Evaporation The two main factors influencing evaporation from an open water surface are : 1) The supply of energy to provide latent heat of vaporization. Solar radiation is the main source of heat energy 2) The ability to transport vapor away from evaporative surface. depends on wind velocity over the surface specific humidity gradient in the air above it

EVAPORATION MEASURMENTS

EVAPORATION MEASURMENTS

Evaporation estimation evaporation estimation is divided into two categories: evaporation from open water surface and evaporation from land. Relevant Basic Terms Flux: is flow rate divided by the area, i.e., flow rate in a unit area.

Evaporation estimation 2. Radiation emission: Radiation is continuously emitted from all bodies at rates linked with their surface temperature. Where: Re: is the emitted energy flux (W/m2) : is the emissivity of the surface σ : is the Stefan- Boltzmann constant ( 5.67x10-8 W/m2.K4 ) T: is the surface temperature in degrees Celsius

Evaporation estimation For a perfect radiator (i.e., black body), the emissivity is =1. Water’s = 0.98 sand =0.9 and soil =0.9 ~0.98. A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence.

Evaporation estimation EX.1: Estimate the radiation from a human body (skin area of 2m2, skin temperature of 33oC and emissivity =1).

Evaporation estimation 3. Net Radiation : When radiation strikes a surface, it is partially reflected and partially absorbed. The reflected fraction is called albedo Deep water absorbs most of the incident radiation with The net radiation flux Rn is the difference between the radiation absorbed and emitted is applicable to both shortwave and long wave radiations.

Evaporation estimation 4. Vapor pressure and relative humidity: Water vapour pressure e is the partial pressure contributed by water vapour. When the pressure is in equilibrium, it is called saturated vapour pressure es. The relative humidity is: Saturation vapour pressure is related to air temperature

Evaporation estimation is the gradient of the saturated vapour pressure curve at air temperature T.

Evaporation estimation

Evaporation estimation 5. Sensible Heat: Sensible heat: responsible for liquid water temperature change. Where: : is sensible heat (J/kg) : is the temperature change : is the specific heat

Evaporation estimation 6. Latent Heat: Used to vapourise liquid water into water vapour. It varies slightly with temperature (vaporisation under higher temperature needs less energy)

Evaporation estimation Evaporation from Open Water Surface: Evaporation from open water surface is influenced by two factors: energy input and vapour transport. Energy (mainly solar energy) provides the latent heat for the vapourisation and vapour transport helps to move the vapour away from the water surface.

Evaporation estimation Energy balance method The energy input (e.g., solar energy) is used to vapourise liquid water, warm up the water and warm up the underlying soil. If the vapour transport is sufficient (i.e., not a limiting factor), the evaporation rate is

Energy Balance Method Consider the evaporation from an evaporation pan An evaporation pan is a circular tank containing water. The rate of evaporation is measured by the rate of fall of water surface. A control surface is drawn around the pan enclosing both the water in the pan and the air above it. Evaporation Pan

Evaporation estimation EX.2: Estimate the evaporation rate (in mm/day) from an open water surface based on the energy balance method. The net radiation is 1000 W/m2 and air temperature is 200C. Assume no sensible heat or ground heat flux. The water density is 1000kg/m3.

Evaporation estimation Aerodynamic method In addition to the energy, vapour transport is also important. The transport rate is governed by the humidity gradient in the air near the surface and the wind speed across the surface. It is not straightforward to derive a general formula and many forms have been proposed depending on the different assumptions. A commonly used formula is

Evaporation estimation Evaporation and wind speed, pressure and deficit ?

Evaporation estimation In practice, a formula derived from Lake Hefner may be used to estimate evaporation from a lake

Evaporation from an open water surface Aerodynamic Method Evaporation from an open water surface

Combined method The energy balance method may be used when transport is not limiting and the aerodynamic method is used when energy supply is not limiting. In reality, both factors may be limiting and a combined method should be used.

Combined method

Evaporation estimation Evaporation from Open Land: On land, evapotranspiration is a combination of evaporation from the soil surface and transpiration from vegetation. In addition to energy and water transport, the availability of soil water is also important. When water availability is not a limiting factor, evapotranspiration reaches its full and is called------?

Evaporation estimation Evaporation from Open Land: The daily reference evapotranspiration recommended (based on the Penman- Monteith Equation) is

Evaporation estimation Evaporation from Open Land: The daily reference evapotranspiration recommended (based on the Penman- Monteith Equation) is

Evaporation estimation Evaporation from Open Land: Actual evapotranspiration depends on the vegetation type and availability of soil water. If soil water is not a limiting factor, actual evapotranspiration for a vegetation cover called crop is :

Evapotranspiration Evaporation from land surface comprised 1) evaporation directly from soil and vegetation surface 2) transpiration through leaves The processes of evaporation from land surface and transpiration from vegetation are collectively termed “Evapotranspiration”. Evapotranspiration is influenced by 1) the supply of energy to provide the latent heat of vaporization Energy Supply 2) the ability to transport the vapor away from the evaporative surface Vapor Transport 3) the supply of moisture at the evaporative surface Moisture

The term Evapotranspiration combines two words: 1) Evaporation of water from the soil 2) Transpiration of water from plants into the air Evapotranspiration means the total loss of water from a crop into the air. Water evaporates from any moist surface into the air unless the air is saturated. Water surfaces in contact with air, such as lakes, plant leaves, and moist soils, all evaporate water.

Crop water is important because it determines how much water must be provided by irrigation or rain. -If there is too little water, the crop yield can diminish. -If there is too much irrigation, then it will waste energy, water, and nutrients and unnecessarily deplete the aquifer.

Reference Crop ET Calculations of the rate of evaporatranspiration For given climatic conditions, the basic rate is the reference crop evapotranspiration, this being “the rate of evapotranspiration from an extensive surface of 8-15 cm tall green grass cover of uniform height, actively growing, completely shading the ground and not short of water” Reference Crop Evapotranspiration Equations: 1) Penman Equation 2) Penman-Monteith Equation 3) FAO-Penman Monteith Equation 4) Doorenbos-Pruitt Equation 5) Bladney-Criddle Equation 6) others Climatic conditions are given

Actual ET The potential evapotranspiration of another crop growing under the same conditions as the reference crop is calculated by multiplying the reference crop evapotranspiration, ETo by a crop coefficient, kc. The value of kc changes with the stage of growth of the crop. The actual evopotranspiration, Et is found by multiplying the potential evapotranspiration by a soil coefficient, ks (0ks1) Potential ET Actual ET The values of the crop coefficient, kc vary over a range of about 0.2kc1.3

ETo ETp ETc Reference Crop Evapotranspiration Potential Evapotranspiration ETc Actual Evapotranspiration

Penman-Monteith Formula Reference ET Reference Crop Weighting Lysimeter 1. Measure climatic conditions 2. Measure water use 3. Calibrate formula to calculate ETo Penman-Monteith Formula

Measure climatic conditions and calculate ETo. Developing Crop Coefficient Evapotranspiration (ET) is an energy driven process. ET increases with temperature, solar radiation and wind. ET decreases with increasing humidity. Measure climatic conditions and calculate ETo. Temperature Humidity Developing crop coefficients to determine actual crop ET

ETo Measurement Crop ET vs Reference ET Reference ET Crop ET Kc=ETc/ETo

Pan Method Previous evaporation methods are analytical methods. Although they provide better results, they involve parameters that are difficult to assess or expansive to obtain. Empirical equation can give approximate value of the correct order or magnitude. Pan measurements find general acceptance for practical applications.

Evaporation Measurement Class A Pan Evaporation Pan Method, E = Kp.Ep E = Evaporation Rate (mm/day) Kp = Pan Coefficient Ep = Measured Evaporation (mm/day)

Ex3. Compute the daily evaporation from a Class A pan if the amounts of water required to bring the level to the fixed point are as follows: If the pan coefficient is 0.70, what is the lake evaporation for the 5-day period for a lake with a 250 m2 surface area?

Ex4. A class A pan was set up adjacent to a lake Ex4. A class A pan was set up adjacent to a lake. The depth of water in the pan at the beginning of a certain week was 195 mm. in that week there was a rainfall of 45 mm and 15 mm of water was removed from the pan to keep the water level within the specified depth range. If the depth of the water in the pan at the end of the week was 190 mm, calculate the pan evaporation. If the pan coefficient is 0.70, what is the lake evaporation in that week with a 100 hectares surface area?

Catchment/reservoir water balance Evapotranspiration may be estimated by creating an equation of the water balance of a catchment. The equation balances the change in water stored within the basin (S) with precipitation P, surface runoff R, groundwater runoff G and storage change .

Methods to Reduce Evaporation Losses 1- Reduction of surface area since the volume of water lost by evaporation is directly proportional to the surface area of the water body, the reduction of surface area reduces the evaporation losses. Like having a deep reservoir in a place of wider ones.

Methods to Reduce Evaporation Losses 2- Mechanical Covers temporary roofs and floating roofs can be adopted. These measures are limited to very small water bodies such as ponds.

Methods to Reduce Evaporation Losses 3- Chemical Films this method consists of applying a thin chemical films on the water surface to reduce evaporation. Effect of heavy wind is the only factor affecting the efficiency of chemical films.

ET Examples 1. What weather variables are needed for calculating evaporation from open water surface with the combined method?

ET Examples 2. Using the Hefner equation, find the daily evaporation rate (in mm/day) for a lake of area 5 km2 given that the mean air temperature is 20oC and water surface temperature is 150C. The average wind speed is 15 km/h, and relative humidity is 20% (all the measures in air are at 2m height). If the same evaporation rate is maintained for a whole year, how much water is lost due to evaporation (m3)?

ET Examples 3. With the same lake in ex2, if the net radiation is 210 W/m2 and the lake is 1000m above sea level, estimate evaporation rate (in mm/day) using the combined method (assume water density is 1000kg/m3).

ET Examples 4. With the same weather conditions as ex 2 and 3 (replacing the lake with a land), estimate reference evapotranspiration using Penman- Monteith equation.

ET Examples 5.The total observed runoff volume during a storm of 6-hr duration with a uniform intensity of 15 mm/hr is 21.6 Mm3. If the area of the basin is 300 km2, find the average infiltration rate and the runoff coefficient for the basin.