Deficits and deluges

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  • The garden of the Lower Lode Inn near Tewkesbury, Gloucestershire, on Wednesday May 2 after flooding across the UK. Photo: Tim Ireland/PA Wire

    Water flowing rapidly down a road in Cheddar Gorge, Somerset, on Tuesday May 1. Photo: Ben Birchall/PA

    Sign at Cherrapunji, Meghalaya state, India. Photo: R Mehra

    Nohkalikai Fall, Meghalaya, India.

  • Deficits and deluges
    14.05.2012 06:41

     

    Debate continues as to whether or not those areas of Britain currently under hose pipe restrictions are still in drought; but there is no argument that there is still a water deficit in parts of eastern and southern England.

    However much it has rained during the past four weeks – and it’s been a lot, with over 200 per cent of the climate normal during April across England and Wales – it needs even more to make up for a couple of particularly dry years. We are roughly half way towards seeing the shortage fully made up.

    The problem stems form dry winters. Even the winter of 2010 to 2011 was dry, despite the amount of snow that fell then seeming to belie that statement.

    During winter the rain is more ‘useful’ in that it can almost immediately start to fill aquifers, rivers and reservoirs. During spring, though, a lot of water is ‘stolen’ by burgeoning plant growth, or evaporates due to greater warmth and through transpiration from increasingly leafy vegetation.

    It’s one of the vagaries of the climate of the British Isles, being perched on the boundary of a continent and an ocean, that we cannot rely on one pattern of weather setting in reliably for a season. We only have to look back at the warmth and sunshine that many regions experienced in March to realise that.

    However, some of these patterns have perhaps been lengthier than usual during the last couple of years, as we have often seen a weakening of the usual westerly flows and a greater number of instances of atmospheric ‘blocking’, where the flow has an exaggerated north-south set up.

    In some parts of the world there is greater predictability, of course. The Atacama Desert in South America will be dry, as will the McMurdo Dry Valleys in Antarctica, the most arid spot on the planet.

    The monsoon rains will come every summer to the Indian Subcontinent, although in some years more reliably than others. It’s what makes Cherrapunji in northeast India’s Meghalaya State the wettest place in the world.

    How much rain falls on the entire Earth in a day, though? Globally averaged annual precipitation is about 990mm. Sufficient for everyone on the planet, you might think – but only a fraction can be collected, and most of it falls over the oceans.

    By: Stephen Davenport