Llansadwrn (Anglesey) Weather Wx-Watch from the observer's notebook

The dry spell in March and April 2003, and concerns about soil water.

A dry March with 35.5 mm rainfall up to midnight on the 31st, brought rainfall for the year to 163 mm making it the 13th driest starts to the year on record since 1928. But as sometimes happens to spoil the record book, rainfall after midnight on 1 April until 09 GMT (10 am BST) of 15.8 mm must be credited to the previous month. This brought the total for March to 51.3 mm and ranked only 29th driest (2001 was 35.0 mm). But the total for the year (January to March) was 179 mm and ranked 10th driest. Totals for January to March in the driest years were 96 mm (1929), 144 mm (1963), 120 mm (1964) and 140 mm (1996). This year's January to March total was 32 mm less than in the driest year (1945), and a massive 164 mm less than that in the wettest (2000) on record.

It may seem surprising that, after the dry start, that water was standing on gardens and fields after a fall of 13.8 mm on the 7th and again after 15.8 mm on 1 April. But most soils remained nearly saturated with water (at field capacity). At the start of the dry spell comments seen in the press were that gardeners would be worried by the lack of water. They should not have worried at that time, however, as most soils still had considerable reserves of water; despite the dry weather there had been little or no evaporation since autumn. As temperatures improved, becoming above average towards the end of March, evaporation increased; there was evaporation from the ground (measured by lysimeter) from the 10th. Potential evapotranspiration (PE) at the end of the month was 22.9 mm, but the PWB (Potential water balance) on the 31st before the heavy rain still positive at 14.9 mm. From January it was 144.6 mm. After the rain the March PWB increased to 28.4 mm while from January it was 158.3 mm

The start of April continued the dry weather and it was not until the 13th that significant rain appeared in the west but little fell elsewhere. Rainfall here was only 3.3 mm (Capel Curig with 8 mm had the most) and brought the total to 5.4 mm. It was not until the 20th that there was further 6.8 mm rainfall that brought the rainfall to 12.2 mm. On both these occasions there was no standing water seen! April is the driest month of the year, with an average of 65 mm, and more rain can be expected statistically by the end of the month. But there have been some very dry April's: in 1980 there was only 6.7 mm while 1984 had 11.7 mm. With unusually high temperatures PE by the 19th was 30.5 mm. Totals for the period September 2002 to 19 April 2003 show that PE reached a total of 81.3 mm and rainfall 675.8 mm, while the PWB was still a healthy 587.7 mm.

Surface soils especially those that are light or sandy will become eventually, without some rain, deficient in moisture. Not all the water held in soils can be easily extracted by plants and trees; there is an amount of soil water (dependant on the type of soil) beyond which extraction is very slow, or cannot take place at all. This is known as the wilting point.

If soils are deficient in moisture sown seeds may not germinate and seedlings would indeed require watering. But overnight dew-falls have been heavy, up to 0.3 mm each night; the surface soil in the morning has been moist only drying later on some days. Recently sown spring barley had germinated and ploughed fields were showing green. Not until the 23rd, however, was the soil surface dry at 09 GMT. Pots of plants around the garden did require some water but other plants did not. In soil deeper rooted plants, and trees just coming into leaf, had no problem, and would have had no problem for a long time, being able to extract water from considerable depths in the soil. Eventually even trees will wilt and even die as in the late summer drought of 1976. The amount of water stored in reservoirs is another matter, but indications were that they were well stocked and even being used to top up river levels to protect salmon runs.

As the dry spell continued, and with 4 days of record temperatures and high rates of evaporation from 14 - 17th April, shallow rooted natural vegetation around coastal cliffs, moorlands and mountains was still looking winter-brown. There was a noticeable effect on spring re-growth, what should be lush and green, was still brown with last autumn's leaf die-back lying on the ground. Bluebells in woodland were flowering but were not in the profusion normal at this time of year. There was also some evidence that birds were delaying nesting activity. The dry surfaces led to many grass fires all over the UK. In South Wales alone the Fire Service had attended over 1000 grass fires and, at fires in Baglan and Wattsville, some homes were evacuated but were saved. In Scotland and the north of England, there were large and serious fires on some moorlands and woodland areas. A large fire on Kinder Scout in the Peak District started on the 16th, affected moorland conservation areas, setting fire to the underlying peat. In Dorset a grass heath fire near Verwood Forest trapped 3 people who had to be rescued by Police helicopter.

In dry spells gardeners should adopt the method of sowing seed I have seen in the drier parts of southern France. For example to sow a row of lettuce; with a corner of rake draw a 1 inch (2.5 cm) deep v-shaped furrow along the line. With a watering can water slowly and well along the furrow. When the water has soaked into the soil sow the seed then cover with soil using the rake. The seed will germinate and should not require further water even though the surface remains dry. This method also saves water; a lightly hoed soil surface will also conserve moisture underneath. A big advantage of a dry surface is that weed seeds tend not germinate. To be enjoyed while it lasted!

Diary

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These pages are designed and written by Donald Perkins © 1998 - 2003 Document dated March 2003 http://www.llansadwrn-wx.co.uk Top of page Return to homepage