Flow duration curve estimation in ephemeral catchments in Portugal

Abstract

The flow regime of a river can be described using the flow duration curve (FDC), which represents the frequency distribution of flows and can be derived from gauged data. Many resource assessments are required where gauged data are limited or unavailable, thus models for predicting FDCs in ungauged catchments are required. In semiarid environments, such as parts of southern Portugal, river flows often become zero for significant periods of time. This makes modelling the discontinuities in the flow regime using commonly used continuous distributions more difficult. This paper presents the derivation of a regionalized model (from a data set of 67 catchments) for predicting FDCs for ungauged catchments in Portugal, which may be ephemeral. The approach uses the theory of total probability to combine a model for predicting the percentage of time the river is dry with a model for predicting an FDC for the non-zero period. These component parts can be modelled separately, relating them to catchment characteristics such as hydrogeology and climate.