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the forenoon relative humidity without any significant spatial and seasonal
heterogeneity. However, results of the test of homogeneity of trend for the
RH
an
indicates no heterogeneity with respect to seasons and stations, and also
a non-significant overall positive (
-
= 0.12) trend.
The result of increasing trend of relative humidity is consistent with that
evidenced by Wang and Gaffen (2001). The increasing trends of the relative
humidity may be attributed to the abundant moisture supply from the Bay of
Bengal. The minimum temperature also contributes to the increase in relative
humidity trends in winter. This explains the increasing trend of relative humidity
in Orissa as the night temperature shows a cooling trend. However, the
pronounced warming trend of daytime temperature could have increased the
moisture load in the atmosphere. Combustion of coal and other fossil fuel,
although in small quantity, produces water vapour (Gaffen and Ross, 1999).
Further, Gorden et al. (2005) reported that the increase in water vapour is
correlated with intensive food production in the Indian subcontinent, and thus
expanding irrigation increases the risk for changes in the monsoon system.
However, the uncertainty in both the temporal and spatial distribution of
water vapour remains to be a hindrance while attributing the trends and
variability of relative humidity (Gaffen and Ross, 1999). Even the day-night
difference in relative humidity is larger than the amplitude of the seasonal
cycle (Wang and Gaffen, 2001).
The radiative effect of water vapour feedback mechanism in climate change
context is comparable to the radiative forcing from CO
2
enrichment (Fasullo
and Sun, 2001). Surface relative humidity regulates the evaporation and
transpiration process, and consequently has connections with both hydrological
and surface energy budgets. Any increase in atmospheric moisture enhances
the moisture convergence into storm, and thus amplifies the intensity of rainfall.
However, the frequency and duration of rainfall decreases with consequent
prevalence of drought because the total precipitation is controlled by the
available surface energy (Trenberth, 1998; Trenberth et al., 2007). Therefore,
the drought occurrence has increased over tropics and sub-tropics partly due
to the reduced rainfall in landmass, and also due to the warming and increased
atmospheric demand for moisture. Increasing trend of relative humidity for
Orissa, although not significant, may also have contributed partially to the
occurrence of intense weather events.
11.5 Conclusions
This study employed two nonparametric statistical tests, i.e., Mann-Kendall
(MK) test and Spearman's rho (SR) test for trend detection to understand
response of three hydroclimatic variables (rainfall, temperature and relative
humidity) of Orissa. The annual rainfall shows that the anomalous events in
terms of both drought and flood are increasingly more conspicuous in recent
years. However, the mean annual rainfall and the linear relationship did not
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