  Citation: Li, Y.; Rodriguez, F.; Berthier, E. Development and Evaluation of a Saturated Zone Module in an Integrated Urban Hydrological Model. Water 2022, 14, 1030. https://doi.org/10.3390/ w14071030 Academic Editor: Maria Mimikou Received: 7 February 2022 Accepted: 18 March 2022 Published: 24 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). water Article Development and Evaluation of a Saturated Zone Module in an Integrated Urban Hydrological Model Yinghao Li 1 , Fabrice Rodriguez 2,3, * and Emmanuel Berthier 1 1 Cerema, Equipe Team, 12 rue Teisserenc de Bort, F-78190 Trappes, France; yinghao.li@mail.com (Y.L.); emmanuel.berthier@cerema.fr (E.B.) 2 Department of GERS/LEE, Campus of Bouguenais, University of Gustave Eiffel GERS-LEE, F-44344 Bouguenais, France 3 IRSTV FR CNRS 2488, rue de la Noe, F-44321 Nantes, France * Correspondence: fabrice.rodriguez@univ-eiffel.fr; Tel.: +33-(0)2-4084-5878 Abstract: Shallow urban groundwater interacts with surface water and underground infrastructures. Low-impact development in urban water management by at-source infiltration should consider shallow urban groundwater in a holistic manner. Traditional hydrological models, however, rarely detail groundwater flows and their interaction with urban runoff and the water budget. In the present study, a new approach is proposed, using the integration of a flow module WTI for the saturated zone in a distributed urban hydrological model URBS-MO. This integration is carried out by paying attention to retaining the initial waterflow subsurface parameterization. The performance of the integrated model is evaluated by piezometric and runoff data in an experimental urban catchment, through a sensitivity analysis and a manual calibration of the main model parameters, as well as a validation step. The new module shows its capacity to improve groundwater flow simulation by assessing more realistic water table variations, along with a very small improvement of flowrate simulation. The bias on the average groundwater level was reduced from +14 to +7% for the one-year validation period. The modelling results show the importance of parameter calibration for distributed physically-based hydrological models. Difficulties in the calibration of parameter values due to spatial heterogeneities are also revealed, as the use of piezometric data for the calibration of a hydrological model is rather innovative. Keywords: urban hydrology; groundwater; drainage; modelling; integrated; sewer network; URBS-Model 1. Introduction Urban water is a complex system combining natural water compartments and artificial infrastructures, particularly water supply and sewer systems that modify the natural water cycle. Water flows in the unsaturated and saturated soil layers interact actively with the other compartments. The assessment of the catchment water balance by quantifying these interactions has been the subject of intense studies, but remains one major challenge in urban hydrology [1]. Moreover, low-impact development (LID) practices are being developed worldwide as a way to mitigate the effects of urbanization by preserving pre- development hydrology [2]. Any LID practice for stormwater runoff mitigation can be really efficient only if it does not come into conflict with the equilibrium of the water balance. Moreover, given that most of these LID are infiltration-based, they may significantly affect the groundwater recharge and the base flow [3,4], even though the relation between stormwater infiltration facility insertion and groundwater elevation modification is not straightforward [5]. The urban water system has particularities that distinguish it from natural hydrology, namely reduced temporal and spatial scales, high spatial heterogeneities on the surface and Water 2022, 14, 1030. https://doi.org/10.3390/w14071030 https://www.mdpi.com/journal/water