Scheduling & ET Calculators
Work out reference and crop evapotranspiration, turn that into a water requirement, and build a schedule that tells you when and how much to irrigate. This group also covers California MWELO landscape water budgets. Each calculator answers one question, shows its formula, and works in US or metric units.
Which scheduling or ET calculator do you need?
Find your question below. Each calculator does one job, so picking the right one gets you a precise answer.
| Your question | Use this calculator | How it works |
|---|---|---|
| Which ET method fits the weather data I actually have? | Evapotranspiration Calculator | Picks a method from full weather, temperatures only, or pan readings, and states which it used. |
| What's today's reference ETo from a full weather station? | Reference ET Calculator | FAO-56 Penman-Monteith from temperature, humidity, wind, radiation and location. |
| How much water does my crop use at its current growth stage? | Crop Evapotranspiration Calculator | ETc = Kc × ETo, using FAO-56 crop coefficients by stage. |
| I only have max and min temperature. Can I still get ETo? | Hargreaves ET Calculator | Hargreaves-Samani method from Tmax, Tmin, latitude and date. |
| I only have mean temperature and daylight hours. What's my ETo? | Blaney-Criddle Calculator | Mean temperature and daylight percentage, with the method's limits stated. |
| I have pan evaporation readings. How do I convert them to ETo? | Pan Evaporation to ET Calculator | ETo = Kp × Epan, with Kp from FAO-56 pan tables. |
| How much of the rain that fell can my crop actually use? | Effective Rainfall Calculator | USDA SCS monthly method, a fixed percentage, or dependable rain. |
| After rain and soil water, how much do I need to apply? | Irrigation Water Requirement Calculator | (ETc − effective rain ± soil water change) ÷ efficiency, plus leaching if needed. |
| Can you build me a full watering schedule? | Irrigation Scheduling Calculator | TAW and MAD set net depth; ETc sets interval; efficiency sets run time. |
| How many days can I go between irrigations? | Irrigation Interval Calculator | Readily available water ÷ daily crop ET, for one quick answer. |
| Can I track my root-zone water day by day? | Soil Water Balance Calculator | Daily or weekly ledger of moisture, ETc, rain and irrigation, flags MAD. |
| What efficiency should I use to convert net depth to gross? | Irrigation Efficiency Calculator | Sourced typical efficiencies by system type, or DUlq × management efficiency. |
| What's a reasonable water budget for my whole property? | Landscape Water Budget Calculator | Hydrozone area × plant factor × ETo, summed into a monthly budget. |
| What's the maximum water my California landscape is allowed by ordinance? | MAWA Calculator | California MWELO formula using ETo, ETAF, landscape area and special landscape area. |
| How much water will my landscape design actually use, and does it comply? | ETWU Calculator | Hydrozone plant factor, area and efficiency × ETo, compared with MAWA. |
| What landscape coefficient should I use for a mixed-plant hydrozone? | Landscape Coefficient Calculator | KL = Ks × Kd × Kmc, the WUCOLS/Costello method. |
All scheduling and ET calculators
Estimating ETo and ETc
- Evapotranspiration CalculatorPicks the right ET method for your weather data
- Reference ET CalculatorDaily ETo by FAO-56 Penman-Monteith from full weather data
- Crop Evapotranspiration CalculatorETc from ETo and a crop's growth-stage coefficient
- Hargreaves ET CalculatorETo from max and min temperature and latitude alone
- Blaney-Criddle CalculatorETo from mean temperature and daylight percentage
- Pan Evaporation to ET CalculatorConverts pan readings to ETo with FAO-56 pan coefficients
Water requirements and schedules
- Effective Rainfall CalculatorThe share of rainfall your crop can actually use
- Irrigation Water Requirement CalculatorNet and gross depth after rain, soil water and efficiency
- Irrigation Scheduling CalculatorFull watering schedule: depth, interval and run time
- Irrigation Interval CalculatorDays between irrigations from available water and crop ET
- Soil Water Balance CalculatorDaily root-zone ledger that flags when to irrigate
- Irrigation Efficiency CalculatorTypical system efficiencies and net-to-gross conversion
Landscape water budgets (California MWELO)
- Landscape Water Budget CalculatorMonthly water budget for a property's hydrozones
- MAWA CalculatorCalifornia MWELO maximum applied water allowance, in gallons
- ETWU CalculatorEstimated landscape water use, checked against MAWA
- Landscape Coefficient CalculatorKL for a hydrozone by the WUCOLS/Costello method
Scheduling and ET basics
What the standard references say, in brief. Each calculator page goes into detail and cites its sources.
Penman-Monteith is the standard ETo method
The Food and Agriculture Organization's Irrigation and Drainage Paper 56 (Allen et al., 1998) maintains the FAO Penman-Monteith method as the sole standard method for computing reference evapotranspiration (ETo) from weather data. Simpler methods, like Hargreaves or Blaney-Criddle, are fallbacks for when full weather data isn't available.
Crop coefficients change through the season
FAO-56's crop coefficient approach uses a different Kc for a crop's initial, development, mid-season and late-season stages, since a young, partly-covering crop uses far less water than a fully grown one at the same ETo.
About half the available water is a common depletion limit
FAO-56 lists typical management allowed depletion (MAD) values around 0.5, or roughly half of total available water, for many field and vegetable crops before yield is affected, with lower fractions for more sensitive crops and higher for drought-tolerant ones.
California fixes the ET adjustment factor by ordinance
California's Model Water Efficient Landscape Ordinance (MWELO) sets the evapotranspiration adjustment factor (ETAF) used in the Maximum Applied Water Allowance formula at 0.55 for residential landscapes and 0.45 for non-residential landscapes.
MWELO compliance also depends on your local jurisdiction's adopted ordinance, which can be stricter than the state model. For the data behind these numbers, see the crop coefficient table.
Related calculator categories
- Lawn calculatorsWeekly watering, schedules and grass-type water needs
- Sprinkler calculatorsPrecipitation rates, zones, spacing and audits
- Soil calculatorsAvailable water capacity, infiltration and root zone depth
- Agriculture calculatorsCrop water requirements, center pivots and field methods
- Cost and savings calculatorsWhat watering to your schedule costs, and what you'd save
- All calculatorsEvery irrigation calculator by category
Sources and review
This page and its calculators are reviewed by Daniel Reeves, PE, CID. Last reviewed . See our methodology and editorial policy, or report an error.
- Allen, R. G., Pereira, L. S., Raes, D., Smith, M. (1998). Crop Evapotranspiration – Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56.
- California Code of Regulations, Title 23, Chapter 2.7. Model Water Efficient Landscape Ordinance (MWELO). California Department of Water Resources.