Assessment of long-term spatio-temporal variability in Rainfall trends: A study of Cauvery river basin, Southern India

Abstract

Water supply, the severity of floods and droughts, and the sizing of hydraulic structures all hinge on rainfall, and this dependence is acute in a basin as large as the Cauvery, which drains parts of Karnataka, Tamil Nadu and Kerala and supports irrigation, hydropower and ecosystem services for millions of residents. Given growing water demand and a variable climate, we evaluated how annual and seasonal rainfall over this basin changed during 1981–2024. Daily CHIRPS rainfall grids were retrieved through Google Earth Engine, the basin outline was taken from the India-WRIS Cauvery Basin Report (2014), and maps were prepared in QGIS. Python scripts in Google Colab handled cleaning and statistics for the annual series and for three seasons: pre-monsoon (March–May), monsoon (June–September) and post-monsoon (October–December). Trend significance and magnitude were assessed with the Mann-Kendall test and Sen’s slope. Rainfall decreases steadily from west to east, peaking over the Western Ghats. Annual totals (Sen’s slope +7.37 mm/year, p = 0.0005) and pre-monsoon totals (+2.41 mm/year, p = 0.0005) rose significantly, while monsoon and post-monsoon totals fluctuated strongly between years with no significant trend. The annual rise agrees with the wetter phase documented for South Peninsular India after the 1980s, yet century-long records for the basin show weak or non-significant decreases, so the detected increase belongs to the recent window rather than the long-term record. The work offers a reproducible, open-source geospatial procedure for rainfall variability assessment and provides evidence useful for water management, infrastructure planning and climate adaptation in the Cauvery Basin.