## 4 Results and Discussion
### Publication Trends  
Over time, the quantity of research articles on ML and MM techniques for groundwater level change prediction has expanded. Between 2000 and 2015, the use of ML and MM approaches for predicting groundwater level changes increased and decreased within the same period (Figure 4), but it began to gain traction again after 2016. From 2000 to 2016, thirty-seven articles accounted for 31.62 percent of all primary research reviewed. There were no papers published in 2001, 2005, or 2013. Based on the study's goal, there was a surge in 2021, which represents 28 (23.93 percent) of the 117 reviewed papers on ML and MM techniques. Since 2016, publications in the domain saw an exponential increase up until 2021, i.e., the steepest slope. In 2019, the number of publications in the area climbed from six to sixteen, increased by one in 2020, and increased by eleven in 2021. As of the time the survey was conducted in 2022, nine articles had been recorded.  
Based on the findings, the primary articles (117 articles) were also slanted towards publishing firms/outlets. Out of the primary articles, Elsevier BV had the most papers with 30 (25.64%), followed by Multidisciplinary Digital Publishing Institute (MDPI) with 20 (17.09%), Springer Science and Business with 14 (11.97%), and Blackwell Publishing Limited with 10 (8.55%). These four publishers published 63.25 percent (74) of the 117 primary articles (Figure 5). The fewest number of articles recorded by a publishing house was one. The remaining publishing houses only have eight or fewer studies.  
We discovered that the primary studies (117 articles) are skewed by country in this systematic review. China had the most corresponding authors with thirty-eight, followed by the United States (US) with twenty-five, and the United Kingdom (UK) with nine (Figure 6). According to the associated writers' addresses, the majority of the papers originated in Asian countries. Asia was responsible for 56 of the 117 articles (47.9 percent). North America had twenty-five articles, while South America had three. Africa was represented by two articles: from South Africa and Egypt. Figure 6 summarizes the distribution of publications based on geographic location.  
The proportion of the top five countries is also shown (Figure 7). China accounted for 32% of all primary studies, the US for 21%, the UK for 8%, Iran for 4%, and Portugal, Japan, and Spain for 3% each.  
Predictor Variables for Modeling GWL Changes  
The primary articles used in this study used input data to predict groundwater level changes and address their impact, and our study will look at the climatic data from those studies as well. According to NASA.gov accessed on 9 June 2022 [91], \"climate change is a shift in the usual weather found in a place\". Groundwater level changes are influenced by climate conditions [92], and climate change is threatening our future. Extreme occurrences such as droughts and floods, as well as rising precipitation variability due to climate change, have a severe impact on groundwater quantity and quality. Precipitation, temperature,  
Figure 4: Publication trends in primary articles from 2000 to 2022.  
Figure 5: Publication outlets from the primary studies.  
rainfall, Gravity Recovery and Climate Experiment (GRACE) data, ENVISAT advanced synthetic aperture radar (ASAR) satellite data, and lithology are among the many predictor variables. The primary studies' predictor factors are provided (see Table 4).  
The groundwater level changes were predicted with the predictor factors from our primary investigations (117 articles) (Table 4). Precipitation was used in many of the studies. According to education.nationalgeographic.org accessed on 9 june 2022 [93], any liquid or frozen water that forms in the atmosphere and falls back to Earth is known as precipitation. Rain, sleet, and snow are only a few examples.