Applied Hydrometeorology

Water, whose importance is rated next to air, is the world’s most precious natural resource and is vital for all forms of life. It generally refers to that part of fresh water which is renewable annually and includes surface water, soil water and underground water. Atmospheric precipitation (rainfall, snowfall, and other forms) constitutes the source of all fresh water on the earth. Rainfall, the primary source of water that occurs as a result of condensation of atmospheric moisture, is governed by the science of meteorology and, therefore, is considered to be one of the most important meteorological elements. Similarly, the occurrence and distribution of rainwater on and beneath the earth’s surface is governed by the science of hydrology and geology. Both meteorology and hydrology are concerned with the hydrologic cycle (also sometimes referred to as water cycle) dealing with the movement and interchange of water between the oceans, the atmosphere and the earth. The striking elements of the hydrologic cycle, which are generally observed and recorded for purposes of water development, are meteorological elements, such as rainfall and evaporation, as well as hydrologic elements of lake and river levels, river flow, infiltration, and ground water.

The atmosphere is closely related to hydrology in a fundamental manner. It will, therefore, be appropriate to make a brief introduction to the subject of atmosphere. As pointed out in chapter 1, atmosphere is a thin shell of gases, which is held close to the earth by the gravitational attraction and is commonly called the air. These gases seem to have originated slowly over the geological ages from the interior of the solid earth by large scale volcanic activity. The atmosphere of the earth, therefore, consists of gases, water vapor as well as solid and liquid particles. All of the weather phenomena of the earth, such as winds, precipitation, clouds, haze, mist, fog, cyclones and anticyclones, tornados, thunderstorms, fronts and so on, are caused in this layer as a result of solar energy transfers and transformations that take place within the earth-atmosphere system. These weather phenomena associated with solar energy obey the laws of physics. It is necessary, therefore, to understand the relevant physical laws that govern the earth-atmosphere system. This chapter discusses the fundamental properties of the atmosphere.

Atmosphere is the most dynamic part of the terrestrial environment. It is driven by the energy received from the sun. Almost all weather phenomena mentioned in Chapter 2 result from the differences in the amount of solar energy received and utilization thereof. It is, therefore, necessary to understand as to how the energy from the sun is converted into heat and shared by the earth and the atmosphere. Thermodynamics provides quantitative relationships between heat and other forms of energy and can thus be utilized to analyze the ways in which changes in the heat content of a substance affects the dynamics of the substance. A typical thermodynamic process involves the addition of heat to a fluid causing pressure and volume to change. The laws of thermodynamics relate changes in heat content, pressure and volume. Thermodynamics has, therefore, wide applications in hydrometeorology. This chapter briefly discusses physical laws which are important for understanding variations in heat, temperature, pressure and density of air which are needed for understanding the various atmospheric processes.

Hydrologic characteristics of an area depend primarily on its climate and geology. Climate is particularly important, because it determines the distribution of rain water for agriculture, hydropower generation, and other human activities with significant economic consequences. Hence, it is essential to have a basic knowledge of the factors that make our climate what it is.

The theme of this chapter is to provide a general discussion of weather systems, such as Inter Tropical Convergence Zone (ITCZ), jet streams, cyclones and anticyclones, tropical depressions, extra tropical cyclones, fronts, trade winds, monsoon winds and easterly waves. Radiation, temperature, wind and pressure are the fundamental factors that govern these weather systems. All these factors are inter-related. Consequently, in the study of weather systems the knowledge of radiation and temperature as well as pressure belts and wind systems of the world is obviously of prime importance. Radiation and temperature were discussed in Chapter 4. Before discussing weather systems, a short discussion of the major pressure belts and wind systems of the earth is given first.

Currently, water is used in five major sectors: domestic (including drinking water), agriculture, industry, power generation and environmental conservation. As a result, sustainable access to safe water and improved sanitation has become an urgent issue in many countries of the world. We know that precipitation brings water to the earth’s surface and the occurrence of precipitation can be considered to be a process of weather systems. More detailed discussion on various weather systems that produce precipitation have been presented in the previous chapter. The theme of this chapter, therefore, is to provide an overview of weather systems and precipitation in the context of an ideal region to get a realistic picture of the status of water availability.

The tropical disturbances having wind speeds below 63 knots are known as depressions, cyclonic storms and severe cyclonic storms. More intense cyclonic storms with winds over 63 knots around a low pressure center spirally form over the tropical oceans at latitudes between 7° and 15°. Such systems are known as cyclones in India, hurricanes in North America and the Caribbean area, and typhoons in Japan. After their formation they start moving at a speed of around 15 km per hour generally to the west over the open waters of the oceans. The tropical cyclones produce heavy clouds, rough seas and very heavy rainfall. A well-developed hurricane is a hazard to ships in its proximity as well as to the coastal area where it strikes. The storm piles up a huge sheet of sea water in its forward sector which can inundate low lying coastal areas causing large scale death and destruction. This is known as storm surge, which can be 80 km wide and four meters deep and is the most devastating feature associated with a hurricane. Nine out of ten hurricane fatalities are caused by storm surges. For example, the infamous tropical cyclone, which struck Bangladesh in November 1970, was responsible for killing nearly 300,000 people as a result of storm surge, while hurricane Mitch that struck central America in October, 1998, caused fatalities in excess of 11000 and the total damage was worth millions of U.S. dollars.

Weather systems of the tropics and extra tropics generate heavy to very heavy rainfall for periods of days in various parts of the world, as discussed in Chapter 5. The heavy rainfall events may last 3 to 4 days and are responsible for causing floods, landslides, levee breaches, dams overtopping, sedimentation, erosion and other such occurrences. Flooding from some of the abnormally heavy rainfall events have destroyed dams, causing enormous loss of life and property damage. This happened in India in 1979 when a heavy rainfall event over the Machhu River catchments totally destroyed the Machhu-2 dam on 11 August. The disaster killed as many as 2000 people and totally devastated urban and rural property downstream of the dam (Purohit et al., 1993). Another instance is the catastrophic flood generated from a heavy rainfall event caused by typhoon Nina of August 1975 in the Hongru River basin in China which destroyed the Banqiao and Shimantan dams (Tan and Lu, 1994). There are many dam failures (see Table 8.11) caused by flooding due to heavy rains in various parts of the world (Lemperiere, 1993). In fact, the first recorded dam failure in the USA occurred on 16 May, 1874 in Williamsburg, Massachusetts. It is evident from the above examples that tropical and extra tropical storms and associated heavy rainfalls cause floods which are undoubtedly one of the mightiest and most devastating forces of nature. However, there are ways to prevent or minimize the impacts of flood disasters from heavy rainfall on people and property by adopting the following measures:

The term precipitation denotes all forms of water that reach the earth from the atmospheric weather systems. In other words, precipitation is the quantity of naturally available water. Precipitation, however, exhibits marked variability in time and space owing to meteorological causes. As a result, the annual total precipitation may range from half of the normal in one year to twice the normal the next year. Also, we have noted a large spatial variation in the distribution of seasonal and annual rainfall of India, as shown in Figs 6.22 to 6.26 (Chapter 6). This variability in precipitation is responsible for many hydrological problems, such as floods, droughts, land slides, erosion, etc. The study of precipitation therefore forms a major portion of the subject of applied hydrometeorology. This chapter deals with various forms and types of precipitation and in particular with the measurement and analysis of precipitation data for their use in hydrological problems.

Construction of dams and reservoirs across rivers for collecting and storing the runoff water to serve the needs of the people has been in vogue for many centuries. It was known that these dams often breached due to overtopping or seepage instability. Later, the discovery of concrete in the late 19^th century gave a tremendous push to the dam construction work throughout the world. In all these dams, a spillway is built that not only allows water to pass over the dam structure in normal day to day operation but is also a safety device to pass the largest flood discharge arising from heavy rainfall after the reservoir is filled so that the dam would not be damaged. The magnitude of the largest flood, called the spillway design flood, was earlier based on professional judgement and historical flood marks. As a consequence, the largest flood that would be expected at the dam site was estimated from flood marks and the spillway capacity was provided accordingly. In the later part of the 19^th century, the magnitude of the spillway design flood was determined by empirical formulae relating discharge to drainage area in the form Q = K A ⁿ , where Q is the peak flood discharge, A is the drainage area, K is an empirical coefficient depending upon the rainfall-runoff characteristics of the drainage area, and n is a constant whose value usually lies between 0.5 and 1.0. The empirical formulae were based on the catchment area, with the assumption of constant coefficients. The degree of conservatism implied by these formulae was obviously unknown. The dams based on these formulae were often breached due to inadequate spillway capacities as pointed out in Chapter 8.

It has been noted in Chapter 10 that an important problem in hydrometeorology is to estimate the frequency of maximum rainfall or design storm rainfall of a specified duration that is likely to occur at a selected station or in a selected river catchment for designing hydraulic structures subject to flooding, such as bridges, culverts and dams. In the same chapter it was also mentioned that the statistical method is one of the four methods for estimating design storms. In this method an estimate of the frequency with which a given magnitude of rainfall may be exceeded in the future is based upon the study of the frequency with which it has been exceeded in the past in a probabilistic sense.

Precipitation or rainfall is the major factor which influences hydrological and agricultural activities of any region, but rainfall is highly dependent on atmospheric weather systems. The weather systems thus play a determinant role in the production of rainfall. Forecast of rainfall is of considerable importance to hydrologists and agriculturists for making operational decisions. As mentioned in Chapter 1 under the scope of hydrometeorology, rainfall forecast is an important input in river flow warning as well as in efficient operation of multipurpose dams. It is well known that rainfall forecast is intimately linked with the safety of dams. In the case of a heavy rainfall warning the magnitude of inflow is determined and the reservoir is operated to accommodate the incoming flood waters. Because rainfall is a highly variable weather element, one of the important aspects of rainfall forecast is to identify those physical or natural factors which have the largest influence over rainfall. If these factors that control the rainfall of any region are identified then they can be used to provide a forecast of the behavior of rainfall well in advance. For example, there is a relation between the pressure in South America and subsequent rainfall in India. If the pressure is unusually high in Argentina and Chile during March and April there is likely to be a heavy monsoon rainfall in India in the following months of July and August.

It has already been stated that precipitation in the form of rain and snow is the major source of water over the surface of the earth. Of this water, a sizable portion is lost as water vapor to the atmosphere by evaporation and transpiration. For example, of the 1170 mm of water that is received annually as precipitation in India about 597 mm (51%) of water is lost to the atmosphere through evaporation and transpiration and only the remainder is available to streams and soils and underground formations. In the United States, of the 750 mm of the average yearly precipitation received, about 70% is evaporated, thus reducing the total water obtained by about three fourth. These data illustrate the importance of evaporation for water resources development and water conservation. In countries where surface water storage is important over large areas, evaporation is even more significant.

Worldwide the general connotation of the term drought in any area is significant shortage of water from the lack of rainfall. The rainfall in a given area is not the same every year and as discussed earlier it may range from half of the normal in one year to twice the normal the next year. This causes either too much of water from high rainfall resulting in floods or too less water from low rainfall resulting in droughts. The periodic cycle of floods and droughts is a bane to economy. In this chapter, we study abnormally low rainfall leading to droughts and in the subsequent chapter we will discuss abnormally high rainfall leading to floods.

Extreme hydrometeorological events that arise from unusually high and low precipitation are characterized by floods and droughts, respectively. In the previous chapter we have discussed unusually low rainfall leading to droughts. Apparently, floods and droughts are weather related phenomena and because of the differences in climate, topography and precipitation, their characteristics, such as magnitudes, duration of occurrence, time of occurrence, frequency of occurrence, number of occurrences, and time interval between occurrences, significantly vary from region to region. The damages caused by these weather events in terms of the loss of life, persons affected, environmental damage, social disruption, and economic losses are all too known. It has been estimated that in the United States, the average annual flood losses is more than four billion dollars and for India, it is of the order of seven billion Indian Rupees. It can, therefore, be said that floods are serious problems in virtually every country of the world. In order to control them and keep damages down to the lowest possible level, their prediction is fundamental. In this chapter we discuss floods and their hydrometeorology for understanding why we have floods?