IMG_20121205_164919It’s a common belief that using rainwater for irrigation in a Mediterranean climate is not practical, because most rains come in the winter and most usage comes in the dry summer. The argument is essentially that you can’t store enough water to irrigate for an entire summer. While this weather pattern is a problem, it doesn’t mean you can’t–or shouldn’t–use rainwater to irrigate.Following are some thoughts on rainwater for irrigation in the Bay Area’s Mediterranean climate as well as my experience for the year Oct 2011 – Sep 2012.

Four factors mitigate the problem from a seasonal rainfall pattern:

  1. There is some overlap of the rainwater collection period and the irrigation period. That is, in warm spring weather, especially with newly planted crops, some irrigation is necessary. But, some rains also come during the spring. So, you can be using stored water during the same period you are using it. What this means is that you don’t need to have storage capacity equal to all of the irrigation water you will use during the year. You can get by with considerably less, as my data below show
  2. The benefits of rainwater collection can be obtained from partial irrigation with rainwater. Any rainwater saved for irrigation is water not sourced from regular sources, and thus a savings. Even if only 25% of irrigation water for a year comes from rainwater, the savings in water and energy used to pump or transport water is saved. It’s not an all-or-nothing proposition.
  3. Urban farmers are likely to be more efficient with water than large-scale rural farmers. In addition, it’s easy for urban farmers to collect and store rainwater. Residential roofs will collect much more rainwater than can be stored. Thus, demand can be lower and supply higher in an urban area.
  4. In a sense, the problem is being able to store sufficient water given the supply. This is being made easier by newer designs of storage containers, as shown in the photograph above. The containers shown each have up to 300 gallons capacity, yet have a footprint only two-feet deep, making them ideal to sit next to a house under an eave. Here they are next to my house but do not block my neighbors driveway. Earlier rainwater storage tanks were typically round and difficult to find room for, especially if they had a large capacity.  See a previous post here, with photos of my four 65-gallon barrels, ie, a similar capacity as the 300-gallon tanks shown, but with a much larger footprint. The eight garbage cans, again with the same amount of storage, take up an even larger footprint.

For the year Oct 2011 – Sep 2012, I had four 65-gallon rainwater collection barrels, plus eight garbage cans for secondary storage, which doubled my capacity to 520 gallons. I bumped this up somewhat with a dozen or so 5-gallon buckets, bringing my total storage to about 600 gallons. During this period, however, I used a total of 971 gallons of rainwater for irrigation. We had a warm spell from January to March, during which I used irrigated exclusively using rainwater. We then had a series of storms in April and May, during which I topped off my storage containers. So, this overlap period in which I both irrigated with and collected rainwater increased my actual usage of rainwater by more than 50% over my actual storage capacity.

In total, I used 1611 gallons of water for irrigation during the year. Thus, 60% of my irrigation was with rainwater. I irrigated exclusively with rainwater from October through June. The warmer months of July through September were with city water.

I have now doubled my storage capacity, to slightly over 1000 gallons. I believe with this total capacity, I may be able to irrigate my garden exclusively with rainwater this year. The experiment continues….

Addendum, 12/19/12: Not included in the above figures is water used to irrigate my perennial fruit trees, canes and vines. That totaled 35 gallons, all using laundry grey water and dripped into the soil using the five-gallon bucket method.