hydrology prelim

Precipitation

Water for atmosphere reaching earth includes rainfall, snowfall, hail, and frost

Rain

It is the most common form of precipitation in tropical countries

Rainfall

Used to describe precipitation in the form of water drop less of size larger than 0.5 MM

Red Rainfall

The rain measurement more than 30 MM rain observed in one hour and expected to continue in the next two hours. Flood possibility is flooding expected in low-lying areas the response evacuation.

Orange Rainfall

Rain measurement 15 to 30 MM rain observed in one hour and expected to continue in the next two hours. Flood possibility flooding threatening response alert for possible evacuation.

Yellow rainfall

7.5 to 15 MM rain observed in one hour and expected to continue in the next two hours. Blood possibility flood is possible. The response is monitor the weather condition.

Snow

Consist of ice crystals, which usually combine to form flakes form when the temperature is below freezing

Drizzle

Fine sprinkle of numerous water droplets to the plates are so small that they appear to float in the air

Glaze

When the rain comes contact with cold, ground the water, droplets freeze and form an ice coating

Sleet

It is frozen raindrops still transparent greens when Rain falls through air at sub freezing temperature

Hail

It is a shower precipitation in the form of irregular pallets or lap of ice hails occur in violent thunderstorm in which currents are very strong

Frontal system

In front is the interface between two distinct air masses when a warm air and cold air meets, The warmer air is lifted over the colder one with the formation of fronts

Cyclonic system

A cyclone is a large of air mass that rotates around a strong center of low atmospheric pressure

Convective system

Convective rain occurs when the sun hits the earths surface causing warm air to rise cool and condense into clouds leading to rainfall often accompanied by thunderstorm

Orographic system

Refers to precipitation that occurs when moist air is forced upward over a mountain range as the air races and close it for clouds leading to rain most of which falls on the windward side of the mountain side known as the rain shadow typically receives less rainfall

Rainfall depth

Total vertical accumulation of rain over a defined area and time period

Rainfall duration

The length of time of which a rainfall event occurs

Light rainfall

<2.5mm/hr

Moderate rainfall

2.5 - 7.6 mm/hr

Heavy Rainfall

>7.6 mm/hr

Hyetographs

A graphical representation showing rainfall intensity or depth versus time for a specific storm event

Instantaneous

Time history of rainfall rate at specific point

Cumulative

Total rainfall depth accumulation over time

Dimensionless

Normalized time and depth (% values)

Standard rain gauge

Collect rainfall in a graduated cylinder

Rain gauge

is an instrument used to measure the amount of rainfall over a specific. Period.

Classification of rain gauges

Non-Recording and recording rain gauges

OBS rain Gauge

Measures rainfall, regardless of wind direction

Hellman Rain Gauge

Designed to measure long-term precise, rainfall at a fixed location

Recording rain gauge

Measures rainfall continuously

Tipping bucket rain gauge

Produces a rainfall graph, which is the hyetograph

Optical and acoustic rain gauge

Uses laser, radar, or sound to detect rain drops

Point rainfall

Is the precipitation measured at a single rain gauge station, representing local conditions only

Area rainfall

Is the average rainfall over defined area ( such as a watershed ), calculated by combining multiple point rainfall measurements using the following methods. Two types of area rainfall, Thiessen Polygon Method and Isohyetal method

Arithmetic mean.

This method is the simplest method of determining area of rainfall it involves averaging the rainfall that recorded at the number of gauges. This method is satisfactory if the gauges are uniformly distributed over the area and the individual gauge measurements do not vary greatly about the mean

Thiessen Polygon Method

is a widely used technique for determining areal average rainfall, it involves dividing the catchment into polygons each surrounding a rain gauge by drawing perpendicular bisector between adjacent gauges

Isohyetal Method

The isohyetal method overcomes some of these difficulties by constructing isohyets, using observed depths at rain gauges and interpolation between adjacent gauges

Double-mass analysis

It is used to the detect if data site have been subjected to a significant change in magnitude due to external factors such as problems with instrumentation observation, practices or recording conditions