Seasonal controls on groundwater irrigation quality in a semi-arid granitic watershed of Telangana, India: an integrated IWQI-GIS-multivariate approach

Groundwater is a vital resource for irrigation in arid and semi-arid regions. This study evaluates groundwater suitability for irrigation in the granitic watershed of Dilawarpur Mandal, Nirmal District, Telangana, India, using 61 groundwater samples collected during pre- and post-monsoon seasons. Major physicochemical parameters (pH, EC, TDS, Ca2+, Mg2+, Na+, K+, HCO3−, Cl−, SO42−, and F−) were analysed. Irrigation suitability was assessed using the Irrigation Water Quality Index (IWQI) in conjunction with EC, %Na⁺, SAR, SSP, KR, MHR, PI, and RSC. Multivariate statistical techniques, including Pearson correlation, Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA), and Multiple Linear Regression (MLR), were applied to identify dominant hydrochemical controls and evaluate predictive relationships. Although elevated EC indicates moderate to high salinity at several locations, most groundwater samples are suitable for irrigation. IWQI results show that 64% of pre-monsoon and 66% of post-monsoon samples fall within excellent to good categories, with post-monsoon improvement attributed to monsoonal dilution of dissolved salts and Na+. Spatial IWQI analysis indicates an expansion of excellent-quality zones from 85% in pre-monsoon to 96% in post-monsoon season. Correlation analysis, PCA, and HCA consistently identify Na+-related indices and salinity as the primary controls on irrigation suitability, while carbonate balance and soil permeability act as secondary modifiers. PCA explains 92.21% and 87.57% of the total variance during pre- and post-monsoon seasons, respectively. MLR reveals stronger and more predictable hydrochemical control during the pre-monsoon season (R2 = 0.71) compared to the post-monsoon season (R2 = 0.33), where dilution and hydrochemical mixing reduce model predictability. The study highlights Na⁺-related processes as the principal constraint on irrigation water quality and emphasizes the need for season-specific groundwater management strategies in semi-arid hard-rock terrains.