The Valle de Bravo reservoir provides 30% of Mexico City’s drinking water. Unfortunately, human activities in the watershed have affected the water quality in this reservoir. Its trophic state changed from oligotrophic in 1980 to mesotrophic in 1987. Research conducted from March 1992 to February 1993 defined the main limnological characteristics of the water body and identified the most highly polluted subbasins in the Amanalco River Watershed. For this purpose, water, sediments and vascular plants were sampled in the waterbody. In the Amanalaco River Watershed land use and nutrient loadings in each subbasin were quantified.
The study dam is warm and monomictic, with a nine-month period of stratification and an overturn in December. Bottom anoxia occurred during the stratified phase. The reservoir was eutrophic during summer and mesotrophic the rest of the year. In the Amanalco basin, the highest phosphorus and nitrogen loadings (P, N in t y-1) entered by way of the Becerra (4.9, 47.2), Pipioltepec (8.6, 61.9) and Candelaria (8.5, 69.5) subbasins. A good agreement was found in a comparison between the export coefficients estimates in the Amanalco River Watershed and the measured amounts of phosphorus and nitrogen transported to 38 US waterbodies. Nevertheless, the phosphorus export coefficients for the forested subbasins within the Amanalco River Watershed were rather high in relation to the typical values found in the literature, and the values for agricultural land were low in relation to the same source.
For the first time in Mexico, export coefficients for phosphorus and nitrogen were estimated, both of which will be useful in assigning priorities in land management. The export coefficients estimates presented in this study can be applied to other scenarios in Mexico which have the same temperate semihumid climate.