Classifying low flow hydrological regimes at a regional scale
- 1School of Geography, University of Leeds, UK
- 2Surface Hydrology and Erosion Group, Geosciences Department Institute of Environmental Assessment and Water Research (IDǼA), CSIC Barcelona, Spain
- 3Centre for Ecology & Hydrology (CEH), Wallingford, UK
- 4Centre for Water and Climate (CWK), Integrated Water Resources Management, Alterra, Wageningen, The Netherlands
- 5Water Research Institute IRSA-CNR, Bari, Italy
- 6MIRAGE project (Mediterranean Intermittent River ManAGEment): 7th EU Framework Programme, coordinated by J. Froebrich, Centre for Water and Climate (CWK), Integrated Water Resources Management, Alterra, Wageningen, The Netherlands
Abstract. The paper uses a simple water balance model that partitions the precipitation between actual evapotranspiration, quick flow and delayed flow, and has sufficient complexity to capture the essence of climate and vegetation controls on this partitioning. Using this model, monthly flow duration curves have been constructed from climate data across Europe to address the relative frequency of ecologically critical low flow stages in semi-arid rivers, when flow commonly persists only in disconnected pools in the river bed. The hydrological model is based on a dynamic partitioning of precipitation 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 estimate 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 discharge on the basis of frequency. The corresponding frequency for pools is then read from the duration curve, using this (0.1%) ratio to estimate pool discharge from bank full discharge. The flow duration curve then provides an estimate of the frequency of poorly connected pool conditions, 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 under alternative climatic scenarios.