Trends in reference evapotranspiration in the humid region of northeast India
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abstract
In the present study, the trends in the reference evapotranspiration (ET O ) estimated through the Penman-Monteith method were investigated over the humid region of northeast (NE) India by using the Mann-Kendall (MK) test after removing the effect of significant lag-1 serial correlation from the time series of ET O by pre-whitening. During the last 22 years, ET O has been found to decrease significantly at annual and seasonal time scales for 6 sites in NE India and NE India as a whole. The seasonal decreases in ET O have, however, been more significant in the pre-monsoon season, indicating the presence of an element of a seasonal cycle. The decreases in ET O are mainly attributed to the net radiation and wind speed, which are also corroborated by the observed trends in these two parameters at almost all the times scales over most of the sites in NE India. The steady decrease in wind speed and decline in net radiation not only balanced the impact of the temperature increases on ET O , but may have actually caused the decreases in ET O over the humid region of northeast India. 2011 John Wiley & Sons, Ltd.
