Effects of historic changes in regional drainage characteristics on the drawdown of groundwater abstractions

Calculating the drawdown of groundwater abstractions for drinking water is usually done considering the current land use and regional drainage characteristics. However, many drinking water abstractions already exist for several decades and abstracted volumes have increased over time. In the Netherlands, especially in the more elevated parts of the country, the drainage characteristics were also significantly altered to prevent water logging and to optimize the water management for agricultural use, often after establishment of the groundwater abstraction site. These changes were intended to lower the phreatic groundwater levels to prevent waterlogging, but unintendedly also made the regions more vulnerable to drought. The question is whether groundwater abstractions for groundwater would have a similar impact in the former historic hydrological context and whether restoring the system to this state would ameliorate current drought problems.

In this study we investigated whether a drinking water abstraction would have the same drawdown if the regional drainage characteristics would not have been altered and whether restoring the historic situation would decrease drought impacts. First, a literature study was conducted to understand the changes to the drainage system over time. These changes were then implemented in a regional groundwater model (based on Modflow) for a conceptual region, representative for the eastern part of the Netherlands. Results from both the literature study and the groundwater model indicated that changes in the drainage system lowered the groundwater levels by tens of centimetres (differences ranged from 20 to 100 cm). Drawdown from the drinking water abstraction was larger in the historical situation than in the current situation, even though groundwater levels were higher. In the historical situation less reduction in transpiration occurred, leading to a lower recharge of the groundwater and thus a larger drawdown. However, when irrigation was applied, this effect was not found.  This implied that a correct estimate of groundwater recharge is crucial to calculate drawdown from abstractions. Recharge depends on actual evapotranspiration, of which the conceptualization in regional models could be improved. Returning the drainage system to the historical situation leads to higher groundwater levels, thereby reducing the drought impact, but also increasing the risk of oxygen stress in crops. More research with regard to the impact on crop yields is needed on local scale, before measures to mitigate drought impacts can be taken.