Solar radiation and Evapotranspiration Omer M. Ahmed MSc, 2 nd Year, 2017 University of Kerala, India.

INTRODUCTION Evaporation is a process by which water changed from the liquid or solid state into the gaseous state through the absorption of heat It is always related to the loss of water from a free surface over a fixed time interval. Either direct observation, or calculation based on the factors involved in the transfer of thermal energy. One of the fundamental component of hydrological cycle Essential requirements in the process are 1.The source of energy to vaporize the liquid water (solar or wind) 2.The presence of gradient of concentration between the evaporating surface and the surrounding air.

Solar Radiation

Solar energy is created at the core of the sun when hydrogen atoms are fused into helium by nuclear fusion. It is the primary source of energy for processes near the Earth's surface. The electromagnetic radiation emitted by the sun contains a broad range of wavelengths (spectrum). The Visible light is a small fraction of the wavelength spectrum.

Solar Radiation

Radiation flux The flux of radiation energy (J r ) received at the outer surface of the atmosphere can be calculated using the Stefan-Boltzmann law. The Stefan-Boltzmann law predicts the amount of electromagnetic energy emitted by an object in relation to its temperature and emissivity. £ is the emissivity which ranges from 0 to 1, with £=1 for a "black body", or perfect emitter, like the sun. is the Stefan- Boltzmann constant (5.6697x10 -8 W m -2 K -4 ), and T is temperature of the energy emitting body (K)

Three atmospheric processes modify the solar radiation passing through our atmosphere destined to the Earth's surface. These processes act on the radiation when it interacts with gases and suspended particles found in the atmosphere. SCATTERING: ABSORPTION: REFLECTION:

SCATTERING: The process of scattering occurs when small particles and gas molecules diffuse part of the incoming solar radiation in random directions without any alteration to the wavelength of the electromagnetic energy. Reduces the amount of radiation reaching the Earths surface.

REFLECTION: A process where sunlight is redirect by 180 degrees after it strikes an atmospheric particle. This redirection causes a 100 % “loss” of the solar radiation. Most of the reflection in the atmosphere occurs in clouds when light is intercepted by particles of liquid and frozen water.

ABSORPTION: Is a process in which solar radiation retained by a substance and converted into heat energy. The creation of heat energy also causes the substance to emit its own radiation. Only part of this emission reaches the earths surface.

Atmospheric interactions About 30% of the incoming solar radiation is reflected back to space, and another portion is absorbed and scattered by the atmosphere. The portion of solar energy that reaches the earth's surface is between 40% and 70% of the extraterrestrial radiation energy, depending primarily on cloud cover.

Radiation balance Radiation reaching the Earth's surface unmodified by any of the above atmospheric processes is termed direct solar radiation. Solar radiation that reaches the Earth's surface as long-wave radiation after it was altered by the process of scattering is called diffuse or sky radiation R sky The radiation flux balance on the earth's surface includes the total (short and long wave) incoming minus total outgoing radiation. The net radiation (R N ) absorbed by a surface is: R S  is the incoming short-wave radiation known as global short-wave radiation and which may include direct and scattered short-wave radiation. R S  is the reflected short-wave radiation. R L  is the diffuse long-wave radiation (R sky  ). R L  is the long-wave radiation reflected and emitted by the surface

Solar radiation is more intense nearer the equator, where rising air condenses and falls back onto the world’s rain forests. It is the driving force behind the hydrologic cycle, including transpiration by plants & evaporation from soil and from bodies of water. The amount and intensity of solar radiation that a location or body of water receives depends on a variety of factors. Such as latitude, season, time of day, cloud cover, aspect, shading, slope and altitude

WATER When water is exposed to excessive amounts of sunlight, the radiation will heat the water. The warmer a body water is, the faster the rate of evaporation will be. This can reduce water levels and water flow. In addition, warm water can not hold as much dissolved oxygen (as cold water) less dissolved oxygen available for aquatic organisms.

Too much infrared light can also cause the enzymes used in photosynthesis to denature, which can slow or halt the photosynthetic process. If radiation from the sun is lower than usual for an extended period of time, photosynthetic production can decrease or be stopped completely. Without sunlight, phytoplankton and plants will consume oxygen instead of producing it. These conditions can cause dissolved oxygen levels in the water to plummet, potentially causing a fish kill.

Evapotranspiration

Transpiration is the process by which water vapor leaves the living plant body and enters the atmosphere. Michel (1978). It is often difficult to separate transpiration from plants and evaporation from a water surface, they are combined together into a term called evapotranspiration. Basically an evaporation process. It is always related to the vegetation and often only that water which is spent on the building of plant tissues is considered as transpiration.

Evapotranspiration Evapotranspiration is the combination of two simultaneous processes: Evaporation and Transpiration (ET). ET sometimes also called water loss. Defined as a process by which water is evaporated from wet surface and transpired by plants. Water evaporated from the intercepted storage on the leaf surface is considered as a part of the evapotranspiration. Also the evaporation from part of the soil surface which is not covered by vegetation is a part of ET.

Unsaturated zone becomes saturated when the ET becomes negligible as compared with the precipitation. ET from deep rooted plants or in areas with a shallow water table may lower ground water levels, (during the summer). In the water balance to solve various regional groundwater problems, ET is the most difficult component to measure. The factor may be estimated based upon the vegetation coverage. If runoff is measured from a “representative” area, and the amount of water that passes below the root and capillary rise depths is also measured, the ET value can be calculated as (precipitation) - (runoff + true recharge).

TYPE OF EVAPOTRANSPIRATION There are two types of ET: POTENTIAL EVAPOTRANSPIRATION (PET) EFFECTIVE EVAPOTRANSPIRATION (EET)

o Theoretical amount. o The amount of moisture which, if available, would be removed from a given land area by evapotranspiration. o Expressed in units of water depth. POTENTIAL EVAPOTRANSPIRATION (PET)

Actual amount of water lost due to evapotranspiration from the soil along with actively growing plant or crop. The value depends upon plant and soil characteristics, and upon the amount of available water in the soil. EFFECTIVE EVAPOTRANSPIRATION (EET)

Estimation of Evapotranspiration Indirect method Open pan Evaporimeter Energy Balance Direct method Lysimetry method Eddy Covariance ESTIMATION OF EVAPOTRANSPIRATION Catchment Water Balance

Direct method

Lysimeter Lysimeter is adevice in which a volume of soil planted with vegetation is located in container to isolate it hydrologically from the surronding soil. Having a weighing device and a drainage system, which permit continuous measurement of excess water and draining below the root zone and plant water use, and hence evapotranspiration. The amount of water lost by ET can be worked out by calculating the difference between the weight before and after the precipitation input. ET = P + (I – D) + S P = Precipitation I = Irrigation D = Drainage S = Soil weight

Eddy Covariance Measure and calculate vertical turbulent fluxes Fast fluctuations of vertical wind speed are correlated with fast fluctuations in atmospheric water vapour density. The technique is mathematically complex, and requires significant care in setting up and processing data. Directly estimates the transfer of water vapour from the land surface to the atmosphere.

Indirect method

Using open pan Evaporimeter An evaporation pan is used to hold water during observations for the determination of the quantity of evaporation at a given location. Varying sizes and shapes, the most commonly used being circular or square The measurement begins with the pan filled to exactly two inches (5 cm) from the pan top and evaporation is measured daily. Evaporation cannot be measured when the pan's water surface is frozen and of limited use on days with rainfall events of >30mm

ETo = KC × E pan Where:  ETo : reference crop evapotranspiration  KC: crop coefficient  E pan: pan of evaporation

Catchment water balance or water budget method ET = P – Q – ΔS - ΔD Where: ET= evaporation and transpiration (mm) P = Precipitation (mm) Q = Stream flow (mm) ΔS= watershed storage variation (mm): S end –S beginning ΔD = Seepage out – seepage in (mm) Used to describe the flow of water in and out of a system. Water balance can also refer to the ways in which an organism maintains water in dry or hot conditions. It is often discussed in reference to plants or arthropods, which have a variety of water retention mechanisms

Energy balance methods Evaporation of water requires relatively large amounts of energy either in the form of sensible heat or radiant energy Therefore the evapotranspiration process is governed by energy exchange at the vegetation surface and is limited by the amount of energy available. The energy arriving at the surface must equal the energy leaving the surface for the same time period. Because of this limitation, it is possible to predict the evapotranspiration rate by applying the principle of energy conservation.

λET = Rn - G - H Where: λ : Latent heat of vaporization of water (Jkg-1) λET : Latent heat flux density (Wm-2) Rn : Net surface radiation flux density (Wm-2) G : Ground heat flux density (Wm-2) H : Sensible heat flux density (Wm-2)

Evapotranspiration from Satellite Data When a surface evaporates, it looses energy and cools itself. That cooling can be observed from space. Satellites can map the infrared heat radiated from Earth, thus enabling to distinguish the cool surfaces from the warm surfaces.

Various Formula for Calculation of Evapotranspiration 1.Thornthwaite equation 2.Hargreaves equation 3.Net radiation (R n ) based method

Thornthwaite equation e = 1.6(10t/I)a WHERE: e = un adjusted potential ET (cm/month) t = mean air temperature (celcius) I = annual or seasonal heat index a = an emperical exponent computed

Hargreaves equation ET = (Tm )(T max – T min )1/2.Ra Where: Tm – daily mean temperature Ra – extra-terrestrial radiation [MJ m -2 day -1 ].

Net radiation (R n ) based method ET = R n T mean Where: R n [MJ m -2 day -1 ] is net radiation.

Factors Affecting Evapotranspiration  Water availability - ET occur only if water is available.  Energy availability - The more energy, the greater the rate of ET.  Wind speed higher the wind speed, greater will be the rate of ET.  Humidity gradient - The rate and quantity of water vapour entering into the atmosphere both become higher in drier air.  Physical attributes of the vegetation - as vegetative cover, plant height and reflectivity surfaces, shape and area of the leaf.  Soil characteristics - include its heat capacity, and soil chemistry and albedo.

CONCLUSION It is the primary source of energy for processes near the earth's surface. Solar radiation is more intense nearer the equator, It affected by surface characteristics, such as slope, aspect, altitude and shading. Evapotranspiration is the combination of two simultaneous processes: Evaporation and Transpiration (ET) (water loss) Two types are there Potential (PET) and Effective (EET) It can be estimated either by directed or non direct methods

REFERENCES J. Balck (1989), groundwater resources assessment, ELSEVIER in co- edition with SNTL, 250P. C.W.Fetter (2014), applied hydrogeology, Dorling Kindersley India Pvt. Ltd, 4 th edition, 612P. (et)

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