Geoscience Reference
In-Depth Information
Importantly, the expected increase in human water demand and the related change in land
use seem to occur faster than any anticipated effect of a changing climate on the avail-
ability of freshwater (Piao et al.
2007
). To comprehend the combined effect of (and
interactions between) increasing human stresses on water supplies, land use and climate
change, the understanding of the hydrological cycle needs to be advanced on all inter-
connected scales from local to global.
Many regional and local studies already exist on how climate change is expected to
impact the different compartments of the hydrological cycle and how societies may best
adapt to these changes by changing land use, water storage or even water use itself
(Ludwig et al.
2008
; Christensen et al.
2004
; Fowler et al.
2007
; Ludwig et al.
2008
; Prasch
et al.
2013
). The majority of these hydrological cycle studies, with few exceptions (e.g.,
Strengers et al.
2010
; Zabel and Mauser
2013
), treat this issue as a unidirectional cause-
effect chain, where a changing climate influences the regional and local hydrological cycle
and thereby affects water availability. Possible effects of dynamic, human-induced land
surface changes on the carbon cycle and regional and global water cycle, which in turn
may result in a changing climate, are, however, neglected here.
All components of the hydrological cycle are involved in different ways in climate
change, either by causing it or by reacting to it, sometimes amplifying each other's action,
sometimes giving rise to negative feedbacks such as atmospheric cooling through larger
Sun shielding cloud formations from increased evaporation. Variations in the hydrological
cycle often take place at regional or even local scale (such as orographic variations in
precipitation, small-scale variations of soil physical properties, ecosystem composition or
run-off processes), but can still trigger modifications that have an upscale effect possibly
leading to global changes in the hydrological cycle. The mechanisms and magnitudes of
the feedbacks of local interferences with the hydrological cycle with the regional and
global hydrological cycle are complex, largely unknown and cannot satisfactorily be
explored with current Earth system models. The feedbacks, e.g., between precipitation,
land use, soil moisture and the resulting evapotranspiration, are especially challenging and
exhibit strong intermittency and interactions at all scales, which makes them often hard to
model with contemporary Earth system models. The strong, small-scale and nonlinear
dependency of the participating processes on topography, soil physical properties and plant
physiology makes it also hard to estimate their magnitudes from sparse rain gauge, sparser
soil moisture and even sparser evapotranspiration measurement networks.
Nevertheless, there are important research issues waiting to be addressed, which are
related to the hydrological cycle and its relation both to the carbon cycle and the global
climate system. They centre on the feedbacks between the accumulated effects of
increasing local human interferences with the hydrological cycle through more intense
agricultural use and the global climate system. Most relevant and probably most difficult to
understand and model is a possible change in precipitation patterns, which results from
land use and evapotranspiration changes in one region and which affects the local and
regional hydrological cycle as well as the water availability elsewhere through telecon-
nections in the global circulation. The described mechanism may be of extreme relevance
to the affected region in cases when rainfall is reduced below a critical value, where
agriculture becomes impossible. Besides the global climate, a substantial part of the global
population may thereby indirectly be affected by collective land-use activities in another
part of the globe. Current understanding of the hydrological cycle, its components and the
feedbacks with the carbon cycle and the climate system is not sufficient to approach this
and similar research questions related to the multi-scale hydrological cycle. Currently
available Earth system models both lack spatial resolution to be able to accumulate the
