I'm planning to do a little rain water harvesting, well actually A LOT of rain water harvesting. I am in Texas and we have drought here that can decimate a farm. One thing I'm trying to decide is how much to harvest. So to help my calculations, I'm breaking it down bit per bit. So let's say I want to water my garden. How much water does a garden need, let's say per square meter (or any other unit measure)? The garden will be an assortment of plants including grains, vegetables, herbs, fruits etc. Probably 1/3 will be grains such as wheat. I'll have some trees scattered about but likely not near the main garden. I'm not looking for an exact measure but rather a general starting point. So for the sake of calculation, let's say water harvest is my only source of water for the garden, no rain. How much water will it need?

A general rule of thumb is that most annual vegetables need the equivalent of 1" of rain per week.

If you rig drip irrigation, you only need to water the individual plants, not the entire garden space.
A good layer of mulch will minimize evaporation.
Never water during the heat of the day.
One deep watering is more beneficial than multiple light waterings.

John Polk wrote:A general rule of thumb is that most annual vegetables need the equivalent of 1" of rain per week.

If you rig drip irrigation, you only need to water the individual plants, not the entire garden space.
A good layer of mulch will minimize evaporation.
Never water during the heat of the day.
One deep watering is more beneficial than multiple light waterings.

Thanks again John for your helpful reply. Drip irrigation sounds like the way to go. As for 1" per week, how does that translate to each plant? It's not 1" per plant is it? I'm trying to get an idea of how I could measure that.

1 cubic foot of water is 7.43 gallons
1 inch of rain across a sqft of land is 1/12 cubic feet=7.43/12=.62 gallons. That's about 2 1/2 quarts.
Water per plant is dependent on how many plants you can squeeze into an area.

I grow my vegetables in beds. 4' wide so I can reach the center without stomping on stuff, 50' long matches the length of drip hose.
Total area is 200 sqft/bed.
To match 1" of rain across the entire bed, I need 200 x .62 gallons = 124 gallons/week.
For more area, just do the math.

Let's add time to the equation...
For a crop that takes 90 days, I'll need to water 13 times.
That same 200 sqft bed needs 124 x 13 = 1612 gallons to produce the entire crop.
You'll need to do the math for your own design.

How much space for rainwater harvesting?
I have a 12x24 garage. That's 288 sqft of roof.
An inch of rain falling on that roof = 288/12 =24 cuft of water.
At 7.43 gallons/cuft, that's 178 gallons.
Not bad for a light shower.

As John Polk referenced, there are ways to decrease the water demand. Hugelkulture will store massive amounts of water. Organic matter in the form of compost, humus, and leaf mold will also hold several times their weight in water. I tested leaf mold and found it will hold 4 times it's own weight in water: 2 pounds of leaf mold will hold 8 pounds of water. That's a gallon. A Cubic yard of compost will run you $20-40 if you had to buy it, and weigh in at around 1000 pounds. Spread across 500 sqft, you end up with about 2 pounds of organic matter per square foot, and will hold 500 gallons of water. That will keep you going for a week after a light shower.

The method by which you deliver the water is important. With overhead spraying from a sprinkler, you can easily lose 30% of your water to wind and immediate evaporation. The sprinkler will deliver the water across a wide area, including the paths between your plants. There goes another third of the water. Landing on top of your plants, the water has time to evaporate before it can make its way into the soil. If you have mulch covering the ground, it will absorb a great deal of water before the soil gets any. It's possible to run the sprinkler all day without adding moisture to the soil.
Drip irrigation can be set up easily. The water is placed directly on the points you want to irrigate. It can be placed under the mulch. In my experience, each drip point will service a radius of a couple of feet. I set up a drip line with a 3/4" supply line down the center of a bed, with a foot long 1/4" line coming out every foot, 1 going left, one going right, with a drip tip at the end. See terrible artwork below.

The tilth of your soil has an impact on water demand. A hard, crusty soil will allow the water to run off before it can percolate into the soil. Soil that is loose and crumbly allows water to penetrate, and has more surface area to hold more water. Here's where drip irrigation shows another advantage. Overhead sprinkling and rain on bare soil will tend to compact the soil, where drip irrigation falls maybe half an inch, under low pressure, landing on the same spot. If you look at a working drip system, you see a wet spot the size of a dime.

MULCH
I've got nothing bad to say about mulch. Mulch covers the soil. It prevents the rainfall from hitting like a sledgehammer, pounding it into a hard surface. Rain can still penetrate, but the mulch is a shock absorber. Mulch acts as insulation, keeping the soil cooler in the hot sun, which translates to less evaporation. If you have mulch in place and want to add drip irrigation, the mulch can be moved to the side pretty easily, then replaced. With mulch ion top of the drip line, the water moving through the tubing does not get hot. Plants like a cool drink on a hot day, just like we do.

Hi Ken. Thanks for the detailed information. Let's say I have an 10,500 sq ft garden, 150' x 70' sectioned off into 4' beds as you have, with 2' in between each bed. That is 7200 sq ft of plants. That is 7200 x .62 = 4464 gallons per week (wow!). I checked on wheat and it's about 17 weeks so let's say two 17 week crops per year = 34 weeks of watering. My average rain fall in my area is 40 inches per year. A terrible drought would be, what, 20 inches per year? I truly have no idea but let's go with that. So 20 of those 34 weeks are handled by nature leaving 14 weeks that I must provide supplemental water. So that's 4464 gallons X 14 weeks = 62496 gallons? Is that correct? If so, wow that's a ton of water! Wait no that's 260 tons of water!

My math says 17 tons, but it's a bunch of water. Lots of homework for you to do now. When does the rain come, when do you need it, how much is a reasonable amount to store, whats the best storage method for your situation?

Weather websites can give you a good idea of how the rain is spread out over the year.
How much can you gather is determined in part by the structures and their distribution, and tarps can be placed inexpensively.

Lots of options for storing the water.
-Surface pool/tank/container
-in ground cistern
-ferrocement vessel
-natural features of the land, cave, gully

You will need a plan to move the water from where it is stored onto the crops. A small tank up off the ground will give you pressure. A pump, hand, wind, or solar PV powered, can do the moving.

To save yourself investment and effort, look for ways of reducing your water demand, and getting more use out of the water. If you can cut your demand in half, the plan got a lot easier.
Does it make sense to raise fish? Irrigate the land and fertilize at the same time, plus get a fish fry out of the deal.
Can you move the water through a radiator inside the house to offer some cooling on its way to the field.
Can you use water stored up high to operate a turbine or mechanical gear on its way down.
How about greywater?

Permaculture has a lot to do with integrating your designs so that one project benefits another.
Raise worms to eat waste. They produce castings for the crops and more worms to feed chickens and fish.
Add worms to the soil, they do the tilling, increase fertility, reduce nematodes, and provide drainage channels when it rains too much.
Raise fish for food. They poop in the water, a portion of the water needs to be changed each day, run that nutrient rich water to the crops.
Raise chickens in tractors. They prepare the land for crops and keep the bugs down.
Crop waste is offered to the chickens or goes to the compost.
Compost is added to the worms to keep them going strong.
Water from the roof is stored to serve the worms, fish, chickens, humans and crops.
When the water comes from the cool cistern, it runs through the house in the summer to cool the place, through the livestock barn in the winter to warm it up.
Water from the cistern is pumped over the roof of the house in the summer for hot water, and to help cool the house.
Straw from the grain is used to build a strawbale structure for the livestock in the winter.
Trees give you shade in the summer, plus fruit and nuts, plus leaves to use as mulch so you use less water.
Sunflowers keep the bees employed, decorate the place, give up seed for chicken or fish feed or sunflower oil. The scraps from making oil is offered to the fish or worms.
Sweet sorgum is drought tolerant, the seed is an excellent grain, the stalks are pressed to produce syrup, the pressed stalks make an excellent fodder for the dairy cow.
Marigolds and nasturtiums help keep bugs off the tomatoes, and offer more flowers for the bees, which pollinate the crops and give you honey.
Herbs drive off pests and adds flavor and diversity to your meals.
Beans give you burritos, baked beans, hummus, and add Nitrogen to the soil.
Peanuts also add nitrogen, plus give you peanut oil, peanut butter, roasted snacks, and peanut hay for the livestock.
It just keeps on going...

Ken Peavey wrote:
Raise fish for food. They poop in the water, a portion of the water needs to be changed each day, run that nutrient rich water to the crops.

Or if you want to skip the step of changing the water, use aquaponics.

all you math folks might like to figure out how much water soil can retain with 2% carbon vs. how much it can retain with 12% carbon.  in Gabe Brown's video he says on his land the difference amounts to a humongous reservoir, millions of gallons of water.  with microbes and organic matter it is quite doable to increase the carbon ration in the soil to 12% with several years.  see Gabe Brown video

https://www.youtube.com/watch?v=y_GEpq59urY
Keys To Building a Healthy Soil - Organic - Permaculture and Polyculture
Gabe Brown Soil Conservationist - Explains how to remediate and build up your soil quality. .
Keys To Building a Healthy Soil - Organic - Permaculture and Polyculture
Gabe Brown Soil Conservationist - Explains how to remediate and build up your soil quality.