Hydrology Learning Packet 1 PDF

1 | Hydrology 1 UNIT 1: INTRODUCTION TO HYDROLOGY The first unit of this module intends to generally introduce what hydrology is. Specifically, it is designed for the students to attain the following learning...

1 | Hydrology 1 UNIT 1: INTRODUCTION TO HYDROLOGY The first unit of this module intends to generally introduce what hydrology is. Specifically, it is designed for the students to attain the following learning outcomes: ▪ define what hydrology is; ▪ identify significant periods in the advancement of hydrology; ▪ distinguish the different processes of hydrological cycle; ▪ solve problems related to water budget equation; ▪ identify the significant features of global water balance studies; ▪ identify the applications of hydrology in engineering; and ▪ classify the sources of hydrological data. 1.1 DEFINITION OF HYDROLOGY Hydrology is the science of water. Considering its etymology, the word hydrology is taken from the Greek words, hydro and logo, which means water and study, respectively (Jain & Singh, 2019). Subramanya (2008) stated that hydrology is the science that deals with the occurrence, circulation, and distribution of water of the earth and earth’s atmosphere. He further emphasized that, as a branch of earth science, it is concerned with the water in streams and lakes, rainfall and snowfall, snow and ice on the land and water occurring below the earth’s surface in the pores of the soil and rocks. Thus, according to him, it is a very broad subject of an inter-disciplinary nature drawing support from allied sciences, such as meteorology, geology, statistics, chemistry, physics and fluid mechanics. However, Subramanya (2008) also noted that hydrology is basically an applied science which is sometimes classified as to its degree of applicability—scientific hydrology or engineering or applied hydrology. The former is mainly concerned with the academic aspects of the subject, whereas the latter deals with engineering applications. Moreover, in a general sense, he described engineering hydrology as the one that deals with the following areas: i. estimation of water resources; ii. the study of processes such as precipitation, runoff, evapotranspiration and their interaction; and iii. the study of problems such as floods and droughts, and strategies to combat them. 1.2 BRIEF HISTORY OF HYDROLOGY Ancient philosophers focused their attention on the nature of processes involved in the production of surface water flows and other phenomena related to the origin and occurrence of water in various stages of the perpetual cycle of water being conveyed from the sea to the atmosphere to the land and back again to the sea. Unfortunately, early speculation was often faulty. For example, Homer believed in the existence of a large subterranean reservoirs that supplied rivers, seas, springs, and deep wells. It is interesting to note, however, that Homer understood the dependence of flow in the Greek aqueducts on both conveyance cross section and velocity. This knowledge was lost to the Romans, and the proper relation between area, velocity, and rate of flow remained unknown until Leonardo da Vinci rediscovered it during the Italian Renaissance.  1 | Hydrology 2 During the first century B.C., Marcus Vitruvius, in Volume 8 of his treatise De Architectura Libri Decem (the engineer's chief handbook during the Middle Ages), set forth a theory generally considered to be the predecessor of modern notions of the hydrologic cycle. He hypothesized that rain and snow falling in mountainous areas infiltrated the earth's surface and later appeared in the lowlands as streams and springs. In spite of the inaccurate theories proposed in ancient times, it is only fair to state that practical application of various hydrologic principles was often carried out with considerable success. For example, about 4000 B.C., a dam was constructed across the Nile to permit reclamation of previously barren lands for agricultural production. Several thousand years later a canal to convey fresh water from Cairo to Suez was built. Mesopotamian towns were protected against floods by high earthen walls. The Greek and Roman aqueducts and early Chinese irrigation and flood control works were also significant projects. Near the end of the fifteenth century, the trend toward a more scientific approach to hydrology based on the observation of hydrologic phenomena became evident. Leonardo da Vinci and Bernard Palissy independently reached an accurate understanding of the water cycle. They apparently based their theories more on observation than on purely philosophical reasoning. Nevertheless, until the seventeenth century it seems evident that little of any effort was directed toward obtaining quantitative measurements of hydrologic variables. The advent of what might be called the "modern" science of hydrology is usually considered to begin with the studies of such pioneers as Perrault, Mariotte, and Halley in the seventeenth century. Perrault obtained measurements of rainfall in the Seine River drainage basin over a period of 3 years. Using these and measurements of runoff, and knowing, the drainage area size, he showed that rainfall was adequate in quantity to account for river flows. He also made measurements of evaporation and capillarity. Mariotte gauged the velocity of flow of the River Seine. Recorded velocities were translated into terms of discharge by introducing measurements of the river cross section. The English astronomer Halley measured the rate of evaporation of the Mediterranean Sea and concluded that the amount of water evaporated was sufficient to account for the outflow of rivers tributary to the sea. Measurements such as these, although crude, permitted reliable conclusions to be drawn regarding he hydrologic phenomena being studied. The eighteenth century brought fourth numerous advances in hydraulic theory and instrumentation. The Bernoulli piezometer, the Pitot tube, Bernoulli’s theorem, and the Chézy formula are some examples. During the nineteenth century, experimental hydrology flourished. Significant advances were made in groundwater hydrology and in the measurement of surface water. Such significant contributions as Hagen-Poiseuille’s capillary flow equation, Darcy’s law of flow in porous media, and Dupuit-Thiem well formula were evolved. The beginning of systematic stream gauging can also be traced to this period. Although the basis for modern hydrology was well established in the nineteenth century, much of the effort was empirical in nature. The fundamentals of physical hydrology had not yet been well established or widely recognized. In the early years of the twentieth century the inadequacies of many earlier empirical formulations became well known. As a result, interested governmental agencies began to develop their own programs of hydrologic research. From about 1930 to 1950, rational analysis began to replace empiricism. Sherman's unit hydrograph,  1 | Hydrology 3 Horton's infiltration theory, und Theis's nonequilibrium approach to well hydraulics are outstanding examples of the great progress made. Since 1950, a theoretical approach to hydrologic problems has largely replaced less sophisticated methods of the past. Advances in scientific knowledge permit a better understanding of the physical basis of hydrologic relations, and the advent and continued development of high- speed digital computers have made possible, in both – a practical and an economic sense, extensive mathematical manipulations that would have been overwhelming in the past. 1.3 HYDROLOGIC CYCLE Subramanya (2008) suggested that a convenient starting point to describe the cycle is in the oceans where the water evaporates due to the heat energy supplied by solar radiation, then clouds are created as water vapor ascends. A portion of the clouds are driven to the land by winds, while the majority of the clouds condense and fall back to the oceans as rain. There they cool and precipitate as rain, snow, hail, sleet, etc. onto the land mass. Even as it is falling, some of the precipitation may evaporate back into the atmosphere. A different portion might be intercepted by vegetation, structures, and other surface modifications, where it might either evaporate back into the atmosphere or descend to the ground's surface. A part of the water that enters the ground through infiltration increases the soil's moisture content and eventually reaches the groundwater body. Through the process of transpiration, vegetation releases some of the water from below the ground to the atmosphere. The precipitation that reaches the ground surface after supplying infiltration and evaporation demands travels across the surface along the natural slope and through a system of gullies, streams, and rivers to the ocean. After sitting for a lot longer than the surface flow, the groundwater may rise to the surface through springs and other outlets. Runoff is the portion of precipitation that travels through various routes above and below the earth's surface to reach the stream channel. Runoff changes to stream flow after it enters a stream channel. The figure shown below is the schematic representation of the hydrologic cycle. Moreover, it depicts the sequence of processes as discussed above. Figure 1.1 The Hydrologic Cycle  1 | Hydrology 4 Additionally, Subramanya (2008) stated that each path of the hydrologic cycle includes one or more of the following elements: (i) transportation of water, (ii) temporary storage and (iii) change of state. For instance, there are vapor movement, vapor storage, and condensation during the atmospheric phase (phase change from vapor to the liquid or solid state). Similarly, water exists in the form of vapor in the atmosphere while primarily existing as saline liquid in the oceans. Futhermore, he broadly classified the main components of the hydrologic cycle as transportation (flow) components and storage components as presented in the following table: Table 1.1 Main Components of Hydrologic Cycle Studying the various processes of hydrologic cycle, including evaporation, precipitation, and groundwater flow, enables one to research hydrological science in a systematic manner. One also recognizes that practically every aspect of the hydrologic cycle is susceptible to human interference, including the creation of artificial rain, the suppression of evaporation, changes in vegetation and land use, the extraction of groundwater, etc. Interference at one point in the cycle can have serious consequences at another. Several sectors, including agriculture, forestry, geography, economics, sociology, and politics are significantly impacted by the hydrological cycle. The design and operation of projects dealing with water supply, irrigation and drainage, water power, flood control, navigation, coastal works, salinity control, and recreational uses of water all involve engineering applications of the hydrologic cycle knowledge, and consequently of the subjects of hydrology. 1.4 WATER BUDGET EQUATION The continuity principle, also known as the water budget equation or hydrologic equation, can be used to describe the quantities of water that move through the various individual channels of the hydrological cycle in a particular system. It is significant to remember that the earth's total water supply is constant and that the hydrologic cycle gets its energy from the sun. Meanwhile, the geographical area that at a specific location drains into a stream or other water course is known as the catchment area. It may also be referred to as a drainage basin or drainage area. It is known as a watershed in the USA. However, a ridge known as a divide in the United States and a watershed in the UK distinguishes a catchment area from its neighboring areas. In identifying the catchment and measuring its overall extent, the ridge on a topographic map is traced using a planimeter. The following figures on the succeeding page show what a ridge of a catchment area is ̶ traced by broken red lines ̶ (Figure 1.2) and how a planimeter looks like (Figure 1.3).  1 | Hydrology 5 RIDGE Figure 1.2 The Catchment Area and its Ridge Figure 1.3 The Planimeter Set For a given problem area, say a catchment, in an interval of time (Δt), the continuity equation for water in its various phases is written as Mass Inflow - Mass Outflow = Change in Mass Storage If the density of the inflow, outflow and storage volumes are the same Vi - V0 = ΔS (1.1) where Vi = inflow volume of water into the problem area during the time period, V0 = outflow volume of water from the problem area during the time period, and ΔS = change in the storage of the water volume over and under the given area during the given period. In applying this continuity equation [Eq. (1.1)] to the paths of the hydrologic cycle involving change of state, the volumes considered are the equivalent volumes of water at a reference temperature. In hydrologic calculations, the volumes are often expressed as average depths over the catchment area. Thus, for  1 | Hydrology 6 example, if the annual stream flow from a 10 km2 catchment is 107 m3, it corresponds to a depth of 107 (10×106 )= 1 m = 100 cm. Rainfall, evaporation and often runoff volumes are expressed in units of depth over the catchment. While realizing that all the terms in a hydrological water budget may not be known to the same degree of accuracy, an expression for the water budget of a catchment for a time interval Δt is written as P - R - G - E - T = ΔS (1.2-a) In this P = precipitation, R = surface runoff, G = net groundwater flow out of the catchment, E = evaporation, T = transpiration and ΔS = change in storage. The storage S consists of three components as S = Ss + Ssm + Sg where Ss = surface water storage; Ssm = water in storage as soil moisture; and Sg = water in storage as groundwater. Thus in Eq. (1.2-a) ΔS = ΔSs + ΔSsm + ΔSg All terms in Eq. (1.2-a) have the dimensions of volume. Note that all these terms can be expressed as depth over the catchment area (e.g. in centimetres), and in fact this is a very common unit. In terms of rainfall-runoff relationship, Eq. (1.2-a) can be represented as R=P-L (1.2-b) where L = Losses = water not available to runoff due to infiltration (causing addition to soil moisture and groundwater storage), evaporation, transpiration and surface storage. Details of various components of the water budget equation are discussed in subsequent chapters. Note that in Eqs. (1.2-a and b) the net import of water into the catchment, from sources outside the catchment, by action of man is assumed to be zero.  1 | Hydrology 7 1.5 WORLD WATER BALANCE The total quantity of water in the world is estimated to be about 1386 million cubic kilometers (M km3). About 96.5% of this water is contained in the oceans as saline water. Some of the water on the land amounting to about 1% of the total water is also saline. Thus, only about 35.0 M km3 of fresh water is available. Out of this about 10.6 M km3 is both liquid and fresh and the remaining 24.4 M km3 is contained in frozen state as ice in the polar regions and on mountain tops and glaciers. An estimated distribution of water on the earth is given in Table 1.2.  1 | Hydrology 8 Table 1.2 Estimated World Water Quantities The global annual water balance is shown in Table 1.3. Table 1.3 Global Annual Water Balance It is seen from Table 1.3 that the annual evaporation from the world’s oceans and inland areas are 0.505 and 0.072 M km3 respectively. Thus, over the oceans about 9% more water evaporates than that falls back as precipitation. Correspondingly, there will be excess precipitation over evaporation on the land mass. The differential, which is estimated to be about 0.047 M km3 is the runoff from land mass to oceans and groundwater outflow to oceans. It is interesting to know that less than 4% of this total river flow is used for irrigation and the rest flows down to sea. These estimates are only approximate and the results from different studies vary; the chief cause being the difficulty in obtaining adequate and reliable data on a global scale. The volume in various phases of the hydrologic cycle (Table 1.2) as also the rate of flow in that phase (Table 1.3) do vary considerably. The average duration of a particle of water to pass through a phase of the hydrologic cycle is known as the residence time of that phase. It could be calculated  1 | Hydrology 9 by dividing the volume of water in the phase by the average flow rate in that phase. For example, by assuming that all the surface runoff to the oceans comes from the rivers, From Table 1.2, the volume of water in the rivers of the world = 0.00212 M km3 From Table 1.3, the average flow rate of water in global rivers = 44, 700 km3/year Hence, residence time of global rivers, Tr = 2120/44700 = 0.0474 year = 17.3 days. Similarly, the residence time for other phases of the hydrological cycle can be calculated (Prob. 1.6). It will be found that the value of Tr varies from phase to phase. In a general sense the shorter the residence time the greater is the difficulty in predicting the behavior of that phase of the hydrologic cycle. Annual water balance studies of the sub-areas of the world indicate interesting facts. The water balance of the continental land mass is shown in Table 1.4(a). It is interesting to see from this table that Africa, in spite of its equatorial forest zones, is the driest continent in the world with only 20% of the precipitation going as runoff. On the other hand, North America and Europe emerge as continents with highest runoff. Extending this type of analysis to a smaller land mass, viz. the Indian subcontinent, the long-term average runoff for India is found to be 46%. Table 1.4(a) Water Balance of Continents mm/year Water balance studies on the oceans indicate that there is considerable transfer of water between the oceans and the evaporation and precipitation values vary from one ocean to another (Table 1.4(b)). Table 1.4(a) Water Balance of Oceans mm/year Each year the rivers of the world discharge about 44, 700 km3 of water into the oceans. This amounts to an annual average flow of 1.417 M m3/s. The world’s largest river, the Amazon, has an annual average discharge of 200,000 m3/s, i.e. one-seventh of the world’s annual average value.  1 | Hydrology 10 India’s largest river, the Brahmaputra, and the second largest, the Ganga, flow into the Bay of Bengal with a mean annual average discharges of 16,200 m3/s and 15,600 m3/s respectively. 1.6 APPLICATIONS IN ENGINEERING Hydrology finds its greatest application in the design and operation of water-resources engineering projects, such as those for (i) irrigation, (ii) water supply, (iii) flood control, (iv) water power, and (v) navigation. In all these projects hydrological investigations for the proper assessment of the following factors are necessary: 1. The capacity of storage structures such as reservoirs. 2. The magnitude of flood flows to enable safe disposal of the excess flow. 3. The minimum flow and quantity of flow available at various seasons. 4. The interaction of the flood wave and hydraulic structures, such as levees, reservoirs, barrages and bridges. The hydrological study of a project should necessarily precede structural and other detailed design studies. It involves the collection of relevant data and analysis of the data by applying the principles and theories of hydrology to seek solutions to practical problems. Many important projects in the past have failed due to improper assessment of the hydrological factors. Some typical failures of hydraulic structures are: (i) overtopping and consequent failure of an earthen dam due to an inadequate spill- way capacity, (ii) failure of bridges and culverts due to excess flood flow and (iii) inability of a large reservoir to fill up with water due to overestimation of the stream flow. Such failure, often called hydrologic failures underscore the uncertainty aspect inherent in hydrological studies. Various phases of the hydrological cycle, such as rainfall, runoff, evaporation and transpiration are all nonuniformly distributed both in time and space. Further, practically all hydrologic phenomena are complex and at the present level of knowledge, they can at best be interpreted with the aid of probability concepts. Hydrological events are treated as random processes and the historical data relating to the event are analyzed by statistical methods to obtain information on probabilities of occurrence of various events. The probability analysis of hydrologic data is an important component of present-day hydrological studies and enables the engineer to take suitable design decisions consistent with economic and other criteria to be taken in a given project. 1.7 SOURCES OF HYDROLOGICAL DATA Depending upon the problem at hand, a hydrologist would require data relating to the various relevant phases of the hydrological cycle playing on the problem catchment. The data normally required in the studies are: Weather records (such as temperature, humidity and wind velocity); Precipitation data; Stream flow records; Evaporation and evapotranspiration data;  1 | Hydrology 11 Infiltration characteristics of the study area; Soils of the area; Land use and land cover; Groundwater characteristics; Physical and geological characteristics of the area; and Water quality data.  1 | Hydrology 12 1.8 ASSESSMENT 1 | Hydrology 13  1 | Hydrology 14 1.9 REFERENCES Subramanya, K (2008). Engineering Hydrology (3rd ed.). Tata McGraw-Hill Publishing Company Limited. Viessman, W., & Lewis, G. L. (2014). Introduction to hydrology. PHI Learning Private. https://www.wikiwand.com/en/Planimeter https://www.youtube.com/watch?v=eAf3VjQTV44