Why is the ratio of pervious runoff to baseflow so different for my project in Hydro v5 vs. v6?

Post by **rcoville** » Thu Apr 11, 2019 9:18 am

We've found various cases where a project run in v5 vs. v6 varies significantly in how each allocates baseflow and pervious flow. This can also result in slightly different total flow estimates. In all cases so far, we've found that the issue is that the project inputs are not identical in these comparisons.

If you've run into this issue, carefully compare the inputs of each project, especially Step 3's hydrological parameters. When we've lined the hydrological parameters up between v5 and v6 projects and all other inputs are the same, the total flow and flow allocation for the projects are much more similar. Below is a list of hydrological parameters that are particularly likely to differ between v5 and v6 projects. We recommend using the values associated with Hydro v6.

Average Annual Flow may have an erroneously limited range in v5 and both projects will be need to set to a value available in both versions
Soil Macropore Percentage may be a different order of magnitude between v5 and v6 projects
"Watershed area where rainfall rate can exceed infiltration rate" is set to 100% by default in v6 projects, but many v5 projects have this set to 65%
Time Constants have been split into additional parameters in v6 and the default values for these are higher than they were in v5. We recommend in v5 setting Sub Surface Flow Routing = 120 and Time Constants for Surface Flow both = 40, equivalent to the defaults in a Hydro v6 project

If the project inputs are identical and you are still having issues with unexpected differences between Hydro versions, please reach out to us (on this post or by emailing info [at] itreetools.org) so we can investigate further. You can also check the i-Tree Change Log for Hydro changes that are known to affect results (e.g. a bug fix with baseflow and ET allocation in v5.0 -> v5.1; refactoring architecture and modified hydrological parameters causing slight changes in estimates in v5.1 -> v6).