Scaling transpiration in apple orchards -- meteorological versus plant based physiological measurements.

摘要

In order to quantify water use of apple orchards, a field experiment was setup near Bonn, Germany, during the 1998 and 1999 summer period. Evapotranspiration from this orchard was estimated using a number of meteorological approaches. Tree water usewas estimated using sap flow measurements scaled to the whole orchard area through leaf area index. Evapotranspiration, the sum of tree and grass strip transpiration as well as soil moisture evaporation was estimated through (1) energy balance measurements (Bowen ratio), (2) similarity theory and (3) turbulent energy exchange (Eddy covariance). Tree transpiration measured by a sap flow technique (heat balance method) allowed for a separation of tree and grass strip contribution and, in the case of a dry surface, a direct comparison of scaling results through plant physiological and meteorologically based measurements. Energy balance and turbulent energy fluxes agreed well with both methods showing similar results. In addition, scaling of single tree sap flow measurements through leaf area index resulted in a comparable estimate of whole orchard transpiration both in daily course and total values when grass strip contribution was negligible due to a dry upper soil layer. Under conditions of a higher soil moisture content and large soil heat flux, the sap flow measurements were greatly overestimating tree transpiration while it appeared that latent heat flux determined by the Bowen ratio method was underestimated.