A novel conceptualization to estimate unsaturated zone mass-fluxes and integrate pre-existing surface- and ground- water models
Authors: Kootanoor Sheshadrivasan Veethahavya , Langhammer Jakub , Class Holger , Lang Ulrich 
Published: March 2022

&lt;p&gt;A good portion of our socio-economic activity relies on groundwater - a seemingly inexhaustible supply of water. Only in the recent past has the true limited nature of groundwater resources drawn mainstream attention. Groundwater resources largely remain an &lt;em&gt;invisible &lt;/em&gt;resource, thus posing a challenge to its management and sustainable use. Replenishment of exploited groundwater reserves is a pressing issue in ensuring water security. The primary pathway of influxes into most groundwater (GW) reserves is via infiltration of surface water (SW), often through the unsaturated zone (UZ). Studying GW fluxes calls for an integrated assessment of the fluxes between GW, UZ, and SW systems - dynamically in both spatial and temporal domains. Such an integrated approach becomes even essential to study the effects of climate change on our hydrosystems.&lt;/p&gt;&lt;p&gt;Numerical modelling of GW-UZ-SW fluxes holds tremendous potential in visualising the &lt;em&gt;invisible &lt;/em&gt;resource. However, numerical modelling of hydrosystems has largely remained fragmented between hydrology and hydrogeology, for various warranted reasons.&lt;/p&gt;&lt;p&gt;Often, the cost of building and running an integrated GW-SW model outweighs its benefits. A good portion of the cost can be attributed to the need to model the UZ fluxes. Farthing and Ogden (2017) outline well, the challenges associated with modelling the UZ.&lt;/p&gt;&lt;p&gt;Although there exist numerous integrated models ranging from physically based ones to conceptual models, they have not yet convinced the mostly fragmented community of hydrologists and hydrogeologists to utilize them as general-purpose modelling tools for local to regional scales. Barthel and Banzhaf (2015) review the state of integrated GW-SW modelling at such scales. Furthermore, the need to acquaint oneself with new modelling tools adds to the cost of utilizing an integrated modelling approach.&lt;/p&gt;&lt;p&gt;In this study, we aim to design a conceptual model to adequately model UZ fluxes with the primary aim of integrating existing GW and SW models along with the help of an explicit coupling scheme. The conceptualization is inspired by the HBV model and utilizes distributed bucket-like storage compartments on which computations are performed at each discrete element and timestep over the model domain. The model primarily sets out to describe the fluxes entering and exiting the UZ, while also partitioning the precipitation as influxes into the three respective storage terms (GW, UZ, and SW), and drawing the evapotranspiration from the affected storage terms, employing the principle of mass-balance. While the UZ storage term is retained in the model, GW and SW levels are read from the respective models at the beginning of the coupling time-step and their subsequent changes are reported as discharges at the end of the time-step, making these storages virtual.&lt;/p&gt;&lt;p&gt;The model has shown promising results in a preliminary application at a peat-bog near Lake Constance. Yet it leaves plenty of room for improvement. Findings from the previous application are planned to be used in (I) testing and validation in a controlled theoretical case, and (II) application, calibration, and validation in an experimental catchment jointly maintained by the Department of Physical Geography and Geoecology of Charles University in the Sumava Mountains.&lt;/p&gt;