HYDROLOGY (midterm coverage)

HYDROLOGY

An earth science that refers to the scientific study of water and its properties, distribution, movements, and effects on Earth's surface, soil, and atmosphere.

Hydrology

* Comes from the Greek words **hudor** meaning **water** and **logy** meaning **a study of.**
* Has origins in the New Latin word **hydrologia.**

Atmospheric processes

cloud condensation,

precipitation

Surface processes

snow accumulation

overland flow

river flow

lake storage.

Subsurface processes

infiltration,

soil-water storage,

groundwater flow.

Interfacial processes

evaporation,

transpiration,

sediment water exchange

* hydropower generation
* navigation
* water quality control
* thermal pollution
* recreation and the protection and conservation of nature
* contributions to **functional design** (e.g., location and height of the dam) by developing design criteria, and to **water resources management** by establishing the hydrological boundary conditions to planning (inflow sequences, water resources assessment).

Hydrologists field of interest also includes:

Hydrologists

are traditionally concerned with the supply of water for domestic and agricultural use and the prevention of flood disasters.

• Sustainable agriculture (foods for the growing population).

• Environmental protection and management.

• Water resources development and management.

• Prevention and control of natural disasters.

• Control problems of tidal rivers and estuaries.

• Soil erosion and sediment transport and deposition

• Mitigation of the negative impacts of climatic change.

• Water supply

• Flood and drought control.

Hydrological science has both pure and applied aspects. Understanding the engineering hydrology science is essential for:

Traditional water management

has focused on providing freshwater resources to the needs of humans, livestock, commercial enterprises, agriculture, mining, industry, and electric power.

1950

Theoretical approaches in hydrology have been increasingly developed due to the development of digital computers since

**Hydrology**

is used in engineering mainly in connection with the design and operation of hydraulic structures.

1. data collection
2. methods of analysis

Two phases of the topics that Hydrology deals with:

* estimates of extremes rarely observed in a small data sample
* hydrologic characteristics at locations where no data have been collected (such locations are much more numerous than sites with data)
* estimates of the effects of human actions on the hydrologic characteristics of an area

Typical hydrologic problems involve:

Water

• is found everywhere on the earth, is one of the most basic and commonly occurring substances.

• the only substance on earth that exists naturally in the three basic forms of matter, i.e., liquid, solid, and gas.

Hydrologic cycle

the constant interchange of water from the oceans to the atmosphere to the land and back to the ocean.

Soil moisture

The effect of infiltration is to increase the_______.

the water percolates downward to become ground water.

If the moisture content becomes greater than the field capacity_____

Field capacity

moisture held by the soil after all gravitational drainage

Evaporate,

Infiltrate, or

Fills the Depressions.

The part of precipitation that falls into puddles and depressions can_____

Depression storage

Low places on the ground where water is held and is not available for overland flow or surface runoff.

Detention storage

* the flow path that must be filled with water before flow can occur overland and in the natural and/or manmade drainage systems.
* it is temporary since most of the water continues to run off after the rainfall ceases.

Precipitation

* in chemistry, it refers to material falling out of suspension.
* when studying weather and from meteorology, it refers to all forms of liquid or solid water particles that form in the atmosphere and then fall to the earth's surface.

Infiltration

the downward movement of water from the land surface into the soil profile.

Evaporation

is the transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere.

Solar radiation

the primary source of energy for evaporation

Evapotranspiration Process (ET)

* used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere.
* an important part of the water cycle

**Evaporation**

accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies.

Transpiration

accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves.

Evapotranspirator

an element such as a tree that contributes to evapotranspiration is called_____.

Condensation Process

the transformation of water vapor to liquid water droplets in the air, creating clouds and fog.

Groundwater

the water which occurs in the saturated zone of variable thickness and depth below the earth’s surface.

a. Precipitation

b. Infiltration

c. Evaporation

d. Evapotranspiration Process

e. Condensation Process

f. Groundwater

Main elements of Hydrologic Cycle

• Atmospheric processes

• Surface processes

• Subsurface processes

• Interfacial processes

Physical properties that are studied by hydrology:

a. Air temperature

b. Pressure

c. Humidity

d. Wind

e. Solar Radiation

Meteorological parameters that are used to study the hydrologic processes:

Air Temperature

is measured by Thermometers located 1.25 m above the ground and sheltered.

Mercury thermometer

measures maximum temperature

Alcohol thermometer

measures minimum temperature

Humidity

the water vapor (moisture) content in the air or other gases

Psychrometer

measures humidity

Relative Humidity (Rh)

the air's capacity of absorbing moisture and can be expressed as a percentage

Anemometer

counts the number of rotations which is used to measures wind speed (W)

Wind vane

measured direction of wind

Wind speed

is measured by units of Knot or mph.

where:
1 Knot = 1.852 km/h
1 mph = 1.61 km/h

Power law profile equation

it expresses the relationship between wind speed and elevation.

Solar radiation

the source of energy on the earth and it is measured by units of Watt/m2 and KJ/m2.

Radiometer

measures solar radiation in micro-meter 10−6 m

Net radiation (Rn)

an important term used in some methods of estimating evapotranspiration.

**-Monsoon**,

-**Inter-Tropical Convergence Zone (ITCZ),**

\-**Tropical Cyclones.**

Rainfall in the Philippines is brought about by different-causing weather patterns and among them— dominating causes are the

Monsoon

the seasonal change in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea.

Northeast Monsoon

Southeast monsoon

Two monsoons in the Philippines:

Northeast Monsoon

* wind from the northeast
* it affects the country from the month of November to March

Southeast monsoon

* wind from the southeast
* begins as early as mid of April and ends in November
depending on location

Inter-Tropical Convergence Zone (ITCZ)

* formed as a point where the trade winds converge forces the
air up into the atmosphere.
* it appears as a band of clouds consisting of rain showers with
occasional thunderstorms.

Northern hemisphere

Trade winds move in southwesterly direction

Southern hemisphere

Trade winds move northwesterly direction

Tropical Cyclones

* are characterized by a low-pressure center where winds of varying intensive blow around the center
* are classified according to maximum winds near the center

Tropical Depression - Winds from 45 to 63 KPH

Tropical Storm - Winds from 63 to 117 KPH

Typhoon - Winds of more than 117 KPH

Classification of Tropical Cyclones

a. Rain

b. Drizzle

c. Snow

d. Hail

e. Sleet

Various forms which Precipitation occurs:

Rain

* is precipitation that is in the liquid state when it reaches the
earth.
* form of precipitation in which separate drops of water fall to
the Earth's surface from clouds.

Drizzle

– liquid precipitation that reaches the surface in the form of drops that are less than 0.5 millimeters in diameter.

Snow

* a frozen water in a crystalline state.
* occurs when the layer of the atmosphere from the surface of
the earth through the cloud is entirely below freezing.

Hail

* a frozen water in a 'massive' state.It is a product of very intense thunderstorms.
* is rarely seen when the surface air temperature is below
freezing.
* forms as a byproduct of strong updrafts that exist in
thunderstorms.

Cumulonimbus clouds

that are associated with thunderstorms can grow to heights where the temperature is below freezing. Drops of water will rise up with the upward directed wind as they collide with other droplets and grow larger. This will eventually result in the droplet freezing into a hailstone.

Sleet

* a melted snow that is an intermixture of rain and snow.
* is nothing more than frozen raindrops.
* occurs when there is a warm layer of air above a relatively
deep sub-freezing layer at the surface.
* usually doesn't last long and mainly occurs ahead of warm
fronts during winter months.

* Dynamic or adiabatic cooling
* Mixing of air masses having different temperatures
* Contact cooling with the Earth
* Cooling by radiation

Condensation may be attributed to one or more of the following causes:

Dynamic cooling

most important cause of condensation

* Dew
* Frost
* Fog

**Condensation forms** commonly associated with radiational and contact cooling are:

a. convective storms

b. orographic storms

c. cyclonic storms.

d. hurricane or tropical cyclone (often added in the types of storms, and is a special case of the cyclonic storm)

Types of storms:

Convective

Precipitation from ______storm results as warm moist air rises from lower elevations into cooler overlying air. Heating of air at the interface with the ground, the heated air expands with a result of reduction of weight and the air will rise

summer thunderstorm

The characteristic form of convective precipitation is the

Orographic

* it results as air is forced to rise over a fixed-position
geographic feature such as a range of mountains.
* it is due to mechanical lifting of moist air masses over natural
barriers such as mountains.

Cyclonic precipitation

is caused by the rising or lifting of air as it converges on an area of low pressure.

Continental storms

occur at the boundaries of air of significantly different temperatures.

**a. Warm front** – warm air is the aggressor

**b. Cold front** – cold air is the aggressor

Type of Fronts:

29°C (84°F)

Hurricanes, typhoons, or tropical cyclones develop over tropical oceans that have a surface water temperature greater than _______

Hurricane

has no trailing fronts, as the air is uniformly warm since the ocean surface from which it was spawned is uniformly warm

a. Standard Rainfall Gauges

b. Automated Rain Gauge

c. Optical Rainfall gauges

d. High Precision Single-Unit Rain Gauge

Rainfall Gauges:

Standard gauges

* measure precipitation at or near the ground, and are observed at least once a day. The sizes of the gauges are made big enough to collect more than the average one-day or maximum 1-2 hours precipitation which differs according to various climatic conditions.
* are also commonly used to measure both rain and snow, and the latter affects fundamentally the form and dimensions of a particular national gauge (snow gauges are bigger).
* **The height of the gauge orifice varies** between **zero and more than 1m** above the ground.

Automated Rain Gauge

An **electronic rain gauges** that measures rainfall and are also self-emptying and frost proof. The basic idea is the rain collector’s measuring spoon being automatically tipped and emptied when the pre-adjusted water weight has been reached.

Weighing bucket

that moves a pen downward with the rainfall accumulating in the collecting bucket.

Optical Rainfall gauges

have a row of collection funnels. In an enclosed space below each is a laser diode and a phototransistor detector. When enough water is collected to make a single drop, it drips from the bottom, falling into the laser beam path. The sensor is set at right angles to the laser so that enough light is scattered to be detected as a sudden flash of light. The flashes can be translated to amount of water and the rate of flashing can represent the time scale.

High Precision Single-Unit Rain Gauge

The bucket, 4" in diameter, measures each rain drop, displays it on the digital display with 3/8" numerals, and then empties itself. Simply place it outside on a hard, level surface and watch it record rainfall up to 99.999 in. Convenient one touch reset button lets you keep annual, monthly, or stormby-storm totals. The unit has no moving parts, gold-plated sensors for reliability, and is not damaged by freezing conditions.

* Data loggers
* Infrared recorders
* Wireless
* Data logging rain gauges

Other types of modern rain gauges are:

Radar

* an object detection system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain.


* was coined in 1941 as an acronym for Radio Detection and Ranging.
* was originally called **Radio Direction Finder (RDF)** in the United Kingdom, in order to preserve the secrecy of its ranging capability.

Radar dish – or antenna

, transmits pulses of radio waves or microwaves which bounce off any object in their path.

**• Transmitter**

**• Waveguide**

**• Duplexer**

**• Receiver**

**• Electronic section**

**• A link to end users**

Components of a radar:

Weather radar

* a type of radar used to locate precipitation, calculate its motion, estimate its type (rain, snow, hail, etc.), and forecast its future position and intensity.

Doppler radars

capable of detecting the motion of rain droplets in addition to intensity of the precipitation.

Precipitation Radar

* the first space borne instrument designed to provide three-dimensional maps of storm structure.
* has a horizontal resolution at the ground of about **3.1 miles (five
kilometers)** and a swath width of **154 miles (247 kilometers).**
* One of its most important features is its ability to provide vertical profiles of the rain and snow from the surface up to a height of about **12 miles (20 kilometers)**
* it can detect fairly light rain rates down to about **0.027 inches (0.7
millimeters) per hour.**
* it can separate out rain echoes for vertical sample sizes of about **820
feet (250 meters)** when looking straight down.
* it carries out all these measurements while using only **224 watts of
electric power**—the power of just a few households light bulbs.
* was built by the **National Space Development Agency (JAXA)** of Japan
as part of its contribution to the joint US/Japan Tropical Rainfall
Measuring Mission (TRMM).

Weather satellite

is a type of satellite that is primarily used to monitor the weather and climate of the Earth.

* City lights
* Fires
* Effects of pollution
* Auroras
* Sand and dust storms
* Snow cover
* Ice mapping
* Boundaries of ocean currents
* Energy flows

Other types of environmental information collected using weather satellites:

Disdrometer

an instrument used to measure the drop size distribution and velocity of falling hydrometeors.

2D video disdrometers

can be used to analyze individual snowflakes.

Weather logger

* developed for measuring and recording in remote areas and it
operates as a completely self-contained system.
* it includes Instrument shelter and needs no power and depend only
on solar/battery supply.
* it has a RAM of a suitable size for storing data that can be
downloaded.

Rain gauge network

should consist of adequate number of rain gauges evenly distributed all over the drainage basin.

* shifting of a rain gauge station 10 a new location
* the neighborhood of the station undergoing a marked change
* change in the ecosystem due to calamities, such as forest fires,
landslides.
* occurrence of observational error from a certain date. The checking for inconsistency of a record is done by the doublemass curve technique.

Some of the common causes for inconsistency of record are:

The arithmetic-mean method

* the simplest method of determining areal average rainfall.
* it involves averaging the rainfall depths recorded at several gauges.

The Thiessen method

* assumes that at any point in the watershed the rainfall is the same
as that at the nearest gauge, so the depth recorded at a given
gauge is applied out to a distance halfway to the next station in any
direction.
* is generally more accurate than the arithmetic mean method, but
it is inflexible, because a new Thiessen network must be
constructed each time there is a change in the gauge network, such
as when data is missing from one of the gages.
* it does not directly account for orographic influences on rainfall.

The Isohyetal method

\
* is superior to the other two methods especially when the stations
are large in number