Irrigation Water Calculator

Irrigation Water is evaluated from Lawn/Garden Area, Climate Zone / Evapotranspiration and Plant Type. The calculation reports Weekly Water Needed, Monthly Irrigation Water and Estimated Monthly Irrigation Cost.

Results

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About the Irrigation Water Calculator

Irrigation Water is treated here as a quantitative relation between Lawn/Garden Area, Climate Zone / Evapotranspiration, Plant Type and Irrigation System Type and Weekly Water Needed, Monthly Irrigation Water, Estimated Monthly Irrigation Cost and Weekly Run Time.

The calculator uses a multi formula configuration. Each reported value is read as a direct evaluation of the stored rules with the declared field formats and units.

Formula basis:
Net water need = ET rate x crop coefficient (inches/week)
Gross applied = net / system efficiency (accounts for evaporation/runoff losses)
Weekly gallons = area x (gross inches / 12) x 7.48 gal/cu ft
Run time = weekly gallons / flow rate (GPM)

Interpret the outputs in the order shown by the result fields. Optional inputs affect only the outputs that depend on those variables.

Formula & How It Works

The calculation applies the following relations exactly as recorded in the metadata:

Net water need = ET rate x crop coefficient (inches/week)
Gross applied = net / system efficiency (accounts for evaporation/runoff losses)
Weekly gallons = area x (gross inches / 12) x 7.48 gal/cu ft
Run time = weekly gallons / flow rate (GPM)

Each output field is produced by substituting the supplied inputs into the relevant relation and then applying the declared rounding or text format.

Worked Examples

Example 1: 2,500 sq ft fescue lawn in Atlanta (moderate ET, spray heads, $4/1,000 gal)

Inputs

area_sqft: 2500 et_rate: 1.00 plant_type: 1.00 efficiency: 0.65 water_rate: 4
Weekly Water Needed: 2,398 gallons/week. Monthly Irrigation Water: 10,382 gallons/month. Estimated Monthly Irrigation Cost: $41.53. Weekly Run Time: 96 minutes/week

With Lawn/Garden Area = 2,500, Climate Zone / Evapotranspiration = 1, Plant Type = 1 and Irrigation System Type = 0.65 as the stated inputs, the result is Weekly Water Needed = 2,398 gallons/week, Monthly Irrigation Water = 10,382 gallons/month and Estimated Monthly Irrigation Cost = $41.53. Each value corresponds to the declared output fields.

Example 2: 5,000 sq ft Bermuda grass in Phoenix (desert ET, rotary sprinklers, $7/1,000 gal)

Inputs

area_sqft: 5000 et_rate: 1.50 plant_type: 0.80 efficiency: 0.75 water_rate: 7
Weekly Water Needed: 4,987 gallons/week. Monthly Irrigation Water: 21,594 gallons/month. Estimated Monthly Irrigation Cost: $151.16. Weekly Run Time: 100 minutes/week

With Lawn/Garden Area = 5,000, Climate Zone / Evapotranspiration = 1.5, Plant Type = 0.8 and Irrigation System Type = 0.75 as the stated inputs, the result is Weekly Water Needed = 4,987 gallons/week, Monthly Irrigation Water = 21,594 gallons/month and Estimated Monthly Irrigation Cost = $151.16. Each value corresponds to the declared output fields.

Example 3: 1,000 sq ft vegetable garden in Chicago (moderate ET, drip irrigation, $5/1,000 gal)

Inputs

area_sqft: 1000 et_rate: 1.00 plant_type: 1.10 efficiency: 0.90 water_rate: 5
Weekly Water Needed: 762 gallons/week. Monthly Irrigation Water: 3,299 gallons/month. Estimated Monthly Irrigation Cost: $16.5. Weekly Run Time: 76 minutes/week

With Lawn/Garden Area = 1,000, Climate Zone / Evapotranspiration = 1, Plant Type = 1.1 and Irrigation System Type = 0.9 as the stated inputs, the result is Weekly Water Needed = 762 gallons/week, Monthly Irrigation Water = 3,299 gallons/month and Estimated Monthly Irrigation Cost = $16.5. Each value corresponds to the declared output fields.

Example 4: 3,000 sq ft native plant xeriscape in Tucson (desert ET, drip, $8/1,000 gal)

Inputs

area_sqft: 3000 et_rate: 1.75 plant_type: 0.50 efficiency: 0.90 water_rate: 8
Weekly Water Needed: 1,818 gallons/week. Monthly Irrigation Water: 7,873 gallons/month. Estimated Monthly Irrigation Cost: $62.98. Weekly Run Time: 61 minutes/week

With Lawn/Garden Area = 3,000, Climate Zone / Evapotranspiration = 1.75, Plant Type = 0.5 and Irrigation System Type = 0.9 as the stated inputs, the result is Weekly Water Needed = 1,818 gallons/week, Monthly Irrigation Water = 7,873 gallons/month and Estimated Monthly Irrigation Cost = $62.98. Each value corresponds to the declared output fields.

Common Use Cases

  • Calculate lawn and garden irrigation water requirements
  • Estimate sprinkler system water usage and cost
  • Determine optimal irrigation schedule based on local ET rates