Sentence:  Isotope hydrograph separations determined by simple conservative-mixing models have shown repeatedly, in a variety of climactic and hydrogeologic environments, that streamflow generated during rainfall or snowmelt is derived primarily from water stored in the catchment prior to the event, a surprising insight that has profoundly changed how hydrologists view the runoff process. ... <WSD>Catchment</WSD> hydrology is more than just the study of water, or rocks, or biota – it encompasses the physical and biogeochemical interactions that control the hydrology and biogeochemistry of the system. 
Meaning of the word marked with <WSD> tags:  Any structure or land feature which catches and holds water; the collection of such water.