Hydrology is a compound word. The first part, 'hydro,' basically means water. The ending, 'logy,' means the study of. Thus your word means the study of water. If you can remember the meanings of these two word parts, and others, you can infer the meanings of countless terms.
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Hydrology is the study of the the water cycle. Included are details about oceans, aquifers, rivers and lakes. The science deals primarily with evaporation, cloud formation, condensation, and precipitation. For more info, check Wikipedia.