This study proposed a criterion for baseflow determination by theory discussion, filed investigation, baseflow calculation and comparision, and public hearing convention. The baseflow of certain area of four representative river, i.e. Xin-dian river (northern part of Taiwan), Zhou-shui river (middle part of Taiwan), Gao-ping river (southern part of Taiwan) and Pei-nan river (eastern part of Taiwan), were also determined and compared by Q95, Q97, Tennent, and experimental methods. It was suggested that the lower value of Q97 and 3 cms should be used as baseflow when hydrographic station data was available. Otherwise, the baseflow should be the summation of downstream required flowrate and 0.135 cms/100Km2 when no hydrographic station data was available. The baseflows of above four rivers were calculated as 0.87, 2.94, 13.50, and 12.05 cms by Q97 method, respectively. In addition, the river channel widths of the above river were 42, 298.1, 225.0 and 200.8 m when parabola shape river channel was used to keep a flow velocity at 30cm/s and river depth of 30cm. Furthermore, when the flowrate was lower than the calculated baseflow, river channel constraint or habit improvement should be used to help the aquatic biology to be survived during the dry season. Finally, the calculation of baseflow should be included in the environmental impact assessment.

Baseflow, dam, environmental impact assessment