The Penman-Monteith (PM) equation is a state-of-the-art modelling approach to simulate evapotranspiration (ET) at site and local scale. However, its practical application is often restricted by the availability and quality of required parameters. One of these parameters is the canopy conductance. Long term measurements of evapotranspiration by the eddy-covariance method provide an improved data basis to determine this parameter by inverse modelling. Because this approach may also include evaporation from the soil, not only the ‘actual’ canopy conductance but the whole surface conductance (gc$g_{c}$) is addressed. Two full cycles of crop rotation with five different crop types (winter barley, winter rape seed, winter wheat, silage maize, and spring barley) have been continuously monitored for 10 years. These data form the basis for this study. As estimates of gc$g_{c}$ are obtained on basis of measurements, we investigated the impact of measurements uncertainties on obtained values of gc$g_{c }$. Here, two different foci were inspected more in detail. Firstly, the effect of the energy balance closure gap (EBCG) on obtained values of gc$g_{c}$ was analysed. Secondly, the common hydrological practice to use vegetation height (hc$h_{c}$) to determine the period of highest plant activity (i.e., times with maximum gc$g_{c}$ concerning CO2-exchange and transpiration) was critically reviewed. The results showed that hc$h_{c}$ and gc$g_{c}$ do only agree at the beginning of the growing season but increasingly differ during the rest of the growing season. Thus, the utilisation of hc$h_{c}$ as a proxy to assess maximum gc$g_{c}$ (gc,max$g_{c,\text{max}}$) can lead to inaccurate estimates of gc,max$g_{c,\text{max}}$ which in turn can cause serious shortcomings in simulated ET. The light use efficiency (LUE) is superior to hc$h_{c}$ as a proxy to determine periods with maximum gc$g_{c}$. Based on this proxy, crop specific estimates of gc,max$g_{c,\text{max}}$ could be determined for the first (and the second) cycle of crop rotation: winter barley, 19.2 mm s−1 (16.0 mm s−1); winter rape seed, 12.3 mm s−1 (13.1 mm s−1); winter wheat, 16.5 mm s−1 (11.2 mm s−1); silage maize, 7.4 mm s−1 (8.5 mm s−1); and spring barley, 7.0 mm s−1 (6.2 mm s−1).
