Estimating water usage for a Market Garden

As part of the planning for starting a Market Garden, I have to estimate my water usage. EBMUD needs this information for capacity planning for their Recycled Water system. They asked me for my heaviest month usage.

A report from the California Irrigation Management Information System lists the following average monthly ETo for Pleasanton, the closest measuring station to me. July, with an average ETo of 7.36 inches is the most demanding month.

Stn Id Stn Name CIMIS Region Jan
(in)
Feb
(in)
Mar
(in)
Apr
(in)
May
(in)
Jun
(in)
Jul
(in)
Aug
(in)
Sep
(in)
Oct
(in)
Nov
(in)
Dec
(in)
Total
(in)
191 Pleasanton SFB
1.50
2.08
3.59
4.85
6.17
6.86
7.36
6.49
4.86
3.32
1.90
1.39
50.37

This data is listed in ETo, which is the EvapoTranspiration for grass. I’m more interested in ETr, which is Evapotranspiration for alfalfa. ETr more closely matches most of the crops I will be growing.

ETr = ETo * 0.83

7.36 * 0.83 = 6.1″ / july acre * 27,154g/in/acre = 165,639 gallons per acre in July.

I plan to start cultivation on 1/2 acre of land. Row density should be 50%.

165,639 gallons / July Acre * 1/2 acre * 50% density = 41,409 Gallons for July, or about 1400 gallons per day.

I believe there should be an additional reduction in usage due to drip-only irrigation, but don’t know how to quantify that at this time.

BTW: At this time, EBMUD charges $3.17 per 100 cubic feet of non-potable water. 100 cubic feet = 748 gallons. I should expect a $175 water bill in July.

References: http://www.agf.gov.bc.ca/resmgmt/publist/500Series/577100-5.pdf

Calculation for full farm usage

Lower Rows: 100 rows @300 ft2/row = 30,000 ft2

Upper Rows: 85 rows @ 210ft2/row = 17,850 ft2

High Tunnels: 4 @ 1,728 ft2 = 7000 ft2

Total growing space (excluding trees): 54,850 ft2 = 1.25 acres.

165,639 g in july/acre * 1.25  = 207,000 gallons for July, or 6,900g/day.

207,000 / 748 = 276 units * $3.17 = $874 water bill in July.

Tree area = 100 trees at 5×12 spacing = 12’x500’=6000 ft2 = 0.13 acres.

Early tree water usage = 80 Gallons Per Week = 4100g/yr
Later tree water usage = 250 Gallons per Week  = 13,000g/yr.

A 5 gpm well provides 7200g/day.

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