Comparison of hydrologically based instream flow methods using a resampling technique

The protection of fish habitat against the impact of water extraction in rivers is a recurring problem in water resources management. As such, a wide range of methodologies is available for the calculation of instream flows. This study focuses on historical streamflow methods that rely solely on hydrometric data for instream flow evaluation. The objectives of the study are to compare different historical streamflow methods and use a jackknife resampling technique to assess the variability of instream flow estimates. Results showed that methods based on a percentage of mean annual flow (MAF) generated higher levels of instream flow protection and showed low spatial and sample size variability. Low spatial variability makes the MAF methods more suitable for calculations of instream flows for ungauged basins. The Q(50) method provided relatively high levels of instream flow protection; however, spatial and sample size variability were higher than those for the MAF methods. Lastly, the results showed that some methods generated low instream flow protection (namely, the Q(90), 7Q2, and 7Q10 methods), especially for small streams, and thus are not recommended for use.