3.2.1 The likely magnitude of meteorological controls driving
observed Ewc losses
Penman-Monteith potential evaporation estimates, \(E_{\text{PM}}\), were
made across the ranges described in section 2.3 above. A representation
of how \(E_{\text{PM}}\) varies with \(r_{a}\_s\) and saturation vapour
pressure deficit (expressed as RH ) is shown in Figure 4. With
respect to Objective 2, given the Penman-Monteith equation and the
assumptions of the analysis, to achieve the higher end of absoluteEwc losses observed (\(\approx\) 20 to 40 mm
d-1) either fairly low RH or very low\(r_{a}\_s\) values are required, or an equivalent combination ofRH and \(r_{a}\_s\). Relative humidity needs to be below
approximately 90% where \(r_{a}\_s\) is around 2 s
m-1 or around 97.5% as \(r_{a}\_s\) approaches 0.5 s
m-1. Figure 4 also highlights
how\(\ E_{\text{PM}}\)becomes increasingly sensitive to small changes in\(r_{a}\_s\) at lower values: i.e. for higher wind speeds and rougher
canopies (as previously shown by Beven, 1979 & Dolman, 1986). At these
low \(r_{a}\_s\) values, \(E_{\text{PM}}\) is also considerably more
sensitive to changes in RH , and even at relatively highRH , the potential for significant evaporation loss exists. Owing
to this extreme sensitivity at low \(r_{a}\_s\) values, uncertainties
associated with estimating effective \(r_{a}\_s\) values andRH become critical in the interpretation of the results presented
here and are discussed in more detail below.