Increasing intermittence of the UK’s chalk streams into the future 

The chalk streams of the UK are globally rare, strongly intermittent in their upper reaches, and highly valued for their biodiversity and historical provision of water resources. Recent projections of river flows and groundwater levels under climate change in the UK, coupled to existing statistical models of hydrological state, enable the projection of spatiotemporal intermittence patterns into the near- and far-future.   Catchments were selected for the study based on the availability of data and the performance of statistical models in the historical period.  Cumulative logit models, previously trained on historical data, were used in conjunction with state-of-the-art ensemble projections of future river flows and groundwater levels to simulate future hydrological state at multiple sites along chalk streams in the south-east of England.  Heatmaps visualise spatiotemporal variations in state, and intermittence metrics quantify the variability.    The results show projected increases in drying into the future, both temporally, with greater duration of drying, and spatially, with intermittence extending downstream.  Some sites are likely to alter substantially, for example, on the river Chess with notable decreases in modelled flow permanence projected, from 75% in the baseline period (2005-2020) to 25% in the far future (2065-2080).  This research provides quantifiable spatiotemporal dynamics of intermittence, informing water resource decisions, drought management and engagement activities on these high-profile streams.  The methods developed are adaptable for transfer to other catchments for which spatiotemporal mapping of intermittence patterns and future projections of driving variables exist.