Irrigation & Record Keeping

This time of year I like to organize all my records and analyze our numbers from the previous season. Bed feet planted, harvest totals, actual days to harvest, etc. Almost 4,000 lbs of baby greens! 1,700 lbs of arugula alone! 32% spike in cherry tomato production! It’s a very exciting month for spreadsheet nerds like myself.

NG greens

Harvest totals are old hat, but this is the first year that I kept track of our irrigation water. All of our overhead water comes from the irrigation ditch that runs behind the orchard, fed by the Jocko river which is fed by snow run-off and a spring. It’s managed by the Flathead Irrigation Project who turns on the ditch every May and off on or before September 15 each year. We have a system of hand line on the farm, and can run only one set of 10 or 12 three-inch pipes at a time with the old pump. Veggies and cover crop areas are irrigated as needed, generally for 3-4 hours twice a week. Each line in the pasture runs overnight once a week. Because we lease our land, and we don’t deal with the water taxes or bills, I haven’t had to think about water use much at all. When we needed it, I just turned on the pump. No problem. This year, with the proposed water compact (read more about that here, it’s complications here and here), Steve, our landlord, asked that we record what we used. It always seems like a lot of water, but how much is it really?

For those of you uninterested in the math, skip down the page to find some reflections, below. For all you ag nerds, carry on.

There’s no question that there are gaping holes in the accuracy of these calculations, worn nozzles and varying pressure being two of many, but a general idea of water use is better than nothing. Here’s what I did: Each time I irrigated, I wrote down the section watered, how many sprinkler heads were used, and how long the pump stayed on. So by the end of the season, I could multiply the sprinkler nozzles by the hours irrigated on each line…

8.24.13    Barn Garden    10 nozzles    4 hrs     =   40 hours combined running time

… and add up every line of my irrigation log. The total for our 5.3 acres came to 8,941 total hours. In other words, if all the ditch water we used this season came from just one sprinkler, that sprinkler would have run for 8,941 hours. Then I could multiply those total hours by the estimated gallons per hour each sprinkler head emits to get the total number of gallons we irrigated with this season.

(total combined sprinkler hours) x (sprinkler nozzle gph) = total gallons of water used

To find the gph of each sprinkler head, I used the following chart and and multiplied the gallons per minute (gpm) by 60. Most of our nozzles are 5/32″ and at 40psi, they emit approximately 4.5gpm. This chart is from the Montana Irrigators Pocket Guide, published by the National Center for Appropriate Technology (which is helpful and generally recommended).


From there, I was able to calculate the the acre feet we used per acre of irrigated land. There are 325,851.43 gallons in an acre foot of water. Therefore:

(total gallons / 325851.43) / acres irrigated = acre feet per acre
in our case:
2,414,070 / 325,851.43 / 5.3 = 1.39 acre ft per acre

It just so happens that 1.39 acre feet is just under the 1.4 acre feet limit of the proposed water compact. That number was painstakingly calculated over 18 years of research monitoring rivers and agricultural land on the reservation. It takes into consideration (among many other things) in stream flows that keep fish and other wildlife at optimum levels and water needed for pasture, crops and livestock.

This area of Western Montana gets an average of 15 inches of rain per year, including snow fall. Compare that to Las Vegas at 4.5 in per year, or the Hudson Valley (where we moved from in 2011) at 48 in per year. It’s dry. In three years, we’ve harvested crops maybe three times in the rain. The pieces of land that we don’t irrigate turn gold and brown by mid July, as do the surrounding hills.

Water management is clearly an important part of responsible growing. I like to think that we keep this land as healthy and as naturally sustainable as possible, monitoring the soil saturation etc., but the natural state of this farm is not green rows of vegetables and crops. We rely completely on ditch irrigation and our drip system which we run off of the well. Without those infrastructures, all that remains is marginal pasture, brush, seasonal wildflowers, and sparse evergreens. With aluminum pipe, electric pumps, and man-made canals, we’re able to grow ten thousand pounds of vegetables annually, and roll in green grass in August. What we get from this soil is what we (and those who came before us) put into it: organic matter, manure, water, time, and care. We create this world that we sustain, balancing it as best as we can, encouraging the fungi, bacteria, insects, and moisture that keep our land producing.

Our ditch feeds a wide corridor of riparian forest, home to birds, bugs, microbes, and animals that all contribute to the diversity of our farm. This winter the cottonwoods are home to two great horned owls, conversing in long low tones all night long. It takes a considerable amount of work to keep this artificial system going, but it’s also incredibly rewarding.


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