The Nyl River floodplain is a seasonal wetland of great conservation importance in Limpopo Province, South Africa. Water resource developments in the upstream catchments are changing the quantity and timing of water delivery to the floodplain, and this is expected to have an ecological impact. Hydrological and hydraulic models have been developed to help assess this impact. This paper describes the calibration and application of hydrological models of the contributing catchments. The hydraulic modelling and scenario applications to predict changes to vegetation habitat associated with upstream developments are presented in two companion papers. Hydrological simulations were performed using two models, one using daily time steps and the other using monthly time steps. The models were calibrated using historical data on 10 gauged catchments. Their performance was similar in producing average monthly and annual flows, although the higher resolution daily model agreed more closely with the historical data. Simulated estimates of mean annual runoff values for individual catchments compared well with results from previous studies. The daily model also predicted acceptable measures of probability distributions of maximum annual daily flows and daily flows exceeding a specified threshold.