Ecological research and large scale land-atmosphere feedbacks: lesson from the Bouchet's complementary relationship

Abstract. Acceleration of the global water cycle over recent decades, which is hypothesized by several studies, remains uncertain because of the high inter-annual variability of its components. Observations of pan evaporation (E_pan), a proxy of potential evapotranspiration (ET_p), may help to identify trends in the water cycle over long time periods. The complementary relation (CR; Bouchet, 1963) states ET_p and actual evapotranspiration (ET_a) depend on each other in a complementary manner, through land-atmosphere feedbacks in water limited environments. Using a long time series of E_pan observations in Australia, we estimated monthly ET_a values using the CR and compared our estimates with ET_a measured at eddy covariance stations in Fluxnet. Our results confirm that CR can be reliably applied to estimate ET_a and produces better results than a global vegetation model run without specific calibration. In addition, our analysis indicated that, on average, ET_a did not show any significant trend between 1975 and 2009 in Australia, but short-term analysis including anomaly periods may give the idea of a rapid climate change that is not perceived in a long-term perspective.