<p>Due to continuous changes in the meteorological conditions of Mediterranean regions, it is becoming increasingly important to improve knowledge of hydrological and hydrogeological recharge processes and their dependency on climate conditions to adapt the use of limited water resources. Within the IsoMed project (isotope hydrology in Mediterranean areas), soil profiles were sampled in November 2018 and February 2019, from various hydrogeological settings in Cyprus to estimate groundwater recharge using stable isotope equilibration methods combined with soil water balance modeling. A total of 11 soil profiles were taken from the Troodos massif (Galata and Platania) and the Mesaoria plain in Deftera, Nicosia. A vertical profile of stable isotopes has been determined with a 2 cm resolution and measured with Tunable Diode Laser spectrometry. Percolation through the soil profile has been estimated based on the convolution of a seasonal input function using advection-dispersion transport models. In Galata, groundwater percolation estimates range from 20-30 mm/y on clayey soil with natural vegetation to 100-120 mm/y at an irrigated terraced orchard. The results in Platania vary from 20-60 mm/y at steep hillslopes under natural vegetation and amount to 220-340 mm/y in the root zone at the irrigated site with olive trees in Deftera. The comparison of groundwater percolation rates based on stable isotope profiles with those derived from soil water balance modeling indicates a significant bias. While percolation rates correspond well to results obtained from a daily soil water balance model for irrigated fine-grained soils in the plain, recharge rates obtained from stable isotope profile methods on coarse-grained hillslopes tend to be much lower than expected. The observed bias suggests that stable isotope methods, regardless of water extraction or equilibration technique, mainly record the isotope signal of matrix flow. Thus, macro-pore and preferential flow components in coarse-grained soils may not be accounted for. Data collected from the same profiles in late autumn and spring suggest that macro-pore and preferential flow constitute a major component of percolation in coarse-grained shallow hillslope soils of Troodos indeed, without leaving measurable isotope traces in the soil water profile. Additional approaches need to be applied in conjunction with methods based on the evaluation of soil water isotope profiles to overcome this limitation.</p>

<p>To improve knowledge of hydrological and hydrogeological flow processes and their dependency on climate conditions it is becoming increasingly important to integrate sensors technology, independent observation methods, and new modeling techniques. Established isotope methods are usually regarded as a supplement and extension to classical hydrological investigation methods but are rarely included in soil water balance models. However, the combination could close knowledge gaps and thus lead to more precise and realistic predictions and therefore to better water management. Within the Wasserpfad project, a project of the Department of Civil Engineering at the TH Lübeck, soil moisture has been measured since May 2018. SMT100 soil moisture sensors from TRUEBNER GmbH are used at depths of 20, 40, 60, and 80 cm. Next to the station a 2m deep soil profile was taken in 2020, to estimate groundwater recharge using stable isotope equilibration methods and cryogenic extraction combined with soil water balance modeling. Vertical profiles of stable isotopes have been determined with a 10-cm resolution and measured with Tunable Diode Laser spectrometry. Percolation through the soil profile has been estimated based on the convolution of a seasonal input function using advection-dispersion transport models. Percolation rate estimate based on environmental isotope profiles results in 230 mm per year. Fitting of the advection-dispersion equation using a sinusoidal isotope input fitted to available time series provides an estimate of 255 mm per year. This difference is due to the dispersion effect on the isotope minima and maxima. The result of modeling the soil moisture data with a soil water balance model integrating the Richards equation for water transport and Penmen-Monteith based calculation of actual evaporation is used to verify the percolation rates. The analysis of soil moisture and isotope data by modeling provides a direct and efficient way to estimate the percolation rate. The combination of isotope methods with classical hydrological measuring techniques offers the possibility to verify results, to calibrate models, or to investigate the limits of isotope methods. Thus, flow processes can be predicted more reliably in the future.</p>

<p>It is becoming increasingly important to develop simple and efficient models for the assessment of groundwater recharge to improve sustainable groundwater management strategies. Reliable data on environmental fluxes can be collected on-site and for corresponding periods, providing information on groundwater recharge only after measurement campaigns covering several years. However, the large amount of hydrological data necessary for some methods to estimate recharge is not always available. To address these issues, a specific model for the evaluation of vertical water fluxes in the critical zone was developed. The model is directly based on soil moisture sensor data in the critical zone and does not require modeling of meteorological processes, nor time series of precipitation, temperature, relative humidity, global radiation, or sunshine hours. Based on the numerical solution of the Richards equation and using Van Genuchten relationships for soil parameterization, percolation rates and subsequent groundwater recharge are derived from potential gradients and variations of volumetric soil moistures at different depths in the critical zone. Model results are evaluated with a detailed performance analysis with several mathematical error indices and graphical methods. The resulting fluxes have been validated with an independent stable isotope profile approach confirming the applicability of the model. The model can be applied to a soil moisture monitoring station with three monitoring sensors in the critical zone to derive daily time series of water fluxes.</p>