During the last five decades, the forests and natural resources have been highly disrupted by human activities, causing a significant decrease of the plant coverage, consequently resulting in an increase runoff and soil erosion. Likewise, since 1989 non-basic information related to these topics has been obtained from the field, which does not allow appropriate hydraulic structure design. The purpose of the current paper is to determine the relation between the Curve Number CN and runoff coefficient C, from information of the 34 meteorological stations located in the Manabi Hydrographic Demarcation (MHD), during the period 1963-2013. This work is developed using information provided by several State institutions, and support of Geographic Information System. The information is processed using lineal correlation analysis to find an equation that relates the CN a C values.

The climate on the Ecuadorian coast depends mainly on the marine currents of Humboldt and El Niño, which appear in the dry and rainy seasons, respectively. The Humboldt current is distinguished by being cold, while that of El Niño is hot. The presence of the El Niño current causes an increase in the evaporation of ocean waters with the consequent appearance of the rainy season. There are anomalous seasons of the El Niño stream, when the water temperature rises above the norm, higher than 25.5 °C, which has been called El Niño phenomena. The appearance of this natural phenomena has proven to be decisive in the climate change of the coast of Ecuador. In order to have technical information, important for the planning, control and development of the water resources of the DHM, in this research a temporal analysis of the monthly rainfall during 55 years, 1963-2017 period, is carried out. The National Institute of Hydrology and Meteorology of Ecuador (Instituto National de Meteorología e Hidrología-INAMHI) at station M005, located in the Botanical Garden of the Technical University of Manabí in Portoviejo, obtained these records. An analysis of the monthly and annual patterns is made, establishing that the El Niño events occurred in 1983, 1997 and 1998, have established guidelines for the change in the production of rainwater in the levels of intensity and temporal distribution, increasing the months of drought, while precipitation levels increase, concentrating in fewer months, basically in February and March. This is a situation that increases the water deficit, especially when there is not enough infrastructure of hydraulic works for the storage and regulation of runoff.