Hydrology Chapter 1,2&3

Hydrology

a multidisciplinary subject that deals with the occurrence, circulation, storage and distribution of surface and ground water on the earth.

Hydrologic Cycle

a continuous process in which water is evaporated from water surfaces and the oceans, moves inland as moist air masses and produces precipitation if the correct vertical lifting exists.

Precipitation

falls from clouds onto the land surface of the earth is dispersed to the hydrologic cycle via several pathways.

Evaporation

he conversion of water to water vapor from a water surface

infiltration

which is a function of soil moisture conditions and soil type, and may reenter channels later as interflow or may percolate to recharge the shallow ground water.

transpiration

the loss of water vapor through plant tissue and leaves.

evapotranspiration

is a maximum value if the water supply in the soil is adequate at all times.

watershed or basin area

physiographic property that determines the volume of runoff to be expected from a given rainfall event that falls over the area.

watershed or basin area

It is an area of land that drains all streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay or any point along a stream channel.

watershed area

The area encompassed by the divide is the

elevation or topographic map

The boundary of the catchment can be determined by using an

Runoff

originates at higher elevations and moves toward lower elevations in a direction perpendicular to the contour lines.

water budget equation

is also known as the hydrologic equation.

water budget equation

the total water resource of the earth is constant and the sun is the source of energy for the hydrologic cycle.

Precipitation

onsidered as the main input in the hydrologic cycle.

condensation

Moist air changes its phase through the process of

atmosphere

plays a crucial role in the process of precipitation.

Orographic precipitation

It occurs due to mechanical lifting of moist air masses over windward side of mountain ranges.

skyscrapers

In urban areas,______ also act as barriers in the mechanical lifting of the moist air mass

Convective precipitation

It is attributed to the intense heating of air at the ground which then leads to expansion and vertical rise of air.

Cyclonic precipitation

It is associated with the movement of large air-mass systems.

Cyclonic precipitation

When this occurs, warm air is lifted over the colder air and the front will slope diagonally. When the warm air is cooled by expansion, the saturation vapor pressure lowers and causes condensation.

Cumulus stage

It is wherein moist air rises, cools and condenses to form cumulus clouds. These clouds continue to grow taller as the rising air condenses and it grows in width.

Mature stage

Rain begins to fall as the updraft cannot support these water particles. As this progresses, drier air around the cloud is being draft to the clouds

entrainment

As this progresses, drier air around the cloud is being draft to the clouds through the process called

Dissipating stage

downdrafts dominate and cut off the supply of warm, moist air to the cloud.

Philippine Atmospheric, Geophysical and Astronomical Services Administration

is the agency in the country tasked to monitor various weather conditions.

rain gage

also termed as pluviometer, ombrometer and hyetometer.

tipping bucket rain gage.

There are two small buckets inside this canister-like equipment.

Radar

provide spatial and temporal variations of rainfall.

Satellite imagery data

provide less direct and less accurate data.

Satellite imagery data

have high spatial resolution and coverage especially in locations wherein rain gages cannot be possible installed and radar data are not available.

World Meteorological Organization

WMO

Mass Curve of Rainfall

This is a plot of accumulated precipitation against time.

Mass Curve of Rainfall

This is useful in extracting information on the duration, magnitude and intensity of the rain.

Hyetograph

This is a plot of intensity against time interval.

Hyetograph

This is used to represent the characteristic of the storm and used to predict extreme floods.

Arithmetic Mean Method

This method is used when the rainfall measured at various stations in a catchment show little variations. This uses only data of stations within the catchment

Thiessen Mean Method

this uses data within the catchment as well as data from stations of neighboring catchments. The calculation employs weighted average on the basis of the area closest to the station.

isohyet

line joining points of equal rainfall magnitude

Rainfall Intensity Duration Frequency Curves

essential tool in hydrology for estimating the design rainfall intensities for different durations and return periods.

Return periods

describe the frequency that an event re-occurs.

IDF curves

show the relationship between rainfall intensity, duration and the frequency of occurrence.

RIDF curves

developed based on historical data, which may not fully represent the changing climate patterns.

RIDF curves

vary significantly from one region to another due to differences in climate, topography and other local factors

Evaporation

is the process in which a liquid change to the gaseous state at the free surface, below the boiling point through the transfer of heat energy.

Evaporation

holds significant importance in the field of engineering and hydrology as it plays a crucial role in the water cycle and has significant implications for water resource management, environmental sustainability, and engineering design.

Evaporation

affects the overall water balance of a region, influencing the volume of water stored in reservoirs, lakes and groundwater aquifers.

Radiation

is the transmission of thermal energy by electromagnetic waves; radiant energy is not heat itself.

Radiant energy

can be reflected, absorbed, or transmitted.

Wind

aids in removing the evaporated water vapor from the zone of evaporation and consequently creates a greater scope for evaporation.

Critical wind speed value

is a function of the size of the water surface.

heat storage

essentially to change the seasonal evaporation rates and the annual evaporation rate is seldom affected.

Evaporimeter Data

most widely used to determine potential evaporation.

Potential evaporation

is the amount of evaporation of a sufficient water source is available.

Colorado Sunken Pan

A standard sunken pan is the

Colorado Sunken Pans

This has a standard dimension of 0.92 meter square and a depth of 0.46m. This pan is made of galvanized iron. This is set in the ground with the rim 0.05m above the ground level. The water level inside the pan must be maintained at or slightly below the ground level.

Colorado Sunken Pans

aerodynamic and radiation characteristics are similar to those of a lake. However, it is difficult to detect leaks in this pan. Extra care is also needed to keep the surrounding area free from tall grass, dust and other debris.

Floating pans

conceptually ideal because the pan exists in the same environment as the water body. However, it is difficult to maintain, somewhat inaccessible for data collection and maintenance, and is subject to splashing action.

US Geological Survey Floating Pan

A standard floating pan is the

US Geological Survey Floating Pan

It has a standard dimension of 900mm on the side and 450mm depth. This is supported by drum floats in the middle of a raft (4.25m x 4.87m). Water level in the pan is the same as that of the lake leaving 75mm space from the rim.

US Weather Bureau Class A Pan

This is a circular pan that is 1.21m in diameter and 0.225m deep. This is made of galvanized iron, gage 22 or monel metal that is 0.8mm thick. This pan is mounted on a wooden open frame platform with its bottom 0.15m above the ground level. This is filled with water to 0.05m below the rim and the water level should not drop to more than 0.075m below the rim.

Penman’s equation

is a widely used method for estimating evaporation from open water surfaces or bare soil.

Penman’s equation

It was developed by Howard Penman in the 1940s and is based on the principles od energy balance and mass transfer.

Penman’s equation

The equation takes into account meteorological variables and surface characteristics to provide a more comprehensive estimate of evaporation compared to simpler methods.

Penman equation

considered more comprehensive than simpler methods (e.g. Thornthwaite method) because it considers both energy and mass transfer aspects of evaporation.