When the weather is fine, then you know it's the time
For messing about on the river
So goes the song, but how do we know? Catchment hydrology is the application of basic physical laws (such as mass conservation) and hydrodynamic theory, at the scale of river basins. It is of vital importance in water resources management and the forecasting and mitigation of floods, a natural hazard which costs the UK £2.5 billion every year: a figure which is likely to increase in the future due to greater urbanisation and climate change. It is inherently a multi-disciplinary and applied subject, with active research ranging from the underlying mathematics to shaping government policy. I will present an overview of the work we do at CEH in this area, with emphasis on the various mathematical models employed and the extensive data resources we curate, many of which are publicly available through our online information gateway.