Surface & Ground Water Resources- 2 | Engineering Hydrology - Civil Engineering (CE) PDF Download

Introduction

Water in our planet is available in the atmosphere, the oceans, on land and within the soil and fractured rock of the earth’s crust Water molecules from one location to another are driven by the solar energy.  Moisture circulates from the earth into the atmosphere through evaporation and then back into the earth as precipitation.  In going through this process, called the Hydrologic Cycle (Figure 1), water is conserved – that is, it is neither created nor destroyed.  

Figure 1.  Hydrologic cycle

It would perhaps be interesting to note that the knowledge of the hydrologic cycle was known at least by about 1000 BC by the people of the Indian Subcontinent.  This is reflected by the fact that one verse of Chhandogya Upanishad (the Philosophical reflections of the Vedas) points to the following:

“The rivers… all discharge their waters into the sea. They lead from sea to sea, the clouds raise them to the sky as vapour and release them in the form of rain…” 

The earth’s total water content in the hydrologic cycle is not equally distributed (Figure 2). 

                        Fresh Water 2.5%

 Saline Water in Oceans 97.5%

Figure 2.  Total global water content 

The oceans are the largest reservoirs of water, but since it is saline it is not readily usable for requirements of human survival.  The freshwater content is just a fraction of the total water available (Figure 3)

                             Lakes, Rivers, and Soil Moisture 0.4%

Figure 3.  Global fresh water distribution 

Again, the fresh water distribution is highly uneven, with most of the water locked in frozen polar ice caps.

The hydrologic cycle consists of four key components

1. Precipitation
2. Runoff
3. Storage
4. Evapotranspiration

These are described in the next sections. 

Precipitation

Precipitation occurs when atmospheric moisture becomes too great to remain suspended in clouds.  It denotes all forms of water that reach the earth from the atmosphere, the usual forms being rainfall, snowfall, hail, frost and dew.   Once it reaches the earth’s surface, precipitation can become surface water runoff, surface water storage, glacial ice, water for plants, groundwater, or may evaporate and return immediately to the atmosphere.  Ocean evaporation is the greatest source (about 90%) of precipitation.

Rainfall is the predominant form of precipitation and its distribution over the world and within a country. The former is shown in Figure 4, which is taken from the site of the Global Precipitation Climatology Project (GPCP) is an element of the Global Energy and Water Cycle Experiment (GEWEX) of the World Climate Research program (WCRP).

Figure 4. A typical distribution of global precipitation (Courtesy: Global Precipitation Climatology Project) 

The distribution of precipitation for our country as recorded by the India Meteorological Department (IMD) is presented in the web-site of IMD One typical distribution is shown in Figure 5 and it may be observed that rainfall is substantially non-uniform, both in space and over time.   

India has a typical monsoon climate.  At this time, the surface winds undergo a complete reversal from January to July, and cause two types of monsoon.  In winter dry and cold air from land in the northern latitudes flows southwest (northeast monsoon), while in summer warm and humid air originates over the ocean and flows in the opposite direction (southwest monsoon), accounting for some 70 to 95 percent of the annual rainfall.  The average annual rainfall is estimated as 1170 mm over the country, but varies significantly from place to place.  In the northwest desert of Rajasthan, the average annual rainfall is lower than 150 mm/year.  In the broad belt extending from Madhya Pradesh up to Tamil Nadu, through Maharastra, parts of Andhra Pradesh and Karnataka, the average annual rainfall is generally lower than 500 mm/year.  At the other extreme, more than 10000 mm of rainfall occurs in some portion of the Khasi Hills in the northeast of the country in a short period of four months.  In other parts of the northeast (Assam, Arunachal Pradesh, Mizoram, etc.,) west coast and in sub-Himalayan West Bengal the average annual rainfall is about 2500 mm.  

Except in the northwest of India, inter annual variability of rainfall in relatively low.  The main areas affected by severe droughts are Rajasthan, Gujarat (Kutch and Saurashtra).

The year can be divided into four seasons:

• The winter or northeast monsoon season from January to February.
• The hot season from March to May.
• The summer or south west monsoon from June to September.
• The post – monsoon season from October to December

The monsoon winds advance over the country either from the Arabian Sea or from the Bay of Bengal.  In India, the south-west monsoon is the principal rainy season, which contributes over 75% of the annual rainfall received over a major portion of the country.  The normal dates of onset  (Figure 6) and withdrawal (Figure 7) of monsoon rains provide a rough estimate of the duration of monsoon rains at any region.