Simulating Climate Change Impacts on the Recharge Dynamics of a Mediterranean Karst Aquifer
Authors: Bresinsky Lysander , Kordilla Jannes , Thoenes Emanuel , Würsch Thibault , Engelhardt Irina , Sauter Martin 
Published: March 2020

&lt;p&gt;&lt;span&gt;Integrated surface-subsurface flow models &lt;/span&gt;&lt;span&gt;solving the Richards&amp;#8217; equation &lt;/span&gt;&lt;span&gt;allow t&lt;/span&gt;&lt;span&gt;o simulate&lt;/span&gt;&lt;span&gt; flow within all compartments (e.g. vadose, phreatic and surface zone) and their reciprocal interaction, &lt;/span&gt;&lt;span&gt;and &lt;/span&gt;&lt;span&gt;provid&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt; a useful tool to investigate the impact of climatic changes on infiltration dynamics. The Mediterranean karst aquifers are in particular prone to shifting climates, as a decrease in mean precipitation with increasing intensity and frequency of short-duration extreme rainfall, may have a significant impact on recharge dynamics and the overall water budget. Here, we use the finite element, distributed, multi-continuum flow simulator HydroGeoSphere (Aquanty, 2015) on a high-performance-computing platform to simulate infiltration and groundwater flow of the Western-Mountain-Aquifer (WMA) c&lt;/span&gt;&lt;span&gt;onsidering&lt;/span&gt;&lt;span&gt; chang&lt;/span&gt;&lt;span&gt;ing&lt;/span&gt;&lt;span&gt; hydrologic conditions. A thin soil cover and abundant exposed bare karstic carbonate rock compose the recharge area, providing efficient pathways &lt;/span&gt;&lt;span&gt;for&lt;/span&gt;&lt;span&gt; fast direct infiltration along karst features (e.g., sinkholes and dolines). The lowered total annual precipitation may not result in a decrease in recharge since the severity and frequency of individual rainfall storms are projected to increase. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;To account for the duality of karst flow dynamics, both in the vadose and phreatic zones, with rapid flow through conduits and slow flow through the fractured rock matrix, we apply a double-continuum approach based on the volume-effective Richards&amp;#8217; equation with van Genuchten parameters. A 2-D friction-based overland flow continuum, coupled via a first-order exchange term to the subsurface, accounts for overland flow d&lt;/span&gt;&lt;span&gt;ue&lt;/span&gt; &lt;span&gt;to&lt;/span&gt;&lt;span&gt; infiltration excess. This allows t&lt;/span&gt;&lt;span&gt;o represent&lt;/span&gt;&lt;span&gt; the partitioning of rainfall into diffuse and rapid direct recharge, e.g., along dry valleys or sinkholes. This modeling approach, therefore, accounts for complex spatially distributed infiltration characteristics of the rock-soil landscape, with focused recharge along karst features and transmission losses of ephemeral streams (wadis) under variable precipitation patterns.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;To get a better understanding of the complex interaction dynamics of the surface and subsurface domain in a coupled unsaturated single- and dual-continuum model we carried out small-scale process-oriented studies. Two types of synthetic karst features, (1) a dry valley averaged from field data (i.e., Wadi Natuf) and (2) an analytical generalized doline, were investigated. Geometries close to natural systems, such as sub-catchments of the WMA, were implemented. Sensitivity studies reveal complex dependencies of domain properties on linear- and log-scales. However, the exchange parameters controlling the coupling between the subsurface continua are insensitive.&lt;/p&gt;