Alley Cat water use

This detailed spreadsheet contains basic measures (such as how many gallons of water soak into the ground, or are collected from a roof, per square foot when an inch of rain falls), average precipitation figures for Minneapolis since the end of the 19th century, and the dimensions of each of our plots: http://bit.ly/igfT9R. This allows detailed estimation of water use, as well as the cost of a low-pressure soaker hose irrigation system.

waterlog

The link for Alley Cat gardeners to our mowing and watering log is http://bit.ly/mvhP80.

Watering Tips

Here is some watering advice found in Organic Gardening magazine's summer 2007 issue:

Watering the soil deeply but infrequently encourages robust root development. They suggest wetting the top 6-8 inches of soil when you do water, which is about 13 minutes of watering. "Water, then wait about half an hour and see how deep the water went," instructs Don Schultz, the horticulture manager at the Water Conservation Garden at Cuyamaca College in El Cajon CA. "Determine the length of watering time based on how deep the water went." You can then assess watering frequency based on how quickly the soil dries out. Wait to water again until the soil has dried at least 2 inches down from the surface.

It's also important to observe your plants. If they look water-stressed then you should water them right away rather then waiting until your next scheduled watering session. As a general guideline, water established plants until soil soaked 6-8 inches down twice a week. More recently planted vegetables may need to be watered more often until they develop larger root systems.

Gardening Matters

5th Annual Parade of Community Gardens 10 a.m. to 2 p.m. August 21, 2010 at community gardens throughout Minneapolis and St Paul

www.gardeningmatters.org/events/parade.html

and an event to watch for next spring: Community Garden Spring Resource Fair

www.gardeningmatters.org/events/fair.html

Rainwater for Community Gardening

Columbia Park residents of Van Buren Street and Architect Avenue have long been aware of a triangular piece of land located at 3606 1/2 Van Buren Street Northeast. This land is now owned by the county. Our neighbor Peter has successfully applied for a community gardening permit and had the soil tested with favorable results. This Spring, several of us are beginning to use the land.

The triangular property is bounded on the east and west-northwest sides by alleyway, and on the south side by the adjacent property, 3600 1/2 Van Buren Street Northeast, on which a garage stands. Old maps show a very short 36th Avenue Northeast connecting Central Avenue with this alley, running east-west through what is now 3601 and 3558 Van Buren. The garage at 3600 1/2 Van Buren, listed as built in 1900, would have been on 36th Avenue Northeast. Some time prior to 1922 (when property records indicate the house at 3558 Van Buren was built), 36th Avenue Northeast must have been removed, leaving both 3600 1/2 and 3606 1/2 Van Buren accessible only by alleyway and on foot.

The garage at 3600 1/2 Van Buren measures 40' 7" x 20'. Its roof has a pitch of about 4 in 12, giving a surface area of 427 square feet on each side for a total of 853 s.f. The building is on a rise of three to four feet above most of the garden area, offering the prospect of favorable water pressure from a storage tank set on the ground next to the building, without the structural complications of elevating it off the ground.

Based on 118 years on record (http://climate.umn.edu/text/historical/msppre.txt), Minneapolis receives an average of 27.6" precipitation per year. 21.9" of this falls from April through October. Based on the last 51 years on record (http://www.weatherbase.com/weather/weather.php3?refer=&s=85627), these months are mostly warm enough to store the water without freezing:

April: 2.2 inches of precipitation, 11 days below 32F

May: 3.4", 1 day

June: 4.2", 0 days

July: 3.6", 0 days

August: 3.5", 0 days

Sept: 2.9", 0 days

Oct: 2.1", 7 days

Collecting rainwater from May through September is most likely risk-free, but beginning to collect rainwater in April risks an average of only 11 days below freezing. While total precipitation from November to March accounts for the equivalent of less than six inches of rainwater, the more important reason to explore starting to collect rainwater as early as possible is the chance to have water on hand immediately after planting.

Based on 0.62 gallons of water collected per square foot of surface area per inch of precipitation (http://www.blurtit.com/q2606817.html), not accounting for splash or evaporation, the following seasonal totals can be estimated:

June-Sept: 3,746 maximum potential gals of water collected from one side of the roof / 7,493 gals from both sides of the roof

May-Sept: 4,638 / 9,276 gals

April-Sept: 5,228 / 10,456 gals

April-Oct: 5,784 / 11,567 gals

Water is used while it's being collected, so a tank wouldn't not have to be able to hold all the water at once that it will collect in a season. Another way to look at rainfall is to note that, on average, it rains about a third of an inch every three days in Minneapolis, April through October, which would result in about 80 gallons of rainwater collected from each side of the roof at 3600 1/2 Van Buren.

One gallon of water dispersed on one square foot of land would be equivalent to a 1.61" rainfall. The same gallon could water 5.38 square feet to the equivalent of the Minneapolis average of 0.3" inches per rainfall. It would take about 52 gallons of water to moisten a 10 x 27' garden plot to the same extent. This morning we staked the first five such plots, with room for at least another seven or eight. 3500 square feet of garden area would require about 650 gallons of water to equal 0.3" of rain, and so forth.

What size tank would be be optimal? A 1000 gallon tank collecting rainwater from the roof at 3600 1/2 Van Buren could offer the garden land envisioned the equivalent of about one and a half average rainfalls worth of additional water, a 2000 gallon tank about three average rainfalls, and so forth.

A 1000 gallon tank could hold 1.9 inches of rainfall collected from both sides of the roof, a bit less than average April rainfall in Minneapolis. (Total rainfall in June, the rainiest month, is 4.16 inches on average.)

1500 gals = 2.8" rainfall collected

2000 gals = 3.8"

2500 gals = 4.7"

3000 gals = 5.7"

Total precipitation in Minneapolis has varied widely throughout the 20th century, in the same month from year to year, as well as annual totals. (See the google docs spreadsheet "Minneapolis rainfall" at

https://spreadsheets.google.com/ccc?key=0AnaaerURnOZAdFdTby1OdWU1WGpzS0g5WU9tbW1jWnc&hl=en). Closer examination of the weather record for extremes in amount of rainfall, which varies widely from sprinkles to thunderstorms, would allow one to say how often a single rainfall has exceeded any given tank capacity. Noting length of dry spells between rainfalls will also be helpful. 

Not every time in spring when the temperature falls below 32F would a tank of water freeze solid, although it's worth a closer look at weather patterns for how far below 32F the temperature falls in April and even late March as well as how long the temperature remains below 32F. In addition, further information is needed on the dynamics of water freezing in tanks and hoses.

Water in motion freezes at a lower temperature than still water so design refinement in future seasons could include a simple wind powered agitator. It could be that the air is still enough overnight that a mechanical agitator is at rest precisely when it's most needed, but ideally this would be of simple mechanical design, using gears to exchange speed at the windvanes for torque in the water, thus avoiding either a conventional electric heater (this contradicts principles of sustainability in addition to requiring grid electric power the site lacks) or a wind-to-electric generator (a more complex system with more components to go wrong, plus energy is lost converting motion to electricity).

Calculations are available in the spreadsheet "rainwater collection" found at https://spreadsheets.google.com/ccc?key=0AnaaerURnOZAdG5nalhRVGYzcFRFZ29xVDRWWEtkcWc&hl=en

paul bernhardt

papadiscobravo at gmail.com