"Snow/ice has a vapour pressure even at sub-zero. "

Good point. And there is some data I found:
"A slower evaporation rate means a longer lifetime for the water, and under ideal circumstances on the surface of Mars, this lifetime can be extended to the point where the water may melt. The project for the summer was to put these theories of evaporation rates to test.

Experimental: The sample was prepared by freezing 1 cm water in a 100-mL graduated cylinder. A thermocouple was placed on the surface of the ice. JSC Mars-1 Martian simulation soil was mixed to homogeneity and poured on top of the ice and thermocouple at depths varying from 0, 0.25, 0.5, 1, 2.5 and 5 cm. The chamber was evacuated of clean of air, filled with dry CO2 gas, and chilled to 0°C using a methanol-dry ice cooling system. The sample was then lowered into the chamber and pumped down to 5.25 torr (7 mbar), Martian surface pressure. Many variables were recorded, including mass, atmospheric pressure, water vapor pressure, and temperatures of sample, chamber atmosphere and walls, all as a function of time. Mass was plotted versus time, and a second order polynomial curve was fit to the data. Taking the derivative of the best-fit curve gave an evaporation rate in g/min, which was converted to mm/hr. This equation is set to time equal zero to determine initial rate of mass loss with no water vapor present in the atmosphere. "

here

Or to download Worddoc:
JenniferHabstract.doc

continuing:
"Results: In total, 40 experiments were run with varying soil depths. At least 5 repetitions were performed at each soil depth. Figure 1 shows the average and standard deviation at each depth, with a second-order best-fit curve. With no soil the evaporation rate was measured to be 0.415±0.078 mm/hr....."

Now, I have don't a huge amount confidence in these experiments, but it's seems somewhat reliable as a baseline [at least until I have something better].

So, if you changed the 3' of snow to say to solid ice it would be about 3-4" or about 8-10 cm. So if the ice remained at about 0 C and in less than Mars atmosphere then it could somewhere around .5 mm/hr [though perhaps a significantly higher rate in the Moon's vacuum] and at this rate it would all evaporate in 160 to 200 hrs.
Though of course would get reduction in the rate the colder the ice or snow was and it would stop at around -100 C.

So it seems to me that it's likely the 3' snow would survive the lunar nite- though it could lose somewhere around 6-12" [more than I expected].

And in the 14 day lunar day [300+ hours] it very likely that all of 3' of snow would evaporate- and perhaps all of it within 100 hours.

"As for the spray, it'd evaporate rapidly in sunlight, or freeze by evaporative cooling in shade. No liquid would make it."

So, you don't think liquid would hit the lunar surface?

Would it matter how much volume water per second was use, say bigger hose or more water pressure?

I don't think the presence or lack of sunlight would make much difference. But anyhow, you think if this was done at nite it would function like a snow machine?