Comparison and regionalization of hydrologically based instream flow techniques in Atlantic Canada

Abstract

Five hydrologically based instream flow assessment methods are compared for 70 rivers in Atlantic Canada; these methods included (i) Tennant method; (ii) 25% mean annual flow (25% MAF); (iii) median monthly flow (Q50) which includes the aquatic base flow (ABF); (iv) the flow equalled or exceeded 90% of time on a monthly flow duration curve (Q90); and (v) the statistical 7-day low flow frequency of a 10-year recurrence interval (7Q10). By comparing the different methods relative to the 25% MAF (the commonly used method in Atlantic Canada), we found that the Q90 and 7Q10 methods predicted extremely low instream flows during winter and summer months. Resource management decisions based on these extremely low flow predictions could have serious adverse consequences. The median monthly flow method (Q50) was recommended for gauged basins, whereas the Tennant method, the 25% MAF method, and the ABF methods were recommended for ungauged basins. For ungauged basins, we conducted a regional study to estimate the 25% MAF and the ABF using multiple regression analysis. Physiographic parameters were used as explanatory variables in the regression analysis. Based on the coefficient of determination, R2, the best regression results were obtained for the 25% MAF with R2 ranging from 0.957 to 0.999. Although the results for ABF were slightly lower than for the 25% MAF, R2 was still in the range of 0.868 to 0.979. Key words: environmental assessment, maintenance flow, low flow, aquatic resources.