Evapotranspiration Quiz: Pan Evaporation vs. Potential ET

Pan evaporation is the direct measurement of water lost by evaporation from a standardized open container, most commonly the US Class A evaporation pan. The pan is filled with water and the daily drop in water level is recorded in millimeters. Pan evaporation is widely used as a simple empirical index of atmospheric evaporative demand because it integrates the effects of temperature, humidity, wind, and solar radiation.

Pan evaporation systematically overestimates evaporation from large water bodies. The small water volume in a pan heats more rapidly than a deep lake, producing higher water temperatures and greater evaporation. Metal pan walls also absorb solar radiation and heat the water. A pan coefficient, typically ranging from 0.6 to 0.8, is applied to convert pan evaporation to lake evaporation estimates accounting for these differences.

Potential evapotranspiration is the rate of water loss that would occur from a well-watered reference surface under prevailing atmospheric conditions if water supply were unlimited. It represents pure atmospheric demand, controlled by temperature, solar radiation, wind, and humidity. It differs from actual evapotranspiration, which is limited by available soil water and plant physiological controls including stomatal regulation.

In arid and semi-arid regions, low soil moisture and plant water stress constrain actual evapotranspiration well below the atmospheric demand. Even when the atmosphere could drive rapid evaporation, there is insufficient water available at the surface to sustain it. The gap between potential and actual evapotranspiration is therefore large, serving as a useful index of drought severity and irrigation water demand in these regions.

The Thornthwaite method, developed in the 1940s, estimates monthly potential evapotranspiration using mean monthly air temperature and the number of daylight hours as the only inputs. While simple and widely applied given its minimal data requirements, the method is criticized for poor performance in humid tropical climates and in regions where the relationship between temperature and energy availability is decoupled from the standard pattern.

Pan evaporation overestimates true lake or reference surface evaporation because the small pan volume heats rapidly and pan walls absorb heat. The pan coefficient corrects for these effects by multiplying pan readings by a value typically between 0.6 and 0.8 depending on pan type, surroundings, and local conditions. Applying the correct pan coefficient is essential before using pan data in water balance studies or irrigation planning.

Both pan evaporation and potential evapotranspiration are driven by the same atmospheric conditions. Solar radiation provides the latent heat needed for evaporation. Wind removes vapor from the surface maintaining the concentration gradient. Vapor pressure deficit reflects how far the air is from saturation. Groundwater depth affects actual evapotranspiration in riparian areas but is not a standard input in either pan measurement or standard PET estimation methods.

The complementary relationship proposes a feedback between land surface drying and atmospheric demand. As soil water depletes, actual evapotranspiration decreases and less water vapor enters the air. The atmosphere becomes warmer and drier, increasing potential evapotranspiration. The sum of actual and potential evapotranspiration is proposed to remain approximately constant, meaning their departures from a wet environment reference are equal and opposite, complementing each other.

Potential evapotranspiration represents the upper limit of water loss under unlimited water supply. Actual evapotranspiration can never exceed this limit because it describes what the atmosphere can maximally drive. When water is plentiful, actual approaches potential. Under water stress, actual falls below potential. The ratio of actual to potential evapotranspiration is therefore a useful indicator of soil moisture status and drought severity.

Pan evaporation measures pure atmospheric evaporative demand from an open water surface but does not account for crop-specific factors including stomatal resistance, canopy architecture, root depth, and phenological stage. Different crops have very different water use efficiencies. Crop coefficients developed through field experimentation must be combined with reference evapotranspiration estimates to convert atmospheric demand into actual crop water requirements for irrigation scheduling.

The Hargreaves-Samani equation estimates potential evapotranspiration using only daily maximum and minimum air temperatures and astronomically calculated extraterrestrial radiation. Because these inputs require minimal instrumentation, the method is widely applied in developing regions and historical analyses where only temperature records are available. Despite its simplicity, it performs reasonably well across diverse climates when calibrated against more comprehensive methods.

Pan evaporation has several recognized limitations. Small pan volume causes overheating relative to large water bodies. Wildlife interaction introduces random errors in water level measurements. Pan rim aerodynamics differ from conditions over open water, affecting evaporation. These issues collectively reduce the accuracy of pan measurements as proxies for natural evapotranspiration and require correction factors and careful siting for reliable estimates.

FAO-56 introduced a standardized reference evapotranspiration based on a hypothetical well-watered grass surface with a fixed height, surface resistance, and albedo. This standard eliminates the site-specific variability that plagued earlier pan-based methods and enables crop coefficients developed at one location to be reliably transferred to other climates. The Penman-Monteith equation applied to this reference surface became the globally accepted standard for irrigation water requirement calculations.

In humid climates with year-round rainfall, soil moisture is generally sufficient to meet atmospheric demand, allowing actual evapotranspiration to approach potential evapotranspiration closely. The ratio of actual to potential evapotranspiration nears one in well-watered humid regions. This contrasts sharply with arid regions where limited water supply keeps actual evapotranspiration far below atmospheric demand for most of the year.

The aridity index, defined by UNEP as the ratio of mean annual precipitation to mean annual potential evapotranspiration, classifies climates from hyperarid through humid. Values below 0.2 indicate hyperarid desert conditions while values above 0.65 indicate humid regions. The index captures the balance between water supply and atmospheric demand and is widely used to map dryland regions, assess desertification risk, and study climate change impacts on global water availability.