London scoping - ukcip

Final Report
19
there have been significant increases in PE (Penman method) at Kew since 1871, and for PE
(Thornthwaite method) using CETs since 1659. The strongest trends exist in spring and
autumn, although all seasons and annual totals show significant increases (Figure 3.7),
consistent with observed temperature rises. Indeed, annual PE totals in southern England often
exceeded 700 mm in the 1990’s – values more typical of western France (Marsh, 2001a).
In contrast, summer relative humidity has generally declined since the 1920’s, with
exceptionally low humidity values in August during the last two decades (Carter and Robertson,
1998). The Cranwell–Waddington relative humidity series indicates that occurrences of low
humidities are becoming more frequent but not necessarily more intense (Lockwood, 2000).
However, exceptionally low summer humidities in 1933/34, 1975/76 and 1989/90 were all
connected with widespread regional drought.
Figure 3.7 Oxford annual potential evaporation anomaly estimated using a modified
Thornthwaite method, 1901-1996. Anomalies are relative to 1961-1990 average.
Evaporation anomaly (%)
10
8
6
4
2
0
-2
-4
-6
1901 1911 1921 1931 1941 1951 1961 1971 1981 1991
3.6 River Flows
The detection of trends in river flow series is highly problematic because most hydrological
records tend to be short (