The Great Stour Project - River and Coastal Flooding

PRECIPITATION, RUNOFF AND BALANCE 1996-99

During the period October 1996 to December 1999 the total precipitation was 103% of the long term average.

There were several months during which precipitation was well above average, particularly June 1997 and September 1999 when over twice the monthly average fell.

Conversely, there were spells when precipitation was well below average, notably the particularly dry periods between December 1996 and April 1997 and during September 1997 and July and August 1998. At these times, the discharge of the Stour was exceptionally low, being sustained by groundwater flow.

A remarkable feature of the water balance is the unusually high loss of water from the system. Of the 2273mm of precipitation that fell between October 1996 and October 1999, only 745mm nourished the Stour. This means that 1528mm were lost to other outputs such as evapotranspiration, abstraction and groundwater recharge.

The magnitude of the loss, unusually high by UK standards is, to a great extent accounted for by the relatively warm climate, low basin relief, high infiltration capacities, rock permeability, dominantly arable land use and the relatively high population density resident within the catchment boundaries.

During only three months did the output of the Stour exceed the precipitation received, notably during the drought of March and April 1997 when the river was sustained by groundwater seepage.

Note: When interpreting the relationship between rainfall and runoff it is important to stress that the Stour catchment has a relatively high storage capacity. Much of the winter rainfall recharges the groundwater reservoir and is not lost to the system but feeds the river during the summer period of water deficit. The catchment therefore has a relatively long lag time between precipitation and runoff. This means that it is essential to calculate water losses over a long period of time to minimise the effect of storage change. It should also be observed that losses through evaporation will be limited by the low summer rainfall when potential evapotranspiration may be higher than actual precipitation.