et al., 2013; Taylor et al., 2013; Ülker, Ergüven
& Gazioğlu2018).
The water resources of Turkey suffer heavy pressure in terms of quality
and quantity because of the rapid population growth and
industrialization as well as the impacts of climate change and the
morpho-topological characteristics of the streams and rivers throughout
the country (Croitoru, Divrak, Xie & Arikan 2016). The available water
in Turkey is expected to shrink from 1519 m3 per
capita to 1120 m3 per capita by 2030 due to the
anthropogenic and natural factors (DSI, 2015). Groundwater is a critical
water resource for Turkey because of the fast social and economic growth
of the country, which requires establishment of an adequate management
and monitoring network with the focus on the sustainability of
groundwater aquifers (Apaydin, 2011). However, overexploitation of
groundwater aquifers in Turkey is a common problem causing an alarming
situation in some regions with regards to declining water tables and
increasing extraction costs. Unfortunately, as groundwater level data is
not widely available spatially and temporally, it is difficult to
present a realistic and unbiased picture of the situation (FAO, 2016).
The direct in-situ measurements of TWS is not common. In addition, the
challenges and restrictions associated with the traditional monitoring
approach of GWS (such as limited monitoring networks and incomplete
temporal resolutions) pushes scientists and researchers into using
remote sensing data collections and management tools and techniques
particularly for regional studies. Remote sensing satellites have been
providing useful data for the scientific community mostly at global
scales. Gravity Recovery and Climate Exchange [GRACE] Program is the
first remote sensing mission of its kind administrated by NASA. Since
2002, GRACE program has been providing gravitational pull signals, which
are later converted into TWS estimations. In this study, the authors aim
at monitoring the temporal variations of different hydro-climatic
variables (including Soil Moisture Storage [SMS], Snow Water
Equivalent [SWE], Terrestrial Water Storage [TWS], Groundwater
Storage [GWS], Precipitation [P] and Temperature [T]) over
Turkey based on GRACE and other remotely sensed data with the focus on
obtaining significant relations between climatic variables and
groundwater storage anomalies [GWSA]. The authors also aim to assess
the impacts of climatic variables on the variations of groundwater
storage over Turkey.
Site Description
Turkey is located between 25°38’ to 44°48’ East and 35°48’ to 42°6’
North [Figure 1] straddling Eastern Europe and Western Asia. Turkey
is known for its diverse topography with different terrain features. The
majority of the country is occupied by high mountains, East Anatolian
Highlands in the East, Black Sea Mountain Range in the North, Taurus
Mountains in the South, and the Anatolian Plateau in the center (Nistor
and Man, 2019). While the general climate of Turkey is known to be of
semi-arid type (Apaydin, 2011), the geographical characteristics of the
country such as proximity to large water bodies and diverse
topographical features create different subclimatic patterns in
different regions of Turkey where coastal areas experience milder
climates whereas hot summers and cold winters describe the dominant
climate of the central Anatolian plateau (Sensoy, Demircan, Ulupınar &
Balta, 2008). Although the climatic parameters of temperature and
precipitation vary from region to region, official statistics of the
Meteorological Service indicates that the mean annual temperature and
the total annual precipitation of Turkey is calculated to be 11.9 °C and
391.9 mm respectively (MGM, 2020).