2. HYDROLOGY
The science of water. It is the
science that deals with the
occurrence, circulation and
distribution of water of the earth
and earth's atmosphere
3. SOME TERMINOLOGIES
• WATER CYCLE – describes the continuous movement of water on, above and
below the earth surface
• EVAPORATION – transformation of water from liquid to gas, it moves from the
ground or bodies of water into the overlying atmosphere
• EVAPOTRANSPIRATION – water transpired from plants and evaporated from
the soil
• CONDENSATION –tansformation of water vapor in the air that cools down and
turns back into liquid watcer
• PRECIPITATION – transformation of water (in the form of rain, snow, hail or
sleet) falls from clouds in the sky.
4. SOME TERMINOLOGIES
• AQUIFER – a water-bearing rock that readily transmits water to wells and
springs
• INFILTRATION – movement of water into the ground from the surface
• RUNOFF – variety of ways by which water moves across the land. As it flows,
water may infiltrate into the ground, evaporate into the air, and become stored in
lakes or reservoir, or be extracted for agricultural uses or other human uses
• SUBSURFACE FLOW – flow of water underground, it can return into the surface
or seep into the ocean
5. TYPES OF PRECIPITATION
• DRIZZLE - a very light, usually uniform, precipitation consisting of numerous
minute droplets with diameters in excess of 0.1 mm but smaller than 0.5 mm.
• RAIN -. precipitation consisting of water drops larger than 0.5 mm. It can be
classified as light rain when the intensity is smaller than 2.5 mm/hr, moderate
when it is between 2.5 and 7.5 mm/hr, and heavy when it exceeds 7.5 mm/hr
• SNOW - precipitation in the form mainly of branched hexagonal or star-like ice
crystals, resulting from direct reverse sublimation of the atmospheric water vapor
• SLEET - (North American usage) is precipitation consisting of transparent pellets or
grains of ice, formed as a result of the passage of raindrops through a layer of
colder air near the ground. In British usage the word sleet refers to precipitation
consisting of melting snow or a mixture of snow and rain.
6. TYPES OF PRECIPITATION
• GLAZE - also known as freezing rain; an ice deposited by drizzle or rain on cold
surfaces.
• SNOW PELLETS- also called granular snow or graupel; a form of precipitation
consisting of white, opaque, small grains with diameters between roughly 0.5 and
5 mm.
• HAIL - consists of balls or irregular chunks of ice with diameters between 5 and
50 mm, or even larger. These lumps of ice can be transparent, or they can consist
of concentric layers of clear and opaque ice
• DEW - consists of moisture in the form of liquid drops on the ground surface and
on the vegetation and other surface elements, as a result of direct condensation of
atmospheric water vapor.
10. EXAMPLE #1:
(Delineation of
Watershed)
The volume of atmospheric water per month is
12,900km3. The evapotranspiration from land is
28,000 km3 per year and river runoff is 50,000 km3
per year. Estimate the storage volume of the
catchment.
11. RAINDROP
DROP TERMINAL VELOCITY
EQUATION:
From fluid mechanics, the force balance is:
D 0.2 0.4 0.6 0.8 1.0 2.0 3.0 4.0 5.0
Cd 4.2 1.66 1.07 0.815 0.671 0.517 0.503 0.559 0.66
Drag Coefficient table
The force balance for a raindrop is:
Fd = Fg – Fb
where:
𝐹𝑑 – drag force
𝐹𝑔 – gravity force
𝐹𝑏 – buoyancy force
12. EXAMPLE #1:
(Raindrop)
What is the terminal velocity for the light rain with a
drop size of 0.6mm at sea level.
Ans. : 2.4708m/s
14. EXAMPLE #1:
(Precipitation
Data)
In the given catchment figure as shown, the theissen
polygon with the rainfall depths recorded by gauge
A, B, and C as 10mm, 8mm, and 9mm. The
corresponding polygon areas are 5.1km2, 3.2km2,
and 5.3km2. Compute the average rainfall depth
express in mm.
Ans. : 9.1397mm
