HYDROLOGY 3

PRECIPITATION

Precipitation

It is one of the most important components of the hydrologic cycle. It represents the primary input of water to the land surface and directly influences streamflow, groundwater recharge, flooding, and water resources management.

1/4

About __ of the total precipitation that falls on continental areas is turned to the seas by direct runoff and underground.

2360

The Philippines receives an average of _____mm of rainfall every year. However, this amount is not evenly distributed over time and space.

Convective Lifting

It is caused by unequal heating of land masses. Warm air rises due to solar heating.

Orographic Lifting

It is the mechanical lifting over mountain barriers.

Frontal Lifting

Lifting of warm air on one side of a frontal surface over colder, denser air on the other side. Warm air is forced upward by a colder, denser air mass.

Cyclonic Lifting

Lifting of air converging into a low-pressure area

Condensation Nuclei

These are tiny particles in the atmosphere that act as a surface for water vapor to condense on, forming cloud droplets and leading to the formation of clouds, fog, and rain.

Collision-Coalescence Process

A process where droplets collide and combine into larger droplets.

Bergeron-Findeisen Process

A process where ice crystals grow at the expense of supercooled water droplets.

Drizzle/Mist

It consists of tiny liquid water droplets, usually with diameters between 0.1mm and 0.5mm, has slow settling rate.

Rain

It consists of liquid water drops mostly larger than 0.5mm in diameter.

Glaze

These are ice coatings formed on exposed surfaces by the freezing of supercooled water deposited by rain or drizzle.

Rime

These are white, opaque deposit of ice granules more or less separated by trapped air and formed by rapid freezing of supercooled water drops impinging on exposed objects.

Snow

These are ice crystals that form when temperatures remain below freezing in the atmosphere and at the surface.

Hail

These are balls or lumps of ice formed in strong updrafts in convective (thunderstorm) clouds.

Sleet

These are frozen raindrops that form when rain passes through a deep-freezing layer before reaching the ground.

Intensity

The rate at which rain falls

Duration

How long the rainfall event lasts.

Frequency

How often a certain rainfall amount occurs (related to return periods)

Depth

Total precipitation over a given storm or period

Temporal Distribution

How rainfall varies over time during an event

Spatial Distribution

How rainfall varies across a region.

Simple Arithmetic

This technique calculates areal precipitation using the arithmetic mean of all the point or areal measurements considered in the analysis. This method is suitable when rainfall is uniformly distributed, and the area is not very large.

Normal Ratio Method

If the normal precipitations vary considerably then Px is estimated by weighing the precipitation at various stations by the ratios of normal annual precipitation.

Thiessen Polygon Method

Allows for non-uniform distribution of gage by providing a weighting factor for each gage; connecting lines are drawn between stations on a map then perpendicular bisectors are drawn on these lines in such a way that the bisectors enclose areas referred to as ???? the area of which is determined by planimetry

Isohyetal Method

Most accurate method; contours of equal precipitation are drawn then the average precipitation is computed by weighting the average precipitation between successive isohyets by the area between isohyets.