Modeling falling groundwater head declines in major cities of the world: current situation and future projection

Over-consumption groundwater use is one of the major drivers in the hydrology of many major cities in the world, particularly in delta regions. However, a global assessment to identify cities with declining groundwater table problems has not been done yet. In this study we used the global hydrological model PCR-GLOBWB (10 km resolution) to do so. Using this model, we calculated groundwater recharge and river discharge/surface water levels, as well as groundwater abstraction. The output of PCR-GLOBWB model was then used to force a groundwater MODFLOW-based model simulating spatio-temporal groundwater head dynamics, including groundwater head declines in all major cities - mainly in delta regions - due to escalation in abstraction of groundwater to meet increasing water demand. Using this approach, we managed to identify a number of critical cities having groundwater table falling rates above 25 cm/year - average in the period 2000-2010 - such as Jakarta, Barcelona, Houston, Los Angeles, Mexico City, Rome and many large cities in China, Libya, India and Pakistan, as well as in the Middle East and Central Asia regions. However, our results overestimate head declines in Tokyo and some other places where groundwater depletion has been aggressively managed (e.g. groundwater abstraction has been minimized and replaced by importing surface water from other places). 

Currently, we are expanding this modeling simulation for the future period (i.e. until 2100), considering different climate scenarios (RCPs: 4.5 and 8.5) and socio-economic conditions (SSPs: 2 and 5). Our simulation results show that new and more cities with falling groundwater head problems would occur in the future, not only due to climate change (i.e. in areas that become dryer), but also due to increasing population, as well as expansion of existing urban areas and development of new urban areas.