A systematic review of agricultural use water quality indices

Water Quality Indices (WQIs) are increasingly being applied for reporting on the suitability of water for a variety of human uses including agriculture. This systematic review identified and compared 42 examples of Agricultural use Water Quality Indices (AgWQIs) for surface waters in published literature. The review confirmed the growing popularity in AgWQI reporting, particularly in the last six years. All studies incorporated the suitability of water for irrigated cropping into their AgWQI with three also addressing stock watering. The review confirmed that all parameter thresholds adopted by AgWQI studies originated from either the Food and Agriculture Organisation of the United Nations publication Water quality for agriculture publication or National Standards. An AgWQI common key was developed to overcome interstudy method variability and facilitate comparative assessment. This assessment determined that all study methods originated from two sources, the Canadian Council of Ministers of the Environment Water Quality Index, and the National Sanitation Foundation Water Quality Index. For studies adopting the latter method, a further three strategies for parameter weightings and eight functions for developing water quality ratings were identified. Our assessment also identified and explored limitations with some equations, including a method known as the proportionality constant. Significant variation in parameters, classes, thresholds, subindices, and weightings between studies was found, but also some areas of agreement. Based on the review findings, a guide has been developed to assist in future AgWQI development.