Updated October 2026. This page extends the first bibliometric snapshot published in April 2026 with an updated citation analysis, the structure of HydroLAB’s scientific collaboration network, and the evolution of its research themes.
Why analyse the scientific impact of HydroLAB
HydroLAB, the hydrology research group led by Salvatore Manfreda at the University of Naples Federico II (UNINA), works on hydrological monitoring, remote sensing, UAV-based and image-based observation of rivers, flood mapping and hydrological modelling. Publication counts alone say little about how this work is used. Two complementary questions are more informative: who cites our research, and where? And who do we produce it with?
This bibliometric analysis addresses both. The first part describes the citation footprint, meaning the documents, countries and disciplines that build on HydroLAB publications. The second part describes the collaboration network, meaning the co-authors, institutions and countries that take part in producing them. A final section looks at how research themes have evolved.
A growing international scientific footprint
According to OpenAlex (retrieved 1 October 2026), the indexed works of Salvatore Manfreda and his co-authors have received 7,140 citations, corresponding to an h-index of 42 and an i10-index of 107. These citations come from 4,948 distinct citing documents. Only 168 of them (3.4%) are co-authored by Salvatore Manfreda. The citation footprint is therefore driven almost entirely by other research groups.
The temporal pattern shows two phases (Figure 2). Until 2016, citations grew slowly and stayed around 100 per year. Between 2017 and 2021 they increased about eightfold, reaching 950 in 2021. Since then they have remained between 730 and 851 per year. This plateau at a higher level indicates that the work is being used steadily, not only during a short peak of attention.
Citations received per year, 2012–2026
All indexed works by Salvatore Manfreda and HydroLAB co-authors · OpenAlex, retrieved 1 October 2026 · hatched bar = 2026, January–September only
Show data table
| Year | Citations received |
|---|---|
| 2012 | 58 |
| 2013 | 105 |
| 2014 | 98 |
| 2015 | 101 |
| 2016 | 114 |
| 2017 | 212 |
| 2018 | 332 |
| 2019 | 433 |
| 2020 | 623 |
| 2021 | 950 |
| 2022 | 730 |
| 2023 | 768 |
| 2024 | 851 |
| 2025 | 817 |
| 2026 | 652 (Jan–Sep) |
Figure 2. Annual citations rose from 58 in 2012 to 950 in 2021 and have remained between 730 and 851 per year since 2022. The 652 citations recorded up to September 2026 already exceed every annual total before 2020. Counts refer to citations received in each calendar year, not to the year of publication of the cited work.
Part of the 2017–2021 growth is associated with a small number of highly cited papers on new observation techniques. The review On the Use of Unmanned Aerial Systems for Environmental Monitoring (Remote Sensing, 2018) has received 711 citations, about one tenth of the total. It is followed by Current Practices in UAS-based Environmental Monitoring (Remote Sensing, 2020; 270 citations) and the MOXXI paper on innovative hydrological measurements (Hydrological Sciences Journal, 2018; 244). Earlier work on geomorphic delineation of flood-prone areas (2011, 207 citations; 2017, 183) and on Mediterranean ecohydrology (2008, 186) continues to be cited, so the footprint does not depend on a single topic.
Where HydroLAB research is being used
The first edition of this page mapped citing documents in Scopus (Figure 1). That April 2026 snapshot found citing authors in 90 countries on six continents, with China (495 citing documents), Italy (470) and the United States (395) as the largest contributors.
The updated analysis uses OpenAlex, which indexes a broader set of journals, conference proceedings and repositories. It identifies citing authors in 130 countries (Figure 3). The ranking of the leading countries changes slightly between the two databases. The overall picture does not: the United States (884 citing documents), Italy (864) and China (846) form a leading group, followed by the United Kingdom (360), Germany (351), India (281) and Spain (250).
Two features of the distribution stand out. First, 81.5% of citing documents with known affiliations have no Italian author (3,815 of 4,679). The research travels well beyond its national context. Second, the footprint is not confined to Europe and North America. Citing authors are based in 40 Asian and 31 African countries, including Ethiopia, Nigeria, Kenya, Tunisia and Morocco.
Where HydroLAB research is cited
Documents citing HydroLAB work, by country of author affiliation (top 15 of 130) · a document is counted once for each country among its authors · OpenAlex, 1 October 2026
Show data table
| Country | Citing documents |
|---|---|
| United States | 884 |
| Italy | 864 |
| China | 846 |
| United Kingdom | 360 |
| Germany | 351 |
| India | 281 |
| Spain | 250 |
| Australia | 210 |
| Netherlands | 194 |
| France | 175 |
| Canada | 161 |
| Iran | 155 |
| Greece | 141 |
| Austria | 118 |
| Japan | 114 |
Figure 3. The 4,948 documents citing HydroLAB work have authors affiliated in 130 countries across six continents. The United States, Italy and China each account for more than 800 citing documents. Asia (40 countries) and Africa (31) together contribute almost twice as many countries as Europe (39).
The citing literature is also diverse across disciplines. About 70% of citing documents are classified under Environmental Science. Engineering (437 documents), Earth and Planetary Sciences (345), Agricultural and Biological Sciences (323) and Computer Science (148) follow.
A global network of scientific collaboration
The second dimension concerns how the research is produced. A network analysis of HydroLAB’s full publication record covers 192 publications written with 409 co-authors in 43 countries on five continents. Italy is the main hub of the network. The United States, the United Kingdom, the Netherlands, Germany and France are its principal international nodes.
Figure 4 shows the distribution of these collaborations at country level, based on the 163 journal articles indexed in OpenAlex. Here the United States is the most frequent partner country (29 joint articles), followed by the United Kingdom, the Netherlands and Germany (17 each), Spain and Chile (10 each). Recurrent institutional partners include Princeton University, the University of Twente, the Helmholtz Centre for Environmental Research (UFZ), Newcastle University, Wageningen University, the University of California Santa Barbara and the University of Concepción. In Italy, the strongest links are with the University of Basilicata, the National Research Council (CNR) and the Polytechnic University of Bari.
Where HydroLAB co-authors are based
Journal articles co-authored with partners in each country, grouped by continent · Italy excluded from the bars · OpenAlex, 163 journal articles, 2001–2026
Show data table
| Country | Co-authored articles | Continent |
|---|---|---|
| United States | 29 | North America |
| United Kingdom | 17 | Europe |
| Netherlands | 17 | Europe |
| Germany | 17 | Europe |
| Spain | 10 | Europe |
| Chile | 10 | South America |
| France | 8 | Europe |
| Greece | 7 | Europe |
| Hungary | 7 | Europe |
| Austria | 7 | Europe |
| Portugal | 5 | Europe |
| Sweden | 5 | Europe |
| Switzerland | 5 | Europe |
| Belgium | 5 | Europe |
| Israel | 5 | Asia |
| China | 5 | Asia |
| Czechia | 4 | Europe |
| Serbia | 4 | Europe |
| Canada | 4 | North America |
| Philippines | 4 | Asia |
| Türkiye | 4 | Asia |
| Romania | 3 | Europe |
| Iran | 3 | Asia |
| Lithuania | 2 | Europe |
| Slovakia | 2 | Europe |
| Denmark | 2 | Europe |
| Saudi Arabia | 2 | Asia |
| Australia | 2 | Oceania |
| Estonia | 1 | Europe |
| Norway | 1 | Europe |
| Poland | 1 | Europe |
| Ireland | 1 | Europe |
| Finland | 1 | Europe |
| Luxembourg | 1 | Europe |
| Russia | 1 | Europe |
| Brazil | 1 | South America |
| India | 1 | Asia |
| Sri Lanka | 1 | Asia |
| Nepal | 1 | Asia |
| Morocco | 1 | Africa |
| Tunisia | 1 | Africa |
| Egypt | 1 | Africa |
| Namibia | 1 | Africa |
| Ethiopia | 1 | Africa |
Figure 4. International co-authorship spans 44 countries outside Italy on six continents. The United States (29 joint articles) is the most frequent partner. The United Kingdom, the Netherlands and Germany follow with 17 each, then Spain and Chile with 10 each. Europe contributes the largest number of partner countries (25 besides Italy), and 21 countries appear in only one or two articles, a sign of a broad and still expanding periphery. Counts are articles with at least one author affiliated in the country.
The network is also the outcome of coordinated community efforts. The IAHS MOXXI working group on innovative hydrological measurements and the HARMONIOUS COST Action on UAS-based environmental monitoring brought together researchers from many countries. Several of the most collaborative articles, including the 2018 UAS review and the MOXXI paper, originate from these initiatives.
The network has grown in step with these activities (Figure 5). Before 2016, most articles were written by small, largely Italian teams. Between 2011 and 2015, only 15% of articles had a co-author abroad, and just two countries were represented. After 2016 the picture changes. In 2016–2020, 52% of articles were internationally co-authored, with partners in 27 countries. In 2021–2026 this share reached 65%, with 38 countries and 227 distinct co-authors. Teams also became larger, rising from an average of 3.8 authors per article in 2001–2010 to 8.9 in 2021–2026.
How the collaboration network has grown
Journal articles grouped by period of publication · OpenAlex, 163 journal articles · Italy included in the country count
Internationally co-authored articles (%)
Countries in the co-author network
Distinct co-authors
Mean authors per article
Show data table
| Period | Articles | International | Countries | Co-authors | Authors/article |
|---|---|---|---|---|---|
| 2001–10 | 32 | 34% | 5 | 42 | 3.8 |
| 2011–15 | 26 | 15% | 2 | 41 | 4.8 |
| 2016–20 | 56 | 52% | 27 | 172 | 7.1 |
| 2021–26 | 49 | 65% | 38 | 227 | 8.9 |
Figure 5. The network has changed in scale and in reach. The share of articles with at least one co-author abroad rose from 15% in 2011–2015 to 52% in 2016–2020 and 65% in 2021–2026. Over the same periods, the countries represented grew from 2 to 27 and then 38. The mean team size more than doubled, from 3.8 authors per article in 2001–2010 to 8.9 in 2021–2026.
Research themes and emerging directions
The bibliometric record mirrors the scientific evolution of the laboratory. HydroLAB’s research has moved along a continuum that links classical hydrology to new observation technologies and data-driven methods:
Hydrology → Earth Observation → Remote Sensing → UAS → Cameras & Image Analysis → AI → Environmental Monitoring → Hydrological Applications
Figure 6 traces this path through article titles. Flood mapping and soil moisture form a stable core across 25 years. From 2016, UAS-based observation and image velocimetry become a second pillar, which coincides with the rise in citations and international co-authorship. After 2020, three lines emerge that were absent before: machine learning and AI (8 articles), water quality and plastic monitoring (5) and nature-based solutions (5). Satellite remote sensing grows steadily throughout.
Research themes over time
Journal articles whose title addresses each theme, by period · an article can belong to more than one theme · keyword classification of OpenAlex titles (163 articles)
| 2001–10 | 2011–15 | 2016–20 | 2021–26 | |
|---|---|---|---|---|
| Flood-prone areas & flood mapping | 11 | 10 | 13 | 9 |
| Soil moisture | 6 | 7 | 7 | 7 |
| Satellite remote sensing | 1 | 2 | 5 | 9 |
| UAS / drones | – | – | 6 | 6 |
| Cameras & image velocimetry | – | – | 8 | 9 |
| Machine learning & AI | – | – | – | 8 |
| Water quality & plastics | 1 | – | – | 5 |
| Nature-based solutions | – | – | – | 5 |
| Vegetation & ecohydrology | 5 | 4 | 3 | 2 |
Figure 6. Flood mapping and soil moisture form a stable core across 25 years. UAS-based and camera-based observation appear from 2016. Satellite remote sensing grows steadily. Machine learning, water quality and plastics, and nature-based solutions emerge as new lines after 2020. Article counts per period: 32, 26, 56 and 49.
These emerging themes connect monitoring to applications. Camera networks and drones generate data that machine-learning models can process at scale. The results feed into flood hazard mapping, hydrological modelling and water-resource management, from the Mediterranean to data-scarce regions.
What the numbers tell us
- Internationalisation. 81.5% of citing documents have no Italian author, and 65% of recent articles are internationally co-authored.
- Capacity to generate collaboration. The number of countries in the co-author network grew from 2 (2011–2015) to 38 (2021–2026), and the number of distinct co-authors more than quintupled.
- Global reach of results. Citing authors are based in 130 countries, including 31 in Africa and 40 in Asia.
- Interdisciplinarity. Citations come from environmental science, engineering, Earth sciences, agricultural sciences and computer science.
- Openness of the community. Only 3.4% of citing documents are self-citations.
- Evolution over time. The citation and collaboration curves both change slope around 2016–2018, when UAS and image-based monitoring entered HydroLAB’s research portfolio.
Taken together, the data describe more than a growing list of publications. HydroLAB operates as a node through which methods, data and people circulate. Collaborators from 43 countries contribute to producing the research, and authors in 130 countries use it in their own work.
Looking ahead
Several current initiatives are likely to shape the next phase of the network. Ongoing projects such as INTEGRATOR and SHARE Africa extend HydroLAB’s monitoring methods to new river basins and partner institutions, particularly in Africa and the Mediterranean. The emerging lines on AI, water quality and nature-based solutions are recent, and their citation impact is not yet visible in the data. We plan to update this analysis periodically and to track whether these themes follow the trajectory observed for UAS and image-based monitoring.
The full list of HydroLAB publications, with DOIs, PDFs and BibTeX entries, is available on the publications page. An overview of our monitoring research is on the hydrological monitoring page.
Data and methods
Citation data were retrieved from OpenAlex (author ID A5023933354, ORCID 0000-0002-0225-144X) on 1 October 2026. Citing documents were identified with the OpenAlex cites filter on all cited works and de-duplicated. Countries refer to author affiliations, with each document counted once per country (whole counting). Figure 1 reproduces the Scopus analysis published in April 2026. The collaboration network figures (192 publications, 409 co-authors, 43 countries) come from a separate network analysis of the full publication record. Country-level distributions in Figures 4–5 are based on the 163 journal articles indexed in OpenAlex. Themes in Figure 6 were assigned by keyword matching on article titles. Values from different databases are not directly comparable because of differences in coverage and affiliation parsing.