FRESH-WATER FLOW INTO A COASTAL EMBAYMENT - GROUNDWATER AND SURFACE-WATER INPUTS

Freshwater discharge to a shallow coastal embayment was measured with two upland hydrologic and three embayment physical methods for 2 yr. Parallel measurements from the five methods ranged from 3,900 (+/-630) to 9,400 (+/-3,400) m(3) d(-1), and four of the methods showed close agreement and averaged 4,800 (+/-670) m(3) d(-1). The most precise estimate of discharge was from a chloride balance, while the best understanding of the rate and pattern of groundwater flow was from a Darcian streamtube approach. Groundwater dominated the freshwater budget, accounting for >95% of the total annual input, and was partitioned almost equally between direct seepage to embayment waters and seepage to a stream with final discharge via surface Bow. Freshwater inputs decreased rapidly toward the mouth of the estuary and >80% entered into the upper half. The lack of fixed watershed boundaries resulted in large errors in both the location and area of the topographically defined watershed when compared to a watershed defined by water-table mapping. Seasonal variations were found in both the boundaries of the watershed (8%) and in groundwater discharge (6-fold) in response to changing water-table gradients due to recharge. Hydrologic alterations of the upland through the import of water and the increased recharge from impermeable surfaces led to an apparent increase in the total freshwater discharge to the embayment of nearly 50% over ''natural'' levels.