Up until this point, there has been no unifying effort to create a global map of groundwater tables. However, a global, comprehensive map of the location and depth of water tables throughout the world can help with finding global patterns of groundwater movement. Fan et al. compiled all existing government records of groundwater tables from over a million well sites around the globe. Where government records were not available, they used data from published literature. The compiled map was not complete though; the water tables in many places around the globe remain unrecorded. In order to look more closely and completely at global, regional, and local trends in groundwater distribution and depth the researchers also used a pre-existing groundwater table model. They found that groundwater tends to be shallowest in the most humid climates, in wetland regions, in arid valleys and along the edges of continents, especially in areas with long, flat, plains of wetlands leading up to the coast. Additionally, the model looked for the influence of three forces—climate, terrain, and sea level—on the water table depth (WTD). While sea level has the strongest influence on WTD globally, regionally climate and topographic gradient are most important, and locally they found that terrain can override climate boundaries and lead to climatic anomalies like oases. —Alison Marks
Fan, Y., Li, H., Miguez-Macho, G. Global Patterns of Groundwater Table Depth. Science 339, 940–943.
Fan et al. began by compiling pre-existing data about groundwater abundance and depth into a global map. The process included using data from 1,603,781 well sites throughout the world. This map, while unique in that these data had never been compiled together before, was not highly comprehensive as it only covered areas that had been surveyed. Many areas in Africa and Asia were completely blank. With the partial map, Fan et al. were able to determine a few trends. They found that WTD is usually less than 5 meters deep, especially in humid climates (wetlands) and arid valleys (all of the water from the area converges to one place).
The researchers then used a previously developed groundwater flow model, which offers a comprehensive, but simpler, view of the movement and depth of groundwater throughout the world. The model also doesn’t consider any possible anthropogenic forces that could be affecting the groundwater supply, including pumping, irrigation, and the dumping of wastewater. Despite its low resolution, the model allowed for the researchers to identify certain trends in WTD across climates, terrains, and relation to sea level. Globally, sea level greatly affects WTD; along the edges of continents and along coastal wetland plains the land tends to be saturated and the WTD tends to be shallow. Regionally, climate is the primary factor that influences WTD. Deserts tend to have deep WTD with low recharge, whereas tropical swamps tend to have shallow WTD with a high rate of recharge. Topographic gradient also influences WTD at a regional level, where especially flat lowlands can lead to large inland freshwater wetlands like those seen in Central America. Finally, the authors found that at a local level, the terrain is the most important factor. Basins, cliffs, flatlands, and mountains can all confound the previously mentioned trends at a local level, leading to anomalies like oases in the middle of arid basins due to concentrated water collection in one place.
Better understanding the depth of water tables is important for many reasons. Not only can understanding the flow and depth of groundwater help better inform our understanding of the water cycle, it can inform how we interact and think about the areas that we are in. Understanding how groundwater moves throughout our world, both locally and on a larger, global scale, can help to inform important decisions about groundwater use. Through compiling a map of existing data, Fan et al. not only created a resource where one did not exist before, but they also pointed out holes in the current understanding of the great pools and rivers of water that exist below earth’s surface. Though the results of this study are not complete or definitive, they hopefully can inspire further research into groundwater data collection and analysis.