## 1 Introduction  
Flood protection in the Netherlands is assured by multiple levels of dikes. A majority of this dike network defending the country from internal waterways referred to as regional dikes, originates from the historic practice of land reclamation, with both dike body and foundation consisting of heterogeneous soft soil deposits, such as clays, peats and silts [1]. The outer and inner water levels of these dikes are almost constant in time, due to careful regulation of the waterway and polder water levels by the waterboards. Therefore, changes in the water content of the dike body depend on the interaction with the atmospheric boundary conditions and the activity of the vegetation [2, 3]. To protect the body of the dike from extreme changes in water content, hence reducing the hazard of cracks and degradation phenomena, they are protected by a top clayey layer underlying the vegetated cover. The combination of a vegetated topsoil with a 50 cm-thick on average clayey layer is claimed to be enough to avoid seasonal swelling or shrinkage, changes in unit weight and changes in the stress state [4; 5]. However, measurements taken in the body of various regional dikes in recent years demonstrate that huge variations are possible under increasing climate pressure. For example, at the Leendert de Boerspolder site, where a stress test was carried out over more than one year, the data showed that despite the clayey top material, interaction with the atmosphere led to oscillations of the piezometric surface of about 0.7 m [6], which are extremely relevant compared to the total 1.9 m height of the water defense. To better understand how the top clayey layer behaves under atmospheric stresses and provide the necessary information to analyze its influence on the time-dependent response of water defense, an experimental and numerical study was undertaken, which included an investigation in the laboratory of the unsaturated behavior of the top clayey layer subjected to drying and wetting cycles in a small physical model set up in a HYPROPTM device. Different from previous research, where HYPROP data were just used to derive an \"average\" water retention curve under simplifying assumptions, the test data were interpreted as a boundary value problem, fully dependent on space and time. A novel test protocol was designed to simulate climate actions, aiming to mimic the behavior observed in the top layer of dikes in the field. Different samples retrieved from the upper part of the regional dike at the Leendert de Boerspolder were tested both upon drying and wetting under relatively controlled boundary conditions. In the device, which was chosen as a small replica of the field cover, two tensiometers allow for direct measurement of suction at two depths and provide an indication of the delayed response with depth during evaporation and infiltration. The total mass of the samples was continuously recorded as well as the temperature at the bottom of the samples. Given that these measurements are taken during a continuous dynamic process, their interpretation is not as straightforward as usually assumed. If the tested material is clay, the material properties (water retention and hydraulic conductivity) cannot be directly inferred from the test data. In fact, the assumption that the average of the two measured suction values equals the suction corresponding to the average water content of the sample is strongly debatable when testing soils with low hydraulic conductivity, where the water content profile is very far from being linear [7]. To overcome the oversimplifying assumptions, the tests were simulated numerically using CODE BRIGHT [8; 9] by means of a finite element discretization of the time-dependent coupled thermohydraulic response of the sample, including both liquid and vapor phases. The numerical simulation allows analyzing the response of the soil sample in detail, and better inferring the inherent properties of the tested material.