5. Dry and Wet Soil Conditions in Southern England
TJ Marsh

Number of days per year when the soil moisture deficit exceeded 60 mm (dry soils)
Number of days per year when the soil moisture deficit was less than 10 mm (wet soils)
at the Institute of Hydrology, Wallingford, Oxon.

Specific Indicator
The Met Office provides monthly and weekly soil moisture data for 40 km by 40 km squares in Britain, based on 120 meteorological stations, which is available at cost (the Met Office Rainfall and Evaporation Calculation System, MORECS). Here we propose, as an indicator, soil moisture levels calculated for a single representative site in England, at the Centre for Ecology and Hydrology Wallingford.
The specific indicators are the number of days per year that the soil moisture deficit at Wallingford exceeds 60 mm (the >dry= soil indicator) or is less than 10 mm (the >wet= soil indicator). The dry soil indicator relates to the calendar year; the wet soil indicator relates to the 12-month period from August to the following July. Soil moisture levels are calculated from daily rainfall, net radiation and mean air temperature.
[Source: The National Water Archive, Centre for Ecology and Hydrology Wallingford OX10 8BB]

Relevance
Soil moisture conditions are important in relation to both water resources and agriculture. Wet soils promote surface runoff and replenishes groundwaters. The length of the winter period when soils are close to saturation defines the window of opportunity for aquifer recharge. This period is normally much shorter in eastern Britain than in the west; in parts of the eastern Chalk it is, on average, less than ten weeks. During the 1988-1997 period the recharge season has been brief in a number of winters - leading to depressed groundwater levels.

Dry soils in the spring and summer can lessen the threat of flooding but threaten agricultural production. The duration (and magnitude) of substantial deficiencies largely determine irrigation needs, which are concentrated in those parts of the country where water resources and river systems are most depleted during hot dry summers.

Sensitivity to climatic and other factors
Soil moisture conditions are sensitive to the balance between rainfall and evapotranspiration losses (which, in turn, reflect a range of climatic parameters - principally temperature and solar radiation, but wind speed can also be an important factor). Differing soil types (which have differing moisture retention capabilities) and land uses (which impose differing transpiration demands) also affect soil moisture conditions. As a consequence, spatial variations in soil moisture conditions can be considerable. The indicators used here for southern England assume a grass cover and a soil of medium water retention capacity.

Change over time
Data presented for the 1962-2002 period at Wallingford display no clear trend in the duration of wet or very dry soil conditions. Wide departures from the average number of days with wet or dry soils has, however, characterised much of the last 40 years. Persistently high soil moisture deficits were a feature of 1988-92 and 1995-1997 but the intervening period was wet - notably the winter – and above average rainfall since mid-1997 has counterbalanced the higher evaporative demands which have characterised the recent past. Although maximum soil moisture deficits have been notably high in the 1990s, the length of time over which substantial deficits have obtained has not been unusual. The dry soil conditions over the winter of 1996/97 greatly restricted the period available for groundwater recharge but, thereafter, the recharge season has been of normal length – allowing groundwater levels to recover to within their normal range.

A tendency towards drier and warmer summers would increase the duration of notably dry soils and, by delaying the seasonal wetting-up of the soil in the autumn, tend to reduce the number wet soil days in the winter. This may be expected to have a negative impact on groundwater replenishment - but increased winter rainfall could produce a counterbalancing effect.