Evaporation and Transpiration

Evaporation and Transpiration

• Discussions on evaporation require the understanding of different fields.
• Hydrology and meteorology are two related fields, with hydrometeorology bridging them.
• Hydrology focuses on water movement on/under the Earth's surface, excluding oceans.
• Meteorology emphasizes oceanography's role in evaporation and Earth's atmosphere.
• Oceans cover 71% of the Earth's surface and play a vital part in moisturizing earth's atmosphere.
• Evaporation data are often classified as unreliable due to multiple error sources.
• Unreliable data includes data availability/accessibility and measurement techniques used.
• Difficulties arise in measuring small amounts of evaporation during precipitation events.
• The techniques used for indirect measurement can result in human errors.
• Evaporation results from soil, snow, and water surfaces, while transpiration occurs through plants.
• Hydrologists study evaporation for reservoir design to anticipate and make decisions.
• Designing reservoirs enhances water supply stability, which is necessary in dry regions.
• Sustainability is important for projects in dry regions, highlighting the need for water storage and reservoir design.
• Meteorological factors, such as temperature, air pressure, humidity, and wind speed, affect evaporation rates.
• Natural evaporation occurs naturally; solar evaporation occurs with human intervention.
• Humans can control evaporation rates; for example, constant or double them by filling shallow lakes with seawater.
• Minimizing evaporation rates during water storage for stabilization is an important process.
• Evaporating surfaces include buildings, vegetation, and paved surfaces.
• Saturated soil has every pore space filled with water and is saturated to its limit.
• There is excess water in the evaporation surface when the rain won't stop.
• Waters called runoff are going either underground as groundwater or streaming water like rivers and lakes.
• Saturated soil, snow surface, and water surface have approximately the same evaporation rate.
• The drier the soil, the less opportunity for evaporation; beyond certain depths, evaporation ceases.
• The saturation of the air and the need to melt snow make winter evaporation rates lower.
• Water-budget is the balance between water inflow, outflow, and storage in a certain area, such as in a reservoir.

Water-Budget Equation

• E = (S1 - S2) + I + P - O - Oo is used to calculate evaporation
• E is evaporation.
• (S1 - S2) is changes in water storage.
• I, is surface inflow.
• P, is precipitation.
• O, is surface outflow.
• Oo, is subsurface seepage.
• Subsurface seepage is difficult to evaluate and is estimated from groundwater levels.
• If subsurface seepage exceeds evaporation, outcomes can become unreliable.
• Determining precipitation is typically straightforward, excluding areas where rugged terrain interferes with measurements.
• Snowfall is a special case where the water-budget is temporarily unreliable due to freezing.
• S1-S2 is changes in water storage, referring to the difference in water level over a specific period.
• Changes in water temperature can affect data due to molecular expansion/contraction.
• Lake Hefner in Oklahoma City meets water-budget requirements with a limited error percentage.
• Lake Hefner provides drinking water to the city while strategically countering rainfall and dry seasons.

Energy-Budget Determination

• Utilizes a continuity equation and solves for evaporation as a residual required to maintain energy.
• An approximate water budget is required, accounting for inflow, outflow, and storage as energy values relative to temperatures.
• The Lake Hefner experiment provided the first controlled test of the energy-budget.
• QsQrQb-Qh-Qe = Q。- Qv is used to determine the energy budget for a lake or reservoir
• Qs is short-wave sun/sky radiation at the water.
• Qr is reflected short-wave radiation.
• Qb is net energy lost by the water body through long-wave radiation exchange related to temperature.
• Qh is sensible-heat transfer (conduction) to the atmosphere.
• Qe is energy used for evaporation.
• Qo is the increase in energy stored in the water body.
• Qv is net energy advected into the water body; all are in calories per square centimeter.

Bowen's Equation

• Was conceived to eliminate sensible-heat transfer from the energy-budget equation.

Equation

• R = 0.61*(((Ts-Ta)/(esea))*P/1000)
• P is atmospheric pressure.
• Ta is air temperature.
• ea is vapor pressure of air.
• Ts is water-surface temperature.
• es is the saturation vapor pressure.
• All temperatures and pressures are in degrees centigrade and millibars.
• Energy advection and storage terms are calculated from a water budget & temperatures.

Mass-Transfer Determinations of Reservoir Evaporation

• Provides an alternative, focusing on water vapor movement into the air instead of energy flows.
• The Thornthwaite-Holzman equation assumes an adiabatic atmosphere and logarithmic distribution.

Equation

• E = (833k²(e1-e2)(V2 - V1))/(T + 459.4)loge(z2/z1)2
• E is evaporation.
• k is von Karman's constant (0.4).
• e is vapor pressure.
• v is wind speed.
• T is layer temperature in degrees Fahrenheit.

Meyer formula for evaporation rates

• E = c(es - ea)(1+V/10)
• Derived numerous empirical formulas express functions of atmospheric elements.
• es and ea are the vapor pressure of the water surface and over running air in. of Hg.
• V is wind speed (mph)
• c = 0.36 when the formula is applied to daily data for an ordinary lake, provided that the wind and humidity observations are about 25 ft above the surface.
• Numerous empirical lake-derived equations exist.

Estimation of Reservoir Evaporation from Pan Evaporation

• Pan evaporation has been used for an extended period of time and is undoubtedly the most widely used evaporation instrument today.

Types of Installations

• Includes Sunken Pans, Floating Pans, and Surface Pans.
• Buried Sunken Pans reduce side-wall radiation effects
• Sunken Pans are subject to trash collection, difficult maintenance, leak detection problems, and vegetation impact.
• Floating Pans directly measure evaporation similarly to the lake itself.
• Floating pans are subject to splashing issues and have high operational costs.
• Surface pans placed on the ground are mainly used for measurement and simple upkeep.
• Surface pans are subject to heat transfer with direct sunlight.

Types of Evaporation Pans

• BPI (Bureau of Plant Industry) Pans, are small and covered with a mesh, its results can be unstable.
• Young Pan is similar to the BPI pan with the same dimensions and purpose.
• Colorado Pan is larger than the BPI pan, therefore offering better measurements.

Standard Weather Bureau Class A pan

• This pan is of a specific size and made of unpainted galvanized iron.
• It is placed on a wooden frame, filled with water to specific depths, and measured using a hook gauge.
• Evaporation rate is determined as the difference between observed water levels.
• Pan evaporation and meteorological factors study weather condition influence.
• Missing data can be filled
• Areas can be unmonitored
• The accuracy & reliability should be kept in mind and validated
• Study differences between pan and actual lake evaporation by comparing

Estimating Evaporation

• Howard Penman developed an equation that eliminates the need for separate water temperature observations.
• It combines an empirical mass-transfer equation with an energy balance equation.

The Penman Equation for evaporation

• E = (1/(Δ + γ)) * (QnA + γEa)
• A equal the saturation, vapor pressure vs, and temperature curve at air temperature Ta.

Bowen ratio

• R = 0.61*((Ts-Ta)/(es - ea))*P/1000

Pan Coefficient

• Describes how much a the relationship between the evaporation measured in lake and the representative pan relates
• Using the pan coefficient, the lake evaporation can easily be determined.
• KP = ELake/EPan

Factors of Pan Coefficient

• One of the factors involves the different conditions between the representative pan and the lake.
• The sides of a pan are warmed by the warm air via advected energy, increasing evaporation.
• The lake will also evaporate rapidly due to the passing wind above its surface.
• The advected energy has factors to consider: Air speed (in miles per day) and Temperature of air (in Fahrenheit)

Finding Advected Energy

• Factors are utilized to determine the utilization of advected energy for evaporation.
• A formula described as: α(Qɛ - Qv) is calculated to determine net advected energy.
• The formula can be utilized with measured or computed theoretical evaporation.

Estimation Techniques

• Compared to other methods, using a pan station is relatively inexpensive and makes a practical choice.
• The pan evaporation method does not require extensive instrumentation or maintenance.
• Pan evaporation measurements can provide satisfactory estimates of annual reservoir evaporation.
• The method can be applied when representative pan data is available.
• The approach allows for adjustments using appropriate coefficients to enhance accuracy.

Increased Water Supplies Through Reduced Evaporation

• Reduced evaporation challenges the water supply, as it significantly impacts usable supply to sustain the region long-term.

Reduce Evaporation:

• Optimized Site Selection and Design - Choosing positions that reduce surface area relative to capacity by comparing deep reservoirs to shallow reservoirs
• Reservoir Covers - Covering reservoirs prevents evaporation with challenges in financial constraints to implement
• Windbreaks: A Limited Solution - Windbreaks found to have negligible results and impractical
• Selective Water Discharge - Releasing warmer surface water instead of cooler bottom water, reducing overall evaporation rates.
• Monomolecular Films - Films decrease evaporation by up to 40% when under certain weather conditions.

Transpiration

• Essential to the hydrologic rotation, is often mixed with evaporation, and is vital to water resource management.
• The rate is influenced by solar radiation, temperature, soil moisture, crop type, transpiration, and plant growth.
• It drives water from plants to the air
• There are different measurements

Lab Transpiration Measurements:

• Closed-Container Method: estimates water loss via moisture increase in sealed container with potted plants.
• Phytometer Method: determines water loss via weighing plant-soil system over time

Evapotranspiration

• ET is the transfer of water from different surfaces and plant discharges to the atmosphere.
• Water loss is a component of the hydrologic turnover.

Estimating:

• Empirical formulas (e.g., climate-based Penman-Monteith equation).
• Evaporation Pans
• Remote Sensing

Data Needs:

• Water use/loss with changes in basin
• Effect due to vegetation change.
• Irrigation amount

Water-Budget Method:

• Measures total water lost from a watershed due to evaporation and plant transpiration.
• Evaluates with: ET=P-Q-ΔS
• ET = evapotranspiration
• P = precipitation
• Q = streamflow (runoff)
• ΔS = change in storage (soil moisture, groundwater, or surface water)

Factors for Water Budget

• Evaluate with Groundwater level, precipitation all year, and long-term reports.
• It should be limited when artificially affected by drought.
• Short-term analysis evaluates with 4.60 rainfall and 2.37 runoff. Then evaluate for difference.
• This will estimate ET.

ET Determination in Field Plots:

• Measurements on soil surface in a field are to be made
• Energy budget requires computing heat stored in soil.
• Need knowledge on the Bowens ratio to test its temperature gradients above vegetation.
• The Thronthwaite-Holzman is a hopeful determination, the issue is limited instrumentation on both ends.
• Rainfall and streamflow is to then have analysis for data.

Determination of Potential Evapotranspiration

• Certain fields must provide always water for consistency on data.
• Measurement techniques give data to a accuracy in determining potentia and is used over a variety of tools.
• Includes Evapotranspirometers, Lysimeters, and Meteorological data to assist and better give data.
• Blaney's Approach assists data output with various climates.

Moisture Deficiency

• Soil is tracked for data/impact reports
• Rates are a complex relations
• Accurate reports will determine evaporation rates depending soil moisture.
• Natural Basins are to consider vegetation diversity
• ET is a decrease rate because of moisture and available
• There are types of vegetation at different stages
• The slope is reflected in ET data.
• Soil is noted for when ET decreases to the moisture content.
• Depression storage from free-water will give lower evaporation and water will run off quickly too