RAINFALL- MEASUREMENT, ESTIMATION, AND FREQUENCY ANALYSIS OF POINT RAINFALL
Precipitation is characterized as the depth of water that collects on the surface of the land during its occurrence. It's usually measured in mm. The term "rain gauge" refers to an instrument that is used to measure precipitation. The characteristics of a rain gauge, such as its kind, installation, and the optimal number of rain gauges required for a watershed, are detailed in-depth below.
Rainfall is typically measured in terms of depth and intensity for hydrological analysis. The following are the descriptions of the two measuring modes:
Rainfall depth: Rainfall depth is quantified in terms of daily rainfall, which is then transformed to monthly, yearly, and so on to describe the temporal distribution of the rainfall. It is measured in mm.
Rainfall intensity: Rainfall intensity is the rate at which rainwater falls over the ground surface, often represented in mm per hour (mm/h). The rainfall intensity is determined by the slope of the mass curve at any given period.
- Rain Gauge- It is an instrument, used for measuring the depth of precipitation at a particular point and time falling over the land surface. Rain gauges are classified into two following categories-
1. Non recording type rain gauge:
This type of rain gauge only records rainfall depth and does not indicate rainfall time. Manual observations are taken at the end of the 24 hours. However, if there is a lot of rain, the observation time can be shifted. The collected rainwater in the "can" is measured in terms of volume using a graduated measuring cylinder in this sort of rain gauge. The volume of rainwater is converted to depth by dividing it by the gauge's aperture area, as shown below:
Rainfall depth (cm) = volume of collected water (cm³) / Aperture area of the gauge (cm²)
Symon's type rain gauge is one of the most common and widely used for determining the depth of rainfall in a watershed.
- Symon type rain gauge: It is a non-recording rain gauge that comprises a cylindrical vessel with a diameter of 12.7 cm, a funnel with an internal diameter of 12.7 cm, and a receiving bottle. The funnel is kept at the top of the cylindrical vessel with its shank above the mouth of the receiving bottle. The receiving bottle's capacity is sufficient to collect the amount of rain that is predicted to fall over 24 hours. The gauge is installed 30.5 cm above the ground surface, vertically on the masonry foundation.
Rainfall measurement is as follows: when it rains, the raindrops fall directly into the funnel, where they are gathered and placed in the bottle. A graduated cylinder is used to measure the collected water. At any given time, a full receiving bottle implies 1.25 cm of rainfall depth.
2. Recording type rain gauge:
Rain gauges of the recording kind are often known as automated, integrated, or self-recording rain gauges. The gathered rainwater is not measured by the graduated cylinder in this rain gauge but is instead recorded automatically by a mechanical mechanism included in the device. A rain gauge chart is wrapped around a clockwise spinning drum on which rainfall depth vs. time is recorded as a curve called a "mass-curve."
Using mass-curve, the following information can be furnished:
- Depth of rainfall at any time.
- The intensity of rainfall at any time during the rainfall.
- The onset of rainfall.
- Cessation of rainfall.
Kinds of Recording Type Rain Gauge:
- Weighing type
- Tipping bucket type
- Float type
- Weighing type rain gauge: It is a typical rain gauge that is used to measure rainfall and moderate snowfall. It expresses the depth of snowfall in terms of equivalent rainfall depth.
It is made up of a receiver or bucket that is kept on the platform and is balanced by a spring or lever. Rainwater collects in the collector before being put in the receiver. The weighing system has a recording mechanism, which consists of a recording pen and a clockwise spinning drum with a rain gauge chart.
When it rains, the rainwater collects in the bucket, which causes the weighing mechanism to depress. The depression is also conveyed to the recording pen through the lever at the same time. Over the chart wrapped on a clockwise spinning drum, the pen sketches the depression in the form of a curve.
- Tipping bucket-type rain gauge: This is a rain gauge that is powered by electricity. It's used to keep track of rainfall in remote places. It comprises a funnel inside a 300 mm diameter sharp-edged cylindrical receiver. One set of buckets is pivoted on a fulcrum beneath the funnel. Each bucket has a capacity of 0.25 mm of rainfall depth. When one of these buckets is filled, it tips down and dumps the water into the storage 'can' located right beneath the tipping bucket. The other bucket then takes the place of the first bucket.
When the device is tipped, an electrical current is activated, causing the pen to trace the mass curve on the rain gauge chart located on the clockwise revolving drum.
- Float type rain gauge: The siphon type rain gauge is another name for it. When it rains, the rainwater flows over a 127 mm funnel that is kept over the float chamber and is eventually gathered in the float chamber. The recording mechanism is connected to the float. The float is pulled up as the volume of rainwater in the float chamber grows, and the movement of the float is conveyed to the pen via an appropriate mechanism. A clockwise revolving drum is provided, which takes 24 hours to complete one full rotation. A chart is draped over the drum, on which a pen tracks the amount of rainfall concerning time.
- Rainfall Measurement by Weather Radar:
Radar is used to determine the storm's magnitude, area, orientation, and velocity. It is based on the echo-sounding idea. Electromagnetic signals are blasted into the sky at a high speed, equal to the speed of light, to monitor rainfall. The rain absorbs a chunk of it. The existence of clouds in the sky is predicted by the difference in outgoing and incoming signals.
- Automatic Radio-Reporting Rain gauge:
It is a specific type of rain gauge that forecasts rainfall information through a radio transmitter at a predetermined time. It is used to measure rainfall in inaccessible and distant catchment areas. It has a range of around 100 km. The rain is collected using a tipping bucket-style rain gauge in this manner. The tipping bucket is distinguished by the fact that it generates an electrical pulse as a result of tipping action, which is stored in the system via a data register. In a nutshell, this rain gauge begins by collecting rainfall data in the form of a 'code message,' which is then transformed into rainfall depth.
- Rainfall measurement by satellite:
Because liquid or ice drops do not respond to space satellites, they do not directly detect rainfall. They detect the presence of clouds in the sky, which is linked to the amount of precipitation detected by rain gauges. The fundamental advantage of this technology is that it can measure rainfall over a vast area.
ESTIMATION OF MEAN RAINFALL:
The precipitation measured by a rain gauge represents the depth of rainfall that has happened at a specific location (i.e. point precipitation) rather than the total rainfall. Rainfall data from numerous rain gauge stations is collected and analyzed to determine the mean areal precipitation of a given watershed. For calculating the runoff of a catchment area, the mean areal precipitation is required.
The following methods are employed for computing the mean areal precipitation:
- Arithmatic Mean Method
It is the simplest method for computing the mean areal precipitation. It consists of computing the mean value of measured rainfall of various rain gauge stations of the watershed. This method computes better, when:
- Rain gauges are uniformly distributed throughout the catchment.
- Rainfall does not differ very much among the rainfall of various rain gauge stations.
- Topography of waterslied is nearly flat.
- Theissen Polygen Method
In this method, the mean acral precipitation is computed by weighing the rainfall depth with the area of the polygon of the respective rain gauge station Due to this reason, sometimes this method is also known as a weighed mean method. Theissen method estimates a more accurate value of mean areal precipitation, when:
- Size of a watershed is between 500 and 5000 km² area.
- Topography is flat.
- There is uniformity in the distribution of rainfall measuring stations.
- There is a small variation in rainfall at different rain gauge stations in the catchment area.
- Isohyetal Method
It gives a more reliable estimate of mean areal rainfall as compared to the other two methods, described above. In this method, isolates are drawn by joining the points of equal rainfall depths on the base map of the catchment. Usually, the isohyets are drawn at a 1 cm interval. Accuracy of this method depends on the isohyet map and skill of the analyst.
This method requires more number of raincoat station in the catchment. this method is more suitable for computing the mean aerial rainfall of hilly and drug and topographical area of more than 5000 km².
FREQUENCY ANALYSIS OF POINT RAINFALL:
The probability of occurrence of an event in this series is studied by frequency analysis. The analysis of annual series, even though described with rainfall as a reference is equally applicable to any other random hydrological process, e.g. streamflow. First, it is necessary to correctly understand the terminology used in frequency analysis. The probability of occurrence of an event of a random variable whose magnitude is equal to or in excess of a specified magnitude X is denoted by P.
The recurrence interval (also known as the return period) is defined as
T = 1/P
The purpose of the frequency analysis of an annual series is to obtain a relation between the magnitude of the event and its probability of exceedance. The probability analysis may be made either by empirical or by analytical methods. The probability P of an event equaled to or exceeded is given by the Weibull formula.
As per the conclusion, the forms of rainfall measurement, methods to estimate average rainfall, and frequency of point rainfall have all been comprehensively discussed in this article based on rainfall measurement, estimation of mean rainfall, and frequency analysis of rainfall data. One of the most popular non-recording rain gauges used in India is the symon type rain gauge, whereas the float type rain gauge is the most common recording type rain gauge used in India. The most often used approach for estimating mean rainfall is the isohyetal method. The subtopics of point precipitation frequency analysis will be explored in the next topic.
Frequently Asked Questions
Mass curve, hyetograph, depth-area-duration curves and intensity-duration-frequency relationship.