Hydraulic models present a great opportunity to assess habitat sensitivity to environmental changes, such as climate change. Here, the 1D fully dynamic model MIKE 11 was used to simulate flows along a 600 m reach of the River Lambourn, Boxford, UK. This is a lowland chalk valley river where in-channel macrophyte growth and its management have an acute effect on water levels. Assessment of climate change impacts relied on using outputs from the UK Climate Projections 2009 ensemble of global climate models for the 2080s to obtain monthly percentage changes in river discharge. Macrophyte growth was incorporated into the hydraulic model by manipulating channel bed roughness (Manning’s n). 

Habitat modelling followed the established PHABSIM methodology. Outputs from the hydraulic model were disaggregated to provide velocity profiles for 41 cross sections along the reach. These were combined with stage outputs, measurements of channel geometry, and integrated with habitat suitability criteria for brown trout (Salmo trutta) to generate a measure of available physical habitat.
The influence of macrophyte growth caused the habitat discharge relationship to be meaningless. Habitat availability was instead evaluated over the modelled period. Impacts of climate change were seen as most pronounced through summer months, and accentuate periods of low flows. These are reflected in a marked reduction in habitat availability, more so for adult than juvenile trout, with implications for river management.