Regionalization of streamflow intermittency in Mediterranean catchments (Southern Italy)

Flow Duration Curves (FDCs) are used to describe streamflow variability and support water-resources planning. In Mediterranean climates, intermittent rivers challenge FDC regionalization because zero-flow periods influence the lower part of the curve and discharge observations are scarce or fragmented in gauged basins. Southern Italy is a representative case where historical hydrological information is discontinuous and uncertain, so regional tools are needed to infer flow behavior in unmonitored catchments [1].

This study utilizes historical daily discharge from 58 gauging stations across three regions in southern Italy, covered by the Departments of Napoli (1925–1994), Bari (1950–1996), and Catanzaro (1950–1984). Stations were retained if they provide at least 15 years of records. Streamflow intermittency is quantified using the Intermittency Ratio (τ), defined as the fraction of non-zero daily discharge observations over the total number of daily observations. In the selected dataset, 20 stations exhibit τ < 1, indicating zero-flow days and intermittent behavior; these stations are considered in the intermittency analysis and regionalization.

For each basin, a hydrologically connected DEM was built by integrating an authoritative 20 m × 20 m DTM with the available river network and basin boundaries, enabling extraction of physiographic and morphologic descriptors in a GIS environment, supported by land-cover and geological and rainfall information [2,3]. A set of physiographic, topographic, and climatic covariates was analyzed, starting with collinearity assessment to reduce redundancy among predictors. The dependence of τ on catchment descriptors was investigated through stepwise regression using a power-law formulation. Results show that a parsimonious model based on two predictors—catchment area (A) and a catchment-shape descriptor (SF)—is sufficient to describe a significant portion of the observed variability in τ across the study region (as seen in figure 1).

Model performance is evaluated using standard statistical indices, including R², PBIAS, RMSE, MAE, and MAPE, which summarize explained variance, bias, and absolute/relative errors. The approach supports practical estimation of intermittency in ungauged Mediterranean catchments and provides a basis to incorporate intermittency into FDC regionalization, improving low-flow and zero-flow representation in data-constrained basins.

 

Figure 1. Observed vs predicted intermittency ratio (τ). Labels: B = Bari, N = Napoli, C = Catanzaro. Dashed line = 1:1.

 

Keywords: Intermittency ratio, Flow Duration Curve, regionalization, regression, stepwise selection, Mediterranean rivers, Southern Italy, GIS

References:

[1] Viola, F., Noto, L. V., Cannarozzo, M., & La Loggia, G. (2011). Regional flow duration curves for ungauged sites in Sicily. Hydrology and Earth System Sciences, 15(1), 323–331.

[2] Mendicino, G., & Senatore, A. (2013). Evaluation of parametric and statistical approaches for the regionalization of flow duration curves in intermittent regimes. Journal of Hydrology, 480, 19–32.

[3] Burgan, H. I., & Aksoy, H. (2022). Daily flow duration curve model for ungauged intermittent subbasins of gauged rivers. Journal of Hydrology, 604, 127249.