Unraveling the Potential of Controlled Drainage: Enhancing Freshwater Availability and Combating Salinization in Dutch Polders

Abstract

Over the last two decades, coastal aquifers have faced intensified saltwater intrusion due to sea level rise and land subsidence. In the Netherlands' low-lying coastal zones, models predict a doubling in salt loads by 2100, with serious implications for freshwater availability for agriculture, consumption, and industry. The impact of climate change on precipitation and evapotranspiration further complicates this issue, potentially reducing rainwater infiltration and increasing the ascent of saline groundwater, jeopardizing crop development. This study focuses on the Dutch polders, where agricultural lands are defended by dikes and managed with subsurface drainage systems that cover a significant portion of the area. Such systems, while essential for streamlining drainage, may also facilitate the upward movement of saline water, thus salinizing the freshwater lens.  This study addresses the urgent need for water conservation strategies in the face of climate change, urbanization, and increased groundwater extraction. We propose a climate-adaptive water management framework that emphasizes the sequential retention, storage, and discharge of water. Central to this framework is the implementation of controlled drainage systems, designed to minimize water loss and mitigate peak discharge flows, thereby conserving water during dry spells and preventing flooding during excessive rainfall. Our research aims to explore the role of drainage networks in mitigating aquifer salinization and improving freshwater management in these polders. By employing density-dependent numerical modeling that integrates unsaturated and saturated zones, we investigate the potential of controlled drainage strategies under varied hydrogeological conditions and design parameters. These include soil characteristics and meteorological scenarios spanning dry to wet years, as well as the design of drainage systems, such as the depth and spacing of drains. The findings aim to elucidate the dynamics of the freshwater lens, including the mixing processes, salinity thresholds at the root zone, and overall groundwater quality. Ultimately, the study seeks to provide a comprehensive framework for determining when and how controlled drainage can maximize freshwater availability and support sustainable agricultural practices in coastal regions vulnerable to saltwater intrusion.