*Article* **Spatiotemporal Variation Characteristics of Groundwater Storage and Its Driving Factors and Ecological Effects in Tibetan Plateau**

**Wenhao Ren 1,2, Yanyan Gao 1,2, Hui Qian 1,2,\*, Yaoming Ma 3,4,5,6, Zhongbo Su 7, Weiqiang Ma 3,4, Yu Liu 8,9 and Panpan Xu 1,2**


**Abstract:** Known as the "Asian Water Tower", the Tibetan Plateau (TP) is a rich water resource and serves an important ecological function. Climate change may cause changes to the water cycle, and these changes may affect the alpine vegetation growth. However, the variation characteristics of groundwater storage (GWS) and its driving factors and associated ecological effects in the TP are poorly understood. In this study, terrestrial water storage changes retrieved by GRACE (Gravity Recovery and Climate Experiment) were combined with GLDAS (Global Land Data Assimilation System) to estimate the GWS changes in the TP. The temporal and spatial variation characteristics of GWS were identified using linear regression and the modified Mann–Kendall (MMK) test, respectively. The analyses showed that the GWS of the TP decreased at an average rate of −0.89 mm/a from January 2003 to December 2021, but since January 2016, it gradually recovered at a rate of 1.47 mm/a. This shows that the GWS in the eastern and northern parts of the TP is decreasing, while the GWS in the western and southern parts is increasing. The influence of climate change on GWS in time and space was determined using the correlation analysis method. Decreased precipitation and permafrost degradation caused by increasing temperatures will lead to a decrease in GWS. On the other hand, rising temperatures may result in an increase in GWS in regions where glaciers are distributed. In this study, the ecological effects were represented by the relationship between GWS and vegetation change. A decline in GWS means that the vegetation will not receive enough water, leading to a decrease in the NDVI and the eventual degradation of grassland to sand, desert, or other kinds of unused land on the TP. On the other hand, an increase in GWS would promote vegetation restoration. The results of this study offer a new opportunity to reveal the groundwater changes in a cryosphere region and to assess the impact of changes in hydrological conditions on ecology.

**Keywords:** groundwater storage; GRACE; GLDAS; climate change; vegetation response

**Citation:** Ren, W.; Gao, Y.; Qian, H.; Ma, Y.; Su, Z.; Ma, W.; Liu, Y.; Xu, P. Spatiotemporal Variation Characteristics of Groundwater Storage and Its Driving Factors and Ecological Effects in Tibetan Plateau. *Remote Sens.* **2023**, *15*, 2418. https://doi.org/10.3390/rs15092418

Academic Editor: Marouane Temimi

Received: 16 March 2023 Revised: 19 April 2023 Accepted: 3 May 2023 Published: 5 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

### **1. Introduction**

The Tibetan Plateau (TP) has important ecological functions such as global water circulation, ecological security, and protection [1–3]. It is one of the most sensitive regions to climate change due to its unique geographic location [4–6]. Due to its rich water resources, the TP is also called the "Asian Water Tower"; it has a profound impact on the survival and development of about two billion people downstream and important implications in the protection and sustainability of water resources [7]. Unlike surface water, groundwater is invisible, and its distribution and change are difficult to understand. Compared with extensive and well-developed studies on other surface-water resources (glaciers [8,9], snow [10,11], lakes [12,13], rivers [14,15], etc.), there are relatively few studies on the TP groundwater [16]. Therefore, groundwater is one of the most challenging but most important components of the "Asian Water Tower".

The traditional groundwater monitoring method is to regularly monitor the changes to the groundwater levels by monitoring wells. However, considering its high altitude, extensive size, remote geographical location, harsh climate, and difficult working conditions, the adoption of this method in the TP is unrealistic. Therefore, a new method must urgently be adopted to enable long-term and large-scale monitoring. The development of satellites means that remote sensing technology can be effectively used as a method to monitor water storage changes. The Gravity Recovery and Climate Experiment (GRACE) gravity satellite and GRACE-FO, which is GRACE's follow-up satellite, launched in 2002 and 2018, respectively, were shown to have significant advantages in monitoring the changes in regional terrestrial water storage (TWS). The TWS in the TP have been confirmed to have undergone significant changes [17,18]. Additionally, the changes in GWS can be obtained by removing known contributors (soil moisture, accumulated snow, and plant canopy surface water) from the changes in TWS which were observed by GRACE/FO [18–23]. This method has been widely used, and relatively accurate results were obtained. The difference between the results obtained using the well-based and GRACE model-based GWS trends was not more than 1.5 cm/year in Poland [24]. Xiang et al. [16] quantitated the GWS changes in the TP and the surrounding area from 2003 to 2009 and showed increasing trend rates in eight basins. Li et al. [7] pointed out that the GWS in the endorheic and exorheic TP basins decreased during 2002–2017, with a rate of 1.17 Gt/a and 4.89 Gt/a, respectively. However, alongside continual climate change, GWS has changed significantly. The exploration of the latest changes in GWS in the TP remains challenging.

Changes in groundwater storage are mainly influenced by climate change and human activities. Different levels of climate change in different regions have different impacts on the changes in GWS. Compared with decreased precipitation, the influence of increased temperatures on the decrease in GWS is much more pronounced in Turkey [19]. However, in arid Central Asia, precipitation in mountainous areas is considered the main factor affecting the water storage in the piedmont area, while human activities may have a significant impact on the water storage in the Turgay Valley [25]. Human activities do have a great impact on GWS in some small basins. In the Shiyang River Basin, there are many large reservoirs, and irrigation agriculture has been developed in several large oases. Therefore, groundwater storage has been decreasing in recent years due to human activities [26]. The TP covers a vast area but is sparsely populated, accounting for one quarter of China's total area but less than 1% of China's total population, and it contains large uninhabited areas. Therefore, from the perspective of the entire TP, human activities have a relatively low impact on GWS [7,18]. It is widely acknowledged that the climate in the TP has changed significantly in the past half century, mainly due to climate warming and wetting [27]. Rising temperatures are not only accelerating the melting of glaciers [28], but also permafrost degradation [29], both of which, together with precipitation, directly or indirectly affect GWS. Therefore, the elicidation of the influence of climate factors on GWS is key.

As an indicator of ecological environment change, vegetation is highly dependent on groundwater [30–32], especially when the groundwater level is lower than the root depth [31]. However, in a large number of studies, the effect of conventional climatic indicators on ecological systems has been studied [32–35]. Zhang and Zhou [36] found that grassland has undergone the largest decrease in area, which has decreased by 9.47%, while the LUCC that has undergone the largest increase in area is unused land, which increased by 7.25% in the TP from 1980 to 2018. However, they attributed this difference to temporal and spatial variations in precipitation. Xu et al. [32] pointed out that the effect of the changes to soil water storage on vegetation in the Three Rivers Source Region was considerably greater than the effects of precipitation and temperature. The response of vegetation to groundwater changes has been poorly studied, and the impact of groundwater changes has been ignored in the TP [37].

Therefore, the objectives of the present study are to (1) identify the spatial–temporal characteristics of groundwater storage in the TP and its ten sub-regions during 2002–2021; (2) clarify the spatial–temporal characteristics of climate factors and the influence of climate factors on GWS; and (3) illustrate the spatial–temporal characteristics of vegetation changes and assess the vegetation responses to GWS changes. The results of the present study can act as a reference for the management of groundwater resources in different sub-regions in the TP.

### **2. Methodology**

### *2.1. Study Area*

The study area was in the geographic domain of the Tibetan Plateau (TP) in China and was composed of ten sub-regions with diverse geographical environments (Figure 1), including the Hexi Corridor (HC), the Qaidam Basin (QB), the Yellow River Basin (YRB), the Yangtze River Basin (YB), the Lancang River (upper Mekong River) Basin (L-MRB), the Nu River (upper Salween River) Basin (N-SRB), the Yarlung Zangbo River (upper Brahmaputra River) Basin (YZ-BRB), the Inner Basin (IB), the Sengezangbu River (upper Indus River) Basin (S-IRB), and the Tarim Basin (TB) [38].

**Figure 1.** Location map of the TP.

These different sub-regions are characterized by different climate and hydrogeological conditions [3,39]. The western, eastern, and southern TP are affected by the westerlies, the East Asian monsoon, and the Indian monsoon, respectively, and other areas are generally controlled by a combination of two or three of these conditions. Therefore, the water cycle patterns in the TP will be significantly affected by large-scale atmospheric circulation [3]. The abundant precipitation caused by monsoons can greatly replenish large rivers on the edge of the TP. During 1980−2018, the annual runoff of these rivers showed different change characteristics, such as a significant increase in the Sengezangbu River (+3.9 Gt per decade), but a stable status in the Yangtze River and Nu River, while the Yellow River recorded a decline (–1.5 Gt per decade) during the same period [3]. In the center of the TP, there are numerous endorheic lakes, rather than large exorheic rivers, because of the lower annual precipitation level [39].
