Reservoir levels are not always an indication of whether water supplies are plentiful or lacking. For instance, during prolonged periods of heavy rainfall, Thames Water will allow our local reservoir, Farmoor, to deplete by not topping it up from the Thames. This is so that chemicals from Thames Valley farmland that gets washed into the river doesn’t then get into the reservoir and cost them to filter it out when processing it for the water supply. Conversely, when we have long periods of dry weather, the reservoir level is kept high (they pump water from the river) so we have sufficient in reserve.
Natural reservoirs are different of course.
River levels are a better indication of droughts IMHO.
