Improving current methodology for mapping the index of potential alteration of water quality for the Colombian National Water Study

In Colombia, the Institute of Hydrology, Meteorology and Environmental Studies (IDEAM) publishes a quadrennial report called the National Water Study (ENA). In this report, there is a section dedicated to informing public and private entities about the state and pressure of water quality of surface water bodies in Colombia. These reports help determine and control the discharges from multiple industries, by limiting their discharges according to the information provided by the Water Quality Index (WQI) and the index of potential alteration of water quality (IACAL). However, IDEAM identified certain weaknesses within the IACAL methodologythat created the opportunity for IHE-Delft to stablish this study, addressing the identified weaknesses and proposing new methodologies to update the IACAL and its reporting in the ENA. IACAL index is based on representing the impact or pressure of five or more economic sectors into the water quality of a given area. Thus, this study has the objective of developing and proposing an improved methodology for this index, based on sensitivity analysis of input data from economic sectors, addition of diffuse sources of pollution using GIS-based modelling and updating the calculation steps by restructuring the range and categorization of the IACAL index based on the National Sanitation Foundation Water Quality Index (NSFWQI). Consequently, obtaining updated graphs and maps of the adjusted methodology for the IACAL index in the study area along with a summarized recommended methodology for implementation in the ENA reports. The sensitivity analysis was performed by the One at a Time (OAT) method using input data to calculate the IACAL, while the estimation of diffuse sources of pollution (DSP) was done using the Pollutant Loading Estimator (PLOAD). The modifications to range and categories were done by adding new curves for five water quality parameters used in IACAL creating representative subindices. These subindices followed multiple mathematical adjustments and their respective aggregation using weights per parameter. The aggregation followed multiplicative and arithmetic methods, to obtain the final IACAL index of the area of study. It was concluded that the modifications showed improvement in data distribution and specificity in the classification of sub-catchments. Simultaneously, the PLOAD model generated estimations of diffuse sources of pollution in the study area and created the opportunity to add Total Nitrogen (TN) and Total Phosphorus (TP) loads into the IACAL calculation at a municipality scale. Overall, the results of this study indicate methodologies to enhance the IACAL calculation, addressing the main opportunities for improvement and therefore generating more specificity and informative results. However, the limitations of data and time hindered validations of the new methodology in more areas of Colombia