Regional flow duration curve estimation in small ungauged catchments using instantaneous flow measurements and a censored data approach

Regional estimation of flow duration characteristics is important for water resources development at the small catchment scale. Regional analyses often fail to adequately represent the variability of the flow regime in small catchments (<50 km2), especially in remote mountainous regions where the calibration data are sparse and refer to much larger catchment scales. This study suggests an approach in which regional data are combined with actual instantaneous flow data to construct a representative daily flow duration curve for small catchments. A regional dimensionless flow duration curve (FDC) is developed for a hydrologically homogeneous region in Western-Northwestern Greece and used to estimate the FDC in two small mountainous catchments within the region. A number of instantaneous flow measurements available at the two sites are used in a statistical representation of the flow regime from which an estimate of the mean annual flow at the sites is made, allowing the construction of the FDC from the regional curve. Results obtained are in good agreement with observed data and show significant estimation improvement over other methods commonly employed in the study region. A sensitivity analysis using Monte Carlo simulation is performed to establish reasonable sampling requirements for small ungauged catchments in the study area and similar mountainous areas in the Mediterranean region.