LESSON 5.1-5.3 Flashcards
The hydrologic equation states that
Rainfall – Losses = Runoff
WATER LOSSES
✓ Evaporation:
a. from water surface, i.e., reservoirs, lakes, ponds, river channels, etc.
b. from soil surface, appreciably when the ground water table is very near the soil surface.
✓ Transpiration—from plant leaves.
✓ Evapotranspiration for consumptive use—from irrigated or cropped land.
✓ Interception loss-due to surface vegetation, i.e., held by plant leaves.
✓ Infiltration—into the soil at the ground surface.
✓ Watershed leakage—ground water movement from one basin to another or into the sea.
the primary abstractions of the hydrological cycle.
Evaporation and transpiration
The combined effect of
evaporation and transpiration is called
evapotranspiration.
The process by which water is changed from the liquid or solid state into the gaseous state through the transfer of heat
energy is known as
evaporation
in general, evaporation is affected by
temperature,
wind,
atmospheric pressure,
humidity,
water quality,
water depth,
soil type and nature, and
shape of surface
Factors affecting lake evaporation:
Vapor Pressure
Temperature
Wind
Atmospheric Pressure
Soluble Salts
Hest Storage in Water Bodies
The amount of water evaporated from a water surface is estimated by the following methods:
(i) using evaporimeter data
(ii) empirical evaporation equations
(iii) analytical methods
Types of Evaporimeters
Class A Evaporation Pan
ISI Standard Pan
Colorado Sunken Pan
US Geological Survey Floating Pan
are water-containing pans which are exposed to the atmosphere and the loss of water by evaporation is
measured in them at regular intervals.
Evaporimeters
Monel metal is
used where corrosion is a problem. The pan is placed on a 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 depth of water with a hook gauge in a stilling well.
Class A Evaporation Pan
The evaporation from this
pan is found to be less by about 14% compared to
that from an unscreened pan. The pan is placed
over a square wooden platform of 1225 mm width
and 100 mm height to enable circulation of air
underneath the pan.
ISI Standard Pan
5.3). The chief advantage of the sunken pan is that radiation and
aerodynamic characteristics are similar to those of a lake. However, it has
the following disadvantages: (i) difficult to detect leaks, (ii) extra care is
needed to keep the surrounding area free from tall grass, dust, etc., and (iii)
expensive to install.
Colorado Sunken Pan
With a view to simulate the characteristics of a large body of water, this square pan (900 mm side and 450 mm depth)
supported by drum floats in the middle of a raft (4.25 m x 4.87 m) is set afloat in a lake.
US Geological Survey Floating Pan
a. Dry zones—One station for every 30,000 km2
b. Humid temperate climates—One station for every 50,000 km2
c. Cold regions—One station for every 100,000 km2
Evaporation Stations
