3.3.1 Penman-Monteith wet-canopy evaporation estimates for
mountainous regions of the UK
Estimates of \(E_{\text{PM}}\) made using the meteorological data for
the 17 sites specified in section 2.3.2 show that, given the assumptions
of our analysis, within-storm conditions for potentially high Ewcloss are possible in mountainous regions of the UK. High wind speeds and
relatively low RH can prevail during days with significant
rainfall. This is illustrated in Figure 5a where hourly average\(r_{a}\_s\) versus RH data are plotted for the sites identified
in Table Supp. 2. The points plotted in Figure 5a relate to hourly
periods within a 24-hour period with over 50 mm of rainfall
and where the hourly rainfall total was above zero. The
estimates of \(r_{a}\_s\), using the 3 scenarios for \(z0\_s\) as
described above, are represented by: black filled-circles
for\(\ z0\_s=\) \(0.1(\text{Zc})\), green filled-circles for\(z0\_s=\) \(0.05(\text{Zc})\) and red filled-circles for \(z0\_s=\)\(0.01(Zc)\). Figure 5a demonstrates that very low \(r_{a}\_s\ \)values
can occur within 24-hour periods where 𝑃𝑔 is greater than 50 mm and that
the majority of these periods were associated with RH values
predominantly in the range 85% to 98% which shows significant overlap
with the conditions required for significant \(E_{\text{PM}}\) estimated
for Objective 2. Meteorological conditions during more extreme events
(>150 mm in 24-hours and where the hourly rainfall
> 0), also shown in Figure 5a as diamonds; this figure
suggests that, particularly for RH, conditions can be even more
favourable for high \(E_{\text{PM}}\) but are associated with the caveat
that there are relatively few observations during very few events of
this magnitude. The potential for high \(E_{\text{PM}}\) is shown more
explicitly in Figure 5b (which uses the same data as Figure 5a). The
difference between the estimates made using the 3 \(z0\_s\) highlights
again how sensitive \(E_{\text{PM}}\) magnitude is to\(\ z0\_s\).
However, fairly high rates of \(E_{\text{PM}}\) are estimated for all\(z0\_s\) scenarios although the \(z0\_s=\ 0.01(\text{Zc})\)scenario is mainly limited to losses of below 12 mm
d-1.