How Long To Water A Lawn: Rainfall + Irrigation Calculation Per Week

Lawn Irrigation Optimization Tips

  1. Most grass types require 1” water per week during the growing season to stay healthy
  2. If you get that much through rain then you rarely need to irrigate
  3. If you use sprinklers then you’ll have to irrigate more than 1” per week to get 1” into the grass due to water loss
    • Evaporation of water droplets in air
    • Wind loss of water in air
    • Evaporative loss of water on grass blades and soil
    • Runoff
    • Overspray

Overspray is usually caused by sprinkler heads that do not conform to the exact shape or length of a lawn space. Some lawns will have a lot of overspray which can easily be the biggest waste of water in the lawn of them all.

I installed an Irrigreen sprinkler system in my lawn to prevent this from being a problem as it can conform to just about any shape lawn of nearly any normal size. Click the link to see my review of Irrigreen.

Evaporative loss of water in air

  • Increases when water droplets are smaller
  • Increases when irrigation is in air longer (further distance or higher height)
  • Increases as wind speed increases
  • Increases under higher water pressure delivery
  • Increases as air temp increases
  • Increases as relative humidity decreases

Minimize evaporative loss in air by using many sprinkler heads over a larger area that spray with larger water droplets and don’t throw water high into the air…only irrigate on the coolest days of the week in times of higher humidity when wind speed is low – this is typically at night just before dawn. I advise using a sprinkler control box or a hose timer to end a sprinkler zone session at dawn. For small properties or zones that don’t need more than one or two sprinklers attaching a pressure reducer before your sprinkler emitters will give you more control of your irrigation pattern, allow you to use multiple hose end (low to the ground) sprinklers and minimize loss over the session because you can fine tune your water delivery. Hose attached rotary sprinklers with wider emitters will be best.

Wind Loss Of Water In Air

  • Increases greatly as height of spray pattern increases
  • Increases greatly as wind speed increases
  • Increases greatly as droplet size decreases

Minimize wind loss by irrigating in low (or zero) wind speeds only and keep your irrigation as low to the ground as possible using sprinklers that diffuse the water less (larger droplet sizes).

Evaporative loss of water on grass blades and soil

  • Increases in higher heat
  • Increases in lower humidity
  • Increases on blades of taller cut grass
  • Decreases on soil under taller cut grass
  • Decreases on blades of short cut grass
  • Increases on soil under short cut grass
  • Increases on a sunny day
  • Increases when small amounts of water fall on dry hydrophobic soil

To minimize evaporative loss of water on grass blades and soil irrigate when temps are low and humidity is high for your area. The best days are overcast and with higher volumes (duration of irrigation) this will force a greater percentage of the irrigated water to the soil where it is needed. Irrigating immediately after mowing (mow in evening then irrigate at the end of the same night) will allow for less blade surface for water to collect on and evaporate away…and if you receive drizzle or small amounts of rainfall this is a great time to apply supplemental irrigation because the negligible rainfall will ensure the soil isn’t hydrophobic.


  1. Increases when sprinkler spray patterns are not optimized
  2. Increases when fewer sprinklers are used per zone
  3. Increases when hoses do not fit tightly
  4. Increases when spay patterns spray water too far
  5. Increases when soil can’t absorb water as fast as it is delivered
  6. Decreases as surface area increases (porous or aerated soil)

Limit runoff by running more sprinklers per zone for better spray pattern optimization. Adjust spray distance using pressure regulators if needed and ensure all hose fittings and connections are not leaking. Teflon tape and fresh hose washers help a lot. Further decrease runoff by aerating the lawn and ensuring it is not hydrophobic prior to irrigating.

How To Waste The Least Amount Of Water Possible

Test each sprinkler zone for water delivery to the soil in a few different locations under the optimal watering conditions.

Set multiple rain collection gauges up, tuna cans, or coffee cups (anything with straight sides) and run the sprinklers on a cool day of the week just before dawn when it is not windy with the most sprinklers and least water pressure are used. If possible use sprinklers that emit water in small droplets rather than a fine spray.

Time how long it takes for 1” of water to collect in your container – this will be your target watering duration per zone if you only water one time per week.

As temperatures climb and/or humidity drops at different times of the year you will need to increase watering duration slightly to offset increased evaporative loss from water flying through the air.

You can limit water loss on the ground by mowing prior to irrigation, core aerating the lawn and top-dressing with water retaining biochar and applying the evaporation slowing product called Hydretain every 12 weeks.

Time your mow and irrigation session to the coolest days of the week based on weather forecasts rather than the hottest days and especially those days that are more humid or overcast. If light rain occurs once in a week then that is usually the best time to irrigate because you can ensure deeper water penetration into the ground and get more water on the ground when it is not hydrophobic.

During the hottest weeks of the year you’ll have to irrigate more frequently because evaporative loss will be greater. Instead of watering deeply once every 7 days you may need to water roughly 60% normal volume per session every fourth day based on your environmental conditions.

There will likely be a big variance between hot humid climates and hot dry climates, humid may be once every fifth day, dry may be once every third day. During the hottest parts of the summer leaving grass blades longer will keep soil temperatures lower and retain more water in the soil than it would in a shorter cut lawn.

Applying root development products to the lawn in the spring like sea kelp and alfalfa meal while fertilizing for stress tolerance using slow release organic nitrogen sources, adequate amounts of potassium, and adding trace minerals via Azomite or Greensand can all help minimize your need to overwater throughout stressful times of the year.

Under high heat periods in the late Spring, Summer, and early Fall you may have to fully increase water delivery to your lawn upwards of 1.5” per week but your target should always be 1” and you should be working towards that year after year.

It’s possible that eventually you can lower you lawns demand for water to even less than 1” per week but that usually requires a lot of effort to get it there over multiple seasons of lawn improvement.