HYDROLOGICAL ANALYSIS OF VOLCANIC TERRANE: LOWER BASIN OF THE RIO GRANDE DE SAN MIGUEL EL SALVADOR

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Abstract
Standard hydrological methods have been used to evaluate the water resources of a volcanic area that comprises the Lower Basin of the Rio Grande de San Miguel, El Salvador. The area of investigation included study of approximately 645 square kilometers. The lower San Miguel Basin consists of a faulted valley situated at the base of a series of old and recent volcanoes bounding the valley on the East, a tuff block to the South, and consolidated volcanics (lahars) on the West and North. The valley fill consists of pyroclastics which diminish in particle size with distance from the volcanoes (Lapilli to ash), in the southern portion; however, the pyroclastics may be interbedded with lake deposits. A principal objective of the evaluation was to assess the feasibility of irrigating 10,000 hectares of the valley. Results based upon the period of study suggest that almost half of the annual precipitation infiltrates into the ground. Of this quantity, about one third is accounted for by the existing hydrological balance between recharge and depletion. The remainder moves from the valley to the San Miguel River as ground-water runoff and, if retained, is sufficient in quantity to meet the full anticipated irrigation demand.

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