I live near Warorot Market and have to get to the airport by around 12:00 on Monday. Being the peak of Songkran, I'm wondering about the availability of Grab/songthaews/taxis as well as the best route to get there as expect gridlock around the moat (tho perhaps that is not an issue till afternoon). Advice please and/or contact details for a taxi I can book in advance.  

Airport Taxi, they know what is going on, a grab driver might not.

If anything, I always found holiday traffic, lite and easy to negotiate, as long as not on the inter province roads when all actually going somewhere.   At 0900 hrs, it should be very lite, and most still hung over and snoozing, if the partying type.

 

No work or school traffic ... BLISS

Both official taxis and grab-like drivers know exactly what is going on. get whichever you prefer, allow for 30min more than usual just in case. There are ways to go from Warorot to Airport avoiding the moat, Huay Kaew, Nimman. e.g. heading to B11, direction of Promenada, then easy splash-free main road towards Airport Plaza. I expect the airport itself to be actually rather quiet since all Songkran arrivals have already happened and departures not yet.

Just plan to go earlier than normal, and you'll have no trouble.
No one is throwing water at 9 AM.

OP says he wants to be at airport about 12 and we have two posters talking about conditions at 9.  That’s a bit early to be relevant, departing about 11 seems more like it.