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be zero in the base scenario (1; assuming that all irrigation water feeds into ET), and if it
were assumed C3km
3
in the comparative test scenario, the corresponding seasonal ET
DET
DS
irr
change ratio would be
D
ET
WS
irr
1, implying equal or greater loss of irrigation water by
ET during the Dry Season, when less irrigation water is used, than during the Wet Season
when more irrigation water is used. This would in turn mean a greater cooling effect by an
irrigation-driven increase in ET and associated latent heat flux during the Dry Season than
during the Wet Season, which is inconsistent with available temperature observation data
(Tables
1
,
2
; Fig.
3
). For the Dry Season, it is physically reasonable to assume in both
scenarios that all of the used irrigation water feeds into ET because runoff from the basin is
negligible and water storage is decreasing rather than increasing in the basin during this
season.
For the base scenario (1), Eq. (
9
) yields
DT
DS
irr
¼
4
7
DT
WS
irr
ð
10
Þ
In addition, inserting Eq. (
10
) into Eq. (
4
) yields DT
WS-irr
= -0.16 C and DT
DS-irr
=-0.09 C for the base scenario. Similarly, for the test scenario, Eq. (
9
) yields
DT
DS
irr
¼
4
5
DT
WS
irr
ð
11
Þ
which, inserted in Eq. (
4
), yields DT
WS-irr
= -0.35 C and DT
DS-irr
=-0.28 C for the test
scenario.
For both scenarios, the annual average T change due to irrigation, DT
ann-irr
, is obtained
as the temporal average of DT
DS-irr
and DT
WS-irr
. The regional manifestation of global
climate change, DT
cl
, can be calculated from either one of the Eqs. (
1
)or(
2
), given the
obtained DT
WS-irr
and DT
DS-irr
values, as well as from the relation DT
cl
= DT
a
- DT
ann-irr
,
where DT
a
is the data-given value of annual average T change. There are thus three
different possibilities for calculating DT
cl
, and all of these possibilities must and do provide
the same resulting DT
cl
value for each scenario. Furthermore, with seasonal (and thereby
also corresponding average annual) DET components given directly from the assumptions
of the different scenarios [see the seasonal temperature change ratios that define the dif-
ferent scenarios in Eqs. (
10
)-(
11
)], associated seasonal (and average annual) latent heat
flux changes DF can be calculated from Eq. (
8
).
With regard to the DET and DF changes that are driven by the regional manifestation of
global climate change, however, one cannot calculate them in the same way as the irri-
gation-driven flux changes, from the climate-driven temperature change DT
cl
. The reason is
that DT
cl
quantifies the total climate-driven T change and not only the latent heat-related
DT contribution, whereas the DT components driven by irrigation are entirely due to the
latent heat flux changes implied by the regional irrigation. In order to also compare the
irrigation-driven DET and DF changes with those driven purely by the observed climate
change, we must use results from more complex, distributed hydrological modeling of a
hypothetical scenario of only climate (i.e., actually observed T and P) change, the results
from which are summarized in Table
1
. The ET change result from this hypothetical
climate scenario is then used to quantify the climate-driven DET, and the associated latent
heat flux change DF can further be calculated from Eq. (
8
).
The total annual average changes of ET and F can finally be calculated as the sums of
the different (irrigation and climate change) components of DET and DF, respectively.
This DET result for the simple base and test scenarios can be compared with the
