midterm Flashcards
is the transfer of water from the liquid to the vapor state
Evaporation
is the process by which plants remove water from the soil and release it into the air as vapors.
Transpiration
is the combined processes of losing water to the atmosphere from water surface, soil surface, and from plants.
Evapotranspiration
Factors affecting the rate of Evaporation (6)
- Vapor pressure at the water surface and above.
- Air and Water Temperatures
- Wind Speed
- Atmospheric Pressure
- Quality of Water
- Size of Water Bodies
The rate of evaporation is proportional to the difference between the saturation vapor pressure at the water temperature,
ew and the actual vapor pressure in the air, ea,
thus,
EL = c (ew – ea)
- Vapor pressure at the water surface and above.
with all factors remaining the same, the rate of evaporation increases with an increase in the water temperature.
Air and Water Temperatures
With aids in removing the evaporated water vapor from the zone of evaporation and
consequently creates greater scope of evaporation. The rate of evaporation increases with the wind speed up to critical speed.
Wind Speed
With all factors remaining the same, a decrease in barometric pressure increases evaporation.
Atmospheric Pressure
When a solute is dissolved in water, the vapor pressure of the solution is lesser than that of
pure water and hence causes reduction on the rate of evaporation
Quality of Water
Deep water bodies have more heat storage than shallow ones. A deep lake may store
radiation energy in summer and release it in winter causing less evaporation in summer and
more evaporation in winter.
Size of Water Bodies
________ is very important in many hydrologic problems associated with planning and operation of reservoirs and irrigation systems. It is also important in areas where water is scarce. The exact measurement of evaporation is one of the most difficult tasks.
Determination of Evaporation
Determination of Evaporation
It can be estimated using the
following methods: (3)
- Using evaporimeter data
- Empirical evaporation equations
- Analytical methods
_________ are water-containing pans which are exposed to the atmosphere and the loss of water by
evaporation is measured in them at regular intervals. Factors affecting the rate of evaporation are also
noted such as humidity, wind movement, air and water temperatures and precipitation.
Evaporimeters
Types of Evaporimeters: (4)
- Class A Evaporation Pan
- ISI Standard Pan
- Colorado Sunken Pan
- US Geological Survey Floating Pan
This consists of 1210 mm width and a depth of 255 mm. It is used by the U.S. Weather Bureau and is known as Class A Land Pan.
The depth of water is maintained below 18 cm and 20 cm. The pan is normally made of unpainted galvanized iron sheet. Monel metal is used where corrosion is problem. The pan is supported by wooden platform of 15 cm height above the ground to allow
free circulation of air below the pan.
Evaporation measurements are made by measuring the
depths of water with a hook gauge in a stilling well.
- Class A Evaporation Pan
This is known as modified Class A Pan. It consists of a pan 1220 mm in diameter with 255 mm of
depth. The pan is made of copper sheet of 0.90 mm thickness, tinned inside and painted white outside. A fixed-point gauge indicates the level of water. A calibrated cylindrical measure is used
to add or remove water, maintaining the water level in the pan to a fixed mark. The top of the pan is covered fully with a hexagonal wire netting on a galvanized iron to protect the water in the
pan from birds and to make the temperature more uniform during the day and night. The evaporation of this pan is found to be less by 14% compared to unscreened pan.
ISI Standard Pan
This is a 920 mm square and 460 mm deep made up of unpainted galvanized iron sheet and buried into the ground within 100 mm at the top. The main advantage of this type is that radiation and aerodynamic characteristics are similar to a lake.
However, it has the following disadvantages:
a. Difficult to detect leaks
b. Extra care is needed to keep the surrounding area free from all grass, dust, etc.
c. Expensive to install
Colorado Sunken Pan
A square pan with 900 mm side and 450 mm depth supported by drum floats in the middle of a raft is set afloat on a lake to simulate the characteristics of a large body of water. The water level
is kept constant leaving a rim of 75 mm. Diagonal baffles provided in the pan reduce the surging
in the pan due to wave action.
US Geological Survey Floating Pan
Evaporation pans are not exact models of large reservoirs and have the following drawbacks:
- They differ in the heat-storing capacity and heat transfer from the sides and bottom.
- The height of the rim in an evaporation pan affects the wind action over the surface.
- The heat-transfer characteristics of the pan material are different from that of the reservoir.
Empirical Evaporation Equations
- Meyer’s Formula
- Rohwer’s Formula
Analytical Method of Estimating Evaporation (3)
- Water-Budget Method
- Energy-Balance Method
- Mass-Transfer Method
Methods for Reduction of Evaporation Losses
- Reduction of Surface Area
- Mechanical Cover
- Chemical Films
The volume of water lost by evaporation is directly proportional to the surface area of the water
body, this reduction of surface area can reduce evaporation loss. Using deep reservoirs in place of wider ones can meet this method of reducing loss of water.
- Reduction of Surface Area
Permanent, temporary of floating roofs can be used to cover reservoirs, but this is only feasible for smaller water bodies.
Mechanical Cover
this method consists of applying a thin chemical film on the water surface to reduce evaporation.
This is feasible for reservoirs of considerable size.
Chemical Films
is the process by which water leaves the body of a living plant and reaches the atmosphere as vapor. The water is taken up by the plant-root system and escape through the leaves.
Transpiration
Factors Affecting Transpiration:
- Atmospheric vapor pressure
- Temperature
- Wind
- Light intensity
- Characteristics of plants
the resulting evapotranspiration when sufficient moisture is always
available to completely meet the needs of vegetation covering the area. These depends essentially on the
climactic factors.
Potential Evapotranspiration (PET)
the real evapotranspiration occurring in a specific situation
Actual Evapotranspiration (AET)
denotes loss by evapotranspiration.
Consumptive Use
the maximum quantity of water that the soil can retain against the force of gravity.
Field Capacity
the moisture content of a soil at which the moisture is no longer available in sufficient quantity to sustain the plants.
Permanent Wilting Point
the difference between the field capacity and the permanent wilting point.
Available Water
Measurement of Evapotranspiration
- Lysimeter
- Field Plot
A special water tank containing a block of soil and set in a field of growing plants. The plants grown
in the ________ are the same as in the surrounding field. Evapotranspiration is estimated in terms of the amount of water required to maintain constant moisture conditions within the tank
measured wither volumetrically through an arrangement made in the lysimeter. _________ should be designed to accurately reproduce the soil condition, moisture content, type and size of
the vegetable of the surrounding area. They should be buried so that the soil is the same level inside and outside the container.
- Lysimeter
In a special plot all the elements of the water budget in a known interval of time are measured
and the evapotranspiration is determined as:
Evapotranspiration = precipitation + irrigation input – runoff – increase in soil storage – groundwater loss
Field Plot
refers to the process of reducing the water volume for runoff. This consists of the interception
process and the depression storage. This abstraction represents the quantity of storage that must be
satisfied first before run-off begins.
Initial loss
is the volume of water caught by the vegetation and the structures and then subsequently evaporated.
Interception
This is the volume of water that fills up all the depressions when precipitation reaches the ground before overland flow occurs. This is lost through the process of filtration and evaporation.
depression Storage
This is the movement of water through the soil surface which plays an important role in the run-off process
by affecting the timing, distribution, and magnitude of the surface run-off. It is also the primary step in the natural groundwater recharge.
Infiltration
is the maximum rate at which the ground can absorb water.
Infiltration Capacity
is a device used to measure the rate of infiltration. It consists of a cylinder driven into the
ground and a buffer cylinder driven into the ground concentric with the cylinder. The cylinders are filled
with water up to the same level and the change in water levels is observed for a time duration. The
purpose of the buffer cylinder is to ensure that the flow of water is truly vertical representing the
infiltration
Infiltrometer
Factors affecting infiltration:
- Type of Soil
- Properties of Soil
- Structure of Soil
- Size of soil particle
- Texture of soil
- Condition of surface
means the draining or flooding off of precipitation from a catchment area through a surface
channel.
Run-off
is the excess precipitation moving over the land surfaces to reach smaller channels.
Overland Flow
a flow travelling all the time over the surface as overland flow and through the channels
as open channel flow and reaching the catchment area.
Surface runoff
is a part of the precipitation that infiltrates and moves laterally through upper crusts of the soil
and returns to the surface at some location away from the point of entry into the soil. This is sometimes
called as **through flow, storm seepage, subsurface flow, or quick return flow. **
________ is classified as:
1. Prompt interflow – an interflow with the least lag
2. Delayed flow
Interflow
is a part of run-off that percolates deeper into the soil and
reaching groundwater storage.
Groundwater flow or Groundwater runoff
Classification of Runoff:
- Direct Runoff
- Baseflow
The part of runoff that enters the stream immediately after the precipitation. It includes surface
runoff, prompt interflow, and precipitation on the channel surface. This is sometimes called as
direct storm runoff or storm runoff.
- Direct Runoff
The delayed flow that reaches the stream essentially as groundwater flow including delayed
interflow.
Baseflow
the entire area of a river basin whose surface runoff (due to a storm) drains into the river in
the basin. Also known as drainage basin, or catchment area.
Watershed
Parts of Watershed
- Drainage Divide
- Concentration point or Measuring point
- Concentration time
the boundary line, along a topographic ridge, separating the two adjacent watersheds.
Drainage Divide
the single point or location at which all surface drainage
from a watershed comes together or concentrates as outflow of the basin in the stream channel.
Concentration point or Measuring point
the time required for the rain falling at the most distant point in a watershed to reach the concentration point.
Concentration time
Physical Characteristics of a Watershed
- The number of streams
- the length of streams
- stream density – which is expressed as the number of streams per square kilometer area of the watershed.
- drainage density – which is expressed as the total length of all stream channels per unit area of the watershed.
Classification of Streams
- Influent Streams and Effluent Streams
- Intermittent and Perennial Streams
when the seepage from the stream feeds the groundwater resulting in
the build up of water mound. This happens when the ground water table is below the bed
of the stream. Such streams will dry up completely in rainless period and are called ephemeral streams.
Influent Streams
the ground water feed the stream. This happens when the ground water table is above the bed of the stream.
Effluent Streams
when the ground water table lies above the stream during the wet
season but drops below the bed during the dry season. The stream flows during wet season due to surface runoff and groundwater contribution but becomes dry during dry seasons.
Intermittent Streams
when the ground water table never drops below the bed of the
stream even in the most severe droughts and therefore they flow throughout the year.
Perennial Streams
is that balance of water, which flows or runs over the natural ground surface after losses by
evaporation, interception and infiltration.
Run-off
the net quantity of water available for storage, after all loses, for the **purposes of water resource utilization and planning, like irrigation, water supply, etc. **
Yield of Catchment
it is the discharge in times of flooding of the catchment area, i.e., when the intensity of rainfall is greatest and the condition of catchment regarding humidity is also favorable for an appreciable run-off.
Maximum Flood Discharge
Run-off Estimation Methods:
- Empirical Formulae, curves, and tables
- Infiltration Method
- Rational Method
- Overland Flow Hydrograph
- Unit Hydrograph Method
By deducting the infiltration loss, i.e., the area under the infiltration curve,
from the total precipitation or by the use of infiltration indices, which are already discussed. These
methods are largely empirical and the derived values are applicable only when the rainfall
characteristics and the initial soil moisture conditions are identical to those for which these are
derived.
Infiltration Method
a rational approach is to obtain the yield of a catchment by assuming a suitable run-off coefficient.
Rational Method
Overland flow occurs as a thin sheet of water over the ground surface
(soon after a storm starts), joins a stream channel, and then flows in the channel to the concentration point.
Overland Flow Hydrograph
The hydrograph of direct surface discharge measured at the outlet of
drainage area, which produces a unit depth of direct runoff (i.e., a Pnet of 1 cm over the entire
area of the catchment) resulting from a unit storm of specified duration (called unit period) is
called a unit hydrograph of that duration.
Unit Hydrograph Method -
