Water quality - nutrients, salts

The water quality index, itself, was built for the Quality Unknown report, and an explanation of how this was constructed is provided in the data appendix: https://openknowledge.worldbank.org/bitstream/handle/10986/32245/211459App.pdf?sequence=5&isAllowed=y (pg 14). Essentially, the 3 parameters were first normalized so that each variable was scaled to a similar support. This is essential since each indicator is originally distributed on a different support, and scaling the indicators on a common support prevents one variable influencing the index more than the others. These normalized values were then added such that higher values correspond to poor water quality. The index is by construction ordinal and it allows one to compare the values of water quality at different locations. The country value here is calculated by taking a population weighted average of all gridcells where the centroid falls within the country. For presentation purpose, the sign of the country value was reverse, so higher values correspond to better water quality.