Evaporation: Beckhead Tarn has disappeared

Beckhead Tarn between Kirk Fell and Great Gable has completely gone. I've never seen anything like it. Perhaps it has trickled through into the ground due to lowering water table in the dry weather. But let's calculate how much energy would be needed to make it evaporate. Let's assume it is circular and of diameter 40 metres. It is only ankle deep so volume of water = pi x 20squared x 0.1 = 125 cubic metres. That's a mass of 125 tonnes of water. Latent heat of vaporisation of water is 2260 kJ per kg. That means 280 million kJ of energy. But we need also to remember that the water needed to be heated up to make it evaporate. It would be extreme to have it heated to boiling point but let's do it: mcdeltatheta for a temperature rise of 90 degrees Celsius needs 125000 x 4200 x 90 = 47 million kJ. Say the Sun was a 1000W per square metre heater, that gives 1.25 million Watts.Total Joules divide Watts gives 261600 seconds = 72 hours of sunshine. It might be feasible since it last rained.