Assessment of spatiotemporal variability and trend analysis of reference crop evapotranspiration for the southern region of Peninsular India.

Accurate estimation of reference evapotranspiration (ET) is an essential requirement for water resource management and scheduling agricultural activities. Several empirical methods have been employed in estimating ET across diverse climate regimes over the past decades. In this study, the Python implementation for estimation of daily and monthly ET values of representative stations of ten agro-climatic zones of Karnataka from 1979 to 2014 using the standard FAO Penman-Monteith method was carried out. The assessment of temporal and spatial variability of monthly ET values across the various agro-climatic zones done by the various statistical measures revealed that the variation in spatial ET values was higher than temporal variation, indicating major difference in ET values was with respect to the stations rather than years under study. The nonparametric Mann-Kendall test conducted at 1% significance level on the annual ET values revealed a statistically significant increasing trend for all the ten stations during the study period. The trend test conducted on the climate variables like mean air temperature, wind speed, relative humidity, and solar radiation signifies their influence on the annual ET values. The magnitude changes in the trends detected by the Theil Sen's slope indicated that increasing values of mean temperature, solar radiation, and decreasing values of relative humidity predominantly contributed to the annual upward trend in ET values for the 10 stations. A trivial impact of wind speed on annual ET values was observed for the stations. Kalburgi and Udupi stations exhibited a positive ET trend with the highest and lowest annual values among ten stations.