On average, evaporation losses account for over 40% of UK rainfall - but the proportion varies greatly from region to region, reaching around 80% in the driest parts of the English Lowlands. Evaporation may occur directly from open water surfaces, from the soil or as transpiration from plants. Potential evaporation (PE) is the maximum evaporation which would occur from a continuous vegetative cover amply supplied with moisture. PE losses exhibit a strong annual cycle, peaking normally in June or July; typically, only 10-20% of the annual PE loss occurs during the October-March period. Given normal rainfall, the increasing temperatures and accelerating evaporative demands through the spring lead to a progressive drying of the soil profile and the creation of what is termed a Soil Moisture Deficit (SMD). Eventually, the ability of transpiration to proceed at the potential rate is reduced as a result of the drying soil conditions, the associated reduced capability of plants to take up water, and the measures plants take to restrict transpiration under such conditions. Thus in the absence of favourable soil moisture conditions actual evaporation (AE) rates will fall below the corresponding PE rates, appreciably so during dry summers. When plant activity and evaporation rates slacken in the autumn, rainfall wets-up the soil profile once more - allowing runoff rates to increase and infiltration to groundwater to re-commence. Knowledge of the soil moisture status and evaporation rates are essential factors in understanding water resource variability. The following commentary on evaporation patterns and soil moisture deficits during 2004 relies, in large part, on monthly figures derived by the Met Office Rainfall and Evaporation Calculation System (MORECS)1.
2004 was another warm year; the average Central England Temperature (CET)2 was close to 10.5°C, approximately one degree above the 1961-90 average. An average of 10.2°C has now been exceeded in 11 years over the post-1988 period, compared with only nine in the preceding 328 years of the CET series. In 2004 mean temperatures were above average in all months apart from July and, relative to the seasonal norm, the late winter (January and February) was notably mild. May and June were warm and the summer half-year (April-September) ranks 25th warmest in the CET series, albeit appreciably cooler than in recent notably warm summers (e.g. 2003 and 1995). Evaporation losses reflect other factors as well as temperatures (e.g. windspeed, humidity and land use) but the, generally, warm conditions through the late spring and early autumn of 2004 were associated with well above average evaporation demands. Figure 1 shows annual potential evaporation (MORECS) losses for 2004 in mm and as percentage of the preceding average. The annual total for England and Wales was around 6% above the 1961-90 average but unremarkable in the context of the preceding 15 years, around half of which had higher annual losses. Potential evaporation (PE) totals ranged from a little above 400mm in the Scottish Highlands to around 700mm in a few, mostly coastal, locations (where wind is particularly influential) in southern Britain. PE totals were 5-15% above average across much of Britain with below average evaporative demands mostly confined to Scotland and south-west Wales. Considering England and Wales as a whole, PE losses for 2004 were above average in the late winter (but still modest in absolute terms) and much more significantly during the summer half year.
Open water evaporation losses were high also in 2004 with daily totals exceeding 5mm during the early summer heat-waves. As notably in a water resources context, the damp summer resulted in transpiration rates being constrained for a much shorter period than normal and contributed to the highest annual Actual Evaporation total for England and Wales in the MORECS series (which begins in 1961). Annual AE totals exceeded the average by more than 20% in many eastern areas (see Figure 2) and the notable contrast with 2003 is evident in Figure 3 - the annual shortfall of AE relative to PE across much of the English Lowlands in 2004 was only 15-30% of that for the preceding year. For Scotland both PE and AE totals for 2004 were, as usual, considerably below the figures for England and Wales and the shortfall of AE relative to PE was narrow - only a couple of millimeters at the national scale. 2004 PE totals exceeded AE losses by a considerably wider (though still modest) margin in the drier eastern catchments.
The development and decay of soil moisture deficits (SMDs) over the 2000-2004 period is illustrated in Figure 3 for six representative MORECS squares; the SMD values relate to the end of each month and assume a grass cover. Monthly PE and AE totals are also shown together with the differences in the annual PE and AE totals. 2004 contrasted markedly with the exceptionally arid soil conditions experienced through the summer and early autumn of the preceding year. After a faltering beginning in the early spring, soil moisture deficit increase followed a relatively normal pattern through the much of the summer but from late July soil moisture variations were atypical in most areas. A notable feature was the substantial decline in SMDs during August. Late August deficits were below average over wide areas with soils close to saturation throughout most of northern and western Britain (see Figure 4). However in September - when a seasonal increase in soil moisture is normally gathering momentum - deficits increased once more in much of southern and eastern Britain (Figure 5). The erratic pattern continued as the sustained October rainfall then caused a brisk increase in soil moisture. Generally, SMDs were very modest by late autumn but the dry end to the year allowed modest deficits to carry over into 2005 in a few, mostly coastal, areas of eastern England.