We propose a statistical framework to generate synthetic rainfall time series at daily resolution, conditional on predictor variables indicative of the atmospheric circulation at the mesoscale. We do so by first introducing a dimensionless measure to assess the relative influence of upper-air variables at different pressure levels on ground-level rainfall statistics, and then simulating rainfall occurrence and amount by proper conditioning on the selected atmospheric predictors. The proposed scheme for conditional rainfall simulation operates at a daily time step (avoiding discrete approaches for identification of weather states), can incorporate any possible number and combination of predictor variables, while it is capable of reproducing rainfall seasonality directly from the variation of upper-air variables, without any type of seasonal analysis or modeling. The suggested downscaling approach is tested using atmospheric data from the ERAInterim archive and daily rainfall measurements from western Greece. The model is found to accurately reproduce several statistics of actual rainfall time series, at both annual and seasonal levels, including wet day fractions, the alternation of wet and dry intervals, the distributions of dry and wet spell lengths, the distribution of rainfall intensities in wet days, short-range dependencies present in historical rainfall records, the distribution of yearly rainfall maxima, dependencies of rainfall statistics on the observation scale, and long-term climatic features present in historical rainfall records. The suggested approach is expected to serve as a useful tool for stochastic rainfall simulation conditional on climate model outputs at a regional level, where climate change impacts and risks are assessed.

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The reliability of quantification (measurement and calculation) of the exchange rate between groundwater and small streams is discussed. Concerning measurements, two measuring techniques (discharge difference meas urements and seepage meter measurements) are investigated. Discharge difference measurements can give reli able results if the length of the reach, at the ends of which the discharge is measured, is appropriate. Infiltrometer measurements are of questionable value when influenced by difficult to detect installation errors. Concerning calculations the following issues are addressed: (a) The sensitivity of the calculated values of the exchange rate to errors in various parameters. This problem is discussed by means of analytical solutions. The analysis shows that, in case of stream beds with relatively small hydraulic resistance, errors in the hydraulic conductivity of the aquifer are important for the exchange rate. In case of stream beds with high hydraulic resistance, exchange rate is strongly influenced by the errors in the hydraulic resistance. In such cases, errors in the parameters characterizing the capillary forces in the porous media can increase the resulting errors considerably. (b) The reliability of estimations of the hydraulic resistance of the stream bed by means of piezometric head measurements near the stream. Investigations in a small stream show that, if the hydraulic resistance of the stream bed is high, its estimation from measurements of the piezometric heads is uncertain. (c) The forecasting of the exchange rate by means of depth integrated models under a hydrologie regime other than the one used for calibration. By means of analytical solutions it is shown that in case of stream beds with high hydraulic resistance and for porous materials in which the capillary forces are negligible, the exchange coefficient used to describe the exchange flow includes only the influence of the clogging layer. In such cases the value of the exchange coefficient estimated from the calibration can be used to forecast the exchange rate under changed hydrological conditions. In cases where the hydraulic resistance of the stream bed is small and for materials with nonnegligible capillary forces, such forecasting can be erroneous. RÉSUMÉCette étude traite de la fiabilité de revaluation, par mesures de terrain et par calculs, du taux d'échange entre l'eau souterraine et les petits ruisseaux. Pour les mesures de terrain , deux techniques ont été évaluées, une mesure par difference de charge et l'autre par mesure de déperdition. Les mesures par difference de charge peuvent donner des résultats fiables si la longueur du troncon au bout duquel on mesure la difference de charge, est appropriée. Les mesures par infiltromètre sont contestables quand elles dependent de la difficulté de détecter les erreurs d'installation. Pour les calculs, les questions suivantesse posent: (a) la sensibilité des variables calculées du taux d'échange aux erreurs de plusieurs paramètres. On traite Ie problème a l'aide de solutions ...

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