Geoscience Reference
In-Depth Information
DT
WS
DT
DS
¼
DT
WS
irr
DT
DS
irr
ð
3
Þ
Inserting the data-given values of DT
WS
= 0.23 C and DT
DS
= 0.30 C in Eq. (
3
)
yields:
DT
WS
irr
DT
DS
irr
¼
0
:
07
C
ð
4
Þ
The T change components due to the Wet Season and Dry Season irrigation are further
related to corresponding changes in latent heat flux DF
WS-irr
and DF
DS-irr
as follows:
DF
WS
irr
¼
M
a
C
p
DT
WS
irr
s
a
ð
5
Þ
DF
DS
irr
¼
M
a
C
p
DT
DS
irr
s
a
ð
6
Þ
where M
a
is the regional air mass that is cooled by the extra ET of the applied irrigation
water, C
p
is the specific heat capacity of the air and s
a
is the average regional residence
time of that air. Assuming, as done in Destouni et al. (
2010
), that the seasonal F and T
changes affect more or less the same regional air mass implies that
DT
DS
irr
¼
DT
WS
irr
DF
DS
irr
DF
WS
irr
ð
7
Þ
The latent heat flux change due to irrigation in either season can in turn be calculated as
follows:
DF
irr
¼
DET
irr
q
W
L
ð
8
Þ
where DET
irr
is the corresponding evapotranspiration change due to irrigation, q
w
is the
water density assumed in present calculations to be 1,000 kg/m
3
and L is the latent heat of
vaporization of 2,260 kJ/kg. Inserting Eq. (
8
) in Eq. (
7
) yields the seasonal DT components
as functions of the seasonal DET components:
DT
DS
irr
¼
DT
WS
irr
DET
DS
irr
DET
WS
irr
ð
9
Þ
The ratio of the seasonal ET changes required to evaluate equation (
9
) can be estimated
from the quantities of irrigation water applied during the respective seasons, that is, 4 km
3
for
Dry Season and 7 km
3
for Wet Season (Asokan
2005
). To investigate different possibilities of
how much of the applied irrigation water leads to actual ET increase in each season, we
consider two different scenarios for the seasonal ET change ratio. (1) The base scenario,
assuming zero water storage change over each season so that all applied irrigation water feeds
into the ET of each season and
D
ET
WS
irr
¼
7
. (2) The test scenario, which accounts for the
possibility that some part of the total applied irrigation water of 7 km
3
during the Wet Season
increases groundwater storage and/or adds to the runoff from the basin rather than feeding
into the ET and then assumes for comparative purposes the non-ET-contributing part to be
2km
3
; this yields the ET change ratio as
DET
DS
irr
DET
DS
irr
D
ET
WS
irr
¼
5
in this scenario.
The assumption of 2 km
3
in the test scenario (2) represents a large (near maximum
possible) contribution of the used irrigation water during the Wet Season to storage change
in the basin and/or runoff from the basin, instead of to ET. This contribution is assumed to
