Hydrological calibration in data-scarce catchments is challenged by non-stationary regimes, fragmented data, and systematic measurement errors. Conventional calibration approaches often assume continuous records and rely on standard performance metrics, which can bias calibration toward high flows and exacerbate parameter equifinality—ultimately reducing robustness under data limitations. This study provides a systematic comparison of three calibration strategies—Kling–Gupta Efficiency (KGE), a non-parametric variant (R_NP), and Flow Duration Curve (FDC)-based calibration—together with their time-consistent counterparts (SKGE, SR_NP, and SRMSE). All schemes are implemented for the lumped HBV-type TUW model across nine catchments in southern Italy and evaluated using independent metrics targeting overall hydrograph agreement, high-flow behavior, and FDC quantile matching (Q5–Q95). The results reveal that the time-consistent KGE-based strategy excels during in calibration (NSE = 0.56, RMSE = 4.65 m³/s) but shows notable declines in validation (NSE = 0.40, RMSE = 3.91 m³/s), indicating sensitivity to non-stationarity. The R_NP-based approach demonstrates enhanced validation robustness (NSE = 0.51, RMSE = 3.60 m³/s) and low-flow accuracy, with NSE_lnQ = 0.30 and low-flow accuracy, leveraging its non-parametric structure. The SR_NP variant further enhances performance in validation (NSE = 0.52, RMSE = 3.42 m³/s), along with superior low-flow performance (NSE_lnQ = 0.48). The FDC-based strategy effectively reproduces flow distributions during calibration (NSE = 0.41, minimal PBIAS = −0.03%) but exhibits limited temporal transferability (validation NSE = 0.25, RMSE = 4.50 m³/s). Time-consistent variants reduce parameter dispersion by approximately 2–8% (relative to full-period calibration) and improve validation metrics by 5–15% across all catchments. Overall, time-consistent calibration provides a practical pathway to increase robustness under non-stationary, data-scarce Mediterranean conditions, highlighting a systematic trade-off between calibration accuracy and validation reliability.

Climate and environmental changes are impacting the hydrological water cycle, affecting water availability and having negative consequences for water security. There are numerous practices in place to address this challenge, one of which is utilizing nature in the form of Nature-based Solutions (NbS). NbS include various interventions, such as green roofs, urban wetlands, permeable pavements, and restored riparian corridors, all inspired by, supported by, or mimicking nature. NbS are emerging as a transformative approach that leverages ecological processes to address societal challenges while delivering multiple co-benefits. However, the application of NbS at a large scale, e.g., Catchment scale, is a challenging task due to constraints in the practicality of these solutions. One major challenge is identifying potential solutions and modeling the impact of these solutions, which seems a straightforward task but presents practical difficulties.This study focuses on identifying and quantifying the impact of solutions on water availability utilizing the DREAM hydrological model. The case study is conducted in the German catchment, the Bode River Basin. Water management in the Bode is a crucial issue for authorities, as it faces extreme events such as droughts and has experienced significant deforestation in recent years. This approach first identified the potential NbS for the catchment using the catchment-scale framework (Sarwar et al., 2025). Then, those selected solutions were modeled, such as the construction of an infiltration basin, using the site's ecological features. Then, to evaluate the effect of these interventions on the water budget, baseline (without solutions) scenarios were compared to scenarios with solutions. Results showed that the total discharge of the basin is significantly affected, with a 5-10 percent decrease in flows. However, in the locations where infiltration basins were constructed, there has been a higher reduction in runoff volume and an increase in groundwater recharge.

The growing number of extreme hydrological events observed has raised the level of attention toward the impact of climate change on rainfall process, which is difficult to quantify given its strong spatial and temporal heterogeneity. Therefore, the impact of the climate cannot be determined on the individual hydrological series but must be assessed on a regional and/or district scale. With this objective, the present work aims at identifying the trends and dynamics of extreme sub-daily rainfall in southern Italy in the period 1970-2020. The database of annual maxima was assembled using all available rainfall data (provided by the National Hydrographic and Mareographic Service - SIMN, and the Regional Civil Protection). However, due to the numerous changes (location, type of sensor, managing agencies) experienced by the national monitoring network, the time-series were found to be extremely uneven and fragmented. Since the spatio-temporal discontinuity could invalidate any statistical analysis, gap-filling techniques (deterministic and/or geostatistical [Teegavarapu, 2009]) were applied to reconstruct the missing data. In particular, the “Spatially-Constrained Ordinary Kriging” (SC-OK) method [Avino et al., 2021] was used, namely a mixed procedure that adopts the Ordinary Kriging (OK) approach with the spatial constraints of the Inverse Distance Weighting (IDW) method. The SC-OK method allows to reconstruct only missing data for stations selected by the IDW method (those with a sufficient number of functioning neighbouring rain gauges). Then, the reconstructed dataset has been used to explore trends and regional patterns in annual maxima highlighting, how rainfall are evolving in the most recent years.REFERENCESAvino, A., Manfreda, S., Cimorelli, L., and Pianese, D. (2021). Trend of annual maximum rainfall in Campania region (Southern Italy). Hydrological Processes, 35.Teegavarapu, R.S.V. (2009). Estimation of Missing Precipitation Records Integrating Surface Interpolation Techniques and Spatio-temporal Association Rules. Journal of Hydroinformatics, 11(2).

In the last few years, several studies have detected changes in the rainfall regime which may have an impact on hydrological extremes and water resources availability. In addition, the lack of continuous observations as well as the significant transformations of river basins limits our ability to fully characterize hydrological response. Therefore, it is urgent to update current methods and tools in order to shed light on the expected hydrological changes. In the present study, we try to construct a detailed description of extreme flow patterns in Southern Italy in the period 1920-2020. For this reason, the dataset of annual maximum discharges was constructed using all available records and extended using indirect measurements (e.g., daily discharge and water levels). The data before 1980 were collected in the SIMN Special Publication No. 17 and in the SIMN Hydrological Yearbooks, which provide the annual maximum flow rates and the annual rating curves. Hydrological observatory have been transferred to the regional Department of Civil Protection, but only water level observations are available for the most recent period. Dataset from different sources requires a significant effort to reconstruct reliable timeseries. In order to extend the historical series of floods annual maxima, we tried to transform mean daily maxima into peak flow values by means of a conversion factor proposed by Taguas et al. (2008). Additionally, the database was also integrated with the most recent data converting water level measurements in annual floods by using the annual maxima flow rating curve. Such rating curve turned out to be quite stable over time as demonstrated by Claps et al. (2010) and it was verified using also hydraulic numerical models. The present study results complement the outcomes of the recent study by Blöschl et al. (2017), who investigated flood trends over the last five decades in Europe. This study provided a clear overview on the recent tendencies in Europe except for southern Italy because of the limited and discontinuous data availability. Therefore, the study allowed to reconstruct a relevant number of timeseries representative of the entire southern Italy. The homogeneity of the reconstructed data have been verified using the Kolmogorov-Smirnov test. Then, the obtained series were analysed in order to detect possible trends by using the Mann-Kendall non-parametric test. Results highlights the dynamics of flood production over the entire southern Italy.   References Blöschl G., Hall J., Parajka J., Perdigão R.A.P., Merz B, Arheimer B., Aronica G.T., Bilibashi A., Bonacci O., [...], Živković N. (2017). Changing climate shifts timing of European floods. Science, 357, pp. 588-59. Claps P., Ganora D., Laio F., and Radice R. (2010) Riesame ed integrazione di serie di portate al colmo mediante scale di deflusso di piena, Atti del XXXII Convegno Nazionale di Idraulica e Costruzioni Idrauliche, Idraulica e Costruzioni Idrauliche, Palermo, 14-17 settembre 2010. Taguas E.V., Ayuso J.L., Pena A., Yuan Y., Sanchez M.C., Giraldez J.V., and Pérez R. (2008) Testing the relationship between instantaneous peak flow and mean daily flow in a Mediterranean Area Southeast Spain, Catena, 75, pp. 129-137. https://doi.org/10.1016/j.catena.2008.04.015