Water estimations

What do you think? how do you measure how much garden a garden will need? We have a landscape architect on board getting specs. Often with hoses in our backyards, we don't have to think of emptying a tank. When having to estimate and take the options of resourcing water the factor becomes crucial, not just for the garden but for the crew of students at Humbolt University designing the water filtration system.

Here is the first attempt! Luckily with suggestion from scientist and permaculture consultant Andrew Faust we have two things on our side: A reminder that the average summer rainfall in NYC is a plentiful amount. Hard to depend on and often in downpours that drainage and flooding becomes a concern, but plentiful, if predictions can allow for it.
Also: the three sister's garden is the BEST for drought tolerant features. Meaning that once established, it needs little to no water (irrigation that is).

Here however are some speculations of water calculation.

Case 1: Using the landscape irrigation formulas that I calculate with at my job, I figured that if a vegetable garden consumed as much water as a lawn, exposed to full sun in New York City, that it would consume 4,040 gallons of water per month if it used drip irrigation. Spray irrigation was even less efficient.
Assumptions:1600 ft2 of garden full sun / maximum exposure New York City regional evapotranspiration rate for the month of July - 4.34Drip irrigation

Case 2: Using the same formulas, my other estimates involved converting units from metric to standard, but i was unsure of some other values. Assuming the worst with a best guess, i calculated that if a garden was equally divided up amongst cabbage, carrots, cucumber, lettuce, 2 kinds of onions, peppers, onions and tomatoes, that it would consume 8,773 gallons per month (I found crop evapotranspiration data for these plants in metric units online).
Assumptions:1600 ft2 of garden full sun / maximum exposure New York City regional evapotranspiration rate for the month of July - 4.34Drip irrigation

Case 3: I looked for another way to figure out the water use consumptions, so I took it back to the basics and tried to find an online resource that talked about inches of water applied when irrigating. This case assumes a traditional spray irrigation - I found one website that said apply 2" of water "often" to vegetables during peak growth. I know that you have to water vegetables AT LEAST every other day during the height of the summer, so i figured conservatively - 2" of water over 1600 ft2, 15x per month equals 29,995 gallons. That seems way too high.
Assumptions:1600 ft2 of garden2" water per irrigation Water applied every other day (15x per month)Spray irrigation

still yet to be finalized...each answer varies the water supply so heavily!
we'll keep you posted