Pan evaporation trends and the terrestrial water balance. I. Principles and observations

Pan evaporation is just that - it is the evaporation rate of water from a small dish located at the ground-surface. Pan evaporation is a measure of the evaporative demand over terrestrial surfaces. Declines in pan evaporation have now been reported in many regions of the world. The trends vary from one pan to the next, but when averaged over many pans, they are typically in the range of -1 to -4 mm a^-2 (mm per annum per annum). In energetic terms, a trend of -2 mm a^-2 is equivalent to -0.16 W m^-2 a -1 and over 30 years this is a change of -4.8 W m^-2. For comparison, the top-of-atmosphere forcing due to doubled CO₂ is estimated by the Intergovernmental Panel on Climate Change (IPCC) to be ∼3.7 W m^-2. Hence, the magnitude of the pan evaporation trend is large. What is of even greater interest is the direction - a decline - given the well-established warming of the last 30-50 years. In this article, the first in a two part series, we describe the underlying principles in using and interpreting pan evaporation data and then summarise the reported observations from different countries. In the second article, we describe the interpretation of the trends in terms of changes in the terrestrial water balance.