By Mark Kinver
BBC News science and nature reporter
Wading birds like lapwings are beginning to feel the heat (P N Watts/English Nature)
Wildlife experts say the two consecutive dry winters are beginning to have an impact on flora and fauna in parts of the UK affected by drought conditions.
The RSPB says 2006 is one the worst breeding seasons on record for wetland birds.
Rainfall in England and Wales over the past 18 months has been the second lowest since 1976, but water levels in parts of central and southern England are at their lowest level since the 1930s.
"If you have a dry winter like the one we have had then you are not able to make the wetlands suitable for the birds you are trying to attract," says Phil Burston, the RSPB's senior water policy officer.
He said fewer birds such as lapwings, redshanks and snipes have arrived at the charity's wetlands reserves.
"Our reserve at Elmley Marshes in Kent would, in a normal year, have about 200 pairs of lapwings.
"Last year it had 80 pairs, and this year it looks as if we have just 60 pairs. So the trend is downwards."
Mr Burston says that a number of birds that did arrive have made no attempt to nest because conditions are too dry.
He says although the breeding season closes towards the end of June, a dry summer will make matters worse.
"The adults and the young have to find food, such as invertebrates in moist soils. So if these wetlands dry up then the waders will not be able to get access to their food. If that continues, they will eventually die."
To help alleviate the problem, the RSPB has been granted a 28-day licence by the Environment Agency to pump water from a nearby creek on to the marsh.
Fears for fish
Elsewhere, the Environment Agency says it is already seeing the consequences of the prolonged dry period.
These include an algal bloom in a lake in Hampshire, and the deaths of hundreds of fish that became trapped in a shallow stream in Berkshire.
"Because it has been relatively wet during April and May, the impacts have been isolated," says Environment Agency water resources manager Glenn Watts.
A long, hot summer will hit aquatic wildlife, experts say
"However, as we head into the summer they will become more obvious. River levels will drop quite quickly this year unless there is relatively high rainfall."
Mr Watts warns that fish populations will be one of the main casualties of a hot summer: "We would see much lower river flows, oxygen levels dropping, resulting in more fish being stressed, if not more deaths."
He says that low water levels would also mean that many species would not be able to reach their spawning grounds, remaining in the main rivers, leaving them within the easy reach of predators.
"We will also see more algal blooms, more weed growth, resulting in the rivers looking quite different," Mr Watts adds.
This month has seen an increase in evaporation rates and soil losing its moisture, which the Centre for Ecology and Hydrology (CEH) believes marks the end of the "recharge season" for this year.
In its latest hydrological summary, the centre says groundwater levels and river flows are set to decline through the summer, with the prospect of exceptionally low flows by the autumn.
"Winter rainfall does control how much water is available in soils to tide plants over dry periods in the summer," says CEH spokesman Barnaby Smith.
However, he says it is too early to be talking in terms of an "ecological crisis".
"If we have a wet summer we will probably not see too many adverse effects, but we will not need a particularly long dry period before some plants start to suffer," Dr Smith adds.
Among the most vulnerable species of trees are birch and beech. The trees' shallow roots mean they are dependent on being able to access water near the surface.
One visible sign that trees were suffering from a lack of water would be the shedding of leaves much earlier than the autumnal norm.
In 1976, millions of trees were reported to have been killed by that year's prolonged summer drought.
"This is the sort of thing that we could see again happening this year, but it would depend upon a very long, hot, dry summer," says Glenn Watts.
Agriculture is one area that straddles both the demand for water abstraction and the environmental impact of water shortages.
"It is too early to tell if the drought this year is going to be a problem for farmers because we have not got to the stage yet where crops are under stress," says National Farmers' Union (NFU) head of policy services, Andrew Clark.
Farmers say crops are not yet showing signs of stress
"The concern for farmers comes in July or August when they want to know whether they will be able to use spray irrigation."
Any water restrictions will particularly affect vegetable crops, such as potatoes and onions, which farmers need to continue watering over the summer months.
"This is to make sure that we get the right size of potatoes, onions, parsnips, carrots etc that the retail market wants us to provide," Mr Clark says.
He says farmers are offered guidance on irrigation. The NFU, Environment Agency, and the UK Irrigation Association have all issued advice on the wise use of water.
"To be honest," says Mr Clark, "water is such a scarce resource that even in a relatively damp period all responsible irrigators would be looking at their methods."
The Environment Agency's Glenn Watts says any savings that can be made now will prove invaluable if rainfall levels remain below average.
"If we had average rainfall over the summer but another dry winter, similar to the previous two, then water companies will face similar difficulties to this year.
"However, if we had a dry summer which was followed by a dry winter then we would face a very difficult situation."