Daily avarage use for soccer arenas

I worked on an ice arena project with a similar scenario, it was filled to peak capacity only a few times a year (though it was a much smaller arena than your Soccer Stadium). For the daily number of visitors, I calculated an average over the number of days that the arena was open per year (so your daily average would be much less than 45,000). I explained in my LEED submission why the daily average number of visitors was significantly less than the peak capacity. Though I do not recall any official USGBC/GBCI protocol to this scenario, my explanation was accepted (LEED NC 2.2 certification). If you want a definite answer, you could submit a CIR.

This credit is concerned with the yearly potable water consumption. Therefore if you can sketch a good approximation of yearly water use via yearly water use FTE, you could approximate the annual water use and savings against the baseline. The calculations actually cancel out the actual consumption inherently, and the credit only concerns itself with the % improvement (the male/female ratio is however important), not the actual water usage in gallons. Further, LEED requires building conditions as per the life time of the building and specifically excludes event based calculations (See MPRs). If you would like information on how the calculations were done for Stadium Manaus, then you should contact gmp Architects in Berlin, Germany.

Jean is correct - you will account for the events, but by averaging the peak out over a typical full year of occupancy.