Preface Soil and water are the two important natural resources for agricultural production and sustenance of life in this earth planet. Out of these two, water is very important. It is vital to life and development of all concerns in all parts of this world. In the developing countries, it plays a crucial role in agriculture and economic sector. With increasing population coupled with industrialisation and urbanisation, the pressure on water for agriculture and domestic uses is increasing at a fast rate. There is a strong need to optimally allocate water for various uses and at the same time creating innovations in research in increasing water resources potential. Hydrology plays a vital role in the development and management of water resources. The basic inputs in the evaluation of water resources are from hydrological parameters. It therefore, forms a part of curricula at under graduate as well as post graduate level in agriculture, agricultural and civil engineering, meteorology, forestry, environmental engineering, geology and earth sciences. Keeping the above points in view, the authors felt a strong necessity to write a text book in simple and lucid style that can help students who do not have sufficient knowledge and exposure to the subject before. The text book contains a lot of basic knowledge in the field of hydrology. Besides, a number of sample calculations in each chapter are presented in the book which will help the students to understand the subject matter very easily. The various chapters of the book are well designed, written in systematic way and are prepared from the class notes for the students besides utilising long practical field experiences of the authors. The book is written covering syllabus of hydrology and hydrologic engineering which is taught in various state agricultural universities and civil engineering departments in various engineering colleges.

1.1 Water and its Importance Water has played a crucial role in sustainace of lives of all living beings including animals, plants and human beings in this earth since time immemorial. It is there since the creation of the nature and will probably remain till the destruction of the nature. It is used in multifarious uses including domestic, industrial, agricultural, recreational and many other uses. It occurs in the nature in practically three forms i.e. solid, liquid and gaseous forms. All the three forms are important to living beings; but the liquid form of water is more beneficial. The weight of the water helps in running hydroelectric turbines, the density of water helps in floating the ships, the steam energy produced by heating the water helps to run the locomotives and thus is beneficial in many ways. It is considered as the most important resource of the society. It has become part and partial in our life so that we cannot live without water. That is why water is called as life.

2.1 Introduction Precipitation is the general term for all forms of moisture emanating from clouds and then ultimately falling on the land surface. It is the total supply of water derived from the atmosphere in the forms of rain, snow, hail, mist, dew, sleet, fog etc. The liquid form of precipitation is called as rainfall whereas the frozen forms of precipitations are snow, hail and sleet. Of all the different forms of precipitation, rain and snow are the two dominant factors that contribute maximum amount of water. In Indian condition, rainfall is the most important form of precipitation that contributes the highest amount of water causing stream flow. Unless otherwise stated, the term rainfall will be used in this book synonymously with precipitation. It is the primary input vector of the hydrologic cycle. Generally it is expressed as the depth water on a horizontal surface over any specified time interval. If the specified time interval is day, then the amount of precipitation is called as daily precipitation. Similarly, the precipitation being contributed over time interval of a month, season and year is termed as monthly, seasonal or annual precipitation. Its form and quantity are influenced by the action of different climatic factors such as wind, temperature and atmospheric pressure. The study of precipitation forms a major portion of the subject of the hydrometeorology.

3.1 Runoff Runoff is one of the important parameter of hydrologic cycle. It is defined as that portion of precipitation which is not absorbed by the deep soil strata but finds its way into the stream after meeting the demands of evapotranspiration, interception, infiltration, surface storage, and surface as well as channel detention. It includes surface runoff received into the channels after precipitation occurs, delayed runoff that enters the stream after passing through the land surface, and other delayed runoff from the temporarily detained pits, depressions, swamps or detained as snow fall. It is thus the flow collected from the drainage basin and appearing at the outlet of the basin. The runoff process is demonstrated in Fig. 3.1 as flow chart. Details of the runoff process are discussed as below.

4.1 Introduction Precipitation is a major component of hydrological cycle. It occurs because of condensation of water vapour molecules in the atmosphere. When the precipitation falls on the grounds, it undergoes several phases before it appears as runoff in the streams. Certain amount of precipitations is intercepted by the foliage of plants and roofs and other parts of the building and other hydraulic structures. This intercepted water is called as interception loss which is evaporated to the atmosphere. Sometimes the dried surface of the buildings and other structures soak certain amount of precipitation while falling on them which also adds to interception loss. After striking the land masses, certain amount of the precipitated water gets infiltrated into the soil surface which then moves both in lateral as well as in vertical direction. This loss is called as infiltration loss. For the study of groundwater, infiltration loss is not a loss but rather it is a gain in terms of groundwater recharge. However for a surface water hydrologist, it is a loss since it decreases the amount of runoff contributing to the stream or any other water bodies.

5.1 Introduction A steam can be defined as a flow channel in to which the surface runoff flowing over the land surfaces and base flow/ground flow flowing below the ground surfaces meet. It receives the total flow from a specified basin. Streamflow representing the runoff phase of the hydrologic cycle is the most important basic data for hydrologic studies. Interestingly, stream flow is the only part of the hydrologic cycle that can be measured accurately. Stream flow is a historical data and like rainfall and runoff, it is a stochastic hydrologic variable. It is also called as discharge or rate of flow. Discharge is defined as the volume of water flowing through a cross section in a unit time.

6.1 Introduction When rainfall strikes a ground surface, some losses like initial and infiltration will occur. After these losses are fulfilled, runoff will begin. The runoff will ultimately reach a stream. The main routes through which runoff flows are (i) overland flow, (ii) interflow and (iii) groundwater flow. Overland flow which is also called as surface flow flows over the land mass and reaches a stream. Overland flow soon reaches a stream and if it occurs in sufficient quantity, it forms an important element in the formation of flood peaks. In small and moderate storms, there may have predominant initial and infiltration losses resulting minimum amount of surface runoff and this is especially true in case of permeable soils. If the soil is impermeable, then there may have reasonable runoff. However, surface runoff is an important factor in streamflow only for heavy or high intensity rainfall.

7.1 Introduction Flood is an unusually high stage in a river resulted due to high rainfall or snow melt which causes large flow of runoff in it. During flood, the river overflows the banks and inundates the adjoining areas. Flood causes heavy damages to buildings and hydraulic structures besides causing catastrophic loss to human life and property. It causes great economic loss due to disruption of many structures and damage to agricultural fields. Many forest and aquatic lives are affected by the flood. Hundreds of crores of rupees are spent every year to control, forecast and manage the flood.

8.1 Introduction The knowledge of statistics and probability is very helpful for correct prediction of hydrological data and correct prediction of hydrological data is useful in giving good performance in all water resources projects. Since hydrological data are highly stochastic varying both in space and time, it is essential to predict them as correct as possible. The hydrologic evens do not depend on physical and chemical laws but depend entirely on nature. They cannot be predicted exactly by using any relationship but can be predicted at different probability levels by fitting suitable probability distribution functions. Application of statistics and probability in the field of hydrology can help to predict the hydrologic events in a better way. In hydrology, objectives of statistics are as follows (Suresh, 2008):

9.1 Introduction Flood water which is in the form of a hydrograph when passes down a river section undergoes several changes in its form. There may be some addition of water to the river section when the flood wave passes down the river section. There may be some diversion of water from the section. Thus, depending on the addition or abstraction of water to the incoming flood water, shape of the flood hydrograph in the downstream section may change. Sometimes channel storage and resistance to flow of water in the channel section also affects the shape of the flood hydrograph.

10.1 Introduction During rain or heavy irrigation, a part of the rainfall or irrigated water moves below the effective root zone of the crop which moves further below and adds to the water table lying above the impermeable bed or hard rock which is called as  ground  water  reservoir. At  times  the  accumulated  surface  water  is  also recharged to the soil reservoir both naturally as well as artificially that add to the underlying ground water reservoir. This groundwater is an important source of water for irrigation as well as domestic uses. It is the most abundant fresh water resource of the earth. Underground water reservoirs contain large stock of fresh water resources. Total quantity of groundwater within 800 m distance below  the  ground  surface  is  more than  30  times  the  total  water  in  all fresh water lakes, more than 60 times the total water in soil and other unsaturated rock materials, more than 300 times the water vapour in the atmosphere and 3000 times the average volume of all rivers and rivulets in the world (Lenka, 2001). But the distribution of groundwater is not uniform.

11.1 Introduction The laws governing the flow of water in wells are referred to as the hydraulics of wells. Just like surface water flows from a higher elevation to a lower elevation by gravity, groundwater also flows from a region of higher water table to a region of lower water table. This difference of water table measured by any two piezometers divided by the longitudinal distance between them is called as hydraulic gradient. Hydraulic gradient plays a dominant role in deciding the flow of groundwater. This hydraulic gradient together with the hydraulic conductivity of the aquifer material forms the main law of well hydraulics called as Darcy’s law. An understanding of Darcy’s law together with some other laws as discussed subsequently in this chapter is essential in order to know the discharge of a well in a particular formation. Darcy’s law is valid for studying groundwater flow in alluvial formations. However, it is not valid for aquifers having hard rock areas.