4.2: Water Use Processes

Understanding how plants use water and evaluating the effect of weather on water use require consideration of fundamental processes. Plants extract water from the soil and transport water to the leaves. The stomata, very small openings, located on the upper and lower surfaces of the leaves, allow for the intake of carbon dioxide required for photosynthesis and plant growth. Water vapor is lost from the plant leaves by evaporation in the stomatal cavity and the flow of water vapor from the stomata into the atmosphere. This is transpiration. Transpiration is necessary to cool the plant and maintain productivity. Converting liquid water to vapor (i.e., evaporation) requires a large amount of energy. If plants did not transpire, the incoming solar energy would heat the plant, perhaps to lethal temperatures. When plants are stressed from lack of water the stomata close, restricting the flow of water and carbon dioxide. When plants are stressed transpiration decreases, but so does photosynthesis. For this reason, crop yield and seasonal transpiration are closely related. Water at the soil surface and on plant leaves or mulch evaporates when solar radiation or hot, dry winds supply energy. Initially, evaporation from a wet soil surface progresses at a maximum energy limiting rate (Figure 4.3). As evaporation continues the soil surface begins to dry and water below the soil surface moves upward replacing soil water lost by evaporation. As the soil dries the resistance to water flow increases. Eventually the rate of water flow in the soil limits evaporation rather than the amount of energy available to evaporate water. This is called the soil limiting phase of evaporation. During the soil limiting phase the rate of evaporation is less than during the energy limiting phase (Figure 4.3). During the soil limiting phase, energy that could have been used to evaporate water is available to heat the soil and air near the soil surface. The heating is most pronounced when there is no crop or when plants are small. If the process persists for a long period, the soil and air become quite hot as in desert climates. Evaporation and transpiration are difficult to measure or predict separately, because water vapor moves from different surfaces into a dynamic environment that varies with time. Measuring devices can alter the local climate around plants and change the actual rate of evaporation or transpiration. Therefore, evaporation and transpiration are usually combined and called evapotranspiration (ET).

Figure 4.3. Example of the stages of evaporation from a bare soil.

Evaporation may be a large component of ET of annual crops early in the season when crops are small, but later in the season, transpiration becomes dominant (Figure 4.4). Evaporation generally constitutes 20 to 30% of the total ET for the crop growing season for irrigated corn in the Great Plains. Transpiration and evaporation from soil, plant leaves, and mulch are evaporative processes. Considerable energy is required to evaporate water. The energy absorbed by plants on a sunny and windy summer day would evaporate enough water to cover the soil surface to a depth of approximately 0.4 inches. For an area of one acre, this would equal about 11,000 gal/d.

Figure 4.4. Example of seasonal patterns of evaporation, transpiration, and ET for irrigated corn in the Great Plains.

The energy available for ET comes from several sources (Figure 4.5). Much of the energy comes from extraterrestrial radiation emitted by the sun. Some extraterrestrial radiation is absorbed or reflected in the atmosphere. The radiant energy that ultimately reaches the crop canopy is called solar radiation. Plant and soil surfaces reflect some solar radiation back into the atmosphere. The portion of the solar radiation absorbed varies depending on the color of the surface and other soil and plant properties. The fraction of the solar radiation reflected to the atmosphere is called the albedo. The albedo for plant and soil surfaces ranges from 35% for snow covered soils to 10% for dark soils that are wet. A commonly used value for the albedo of actively growing crops is 23%. In this case, 77% of the solar radiation is absorbed and used for ET and photosynthesis.

Figure 4.5. Diagram of energy sources for evapotranspiration.

Long-wave radiation is the second component of the radiation balance. Energy is transferred due to the temperature difference between objects. In both cropped and uncropped landscapes, the exchange is between the plant and soil surfaces to the atmosphere. Because the atmosphere is cold relative to the surface of the earth, long-wave energy is lost from the plantsoil system. Radiant energy available for ET is called net radiation equal to the absorbed solar radiation minus the emitted long-wave radiation. Advection is the lateral or horizontal transfer of mass, heat, or other property. Hot, dry winds supply energy for ET due to advection. The amount of energy transferred depends on the wind speed and the vapor pressure of the air. According to Dalton’s law of partial pressure, the pressure exerted by a mixture of gases is equal to the sum of the pressures exerted by each gas if it alone occupied the space. Moist air obeys Dalton’s law. The portion of the barometric pressure due to water vapor is independent from the other gases. The partial pressure due to water vapor is the vapor pressure of the air. At an air water interface, water molecules continually flow from the water into the air and from the air back into the liquid. If the air is dry, more molecules leave the liquid than enter the liquid resulting in evaporation. If air in a sealed container is left in contact with water long enough, the rate of molecules leaving and entering the liquid surface reach an equilibrium. When equilibrium exists with pure water, the air is saturated with water vapor. The pressure exerted by vapor at this equilibrium condition is the saturation vapor pressure. The saturation vapor pressure depends on the air temperature. The ratio of the actual vapor pressure to the saturation vapor pressure when expressed as a percentage is the relative humidity. Air in the soil and the stomatal cavity of plants is often near saturation; thus, it has a high vapor pressure. If air surrounding the plant and soil is at the same temperature, but much drier, the vapor pressure will be lower. Water vapor moves from locations of high vapor pressure toward locations with low vapor pressure. If the air around the crop were contained in a chamber, it would become saturated with water vapor and ET would then be negligible because the air could not hold any additional water. If the saturated air were replaced with dry air, ET would resume. The more rapidly the air is exchanged and the drier the air, the higher the ET rate. In windy-arid locations, advection may contribute as much to ET as radiation. However, in humid locations or in areas with little wind, advection may be quite low. Two other energy sources for ET are the exchange of heat between plants and the soil (called soil heat flux), or between plants and the surrounding air. For example, if the soil is warmer than plants, energy is transferred from the soil to the plants. This energy may increase transpiration. Conversely, if the canopy is warmer than the soil, energy flows toward the soil and transpiration may decrease. The same type of energy transfer occurs between plants and air. Plants that are not stressed for water are generally cooler than the ambient air during the middle of the day. However, if stressed for water, plants will often be warmer than the ambient air (USDA-SCS, 1993). Two additional factors impact ET. First, there must be a source of water in the soil to supply that used by plants. Second, water must move from the soil to the point where evaporation occurs or into and through the plant to the stomatal cavity where transpiration occurs. If the soil is dry, there is more resistance to water transport in the soil. Also, as plants are stressed, the stomata begin to close and the resistance to water flow from the plant increases. Therefore, ET can be limited by either the amount of evaporative energy or amount of water in the soil.