Pan evaporative changes in transboundary Godavari River basin, India

Pan evaporative changes are one of the key components of water resources management of a river basin under changing climate and anthropogenic-induced warming. The present study was undertaken for transboundary Godavari River basin (India) to identify the trends in pan evaporation (E-pan) by non-parametric Mann-Kendall (MK) test in annual, seasonal and monthly time scales. The Pettitt test, a non-parametric test, was also used in this study to detect abrupt changes in the mean of the distribution of the E-pan in the Godavari basin. Furthermore, the stepwise regression analysis was performed to find out the cause of the observed E-pan changes in the basin. On seasonal (monthly) time scales, statistically significant decreasing trends in E-pan were witnessed in the pre-monsoon season (in the months of March, April and May) in all the seven sites of the Godavari River basin. On the other hand, four sites witnessed statistically significant increasing trends in minimum temperature (maximum temperature) in the month of July (December) and in the monsoon (post-monsoon) season in the Godavari River basin. Statistically significant decreasing (increasing) trends in wind speed (relative humidity) in the pre-monsoon season and in the month of March at these seven sites support the observed decline in the evaporative demand in the basin leading to possible enhancement in the total yield of the Godavari River basin. Results of stepwise regression analysis showed that wind speed followed by relative humidity was found to be two main causative parameters of the observed decline in the E-pan under the warmer environments in the Godavari River basin. Pettitt's test shows year 1991-1992 to be the probable year of change in the E-pan in the Godavari River basin.