evaporation and transpiration Flashcards
is a natural process by which a liquid change into a gas or vapor. This process plays a significant role in various fields such as environmental science, medicine, and engineering.
Evaporation
FACTORS CONTROLLING THE EVAPORATION PROCESS
- Temperature
- Humidity
- Wind Speed
- Surface Area
- Solar Radiation
- Air Pressure
- Type of Liquid
- Presence of Solutes
Higher temperatures generally lead to increased evaporation rates. As temperature rises, the kinetic energy of water molecules also increases, causing more of them to escape the liquid phase and enter the vapor phase.
- Temperature
refers to the amount of water vapor present in the air. When the air is already saturated with water vapor (high humidity), the evaporation rate slows down because there is less room for additional water molecules to enter the air.
- Humidity
Wind accelerates evaporation by carrying away the water vapor that accumulates near the evaporating surface. This process helps maintain a lower humidity level, allowing more water molecules to evaporate from the surface.
- Wind Speed
Larger surface areas provide more space for water molecules to escape from the liquid to the vapor phase. In natural systems, bodies of water with larger surface areas tend to have higher evaporation rates.
- Surface Area
Sunlight provides the energy required for evaporation to occur. Regions with more intense and prolonged sunlight experience higher evaporation rates.
- Solar Radiation
Lower air pressure at higher altitudes can increase evaporation rates. This is because the lower air pressure allows water to boil at lower temperatures, leading to more rapid evaporation.
- Air Pressure
The physical properties of the liquid itself, such as viscosity and vapor pressure, can influence evaporation rates. Liquids with higher vapor pressures tend to evaporate more quickly.
- Type of Liquid
Adding solutes to water can affect the evaporation rate. In some cases, solutes may lower the rate of evaporation (e.g., dissolved salts in seawater).
- Presence of Solutes
MASS TRANSFER EVAPORATION OF RESERVOIR EVAPORATION
Mass Transfer Method
Evaporation
Wind Speed
Vapor Pressure
Saturation Vapor Pressure:
It is the method for estimating the actual evaporation from a body of water, assuming it is proportional to the product of wind velocity, the difference between the saturation vapor pressure at water surface temperature and the vapor pressure of the ambient air, and an empirical mass-transfer coefficient.
Mass Transfer Method
it is the process by which water is changed from the liquid state to its gaseous equivalent.
Evaporation
Also known as wind flow velocity, is a fundamental atmospheric quantity caused by air moving from high to low pressure, due to changes in temperature.
Wind Speed
It is the pressure of a vapor in contact with its solid or liquid form.
Vapor Pressure
The pressure vapor at which the gaseous phase of a substance can coexists with the liquid or solid phase in equilibrium at a given temperature.
Saturation Vapor Pressure:
These are pans containing water which are exposed to the atmosphere. Loss of water by evaporation from these pans are measured at regular intervals (daily). Meteorological data such as humidity, wind velocity, air and water temperatures, and precipitation are also measured and noted along with evaporation.
Evaporimeter
Types of Evaporimeter.
- USWB Class-A pan evaporimeter
- ISI standard pan evaporimeter
- Colorado Sunken pan
- USGS Floating Pan
Is the most widely used evaporimeter in the world for finding evaporation from the free water surface. It consists of a 121.5 cm diameter and 25.4 cm deep pan made of 20-gauge galvanized iron sheet with a stilling well. A vertical pointer is provided in the stilling well to show the level of water maintained in the pan. The pan is painted white and is placed on a wooden frame so that air may circulate beneath the pan.
- USWB Class-A pan evaporimeter
Is widely used for measuring evaporation rates in India. It is a cylindrical pan made of galvanized iron and has a diameter of 122 cm and a depth of 25 cm. The pan is painted white to reflect solar radiation and has a water capacity of approximately 50 liters.
- ISI standard pan evaporimeter
A type of evaporation pan that is about 1 m (3 ft) square and 0.5 m (18 in.) deep. This pan is sunk into the ground to within about 5 cm (2 in.) of its rim, and the water is maintained at about ground level. It is made of unpainted galvanized iron. The pan coefficient, on an annual basis, is about 0.8.
- Colorado Sunken Pan
Is a square pan of 90cm sides and 45cm deep. Supported by drum floats in the middle of a raft size 4.25 x 4.87 m, it is set afloat in a lake with a view to simulate the characteristic of a large body of water
- USGS Floating Pan
is a measure of the evaporative efficiency of the pan. It is defined as the ratio of the actual evaporation rate from the pan to the theoretical evaporation rate from a free water surface under the same climatic conditions. The pan coefficient depends on various factors such as wind speed, air temperature, humidity, and solar radiation.
PAN COEFFICIENT
are complete systems used to measure the amount of water lost each day to evaporation.
EVAPORATION STATIONS
WMO recommends the following values of minimum density of evaporimeters
• Arid Zones – 1 station for every 30,000 sq.km
• Humid Temperate Zones - 1 station for every 50,000 sq.km
• Cold regions - 1 station for every 100,000 sq.km
A typical hydro-meteorological station has the following:
• Recording rain gauge and non-recording rain gauge
• Stevenson box with maximum, minimum, wet, and dry bulb thermometers
• Wind anemometer and wind vane
• Pan evaporimeter
• Sunshine Recorder
Is a semi-empirical equation combining mass transfer (Ea) and energy balance (er) methods. The formula was developed by Penman in 1948 and is still widely used for calculating the potential evaporation using synoptic meteorological data.
PENMAN EQUATION
SUMMARY AND APPRAISAL OF TECHNIQUE FOR ESTIMATING RESEVOIR EVAPORATION.
WATER BUDGET METHOD
MASS TRANSFER MODELS
PAN EVAPORATION METHOD
estimates reservoir evaporation by considering the change in water levels, inflows, outflows, and precipitation. This method can be practical for long-term estimates and does not rely solely on evaporation data. However, it may not be accurate for short-term assessments, especially if other water balance components are not accurately measured.
WATER BUDGET METHOD
use principles of heat and mass transfer to estimate evaporation rates from reservoir surfaces. These models can be more accurate than empirical methods, especially when combined with detailed meteorological data.
MASS TRANSFER MODELS
involves measuring the rate of water evaporation from a standard evaporation pan, usually placed near the reservoir. This method is simple and cost-effective, making it widely used. It provides an estimation of potential evaporation and needs a pan coefficient to convert it to reservoir evaporation, introducing some level of uncertainty.
PAN EVAPORATION METHOD
Reducing evaporation from water bodies can indeed lead to an increase in available water supplies. Evaporation is a natural process by which water is converted from liquid to vapor and lost to the atmosphere.
INCREASED WATER SUPPLIES THROUGH REDUCED EVAPORATION.
Strategies to reduce evaporation and enhance water supplies.
- Floating Covers
- Windbreaks and Vegetation
- Water Conservation Practices
- Increased Water Recycling and Reuse
- Water Harvesting and Rainwater Collection
- Liners and Sealing
These covers are designed to float on the surface of water bodies, such as reservoirs, ponds, lagoons, or industrial tanks. They are typically made from durable and impermeable materials like geomembranes or geosynthetic materials.
- Floating Covers
Planting windbreaks or vegetation around water bodies can create a barrier against wind, reducing air movement over the water surface and lowering evaporation rates.
- Windbreaks and Vegetation
Promoting water conservation practices, such as using more efficient irrigation methods, can reduce overall water demand, leading to less water withdrawal from water bodies.
- Water Conservation Practices
Expanding water recycling and reuse practices can reduce the need for freshwater withdrawals from natural sources, consequently reducing evaporation losses.
- Increased Water Recycling and Reuse
Harvesting rainwater and using it for various purposes can reduce the dependency on open water bodies.
- Water Harvesting and Rainwater Collection
For some water bodies, using liners or sealing materials to reduce seepage can help retain water and minimize evaporation
- Liners and Sealing
is the process through which plants lose water in the form of vapor from their leaves and stems. It plays a crucial role in plant physiology, as it is responsible for water movement from the roots to the aerial parts of the plant, maintaining cell turgidity, nutrient uptake, and cooling of leaves. Various factors influence the rate of transpiration in plants, and understanding these factors is essential for optimizing plant growth, water use efficiency, and agricultural practices.
Transpiration
FACTORS AFFECTING TRANSPIRATION
a. Environmental Factors
1. Temperature
2. Humidity
3. Wind Speed
4. Light Intensity
b. Plant Factors
1. Leaf Structure
2. Stomatal Density
3. Stomatal Conductance
c. Plant Physiology
1. Water Availability
2. Plant Size and Growth Stage
d. Soil Factors
1. Soil Moisture
2. Soil Composition
Higher temperatures generally increase transpiration rates due to increased evaporation and higher vapor pressure deficit. Hotter environments result in greater water loss from the plant.
- Temperature
High humidity reduces the vapor pressure deficit, leading to slower transpiration rates. Conversely, low humidity enhances transpiration as it increases the gradient between the leaf and the surrounding air.
- Humidity
Increased air movement results in enhanced evaporation of water from the leaf surface, leading to higher transpiration rates.
- Wind Speed
Higher light intensity stimulates stomatal opening and increases transpiration rates. This is because light is required for photosynthesis, which drives the opening of stomata.
- Light Intensity
Leaves with a large surface area and a thin cuticle facilitate transpiration. Additionally, the presence of stomata on leaf surfaces allows for gas exchange and transpiration.
- Leaf Structure
The density and size of stomata on leaf surfaces affect the rate of transpiration. Higher stomatal density generally results in higher transpiration rates, while smaller stomata may reduce transpiration.
- Stomatal Conductance
refers to the ease with which water vapor diffuses through stomata. Factors such as stomatal opening/closure and the presence of stomatal guard cells influence stomatal conductance and consequently affect transpiration rates.
- Stomatal Conductance
The availability of water in the soil affects plant water potential, which influences the rate of transpiration. Water-stressed plants exhibit reduced transpiration rates as a survival mechanism.
- Water Availability
Larger plants with more extensive leaf surfaces tend to have higher transpiration rates. Additionally, the growth stage of a plant can affect transpiration, with younger plants often having higher rates due to their higher metabolic activity.
- Plant Size and Growth Stage
Adequate soil moisture is necessary for the absorption and transport of water to the plant. Water deficit in the soil can lead to reduced transpiration rates as plants regulate water loss to conserve resources.
- Soil Moisture
The composition of soil, including factors such as texture and organic matter content, affects water-holding capacity. Soils with higher organic matter content generally retain more moisture, potentially reducing transpiration rates.
- Soil Composition
MEASURING OF TRANSPIRATION
- POTOMETER METHOD
- GRAVIMETRIC METHOD
- LYSIMETER METHOD
- HEAT BALANCE METHOD
- SAP FLOW MEASUREMENT
The potometer is a device that measures the rate of water uptake by a plant. It consists of a glass tube connected to the plant stem through a rubber tubing.
- POTOMETER METHOD
This method involves measuring the change in weight of a potted plant over a specific period. The plant is weighed at the beginning and end of the experiment, and the difference in weight represents the amount of water lost through transpiration.
- GRAVIMETRIC METHOD
A lysimeter is a device used to measure the amount of water that passes through a specific area of soil. It consists of a container with a plant or soil sample placed on top, and a collecting system to measure the water that drains from the lysimeter.
- LYSIMETER METHOD
This method involves measuring the heat exchange between the plant and the surrounding environment. By measuring the temperature difference between the plant and a reference surface, along with other environmental parameters such as air temperature and wind speed, transpiration can be calculated using heat and energy balance equations.
- HEAT BALANCE METHOD
This technique directly measures the movement of sap in the xylem vessels of the plant. It involves inserting probes or sensors into the plant stem to monitor the rate of sap flow. By measuring the flow rate, transpiration can be estimated.
- SAP FLOW MEASUREMENT
Involves the transformation of water from its liquid state to its gaseous state.
Evaporation
Involves the loss of water in the form of vapor from the aerial parts of plants like leaves
Transpiration
Is the combined process of water surface evaporation, soil moisture evaporation, and plant transpiration.
Evapotranspiration (ET):
Often use to describe a total water loss in the soil due to evapotranspiration .
Evaporation Rate
4 FACTORS USED TO DETERMINE EVAPOTRANSPIRATION
- Temperature
- Solar Radiation
- Humidity
- Wind Speed
Also known as Water Balance Equation, Continuity equation or conservation equation. It is the balance of the input and output of water within the given area taking into account net changes in storage.
Water Budget equation
Chang in flow Storage
Mass inflow – Mass outflow
Change in Storage
Volume inflow – Volume outflow (if the density of inflow and outflow is the same)
main source of inflow
precipitation
main source of outflow
surface runoff, evapotranspiration, interception, etc.
Is a concept often linked to mass balance equation for water. This is especially true for water and ET, because evaporation impact the energy and water balance simultaneously.
Energy Balance
is the ratio of sensible to latent surface energy flux.
Bowen Ratio
The Thornthwaite equation is a method developed by Thornthwaite (1948) based on an empirical approach in order to estimate potential evapotranspiration. Evapotranspiration is originally destinated to be computed on a monthly basis, but daily estimations are possible as well.
Thornthwaite Method
METHODS FOR MEASURING POTENTIAL EVAPOTRANSPIRATION
- Lysimeter
- Field Experiment
- Soil Moisture Depletion Study
It involves the growing of crops in large containers (lysimeters) installed in cropped fields and measuring their water loss and gains. The soil and crop conditions in these lysimeters should be as close to the surrounding field conditions as possible. The measurements involve weighing of the lysimeter. It may be done with scales or by floating the lysimeters in water or a suitable heavy liquid in which case the change in liquid displacement is computed against water loss from the lysimeter.
- Lysimeter
Measurements of water supplied to the field and changes in soil moisture contents of field plots are sometimes more dependable for computing seasonal water requirement of crops than measurements with small tanks or lysimeters not free from limitations.
- Field Experiment
The soil moisture depletion method is usually employed to determine the consumptive use of irrigative field crops grown on the fairly uniform soil when the depth to the groundwater is such that it will not influence the soil moisture fluctuation within the root zone.
- Soil Moisture Depletion Study
ESTIMATING POTENTIAL EVAPOTRANSPIRATION FROM METEOROLOGICAL DATA
Thornthwaite
Blaney
Lowry and Johnson
Has derived a somewhat involved procedure using only temperature and duration of possible sunshine.
Thornthwaite
involve the same two factors but was designed primarily to transpose observed consumptive use data for irrigated areas to other localities on the basis of derived coefficient.
Blaney
Found high correlation between consumptive use and accumulated degree days during the growing season.
Lowry and Johnson
MOISTURE DEFICIENCY ACCOUNTING
1.) There are numerous types of vegetation at varying stages of development and having roots extending to various depths.
2.) Differences in slope and aspect are reflected in evapotranspiration rates so that, all other things being equal, the wilting point should be reached sooner in some areas than others.
3.) There are areas of bare soil where evaporation decreases with decreasing moisture content. also, evaporation from other surfaces (buildings, streets, etc.) decreases with time subsequent to rainfall.
4.) Depression storage is a source of free-water evaporation. the area of such water surfaces decreases as evaporation and infiltration proceed subsequent to rainfall.
1.) There are numerous types of vegetation at varying stages of development and having roots extending to various depths.
TRUE
2.) Differences in slope and aspect are reflected in evapotranspiration rates so that, all other things being equal, the wilting point should be reached sooner in some areas than others.
TRUE
3.) There are areas of bare soil where evaporation decreases with decreasing moisture content. also, evaporation from other surfaces (buildings, streets, etc.) decreases with time subsequent to rainfall.
TRUE
4.) Depression storage is a source of free-water evaporation. the area of such water surfaces decreases as evaporation and infiltration proceed subsequent to rainfall.
TRUE
