A large-scale analysis of existing water resource models used around the world has found some key processes are missing, which means estimates of water availability could be wrong and cannot be reliably used for making water management decisions.
The international team of scientists found that important climate change impact models are wrong in some areas. One finding was that groundwater recharge estimates for the karst regions of Europe, the Middle East, and North Africa are much higher than previously estimated from current large-scale models. These areas are home to some 560 million people.
The researchers say that this “suggests that more work is needed to ensure sufficient realism in large-scale hydrologic models before they can be reliably used for local water management.”
Groundwater Recharge Rates
Groundwater, water trapped below the earth’s surface in fissures and crevasses, is a key resource in many areas. When precipitation falls, any that is not used by plants may flow into lakes or other surface bodies of water. Some may evaporate. Other water seeps underground into aquifers.
Pumping too much water out of an aquifer before it can be naturally replenished can overdraft the groundwater, causing wells to run dry. Accurately assessing the groundwater recharge rate is essential to ensuring existing supplies are adequate for drinking water and irrigation.
In their assessment, researchers compared two hydrological models for simulating groundwater recharge over large regions. One, an established global model, doesn’t fully account for differences in soil, such as density and permeability. This may affect recharge rates. This model was compared with one the researchers developed, which is based on continental landmasses.
The models were compared using the karst regions of Europe, North Africa and the Middle East. These areas have geologically diverse features created by carbonate rock, which is very susceptible to chemical weathering. This process, karstification, accounts for a wide variation in soil depth and permeability. The researchers explained:
Aquifers from these regions contribute up to half of the drinking water supply for some European countries. Our results suggest that water management for these regions cannot rely on most of the presently available projections of groundwater recharge because spatially variable storages and spatial concentration of recharge result in actual recharge rates that are up to four times larger for present conditions and changes up to five times larger for potential future conditions than previously estimated. These differences in recharge rates for strongly heterogeneous regions suggest a need for groundwater management strategies that are adapted to the fast transit of water from the surface to the aquifers.
More Groundwater Available
The new model accounts for less surface runoff and evaporation, which means the groundwater recharge rate is greater. Regions where existing models have been used may have significantly more groundwater available.
The researchers compared the models with independent groundwater recharge observations at 38 sites. They concluded that their model, which takes into account differences in soil substrata, produces more realistic estimates.
Thorsten Wagener, who worked on the project, is a professor of Water and Environmental Engineering in the Department of Civil Engineering at the University of Bristol in the United Kingdom. He said:
When applied to the example of karst regions, our approach shows how it is possible to adapt global models used to predict water shortages, drought or floods to account more realistically for regional conditions. This approach will ultimately enable both local management and large-scale assessments within the same model.
Along with the University of Bristol researchers, scientists from the University of Freiburg, Germany; the University Victoria, Canada; and the International Institute for Applied Systems Analysis, Austria, contributed to the work.
The research — “Enhanced Groundwater Recharge Rates and Altered Recharge Sensitivity to Climate Variability Through Subsurface Heterogeneity” — was published in Proceedings of the National Academy of Sciences.