I can't speak for the club, as I'm no longer involved with the track prep, however I can tell you there are 6 drain pots on the infield, I am aware that atleast 5 of them are piped to the 40ft container buried near the 2nd bend, each pot used to hold about 6000 ltrs, couldn't tell you what the container holds, sure someone could work it out, but they will most likely be empty as there has been very little rain of late.
As for the pond outside, that is just tap fed, not great, it would take a good 24 hours to fill from empty. It's the same water feed that supplies the stadium, so when the public enter the stadium that tap has to be turned off, otherwise the toilets ect would not work.
For Thursday, if it was me I would of had the tap on all day today, taken water from the pond and put it in the big tank, do that for the next couple of days and it should be fine, then you have a full pond plus tank, water the track from the tank during the day of the meeting, then the pond at night time, if the pond is clear of silt and rubbish you should get 12 to 14 bowsers, and probably 6 from the tank at guess.
I'm sure it's already been thought of so should be OK.