Geoscience Reference
In-Depth Information
Donohue et al.
2007
; Shibuo et al.
2007
; Asokan et al.
2010
; Destouni et al.
2013
). In
addition to impacts from global climate change, human changes in land uses and water
uses that affect vegetation will also affect the Earth's water cycle (Foley et al.
2005
;
Shibuo et al.
2007
; Piao et al.
2007
; Weiskel et al.
2007
; Wisser et al.
2010
; Destouni et al.
2013
). In particular, all agricultural developments that increase the cultivated area or
change the biomass production in a given land area, for instance by irrigation, are asso-
ciated with vegetation changes that will then also affect regional ET rates (Kvalevag et al.
2010
; Destouni et al.
2013
).
Previous studies have shown different types of land-use and water-use changes that may
lead to considerable hydro-climatic change. For instance, deforestation may decrease ET
and increase R, while opposite impacts may result from new forest establishment on
previously sparsely vegetated land (Vanlill et al.
1980
; Gordon et al.
2005
; Loarie et al.
2011
). Furthermore, the conversion of natural unplowed land to cultivated land may often
increase ET (Loarie et al.
2011
; Destouni et al.
2013
), but such conversions may under
some conditions also decrease it (Schilling et al.
2008
). A change from agriculture to forest
may further initially decrease ET (Qiu et al.
2011
) and later increase it (Donohue et al.
2007
). Regarding irrigation of agricultural areas, the direct withdrawal of freshwater for
the irrigation, in addition to the actual land irrigation itself, has been found to affect water
and vapor fluxes at the land surface, both globally (Foley et al.
2005
; Gordon et al.
2005
)
and regionally (Shibuo et al.
2007
; Lobell et al.
2009
; Asokan et al.
2010
; Destouni et al.
2010
; Lee et al.
2011
;T
¨
rnqvist and Jarsj
¨
2012
; Jarsj
¨
et al.
2012
). With close to one
billion people living in regions where irrigation is already used and may be used
increasingly in the future to further enhance agricultural yields and ensure food safety for
growing populations, it is important to understand and distinguish different aspects and
magnitudes of hydro-climatic changes and their water resource impacts driven by irrigation
under different regional conditions (Keiser et al.
2005
; Lobell and Field
2007
).
In order to realistically understand, project and efficiently mitigate or adapt to the
adverse hydrological effects of both global climate change and regional changes in land
use and water use, including irrigation, the dominant drivers, processes and effects need to
be understood and quantified across different scales, including the relevant water man-
agement scales, which are commonly those of regional drainage basins. Regionally, the
hydrological impacts from changes in global atmospheric circulation and climate overlap
with the impacts from regional irrigation and other land-use and water-use changes (Lobell
and Field
2007
). This overlap makes it difficult to distinguish different hydrological cause
and effect relations (Milly et al.
2002
; Piao et al.
2007
; Destouni et al.
2008
). However, the
topographical water divides that define regional drainage basins are also physical bound-
aries, which constrain reasonably well the flows of water and waterborne substances
through the landscape, and associated environmental impacts of man-made changes to
these flows (Jarsj¨ and Destouni
2004
; Darracq et al.
2005
; Shibuo et al.
2007
; Destouni
and Darracq
2009
;T¨ rnqvist et al.
2011
; Jarsj¨ et al.
2012
; Visser et al.
2012
; Destouni
et al.
2013
). The understanding of and distinction between different hydrological change
components and their drivers and effects can therefore be greatly aided and improved by
honoring and accounting for the water flux constraints implied by the fundamental water
balance quantification ET = P - R - DS, which applies to all hydrological drainage
basins. Here, DS is water storage change within the basin, the effects of which are often
small so that this term may be neglected for long-term changes over temporal scales that
are much larger than the annual scale (Destouni et al.
2013
).
The basin-wise water balance constraints imply that the ET, which is difficult to
measure and quantify on large scales, can be derived from directly measured P data across
