Investigating the response of groundwater and streamflow to rainfall in a tropical catchment using non-parametric methods

The present study investigates the monthly and seasonal patterns of groundwater levels and streamflow in response to rainfall in a humid tropical catchment in central Kerala, India. The trends were investigated at a temporal scale using data from eight groundwater-level observation wells, four stream-gauging stations and rainfall records of three subcatchments, viz. Cheripad, Pala and Kidangoor. The exercise of trend detection was carried out using two widely adopted non-parametric tests, viz. the Mann–Kendall (MK) test, Spearman's Rho (SR) test and a relatively new visual graphical approach, innovative trend analysis (ITA). The results revealed predominantly rising trends (monthly and seasonal scale) within the catchment, across all three hydrological components examined. Notably, the ITA test identified specific changes in the trend direction within the time series (positive to negative or vice versa), which were not observable in the MK or SR tests. Data published in the Groundwater Book of Kerala for 2021–22 corroborated this increasing trend, showing, on average, a rising trend in well water levels throughout the watershed across pre-monsoon and post-monsoon seasons. This trend was further confirmed using ITA by pairing the observations from the groundwater-level monitoring station at KTM-OW-11 with rainfall data from the nearby gauging station at Erattupettah.