Module 2 The Science of Surface and Ground Water Lesson 1 Precipitation And Evapotranspiration Notes | EduRev

: Module 2 The Science of Surface and Ground Water Lesson 1 Precipitation And Evapotranspiration Notes | EduRev

 Page 1


 
 
 
 
 
 
 
 
Module 
2 
  
The Science of Surface 
and Ground Water 
Version 2 CE IIT, Kharagpur 
 
Page 2


 
 
 
 
 
 
 
 
Module 
2 
  
The Science of Surface 
and Ground Water 
Version 2 CE IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
Lesson  
1 
 
Precipitation  
And Evapotranspiration 
 
Version 2 CE IIT, Kharagpur 
 
Page 3


 
 
 
 
 
 
 
 
Module 
2 
  
The Science of Surface 
and Ground Water 
Version 2 CE IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
Lesson  
1 
 
Precipitation  
And Evapotranspiration 
 
Version 2 CE IIT, Kharagpur 
 
Instructional Objectives 
 
On completion of this lesson, the student shall learn: 
1. The role of precipitation and evapotranspiration with the hydrologic 
cycle. 
2. The factors that cause precipitation. 
3. The means of measuring rainfall. 
4. The way rain varies in time and space. 
5. The methods to calculate average rainfall over an area. 
6. What are Depth – Area – Duration curves. 
7. What are the Intensity – Duration – Frequency curves. 
8. The causes of anomalous rainfall record and its connective measures. 
9. What are Probable Maximum Precipitation (PMP) and Standard Project 
Storm (SPS). 
10. What are Actual and Potential evapotranspiration. 
11. How can direct measurement of evapotranspiration be made. 
12. How can evapotranspiration be estimated based on climatological data. 
 
 
2.1.0 Introduction 
Precipitation is any form of solid or liquid water that falls from the atmosphere 
to the earth’s surface. Rain, drizzle, hail and snow are examples of 
precipitation. In India, rain is the most common form of precipitation.  
 
Evapotranspiration is the process which returns water to the atmosphere and 
thus completes the hydrologic cycle. Evapotranspiration consists of two parts, 
Evaporation and Transpiration. Evaporation is the loss of water molecules 
from soil masses and water bodies. Transpiration is the loss of water from 
plants in the form of vapour. We proceed on to discuss precipitation, and its 
most important component in India context, the rainfall. 
 
 
2.1.1 Causes of precipitation 
For the formation of clouds and subsequent precipitation, it is for necessary 
that the moist air masses to cool in order to condense. This is generally 
accomplished by adiabatic cooling of moist air through a process of being 
lifted to higher altitudes. The precipitation types can be categorized as. 
 
Version 2 CE IIT, Kharagpur 
 
Page 4


 
 
 
 
 
 
 
 
Module 
2 
  
The Science of Surface 
and Ground Water 
Version 2 CE IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
Lesson  
1 
 
Precipitation  
And Evapotranspiration 
 
Version 2 CE IIT, Kharagpur 
 
Instructional Objectives 
 
On completion of this lesson, the student shall learn: 
1. The role of precipitation and evapotranspiration with the hydrologic 
cycle. 
2. The factors that cause precipitation. 
3. The means of measuring rainfall. 
4. The way rain varies in time and space. 
5. The methods to calculate average rainfall over an area. 
6. What are Depth – Area – Duration curves. 
7. What are the Intensity – Duration – Frequency curves. 
8. The causes of anomalous rainfall record and its connective measures. 
9. What are Probable Maximum Precipitation (PMP) and Standard Project 
Storm (SPS). 
10. What are Actual and Potential evapotranspiration. 
11. How can direct measurement of evapotranspiration be made. 
12. How can evapotranspiration be estimated based on climatological data. 
 
 
2.1.0 Introduction 
Precipitation is any form of solid or liquid water that falls from the atmosphere 
to the earth’s surface. Rain, drizzle, hail and snow are examples of 
precipitation. In India, rain is the most common form of precipitation.  
 
Evapotranspiration is the process which returns water to the atmosphere and 
thus completes the hydrologic cycle. Evapotranspiration consists of two parts, 
Evaporation and Transpiration. Evaporation is the loss of water molecules 
from soil masses and water bodies. Transpiration is the loss of water from 
plants in the form of vapour. We proceed on to discuss precipitation, and its 
most important component in India context, the rainfall. 
 
 
2.1.1 Causes of precipitation 
For the formation of clouds and subsequent precipitation, it is for necessary 
that the moist air masses to cool in order to condense. This is generally 
accomplished by adiabatic cooling of moist air through a process of being 
lifted to higher altitudes. The precipitation types can be categorized as. 
 
Version 2 CE IIT, Kharagpur 
 
• Frontal precipitation 
This is the precipitation that is caused by the expansion of air on 
ascent along or near a frontal surface. 
 
• Convective precipitation 
Precipitation caused by the upward movement of air which is 
warmer than its surroundings.  This precipitation is generally 
showery nature with rapid changes of intensities. 
 
• Orographic precipitation 
Precipitation caused by the air masses which strike the mountain 
barriers and rise up, causing condensation and precipitation.  The 
greatest amount of precipitation will fall on the windward side of the 
barrier and little amount of precipitation will fall on leave ward side. 
  
For the Indian climate, the south-west monsoon is the principal rainy season 
when over 75% of the annual rainfall is received over a major portion of the 
country. Excepting the south-eastern part of the Indian peninsula and Jammu 
and Kashmir, for the rest of the country the south-west monsoon is the 
principal source of rain. 
 
From the point of view of water resources engineering, it is essential to 
quantify rainfall over space and time and extract necessary analytical 
information. 
 
 
2.1.2 Regional rainfall characteristics 
Rain falling over a region is neither uniformly distributed nor is it constant over 
time. You might have experienced the sound of falling rain on a cloudy day 
approaching from distance. Gradually, the rain seems to surround you and 
after a good shower, it appears to recede. It is really difficult to predict when 
and how much of rain would fall. However it is possible to measure the 
amount of rain falling at any point and measurements from different point 
gives an idea of the rainfall pattern within an area. 
 
In India, the rainfall is predominantly dictated by the monsoon climate. The 
monsoon in India arises from the reversal of the prevailing wind direction from 
Southwest to Northeast and results in three distinct seasons during the course 
of the year. The Southwest monsoon brings heavy rains over most of the 
country between June and October, and is referred to commonly as the ‘wet’ 
season. Moisture laden winds sweep in from the Indian Ocean as low-
pressure areas develop over the subcontinent and release their moisture in 
the form of heavy rainfall. Most of the annual rainfall in India comes at this 
time with the exception of in Tamil Nadu, which receives over half of its rain 
during the Northeast monsoon from October to November. 
 
The retreating monsoon brings relatively cool and dry weather to most of India 
as drier air from the Asian interior flows over the subcontinent. From 
Version 2 CE IIT, Kharagpur 
 
Page 5


 
 
 
 
 
 
 
 
Module 
2 
  
The Science of Surface 
and Ground Water 
Version 2 CE IIT, Kharagpur 
 
 
 
 
 
 
 
 
 
Lesson  
1 
 
Precipitation  
And Evapotranspiration 
 
Version 2 CE IIT, Kharagpur 
 
Instructional Objectives 
 
On completion of this lesson, the student shall learn: 
1. The role of precipitation and evapotranspiration with the hydrologic 
cycle. 
2. The factors that cause precipitation. 
3. The means of measuring rainfall. 
4. The way rain varies in time and space. 
5. The methods to calculate average rainfall over an area. 
6. What are Depth – Area – Duration curves. 
7. What are the Intensity – Duration – Frequency curves. 
8. The causes of anomalous rainfall record and its connective measures. 
9. What are Probable Maximum Precipitation (PMP) and Standard Project 
Storm (SPS). 
10. What are Actual and Potential evapotranspiration. 
11. How can direct measurement of evapotranspiration be made. 
12. How can evapotranspiration be estimated based on climatological data. 
 
 
2.1.0 Introduction 
Precipitation is any form of solid or liquid water that falls from the atmosphere 
to the earth’s surface. Rain, drizzle, hail and snow are examples of 
precipitation. In India, rain is the most common form of precipitation.  
 
Evapotranspiration is the process which returns water to the atmosphere and 
thus completes the hydrologic cycle. Evapotranspiration consists of two parts, 
Evaporation and Transpiration. Evaporation is the loss of water molecules 
from soil masses and water bodies. Transpiration is the loss of water from 
plants in the form of vapour. We proceed on to discuss precipitation, and its 
most important component in India context, the rainfall. 
 
 
2.1.1 Causes of precipitation 
For the formation of clouds and subsequent precipitation, it is for necessary 
that the moist air masses to cool in order to condense. This is generally 
accomplished by adiabatic cooling of moist air through a process of being 
lifted to higher altitudes. The precipitation types can be categorized as. 
 
Version 2 CE IIT, Kharagpur 
 
• Frontal precipitation 
This is the precipitation that is caused by the expansion of air on 
ascent along or near a frontal surface. 
 
• Convective precipitation 
Precipitation caused by the upward movement of air which is 
warmer than its surroundings.  This precipitation is generally 
showery nature with rapid changes of intensities. 
 
• Orographic precipitation 
Precipitation caused by the air masses which strike the mountain 
barriers and rise up, causing condensation and precipitation.  The 
greatest amount of precipitation will fall on the windward side of the 
barrier and little amount of precipitation will fall on leave ward side. 
  
For the Indian climate, the south-west monsoon is the principal rainy season 
when over 75% of the annual rainfall is received over a major portion of the 
country. Excepting the south-eastern part of the Indian peninsula and Jammu 
and Kashmir, for the rest of the country the south-west monsoon is the 
principal source of rain. 
 
From the point of view of water resources engineering, it is essential to 
quantify rainfall over space and time and extract necessary analytical 
information. 
 
 
2.1.2 Regional rainfall characteristics 
Rain falling over a region is neither uniformly distributed nor is it constant over 
time. You might have experienced the sound of falling rain on a cloudy day 
approaching from distance. Gradually, the rain seems to surround you and 
after a good shower, it appears to recede. It is really difficult to predict when 
and how much of rain would fall. However it is possible to measure the 
amount of rain falling at any point and measurements from different point 
gives an idea of the rainfall pattern within an area. 
 
In India, the rainfall is predominantly dictated by the monsoon climate. The 
monsoon in India arises from the reversal of the prevailing wind direction from 
Southwest to Northeast and results in three distinct seasons during the course 
of the year. The Southwest monsoon brings heavy rains over most of the 
country between June and October, and is referred to commonly as the ‘wet’ 
season. Moisture laden winds sweep in from the Indian Ocean as low-
pressure areas develop over the subcontinent and release their moisture in 
the form of heavy rainfall. Most of the annual rainfall in India comes at this 
time with the exception of in Tamil Nadu, which receives over half of its rain 
during the Northeast monsoon from October to November. 
 
The retreating monsoon brings relatively cool and dry weather to most of India 
as drier air from the Asian interior flows over the subcontinent. From 
Version 2 CE IIT, Kharagpur 
 
November until February, temperatures remain cool and precipitation low. In 
northern India it can become quite cold, with snow occurring in the Himalayas 
as weak cyclonic storms from the west settle over the mountains. Between 
March and June, the temperature and humidity begin to rise steadily in 
anticipation of the Southwest monsoon. This pre-monsoonal period is often 
seen as a third distinct season although the post-monsoon in October also 
presents unique characteristics in the form of slightly cooler temperatures and 
occasional light drizzling rain. These transitional periods are also associated 
with the arrival of cyclonic tropical storms that batter the coastal areas of India 
with high winds, intense rain and wave activity. 
 
Rainfall and temperature vary greatly depending on season and geographic 
location. Further, the timing and intensity of the monsoon is highly 
unpredictable. This results in a vastly unequal and unpredictable distribution 
over time and space. In general, the northern half of the subcontinent sees 
greater extremes in temperature and rainfall with the former decreasing 
towards the north and the latter towards the west. Rainfall in the Thar Desert 
and areas of Rajasthan can be as low as 200mm per year, whereas on the 
Shillong Plateau in the Northeast, average annual rainfall can exceed 10,000 
mm per year. The extreme southern portion of the country sees less variation 
in temperature and rainfall. In Kerala, the total annual rainfall is of the order of 
3,000 mm. 
 
In this lecture, we discuss about rainfall measurement and interpretation of the 
data.  
 
 
2.1.3 Measurement of rainfall 
One can measure the rain falling at a place by placing a measuring cylinder 
graduated in a length scale, commonly in mm. In this way, we are not 
measuring the volume of water that is stored in the cylinder, but the ‘depth’ of 
rainfall. The cylinder can be of any diameter, and we would expect the same 
‘depth’ even for large diameter cylinders provided the rain that is falling is 
uniformly distributed in space. 
 
Now think of a cylinder with a diameter as large as a town, or a district or a 
catchment of a river. Naturally, the rain falling on the entire area at any time 
would not be the same and what one would get would be an ‘average depth’. 
Hence, to record the spatial variation of rain falling over an area, it is better to 
record the rain at a point using a standard sized measuring cylinder. 
 
In practice, rain is mostly measured with the standard non-recording rain 
gauge the details of which are given in Bureau of Indian Standards code IS 
4989: 2002.  The rainfall variation at a point with time is measured with a 
recording rain-gauge, the details of which may be found in IS 8389: 2003.  
Modern technology has helped to develop Radars, which measures rainfall 
over an entire region.  However, this method is rather costly compared to the 
Version 2 CE IIT, Kharagpur 
 
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