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).