Citation: Caputo, M.C.; De Carlo, L.; Turturro, A.C. HYPROP-FIT to Model Rock Water Retention Curves Estimated by Different Methods. Water 2022, 14, 3443. https:// doi.org/10.3390/w14213443 Academic Editor: Achim A. Beylich Received: 13 September 2022 Accepted: 23 October 2022 Published: 29 October 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 HYPROP-FIT to Model Rock Water Retention Curves Estimated by Different Methods Maria Clementina Caputo * , Lorenzo De Carlo and Antonietta Celeste Turturro CNR National Research Council, IRSA Water Research Institute, Via Francesco De Blasio 5, 70132 Bari, Italy * Correspondence: maria.caputo@ba.irsa.cnr.it; Tel.: +39-080-582-0536 Abstract: The water retention curve, which relates the matric potential, ψ, to the water content, θ, is essential to describe the flow processes in the unsaturated zone and provides useful information for environmental and engineering applications. There are few studies devoted to measuring the rock water retention curves due to the rock’s tightness, which makes it more technically difficult to use specific methods. In this study, we tested four different methods to measure water retention curves of two lithotypes of carbonate porous rocks with the aim to find the most effective to be applied to rock samples. Suction table, evaporation, Quasi-Steady Centrifuge, and WP4-T dewpoint potentiometer methods have been applied. The Quasi-Steady Centrifuge method proved to be the only one capable of determining water retention curves in the entire water content range and capturing the bimodality of the tested media with respect to the other methods. The measured water retention data were fitted with HYPROP-FIT software that allows us to accurately describe the WRCs and obtain critical parameters for the numerical simulation of flow and transport through the vadose zone, which plays a key role in various environmental issues. Keywords: unsaturated carbonate rock; Quasi-Steady Centrifuge method; WP4-T dewpoint potentiometer; evaporation method; suction table method; bimodal hydraulic functions 1. Introduction The unsaturated zone, the portion between the land surface and the top of the phreatic zone, is critical because (i) it controls the water movement from the land surface to the groundwater, (ii) it strongly affects the aquifer recharge rate, and (iii) it controls, in terms of flow rates and chemical reactions, whether, where, and how fast contaminants reach the groundwater. Understanding the flow and transport processes in the unsaturated zone is therefore crucial in determining the quantity and quality of groundwater available for human use. Knowledge of the water retention function, ψ(θ), which correlates the matric potential, ψ (kPa), to the water content, θ (m 3 ·m −3 ), yields information for evaluating, analyzing, and predicting unsaturated flow [1]. Several methodologies have been tested over the years to estimate the water retention curves (WRCs). These methods typically work well for unconsolidated media, generally soils, but the experimental determination of the rock water retention curve is still very difficult [2]. Because the unsaturated zone is often constituted by different layers of rock, it is important to increase the knowledge of the applicability of various experimental methods to rock samples and investigate better models to fit the measured data. For this reason, in recent decades, several studies consisting of WRCs determination on rock samples with different aims have been carried out. Luquot et al. [3] monitored the WRCs and other hydraulic properties of carbonate rocks during different dissolution experiments. They used the centrifugal method and reported on dissolution experiments induced by injecting water with different pH values into four core samples. Rotting et al. [4] investigated the change in WRCs of sedimentary carbonate rocks attacked by permeation Water 2022, 14, 3443. https://doi.org/10.3390/w14213443 https://www.mdpi.com/journal/water