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Abstract. The Soil Conservation Service Curve Number (SCS-CN) method is widely used for predicting direct runoff volume for a given rainfall event. The applicability of the SCS-CN method and the direct runoff generation mechanism were thoroughly analysed in a Mediterranean experimental watershed in Greece. The region is characterized by a Mediterranean semi-arid climate. A detailed land cover and soil survey using remote sensing and GIS techniques, showed that the watershed is dominated by coarse soils with high hydraulic conductivities, whereas a smaller part is covered with medium textured soils and impervious surfaces. The analysis indicated that the SCS-CN method fails to predict runoff for the storm events studied, and that there is a strong correlation between the CN values obtained from measured runoff and the rainfall depth. The hypothesis that this correlation could be attributed to the existence of an impermeable part in a very permeable watershed was examined in depth, by developing a numerical simulation water flow model for predicting surface runoff generated from each of the three soil types of the watershed. Numerical runs were performed using the HYDRUS-1D code. The results support the validity of this hypothesis for most of the events examined where the linear runoff formula provides better results than the SCS-CN method. The runoff coefficient of this formula can be taken equal to the percentage of the impervious area. However, the linear formula should be applied with caution in case of extreme events with very high rainfall intensities. In this case, Correspondence to: K. X. Soulis (soco@aua.gr) the medium textured soils may significantly contribute to the total runoff and the linear formula may significantly underestimate the runoff produced.

Rainwater harvesting gains more and more ground as a modern, relatively inexpensive and simple water-saving technology, and as a sustainable water management practice, which saves water, and reduces stormwater runoff and peaks and non-point source pollution. In this paper, in order to determine the optimal size of rainwater harvesting tanks, two methods, the daily water balance method and the dry period demand method, are used in 75 regions of Greece to meet 30, 40 and 50 % of total water demands of households of 3 to 5 residents. The daily water balance method was developed based on a heuristic algorithm which uses the daily rainfall data, the rainfall collection area, the runoff coefficient, the available storage volume and the water demands, allowing excess water to overflow and setting public water supply to zero. The dry period demand method is based on meeting demand for the longest annual average dry period. According to the daily water balance method, in the majority of the 75 Water Resour Manage Greece regions studied, tank sizes up to 50 m 3 can meet a 240 L/day demand (40 % of total daily demand of 4 residents) with roof area not exceeding 300 m 2 . More than 50 m 3 tank size is needed to meet demands of 300 L/day (40 % of 5 or 50 % of 4 residents) or 375 L/day (50 % of 5 residents). Results demonstrate that the tank size is strongly affected by the dry period length; small dry periods lead to small tanks, with the exception of low rainfall-high demand (300-375 L/day) case, where low rainfall increases sizes, having the dominant role. Comparison among the dry period demand and the daily water balance methods showed that in all cases, the dry period demand method calculates smaller tanks, with the exception of areas with medium-high rainfall and high dry period or low-medium demand (135-225 L/day) and high roof areas (more than 300 m 2 ). Therefore, the main conclusion is that the rainwater harvesting tank capacity is strongly affected by various local variables and cannot be formulated. However, the method presented here can be programmed in a spreadsheet with no much effort, making harvesting tank computations easy.

One of the most widely used methods in determining soil water diffusivity, D, as a function of the volumetric water content, θ, is the one‐step outflow method. Unfortunately, the calculation of D(θ) according to direct methods usually requires the estimation of the first and second derivative of the outflow volume, V, dV/dt and d2V/dt2 of the original measured cumulative outflow data V(t). These derivatives are usually approximated by applying a finite difference technique to two or three consecutive outflow measurements. Therefore, the accuracy of the results depends essentially on the accuracy of local outflow measurements. Inaccuracies are more intensive in the last portion of the outflow curve, where small errors of measurement may yield large inaccuracies in the first and second derivatives of the outflow curve. Furthermore, the entire calculation procedure for estimating dV/dt and d2V/dt2 is rather laborious and complicated. We developed a direct method based on a simple curve‐fitting procedure applied to the experimental outflow data for a simple power and an extended power form function, which leads to direct calculation of the soil water diffusivity function from explicit formulae. The only parameters involved in these formulae are the empirical parameters obtained by the curve‐fitting procedure. The proposed method was verified from one‐step outflow experiments performed in nine porous materials (one sand, two soils, and six substrate mixtures). Comparison with previous methods indicated a good performance of the proposed explicit algebraic formulae.

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