Groundwater budget rationale, time, and regional flow: a case study in Istanbul, Turkey

The groundwater balance is based on the principle that the water entering and leaving any aquifer is considered to be equal over a certain time-interval, taking into account the change in storage. However, in reality, determining a groundwater balance is complicated by the fact that it may take a very long time-interval to achieve a water balance in an aquifer and circulating groundwater may vary greatly in age, and may have been recharged under different climatic conditions in the past. Accounting for all of the inflows and outflows from a groundwater flow system including changes in the future is called a groundwater budget. Until the 1950s, sustainable groundwater management was made with the understanding that groundwater abstraction should not exceed the natural recharge, and the permission for groundwater abstraction is still given with the same approach in Turkey. This approach has globally been declared archaic since the 1980s, and the calculation of water captured from discharge, and recharge has come to an approach that ignores the recharge. Not fully understanding the overlapping groundwater communicating time and regional fast-flow pathways are the main reason for the water budget controversy and modeling mistakes. Estimating the groundwater budget correctly is crucial to controlling global groundwater depletion and floods. It is imperative to revisit the groundwater budget discussions due to these two major environmental problems seeming to be in opposition.