COMPARISON AND REGIONALIZATION OF HYDROLOGICALLY BASED INSTREAM FLOW TECHNIQUES IN ATLANTIC CANADA

Five hydrologically based instream flow assessment methods are compare d for 70 rivers in Atlantic Canada; these methods included (i) Tennant method; (ii) 25% mean annual flow (25% MAF); (iii) median monthly flo w (Q(50)) which includes the aquatic base flow (ABF); (iv) the flow eq ualled or exceeded 90% of time on a monthly flow duration curve (Q(90) ); and (v) the statistical 7-day low flow frequency of a 10-year recur rence interval (7Q10). By comparing the different methods relative to the 25% MAF (the commonly used method in Atlantic Canada), we found th at the Q(90) and 7Q10 methods predicted extremely low instream flows d uring winter and summer months. Resource management decisions based on these extremely low flow predictions could have serious adverse conse quences. The median monthly flow method (Q(50)) was recommended for ga uged basins, whereas the Tennant method, the 25% MAF method, and the A BF methods were recommended for ungauged basins. For ungauged basins, we conducted a regional study to estimate the 25% MAF and the ABF usin g multiple regression analysis. Physiographic parameters were used as explanatory variables in the regression analysis. Based on the coeffic ient of determination, R(2), the best regression results were obtained for the 25% MAF with R(2) ranging from 0.957 to 0.999. Although the r esults for ABF were slightly lower than for the 25% MAF, R(2) was stil l in the range of 0.868 to 0.979.