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NASA Technical Reports Server (NTRS) 20140017639: Irrigation as an ...
by NASA Technical Reports Server (NTRS)
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Irrigation is the single largest anthropogenic water use,
a modification of the land surface that significantly
affects surface energy budgets, the water cycle, and
climate. Irrigation, however, is typically not included
in standard historical general circulation model (GCM)
simulations along with other anthropogenic and natural
forcings. To investigate the importance of irrigation as
an anthropogenic climate forcing, we conduct two 5-member
ensemble GCM experiments. Both are setup identical to the
historical forced (anthropogenic plus natural) scenario
used in version 5 of the Coupled Model Intercomparison
Project, but in one experiment we also add water to the
land surface using a dataset of historically estimated
irrigation rates. Irrigation has a negligible effect on
the global average radiative balance at the top of the
atmosphere, but causes significant cooling of global
average surface air temperatures over land and dampens
regional warming trends. This cooling is regionally
focused and is especially strong in Western North
America, the Mediterranean, the Middle East, and Asia.
Irrigation enhances cloud cover and precipitation in
these same regions, except for summer in parts of Monsoon
Asia, where irrigation causes a reduction in monsoon
season precipitation. Irrigation cools the surface,
reducing upward fluxes of longwave radiation (increasing
net longwave), and increases cloud cover, enhancing
shortwave reflection (reducing net shortwave). The
relative magnitude of these two processes causes regional
increases (northern India) or decreases (Central Asia,
China) in energy availability at the surface and top of
the atmosphere. Despite these changes in net radiation,
however, climate responses are due primarily to larger
magnitude shifts in the Bowen ratio from sensible to
latent heating. Irrigation impacts on temperature,
precipitation, and other climate variables are regionally
significant, even while other anthropogenic forcings
(anthropogenic aerosols, greenhouse gases, etc.) dominate
the long term climate evolution in the simulations. To
better constrain the magnitude and uncertainties of
irrigation-forced climate anomalies, irrigation should
therefore be considered as another important
anthropogenic climate forcing in the next generation of
historical climate simulations and multimodel
assessments.
Date Published: 2016-11-16 21:30:02
Identifier: NASA_NTRS_Archive_20140017639
Item Size: 14738784
Language: english
Media Type: texts
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