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.