**1. Introduction**

Approximately 30% of continental land area is characterized as arid and semiarid [1]. Water cycle conditions and vegetation ecosystems are fragile and sensitive in these areas [2], and increasing water demand from all kinds of water users has seriously impacted vegetation ecosystems. Along with climate change, this increase in demand has greatly changed hydrological factors and water balance in semiarid areas [3,4]. The Xiliaohe River Basin (XRB) has experienced dramatic changes in its hydrological cycle and water balance [5–7]. Distribution and growth of vegetation have changed observably in the area, which may be related to the important role of hydrological factors in a vegetation ecological environment [8,9]. It is within this context that hydrological changes and their impact on vegetation are among the key issues in semiarid areas, where water resources are scarce and the ecological environment is fragile.

XRB is an agropastoral ecotone in a semiarid area that has experienced significant climate change [10], with an average annual temperature (T) increase of 0.5–0.7 ◦C. At the same time, the groundwater level in the study area decreased notably from 2 m to 6 m over the past 30 years due to unrestrained development and water resource use [11]. Water cycle at the regional scale has been directly affected by regional climate change and groundwater overexploitation, which had impacts in local vegetation change and distribution [12]. Precipitation (P) at 46 meteorological stations decreased from 1960–2012 in Inner Mongolia [13], and P in XRB showed a similar decreasing trend, which led to decreased discharge from the four inbound rivers and a significant decrease in the groundwater level from 1951 to 2007 [14]. Meanwhile, evapotranspiration (ET) (determined by the Penman-Monteith method) varied significantly in both time and space in the area [15,16]. Relationships between hydrological factors were analyzed based on the results at a few observation points [17,18] and thus could not well represent the spatial heterogeneity because of the limited observation points. Some conventional hydrological and climatic indicators have also been used to analyze the hydrological variations and their impacts on the vegetation [13,19]. However, these results could not reveal the balance of regional water resources and its impact on vegetation change, especially on a different scale. Moreover, due to the limited number of observations at the regional scale and in remote areas, data on certain hydrological and meteorological factors may not be available, such as the change in the total water storage change (TWSC), soil moisture change (SMC), groundwater storage change (GWC), etc. Thus, determining the hydrological, climatological and ecological processes may be difficult. Now, this is possible using satellite techniques for monitoring land meteorological and hydrological characteristics [11,20]. Since Gravity Recovery and Climate Experiment (GRACE) satellite launch in March 2002, it has provided a unique way to monitor changes in the earth's gravitational field, especially terrestrial water reserve changes at a regional scale [21]. At present, many achievements have been harvested in related fields, such as hydrological characteristics of TWSC, which were estimated in many regional basins, e.g., China [22], Tarim River basin [23] and so on. Moreover, GWC could also be detected on different spatial scales [24,25]. Zhong et al. [11] found that the GWS showed a prolonged declining rate of −17.8 ± 0.1 mm/yr. during 1971–2015 in the North China Plain, based on in situ groundwater-level measurements and satellite observations. Han et al. [26] discussed the GWC dynamic at multi-timescales in Yunnan Province and the correlations with extreme meteorological factors. Lv et al. [27] found that human factors were the main influencing factors of regional hydrological characteristics, through analyzing the quantitative attribution of terrestrial water storage (TWS) variation from hydroclimatic and anthropogenic factors. In addition, the accuracy of GWC retrieved from the GRACE satellite data in a semiarid area was verified by a comparison with in-situ data [11]. In these analyses, regional hydrological characteristics dynamics and their correlation with meteorological factors are analyzed, thus ignoring the lag time between them. In contrast, more attention to the correlations between regional hydrological characteristics and meteorological factors at multiple-time scales were paid in this paper. Furthermore, in order to fully reveal the impacts of hydrological variations on vegetation in semiarid areas, their evolution characteristics at multiple-time scales and spatial scale were examined.

In summary, spatiotemporal changes of hydrological and meteorological factors, such as P, ET, T, TWSC, SMC and GWC, etc., especially their impacts on vegetation in XRB, have rarely been comprehensively discussed. This paper analyzed regional water balance and vegetation factors based on data from multiple satellite observations. Comprehensive correlations among climatological, hydrological, and vegetation factors in XRB were simultaneously analyzed using time-series data from 2002 to 2014 at the regional scale. The main objectives of this study are (1) to analyze spatiotemporal

**2. Materials and Methods** 

dynamic of hydrological factors and normalized difference vegetation index (NDVI) in agropastoral ecotone of semiarid region based on multiple satellite data, (2) analyze response relationships among regional hydrometeorological factors at multiple-time scales, and (3) evaluate the impacts of hydrological variations on vegetation. *2.1. Study Area*  Xiliaohe River Basin (XRB), Inner Mongolia Autonomous Region,northeast part of China, lies between the latitude 42° 30′ to 45° 00′ N and longitude 120° 00′ to 123° 30′ E (Figure 1), which has an

*Remote Sens.* **2020**, *12*, x FOR PEER REVIEW 3 of 19

in agropastoral ecotone of semiarid region based on multiple satellite data, (2) analyze response relationships among regional hydrometeorological factors at multiple-time scales, and (3) evaluate

#### **2. Materials and Methods** area of 3.2 × 104 km2 and an average elevation of 800 m (400~1300 m). Three main rivers once flowed through the study area: Xiliao River, Jiaolai River, and Xinkai River. However, the rivers' discharge

#### *2.1. Study Area* has been reduced and may even dry up either seasonally or perennially [28]. Moreover, the increased

Xiliaohe River Basin (XRB), Inner Mongolia Autonomous Region, northeast part of China, lies between the latitude 42◦300 to 45◦000 N and longitude 120◦000 to 123◦300 E (Figure 1), which has an area of 3.2 <sup>×</sup> <sup>10</sup><sup>4</sup> km<sup>2</sup> and an average elevation of 800 m (400~1300 m). Three main rivers once flowed through the study area: Xiliao River, Jiaolai River, and Xinkai River. However, the rivers' discharge has been reduced and may even dry up either seasonally or perennially [28]. Moreover, the increased water demand from irrigation in recent decades has led groundwater to become the main total terrestrial water storage source supplied to meet agricultural, industrial, and domestic water demand. The overexploitation and utilization of groundwater has caused various environmental problems [29], e.g., regional groundwater table and pollution, land subsidence, and ecological environment deterioration. water demand from irrigation in recent decades has led groundwater to become the main total terrestrial water storage source supplied to meet agricultural, industrial, and domestic water demand. The overexploitation and utilization of groundwater has caused various environmental problems [29], e.g., regional groundwater table and pollution, land subsidence, and ecological environment deterioration. In XRB, annual precipitation ranged from 350 mm to 450 mm, the annual average temperature was 6.0°C, and the pan evaporation (Φ 20 cm) was 1817 mm [30]. Approximately 80% of the total precipitation occurs in summer (June to September). As a typical ecotone between Farming and Animal Husbandry in semi-arid area, crops, trees and grassland constitute the main vegetation types [31]. In order to better understand land distribution and vegetation types, the field survey in July of 2015 and August of 2016, were launched.

**Figure 1.** Location of the study area (Land-Use and Land-Cover Change data from 2015 are from Data Center for Resources and Environmental Sciences, Chinese Academy of Sciences **Figure 1.** Location of the study area (Land-Use and Land-Cover Change data from 2015 are from Data Center for Resources and Environmental Sciences, Chinese Academy of Sciences (http://www.resdc.cn)).

(http://www.resdc.cn)). In XRB, annual precipitation ranged from 350 mm to 450 mm, the annual average temperature was 6.0◦C, and the pan evaporation (Φ 20 cm) was 1817 mm [30]. Approximately 80% of the total precipitation occurs in summer (June to September). As a typical ecotone between Farming and Animal Husbandry in semi-arid area, crops, trees and grassland constitute the main vegetation types [31]. In order to better understand land distribution and vegetation types, the field survey in July of 2015 and August of 2016, were launched.

#### *2.2. Data*
