The Environment Agency routinely measure, monitor and report on a range of hydrological parameters to assess the water situation across England. They do this using their own hydrometric data, together with data provided by the Met Office and water companies.
They do this for:
the amount of rain that falls
how dry the soils are and how much rain they can soak up
the amount of water flowing in rivers
the amount of water stored below ground in aquifers and above ground in reservoirs
the outlook for river flow and groundwater over the coming months