World’s Soil Resources

The geographical distribution of these soil moisture and evapotranspiration regimes can be estimated
with various methods, as discussed in Seneviratne et al. (2010). As an illustration, Figure 7.8 displays the
correlation of annual mean evapotranspiration with radiation and precipitation in an observation-driven
land surface model using a two-dimensional colour map. This analysis illustrates the existence of distinct
evapotranspiration regimes, with most regions clearly displaying either the characteristics of a soil moistureor
energy-limited evapotranspiration regime.
One should note that the relationship displayed in Figure 7.7 is qualitative, and is affected (both in space and
time) by variations in soil parameters, land cover characteristics, and other factors (e.g. Teuling et al., 2010;
Koster and Mahanama, 2012; Guillod et al., 2013).
The water and energy balances of land are tightly connected through the process of evapotranspiration. It
follows that the soil moisture effects on evapotranspiration (illustrated in Figure 7.8) are also highly relevant
for land energy exchanges at the land surface. This link makes soil moisture a strong control of temperature
variability and temperature extremes on land (e.g. Seneviratne et al., 2006; Fischer et al., 2007; Vautard et
al., 2007; Mueller and Seneviratne, 2012). Modelling estimates suggest that soil moisture feedbacks affect
about 60 percent of temperature variability in the present Mediterranean climate in summer (Seneviratne et
al., 2006) and that they induced additional temperature anomalies of the order of 2°C in Central Europe during
the 2003 European summer heat wave (Fischer et al., 2007). Observation-based analyses also confirm the
existence of strong correlations between the occurrence of hot extremes in regional hottest months and prior
precipitation deficits in regions with soil moisture-limited evapotranspiration regimes (Hirschi et al., 2011;
Quesada et al., 2012; Mueller and Seneviratne, 2012). The example of the European summer heat wave shows,
moreover, that these feedbacks can be relevant in extreme years even in regions like Central Europe which
have a dominant energy-limited evapotranspiration regime under the present climate.
For present climate conditions, the relationship between soil moisture deficits and hot extremes implies
that information on soil moisture deficits could be used for improved forecasting of temperature mean and
Dry
Transitional
Wet
EF max
Soil Moisture Limited Energy limited
EF=λE/R n
0
θ WILT
θ CRIT
Soil moisture content
Figure 7.7 Definition of soil moisture regimes and corresponding evapotranspiration regimes. Source: Seneviratne et al., 2010.
EF denotes the evaporative fraction, and EFmax its maximal value.
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