Lecture 6 Evaporation
Evaporation is the process which changes liquid into gas.
Environmental lapse rate an actual decrease in temperature with an increase in altitude through the troposphere
Water Vapor when liquid water is evaporated to form water vapour, heat is absorbed. This helps to cool the surface
of the Earth. This ‘’latent heat of condensation’ is released again when the vapour condenses to form cloud water.
This source of heat helps drive the updraft in clouds and precipitation systems, which then causes even more water
vapour to condense into cloud, and more cloud water and ice from precipitation
Sensible heat energy required to change the temperature of a substance with no phase change
Latent Heat energy absorbed by or released from a substance during a phase change from a gas to a liquid or a
solid or visaversa
Conduction heat transfer by conduction occurs when energy is transferred by direct contact between molecules of a
single body ot among molecules of two or more bobies in physical contact with each other. Conduction takes place
from the area of the higher temperature to that of the lower temperature
Convection transfer of heat by the movement of a substance (gas or liquid) through a space. Examples of heat
transfer by convection include a current of warm air in a room and warm air rising from hot water
Process of Evaporation
1. Provision of sufficient energy at the evaporating surface for the latent heat of vaporization
2. The operation of diffusion processes in the air above the evaporating surface to provide a means for removing the
water vapour produced by evaporation
Factors affecting evaporation
Thermodynamic concerned with estimating the latent energy available for water to change in state from liquid to
gas. This approach involves two main steps:
● Determine the “available energy” at the evaporating surface
● Apportioning this energy into latent and sensible heat transfer
● Electromagnetic spectrum. Most infrared radiation, which carries heat energy, passes straight through the
lower atmosphere and heats the ground. In other words, the lower atmosphere is heated from below, not
● It is constantly in motion,. The lower 20 km of atmosphere (the troposphere) is fairly turpulent
● It is a very poor conductor of heat
Evaporation of water from a given surface is greatest in warm, dry conditions; and least in cold, humid conditions.
When air is warm, saturation vapour pressure (Es) of water is high; when air is dry, the actual vapour pressure (Ea) is
low; the saturation deficit (Es Ea) is large. There is an underlying relationship between the size of saturation deficit
and the rate of evaporation.
Bowen Ratio Bowen (1926) proposed that the ratio of the sensible and latent heat fluxes (H/λE). It is the type of heat
transfer in a water body. It is a mathematical method generally used to calculate heat lost (or gained) in a substance;
it is the ratio of energy fluxes from one state to another by sensible and latent heating respectively
Dalton’s Law: Evaporation from wet surface
E = f(u)(es – ea)
Where f(u) = wind speed, Es = saturation water vapour pressure at the surface and Ea actual vapour pressure.
Evaporation is proportional to the windspeed and the vapour pressure deficit
Laminar motion: air movement in a straight line or along smooth, regular curves in one direction; occurs on the
lowes few millimeters of the atmosphere. Turbulent motion: friction between the air and the ground surface induces eddies and air allow’s irregular, tortuous, fluctual paths .
● The depth and strength of turbulance are largely dependent upon the roughness of the ground surface and
the strength of the wind.
● The greater the intensity of turbulence , the most effectively are water vapour molecules dispersed or
diffused upwards into the atmosphere.
● The turbulent mixing in the boundary layer acts to homogenize the air, since it transfers atmospheric
properties (such as heat and water vapour) from places of high concentration to those of low concentration
so acts to equalize them at all heights.
Evaporation from different surfaces
Open water it is the simplest situation, with little limitation on evaporation. The rate of evaporation over the open
water can depend on following conditions:
● Size of water surface
● Water Depth
Vegetation when precipitation falls on vegetation cover, a proportion of rain is intercepted by the leaves and stems
of the vegetation canopy and temporarily stored on its surface, it is called Interception.
Interception precipitation → canopy interception → stemflow/throughfall→ infiltration
The magnitude of interception and storage is determined by type and growth stage of the vegetation
Ec = Pg (Th+St), where
Ec = canopy interception, Pg = total precipitation, Th throughfall; St = Stemflow
Heat budget the earth’s climate is a solar powered system. Globally, over the course of the year, the Earth system
land usrfaces, oceans, and stmosphere absorbs an average of about 240 Watts of solar power per square meter (1W
● The sun does not heat the Earth evenly
● The atmoshpete and ocean work nonstop to even out solar heating imbalances through evaporation of
convection, rainfall, winds and ocean circulation. This coupled with atmosphere ond ocean circulation is known as
earth’s heat engine
● The heat engine must not only redistribute solar heat from the equator toward the poles, but also from the
Earth’s surface and lower atmosphere back to space.
● Earth’s temperature does not infinitely rise because the surface and the atmosphere are simultaneously
radiating heat to space. This net flow of energy into and out of the Earth system is Earth’s energy budget.
● The energy that Earth receives from sunlight is balanced by equal amount of energy radiating into space.
The energy escapes in the form of thermal infrared radiation
● When a flow of incoming solar energy is balanced by an equal flow of heat to space, Earth is in radiative
equilibrium, and global temperature is relatively stable.
● Anything that increases or decreases the amount of incoming or outgoing energy disturbs Earth’s radiative
equilibrium; global temperatures rise or fall in response
Earth’s Heat Distribution
● Energy from sunlight is not spread evenly over Earth. One hemisphere is always dark, receiving no solar
radiation at all.
● On the daylight side, only the point directly under the Sun receives fullintensity solar radiation. ● From the equator to the poles, the Sun’s rays meet Earth at smaller and smaller angles, and the light gets
spread over larger and larger surface areas (red lines)
Estimation of Evaporation
● Evaporation Pan
Twofold distinction of evaporation
● Potential evaporation is the maximum evaporation rate (mm/day) when the moisture content of the soil and
vegetation conditions do not limit evaporation
● Actual evaporation is the evaporation rate (mm/day) under existing atmospheric, soil and vegetation
● The actual evaporation is therefore always less than or equal to the Potential evaporation
Advantages of Water Balance Estimates
● Such models have been used to estimate the water balance for the following purposes:
○ develop climate classifications
○ estimate soilmoisture storage and runoff and irrigation demand
○ evaluate the hydrologic effects of climate change
Water Balance components
● Precipitation (P) Precipitation in the form of rain, snow, sleet, hail, etc. makes up the primary supply of
water to the surface. In some very dry locations, water can be supplied by dew and fog.
● Evaporation phase change from a liquid to a gas releasing water from a wet surface into the air above.
● Transpiration phase change when water is released into the air by plants
● Evapotranspiration combined transfer of water into the air by evaporation and transpiration.
● Potential Evapotranspiration (PET) the amount of water that would be evaporated under an optimal set
● Soil Moisture Storage (ST) the amount of water held in the soil at any particular time. The amount of water
in the soil depends soil properties like soil texture and organic matter content. The maximum amount of water
the soil can hold is called the field capacity.
○ Fine grain soils have larger field capacities than coarse grain (sandy) soils. Thus, more water is
available for actual evapotranspiration from fine soils than coarse soils. The upper limit of soil
moisture storage is the field capacity, the lower limit is 0 when the soil has dried out.
Water Balance Calculation (Lab Assignments)
Lecture 7 Watershed Hydrology
Drainage basin/watershed/catchment area an area that drains all precipitation received as either runoff or base flow
into a particular river or set of rivers. The boundary of a drainage basin is defined as the ridge beyond which water
flows in the opposite direction.
● Watershed is most commonly used unit for hydrological modelling
● Each drainage basin acts as an individual hydrological system, receiving quantifiable inputs of precipitation
which are transformed into flows and storages and into outputs of evaporation and runoff
● Also it is a natural geomorphological until for the operation of fluvial and some fluvioglacial processes
● In both the hydrological and geomorphological sense, drainage basins are dynamic rather than static entity.
Impacts on watershed
● Population growth
○ Demand for more fresh water: do we have enough?
○ Demand for infrastructure: Municipal, agricultural, industrial, recreational, transportation
● Precipitation Changes
○ Rainfall has increased in the mid and high latitudes but decreased in subtropics and tropics ● Sea level Rise
○ Global sea levels have risen between 1025 cm largely due to thermal expansion of the oceans and
to lesser extend due to the melting of glaciers
● Extreme Events
○ Extreme weather events, such as prolonged heat waves and periods of drought are intensifying
○ A recent study has linked the earth’s warming to an expected future increase in the intensity of
● Slowing of Thermohaline Circulation
○ This ocean's “conveyor belt” , which originates in the Gulf of Mexico, brings warm water and
weather to northwest Europe
○ With glaciers and ice caps melting, flows of water to North Atlantic ocean have increased, slowing
the thermohaline circulation by 30% from 1957 to 2004. Disruption to this ocean current could result
in the cooling of northwestern Europe by several degrees
● Transboundary problems, including atmospheric and aquatic pollution and the sharing of water resources
● need for better institutional frameworks to support the assessment and implementation of appropriate water
● Better flow monitoring networks to support international action in foresting “thought preparedness centres”
● Dryland degradation, which remains a key interdisciplinary challenge across many countries
Lecture 10 Properties of Water
Unusual characteristics of water
● Colourless, tasteless and odourless
● Under normal climatic conditions, it is commonly found in all three phases: solid, liquid and gas
● Changes in phase absorb or release more latent heat than most other common substances
● High specific heat capacity
○ acts as a cushion against extreme temperatures
● Universal solvent because almost all substances are soluble to some extent in water
● Covalent bond:
○ each molecule of water share two electrons between the oxygen and two hydrogen atoms
○ the hydrogen (positive) atoms are basically on one side of the molecule, whereas the oxygen
(negative) atom is on the other side creating an electrostatic charge
■ electrostatic charge effectively creates a very strong bond between adjacent water
molecules resulting in high latent and specific heat values
■ helps to create water cohesion qualities and surface tension which enables it to move
through materials like soil via capillary action
● Generally weaker atomic bonding in most inorganic (noncarbon based) substances allow them to be soluble
● The solubility of water is the key characteristic allowing it to be a medium by which necessary chemicals and
nutrients are transported in plants and animals
● This also allows pollutants and toxic substances to move through the environment
● Water quality can be thought of as a measure of the suitability of water for a particular use based on
selected physical, chemical and bio characteristics
● Water is never absolutely pure. Each component of water cycle changes water quality. Natural and human
activities the quality of water
● TO determine the quality, scientists first measure and analyze characteristics of the water:
○ mineral content ○ number of bacteria
● Those characteristics are then compared to standards and guidelines
Hydrology plays an important role in determining solute compositions and concentrations
Water is generally in movement and its quality is variable, determined by the speed at which it is moving
Water quality is also impacted by other variables including flushing frequency and interstorm period
Measuring water quality
● Water Sampling
● Measure basic water parameters
○ Oxygen content
○ Chemical content
To identify the substances in a stream or lake, scientists collect samples of the water, living organisms a