Changes in pan evaporation in Mexico from 1961 to 2010

An important driver of the terrestrial hydrological cycle is atmospheric evaporative demand. Recent studies using measurements of pan evaporation have found evidence that the atmospheric evaporative demand has been declining over the second half of the 20th century. This work analyses long-term time series of pan evaporation obtained from approximately 150 +/- 30 weather stations located in Mexico with aridity indexes ranging from 0.3 to 10 for 1961-2010. The results show a consistent decline in annual pan evaporation for 1960-1990 (-3.8mmyear(-2)) and for 1990-2010 (-2.6mmyear(-2)) periods whereas the average change during the complete period corresponds to -3.3mmyear(-2). Statistically significant negative changes using the non-parametric Mann-Kendall test were found in 43% of the stations for the early and 27% for the recent periods, respectively. The temperature, relative humidity, radiative and aerodynamic controls attributed to the observed changes are analysed with the Noah model output from the Global Land Data Assimilation System Version 2 (GLDAS-2). Among the climatological variables extracted from GLDAS-2, it was the annual wind speed and net radiation that gave the highest statistical correlations. This work agrees with previous studies that pan evaporation rates have been in a declining trend during the second half of the 20th century though milder decline rates have been observed over the last 20 years. Finally, we show that the magnitude of change in regions dominated by wind and in those dominated by radiative processes can strongly differ.