Precipitation

The term precipitation denotes all forms of water that reach the earth from the atmosphere. The study of precipitation forms a major portion of the subject of hydrometeorology. As the evaporation continues, the amount of vapor present in the atmosphere goes on increasing. After reaching a certain amount, the vapor will then condense and comes to Earth's surface in solid/liquid form.

For precipitation to form; (1) the atmosphere must have moisture, (ii) there should be sufficient nuclei present to aid precipitation, (iii) weather conditions must be good for condensation of water to take place, and (iv), the products of condensation must reach the surface of the earth. Precipitation is greatly affected by the climate, geography, and ocean surfaces.

Precipitation can be classified into two types namely; liquid and solid precipitation. Liquid precipitation can be in the form of drizzle and rainfall. Meanwhile, solid precipitation occurs in the form of snow, hail, glaze, and sleet.

weather systems for precipitation

For the clouds and subsequent precipitation to form, it is necessary that the moist air masses should cool first to form condensation. Here are some of the processes connected with the weather systems associated with precipitation

Convective Precipitation - In this type of precipitation, a packet of air that is warmer than the surrounding air due to localized heating rises because of its lesser density.
Cyclonic Precipitation - It occurs due to a large low-pressure region with a circular wind motion.
Orographic Precipitation - The moist air masses may get lifted up to higher altitudes due to the presence of mountain barriers and consequently undergo cooling processes.

characteristics of rainfall

RAINFALL depth

It is vitally important to the overall hydrologic cycle as it helps for the replenishment of water. It refers to the accumulation of rainfall obtained by multiplying rainfall density and its duration

RAINFALL INTENSITY

It is the measure of the amount of rain that falls over the given time interval. Higher rainfall intensity produces larger raindrop sizes which has more impact energy and higher intensity storms can damage delicate vegetation.

rainfall duration

It refers to the length of time on which rainfall occurs. Longer duration rainfall significantly affects infiltration, runoff, and soil erosion processes.

hyetograph

Hyetograph is the plot of intensity of rainfall against the time interval. It is derived from a mass curve, which is usually represented in a bar chart. Thus, it conveniently represents the storm characteristics that are essential in the hydrological studies in order to predict extreme floods.

point rainfall measurements

Point rainfall or station rainfall is the rainfall during the given time interval measured in a raingauge, or an estimate of the amount which might have been measured in a given point. Point rainfall data is often discerned by method of moving averages. It is a technique for smoothening out the high frequency fluctuations of a time series.

frequency analysis

Frequency analysis deals with the chance of occurrence of an event over a specified period of time. It is used as an aid in determining the design discharge and design rainfall. Thus, it can be used to calculate the frequency of hydrologic or non-hydrologic events.

weibull formula

It is the most commonly used formula for plotting position. The Weibull distribution was derived using the principle of maximum entropy, which is useful in representing probability distributions of rainfall depths and durations.

different types of rain gauges

Rain gauges are instruments used to measure the precipitating rain in a given amount of time per unit area. Measuring rainfall can be done in different ways using different types of rain gauges.

rain gauge standard

The recording of rainfall using the rain gauge standard is done manually. This type of gauges work by catching the falling rain in a funnel-shaped collector that is attached to a measuring tube.

non recording gauges

It is commonly used by meteorological department. An example of non recording type is the Symons rain gauge, in which the funnel is provided with a circular brass rim which is 127mm exactly for it to fit well into the vessel.

recording rain gauges

Recording gauges produce a continuous plot of rainfall against time and provide valuable data of intensity and duration of rainfall for hydrological analysis of storms. The following are the commonly used recording rain gauges:

Tipping Bucket Type
Weighing Bucket Type
Floating or Natural Syphon Type

estimation of missing point data

Missing rainfall data usually occurs due to absence of the observer or damage/fault in a raingauge, which affects the continuity of record. The missing data can be estimated by using the data of its neighbouring stations

methods used to estimate rainfall data

Arithmetic Mean Method
Normal Ratio Method
Inverse distance Method

conversion to point rainfall to areal rainfall

A dense network of point measurements or radar estimates can provide a better representation of the true volume over the given area. Since rain gauges represent only the point sampling of the areal distribution of a storm, hydrological analysis still requires knowledge of the rainfall over the catchment area

To convert the point rainfall values at various stations into an average value over a catchment, the following three methods are used:

Arithmetical-mean Method
Thiessen-Polygon Method
Isohyetal Method

double mass analysis

Double Mass Analysis is used to detect if the data at a site is subjected to a significant change in magnitude due to external factors such as problems concerning instrumentation, observation, practices, and recording conditions. It can be used to investigate the behavior of records as well as to determine whether there is a need for the correction of data.

STATION INCONSISTENCY

Some of the common causes for inconsistency of records are: (1) Shifting of a raingauge station to a new location, (2) The neighborhood of the station undergoes a marked change, (3) change in the ecosystem due to calamities, and (4) occurrence of observational error from a certain date.

The checking of record inconsistency is done using the double mass curve technique. Here are the steps used when presenting a double mass curve:

The precipitation values at a certain station beyond the period of change of regime can be corrected using the following relation given:

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reflection:

Precipitation is one of the most important events in hydrology. Variations in precipitation are known to be responsible for many hydrological problems such as floods and droughts. It is relatively important for civil engineers to recognize the importance of precipitation study for it serves as a contributing factor in water resources management, water supply schemes, and hydrologic data study for hydraulic structures. Thus it is necessary to impart various aspects of precipitation for society to be aware of the global situation we're currently facing with. It is considered as a basic input to modern hydrology due to the environmental effects of landscape evolution and other development projects to water resources management.

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