Reviewed by: Stephanie Ben-Joseph

Net Radiation R_n (MJ/m²/day): Soil Heat Flux G (MJ/m²/day): Mean Air Temperature T (°C): Wind Speed u₂ (m/s at 2 m): Saturation Vapor Pressure e_s (kPa): Actual Vapor Pressure e_a (kPa): Calculate ET₀ Copy Result

Understanding Reference Evapotranspiration

Evapotranspiration, often abbreviated as ET, merges two vital processes by which water moves from the land surface to the atmosphere. Evaporation is the direct conversion of water to vapor from soils, open water, or wet plant surfaces, while transpiration describes the release of water vapor from plant leaves as part of photosynthesis and plant cooling. When agronomists talk about reference evapotranspiration (ET₀), they mean the rate at which a hypothetical, well-watered, short grass surface loses water under given weather conditions. This standardized definition allows researchers and farmers to compare water demand across regions and seasons without the confounding effects of specific crops or management practices. ET₀ acts as a benchmark: actual crop water use can be estimated by multiplying ET₀ by a crop coefficient that accounts for species and growth stage. Because water scarcity and irrigation efficiency are global concerns, tools that compute ET₀ have become indispensable in both high-tech agricultural operations and classroom exercises exploring the water cycle.

The FAO Penman‑Monteith equation is widely regarded as the gold standard for estimating ET₀ from routine meteorological observations. It combines energy-balance principles with aerodynamic theory, acknowledging that water loss depends not only on available energy but also on the capacity of air to transport vapor away from the surface. The equation expresses ET₀ in millimeters per day, equivalent to liters per square meter. A simplified form for a reference surface reads:

$$\frac{0.408\Delta (R_n-G_{})+\gamma \frac{900}{(}{u}_2(e_s-e_a)}{\Delta +\gamma (1+0.34u_2)}$$ In this equation, $\Delta$ is the slope of the saturation vapor pressure curve at temperature $T$ in kPa per degree Celsius, and $\gamma$ is the psychrometric constant with a value near 0.066 kPa/°C for air at sea level. The net radiation $R_n$ represents the balance of incoming and outgoing shortwave and longwave energy, while $G_{}$ captures ground heat flux, often negligible over daily periods. The aerodynamic term uses the wind speed $u_2$ measured at two meters above ground and the vapor pressure deficit $e_s-e_a$, which quantifies how thirsty the air is.

To implement the formula, this calculator first determines Δ from the input temperature using $e_s$ to derive the saturation pressure curve, then combines the energy and aerodynamic terms as shown. All calculations occur instantly in your browser. If you enter a net radiation of 15 MJ/m²/day, temperature of 25°C, wind speed of 2 m/s, and a vapor pressure deficit of about 1.07 kPa, the resulting ET₀ is roughly 4.8 mm/day. That means a well-watered reference lawn would lose nearly five liters per square meter that day. By experimenting with the inputs—perhaps raising wind speed or lowering humidity—you can see how ET₀ responds to changing weather.

Typical ET₀ Values for Different Climates

Because ET₀ depends heavily on weather, values vary by climate and season. The table below lists ballpark daily ET₀ ranges for several representative climates during peak growing months. These values derive from long-term meteorological records and serve as rough expectations rather than precise forecasts.

Climate Type	Typical ET0 (mm/day)	Notes
Humid temperate	3–6	Mild temperatures and moderate humidity keep ET modest.
Arid desert	6–10	High solar radiation and low humidity drive intense water loss.
Tropical wet	4–8	Abundant sun but frequent clouds and high humidity.
High altitude	2–5	Cooler temperatures offset strong solar radiation.
Coastal maritime	2–4	Persistent cloud cover and breezes reduce ET0.

These ranges help irrigators plan roughly how much water their crops might require. A lettuce farmer in a humid temperate region might expect about 4 mm/day, whereas a desert orchard manager could face demands over 8 mm/day. Such distinctions highlight why a one-size-fits-all irrigation schedule is ineffective. Local weather stations, on-farm sensors, or regional agricultural services often provide daily ET₀ updates to guide water application, but having a calculator allows students to explore the underlying physics.

Why ET₀ Matters

Reference evapotranspiration plays a central role in sustainable water management. In irrigated agriculture, applying water in line with ET₀ prevents both drought stress and wasteful overwatering. Water utilities use ET₀ forecasts to schedule deliveries, and conservation agencies rely on the metric to design drought restrictions. Beyond farming, ET₀ influences landscape ecology, hydrology, and climate modeling. Urban planners may evaluate how tree planting or green roofs alter ET₀ and thereby cool neighborhoods. Hydrologists subtract ET from precipitation to estimate groundwater recharge or streamflow generation. Even climate scientists utilize ET a datasets to validate land-surface models within global circulation simulations. Because the equation integrates radiation, temperature, humidity, and wind, it encapsulates the combined effect of many atmospheric variables, offering a window into how climate change might alter water demand.

Although powerful, the Penman‑Monteith approach carries assumptions. It presumes a uniform reference surface and does not account for soil moisture limitations, advection of hot dry air from surrounding regions, or heterogeneities in canopy structure. In reality, crops taller than grass experience different aerodynamic resistance, and bare soil may have lower albedo, absorbing more energy. Nevertheless, the ET₀ concept remains useful precisely because it provides a common baseline. By applying crop coefficients, practitioners tailor ET₀ to vineyards, orchards, turfgrass, or any vegetation of interest, while remembering that coefficients themselves vary with growth stage, management, and stress conditions.

The calculator's reliance on user-supplied data underscores the importance of accurate measurements. Net radiation ideally comes from pyranometer readings that capture both shortwave and longwave fluxes. Many stations approximate it from sunshine duration, temperature, and geographical factors using empirical formulas. Wind speed should be measured at two meters height over grass; readings taken near buildings or at different heights require adjustment. Vapor pressure data might derive from humidity sensors or dew point calculations. When high precision is necessary, such as in research or high-value agriculture, each parameter warrants careful calibration. For classroom exploration, however, reasonable estimates provide valuable insight into how weather drives water use.

In conclusion, this Reference Evapotranspiration Calculator offers an accessible gateway to a foundational concept in environmental science and agricultural engineering. By experimenting with parameters, readers can observe how sunny skies, windy afternoons, and dry air conspire to pull moisture from the land. The long explanation and accompanying table aim to demystify the Penman‑Monteith method, empowering students to connect equations with real-world water challenges. Whether planning irrigation, studying hydrology, or simply curious about how the atmosphere drinks from the earth, ET₀ is a lens through which the interplay of energy and moisture becomes clear.