Groundwater potential mapping based on long time series remote sensing data in Penghu Islands, China

Islands are special bodies of land surrounded by seawater. Many islands are facing water shortages as the demand for groundwater increases, thus, highlighting the importance and urgency of the assessment, planning, and management of island groundwater resources. To facilitate the same, the groundwater potential of the Penghu Islands in China was assessed for the first time using remote sensing (RS), geographic information system (GIS), and the analytic hierarchy process (AHP). Long‐term series remote sensing studies were also conducted to monitor the dynamic change in groundwater potential and to identify groundwater potential areas. In this study, the geological environment factors were normalized, and their weight was allocated based on the AHP of RS and GIS. The groundwater potential assessment (GPA) index, established using the weighted comprehensive algorithm, was used to analyse groundwater potential and demarcate groundwater potential zones. Finally, the trend in the spatio‐temporal variation of groundwater potential was analysed using long‐term series remote sensing data. The results from 2015 to 2021 showed that the Penghu Islands can be divided into five groundwater potential zones of different grades, such as 6.3%–8.1% of grade I, 22.2%–24.2% of grade II, 23%–24.4% of grade III, 19.6%–21% of grade IV, and 25.1%–25.5% of grade V. The results of the GPA were verified by comparing them with the abundance of water‐content rock formation in hydrogeologic maps, which yielded correlation coefficients of 78.3%–88.9%. Overall, the groundwater potential zones of different grades in the study area tended to be stable all year‐round, including the zones of grades I and II. The results of this study highlighted the reliability of predicting groundwater potential based on long‐term series remote sensing data and its usability to local personnel in appropriate groundwater resource planning and management.


| INTRODUCTION
Water is an important natural resource and a material basis for the socio-economic development of a country or region (Peng & Deng, 2020;Wang, Xiao, et al., 2021;Wang, Xu, et al., 2021;Zhang, Jin, & Yu, 2018;Zhang, Hu, et al., 2018).With groundwater being an important resource restricted by natural conditions such as climate, topography, and geological environment, current water resource management strategies focus on its full and reasonable development and utilization to alleviate the shortage of water resources (Coyte et al., 2018;Gleeson et al., 2020;Lima et al., 2022;Wang et al., 2020).With more than 7300 islands measuring over 500 m 2 in area, China has the most islands in the world (Lei & Sui, 2022).However, the lack of freshwater means that less than a quarter of these islands have permanent residents.Due to their limited topography, most islands lack natural surface storage space, making it difficult to directly utilize rainwater and surface runoff without building water conservancy projects.In contrast, groundwater resources are stored in aquifers, which are naturally adjustable and reproducible, and have low development and utilization costs, making them an ideal water source for islands (Tang et al., 2021;Yao et al., 2019;Zhang, Hu, et al., 2018;Zhang, Jin, & Yu, 2018).The Penghu Islands in China are far away from the mainland, and many of them are scattered, resulting in them being neglected in the planning and management of groundwater resources (Lu et al., 2019;Wang et al., 2009).However, they are of great significance to China in military, political, economic, historic, and cultural aspects.In this context, the investigation and assessment of groundwater potential have become an important step in realizing the sustainable management of groundwater resources in the Penghu Islands.
Groundwater Potential Assessment (GPA) is an assessment of the relative availability of groundwater in a specific area under certain technical, economic, and environmental constraints within a certain evaluation depth.The groundwater potential is closely related to the occurrence of groundwater, which is restricted in bedrock islands by a variety of geological environment factors.These factors controlling the occurrence of groundwater have significant differences in spatial distribution, resulting in the spatial difference of groundwater potential (Ajibade et al., 2021;Andualem & Demeke, 2019;Machiwal et al., 2011;Nag & Kundu, 2018;Wang, Xiao, et al., 2021;Wang, Xu, et al., 2021;Zhu & Abdelkareem, 2021).Therefore, a comprehensive and systematic approach is needed to analyse and quantify the contributions of different factors to groundwater potential.There are many methods to obtain information on factors related to GPA.From the perspective of method application, drilling and geophysical exploration technology can realize the inversion of underground aquifers by directly detecting groundwater status and then evaluating groundwater potential.However, the cost of this method is high, and the spatial distribution is relatively sparse, leading to low accuracy (Lin et al., 2017;Saad et al., 2012).The changes in groundwater reserves can be monitored using interferometric radar and satellite gravity data to help evaluate groundwater potential.However, this method can only be applied to scales of thousands of kilometres (Hoffmann et al., 2003;Yeh et al., 2006).
Establishing an effective groundwater model to simulate groundwater level change can help predict groundwater potential change; however, its application will require the determination of many ground parameters (Hussain & Abed, 2019).However, remote sensing technology has great advantages in regional groundwater potential assessment due to its low cost, large survey scope, and certain accuracy (Andualem & Demeke, 2019;Ilia et al., 2018;Peña-Arancibia et al., 2016).Many researchers use the analytical hierarchy process (AHP) to analyse groundwater potential and delineate different groundwater potential areas, displaying the reliability of the technique for groundwater resource planning and management in different geographic environments (Achu et al., 2020;Arunbose et al., 2021;Kaur et al., 2020).Although the above methods make full use of remote sensing data and contain rich spatial information, they cannot accurately reflect the dynamic changes of groundwater in the long term.
Compared with single-period images, long-term series images have more information and temporal dimension.Hence, by using long-term series remote sensing images, regional information characteristics can be acquired over time, greatly reducing the dependence on effective measured data on a limited spatio-temporal scale.To date, researchers have used many remote sensing time series products of various surface parameters to study land use, climate, environmental change, and other aspects (Bishop-Taylor et al., 2018;Sun et al., 2017;Wei et al., 2021;Zeng et al., 2020).However, there is no study on groundwater potential assessment and dynamic change using long-term series remote sensing data.Based on the AHP of remote sensing (RS) and geographic information system (GIS), this study uses long-term remote sensing data to evaluate groundwater potential and analyse its dynamic changes.The results of the survey are valuable for sustainable groundwater management in the region and provide a new idea for GPA and dynamic change detection of bedrock islands with limited effective measured data.

| Study area
The Penghu islands of China extend to 23°45.7′N in the north, 23°09.7′N in the south, 119°42.9′E in the east and 119° 18.1′ E in the west, covering an area of 126.9 square kilometres (Figure 1).
The annual rainfall of Penghu islands is only about 1000 mm, but the evaporation is 1800 mm, especially in winter, the monsoon can reach 20 m/s, which leads to the sparse vegetation on the surface, mostly consisting of short grass, shrubs and a few trees.The base of Penghu islands is an uplifted basalt square mountain, the overall terrain is low and broken.The main body and high parts of Penghu islands are made up of basalt, and part of the coast is made up of sedimentary rocks mixed with fine sand or gravel.

| Data
Landsat-8 carries two sensors and a total of 11 bands.Landsat8 took only 9 days to cover China, which helped to obtain timely images for multi-time relative studies.The ASTER GDEM data product is based on the advanced Spaceborne Thermal Emission and Radiometer (ASTER) data and is currently the only high-resolution elevation data.
In this paper, the remote sensing data mainly used are Landsat8 and DEM data from 2015 to 2021, as shown in Table 1.

| Methodology
In this paper, the analytic hierarchy process based on RS and GIS is used to evaluate groundwater potential, and the specific process is shown in Figure 2.
2.3.1 | Introduction of GPA by RS and GIS When using remote sensing data to evaluate groundwater potential, the pixel of remote sensing image is taken as the sample set of evaluation, qualitative reasoning and semiquantitative methods are adopted to combine with hydrogeological data to obtain groundwater information and analyse it.In bedrock islands, lithology is the most important controlling factor of groundwater.The recharge source of groundwater is atmospheric precipitation, which is mainly affected by relief and slope.Groundwater depth has a great influence on soil humility and surface vegetation, and the The map of study area surface water and groundwater are always interrelated and replenishment in the region.The presence of groundwater also causes changes in surface temperature.Therefore, This paper determined to formation lithology (L) as evaluation index of groundwater in space, relief (R) and slope (S) as evaluation index of groundwater recharge condition, water density (WD), land temperature (LT), soil humility (SH) and vegetation fraction (VF) as evaluation index of groundwater in surface indicator, the set of evaluation indicators is as follows: In order to achieve long-term groundwater potential assessment in the study area, the above evaluation indexes from 2015 to 2021 were extracted and normalized, and each evaluation index was normalized to the interval [0,1].

| The construction of judgement matrix
In this paper, the groundwater potential assessment is divided into five levels, namely level I (very good), level II (good), level III (medium), level IV (poor) and level V (very poor), and the membership functions are F 1 (y), F 2 (y), F 3 (y), F 4 (y), and F 5 (y), respectively.The judgement matrix is constructed as follows: Then, we need to consider the weighting ratio of each indicator to Wi for the target layer (GPA).The weight set is as follows: (1) Y = {yL, yR, yS, yWD, yLT, ySH, yVF} (2)

| The construction of comprehensive assessment
Comprehensive evaluation method is adopted to calculate, and the formula is as follows: Multiply the weight matrix by the comprehensive evaluation matrix, and the formula is: Finally, the comprehensive assessment score is: (3) W = {wL, wR, wS, wWD, wLT, wSH, wVF} (4)

| RESULTS AND DISCUSSION
Groundwater potential assessment is to extract hydrogeological and environmental information from remote sensing images and deduce groundwater-rich areas by using remote sensing technology.Therefore, it is of great significance to select appropriate indices for groundwater potential assessment.In this paper, remote sensing indicators of the study area from 2015 to 2021 were extracted and normalized.Here, 2015 was taken as an example.

| Lithology index
The water abundance of strata is closely related to lithologic composition, and different lithology means different water storage structure or possible water reserve.In Penghu islands, basalt is the main body, while the area of sedimentary rocks mixed with fine sand or gravel is not significant, and only developed near local shorelines (Figure 3).The water-bearing capacity of different types of rocks varies, among which the sedimentary strata mixed with fine sand or gravel have good groundwater potential.

| Relief index
Relief controls the recharge conditions of groundwater.In terms of relief, the Penghu Islands are broad, low, and flat rock platforms vulnerable to weathering, while some coastal zones are sandy beaches vulnerable to seawater.Relief changes show significant discontinuity, especially in the form of isolated patches on the sloping topography of rocky platforms, indicating surface fragmentation (Figure 4).Among them, the surfaces of the sloping topography of rocky platforms are considered to have moderate to good groundwater potential.

| Slope index
Slope mainly affects the process of surface runoff formed by rainfall.For the site with large slope, rainfall is difficult to stay at this point, so it cannot recharge groundwater.In The Penghu islands, slope is generally gentle, mainly concentrated in the interior of the Penghu islands.The area of steep slope is not significant, mainly concentrated in the shoreline area of some islands or in the form of isolated patches inside the islands (Figure 5), indicating that slopes are relatively broken.Among them, the groundwater in gentle slope area has better potential.

| Water density index
Surface water affects the recharge and discharge of groundwater to a certain extent through infiltration, especially when a large amount of local water is extracted, a large amount of surface water often recharge groundwater through infiltration.In the Penghu islands, water density is generally low, which is consistent in the interior of the Penghu Islands.The high water density area is scattered in the local shoreline area of the islands (Figure 6).It shows that the recharge effect of surface water on groundwater is weak in the main part of Penghu islands, but strong in some coastal zones.

| Land temperature index
Under the effect of capillarity and heat conduction, the different reserves and buried depth of groundwater will affect the surface temperature.Therefore, the groundwater enrichment can be reflected by the land temperature.In the Penghu islands, land temperature is generally high, but low in the coastal zone, especially in the southeast coastal area of the Penghu islands (Figure 7).The groundwater is relatively enriched in the coastal zone with obvious surface temperature variation in Penghu islands.

| Soil humility index
Under the influence of capillarity and evaporation, groundwater will form a capillarity zone above the saturated zone, leading to the change of soil humility.Therefore, the groundwater enrichment can be indicated by soil humility.In Penghu islands, soil humility is generally low, with high values in some coastal zones and sporadic areas in the central part of the island (Figure 8).It shows that the regions with good groundwater potential are scattered and isolated in the coastal zone and the central part of the island.

| Vegetation fraction index
To a certain extent, the change of vegetation in an area can reflect the change of local groundwater.The vegetation fraction is unevenly distributed in the study area, and only the areas with high vegetation fraction are scattered in the interior of the island.In most areas, the vegetation fraction is high to low, especially in the coastal zone vulnerable to seawater erosion, and the vegetation fraction is very low (Figure 9).This indicates that the sporadic areas within the islands less affected by seawater erosion have better groundwater (freshwater) potential.The weight obtained by AHP was used to evaluate the groundwater potential of the study area from 2015 to 2021 and delineate the groundwater potential zones of different grades.Formula ( 5) is used to estimate the groundwater potential of the study area, and five evaluation grades are divided according to the GPA value region in Table 2, corresponding to five groundwater potential zones with different levels of water abundance, namely, grade I (red), grade II (yellow), grade III (green), grade IV (blue), and grade V (purple).For example, in 2015 (Figure 10a), grade I groundwater potential zone only accounted for 7.255% of the total study area, while grades II, III, IV and V accounted for 24.265%, 23.406%, 19.637% and 25.437%, respectively, with different grades of groundwater potential zones being scattered in the form of spatial clusters.
Comparing the index data of the GPA in this study with previous studies, in different environmental settings in other parts of the world, found that different studies have different weights of leading indicators.This difference indicated that the relative advantage of leading indicators could also have spatial variability in different regions.Lithology, relief, and slope are the main indexes that affect the degree of groundwater accumulation in the study area.Lithology is the highest (0.3448), followed by relief (0.3153) and slope (0.2832).The gentle slope area of the basalt platform had better groundwater potential.This implied that although the groundwater storage capacity of sandy sedimentary strata is stronger because the area of sandy sedimentary rock is very small, relief and slope have significantly greater practical control on groundwater potential in the study area.Therefore, changes to the relief and slope of the study area may pose severe challenges to groundwater planning and management therein.

| Validation of results
The accuracy of groundwater potential assessment of Penghu islands was verified by analysing the correlation between GPA index and the water-rich degree of water-bearing rock groups in the hydrogeological map of the study area (Table 3).The correlation coefficients of the results from 2015 to 2021 were 0.7827, 0.8678, 0.8616, 0.8617, 0.889, 0.8413, and 0.8514 respectively (Figure 11), indicating that GPA index was consistent with the actual groundwater enrichment degree in the study area.

| Statistics and discussion
After dividing the groundwater potential zones of different grades in the study area, the observation shows that the groundwater distribution grades of the selected study area from 2015 to 2021 show dynamic changes, with the majority of grade IV and V regions, and the minority of grade I and II regions (Figure 10).Then, histogram statistics are conducted based on the statistical results of grade I and II.The area with high groundwater distribution grade in this region is about 270-309 km 2 , and groundwater potential zones in 2015-2016 accounted for 31%, which is the highest period (Figure 13).Therefore, groundwater exploitation can be guided by areas with high GPA.
For further analysis, we obtained two key data in the map of groundwater resources in the study area: natural recharge and exploitable groundwater resources, which can reflect the situation of groundwater resources in the study area.The natural recharge resource of groundwater refers to the increased groundwater amount under natural conditions, including precipitation infiltration, surface water infiltration and groundwater inflow from adjacent hydrogeological units or adjacent aquifers, which is stable at 0.16 × 108 m 3 /a all year round.The maximum amount of water that can be extracted from the aquifer without causing ecological environment deterioration is stable at 0.10 × 108 m 3 /a all year round (Table 4).According to the statistics of the area proportions of groundwater potential areas of different grades in the study area, the statistics show that the respective proportions of groundwater potential areas of different grades in the study area tend to be stable as a whole from 2015 to 2021 (Figure 12).This is consistent with the variation of groundwater resources in the groundwater resources map.Based on the AHP of RS and GIS, the groundwater potential of the Penghu islands was evaluated from 2015 to 2021, and the groundwater potential zones of different grades were divided.The study shows that only 6.3%-8.1% of the total area of the Penghu islands has very good groundwater potential (grade I), while 22.2%-24.2%,23%-24.4%,19.6%-21%, and 25.1%-25.5% of the area has good (grade II), moderate (grade III), poor (grade IV), and very poor (grade V) groundwater potential, respectively.The groundwater potential zones of different grades are not concentrated, appearing sporadically or in patches, which is mainly caused by surface fragmentation in the study area.The correlation between the GPA index and abundance of water-content rock formation in hydrogeologic maps shows that the assessment and delineation of groundwater potential in this area are effective, and hydrogeological mapping is suitable for determining the groundwater potential of bedrock islands with limited effective measured data.Further analysis of groundwater changes in the study area forecasts the developmental trend.The study shows that the groundwater potential areas of different grades in the study area tend to be stable all year round, which is consistent with the change rule of groundwater resources in the map of groundwater resources.The results are very resourceful in filling the gap in groundwater potential research in the study area and exploring the feasibility of studying the dynamic changes of groundwater potential using long-term series remote sensing.This study considers the combination of RS, GIS, and AHP, based on long-term series remote sensing data, as an effective tool for long-term sustainable groundwater management in bedrock islands under limited field data conditions.

1
Details of remote sensing data acquisition in each detection year srtm.csi.cgiar.orgF I G U R E 2 Flow chart of groundwater potential assessment based on long-time series remote sensing data Through the judgement matrix, the weights of the seven indexes are obtained as follows: w L : w R :w S :w VF :w LT :w-WD :w SH = 0.3448: 0.3153: 0.2832: 0.0184: 0.0327: 0.0035: 0.0021 。.
E 3 The normalization of lithology index in the study area F I G U R E 4 The extraction and normalization of relief index in the study area

F
The extraction and normalization of slope index in the study area F I G U R E 6 The extraction and normalization of water density index in the study area

F
The extraction and normalization of land temperature index in the study area F I G U R E 8 The extraction and normalization of soil humility index in the study area

F(
Results of groundwater potential assessment in the study area from 2015 to 2021

T A B L E 3
Water-rich degree of water-bearing rock groups in the hydrogeological map F I G U R E 1 1 Correlation analysis of assessment results from 2015 to 2021 T A B L E 4 Quantity of groundwater resources Period of time Natural resources (10 8 m 3 /a) Exploitable groundwater resources (10 8 m 3 /a) Statistical pie chart of groundwater prediction results in the study area from 2015 to 2021