How does rainfall differ from month to month, year to year and place to place?

Within the Kentish Stour catchment the average annual rainfall during the period 1961-1990 was 714mm with an effective rainfall of 266mm. Annual rainfall totals vary throughout the catchment. The Dover area receives the highest average annual rainfall for the catchment at more than 800mm, and the Thanet area receives the least, with less than 600mm. This is broadly in line with the pattern of relief with the highest rainfall totals recorded on the North Downs.

Rainfall is unevenly distributed throughout the year with 56% of precipitation received in the winter half year, from October to March.

November is usually the wettest month when the frequent passage of depressions from the Atlantic may bring extended periods of rainfall. From December to February, the area occasionally comes under the influence of continental blocking anticyclones which may persist for several days. Whereas continental air streams produce lower precipitation totals, they can bring bitterly cold weather and blizzards.

March to July are the driest months of the year with an average of less than 50mms precipitation. In the summer half year, depressions follow a more northerly track resulting in comparatively low rainfall in the south east. In the late summer, however, humidity and high temperatures often bring heavy storms, accompanied by thunder and lightning. The intense rainfall that they bring can result in localised flooding where the intensity exceeds local infiltration capacities, notably in urban areas.

Surface drainage is strongly influenced by the permeable character of the catchment’s geology, most of the area being underlain by chalk, with Lower Greensand outcropping in the headwaters of the Great Stour. These aquifers provide natural reservoirs replenished by winter rainfall and supporting a predominantly spring fed river upstream. There are few surface streams and they all display a high degree of seasonal regulation; the effect of aquifer storage. This is reflected in the relatively small winter-to-summer flow ratio compared with a clay catchment.

Precipitation, runoff and balance 1996-99 - A case study

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.

When in the year is the Stour most likely to flood?

Canterbury, a summary of flooding events