Stochastic index model for intermittent regimes: from preliminary analysis to regionalisation

Abstract. In small and medium-sized basins or in rivers characterized by intermittent discharges, with low or negligible/null observed values for long periods of the year, the correct representation of the discharge regime is important for issues related to water management and to define the amount and quality of water available for irrigation, domestic and recreational uses. In these cases, only one index as a statistical metric is often not enough; it is thus necessary to introduce Flow Duration Curves (FDC).

The aim of this study is therefore to combine a stochastic index flow model capable of reproducing the FDC record period of a river, regardless of the persistence and seasonality of the series, with the theory of total probability in order to calculate how often a river is dry.

The paper draws from preliminary analyses, including a study to estimate the correlation between discharge indicators Q₉₅, Q₅₀ and Q₁ (discharges exceeding 95%, 50% or 1% of the time, respectively) and some fundamental characteristics of the basin, as well as to identify homogeneous regions in the target area through the study of several geo-morphological features and climatic conditions. The stochastic model was then applied in one of the homogeneous regions that includes intermittent rivers.

Finally, the model was regionalized by means of regression analysis in order to calculate the FDC for ungauged basins; the reliability of this method was tested using jack-knife validation.