This study describes the application of a water run-off model for Lake Santa Ana, Mexico, developed through the combined application of two simulation models: the Watershed Modelling System (WMS) and the Hydrologic Modelling System (HEC-HMS). The WMS was used to estimate the geometric parameters of the hydrographic basin from a digital elevation model and geographical information describing the location of the hydraulic infrastructure developed in the basin in recent years. The HEC-HMS was used to estimate the run-off hydrographs, by using historical rainfall data and calibrating the model with the observed hydrometric data. The lake bathymetry was taken into account in order to estimate the lake's water balance. Recent observations indicate that anthropogenic activities have modified the natural run-off features of the lake basin. The magnitude of these modifications was estimated by linking the WMS and HEC-HMS models. Application of these linked tools allowed the successful estimation of the modified basin limits and hydraulic behaviour of Lake Santa Ana.

Lake Santa Ana is located in the Mexican central plateau, a semiarid region characterized by low rainfall and high evaporation rates. Because of the area's climatic conditions, most of its few existing lakes and reservoirs are ephemeral, with significantly reduced surface areas during the dry season. Mexican authorities currently pay little or no attention to these special aquatic ecosystems. As a consequence, several of these waterbodies are severely degraded in regard to both water quantity and quality. As a result, this report describes Lake Santa Ana as a case study. Several water quality parameters, including physicochemical, microbiological and heavy metal concentrations, were measured at eight sampling sites in the lake, and four sites in the lake basin, in 2005. Some dissolved ions were also analysed in 2007 in the lake, and in a well located close to the lake, in order to evaluate possible interactions between groundwater and surface waters. These study results indicate that the lake does not have significant interactions with groundwater in the local aquifer. Most of the measured water quality parameters do not meet the international suggested standards for inland surface waters to sustain aquatic life, and preserve the habitat for both endemic and migratory waterfowl. Loss of aquatic habitats puts at risk the value of the central portion of Mexico as a migratory route for North American waterfowl species.