**4.2. The BMWP index**

Nowadays, the BMWP index is widely used in various countries of Europe (UK, Spain, Portugal, Turkey, Poland, among others) [8]. This index, also, has been used in some countries of Latin America [5, 14]. However, the procedure for calibrating the scores of the BMWP index has not been detailed. In the present study, we follow several steps for calibrating the BMWP values. The first step included the calculation of the minimum tolerance scores for each macroinvertebrate family from the study area. The *Pcq* index was used to obtain the bioindi‐ cation values for each macroinvertebrate family. The factor analysis displayed the major explanatory variables, which are related with organic matter (N, P, BOD, DO, among others). These variables highlight the role of the different human activities that impinge on river water quality in the study area and can affect the abundance and distribution of aquatic macroin‐ vertebrates families. Due to the particular conditions of each basin or biogeographical region, the relative importance of the physicochemical variables will vary, between study areas (basins), because of unique geological nature, land uses, as well as contamination and dis‐ turbance histories. Among changes in land use, those related to agricultural activities can affect several parameters of water quality (NO3, NO2, NH3, TN, TP, among others), mainly because runoff contributes with the addition of agrochemicals into river, modifying water quality [33]. However, urban wastewater discharges can exert major impairments in the water quality due to their high load of organic matter that can deplete the DO and increase the DBO5 [34]. The data set from the studied rivers included a wide spectrum of conditions: from those with low impact (clean) to those very affected (highly polluted), giving rise to a wide range of the *Pcq* scores and water quality, data very valuable for the calibration scores of the BMWP index.
