Musings about water levels and rain

In the Community Facebook page a comment has been made that the day after the opening was made the level of the Inlet has risen by 50 mm.

That led me to wonder how that relates to recent rainfall.  In what follows many of the necessary bits of data are not available but I think it sort of hangs together.

My weather station (Angophora Drive) has recorded somewhat over 50mm in the last 7 days.  One could make an assumption that this is about average for the catchment of the Inlet  - and checking against BoM data for Cann River and Cathcart that seems not unreasonable.  There are 3 things that could happen to the rain:

1. Sink into the ground
2. Run off into the the river system; 
3. Evaporate.

Given recent weather I think evaporation is not a major player.  I have no idea of the split between points 1 and 2: on the one hand the soil currently has plenty of capacity (favouring (1)) but on the other hand a significant fall in a short time will favour (2).  For a mind-game, I assume 10% of the rain has run off.

It is going to take a while for the water to get here.  The headwaters of the Genoa are at least 45 kms from the mouth of the Inlet in a straight line and probably double that in river kilometres.  That means it is going to take a while  - probably 3-4 days (??) - for some of the run off water to get here.  As most of the rain was several days ago one could assume that most of it has now got here, probably arriving in the last two days

I have read somewhere today that the surface area of the Inlet is  25 sq km (or 2,500 Ha).  Eyeballing Google Earth and following the course of the Genoa and Wallagaraugh Rivers suggests that the catchment area of the Inlet is about 40 km x 25 km (~80,000 Ha).  For the sake of argument, say I have got that way wrong and it is only 40,000 Ha.  That still means the area of the catchment is 16 times that of the Inlet.  

Bringing these guesses together we have 50mm of rain x 10% run-off x 16 times area (50*0.1*16)= an "expected" rise of 80 mm.

Then bring in the new factor of the opening to the sea.  which is about 10m wide.  I have run out of imagination in trying to assess how much water is flowing out in any given time period, but if it has released close to half the water that has arrived through the Narrows in the last 24 hours I reckon that is both believable and beneficial.