RESUMENUno de los grandes problemas que enfrenta el manejo del recurso agua en México es el desconocimiento de su variabilidad espacio-temporal. Dicho manejo se ha enfocado en el consumo humano y sus actividades económicas, sin considerar aquel volumen requerido para los ecosistemas y los servicios ambientales que finalmente redundan en la sustentabilidad. Dado lo anterior, las instituciones gubernamentales, académicas y organizaciones de la sociedad civil propusieron una metodología para determinar el caudal ecológico en una serie de cuencas consideradas como futuras reservas de agua, con impacto tanto para la conservación como para su recuperación. Con el empleo de esa metodología, este trabajo tiene como objetivo el estimar al caudal ecológico ordinario estacional (Qecol*) de varios ríos prístinos o con diferentes grados de impacto y con condiciones climáticas distintas, tanto de la vertiente del Golfo de México como del Pacífico mexicano. En las cuencas estudiadas del Golfo de México, se destacaron por su Importancia Ecológica "Muy Alta," con un porcentaje de caudal ecológico (Qecol*): Pánuco (74%), Papaloapan (78%), Grijalva (68%), Palizada (81%), y Candelaria (62%). Por el lado del Pacífico sobresalieron por tener la "Mayor Presión del Uso del Agua" y un porcentaje de caudal ecológico bajo, las cuencas de Culiacán (4%) y Tehuantepec (15%). ABSTRACTOne of the major problems being faced by the management of water resources in Mexico is the lack of knowledge about temporal and space variability. Management has focused on human consumption and its economic activities, without considering the volume required for ecosystems and environmental services that ultimately lead to sustainability. Governmental and academic institutions, as well as civil organizations, have proposed a methodology to determine the environmental flow in a series of basins considered as future water reserves, with impact both for conservation and recovery. With the use of this methodology, the present work aims to estimate the ordinary seasonal environmental flow (Qecol*) of several pristine rivers with different degrees of impact and with different climatic conditions, from both the Gulf of Mexico and the Mexican Pacific. The studied basins of the Gulf of Mexico were noted for their "Very High" ecological importance and environmental flow (Qecol*): Pánuco (74%), Papaloapan (78%), Grijalva (68%), Palizada and Candelaria (62%). On the Pacific side, due to the greater pressure for water use and a lower percentage of Qecol*, the Culiacan (4%) and Tehuantepec (15%) basins stood out. PALABRAS CLAVE caudal ecológico; cuencas; México; recursos hidrológicos; ríos; sustentabilidad ecológico KEYWORDS ecologic sustainability; environmetal flow; Mexico; hydrologic resources; river basins; water level CONTACT Guadalupe de la Lanza Espino gdlle@unam.mx Apartado Postal 70-233, Ciudad de México, México CP 04510

El Sol is a high-mountain crater lake situated in the caldera of Nevado de Toluca Volcano, Mexico. The climatic conditions at El Sol are intermediate between those of high plains (pdramo) and arid tablelands (puna). The volcanic environment of the lake basin, its small drainage area and the high percentage oxygen saturation favor the maintenance of highly transparent water. A combination of tropical conditions with high altitude produce thermal characteristics intermediate between temperate and tropical lakes. Of these characteristics, a low average temperature is outstanding, despite the large solar radiation input to the system. Freezing frequency is intermediate between the annual freezing of temperate lakes at high latitudes and the sporadic freezing of equatorial high-mountain lakes. The radiation income is favored by water transparency and generates persistent masses of hot water in the lake's deepest parts whose temperatures differ up to 2.5 °C from those at higher levels. Rain is the principal contributor of solids to the lake through erosion of the crater walls. The drop in temperature during winter produces an important supply of ammonia from decaying organisms. The relationship N: P < 16 shows N to be limiting a primary productivity which is mainly due to benthic algae. The peak in phytoplankton chlorophyll coincides with the nutrients maximum in August, the middle of the rainy season. El Sol has various characteristics in common with other high-mountain lakes, such as polymictic, oligotrophic, soft waters susceptible to acidification.A diverse algal community, composed of desmids (Closterium, Desmidium, Pleurotaenium, Euastrum), dinoflagellates (Peridinium), diatoms, and a great number of benthic chlorophytes (Nitella, Oedogonium, Zygnema), was found throughout a euphotic zone which reaches the bottom at 14-15 m.

The water availability in the Usumacinta River sub-basin is determined through the analysis of the amount and frequency of precipitation, associated with the quantity, frequency and magnitude of flow regimes (environmental flows). The river is located in south-eastern Mexico. The precipitation of preimpact (1960-1983) and post-impact (1984-2008) periods was analysed using a climatological mesh database created by CICESE (Center of Scientific Research and Higher Education of Ensenada) covering the period 1960-2008. For the analysis of flows, the hydrometric information the IHA V7 program was used to define the main trends in the temporal variation of the daily flows of the pre and post-impact periods. A number of several factors such as: the increase in monthly precipitation in the rainy season and a decrease of the precipitation in the dry season in the post-impact period, the significant increase in the number of days with zero precipitation, the increase in the number of days a year with a greater amplitude in the maximum rainfall, a positive tendency of precipitation in rainy season and a significant decrease in the dry season; implies that now in the wet period it rains more and in dry season it rains less, indicating that the climate is more extreme, aspects that can be associated with the effects of climate change. Also, torrential rains that can be associated to the changes in the precipitation rates are due to the effects of climatic change. The natural flow shows a slight decrease in the flow during the rainy season, or a significant decrease in the flow rates for some months in the post-impact period. This condition does not coincide with the increase in precipitation for this period and in this part of the basin, a situation that may be related to the anthropic use of the resource. The Usumacinta basin is relevant for the environmental services that it provides.

A Multidisciplinary methodology that describes for the watershed of Laguna de Tuxpan the climate, topography, geology, soil, vegetation and land use through GIS. The topography (maximum altitude of 1731 m). The bathymetry(capacity of 18.89 Mm 3 , an area of 4.1 Mm 2 and a maximum depth of 7.86 m). The physicochemical and environmental parameters, show the generation of 107.01 kg of nitrogen per year (22.93% by point sources and 77.07% by diffuse sources). Tomatal river produces 18.7 t of sediment on the regions of average sloping, this equates to a lost of 7.45 t ha-1 on average throughout the basin. The water quality is within the maximum permissible limits for the development of aquatic life. The lake is classified as warm water body, shallow and tropical, with a holomíctic blend, or a water body mesotrophic-eutrophic, through plankton community. The pollution and toxicity by Vibrio fischeri, Daphnia magna, Selenastrumcapricornutum and Microcistinawas not present in the lake. The presence of six types of phthalates, plus the incidence of herbicide atrazine in the lake, point out as source of pollution to the trash of the area and agricultural activity. There are contamination by the presence of fecal coliform and total suspended solids of human and natural origin in the watershed. Is proposed a strategic plan of recuperation: 1) a program of promotion and adoption of conservation practices for hillside land, 2) construction of little dams within channels for the control sediment and runoff infiltration to help the recharge the aquifer, 3) the river corridor recovery through of reforestation, 4) management of less polluting agrochemicals, 5) relocation of human settlements of the river corridor, 6) rechanneling of the river,7) control of invasive species management of the corridor and the lake, and 8) management of the water urban residuals through social participation.

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