Trend analysis of evapotranspiration and its response to droughts over India

Six temperature homogeneous regions of India were chosen to calculate evapotranspiration (ET) using Hargreaves and Samani method on a monthly basis. Break Trend Analysis has been applied to the annual and seasonal ETs of the respective homogeneous regions as well as for whole India and the factors contributing for the changes in ET have been analysed. By considering the data of Reconnaissance Drought Index (RDI), Rainfall, Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI) for South West (SW) monsoon (June to September) and Rice yield (major Khariff crop yield) of All India, we investigated the variations in ET during drought years. The trend analysis of seasonal and annual ET over the test regions have shown significant (levels of 0.05 to 0.01) increase over the past 107 years of the study period (1901 to 2007). The enhancement in ET is found to be 2.9 mm/decade over India and with a maximum in West Coast India (6 mm/decade), followed by Northwest India (3.9 mm/decade). The increasing trends in ET may be due to the increase in difference of maximum and minimum temperatures over these regions, which is of the order of 0.64 degrees C/decade on all India scale. The analysis of ET in drought years consistently shows higher values in the years when India suffered under moderate and severe droughts. It is also observed that lower rainfall values in the drought years such as 1983, 1987 and 2002 were associated with the higher ETs accompanied by lower NDVI and Rice yields. The analysis has been extended to the comparison of ET with rainfall through wavelet spectrum to understand the interannual variations which inferred the El Ni (n) over tildeo impact on ET. The correlation analysis of ET over the Interior Peninsular region with the number of days falling in active and break spells of monsoon by which drought can be characterised, revealed the significant negative and positive values, respectively.