Watering Your Lawn During a Heat Wave or Drought
A heat wave raises how much water grass loses, while a drought limits how much you can give it. This guide shows the weekly depth, run times and dormancy rules that keep a lawn alive on less.
By Daniel Reeves, PE, CID
Irrigation Design Engineer
Short answer
Water a cool-season lawn at about 60% of its reference evapotranspiration (ETo) to carry it through drought, or give a dormant lawn 0.5 inch (13 mm) every 2 to 3 weeks.
The target is a share of ETo, so a heat wave raises it: at 0.30 inch of ETo a day, deficit watering of fescue or bluegrass is 1.26 inches (32 mm) a week, and warm-season grass needs 0.83 inch (21 mm). When watering your lawn during a heat wave or drought, apply that in one or two deep waterings before 9 a.m., split into cycles if water runs off.
Inches and minutes for your own grass and weather come from the lawn water deficit calculator; the sections below show how those figures are built. Most drought advice stops at "water deeply and less often", and the numbers fill in the rest.
Below are the three watering levels extension programs use, how a heat wave raises the total, run times for one or two allowed days, dormant-lawn watering, and a worked example for tall fescue.
Key takeaways
- Drought watering is a share of ETo. Deficit watering is about 60% of ETo for cool-season grass and 40% for warm-season grass, and survival watering is about 40% and 20%.
- A heat wave raises the total, not the share. When ETo rises from 0.20 to 0.30 inch a day, cool-season deficit watering goes from 0.84 to 1.26 inches (21 to 32 mm) a week.
- Apply the weekly depth in one or two deep waterings before 9 a.m., split into cycles when water starts to run off.
- A dormant cool-season lawn tolerates about 3 to 6 weeks without water. After that, universities advise 0.25 to 1.5 inches every 2 to 4 weeks.
- Hand-water dry patches, mow high, and hold nitrogen, aeration and new seed until the weather breaks.
- Local watering rules override any schedule here.
How much is enough when watering your lawn during a heat wave or drought?
Grass loses water by evaporation and transpiration, and irrigation engineers measure that loss against a reference grass as ETo, which depends on sunshine, temperature, humidity and wind.1 A lawn's use is ETo times a crop coefficient, and UC sets optimum irrigation at 0.8 of ETo for cool-season grasses and 0.6 for warm-season grasses.2 For a local figure, run your grass type and ET through the lawn water requirement calculator.
UC then defines three levels, each a share of the optimum.2 Optimum irrigation gives the most growth and best appearance. Deficit irrigation keeps an adequate appearance with less growth, at 75% of optimum for cool-season grass and 66% for warm-season grass. Survival irrigation only keeps the lawn alive for recovery, at 50% and 33% of optimum, with growth and quality sharply reduced.
Weekly water reaching the roots, by grass type and irrigation level
| Grass type | Level | Share of optimum | Share of ETo | Hot week (ETo 0.20 in/day) | Heat wave (ETo 0.30 in/day) |
|---|---|---|---|---|---|
| Cool-season | Optimum | 100% | 80% | 1.12 in (28 mm) | 1.68 in (43 mm) |
| Cool-season | Deficit | 75% | 60% | 0.84 in (21 mm) | 1.26 in (32 mm) |
| Cool-season | Survival | 50% | 40% | 0.56 in (14 mm) | 0.84 in (21 mm) |
| Warm-season | Optimum | 100% | 60% | 0.84 in (21 mm) | 1.26 in (32 mm) |
| Warm-season | Deficit | 66% | about 40% | 0.55 in (14 mm) | 0.83 in (21 mm) |
| Warm-season | Survival | 33% | about 20% | 0.28 in (7 mm) | 0.42 in (11 mm) |
Shares of optimum are UC's. Shares of ETo are the optimum coefficient times the level (0.6 × 0.66 = 39.6% and 0.6 × 0.33 = 19.8%, rounded). A week at 0.20 and 0.30 in/day is 1.4 and 2.1 inches; both are planning values, not measurements. Depths are before rain and uniformity.2
UC reports that most lawns can survive on 25% to 50% less irrigation than they usually get, as long as they are watered deeply once or twice a week.2 If your lawn is green and healthy today, a first cut of 10% to 20% for cool-season grass, or up to 30% for warm-season grass, usually does no serious harm.5 Once brown areas appear, stop cutting: UC warns that further reductions can permanently damage a lawn that already shows them.5 Species differ widely, and the grass type water needs calculator and the lawn watering by grass type chart show by how much.
How does a heat wave change the water a lawn needs?
A heat wave changes the base, not the percentage. A week at 0.20 inch of ETo a day totals 1.4 inches (36 mm). The same week at 0.30 inch a day totals 2.1 inches (53 mm), which is 50% more, and every level in the table rises with it. Sunshine, wind and dry air raise ETo along with heat, so use the measured or forecast value for your area, or estimate it with the reference ET calculator.1
Duration decides the strategy. A heat wave of a week or two is well inside the 3 to 6 weeks most turf grasses survive dormant,2 so you can keep the lawn green at the optimum level or hold it at deficit and accept some thinning. A drought can last months, and that is when deficit levels, restrictions and dormancy matter.
Aim each watering at the root zone. UC says deep, infrequent watering that wets the top 6 to 8 inches of soil promotes healthy roots, while light sprinkling only encourages roots near the surface.6 Daily use and soil storage together set the days between waterings, which the irrigation interval calculator works out.
How to calculate drought watering depth and run times
The method adds one multiplier to the standard schedule calculation.
Formula
Weekly depth to apply = ETo (in/week) × Kc × level ÷ DU
Kc is 0.8 for cool-season and 0.6 for warm-season grass. Level is 1.0 for optimum, 0.75 (cool) or 0.66 (warm) for deficit, and 0.50 (cool) or 0.33 (warm) for survival. DU is distribution uniformity as a decimal; use 0.7 if untested. For minutes, divide the depth per watering by the precipitation rate in in/hr and multiply by 60.
- Get weekly ETo. Add up the daily values from a local weather network, or use the figure you estimated above.
- Apply Kc and the level. Multiply by 0.8 or 0.6 for optimum, then by the deficit or survival share. The result is the depth the roots need to receive.
- Subtract rain that soaked in. In a drought it is often zero. Light showers often count for little, and the effective rainfall calculator shows what share of a storm the lawn can use.
- Correct for uneven coverage. Divide by DU. A catch-can test measures it, and 0.7 is a fair planning value if you have not tested.
- Split across watering days. Divide the weekly depth by the number of days your schedule or city rule allows.
- Convert to minutes. Divide the depth per watering by your measured sprinkler precipitation rate, then multiply by 60. The how long to water lawn calculator does this for each zone.
Enter grass, soil, weather and what you can apply. The calculator shows how your watering compares with what the lawn uses.
Worked example: tall fescue in a heat wave with 2 watering days
Picture a 6,000 sq ft (557 m²) tall fescue lawn in a heat wave: ETo of 0.30 inch a day, no rain, a city limit of 2 watering days a week, rotors at 0.5 inch an hour, and an untested DU of 0.7. The goal is deficit irrigation.
- Weekly ETo0.30 × 7 = 2.1 in
- Optimum use (Kc 0.8)2.1 × 0.8 = 1.68 in
- Deficit target (75% of optimum)1.68 × 0.75 = 1.26 in
- Applied per week (DU 0.7)1.26 ÷ 0.7 = 1.8 in
- Applied per watering (2 days)1.8 ÷ 2 = 0.9 in
- Minutes per watering0.9 ÷ 0.5 × 60 = 108 min
- Cycles108 ÷ 3 = 36 min each
- Weekly volume6,000 × 1.8 × 0.623 ≈ 6,730 gal
2 waterings a week, 108 minutes each, run as three 36-minute cycles
Metric: 1.26 in is 32 mm reaching the roots, 1.8 in is 46 mm applied, about 23 mm per watering, or roughly 25,500 liters a week over 557 m².
Check the result against the lawn. Each watering delivers 0.9 × 0.7 = 0.63 inch to the roots, so the week delivers 1.26 inches against a heat-wave ETo of 2.1 inches. That is 60%, the UC deficit level for cool-season grass. If runoff begins before the first 36 minutes end, shorten the cycles and add more of them.
At survival level (50% of optimum) the same lawn needs 0.84 inch at the roots, which is 1.2 inches applied (0.84 ÷ 0.7) and two 72-minute waterings. That uses about 4,490 gallons (17,000 liters) a week instead of 6,730, at the cost of drastically reduced growth and quality.2
How to water when you only have one or two days a week
Many cities cap watering days in a drought, and UC's deficit and survival levels assume deep watering once or twice a week anyway.2 The weekly depth stays the same; only the split changes. The table applies the worked example's 1.8 inches (46 mm) a week to a cool-season lawn.
Run time per watering day for 1.8 inches applied a week
| Watering days per week | Depth per watering | Rotor, 0.5 in/hr (13 mm/hr) | Spray head, 1.5 in/hr (38 mm/hr) |
|---|---|---|---|
| 1 day | 1.8 in (46 mm) | 216 min | 72 min |
| 2 days | 0.9 in (23 mm) | 108 min | 36 min |
| 3 days | 0.6 in (15 mm) | 72 min | 24 min |
| 4 days | 0.45 in (11 mm) | 54 min | 18 min |
Cool-season lawn at the deficit level, heat-wave ETo of 0.30 in/day, DU 0.7, no rain. Minutes are the total per watering day and should be split into cycles whenever runoff starts. Run time = depth per watering ÷ precipitation rate × 60.
Long totals have to be broken up. A 216-minute rotor run puts 1.8 inches on the lawn at once, which usually exceeds what the root zone can store. Cycle and soak solves this: run each zone until just before runoff, let the water soak in, then repeat. UC recommends several short cycles to avoid runoff,2 and the cycle and soak calculator sets the cycle length. On very sandy soil a single weekly watering rarely fits; the available water capacity calculator shows how much your soil can store.
If your controller uses a percentage budget, set it from the optimum program: 75% for cool-season deficit, 66% for warm-season deficit, and 50% or 33% for survival. UC's instruction is to reduce the optimum watering time by those percentages.2 The seasonal adjustment calculator applies the same idea by month, and the lawn watering schedule calculator returns days and minutes together.
Municipal rules on days and hours override every schedule here. Large commercial turf can run short of controller time or pressure under tight windows, and a Certified Irrigation Designer should review that design.
What maintenance watering does a dormant lawn need?
Dormancy is the lawn's own drought plan. Shoot growth stops and leaves turn brown, but the crown, rhizomes and roots stay alive.4 UC says most turf grasses survive dormant for 3 to 6 weeks, depending on soil moisture and temperature,2 and Iowa State puts it at 4 to 6 weeks with little long-term damage.3 Past that point, universities advise a small maintenance watering that keeps the crowns alive without greening the lawn.
Maintenance watering for a dormant cool-season lawn, by university
| Source | Water per application | Interval | Weekly equivalent |
|---|---|---|---|
| UC Master Gardeners (California) | 0.5 in (13 mm) | Every 2 to 3 weeks | 0.17–0.25 in (4–6 mm) |
| SDSU Extension (South Dakota) | 0.25–0.5 in (6–13 mm) | Every 2 to 4 weeks | 0.06–0.25 in (2–6 mm) |
| Iowa State Extension | 1–1.5 in (25–38 mm) in one application, after 4 to 6 weeks without water | About every 4 weeks | 0.25–0.38 in (6–10 mm) |
Weekly equivalent = amount ÷ interval in weeks. All three keep crowns alive and none greens the lawn. The ranges differ because climates and soils differ; as a planning judgment, use the upper end on sandy soil, slopes and desert heat.234
The trade is appearance for water. Survival irrigation of a cool-season lawn in the heat-wave week costs 0.84 inch (21 mm) and keeps some growth, while dormancy maintenance costs 0.17 to 0.38 inch (4 to 10 mm) a week on the UC and Iowa State figures, roughly one-fifth to just under one-half as much, and the lawn turns brown.
Decide once, then stay with it
SDSU warns that breaking dormancy early drains the plant's reserves if dry weather returns.4 Do not water a brown lawn back to green until the weather has turned.
What else helps a lawn survive heat and drought?
Water is only part of the plan. Mowing height, fertilizer timing and sprinkler condition decide how much of it helps.
Do
- Mow at the top of your grass's range. Iowa State advises 3.5 to 4 inches (9 to 10 cm) for cool-season lawns,3 and UC lists tall fescue up to 3 inches.2
- Remove no more than one-third of the blade at each mowing, and sharpen the blade, since SDSU reports a dull one makes grass use 40% to 60% more water to recover.4
- Water between 5 a.m. and 9 a.m., with sprinklers aimed at the lawn only.4
- Hand-water the spots that wilt first instead of lengthening every run.2
Don't
Tip: fix the sprinklers before cutting the water
UC notes that limited brown areas under reduced irrigation often point to uneven coverage rather than too little water.5 A catch-can test finds the dry spots, and hand-watering them costs less than lengthening every run.
How do you bring a lawn back after a drought?
Let the weather lead. Shoot growth resumes and the lawn greens up once it cools down or rain starts to fall,4 and UC says a dormant lawn can usually be revived with regular, deep watering.2 Return to the optimum level, not above it, and wait a few weeks before judging patches.
Areas that do not recover can be reseeded in early fall. Iowa State also suggests a more drought-tolerant species or a smaller lawn for the next dry spell.3 For fertilizer, SDSU says to wait until early September, and UC's drought schedule for cool-season grass resumes nitrogen in September or early October.4
Frequently asked questions
Is it okay to water the lawn at midday during a heat wave?
It wastes the most water of any time of day. Iowa State notes that more water is lost to evaporation during the heat of the day, and SDSU puts the best window at 5 a.m. to 9 a.m. If a local rule forces another time slot, keep the weekly depth the same and follow the rule.
How long can a lawn go without water before it dies?
A cool-season lawn usually survives about 3 to 6 weeks dormant, depending on soil moisture and temperature, and Iowa State puts it at 4 to 6 weeks with little long-term damage. After that, universities advise 0.25 to 1.5 inches every 2 to 4 weeks. Warm-season grasses are more drought tolerant than cool-season grasses.
Is a brown lawn dead?
Not necessarily. In dormancy the leaves turn brown, but the crown, rhizomes and roots stay alive, and growth resumes when temperatures fall or rain returns. Patches that do not recover after several weeks of regular, deep watering in cooler weather may be dead. Iowa State suggests reseeding dead areas in early fall.
Should I water every day when the grass is wilting?
Usually not. UC says light sprinkling only encourages roots near the surface. Footprints that stay for several minutes, wilting blades and a blue-gray color mean the lawn needs one deep watering, not a daily sprinkle. Wet the top 6 to 8 inches of soil, then wait until the top 2 inches have dried out.
Should I fertilize my lawn during a drought?
No. Excess nitrogen speeds growth and raises water use. UC advises avoiding nitrogen on cool-season lawns from May through September, Iowa State says not to fertilize during dry periods, and SDSU suggests waiting until early September. Resume feeding once temperatures cool and the lawn is actively growing again.
Do warm-season lawns need less water than cool-season lawns in a drought?
Yes. UC sets optimum irrigation at 60% of ETo for warm-season grass against 80% for cool-season grass, which is 25% less. UC also reports that warm-season grasses need about 20% less water than tall fescue, and they tolerate drought better.
Sources
- Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. (1998). Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56. Food and Agriculture Organization of the United Nations.
- UC Master Gardener Program of Contra Costa County. Your Lawn During Drought. University of California Agriculture and Natural Resources.
- Steil, A. Watering the Garden & Lawn During Periods of Drought or Water Restrictions. Yard and Garden. Iowa State University Extension and Outreach.
- Edwards, L. and Schnabel, C. Lawn Care During a Drought. SDSU Extension, South Dakota State University. Updated June 22, 2023.
- UC Center for Landscape & Urban Horticulture. Questions & Answers About Drought & Water Conservation. University of California Agriculture and Natural Resources.
- UC Statewide IPM Program. How to water. The UC Guide to Healthy Lawns. University of California Agriculture and Natural Resources.
Written by Daniel Reeves, PE, CID. Figures in this article were calculated from the cited sources and checked against our lawn watering calculator. Read how we calculate and our editorial policy. Spotted an error? Tell us.