Hydrology Midterm

Hydrology

• Comes from the Greek words hudor meaning water and logy meaning a study of.

• Has origins in the New Latin word hydrologia

• 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.

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** are traditionally concerned with the supply of water for domestic and agricultural use and the prevention of flood disasters. However, their field of interest also includes:

• 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

water returns to the atmosphere through soil evaporation and by transpiration from plants and trees.

If the moisture content is less than the field capacity of the soil

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.

seepage into streams and lakes, as capillary movement back into the root zone, or it is pumped from wells and discharged into irrigation systems, sewers, or other drainage ways

The precipitation that percolates down to ground water is maintained in the hydrologic cycle as

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.

Ddry soil (or just soil)

has a defined capacity for infiltrating water. The capacity can be expressed as a depth of water that can be infiltrated per unit time, such as inches per hour.

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. Evapotranspiration is an important part of the water cycle.

Evapotranspirator

an element such as a tree that contributes to evapotranspiration.

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

• Solar radiation

d. Evapotranspiration Process (ET)

• Evaporation

• Transpiration

• Evapotranspirator

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. Precipitation and runoff (visible components)

b. Infiltration

c. Transpiration

d. Percolation

e. Groundwater recharge

f. Interflow

g. Groundwater discharge

The hydrologic cycle, illustrated in Figure 2, shows the pathways where water travels as it circulates throughout global systems by various processes. **The components of this hydrologic cycle:**

a. Air temperature

b. Pressure

c. Humidity

d. Wind

e. Solar Radiation

Meteorological parameters that are used to study the hydrologic processes:

• Precipitation

• Runoff

• Transpiration

• Evaporation

These hydrologic processes are:

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

Dew point (Td)

\- the temperature at which 𝑒𝑣 reaches 𝑒𝑠 for the same conditions and water vapor starts to condense.

\- It can be computed using eq. 1 if T is replaced by Td and the normal vapor pressure is considered as saturated one.

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**, the **Inter-Tropical Convergence Zone,** and **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

Water budget

• relation between water volume in the globe that is considered to be constant but changes from a phase to another

• states that the change in the storage within a certain domain is equal to the summation of the inflow, outflow, underground flow, evaporation, and precipitation.

• is accounting of the volume of flow rate of water in all possible locations.

Precipitation

• in chemistry 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

• to the hydrologist, it is the general term for all forms of moisture emanating from the clouds and falling to the ground.

• the primary input parameter of the hydrologic cycle.

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. Hail 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:

• Latitude (high in latitudes of rising air and low in latitudes of descending air)

• Elevation (precipitation increases with elevation).

• Distance from moisture sources.

• Position within the continental land mass.

• Prevailing wind direction

• Relation to mountain ranges (more rain on windward sides than leeward sides)

• Relative temperatures of land and bordering oceans

The average annual precipitation on certain locations is a function of:

**Precipitation**

can be classified by the origin of the lifting motion that causes the precipitation. Each type is characterized by different spatial and temporal rainfall regimens.

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 convective storms 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.

Extra-tropical cyclones or continental storms.

Air moves from areas of higher pressure toward areas of lower pressure. In the middle latitudes, cyclonic storms generally move from west to east and have both cold and warm air associated with them. These mid-latitude cyclones are sometimes called

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:

Front

type of air movement, where one has warm air being pushed into an area of cool air, while the other has cool air pushed into an area of warmer air.

Cold front

shows a much steeper slope of the interface of warm and cold air usually resulting in rainfall of shorter duration and higher intensity. Some depressions are died-out cyclones.

surface water temperature greater than 29°C (84°F)

Hurricanes, typhoons, or tropical cyclones develop over tropical oceans that have a

Hurricane

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

**non-recording cylindrical container type** or the **recording weighing type, float type and tipping-bucket type.**

Measuring precipitation covers rain, hail, snow, rime, hoar frost, and fog, and is traditionally measured using various types of rain gages such as the

• Attempting to collect rain data in a hurricane can be nearly impossible and unreliable (even if the equipment survives) due to wind extremes.

• Rain gauges only indicate rainfall in a localized area. For virtually any gauge, drops will stick to the sides or funnel of the collecting device, such that amounts are very slightly underestimated, and those of .01 inches or 0.25 mm may be recorded as a trace.

• Another problem encountered is when the temperature is close to or below freezing. Rain may fall on the funnel and freeze, or snow may collect in the gauge and not permit any subsequent rain to pass through. Rain gauges, like most meteorological instruments, should be placed far enough away from structures and trees to ensure that any effects caused are minimized.

Limitations of Rain gauges:

• The rim of the collector should have a sharp edge.

• The area of the aperture should be known with an accuracy of 0.5%.

• Design is such that rain is prevented from splashing in or out.

• The reservoir should be constructed to avoid evaporation.

• In some climates the collector should be deep enough to store one day's snowfall.

The requirements for gauge construction are:

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

There are **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

These 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.