It's true what they say, the west coast gets one heck of a lot of rain...

... in the winter.

The rain stops roughly May 1st, and although other parts of town might get some showers in June and another in August, on our farm, it doesn't rain again until the middle of Oct.

I've been working on improving the shape and quality of the land to capture much of the moisture.  We have a few of those ugly blue rain barrels at each downspout.  They just run out so fast, that they are usually empty by June.  I think it's time to go bigger, especially now that solar powered plant watering systems are getting so cheep. 

I would like to water the kitchen garden, one larger area, and the livestock from the rainwater storage.  Since we are at our peak water needs right now, I figure it's a good time to calculate how much water I want to store this winter.  What should I be looking at to calculate this?

Then I need to know if I can fill up the barrels with the roof surface I have.  On our smallest roof, two of the blue barrels fill up from dry in about 18 hours of heavy rain or two days of light rain.  The larger roofs, they can fill up in an hour to a day. 

Any thoughts on where I should start learning about this?

We have used
Number of months without rain (longest dry period) x water use per month

This assumes of course that you have a roof that is big enough and sufficient rainto capture that...

We arrived at 40-50.000L tank as we have 3-5 months without rain and use ~8.000L/month (we collect rainwater for household use and not for irrigation, we will be irrigating with cleaned waste water). Currently we use around 8.000L/month (4 people) - but that is in a leaky tank ... wonder what it will look like once we get a tank that will hold water.

How long is a piece of string ? You will never have enough maybe a better way to look at this is how much can you collect ecomonically and base your usage on that . Some plants will take all you can give them them some more others need lots otherwise they die others seem not to be bothered .

David

As a general rule, I assume a 1 inch rain on 33 square feet of roof collects 20 gallons of water.

Of that 20 gallons I assume that I'll collect 80% of that.

Example:
A 12×24 roof has 288 square feet of collection area.

288/33=8.73

8.73×20=174.5

174.5 ×0.8=140 gallons of collectable rain

Mileage may vary.

In SI units calculation is pretty straight forward. 1 mm of rain for 1 m2 roof area is 1 liter, and 1000 liters equal to 1 ton of water. 80% efficiency is reasonable for mediterraean climate.
Though there are couple of issues to consider.
Unlike other regions we have a big first flush ecent following the first rains in september. As probably known, first flush brings a lot of debris which have to be diverted. During the 6 month dry period, our roof gets covered with dust, dead criters, leaves etc. I connected 800m2 - ~8000ft2 roof area to the first flush tank (settlement tank) (2m3 ~ 2 cubic yard), it fills up with debris after september rains. Thus it is better to clean tanks after 2 - 3 weeks of "first" rains (september rains).
Rain water is gray water, meaning that it is not clean enough to store in a tank. Processing rain water to store is expensive. It just doesn't cut the costs. Storing rain water in soil is one option, storing in a pond/ lake is another. I chose to store in soil. Rain water travels to the main collection tank after the settlement tank. Water does not stay more then couple of days. Sprinkles kick in 2 days after the rain event and if there is more water incoming then the capacity of the collection tank (30m3) another pump pumps the excess to swales.
I used to spend 10 cubic meter of water per day on average.  4 months equals to 1200m3 of water to be stored. Those tanks will be expensive for sure. This year the average droped to 1.2 m3.
Solution I came up with:  store water in soil, increase water holding capacity of the soil by increasing organic matter, mulch mulch mulch, shade shade shade.
My stiuation doesn't correspond to yours, but I thought it might give an idea or two. Hope it helps

I love the idea of storing rainwater in the ground!


My garden sets on a N-S swale made in the 1930's by the WPA. Unfortunately, it's been hardpan clay since the mid 70's

In the 1980s,  the current landowner  made multiple cuts through the swales in the hopes of increasing rain run off to a newly installed (and leaky) pond.


I've dug silt ponds (1m deep by 2m diameter) at the swale cuts to hold water higher.

This has helped immensely, as the water is being held higher in the landscape and allows the plants greater access to water.

As biomass holds about 4 times it's weight in water, storing water in well prepared ground is preferable in my opinion.

Having done drylands horticulture with last drops irrigation over three decades, between high desert New Mexico and coastal southern California plus a few places further afield, and being in a business startup process for rain-water 1st/ grey-water 2nd/ metered-water least irrigation conservation, my suggestions are:

- reduce your water needs to a quantifiable optimum
- swale all your rainwater runoff/ overflows directly across grow spaces & direct-to-soil
- large tanks are unnecessary with summer monsoon during growing season (NM), except when higher-quality water is needed out-of-season
- large tanks are very helpful with winter precipitation/ poor water quality (CA), but will cost you ~$0.50/ gallon capacity
- in most households grey-water will be a larger & cheaper source, requires more critical management for quality but can also reach level of fertigation
- prefer gravity-fed over pumped
- managing & integrating the three sources under highly variable weather, climate, & production situations can be complicated

At NM 1456sf R&D homestead using all 8" avg rainfall but no greywater I am deep-watering 1/10 ac of 20-yr edible plantscape/ orchard-gardens/ (nursery/ starts/ chickens) with 2400 gals once every 3 weeks = 16"/ yr, or half a traditional acequia irrigation right under 100" evaporation. 
At CA 1200 sf demo house under 1/3 evaporation I am putting in 4x365 gals under downspouts & 2x5000 gals for their overflow = 11,460 sized to hold one year's historical average roof runoff, though I'll also be overflowing both neighbours' runoff through system, for smaller plantscape/ gardens but poorer water quality & soil conditions.  Greywater system with 24-hr tank pumped up to hillside orchard tank for 14-zone bi-weekly rotation will produce twice the water, with my & son's bio-compatible soaps but wife's beauty products. 

Around here, we base our irrigation systems on the premise that the plants require one inch of water per week (0.6 gallons per square foot per week) (25 liters per square meter per week)

Thanks guys.  All good things to think about. 

The problem I have is that it's hard to know what questions to ask when I don't know enough about the subject.


A little bit more about my location.

these dates are for our farm's microclimate, not our city which is quite a bit different. Our last killing frost can be as late as April 20th but our soil doesn't warm up for summer crops until the end of May.  The rain stops on or before May 1st.  The rest of the city gets a downpour in June and another one at the end of Aug, but it misses us most years.  Our rain starts on our around Oct 15th, although we may get light showers the week before.  That's about 165 days with zero rain in a normal year. 

We are one of the last houses to draw on this aquifer.  Uphill there are more and more demands on this water seam with increasing housing and farming (read heavy irrigation).  It has a noticeable effect on how our well performs. 

Water conservation methods already in place

low flow everything plus awareness of water use in the home. 

we did grey water collected from the house before the city put a stop to that.  Rainwater collection is encouraged, but not household grey water.

earth works to keep the moisture in the soil - all the growing surfaces are terraced.  We are working on improving our soil to retain more moisture and situate the growing spaces to increase dew collection.  This means we can grow most of the farm without water until mid-July.  Other farms in our area start irrigating in April when the rains lessen.

airwells - we are experimenting with airwells around newly planted trees - with impressive results of over 90% survival rate on spring-planted trees given zero irrigation

plant breeding - inspired by Joseph Lofthouse's work, we've been breeding for drought tolerance and early frost tolerance.  We are able now to direct seed squash at the start of April and grow it to harvest with zero water.

staple crop choice - we are experimenting with staple crops like chickpeas and oats that can be planted during the rainy season and dry down during the dry period

only our small greenhouse and kitchen garden are under irrigation - the greenhouse is watered once a week for about 20 min with drip irrigation, and the kitchen garden gets a light sprinkling around the plants that need it each evening to cool the soil and encourage dew collection (which works amazingly well).

selective weeding - for parts of the farm that get better dew collection, we let the weeds grow up with the seeds.  The weeds help with the dew collection and only when the plants are far enough along that they can collect their own dew, we weed and give them more light and space.  It is surprising how well this works in the low parts of the farm where the dew is strongest.

capturing rainwater - we have two to four of those ugly blue rainwater barrels at every down spout.  This is a great help but not enough.

Water conservation plans for this year:

continue with plant breeding for drought resistance

rework the kitchen garden into a series of smaller beds - of different varieties to experiment and see which gives us the most rewards for our conditions.

keep experimenting with mulch to see if there is one that will work in our conditions - so far all the mulches we've tried need more water in the second half of the summer than mulch-less areas.

collect more rainwater in the winter

I think we're off to a good start conserving water.  What I'm hoping to discover is what questions I need to ask myself.  What do I need to observe now so that I can see how much water I need next year?  Do I need to go and measure each space I want to irrigate?  Measure the amount each sheep drinks per day and then multiply it by the expected number of sheep?  I don't know.  We have lots of roof, so I can take our precipitation numbers and size of each roof.  But it's no use buying more rainwater storage than I need (which is $1 CDN for each American size gallon).  I also need to discover what kind of fixtures I need to gather the water with and then discover if I need anything to get the water out of the tanks.  Some of it can be gravity fed, but not all. 


So far, I think my water needs are

The challenge is, next spring I'm growing a crop for someone else.  This is going to need irrigation from May to about halfway into July, probably by overhead watering for half an hour twice a week.   I did a green manure crop there over the winter and am working in manure and organic matter this summer, with another green manure crop planned this winter.  So by then, the soil should have decent moisture retention. 

There's also another terrace that I want to improve, but this is an area with poor dew collection and depends on having water available all summer.  Once I get the soil repaired there, I plan to grow drought tolerant, deep rooted plants like woad, teasel, and other textile and medicinal crops. 

I don't know how to calculate these water needs.  The first one is on a similar elevation to where the water storage would be and the second can be gravity fed.

animal water needs - this I can observe now how many gallons a day I use in the summer v. average water per day.  Right now, I'm sitting at between 40 and 50 gallons a day.

kitchen garden - this would be so much more successful with more water.

dyeing - I would also like to use rainwater for natural dyeing - this would be about 10 gallons a week in the summer.

What other questions should I be asking myself now?  What else do I need to observe?