Research output trend
As per the extraction process of publications, 65,461 documents were identified as groundwater-related in the Scopus database. On fixing the study period from 1970-2020, 61,263 documents were obtained, and the exclusion of 355 documents published as erratum and 37 documents as retracted lead to a net total of 60,871 documents. When the search was limited to the South and Southeast Asian countries, the produced figure is 7,895 documents, which is 13% of the total global research productivity in the groundwater field for this period.
The two curves shown in Fig. 2 show the growth in groundwater-related publications globally and from South and Southeast Asian countries. The number of groundwater-related publications has increased over the past 50 years. At the global scale, the total annual groundwater-related publications were only 80 in 1970, whereas 3,930 documents were published in 2020, with an average annual growth rate (AAGR) of 9%. Meanwhile, for South and Southeast Asian countries, the total number of annual publications significantly increased from 1 (in 1970) to 894 (in 2020), with a 37% AAGR. However, for South and Southeast Asian countries, the growth in publication numbers from 1970-2002 was slow. The groundwater-related research gained momentum in the 21st Century, and the contribution of groundwater publications from South and Southeast Asian countries to the global groundwater-related output almost doubled from 2003 (12%) to 2020 (23%).
Among the various document types, articles constitute a majority (82%) of documents in this survey. The articles are published in the English language predominantly (99%). Table 1 summarizes by type in the final database. To ensure data consistency, our analysis is further restricted to research articles published in journals only.
Table 1 Publications by document type on groundwater from 1970-2020 from South and Southeast Asian countries
Document Type
|
Numbers
|
Percentage
|
Articlea
|
6,478
|
82.05
|
Conference Paper
|
848
|
10.74
|
Book Chapter
|
260
|
3.29
|
Review
|
159
|
2.01
|
Letter
|
38
|
0.48
|
Erratum
|
33
|
0.42
|
Note
|
33
|
0.42
|
Data Paper
|
20
|
0.25
|
Book
|
13
|
0.16
|
Editorial
|
6
|
0.08
|
Short Survey
|
3
|
0.04
|
Retracted
|
3
|
0.04
|
Undefined
|
1
|
0.01
|
Total Documents
|
7,895
|
a The number of articles presented here are before the screening/ pre-processing stage
|
Further analyzing the article trends obtained after the screening of articles, from Fig. 3, it can be seen that the total number of articles and total citations for groundwater-related research has increased dramatically in the last two decades. The entire 50-years study period can be divided into three distinct periods based on the number of articles published.
- Period 1 (1970-1980) introduces groundwater research in the South and Southeast Asian Countries focused on groundwater exploration studies. As this period was an early Green Revolution era, many South Asian countries started to utilize groundwater sources for irrigation. As documented in the Scopus database, the first groundwater-related article published was “Some geoelectrical investigations for potential groundwater zones in part of Azamgarh area of U.P.” It was authored by Singh C.L. and Singh S.N from the Banaras Hindu University, India and was published in Pure and Applied Geophysics in 1970 (Singh and Singh 1970).
- Period 2 (1981-2002) recorded publications from varied disciplines and themes. Increasing agricultural demand resulted in groundwater over-exploitation and groundwater contamination. Research efforts were made to understand the fluctuation in the groundwater levels and to identify the sources and transport of groundwater contaminants.
- Period 3 (2003-2020) showed the most growth in the groundwater domain. Employing remote sensing and GIS tools for groundwater mapping, pollution assessment and management options were actively explored during this period. The launch of NASA’s GRACE satellite 2003 also provided new perspective to monitoring of groundwater resources. Along with the extended detailed research on groundwater contamination and groundwater levels, focus on how to manage the resource sustainably became the utmost priority to achieve the targets of Millennium Development Goals (2000) and Sustainable Development Goals (2016).
The groundwater related studies were sporadic in the earlier years; however, the number of articles in the last 18 years (2003-2020) is 100 times that in 1970-1980. Additionally, the groundwater-related articles published during these last 18 years accounted for 90% during the entire 50 years.
Geographic and institution distribution analysis
Analyzing the data at the country level, as shown in Table 2 and Fig. 4, revealed a strong predominance of articles from India (4,584 articles, 72%), followed by Pakistan (309 articles, 5%), and Malaysia (289, 5%). Interestingly, no groundwater-related articles for Bhutan, Maldives, and Timor-Leste were found in the database. The publication numbers for the top 5 productive countries showed a significant increase from 2000 onwards.
Of the total 6,361 groundwater-related articles, 4,740 (75%) were single-country articles, and 1,621 (25%) were international collaborations. Single-country articles make up the largest share of the total number of articles in India, Indonesia, Pakistan and Malaysia. The highest collaboration rate was observed between South and Southeast Asian countries and the United States, followed by Japan and China. The total h-index of the retrieved articles for all the South and Southeast Asian countries was 128, implying that 128 articles had been cited at least 128 times during the period 1970 -2020. The highest h-index was 104 for India, followed by Bangladesh with 50 and Malaysia and Pakistan with 34.
Table 2 Bibliometric analysis for 6,361 groundwater-related articles published for South and Southeast Asian countries from 1970-2020
SCRa
|
Country
|
TA
(%)b
|
h-index
|
TCc
|
IA
(%)d
|
Most collaborated country
|
CC
(%)e
|
1
|
India
|
4,584 (72.08)
|
104
|
79,122
|
638
(13.92)
|
United States
|
141 (22.10)
|
2
|
Pakistan
|
309
(4.86)
|
35
|
5,093
|
125
(40.45)
|
China
|
39
(31.20)
|
3
|
Malaysia
|
289
(4.54)
|
35
|
4,796
|
138
(47.75)
|
Iran
|
34
(24.64)
|
4
|
Bangladesh
|
267
(4.20)
|
50
|
11,229
|
186
(69.66)
|
United States
|
67
(36.02)
|
5
|
Thailand
|
154
(2.42)
|
23
|
2,400
|
79
(51.30)
|
United States
|
25
(31.65)
|
6
|
Indonesia
|
139
(2.19)
|
18
|
1,322
|
54
(38.85)
|
Japan
|
24
(44.44)
|
7
|
Vietnam
|
122
(1.92)
|
30
|
4,109
|
106
(86.89)
|
Japan
|
26
(24.53)
|
8
|
Sri Lanka
|
97
(1.53)
|
23
|
2,157
|
53
(54.64)
|
Japan
|
13
(24.53)
|
9
|
Singapore
|
51
(0.80)
|
21
|
1,448
|
44
(86.27)
|
China
|
18
(40.91)
|
10
|
Philippines
|
49
(0.77)
|
14
|
604
|
24
(48.98)
|
United States
and Taiwan
|
8
(33.33)
|
11
|
Nepal
|
34
(0.53)
|
14
|
631
|
24
(70.59)
|
Japan
|
14
(58.33)
|
12
|
Cambodia
|
24
(0.38)
|
13
|
1,341
|
23
(95.83)
|
United States and United Kingdom
|
7
(30.43)
|
13
|
Laos
|
8
(0.13)
|
5
|
50
|
5
(62.50)
|
France
|
2
(40.00)
|
14
|
Afghanistan
|
7
(0.11)
|
5
|
87
|
7
(100.00)
|
United States, Turkey and Iran
|
2
(28.57)
|
15
|
Myanmar
|
4
(0.06)
|
3
|
42
|
3
(75.00)
|
United States and United Kingdom
|
2
(66.67)
|
16
|
Brunei Darussalam
|
2
(0.03)
|
1
|
6
|
1
(50.00)
|
Oman
|
1
(100.00)
|
17
|
Multi- Countryf
|
221
(3.47)
|
40
|
6,111
|
111
(50.23)
|
United States
|
25
(22.52)
|
a SCR – standard competition ranking
b TA (%) – total number and percentage of groundwater-related articles by a country
c TC – Total citations
d IA (%) – total number and percentage of articles with international authors
e CC (%) – total number and percentage of articles with most collaborated country
f Multi-country includes articles written in collaboration between different South and Southeast Asian countries
|
|
At the institutional level, 160 institutions contributed to articles related to groundwater research and the top 20 institutions are presented in Table 3. The table shows the dominant position of India in groundwater research from South and Southeast Asian Countries. Among the top 20 institutes, 17 institutes are from India, two from Malaysia and one from Bangladesh. It should be noted that the Central Ground Water Board and the National Institute of Hydrology have several regional centers. For this analysis, the different branches are pooled under one major institute heading. The articles, if divided into their branch institutes, would yield different rankings.
As per the analysis, the National Geophysical Research Institute, India published the highest number of total articles, single-institution articles, inter-institutionally collaborative articles, and first author/institution articles in the groundwater domain. As of the academic efforts of these institutions during the 50 years, the Indian Institute of Technology, Kharagpur, has the highest quality index (h-index) of 43 amongst the top 20 institutes. This is followed by the National Geophysical Research Institute, India (39) and Jadavpur University (37) and the University of Dhaka (37).
Table 3 Top 20 most productive institutions publishing in the field of groundwater
SCRa
|
Institutes
|
TA
(%)b
|
SI
(%)c
|
CI
(%)d
|
FA
(%)e
|
CA
(%)f
|
h-index
|
1
|
National Geophysical Research Institute India
|
245
(3.85)
|
95
(38.37)
|
151
(61.63)
|
148
(60.41)
|
97
(39.59)
|
38
|
2
|
Anna University
|
207
(3.25)
|
85
(41.06)
|
122
(58.94)
|
100
(48.31)
|
107
(51.69)
|
32
|
3
|
Indian Institute of Technology Kharagpur
|
174
(2.74)
|
59
(33.91)
|
115
(66.09)
|
66
(37.93)
|
108 (62.07)
|
43
|
4
|
Central Ground Water Board
|
140
(2.20)
|
47
(33.57)
|
93
(66.43)
|
71
(50.71)
|
69
(49.29)
|
29
|
5
|
Annamalai University
|
126
(1.98)
|
30
(23.81)
|
96
(76.19)
|
84
(66.67)
|
42
(33.33)
|
27
|
6
|
Andhra University
|
109
(1.71)
|
60
(55.05)
|
49
(44.95)
|
74
(67.89)
|
35
(32.11)
|
30
|
7
|
Jadavpur University
|
105
(1.65)
|
46
(43.81)
|
59
(56.19)
|
46
(43.81)
|
59
(56.19)
|
37
|
8
|
National Institute of Hydrology India
|
103
(1.62)
|
37
(35.92)
|
66
(64.08)
|
58
(56.31)
|
45
(43.69)
|
23
|
9
|
Bhabha Atomic Research Centre
|
102
(1.60)
|
29
(28.43)
|
73
(71.57)
|
32
(31.37)
|
70
(68.63)
|
20
|
10
|
University of Dhaka
|
101
(1.59)
|
6
(5.94)
|
95
(94.06)
|
14
(13.86)
|
8
(86.14)
|
37
|
11
|
Universiti Putra Malaysia
|
95
(1.49)
|
23
(24.21)
|
72
(75.79)
|
44
(46.32)
|
51
(53.68)
|
28
|
12
|
Jawaharlal Nehru University
|
88
(1.388)
|
42
(47.73)
|
46
(52.27)
|
42
(47.73)
|
46
(52.27)
|
36
|
13
|
Indian Institute of Technology Indian School of Mines, Dhanbad
|
87
(1.37)
|
35
(40.23)
|
52
(59.77)
|
55
(63.22)
|
32
(36.78)
|
20
|
14
|
Indian Institute of Technology Roorkee
|
74
(1.16)
|
21
(28.38)
|
53
(71.62)
|
41
(55.41)
|
33
(44.59)
|
18
|
15
|
Indian Institute of Technology Bombay
|
71
(1.12)
|
36
(50.70)
|
35
(49.30)
|
43
(60.56)
|
28
(39.44)
|
17
|
16
|
Banaras Hindu University
|
67
(1.05)
|
32
(47.76)
|
35
(52.24)
|
42
(62.69)
|
25
(37.31)
|
16
|
17
|
Osmania University
|
60
(0.94)
|
25
(41.67)
|
35
(58.33)
|
36
(60.00)
|
24
(40.00)
|
15
|
18
|
ICAR - Indian Agricultural Research Institute, New Delhi
|
59
(0.93)
|
20
(33.90)
|
39
(66.10)
|
29
(49.15)
|
30
(50.85)
|
23
|
19
|
Indian Institute of Science
|
55
(0.86)
|
14
(25.45)
|
41
(74.55)
|
15
27.27%
|
40
(72.73)
|
17
|
20
|
Universiti Sains Malaysia
|
53
(0.83)
|
11
(20.75)
|
42
(79.25)
|
23
(43.40)
|
30
(56.60)
|
15
|
a The institutes are arranged as per SCR (standard competition ranking) based on the total number of articles produced by an institute.
b TA (%) –Total number and percentage of groundwater-related articles by an institution
c SI (%) – number and percentage of single-institution articles
d CI (%) – number and percentage of inter-institutionally co-authored articles
e FA (%) – number and percentage of first author articles
f CA (%) – number and percentage of co-authored articles
Journal and subject area contribution analysis
Overall, groundwater-related research articles from the South and Southeast Asian countries have appeared in 951 academic journals, and Table 4 shows the top 20 active journals. These top 20 journals (2% of the 951 journals) account for 38% of the total article productivity. Overall, the majority of the groundwater-related articles from South and Southeast Asian countries are published in the Environmental Earth Sciences, followed by the Indian Journal of Environmental Protection, Journal of the Geological Society of India, Pollution Research, and Environmental Monitoring and Assessment. These journals are indicative of the fact that most popular themes of research are, hydrogeology, groundwater quality, groundwater contamination and pollution and its control. However, the journals have a different starting year; thus, to understand the journals' contribution to the groundwater field on a long-term basis, we calculated the average number of publications per active year. Therefore, normalizing by active years, we find that Environmental Earth Sciences still remains the top-performing journal with an average of 24 articles per year; however, the Groundwater for Sustainable Development has taken over for the second spot, followed by the Arabian Journal of Geosciences.
|
Table 4 List of top 20 most productive journals with production statistics
SCRa
|
Journal Title
|
TA
(%)b
|
IFc
|
Active Years
|
Avg. no. of publications per each active year
|
Starting year of journal
|
1
|
Environmental Earth Sciences
|
284
(4.46)
|
2.784
|
12
|
24
|
2009
|
2
|
Indian Journal of Environmental Protection
|
258
(4.05)
|
NAd
|
23
|
11
|
1981
|
3
|
Journal of the Geological Society of India
|
211
(3.32)
|
1.459
|
26
|
8
|
1959
|
4
|
Pollution Research
|
184
(2.89)
|
NAd
|
22
|
8
|
1982
|
5
|
Environmental Monitoring and Assessment
|
182
(2.86)
|
2.513
|
24
|
8
|
1981
|
6
|
Arabian Journal of Geosciences
|
160
(2.51)
|
1.827
|
11
|
15
|
2008
|
7
|
Journal of Hydrology
|
120
(1.89)
|
5.722
|
35
|
3
|
1963
|
8
|
Groundwater for Sustainable Development
|
118
(1.85)
|
NAd
|
6
|
20
|
2015
|
9
|
Hydrogeology Journal
|
97
(1.52)
|
3.178
|
21
|
5
|
1992
|
10
|
Current Science
|
95
(1.49)
|
1.102
|
24
|
4
|
1932
|
11
|
International Journal of Earth Sciences and Engineering
|
91
(1.43)
|
NAd
|
7
|
13
|
2008
|
12
|
Environmental Geology
|
89
(1.40)
|
NAd
|
16
|
6
|
1975
|
13
|
Nature Environment and Pollution Technology
|
86
(1.35)
|
NAd
|
11
|
8
|
1994
|
14
|
Ecology, Environment and Conservation
|
84
(1.32)
|
NAd
|
22
|
4
|
1982
|
15
|
Water Resources Management
|
72
(1.13)
|
3.517
|
22
|
3
|
1987
|
16
|
Science of the Total Environment
|
70
(1.10)
|
7.963
|
17
|
4
|
1972
|
17
|
Journal of the Indian Society of Remote Sensing
|
62
(0.97)
|
1.563
|
27
|
2
|
1969
|
18
|
Journal of Earth System Science
|
60
(0.94)
|
1.371
|
13
|
5
|
1934
|
19
|
Sustainable Water Resources Management
|
58
(0.91)
|
NAd
|
6
|
10
|
2015
|
20
|
International Journal of Civil Engineering and Technology
|
55
(0.86)
|
NAd
|
4
|
14
|
2016
|
a The journals are arranged as per standard competition ranking (SCR) based on the total number of articles published in the journal.
b TA (%) – Total number and percentage of groundwater-related articles published by a journal
c IF – Impact Factor extracted from the journal homepage for the year 2020
d NA – impact factors not available for journal
|
Groundwater theme requires a more interdisciplinary approach that connects the physical and social aspects of groundwater resources availability and accessibility. Fig. 5 shows the subject areas on which groundwater related research has been focused. The top 3 disciplines are Environmental Science, Earth and Planetary Sciences and Agricultural and Biological Sciences. Although groundwater-related studies have now interlinked natural science with engineering and social sciences, the central focus is still on environmental sciences, geological and agriculture science. This shows the lack of actual status of interdisciplinary collaboration between natural and social sciences and even economics and policy studies.
Author contribution analysis
To analyze the most productive authors, the frequency of occurrence for the authors was calculated. Fig 6 and Table S1 (Supplementary File) shows the top 20 most productive authors during 50 years’ study period. Most of the authors in the list are affiliated with Indian institutions. As seen in the figure, Elango L. has the highest number of publications, followed by Ahmed S., Chidambaram S., Ahmed K.M., and Mukherjee A. However, based on the quality performance of authors, Ahmed K. M. from Bangladesh has the highest h-index of 33, followed by Indian authors Chakraborti, D. (27), Chidambaram, S. (25), and Subba Rao, N (25). This analysis also reveals the predominance of male researchers with only one female researcher (Brindha K.) making it to the top 20 most productive authors list.
Additionally, author collaboration is equally essential for the growth of the research sector. Thus, the number of authors mentioned in the articles was counted and analyzed to identify the extent of research conducted by authors individually and in collaboration. Fig. S1 (Supplementary File) reveals the authorship pattern for groundwater-related research from South and Southeast Asian countries researchers during 1970-2020. It is clear from the analysis that articles with multiple authors (93%) have a majority over the single-authored articles (7%). Amongst the multiple authorship, two authors (24%) and three authors (24%) collaborations form the most significant contribution to groundwater-related research. This was followed by articles with four authors (18%), more than five authors (16%) and five authors (11%). Thus, this demonstrates that more and more researchers are collaborating to assess and manage the groundwater challenges that have emerged in all the South and Southeast Asian countries.
Further to investigate the collaborative patterns, co-authorship analysis was carried out using the full counting method in the VOSviewer software. A threshold of 15 minimum number of documents by an author and 15 minimum citations of an author was fixed. Basis the threshold, of the 12,184 authors, 96 authors met the threshold, and for each of these 96 authors, the total strength of co-authorship links with the other authors was calculated, and the network graph was obtained (Fig. 7). Based on the author link strength, 8 clusters were formed. The biggest clusters are Cluster 1 (shown in red color) and Cluster 2 (shown in green color) with 24 authors each, followed by Cluster 3 (shown in blue color) and Cluster 4 (shown in yellow color) having 13 and 9 authors respectively. As seen from the Fig. 7, Mukherjee A. has co-authored with the maximum number of authors (21), appearing in the Cluster 2. Following Mukherjee A., Ramanathan A.L. (from Cluster 2) co-authored with 20 other authors and Ahmed S. (from Cluster 3), co-authored with 19 authors. From this network analysis, Prasanna M.V. and Chidambaram S. from Cluster 5 (shown in purple color) co-authored in highest number of groundwater-related articles (45). Thus, it shows that in the field of groundwater research, researchers have formed an academic pattern of close contact and mutual cooperation.
Keywords analysis
For this study, only author keywords have been considered and keyword cloud and frequency analysis were conducted to present a general overview of the most prevalent research themes for the South and Southeast Asian countries.
Of the total 10,240 keywords, 185 keywords (~2%) have been shortlisted, which have been used more than fifteen times in the groundwater-related articles. The most frequently used keywords for the 50 years of this study are illustrated in Fig. 8, and the top 20 keywords are listed in Table 5. Except for Groundwater, which is the top search keyword for this study, the top five most frequently used keywords are “Groundwater quality”, Geographic information system”, “India”, “Arsenic”, and “Fluoride”. Interestingly, from the country level perspective, India and Bangladesh are the two countries occurring the most frequently within the keywords, possibly for India it is due to a relatively higher share of publications in comparison to other countries. Additionally, both of these countries face serious groundwater challenges due to agricultural intensification, industrial development, population growth, climate change, and thus extended research.
Table 5 Top 25 most frequently used author keywords for the study period
SCRa
|
Keyword
|
Frequency
|
1
|
Groundwater
|
1996
|
2
|
Groundwater quality
|
495
|
3
|
Geographic information system
|
429
|
4
|
India
|
337
|
5
|
Arsenic
|
302
|
6
|
Fluoride
|
267
|
7
|
Water quality
|
249
|
8
|
Water quality index
|
222
|
9
|
Heavy metal
|
171
|
10
|
Hydrogeochemistry
|
160
|
11
|
Remote sensing
|
155
|
12
|
Contamination
|
139
|
13
|
Irrigation
|
126
|
14
|
Nitrate
|
117
|
15
|
Groundwater recharge
|
110
|
16
|
Aquifer
|
108
|
17
|
Hydrochemistry
|
107
|
18
|
Bangladesh
|
106
|
19
|
Groundwater pollution
|
104
|
20
|
Groundwater management
|
91
|
a Keywords are arranged as per standard competition ranking (SCR)
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From Table 5, it is evident that groundwater research in groundwater quality has got the most attention over the 50 years in South and Southeast Asian countries. Besides Singapore and Malaysia, the rest all the countries come under developing countries categories who have undergone intensive agriculture and industrial development over the 50 years and thus are under greater threat of groundwater pollution and contamination (Mukherjee, 2018). Researchers from South and Southeast Asian countries have investigated widely in this domain, catering to different pollutants, sources, concentrations, pathways of transport, impacts on human health and the environment, control and removal/ remediation. Under this research theme, research trends on the most studied contaminants are shown in Fig. 9.
Among the vast list of groundwater contaminant studies, arsenic and fluoride are most widely researched in South and Southeast Asian countries, with 606 and 443 articles published, respectively. The number of studies on arsenic has increased quite rapidly post 2002, with an average growth rate of 21%. On the other hand, the research on fluoride has increased significantly post 2005, with an average growth rate of 31%. Heavy metals, nitrate and salinity related publications also have displayed an increasing trend post-2007.
The research contribution from different countries on different groundwater contaminants was also explored (Fig. S2 – Supplementary file). The analysis shows that South Asian countries India and Pakistan have done extensive research on groundwater contamination. The arsenic-related publications from India, Bangladesh, Pakistan, and Vietnam showed an overall fluctuating or decreasing growth trend. Fluoride publications were mainly contributed by researchers based in India, Pakistan and Sri Lanka. Significant contributions to nitrate-related studies came from Indian researchers, and the number of studies has shown an increasing trend. India, Pakistan and Bangladesh are the three major contributors to salinity related studies; however, research productivity has been quite fluctuating. In India, Pakistan and Malaysia, the research output on heavy metals research has continuously increased from 2010 onwards.
To assess and express the groundwater quality and its suitability for different purposes, Water Quality Index (WQI) has been extensively used by researchers. The WQI utilizes several physicochemical and biological parameters and transforms them into a single expression. Due to its concise and straightforward application and interpretation of water quality status, its significance has been widely accepted and appreciated (Ram et al. 2021), thus its prominence among keywords.
In Asia, 15 out of 19 South and Southeast Asian countries are among the largest groundwater consumers for agriculture worldwide. One of the primary reasons is the introduction of the Green Revolution in select countries (India, Pakistan, Bangladesh, Philippines, Afghanistan, Sri Lanka, Indonesia, and Thailand), resulting in the spread of groundwater irrigation (Hirji et al. 2017b). For countries like Nepal (Yadav 2018), Bhutan (Dorji 2016), Vietnam (Ha et al. 2015), climate variability and decline in surface water availability led to the expansion of groundwater irrigation. Thus, irrigation becomes one of the most researched keywords and the themes that have been studied mainly include (i) groundwater quality suitability analysis for irrigation purposes; (ii) estimating the groundwater abstraction for irrigating different crops in different agro-ecological zones; (iii) cost-effective technological solutions for irrigation for different categories of farmers as well as for optimum groundwater utilization.
Additionally, groundwater also serves as a vital source of drinking water supply in many rural and urban areas in the South and Southeast Asian Countries (Mukherjee 2018; Carrard et al. 2020). Groundwater composition is majorly driven by geological structure and anthropogenic activities. However, agricultural practices, industrial and commercial activities, and climate change has progressively deteriorated the groundwater quality in many regions (Li et al. 2021). Consumption of the polluted water has raised many health problems such as toxicity of arsenic, fluoride, nitrate, boron etc. (Sikdar 2019; Shahid et al. 2020). Thus, research studies on assessing the suitability of groundwater quality for drinking water have also begun to gain importance to diminish health related problems and protect the groundwater resources.
Thus, looking at the attention received to water quality and contamination, Hydrochemistry and Hydrogeochemistry emerged as the two major fields of inquiry in groundwater research. The research productivity for hydrogeochemistry has increased constantly from 1999 onwards while the usage of hydrochemistry has been fluctuating and decreasing. This clearly shows the increased consideration given to hydrogeochemical processes that govern the water quality to assess its suitability for different purposes.
Aquifer is the most frequently used keyword in groundwater research, as it is vital to understand groundwater occurrence within rocks and other geological formations. Different studies under this keyword category include (i) to investigate and estimate aquifer parameters using different hydrogeological and geophysical methods (such as vertical electrical sounding); (ii) to map groundwater potential zones by studying the occurrence, distribution and movement of groundwater flow; (iii) to quantify and predict contaminant transport. Linked with the aquifer, quantifying groundwater recharge and storage are historically the most productive research topic. Different methods such as isotopic investigations, modelling techniques, hydrographic analysis and remote sensing have been explored. The first research article on groundwater recharge estimation was published in 1973 and the study was conducted using tritium isotope. However, the research on groundwater recharge was still at a very nascent stage, and on average, there was only one publication coming up per year till 1995. From 1996 - 2006 there was an increase in the number of articles originating mainly from the South Asian countries – India, Pakistan and Sri Lanka. This rise in research interest in India and Pakistan can be due to the aftermath of the Post Green Revolution Era (1990 onwards), which could have led to over-abstraction of groundwater for agriculture leading to declining availability of groundwater resources. Therefore, efforts to evaluate and enhance the groundwater recharge became the topic of concern, and publications grew in number with an average annual growth rate of 34%. Post-2004, the groundwater recharge studies increased furthermore, and contribution came in from both South and South-East Asian countries with an AAGR of 23%
Remote sensing (RS) and Geographic Information System (GIS) are among the top 10 most frequently used keywords during the study period. Together they have played an increasingly important role for the hydrologic community. They have proved to be the cost-effective technologies over traditional methods by producing valuable data for monitoring and management for the current groundwater research (Masud and Bastiaanssen 2017; Thakur et al. 2017). The studies based on the application of RS data and GIS techniques in groundwater research saw the light in the late 1970s with its first research on the delineation of hydromorphic units for groundwater studies using Landsat Multispectral Scanner (Roy 1978). However, the research using these techniques grew only in 21st Century. At present, the integrated application of the RS and GIS has become a breakthrough in groundwater research, for both quality and quantity assessment and management, with the publication growing at an average growth rate of 34%.
Despite the major threats posed to groundwater availability and quality in the South and Southeast Asian Countries, as seen through the keyword analysis, the research on groundwater management has gained lesser attention. The concept of groundwater management was introduced in research in the early 1980s (1985); however, the studies on groundwater management gained momentum post-2000 only. Fig. 10 shows trends for the major themes that are being studied under groundwater management.
Under groundwater management, evaluating the impacts of climate change and climate variability on groundwater resources is critical because it directly and indirectly affects the hydrogeological processes (Green 2016). However, the studies on how groundwater resources respond to climate change and anthropogenic activities have started to pick up from the last decade only, with notable contributions coming from India, Thailand and Bangladesh. The themes mainly included (i) impact of climate change on groundwater levels and recharge under present and future scenarios; (ii) building resilience and adaption for agriculture and livelihood under climate change conditions and natural disasters (iii) impact of climate change on groundwater quality.
Vulnerability, adaptation and resilience-based studies, forming the basis for the sustainable management of groundwater, are also being explored by researchers from South and Southeast Asian countries. Groundwater vulnerability studies done in the past decade mainly focus on mapping vulnerable zones to select groundwater contaminants using GIS and DRASTIC model approaches. A growing scientific base on Managed Aquifer Recharge (MAR) or Artificial Groundwater Recharge supports its rapidly increasing use as an important water adaptation and management strategy to enhance and secure groundwater systems impacted from climate change and natural disasters and the hydrological variability (Dillon et al. 2019). However, resilience-based studies that should be focusing on the rural communities are still to be explored.
Since a majority of the South and Southeast Asian countries are bordered around major seas (the Arabian Sea, Bay of Bengal, Jawa Sea and the South China Sea), Seawater intrusion or saltwater intrusion has become one of the major environmental issues in their coastal regions (Tianzheng et al. 2021). Over the last 20 years, saltwater intrusion research has been carried out to manage coastal groundwater resources. An increased recognition is also being given to understand the importance of surface and groundwater interactions and to understand and control the impact of groundwater salinization. Understanding their interactions has also been helpful in formulating effective conjunctive water resource management plans. It provides an effective adaptation to meet the increasing demands and shortages problems, especially in arid and semi-arid regions (Zhang 2015).
Furthermore, co-occurrence analysis was conducted using the author keywords to understand theme distribution and analyze the relationship and structure between different keywords. Fig. 11 shows the author keyword co-occurrence network visualization map. A threshold of 15 as minimum number of occurrences of a keyword was set. Out of the 10,240 keywords, 195 meet the threshold and based on that the number of co-occurrence links was calculated. Based on the keyword strength, 7 clusters were formed. Cluster 1 (shown in red) has 56 keywords and highlights on issues related to depleting groundwater levels, discusses its assessment through modelling approaches, and the management techniques and approaches for future scenarios. Cluster 2 (shown in green) and Cluster 3 (shown in yellow) contains 30 and 23 keywords, respectively. The keywords in these clusters are connected with the groundwater quality and suitability assessment based on several physico-chemical parameters and evaluated through WQI and multivariate statistical techniques. Cluster 4 (shown in blue) includes 28 keywords present the importance and application of remote sensing and GIS in evaluation groundwater resources for quantity and quality. Cluster 5 (shown in purple) includes 22 keywords which highlights on the health and risk assessment associated with the consumption of contaminated groundwater, mainly with heavy metals. Cluster 6 (shown in cyan) and Cluster 7 (shown in orange) contains 22 and 10 keywords, respectively. The two clusters emphasize on the groundwater contamination by arsenic and fluoride transport, assessment and its removal.
Overall Research Implications and Outlook
The bibliometric analysis showed that the groundwater-related research from South and Southeast Asia started very late compared to the global groundwater research contribution. Globally, the first groundwater-related document was published in 1889 on “Disinfection of springs, and number of germs in ground-water” in the Science journal (Science 1889). However, the first publication from South and Southeast Asia came only in 1957, i.e., 68 years later, on studying the geological and hydrological conditions and the groundwater development potential in the Narmada Valley in Madhya Pradesh, India (Roy 1957). The next thirteen years (1957-1970) had shown almost no growth in groundwater research from South and Southeast Asia. But since the 1970s, groundwater research in the South and Southeast Asian countries showed significant growth over the next 50 years. The growth rate for groundwater-related studies from the region was more than the global growth, signifying the sensitization of the groundwater issues and concentrated efforts made by the researchers from South and Southeast Asian countries to assess and manage their groundwater resources and contribute to global literature.
The bibliometric analysis showed that the contribution from South Asian countries was much higher than the Southeast Asian countries. From 1970-1983, groundwater-related studies mainly originated from India, but from 1983-1999, contributions began to come in from Pakistan, Bangladesh, Thailand, Indonesia, Sri Lanka, Vietnam, Sri Lanka and the Philippines. Some of these countries benefitted from the Green Revolution and improved agriculture productivity by exploring groundwater resources. Post-2000, a remarkable increase in the number of publications is observed and contributions from the other countries. Particularly for India, the contribution to groundwater research has been exceptionally high, and India also became the third-largest contributor in the early 2010s globally (Jia et al. 2020). The high contribution is because India’s groundwater usage is more than the combined usage by the United States and China, the second and third largest groundwater users (Hirji et al. 2017b). The groundwater usage has drastically increased over the years and has enabled higher crop yields, which helped the country achieve its food security and reduce poverty. However, the rising demand for food and intensive levels of groundwater pumping for irrigation have led to overexploitation of the resource followed by severe groundwater depletion and contamination issues, which needs extensive research for better planning and management. Additionally, the involvement of several central and state universities as well as government-operated research institutes showed high productivity and impact in the groundwater research domain.
Research trends established through author keywords analysis provided an insight into the significant groundwater issues South and Southeast Asian countries are facing. Groundwater quality assessment gets major attention, specifically arsenic. India, Bangladesh, Pakistan and Vietnam are amongst the most polluted arsenic areas (Abejón and Garea 2015; Mukherjee 2018). Thus, studies focusing on quality assessment, mapping of hotspot areas, contaminant transport and remediation technologies have been extensively researched in recent years. The research efforts also resonate in the high research productivity of the top 20 journals like - Indian Journal of Environmental Protection, Pollution Research, and Environmental Monitoring and Assessment publishing in this domain. It is also important to note that groundwater quality studies have mainly been done for inorganic and microbiological contaminants. Since most of these countries are agro-based, emphasis on organic contaminants should also be laid.
The number of groundwater studies related to groundwater depletion and management displayed increasing trends. Current rate of overexploitation of groundwater resources have posed serious threats to the long-term food, water and energy security and livelihood of the South and Southeast Asian countries-especially in the semi-arid and arid regions (Rasul, 2016). Thus, research efforts have been made to understand the groundwater behavior using modelling approaches, quantify the groundwater reserves through remote sensing, explore interventions and technological solutions to enhance groundwater recharge, implement better irrigation practices for sustainable management of groundwater resources.
As highlighted through keyword analysis, groundwater issues are interrelated with a wide range of disciplines and subject areas; thus, more collaborated research needs to be done beyond the environmental and engineering aspect to understand the challenges and opportunities and expand the horizon of groundwater research.
With data and information acknowledged as critical for effective groundwater management, there is an urgent need to invest in more extensive monitoring networks to improve the frequency and quantity of groundwater data (Jia et al. 2020). Integrated satellite-based investigations using remote sensing and GIS techniques is one solution that has increased groundwater data availability and spatial and temporal resolutions. Crowdsourcing is another solution that has become a promising approach in recent years to obtain supplementary data by involving citizens (Maheshwari et al. 2014). These solutions need support in order to upscale and be useful at the national level. Along with this, these solutions will lead to more data generation, and thus, data mining techniques and machine learning algorithms will have to be explored.
Through this analysis, it was also observed that the number of female researchers in groundwater research lags behind the number of male researchers, and it was more pronounced in the most productive authors and author collaboration groups. The research community should make efforts to nurture the junior female researchers and encourage and support the senior female researchers at every stage of their careers to reduce the gender imbalance.