Autumn 2000 floods

Could the answer lie in the soil?

The soil acts as a buffer (like a sponge) between the falling rain the water that finds its way into rivers and aquifers. When the aquifers are full and the soil is saturated the rainfall has nowhere to go but into rivers. The Met Office Rainfall and Evaporation Calculation System (MORECS) produces an estimate of soil moisture and hydrologically effective rainfall (HER) for the whole country each week. Estimates for North Yorkshire, the Welsh Borders and Kent have been analysed to see what explanations can be offered for the current floods.

The story begins with a wet spring and early summer which severely hampered the drying out of the soils. This can be seen in the following three graphs, where the soil moisture deficit (SMD) represents the amount of rainfall needed to bring the soil to a saturation level; the higher the SMD, the drier the soil. 2000 has been compared with average and the year with the lowest SMDs on record.

The charts for all three regions show that the soil dried out much more in the summer of 2000 than in the worst years of the last 40, but for the early summer was wetter than average. Only North Yorkshire failed to dry to average levels in the late summer. What is noticeable in 2000 is the manner in which a substantial SMD is reduced abruptly to zero by the autumn rainfall. Taking all three locations together only 1976, 1993 and 1966 returned to saturation sooner in the last 40 years.

The rain that brought the speedy return to saturation has continued and become hydrologically effective. MORECS cannot differentiate between aquifer recharge and run-off to rivers, but given the generally above-average rainfall experienced in the last year or so, comparatively little will be filling aquifers by now. Nevertheless this must be appreciated when considering the charts below, which compare HER in Autumn 2000 with average and other wet years.

North Yorkshire has already equalled its highest level of HER before the year end, Kent has exceeded the previous highest value at this time of year, and the Welsh Borders have equalled it. In all instances 1976 was the previous worst when, following the drought, much more HER would have gone into aquifer recharge rather than runoff.

These graphs, based on MORECS data, add some further insight into the reasons behind the floods, and the wide geographic area covered. Nevertheless, one cannot ultimately get away from the fact that when it rains this much floods are inevitable.