PRECIPITATION

Introduction

Precipitation variability

Rainfall measurements techniques

Design of precipitation gauging network

Consistency of rain record

Filling up of missing record

Estimation of mean areal rainfall

IDF and DAD analysis

Snow measurement and determination of snow melt

1.0 INTRODUCTION

Precipitation denotes all forms of water that reach the earth from the atmosphere. The usual forms are rainfall, snowfall, hail, frost and dew.

The essential requirements for precipitation to occur are

The atmosphere must have moisture,

Presence of nuclei around which condensation of vapor takes place

Weather conditions must be good for condensation of water vapor to take place, and

The products of condensation must reach the earth.

Condensation

Condensation is the change of water from its gaseous form (water vapor) into liquid water. Condensation generally occurs in the atmosphere when warm air rises, cools and looses its capacity to hold water vapor. As a result, excess water vapor condenses to form cloud droplets.

Forms of Precipitation

RAIN Drop size - 0.5 mm to 6 mm. Rain is considered as light rain intensity < 2.5 mm/h moderate intensity caries from 2.5 to 7.5 mm/h heavy intensity > 7.5 mm/h

SNOW in the form of ice crystals, hexagonal in shape; density of snow = 0.1 g/cm3

DRIZZLE droplets of size < 0.5 mm; Intensity < 1 mm/hr

GLAZE it is the drizzle, which freezes immediately in contact with cold objects of the earth’s surface

SLEET where rain falls through the air of subfreezing temperature, the drops freezes to form grains of ice, called sleet.

HAIL It is the precipitating rain in the form of any irregular form of ice with size > 6 mm

DEW During nights the moisture present in atmosphere condenses on the surface of the objects forming water droplets called dew.

Types of Precipitation

Precipitation classified according to the factors responsible for lifting the air mass

1.0 CONVECTION

Convection refers to atmospheric motions in the vertical direction. As the earth is heated by the sun, different surfaces absorb different amounts of energy and convection may occur where the surface heats up very rapidly. As the surface warms, it heats the overlying air, which gradually becomes less dense than the surrounding air and begins to rise.

2.0 OROGRAPHIC

Air is lifted by the earth itself. When air encounters a mountain range, for example, air is forced to rise up and over the mountains and if enough lifting occurs, water vapor condenses to produce orographic clouds.

FRONT - is the interface between two distinct air masses. Under certain favourable conditions when a warm air mass and cold air mass meet, the warmer air mass is lifted over the colder one with the formation of front. The ascending warmer air cools adiabatically with the consequently formation of clouds and precipitation.

3.0 CYCLONE

It is a large low pressure region with circular wind motion. Two types of cyclones are r Tropical cyclones and Extratropical cyclones.

Tropical cyclone: A tropical cyclone, also called cyclone is a wind system with an intensely strong depression with MSL pressures sometimes below 915 mbars. Areal extent of a cyclone is about 100-200 km in diameter. Winds are antilock wise in the northern hemisphere. The centre of storm called the eye, extend to about 10-50 km in diameter.

Extratropical Cyclone

These are cyclones formed in locations outside the tropical zone. Associated with

a frontal system, they possess a strong counter-clockwise wind circulation in the

northern hemisphere. The magnitude of precipitation and wind velocities are

relatively lower than those of a tropical cyclone. However, the duration of

precipitation is usually longer and the areal extent also is larger.

Anti Cyclones

These are regions of high pressure, usually of large areal extent. The weather is

usually calm at the centre. Anticyclones cause clockwise wind circulations in the

northern hemisphere. Winds are of moderate speed, and at the outer edges,

cloudy and precipitation conditions exist.

2. PRECIPITATION

VARIABILITY

GANGA BASIN

The annual hydrographcharacterised by low flows during post- and pre-monsoon seasons and extremely high flows during the monsoon season

The annual variability in maximum flood discharge rates and volumes of the Ganga River for 19 years

Rainfall recorded on hydrographic basins

The rainfall recorded on a hydrographic basin represents values obtained by taking into account the hypotheses regarding the spatial distribution of the uneven distribution of the precipitations. The rainfall fallen in a time unit is maximum in the centre of the rain and decreases in a non-linear manner towards the outskirts of the area.The following methods are used for the calculation of the average rainfall for a basin

•arithmetic mean; •the method of the Thiessen polygons; •the isohyets method; •the square grid method.

When the area is physically and climatically homogenous and the required accuracy is small, the average rainfall for a basin can be obtained as the arithmetic mean of the hi values recorded at stations.

The method of Thiessen polygons The method of Thiessen polygons consists of attributing to each station an influence zone in which it is considered that the rainfall is equivalent to that of the station. The influence zones are represented by convex polygons. These polygons are obtained using the mediators of the segments which link each station to the closest neighbouring stations (figure 3.9). The ratio can be written as: Wi=Fi/F

where:

 Fi represents the area of the Thiessen included partially or wholly in the

considered basin.

  F is the area of the basin

The water volume attributed to each area is: Vi=Fi hi

where hi represents the rainfall recorded at station i.

The average value of the rainfall is the ratio between the total volume of the rainfall (as the sum of the partial volumes) over the area of the basin:

The weights Wi are directly proportional to the area of the corresponding

Thiessen polygon.

Figure 3.11 - The Thiessen polygons method

If the surfaces Fi are equal and the number of the stations is n, then

and the ratio Fi/F, which represents the weight Wi, equals to 1/n.

When the surfaces of the Thiessen polygons are equal, the method consists in calculating of the arithmetic mean for the respective basin.

The Thiessen polygons method takes into account the rainfall at stations not included in the considered basin. The relation describing the average rainfall can be written as follows:

where:

  i refers to stations in the considered basin,

  j refers to stations outside of the considered basin.

The isohyets methodThe isohyets method involves a linear interpolation between the punctual values of rainfall recorded at stations. The isohyets are lines of equal rainfall value. Considering that the area between the isohyets hi and hi+1 is

fi, the partial volume Vi of the rainfall over this surface is

The ratio between the partial volumes and the basin surface F represents the average rainfall:

In reality interpolations should be done in a non-linear fashion, taking into account the characteristics of the basin, that is: the geographic position, the vegetation type, the altitude, the topography, etc. One should have a good knowledge of the area from the climatic and physical point of view. The method is difficult to use for short time intervals.

The square grid method

The square grid method is used to calculate quickly the average rainfall for hourly time intervals. The method takes into account the nonlinearity of the rainfall distribution over the basin area. The basin is divided in a network of square elements with a fixed step size. The rainfall is calculated in each node using interpolation, as a function of the measured values at the adjacent stations.

Figure 3.10 - The square grid method

First we consider the closest stations of the considered node in the four quadrants (figure 3.10). There are not two stations in the same quadrant. The amount of rainfall in the i node is:

where:

 hj represents the recorded rainfall in the nodes 1, 2, 3, 4

 hi are computed values.

 dij distance between nodes i and j (j = 1, 2, 3, 4)

The following ratio signifies a weight, representing the influence of the stations j against the stations i:

With this notation hi becomes:

The values hi are computed for every Δt time interval; the average rainfall

over the basin for the current interval is computed as the arithmetic mean of the hik values; the calculation for each node is:

where: N is the number of nodes of the basin k the index of the time step.

I-D-F curves

The calculation of maximum rainfall is necessary for the designing of evacuation works of rainwater in cities, or on the premises of storm flow correction, or constructions and hydrotechnic installations. For this purpose one can use the intensity-duration-frequency lines (Figure 3.11). The intensity of calculated rainfall is a function of the standardized frequency and the duration of the calculated rainfall.

Figure 3.11 - The intensity-duration-frequency curves (Musy, 2001)

The standardized frequency is the annual number of rains of duration t, whose intensity exceeds the computed intensity. The computed frequency is calculated as a function of the class importance of the analysed objective. Thus for populated centres and industrial units we have the following values of standardized frequencies (table 3.1).

Table 3.2 Standardized frequencies

Class of the importance of the objective

Industrial units and production units of a

different naturePopulated centres

I 1/5 1/2...1/1

II 1/3...1/2 1/1...2/1

III 1/2...1/1 2/1

IV 1/1...2/1 -

V 2/1

In expressing frequency the numerator represents the numbers of rains and the denominator represents the number of years. The values in the table represent frequencies, not probabilities.