Regionalization of flow duration curves

A method for estimating the flow duration curve (FDC) in an ungauged basin (i.e. regionalization of FDCs) is developed. An FDC can be described by the mean value and the coefficient of variation either in terms of empirically determined regional FDCs or as fitted theoretical regional curves. The development of these two first order moments of the FDC along the river network and with the spatial scale, i.e. the basin area, is analysed and a scheme for their interpolation along the river network is elaborated. Daily runoff data records from Costa Rica are used to demonstrate the findings. The estimation errors are highest in relative terms (∼30%) for the durations longer than 85%, somewhat smaller for durations less than 20% (-10%) and small for central parts of the FDC (∼8%). Differences between the empirical and theoretical curves are minor, although the latter give systematically better results especially in central parts of the FDC.