Publication Details

Category Text Publication
Reference Category Journals
DOI 10.3390/w9080607
Title (Primary) Agreement of four equations for computing dewfall in northern Germany
Author Xiao, H.; Meissner, R.; Borg, H.; Wang, R.; Cao, Q.
Source Titel Water
Year 2017
Department BOPHY
Volume 9
Page From art. 607
Language englisch
Keywords dewfall; energy balance; turbulent vapour transport; Penman-Monteith equation; Bowen ratio; vapour conductance
UFZ wide themes RU1
Abstract The energy balance (EB), turbulent vapour transport (TVT), Penman-Monteith (PM) and
Bowen ratio energy balance (BREB) equation were used to estimate dewfall based on meteorological
data. Initially there were big disagreements between the estimates from these four equations.
However, after multiplying the heat and vapour conductance terms by 0.33 the agreement was
much better. This implies that the disagreements derived from improper conductance values.
Initially we did not consider the effect of atmospheric stability on the conductances. With stability
correction the conductances were on average 0.5 times the values without stability correction. To
arrive at the aforementioned 0.33, the conductances with stability correction still need to be lower
by a factor of 0.66. The value of the von Karman constant and the relationships for the zero plane
displacement and the roughness length we used in our conductance computations are widely used,
but not the only possible ones. With different values and relationships also suggested in the
literature one can reach this factor. However, it is also possible that our wind speed data contributed
to the fact that the conductances we computed were too high. Their computation for a given
canopy—atmosphere system requires wind speeds from a wind profile in equilibrium with the
vegetation. This in turn requires an adequate fetch around the investigated surface. The highly
varied vegetation in and around the site where the study was conducted makes adequate fetch
rather doubtful. To obtain valid conductance values the atmospheric stability conditions must be
considered, the appropriate values for the von Karman constant, the zero plane displacement and
roughness length must be used, and there must be adequate fetch. The BREB equation does not
contain a conductance term and therefore does not suffer from the problems just stated. The other
three equations do. However, the BREB, like the EB and TVT equations, need the surface
temperature which is not routinely measured. This then leaves the PM equation from which this
temperature has been eliminated as the only option. Hence, in a future study dewfall estimates from
the PM equation should be compared with direct measurements with a high precision weighing
lysimeter.
Persistent UFZ Identifier https://www.ufz.de/index.php?en=20939&ufzPublicationIdentifier=19092
Xiao, H., Meissner, R., Borg, H., Wang, R., Cao, Q. (2017):
Agreement of four equations for computing dewfall in northern Germany
Water 9 , art. 607 10.3390/w9080607