Helena Mitasova wrote: > Maybe I am overlooking it but the man page for r.watershed does not say what > units > the option "flow" should be in - I am guessing it is fraction of 1 (or > percent given as decimal - e.g. 50% will be 0.5) > > so should flow amount read as fraction or something like that, so that if > all rainfall turns to runoff = 1 > only 50% rainfall turns to runoff (rest is captured by vegetation or > infiltrates) = 0.5
I was thinking about this flow option a bit more, and understand it even less. Using infiltration rates or some value representing the amount of rainfall captured by vegetation might not make sense here because these values have a time component, e.g. x liter per squaremeter and hour are absorbed by soil/vegetation, and surface runoff as calculated by r.watershed does not have a time component. If all but one cell have flow = 1 and this cell has flow = 0.5, and this cell receives surface runoff from 1000 upstream cells, this cell would distribute a surface runoff value of 1000.5 downstream, assuming that none of the water coming from upstream is absorbed, only the rainfall this particular cell receives is absorbed, in this example 50%. The real absorption capacity might well allow to absorb some of the runoff received from upstream. IOW, considering that surface runoff is cumulative and time-independent, I don't know what hydrological mechanism/theory would give me different initial surface flow values for different cells (e.g. the Montgomery option in r.stream.extract weighs the accumulated surface flow (SCA to be precise), no the initial one), but then I am not a hydrologist. The reason why a given input flow map is read as CELL is most probably that back then (1991, version 4.0, r.watershed no longer alpha as in 3.2) GRASS supported only CELL raster maps. Markus M _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
