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5 Conclusion
The present quantification and cross-regional comparison of hydro-climatic change has
shown that significant differences in seasonal temperature changes, indicating the cooling
effects of irrigation, are exhibited in different land regions. The relatively simple approach
proposed by Destouni et al. ( 2010 ) may therefore be widely useful in distinguishing and
quantifying such irrigation effects. An important insight gained from the present com-
parative study is that irrigation-driven changes to the climatically important vapor and
latent heat fluxes and associated temperature change effects at the land surface may be
greater in land regions with small relative changes to water resource availability in the
landscape (like the MRB) than in regions with large such changes (like the Aral region).
The role and importance of various natural and anthropogenic change drivers and their
impacts may thus differ greatly both among different regions and between different per-
spectives on continental water change. A focus on vapor and latent heat fluxes and tem-
perature changes at the land-atmosphere interface represents one perspective on the
continental part of Earth's hydrological cycle. Focus on the availability and flow of con-
tinental water in the landscape represents another such perspective. Regardless of per-
spective, however, the change constraints that are implied by basin-wise water balance
should be accounted for in order to accurately understand and quantify continental hydro-
climatic changes and their variability across different land regions.
Acknowledgments This work has been carried out within the framework of the strategic environmental
research project EkoKlim at Stockholm University. The study was inspired by and written for the docu-
mentation of the ISSI conference The Earth's Hydrological Cycle, Bern, Switzerland, February 6-10, 2012.
Appendix: Significance Testing
The null hypothesis tested is that there has been no change in the long-term average values
of investigated variables from the time period in the beginning to that in the end of the
twentieth century, with the periods being those listed in Table 1 for each region. The null
hypothesis is expressed as l b ¼ l e ¼ l, where l b and l e are the average values of each
investigated variable in the beginning and the end of the twentieth century, respectively.
The hypothesized same average value l for the two periods is estimated from available
data for the end-of-century period, that is, as l ¼ l e . The alternative hypothesis that there
is significant change in the long-term average values between the two averaging time
periods is expressed as l b l e .
The standard normal test variable is as follows:
z ¼ x l
j
j
r = n
p
which is normally distributed with mean 0 (when the null hypothesis is true) and variance r 2 /n,
where r is the standard deviation of each investigated variable, estimated consistently with l
from available data for the end - of-century period with n being the sample size (number of
years with d a ta in that period), x is the sample mean value in the b e ginning-of-century time
period and x j j is the absolute value of the difference between x and l. The value of the
standard normal test variable z is computed and listed for different investigated variables in
Table 3 , along with the confidence level (p) at which the null hypoth es is of no change is
rejected, and hence, the change in variable average value indicated by x l
j
j is significant.
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