Low flow estimation in Scotland

This report describes the results of a low flow study of Scotland commissioned by the Scottish Development Department and carried out by the Institute of Hydrology. The main objective of the study was to improve techniques for low flow estimation at the ungauged site. The study was based on mean daily discharge data for 232 stations held on the UK surface water archive. The authors would like to acknowledge the assistance of the River Purification Boards of Scotland not only for collecting and processing the data used in the study, but also for their contnbution to the production of a Base Flow Index map of Scotland. This report is part of a series of Low Flow Study Reports the first of which was published by the Institute of Hydrology in 1980

A new approach to estimating Mean Flow in the UK

Traditionally, the estimation of Mean Flow (MF) in ungauged catchments has been approached using conceptual water balance models or empirical formulae relating climatic inputs to stream flow. In the UK, these types of models have difficulty in predicting MF in low rainfall areas because the conceptualisation of soil moisture behaviour and its relationship with evaporation rates used is rather simplistic. However, it is in these dry regions where the accurate estimation of flows is most critical to effective management of a scarce resource. A novel approach to estimating MF, specifically designed to improve estimation of runoff in dry catchments, has been developed using a regionalisation of the Penman drying curve theory. The dynamic water balance style Daily Soil Moisture Accounting (DSMA) model operates at a daily time step, using inputs of precipitation and potential evaporation and simulates the development of soil moisture deficits explicitly. The model has been calibrated using measured MFs from a large data set of catchments in the United Kingdom. The performance of the DSMA model is superior to existing established steady state and dynamic water-balance models over the entire data set considered and the largest improvement is observed in very low rainfall catchments. It is concluded that the performance of all models in high rainfall areas is likely to be limited by the spatial representation of rainfall.</p> <p style='line-height: 20px;'><b>Keywords: </b>hydrological models, regionalisation, water resources, mean flow, runoff, water balance, Penman drying curve, soil moisture model</p

A region of influence approach to predicting flow duration curves within ungauged catchments

International audienceThe development of regionalised hydrological models or procedures for estimating flow duration statistics has been the subject of international research since the 1970s. Historically these models have been based on multivariate statistical models that relate flow statistics to the physical and climatic characteristics of a catchment. The a priori classification of catchments has often been a component of this analysis. This paper discusses the background to the development of such models, with particular emphasis on the United Kingdom; it describes a new region of influence approach to estimating flow duration statistics and compares the performance of this method with current multivariate regression based methods for estimating flow duration statistics within the United Kingdom. Keywords: hydrological models, regionalisation, river networks, water resources, flow duration curves, region of influence</p

Spatial and temporal variations in the occurrence of low flow events in the UK

International audienceInformation on the magnitude and variability of low river flows at the river reach scale is central to most aspects of water resource and water quality management. Within the UK, river stretches with permanent gauging stations represent less than one percent of the total number of river stretches mapped at a scale of 1:50,000 and fewer that 20% of gauged catchments can be regarded as having natural flow regimes. This has led to the development of simple, multivariate models for predicting average annual natural flow duration statistics through relationships with catchment characteristics. One assumption within these models is that low flows occur at the same time at all points within a catchment, irrespective of the hydrogeological nature and climatic condition of the catchment. This paper discusses the implications of spatial variations in the timing of low flow events for this type of model. Differences in the timing of the mean day of occurrence of the annual Q95 flow in UK catchments can be identified with low flows occurring earlier in the year within impermeable dry catchments and later in the year for wet permeable catchments. However, any differences in the mean day of occurrence between different catchments are generally masked by the magnitude of the inter-year variability in the day of occurrence. From analysis of linear combinations of flow statistics from nearest-neighbour gauged catchments, the paper demonstrates that the assumption of temporal coherence of low flows will generally result in an under-estimate of Q95; these underestimates are more significant for pairs of impermeable catchments than for combinations of permeable catchments and impermeable-permeable catchments

Low flow study of Northern Ireland

This report describes the results of a low flow study of Northern Ireland. The work was commissioned by the Departme nt of the Environment(N.I.) and was carried out by the Institute of Hydrology. The main objective of the study was to improve techniques for low flow estimation at the ungauged site. The study was based on mean daily discharge for 21 flow records using data provided by the Water Data Unit, Water Services Department. The main conclusion of the study was that relationships between low flow statistics and drift geology can be used to estimate low flows at sites without flow records. It is recommended that the estimation equations are revised when the current revision of the water archive for Northern Ireland is complete