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1. Summary - April 2025

Only 62% of the long term average (LTA) rainfall fell during April and the Hertfordshire and North London area (鈥渢he Area鈥� )experienced its second month in a row with lower than average rainfall. With the low rainfall and warm temperatures during April, there was very little effective rainfall. The Area鈥檚 soils continued to dry throughout the month, ending April with significantly higher than normal soil moisture deficits. River baseflows and groundwater levels across the Area continued to decline during April.

2. Rainfall

Only 62% of the LTA rainfall fell during April and the Area experienced its second month in a row with lower than average rainfall. Both the areal rainfall units of North London and Chilterns-East-Colne had totals which fell into the below normal band for April. Over the last 3 months these same units had seen notably low rainfall for that period. With an average of 24 dry days (<0.2mm rain) most of the monthly rainfall fell in just a few days mid-month, between 13 to 15 April and 20 to 22 April. The largest daily rainfall total of 24mm was recorded at Chenies rain gauge (Chilterns-East-Colne) on the 22 April.

3. Soil moisture deficit and recharge

With the low rainfall and warm temperatures during April, there was very little effective rainfall across the whole Area. Soils continued to dry throughout the month with soil moisture deficits doubling during April. Soil moisture deficits ended the month three times greater than would normally be expected, with North London recording the largest deficit.

3.1 聽聽聽聽聽聽聽 River flows

Baseflows across the Area continued to decline during April, with only small peaks in river flows in response to the mid-month rainfall. As with March, there was a clear split in fortunes between the chalk rivers of the Chilterns and Upper Lee, and with the north London and Essex rivers. The majority of chalk river flow indicator sites experiencing monthly mean flows in the above normal band, with the Ver at Colney Street (with its 5th highest April mean flows on record (1956-present)) and the Mimram at Panshanger (with its 4th highest April mean flows on record (1952-present)) recording flows in the exceptionally high band. In contrast the runoff dominated rivers of north London and Essex only recorded monthly mean flows in the below normal band.

4. Groundwater levels聽 聽

Across the month, groundwater levels continued their seasonal decline. However, groundwater levels remained buoyant with levels in the normal band or higher across the Mid-Chilterns Chalk, while the groundwater indicator sites in the Upper Lee Chalk were all in the notably high band for April. Of note, the groundwater at Lilley Bottom OBH (Upper Lee Chalk) recorded its fourth highest end of April level on record (1979 to present).

5. Reservoir stocks

Despite the warm weather and lower than average rainfall, reservoir stocks remained slightly above average in the Lee Valley and close to average in the Lower Thames reservoirs.

6. Environmental impact

In the Colne catchment, the locations of chalk river sources showed little to no change since February, the:

• River Ver started flowing close to Kensworth Lynch
• River Gade started flowing at Hudnall Corner
• River Bulbourne was flowing upstream of Dudswell village
• source of the River Chess remained upstream of Chesham
• River Misbourne flowed continuously from Mobwell pond, above Great Missenden

The chalk river sources in the Upper Lee showed little to no change since February, the:

• River Mimram started flowing above Whitwell Gas Compound
• River Beane started flowing above Cromer
• River Rib was flowing intermittently from Reed End, before gaining a steadier flow upstream of Buntingford
• River Ash (Herts) was flowing intermittently from Meesden, before gaining a steadier flow at Little Hadham
• River Stort was still flowing from its source above Langley Lower Green

To protect the environment, during April 2 (out of a summer maximum of 30) abstraction licence flow constraints were in force.

Author: Groundwater and Hydrology, [email protected]

Contact Details: 03708 506 506

All data are provisional and may be subject to revision. The views expressed in this document are not necessarily those of the Environment Agency. Its officers, servants or agents accept no liability for any loss or damage arising from the interpretation or use of the information, or reliance upon views contained within.