March was a mild and relatively dry month, parts of Scotland apart, with some notable rainless interludes. These contributed to significant two-month rainfall deficiencies across much of the country at an important time for the water resources outlook; increasing evaporation demands normally curtail the aquifer recharge season during the late spring. Overall, reservoir stocks for England & Wales were above average for the 23rd successive month - but only marginally so and stocks were considerably below the early April average in a few major impoundments (e.g. in north-west England). In Scotland and Northern Ireland, stocks in index reservoirs are well within the normal seasonal range. River flow recessions extended through much of March and runoff rates were depressed in many responsive rivers by early April. With limited rainfall in most outcrop areas and soil moisture deficits beginning to build, aquifer recharge to most major aquifers was modest and seasonal groundwater level recessions have become established in many areas. Nonetheless, March groundwater levels were well within the normal range across most of the country. The last ten weeks has seen an appreciable deterioration in the water resources outlook but overall surface and groundwater resources remain typical of the late spring (in part, this is a reflection of the unusually high runoff and recharge during the autumn of 2008).
Overall, March was warmer and sunnier than average with high pressure dominant throughout much of the month across the majority of the country. The first week was very unsettled with significant snowfall in Scotland on the 3rd and 5th (when, more exceptionally, some locally heavy snowstorms - with accumulations up to 15cm - were recorded in south-west England). Generally however, notable storm rainfall totals were rare in March and the most hydrologically significant features were the extended dry episodes in mid-month. Large areas reported accumulated rainfall totals of < 2mm over periods of 18 days or more; some central southern areas (e.g. south Oxfordshire) reported less than a third of their average rainfall over the eight weeks to early April. This dry spell is reflected in the March rainfall totals. Whilst much of northern Scotland registered well above average rainfall, March totals declined markedly to the south with substantial parts of Wales and southern England reporting less than 50% of average. More significantly, 2-month deficiencies are exceptional in a number of regions. Over the February-March period, provisional data indicate that Northern Ireland equalled its lowest rainfall total in the last 56 years and the Welsh Region its 2nd lowest in the last 53 years. Much of northern England was also notably dry in this timeframe. Substantial rainfall deficiencies are more extensive (encompassing much of north-east England and eastern Scotland) over the November-March period. Fortunately from a water resources perspective, regional rainfall accumulations over the last 12-months (and for longer periods) are generally above average.
In most index catchments the sustained February river flow recessions were arrested during the first week of March and a few notable spates were registered (e.g. on the Mole and Medway). Flows then declined steeply through the middle of the month and, entering April, flows in responsive rivers were seasonally depressed over wide areas. In parts of western and northern Scotland however, above average rainfall and snowmelt contributions maintained very healthy runoff rates - flows in the River Ewe remained above average throughout March. Across much of southern Britain, geological control over runoff rates was particularly evident during March. Flows in many impermeable catchments (e.g. the Soar, Lymington and Warleggan) were substantially below the early spring average by month-end. In contrast, flows in many groundwater-fed streams and rivers (e.g. the Mimram and Lambourn) remained appreciably above average - a consequence of the delayed benefit of exceptional recharge over the latter half of 2008. Relative to the long term average, March runoff totals displayed marked spatial variability. Categorising broadly: above average runoff characterised northern Scotland, whilst runoff in the English Lowlands was typical of the early spring but, elsewhere, runoff totals were well below average notably so in a few western catchments. The Teifi reported its 3rd lowest March runoff in 47 years, the Annacloy its 2nd lowest in a 30-year series. Two-month accumulations are also generally below average but medium and longer term accumulations remain healthy.
A wet start to March allowed some useful infiltration but, thereafter, soils began to dry out once more and, by month-end, soil moisture deficits exceeded the late-March average across the outcrop areas of most major aquifers (much of Norfolk was an exception). With March rainfall falling below 70% of average in most outcrop areas, aquifer replenishment was typically less than half the average. The corresponding decline in groundwater levels in the South Downs almost certainly marks the end of the at-risk period for localities vulnerable to groundwater flooding. More generally, the dryness of the early spring is reflected in the steep recessions which characterise a number of index wells in the limestone (e.g. Ampney Crucis and Alstonfield) and Permo- Triassic sandstones (e.g. Newbridge) outcrop areas. Similarly brisk recessions are found in the responsive (mostly western and southern) outcrop areas of the Chalk (see West Woodyates for example). Levels were still rising during March in the slower- responding aquifer units (e.g. at Heathlanes and in the deep Therfield well in the eastern Chalk). The dry start to April may signal the end of the 2008/09 recharge season in many eastern and central aquifer outcrop areas but, fortunately, the seasonal peak groundwater levels registered in the late winters were generally above average providing at least a partial counterbalance to the early onset of the 2009 recessions. Given normal rainfall patterns, a typical seasonal decline in groundwater levels is in prospect but the magnitude and distribution of late spring rainfall may well influence the steepness of the recessions.