Trend analysis of evapotranspiration over India: Observed from long-term satellite measurements

Owing to the lack of consistent spatial time series data on actual evapotranspiration (ET), very few studies have been conducted on the long-term trend and variability in ET at a national scale over the Indian subcontinent. The present study uses biome specific ET data derived from NOAA satellite's advanced very high resolution radiometer to investigate the trends and variability in ET over India from 1983 to 2006. Trend analysis using the non-parametric Mann-Kendall test showed that the domain average ET decreased during the period at a rate of 0.22mm year(-1). A strong decreasing trend (m = -1.75mm year(-1), F = 17.41, P < 0.01) was observed in forest regions. Seasonal analyses indicated a decreasing trend during southwest summer monsoon (m = -0.320mm season(-1) year(-1)) and post-monsoon period (m = -0.188mm season(-1) year(-1)). In contrast, an increasing trend was observed during northeast winter monsoon (m = 0.156mm season(-1) year(-1)) and pre-monsoon (m = 0.068mm season(-1) year(-1)) periods. Despite an overall net decline in the country, a considerable increase (>4mm year(-1)) was observed over arid and semi-arid regions. Grid level correlation with various climatic parameters exhibited a strong positive correlation (r > 0.5) of ET with soil moisture and precipitation over semi-arid and arid regions, whereas a negative correlation (r<-0.5) occurred with temperature and insolation in dry regions of western India. The results of this analysis are useful for understanding regional ET dynamics and its relationship with various climatic parameters over India. Future studies on the effects of ET changes on the hydrological cycle, carbon cycle, and energy partitioning are needed to account for the feedbacks to the climate.