Observer: There is a common saying in Spain that during a drought, the trees chase after the dogs. Now it is ringing true as the country struggles to deal with the worst drought since the Forties: reservoirs stand at 46 per cent of capacity and rainfall over the past 18 months has been 40 per cent below average. But months before the scorching summer sun threatens to reduce supplies to a trickle, a bitter political battle is raging over how to manage Spain’s scarcest resource – …