Atmospheric evaporative demand (AED) refers to the combined evaporation and transpiration over a land surface. It encompasses different concepts of evapotranspiration (ET): estimates of potential evapotranspiration (ET_p) and reference crop evapotranspiration (ET₀) as well as measured pan evaporation (E_pan). 

With a large percentage of the farmland used for water-consuming rice crops as well as vast tracts in semiarid areas with high irrigation demand, the precise knowledge of AED rates and its potential future development is of prime importance. 

Prof. FAN Zexin from Xishuangbanna Tropical Botanical Garden (XTBG) and Prof. Axel Thomas from Germany studied spatiotemporal variability of attribution patterns over several climate zones and a large latitudinal extent.  

They estimated FAO-Penman-Monteith reference crop evapotranspiration rates for 644 meteorological stations over China during 1960–2011 to analyze spatial and temporal attribution variability. 

They found that reference crop evapotranspiration and observed potential evapotranspiration rates have in general been declining during the last decades, both in China and on a global scale, even though temperatures have increased. 

The attribution of the four climatic variables (including sunshine duration, wind speed, relative humidity and minimum as well as maximum temperature) to reference crop evapotranspiration rates in China varied geographically and on a seasonal basis. 

Spatially distributed attribution analysis showed that the position of the center of maximum contribution of sunshine duration has shifted from Southeast to Northeast China, while in West China, the contribution of wind speed has decreased dramatically. 

They further found that maximum temperature never contributed substantially to reference crop evapotranspiration.  

Their results confirmed that estimating reference crop evapotranspiration rates with simple temperature based estimation methods and without taking into account wind speed will likely give unreliable results, particularly in arid climates. 

The study entitled "Decadal changes of reference crop evapotranspiration attribution: Spatial and temporal variability over China 1960–2011" has been published online in Journal of Hydrology.