April was generally an unsettled month with some notably cold episodes, appreciable snowfall, and substantial regional variations in precipitation totals. Significant replenishment, particularly around the month-end, ensured that (Northern Ireland aside) the great majority of major reservoirs were close to capacity in early May when estimated overall stocks for England & Wales were the highest for seven years (for early May). April river flows were generally well within the normal seasonal range and accumulated runoff totals, over a range of durations, are mostly above average. Accelerating evaporative demands triggered the development of appreciable soil moisture deficits but a wet end to the month ensured that they were mostly below the late April average. Aquifer replenishment was modest and groundwater levels are now generally in recession - and in most areas close to, or above, typical late- spring levels. The seasonal decline in runoff and recharge rates is likely to become firmly established in May but the general water resources outlook for the summer remains very healthy.
Active low pressure systems brought significant rainfall early and, particularly, late in the month but, in between, weather conditions were dictated largely by airflows from a northerly quadrant. These produced seasonally low temperatures and showery conditions with a wide variety of precipitation types; on the 6th significant snowfall extended into southern England; a total of 6cm was recorded at Wallingford (Oxon) with nearly twice this in parts of southern England. Nonetheless, most storms yielded only modest precipitation totals until a south-westerly airflow introduced milder, wetter conditions around the 25th; rainfall totals of 10-25mm were recorded over wide areas on the 29/30th. This wet interlude helped April rainfall totals to exceed the monthly average across most of Great Britain. Parts of the Cairngorms and the North East reported >200% but, by contrast, many western catchments (e.g. in the South West and Lancashire) registered totals in the 80-100% range. On a provisional basis, Northern Ireland recorded its 3rd driest April in the last 20 years (albeit significantly wetter than 2007). With the exception of the South-West (Cornwall particularly), accumulated rainfall totals for the year thus far are considerably above average for all regions, and exceptional in Scotland - which registered its 2nd highest Jan-Apr rainfall in a series from 1914.
River flows generally remained well within the normal range throughout April and most index rivers exhibited broadly similar flow patterns: sustained recessions after the late-March spates followed by modest (and brief) recoveries at month end. These flow increases were more notable in many impermeable catchments (e.g. the Mole, and the Wallington in Hampshire) and postponed the onset of the seasonal recessions in many spring-fed streams and rivers. April runoff totals were within the normal range for a remarkably high proportion of index rivers across the UK. However, in much of northern and eastern Scotland, snowmelt contributed to well above average April runoff totals; the Ness reported its 3rd highest April mean flow in a 35-yr series. By contrast, modest April flows characterised many western catchments; the Annacloy (NI) registered <50% of the April average and, in Cornwall, runoff in the Kenwyn was below average for the 8th successive month. The South-West excepted, runoff totals for the year thus far are above average across the great majority of the country, notably so in northern Britain where the Naver, Tweed (at Norham) and Wharfe each recorded new Jan-Apr runoff maxima. The legacy of the extraordinary summer runoff in 2007 helps ensure that most 12-month runoff totals also exceed the average.
Despite the unsettled conditions, soil moisture deficits began to build through April - particularly in some central Chalk outcrops where rainfall totals were below average - but were reversed by the notable wet spell over the final few days. As a result, soils were wetter than average in almost all areas at month-end. As is normally the case in April, infiltration was patchy and, apart from the slowest-responding aquifer units, groundwater levels were generally in brisk recession. Correspondingly, the ranking of the groundwater levels needs to be treated with caution - being very sensitive to the date(s) on which levels were registered at individual index boreholes. The dryness of the recent past in Northern Ireland is reflected in the relatively depressed April levels for the Chalk but, in E&W, groundwater levels remain well within the normal range, and in most cases above the late spring average. Typical late-spring levels also characterise most limestone and Permo-Triassic sandstones outcrops. Aquifer outflows via springs and seepages have broadly balanced recharge over the last three months in some areas resulting in unusually level hydrographs (e.g. Well House and Stonor in the Chalk, Bussels in the PT sandstones). The scope for further late-spring infiltration was appreciably reduced by the warm dry spell in early May but most summer recessions are likely to track close to, or above, the seasonal average.