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 0.22 mm year⁻¹. A strong decreasing trend (m=−1.75 mm year⁻¹, F=17.41, P 0.01) was observed in forest regions. Seasonal analyses indicated a decreasing trend during southwest summer monsoon (m=−0.320 mm season⁻¹year⁻¹) and post-monsoon period (m=−0.188 mm season⁻¹year⁻¹). In contrast, an increasing trend was observed during northeast winter monsoon (m=0.156 mm season⁻¹year⁻¹) and pre-monsoon (m=0.068mm season⁻¹year⁻¹) periods. Despite an overall net decline in the country, a considerable increase ( 4 mm 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.

Isotopic composition of atmospheric water vapor provides information on transport, mixing and phase change of water in the atmosphere. It provides a useful tool for understanding various aspects of the hydrological cycle. SCanning Imaging Absorption Spectrometer for Atmospheric CHartographY (SCIAMACHY) onboard ENVISAT-1 was a spectrometer designed to measure the composition of trace gases in troposphere and stratosphere. It provided global measurements of total columnar HDO and $H_{2}O$ concentrations using the spectral window between 2338.5 and 2382.5 nm. Temporal variability of columnar $\delta\rm{D}$ was studied over Northeast (NE) India and mean columnar $\delta\rm{D}$ for pre-monsoon and monsoon months were correlated with precipitation data obtained from Global System for Mapping of Precipitation (GSMaP). It was observed that $\delta\rm{D}$ during the pre-monsoon months of April–May showed good correlation ($r$ > 0.7, $p$ < 0.05) with total precipitation during June–August for the corresponding year over forested regions of Meghalaya and parts of Assam. Analysis was also carried out to understand the relationship between SCIAMACHY derived gridded monthly $\delta\rm{D}$ and Multivariate El-Ni$\tilde{n}$o Index (MEI) with zero and one month lag periods. Positive correlation was observed between $\delta\rm{D}$ and MEI over parts of Central India, Myanmar and Thailand. Isotope ratio of water vapor provides additional information compared to traditional meteorological observations and holds the potential to improve forecasting models.