Are Existing Water Quality Indices (WQIs) Fit for Purpose? Evaluating Their Applicability to Irrigation Surface Water Quality

Globalization-driven food demand and rapid urbanization have intensified agricultural intensification and land-use changes, leading to widespread deforestation, increased human settlements, and a heavy reliance on chemical fertilisers, pesticides, and animal manure. These practices have significantly degraded surface and groundwater quality through nutrient-laden agricultural runoff, particularly nitrate and ammoniacal nitrogen, across many watersheds. In light of the deteriorating water quality for irrigation, indexing techniques have been regarded as the most advanced methods for assessing water quality since the late 20th century. This study evaluates the suitability of conventional irrigation water quality indices for assessing surface water used for irrigation in the heavily impacted Hindon River Basin, India. Monthly water samples were collected from 16 strategically selected sites following standard protocols. Key irrigation suitability parameters and indices were computed, including sodium adsorption ratio (SAR), percentage sodium (%Na), permeability index (PI), Kelly ratio (KR), residual sodium carbonate (RSC), and magnesium adsorption ratio (MAR). Results revealed that Irrigation Water Quality Index (IWQI) values at most sites ranged between 25 and 50, classifying the water as marginally suitable with minor treatment required. In contrast, a few downstream sites exceeded 75, indicating severe unsuitability. A marked deterioration in water quality was observed during the pre-monsoon and monsoon periods compared to the post-monsoon period, largely attributed to runoff and leaching processes. However, despite mathematically acceptable index values at several locations, the water remains unsuitable for sustained irrigation due to elevated concentrations of toxic and emerging contaminants that are not incorporated into existing irrigation indices. The study highlights a critical limitation of conventional irrigation water quality indices: their inability to account for trace elements and other non-classical pollutants that pose long-term risks to soil health and crop safety. Findings underscore the urgent need to develop modified or composite indices that account for the trace elements and tailored to agro-industrial basins and to safeguard irrigation water quality, ensure agricultural productivity, and promote environmental sustainability in rapidly urbanizing catchments.