Geography Reference
In-Depth Information
Goldewijk et al.
2011
). More attention has been paid to the influence of farmland
management on climate, e.g., irrigation, no-till agriculture and crop rotation,
among which the influence of irrigation received the most attention. The change in
irrigation has been expected to influence local climate since it directly influences
soil moisture, which will further affect land surface albedo, evaporation and var-
iation in regional temperature and precipitation. The irrigation area has increased
very rapidly during the past centuries. For example, global irrigation area was
8.0 Mha in 1800, it increased to 40 Mha in 1900 and further increased 2.7 Mha in
2000 (Kueppers et al.
2007
). The global irrigation water accounts for 70 % of the
water used by human beings (Douglas et al.
2009
). The irrigation districts mainly
include
China,
India,
Pakistan,
Thailand,
North
America
and
the
Aral
Sea
watershed (Kueppers et al.
2007
).
There have been very few quantitative studies of how the climate responds to
irrigation. The simulation studies in many regions indicated that increased soil
moisture due to irrigation will generally lead to significant decrease of local
average and maximum temperature (T
max
), while the change of the minimum
temperatures (T
min
) varies among regions (Kueppers et al.
2008
). Irrigation will
make the regional moisture in the atmosphere increase significantly, and conse-
quently lead to regional precipitation increase under the appropriate weather
conditions (Douglas et al.
2009
). Besides, irrigation plays a role in reducing the
difference between regional temperature and daily temperature (Oyama and Nobre
2004
; Kueppers et al.
2008
; Douglas et al.
2006
; Lobell et al.
2009
).
Irrigation affects surface climate mainly through the redistribution of latent heat
and sensible heat. When the irrigation area increases, the latent heat will increase
while the sensible heat will decrease. The increase of latent heat will further lead to
the increase of cloud coverage and decrease of the net radiation of land surface. The
researches have indicated that there are obvious seasonal variations of the tem-
perature decrease due to the irrigation. The temperature will reduce significantly in
the dry season and insignificantly in rainy season (Douglas et al.
2006
; Lobell et al.
2009
). Besides, there is obvious regional heterogeneity of the temperature decrease
due to the irrigation (Lobell et al.
2009
). The simulation researches indicated that
the difference in the irrigation area will lead to the difference in soil moisture. The
difference in the response of the cloud to the irrigation is the main reason for the
difference in the regional climate. The cooling effect of irrigation in some regions
(e.g., North America, northwest part of India, northeast part of China) is comparable
to the warming effect in the magnitude, and consequently plays a role in alleviating
the climate warming (Kueppers et al.
2007
). However, according to the prediction
of land use changes, the irrigation area in these regions will show a decreasing trend
in the future decades, which will aggravate the climate warming.
