3.1. Relationship between GASEMT Attributes and Altmeric Score
Almost 85% of records in the GASEMT database have an AS = 0. This finding already allows us to draw a first conclusion, which is that soil erosion modelling is not a (scientific) topic of high interest for the general public. This appears very evidently when we compare the AS values with the one of topics that are more frequently discussed in media, such as climate change [
14] or COVID-19 [
21,
22,
23]. For example, Rahimi et al. [
14] showed that around 50% of papers about climate change were also detected by altmetric.com. Using the GASEMT data, it was found that only 15% of records have an AS larger than 0, and only around 1% have an AS larger than 10 (
Table 1). Furthermore, almost all records with AS larger than 0 were published in journals and not conference proceedings or book series (
Table 1). Thus, it seems that conference proceedings and book series publications are not recognized by the altmetric.com tracking engines in the case of soil erosion modelling studies.
Back in 2015, Costas et al. [
24] confirmed that the presence and density of (social) media altmetric counts were still very low and not very frequent among scientific publications, with 15–24% of the publications presenting an Altmetric Score. These numbers are similar to the ones observed by the GASEMT database. It was also found that around 60% of the soil erosion papers with an AS > 0 were published in the last 5 years, showing an increasing trend in altmetrics. These results are related to the assumption of whether or not scholars populate their results in social media and manage to raise the importance of their findings both in the news (electronic newspapers, blogs, etc.) or in policy documents. It is also clear that GASEMT records related to a higher AS yield more citations and are generally published in journals with a higher CiteScore (
Table 1). Larger AS are also associated with more co-authors (
Table 1), as more intense interaction [
25] is achieved by their response to online comments in diverse social media. No clear relationship between number of authors and number of citations was detected by Bezak et al. (2021) [
8].
Global studies have an AS larger than 0 and also larger than 10 more frequently compared to regional/local studies (
Table 2), suggesting that such large-scale studies attract more attention in the news, blogs and social media. However, for local communities, smaller-scale studies can be as important as global studies since the former deal with specific problems that attract attention from the local community. Global studies were also related to a higher number of citations [
8]. Moreover, studies that focused on South America, Africa, Asia and Oceania have a decreasing percentage of modelling records with increasing AS (
Table 2). There are only a few studies that focused on these continents and have an AS of more than 10 (
Table 2). Furthermore, it is clear that case studies conducted in Europe and North America have larger AS (
Table 2) compared to other continents. This could be related to the fact that different types of media (e.g., social or mass) are more frequently used or more developed in these two continents. More specifically, online journals, news, other media and social media users [
26] pick up research and environmental aspects in Europe and USA more frequently than in Africa or Asia. Similar results were obtained using the scale of the study area (
Table 3). Increasing percentages for higher AS values were observed for global, continental and hillslope studies while opposite results were detected for plot, national and farm/landscape scales (
Table 3).
In terms of applied models, it can be seen that RUSLE and WEPP have higher percentages of modelling records for AS > 10 (
Table 4). The higher percentage of the WEPP model also probably explains the higher percentage for the hillslope studies (
Table 3). On the other hand, the WaTEM/SEDEM model that was associated with the highest number of normalized citations [
8] does not have a very high percentage of modelling records for the AS > 10 (
Table 4). The higher percentage of the RUSLE model can be attributed to some studies with relatively high AS [
27,
28]. Furthermore, the percentage of entries for the USLE model is decreasing with increasing AS (
Table 4). However, it should be noted that RUSLE and USLE model applications represent around 1/3 of all entries in the GASEMT database [
6]. As the percentage of “other models” significantly decreases for AS > 10, this means that other less frequently applied models attract less attention than more frequently applied ones such as RUSLE, USLE, and WaTEM/SEDEM (
Table 4). As altmetrics developed only a decade ago, temporal variations are present and more clear trends may only be observed in years to come. The only obvious trend is the one that the total social media activity is increasing, and the percentage of scientific papers that are showing such activity of any form is also increasing. Therefore, scientists are more and more active on social media where they present their publications.
In terms of field activities, model calibration, soil sampling activities and model validation, no clear pattern could be detected in terms of the AS values (
Table 5). It was shown by Bezak et al. (2021) [
8] that the soil erosion modelling research community should give more attention to these soil erosion modelling characteristics since the number of citations was not significantly higher in cases where these activities were included in soil erosion modelling studies. Therefore, investigation using altmetric.com only confirms this conclusion. Moreover, the percentage of studies dealing with the present decreases with increasing AS; the opposite trend is found for the studies dealing with past and future (
Table 6). This former result could be related to the fact that climate change studies clearly have a relatively big presence in different social media. In addition, the online journal would have a much easier study with future erosion projections (
Table 6) than ones referring to the past, as we can notice with the recent papers published by Panagos et al. (2021) [
29] and Borrelli et al. (2020) [
1]. However, it should be also noted that the absolute number of studies dealing with future soil erosion scenarios is substantially smaller compared to the number of studies focused on past or present in the soil erosion modelling community [
6,
8]. For example, Rahimi et al. (2017) [
14] showed that around 50% of papers were also detected in altmetric.com, while this percentage is much lower in cases of soil erosion modelling studies (
Table 1). According to Bornmann (2014)’s [
30] study, soil erosion modelling seems to be a scientific domain and not a topic having attracted interest from a wide audience.
3.2. Relationship among Almetric.com Sources and Number of Citations
We also investigated the possible correlation between different altmetric.com sources and total and normalized number of citations in GASEMT database records. In the case of soil erosion modelling, the highest correlation is observed between number of Mendeley readers and number of citations (
Figure 1 and
Figure 2). This is then followed by policy mentions, Wikipedia mentions, and Twitter mentions. Relatively similar results were obtained for AS > 0 and AS > 10 and by using all citations and normalized number of citations. Several other studies have investigated these relationships [
31,
32,
33,
34] but not in the field of soil erosion modelling. According to Haustein et al. (2014) [
32], Mendeley bookmarking was moderately correlated (0.45) with Scopus citation counts. Similar studies showed that altmetrics is still under development with the option to include other social media platforms and tools in the future (LinkedIn, Researchgate, Academia.edu, Mendeley, Google Scholar, etc.). Peoples et al. (2016) [
33] studied 1599 primary research articles from 20 ecology journals published in the period 2012–2014 and found a strong positive relationship between Twitter activity (i.e., the number of unique tweets about an article) and number of citations in Web of Science. Biljecki (2016) [
18] studied 12,436 papers published in 20 GIS science journals in the period 2000–2014, and nearly all (97.2%) were bookmarked by at least one reader in Mendeley, two thirds (64.4%) having at least 10 readers. This rate of attention is significantly better than any other discipline (with an average of 66.2%) [
31,
32,
34]. In the case of soil erosion modelling, around 15% of papers included in the GASEMT had at least one Mendeley reader. Around 15% of GASEMT studies had more than 10 Mendeley readers. Hassan et al. (2017) [
20] measured social media activities in 15 broad scientific disciplines indexed in the Scopus database using altmetric.com data. They concluded that altmetric indices can be a good indicator for highly cited publications. Zou and Han (2017) [
17] who studied the top 100 papers from altmetric.com in 2016 found no correlation whatsoever between AS and citation counting (Pearson correlation coefficient was 0.036). A stronger correlation between AS and number of citations was observed in this study (
Figure 3).
Table 7 shows some descriptive statistics of different altmetric.com sources for the GASEMT modelling records with AS larger than 0. Overall, the total number of policy mentions of the studies included in the GASEMT database totalled 247. Policy mentions are three times higher than the news mentions (
Table 7). In addition, Twitter is dominant among social media for soil erosion study outreach. More specifically, one can notice that on average, a soil erosion modelling paper with an AS > 0 had one policy mention, with the highest number (
n = 14) scored by Panagos et al. (2015) [
35]. The selected papers with the highest AS will be studied in more detail in the following section (i.e.,
Section 3.3). It should be noted that only 5% of papers with an AS > 0 appeared in news and around 9% in blogs. This percentage was much higher for Twitter and policy mentions, 44% and 48%, respectively.
Moreover, we analysed the temporal distribution of the GASEMT modelling records for two different subsets based on the AS value (
Figure 4). It can be seen that there were quite some GASEMT records that were published in the period before 2010 (i.e., before the significant development of social media and altmetric trackers). Thus, the higher AS of these studies can mainly be attributed to the above average number of policy mentions [
28,
36]. This could also be related to the topics of the papers that are related to the AS > 10 (
Figure S1). The trend of AS > 0 is exponential during the last 6 years (2011–2017) of the GASEMT as the number of studies quadruples compared to the period before 2011. It is clear that these papers more frequently deal with agriculture, that is, the term that does not appear on the world cloud in the case that the AS > 0 subset is shown (
Figure S1).
3.3. The Impact and Outreach of Soil Erosion Modelling Papers with Top Altmetric Score
In the following section, detailed investigation of the selected papers with an AS > 10 is provided (
Table 8). As is inferable from
Table 8, the number of Twitter and policy mentions are non-uniformly distributed, with some papers having a much larger number of mentions (
Table 8). For example, Pearson’s correlation coefficient between Twitter and news mentions for papers shown in
Table 8 totals to 0.49. It can also be noticed that some papers with a slightly lower number of citations and Mendeley readers tend to have a relatively high number of Twitter and policy mentions (
Table 8). The data reported in
Table 8 help us to notice a time lag between Twitter mentions and Scopus citations, estimated in approximately 1–2 years. It should be noted that the majority of social media activity such as tweets and news mentions occur immediately after publication, while the policy mentions usually occur 2–5 years after publication. In terms of Twitter mentions, the papers with the most mentions [
25,
35,
37,
38] were mostly mentioned by people or institutions from Italy, the UK, Chile, Spain, Belgium, the USA, and Australia. In terms of policy documents, there was no clear time lag between Scopus citations and policy mentions. Thus, these are more overlapping than the Twitter (or other social media) mentions and number of citations. In terms of origin of policy documents, the following countries or institutions are mostly using the results of research studies in different policy documents: Italy, Luxemburg (European Commission) and the USA [
25,
35,
38]. In terms of other sources that are tracked by altmetric.com, one can also mention news, which has a slightly higher absolute number of mentions, while for other sources (e.g., Reddit, Google+), there was almost no activity for the soil erosion modelling studies included in the GASEMT database. For example, Gocht et al. (2017) [
27] had more than 20 news mentions (mostly from the USA) but only one policy mention and two Twitter mentions. This larger number of news mentions also resulted in higher AS (i.e., 189) since the weight for the news mentions is also higher than, for example, for the Twitter mentions.
Since the number of Twitter mentions was higher than other media, or policy mentions, we also investigated the characteristics of the Twitter users that tweet about specific papers.
Figure 5 shows the number of Twitter posts for the GASEMT papers with AS > 10 (
Table 8). As discussed in
Section 2.1, social media such as Twitter are more widely used in Europe and North America than Africa and Asia (
Figure 5). Additionally,
Figure S2 shows the number of followers for Twitter account users that tweeted about the selected papers for the GASEMT database that have at least 10 Twitter mentions. Additionally, we select as benchmark (i.e., papers with high outreach in terms of Twitter activity), one paper with the highest AS from the
Land Use Policy journal [
39] and one from the
Landscape and Urban Planning journal [
40] (
Figure S2). It can be seen that the paper with the highest number of Twitter mentions from the GASEMT database (i.e., Borrelli et al., 2017) [
25] is comparable or even performs better than the selected two benchmark papers that are the two top papers from the selected journals. This means that the outreach of these twitter activities can be relatively significant since tweets of users with more followers are seen by a larger number of people. Furthermore, the second-ranked GASEMT paper (based on the number Twitter mentions) [
35] is also comparable to the benchmark paper published in the
Land Use Policy journal [
39]—this somehow stresses the importance of soil erosion as a theme comparable to other topics published in these two journals.
Moreover, bibliographic and altmetric characteristics of journals where soil erosion modelling studies are mostly published [
8] are also shown in
Table 8. The
Land Use Policy journal has the highest total number of policy mentions and the highest percentage of papers that are tracked by altmetric.com (
Table S1). This journal also has the second highest number of total mentions, while
Journal of Hydrology is ranked first, but it also publishes around three times as many papers. Furthermore, it can also be seen that there exist quite some differences among the journals where soil erosion modelling studies are more frequently published (
Table S1). Papers included in the GASEMT database were overall mentioned 247 times in policy documents and 1333 times on social media (
Table 8). Some journals have higher and some lower numbers of policy documents and social media mentions (
Table S1) than the GASEMT database. The total number of mentions was 1712. Hoverer, only 15% of GASEMT papers could be linked with some altmetric.com activities. A similar percentage was characteristic of Catena and Geoderma, while most other journals (
Table S1) had higher percentages.