April 2016


Period: From April 2016      Issued on 12.04.2016 using data to the end of March 2016

Summary

The outlook for the coming months appears rather complex but reflects the differing conditions at the start of the month and the varying response times of river catchments and aquifer units. In general river flows are likely to be above normal in the south-east of the UK and normal in the north and west. The exception to this is for chalk-fed rivers in the south-east which are also most likely to be in the normal range. Groundwater levels are likely to remain normal across most of the UK with two exceptions; groundwater levels in the northern Permo-Triassic sandstones are likely to remain at notably high levels for some months, and levels in parts of the southern Chalk are likely to be above normal. Whereas this variation in groundwater levels is likely to persist, perhaps until early autumn, river flows are more responsive to rainfall and the longer term outlook is less settled.

Rainfall

Based on projections released by the Met Office on 23rd March: For April, and April-May-June as a whole, above-average precipitation is considered more probable than below-average. Overall, the probability of the UK-average precipitation for April-May-June falling into the driest of our five categories is between 15 and 20%. The probability of UK-average precipitation falling into our wettest category is around 25% (the 1981-2010 probability for each of these categories is 20%).

River flows

River flows in April are likely to show a clear north-west/south-east contrast with normal flows to the north and west, and above average flows to the south and east. This pattern is driven by the rainfall distribution in March combined with a forecast for April of average to above-average rainfall. The exception to this pattern is for chalk-fed streams in the south and east which are likely to have normal flows. This pattern of flows is likely to continue in the coming months. At the start of April it seemed possible that flows in Scotland might be below average, but significant rainfall in the first ten days of April in southern and eastern Scotland means that this is now unlikely to be the case except in the most northerly parts.

Groundwater

As the winter recharge ends, and spring recessions begin to be established, levels across the Chalk aquifers are likely to remain normal, or above normal over the next months. Levels in the North East, raised by March rainfall, are recessing, and also likely to be near normal. In other aquifers, responsive limestones, are likely to be high over the next month, and then return to seasonal norms over the summer.  High levels in the northern Permo-Triassic sandstone will drop slightly over the same period. Groundwater conditions present in April tend to persist until early autumn, in the absence of dramatic recharge events, so most outlooks show a high probability of normal to above normal conditions until October for fast responding Chalk aquifers, and later in the Permo-Triassic sandstones.

Further information - April 2016

Comprehensive further information that supports the production of the Hydrological Outlook UK is available here.