Trend analysis in reference evapotranspiration using Mann-Kendall and Spearman's Rho tests in semi arid region of North Gujarat

Evapotranspiration (ET) is an important element of the hydrological cycle, and changes in ET holds a great significance to agricultural water-use planning, irrigation system design and management. In this study, monthly trends of reference evapotranspiration (ET₀) were examined using Mann-Kendall (MK) test, Spearman's Rho test (SR) and the Sen's slope estimator for Sardarkrushinagar, Gujarat, India. For this purpose, meteorological observations were collected over a period of 29 years (1988-2016). The results indicated that trends of ET₀ for some months were increasing (positive) however, for some months, there were decreasing (negative) trends. The significant and highly significant ET₀ trends on a monthly time scale occurred during the months of January, February, March, August, October and December, but no significant trend were observed for April, May, June, July, September and November months. The highest positive ET₀ trend (0.81 mm/month) and decreasing ET₀ trend (0.45 mm/month) were found during March and September, respectively. In general, the performances and abilities of the MK, SR and Sen's slope estimator were consistent at the verified significant level.