The paper uses a simple water balance model that partitions the precipitation between actual evapotranspi-ration, quick flow and delayed flow, and has sufficient com-plexity to capture the essence of climate and vegetation con-trols on this partitioning. Using this model, monthly flow du-ration curves have been constructed from climate data across Europe to address the relative frequency of ecologically criti-cal low flow stages in semi-arid rivers, when flow commonly persists only in disconnected pools in the river bed. The hy-drological model is based on a dynamic partitioning of pre-cipitation to estimate water available for evapotranspiration and plant growth and for residual runoff. The duration curve for monthly flows has then been analysed to give an esti-mate of bankfull flow based on recurrence interval. Arguing from observed ratios of cross-sectional areas at flood and low flows, hydraulic geometry suggests that disconnected flow under "pool" conditions is approximately 0.1 % of bankfull flow. Flow duration curves define a measure of bankfull dis-charge on the basis of frequency. The corresponding fre-quency for pools is then read from the duration curve, us-ing this (0.1 %) ratio to estimate pool discharge from bank full discharge. The flow duration curve then provides an Correspondence to: M. estimate of the frequency of poorly connected pool condi-tions, corresponding to this discharge, that constrain survival of river-dwelling arthropods and fish. The methodology has here been applied across Europe at 15 km resolution, and the potential is demonstrated for applying the methodology un-der alternative climatic scenarios.