The Basics of Evapotranspiration

Evapotranspiration (ET) represents the loss of water from the Earth’s surface from the combination of direct evaporation and plant transpiration. ET is usually expressed as a rate such as inches per day. Knowledge of ET is important for irrigation scheduling but it is also an important factor for other land use applications such as septic tank drain fields, watershed level budgeting, and climate and weather models. ET can be used as a historical tool but usually it is predicted or used in a forecast to help irrigators optimize irrigation.

Factors Affecting ET

Current weather conditions, plant type, soil conditions (soil chemistry and soil salinity), and geographical location are the primary factors affecting ET rates.

Weather

Conditions include wind speed, air temperature, relative humidity, and sunlight. The effect weather has on ET rate is rather intuitive. Warm, sunny, and dry weather with a lot of wind will have greater ET rates.

Plant Type

The ET rates between different plant species can vary greatly. For example, needle leaf trees such as pine trees will have a much greater ET than a deciduous tree such as an oak tree. Even though the pine needle can be very small, the needles have much more surface area than deciduous leaves allowing for more transpiration.

Soil Chemistry

The chemical makeup of soil is also a major factor affecting ET rates. At the molecular level, most clays will have a chemical affinity (attraction) for water. Clay’s chemical affinity for water results from a planar geometry and the charge distribution at the molecular level. Chemical affinity for water is much less for sands and silts. The chemical interaction between the clay and the water impedes both the evaporation and the plant transpiration. The evaporation rate of a soil that is mostly sand on the other hand will be close to what the evaporation rate would be out of a pan. For example, if the soil moisture is 20% by volume, the ET rate in sand would be very high and very low in clay under the same weather conditions. On the other hand, if the soil moisture is 30% by volume, the sandy soil will dry out quicker than the clay rich soil and the clay rich soil will have a higher ET rate over a longer period of time.

Soil Salinity

Another factor that affects the plant transpiration is the salt content of soil. Plant transpiration is the movement of water from the roots and the subsequent loss of water vapor from the stomata in the leaves. The primary driving force of the movement of water from the roots to the leaves is osmosis. Osmosis is the diffusion or movement of water that is driven by a salt concentration gradient. Increasing soil salinity will decrease plant transpiration because the salt concentration gradient will diminish thus affecting the overall ET.

Geographical Location

In general ET rates increase toward the equator and will be higher in summer than in winter. In areas where ET is greater than precipitation, there will be very little recharge to the aquifer and a net upward movement of water. This net upward movement of water followed by ET will cause salts and minerals to accumulate near the surface. This explains why soils in arid lands tend to have a high pH and a higher salinity. Elevation, longitude, latitude and time of year (seasonality) also have an effect on ET.

Conversely, if precipitation is greater than ET, minerals and nutrients will be leached out of the soil, and there will be a shallow aquifer. Highly leached soils will develop more clay loam textured soils and will typically have a lower base saturation.

The simplest method for approximating ET is the pan evaporation method. Measuring the rate of evaporation of water in a pan provides a quick and reasonable approximation of ET with little or no cost. While convenient, the pan evaporation method does not take into account the contribution of plant transpiration, and it makes the assumption that water evaporates out of the soil at the same rate it would out of a pan.

Evaporation Pan

Developed in the late 1940s, the Penman-Monteith method provides a correlation between ET and the energy the earth’s surface receives. One of the problems with the previous ET models such as the pan evaporation method does not take into account the variability of ET from one kind of crop to the next. The Penman-Monteith method became a widely accepted ET method because a reference ET could be obtained from weather data. A reference ET is the calculated ET for a standard grass of a standard height. The ET for a specific crop could then be calculated by simply multiplying the reference ET by a crop coefficient.

With the development of better and more reliable weather instruments, farm agencies could publish reference ET values along with the crop coefficients so the ET could be determined for the entire crop in a particular region. Despite the advances in weather measurements in the 1950s, the Penman-Monteith method had a few drawbacks. The reference ET calculation was mathematically tedious and few computers of the time had the computational power to calculate it and few mathematicians were up to the challenge of computing it with pencil and paper.

In the 1980s the Priestly-Taylor method was introduced as an alternative to Penman-Monteith method. Based on the same heat budget principles as the Penman-Monteith Method, the Priestly-Taylar method relies on sev- eral assumptions and simplifies the calculation of reference ET.

However, with advances in computers in the 1990s the Priestly-Taylor method was short lived and the Penman- Monteith Method became the standard method for calculating ET by the scientific community and organizations such as the American Society of Civil Engineers and the Food and Agriculture Organization of the United Nations.

While much emphasis has been placed on calculating ET from weather calculation models such and the Penman- Monteith method, a previously overlooked method for calculating ET using Darcy’s Law and soil hydrology has been slow to emerge.

ET rates can also be measured with soil moisture sensors by looking at the water balance in the soil. For example, the amount of water recharging the water table can be measured with a deeply placed soil moisture sensor while changes in soil moisture can be monitored with soil sensors throughout the profile. By examining changes in soil moisture through the soil profile over a period of time, an ET rate can be calculated.

Evapotranspiration Measurement Networks

An ET station typically consists of weather sensors, a data logger, a solar panel to provide electric power and radio telemetry. The radio telemetry communicates the weather data in real time back to a computer where a computer program calculates the reference ET. The typical sensors included on an ET station are an anemometer for wind speed and direction, a relative humidity sensor, air temperature sensor, pyranometer for solar radiation measurements, a barometer, and rain gauge.

Federal agencies such and the US Bureau of Reclamation, The US Department of Agriculture National Resources Conservation Service (NRCS) and state soil water conservation districts provide daily ET rates along with crop coefficients for local crops for network of ET stations. One of the most comprehensive ET networks in the western US is the AgriMET network provided by the US Bureau of Reclamation. In the Midwestern US, many states provide ET rates from State funded Mesonets.

ET and Irrigation Management

Perhaps the most important application for ET rates is irrigation scheduling. The weekly irrigation schedule can be calculated from forecasted ET rates, sprinkler efficiencies, root zone depth, and the available water capacity of the soil. With ET irrigation scheduling, an irrigator will know the approximate amount of water the crop will need for any given week. While ET based irrigation scheduling has shown to save water and to optimize crop yields, ET based irrigation scheduling is based on many assumptions to give an approximation of the actual water requirements of the crops. Micro climates and highly varying soil conditions introduce a lot of error in weather station based ET calculation which in turn will increase error in the scheduling. Because of this uncertainty, crop advisors and irrigation scheduling calculators will manipulate factors in the ET calculation so that the schedule will over irrigate by 10 to 20%. In other words, if there is a 10 to 20% error in the ET based irrigation schedule calculation, adding 10 to 20% more water will insure that the crop will not be stressed.

The ultimate goal for an irrigation schedule is to keep the soil moisture at a level that is best for the crops. ET based calculations basically estimate what the soil moisture would be based on weather data.

Basically, with almost any crop, the soil moisture needs to be maintained above permanent wilting point and in general stay below field capacity. Permanent wilting point is a condition where the soil moisture is at low level where a plant can’t uptake any water. Field capacity is the amount of water that can be held in the soil before gravity will begin to drain the soil. Deep infiltration and aquifer recharge will occur if the soil moisture stays above field capacity.

In precision agriculture, the most accurate irrigation schedules use regional ET forecasts in combination with soil moisture sensors to ensure that the soil moisture values stay at a level that is best for the crop health and yields. The ET forecasts help the irrigator create a weekly irrigation schedule, while soil moisture data takes into account microclimates and soil conditions and fine tune the weekly schedule on a daily basis.

With the growing demands on agricultural products and the growing importance to protect and restore natural aquatic habitats, water conservation is more important than ever. Proper irrigation management is key to ensuring healthy, high quality crops while protecting valuable water resources. Knowledge of ET is critical for environmental and economical best irrigation management practices.
In addition to ET models to help manage crops, products such as the Stevens Hydra Probe or SAM (Stevens Agricultural Monitoring) system can further help define exactly how much water is needed and when it should be applied. Combining actual soil moisture measurements with a local ET forecast can give the user an incredibly detailed look at actual soil moisture information, leading to better managed crops with increased yield and reduced waste.