isochrone maps - are they widely available?

[WaterEngineer]

I need isochrone maps for a water contamination analysis. Does anyone know if these are widely available? A quick google search hasn't given me much, just information on what they are.

I could use the rational method to calculate the runoff instead, but I would rather the time-area method, for which I need the isochrone maps.

Ideally I would like to calculate the amount of runoff going into the river at different points along it.

[Studiot]

Would these be for the Sahara Desert?

I am, of course guessing from your time of posting and therefore your probable time zone, in the absence of better information.

[Bobbywhy]

From our EPA, this seems to deal with runoff calculations into rivers and lakes. I am not sure if this will be useful or not...

http://nepis.epa.gov/Exe/ZyNET.exe/20008PHL.txt?ZyActionD=ZyDocument&Client=EPA&Index=1991%20Thru%201994&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&UseQField=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5CZYFILES%5CINDEX%20DATA%5C91THRU94%5CTX T%5C00000012%5C20008PHL.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=p%7Cf&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=5&ZyEntry=1