Determination of decadal climatic cycles in runoff fluctuation of a hydrologic unit

A group of 60 runoff and 15 precipitation time-series, all belonging to a h ydrological unit in Southwest Mexico, in the Pacific watershed was analyzed . The internal homogeneity of the time series was tested and then they were completed for the calculation period using suitable statistical methods. T hree homogeneous regions were defined in the studied Bone through correlati on matrices. The climatic decadal cycles were determined with the help of t he integrated differences curves. The anthropogenic impact on the runoffs w as estimated through the homogeneity curves. The length of the cycles were ranged from 30 to 40 years. The structure of the decadal cycles permitted t he estimation of the duration of deficiency, abundance, and normality runof f periods in each region. The cyclical structure of the regional runoffs ca n be explained by precipitation. The anthropogenic impact was important in the middle of the general streamflow, where the two main streamflows connec t through Chapala Lake. It was demonstrated that, in spite of the anthropog enic impact on part of the hydrologic system, it continues to be naturally regulated by climatic factors and still constitutes a hydrological unit. Th e estimation of the lengths of different phases of the decadal cycle can be used for the management of the regional water resources.