Stream Flow

Flow Characteristics of a Stream

The flow characteristics of a stream depend upon (i) the intensity and duration of rainfall besides spatial and temporal distribution of the rainfall, (ii) shape, soil, vegetation, slope and drainage network of the catchment basin and (iii) climatic factors influencing evapotranspiration. Based on the characteristics of yearly hydrograph (graphical plot of discharge versus time in chronological order), one can classify streams into the following three types.

i) Perennial streams which have some flow, at all times of a year due to considerable amount of base flow into the stream during dry periods of the year. The stream bed is lower than the ground water table in the adjoining aquifer (i.e., water bearing strata which is capable of storing and yielding large quantity of water).

Fig. 1 Temporal Variation of Discharge in Perennial Streams

ii) Intermittent streams have limited contribution from the ground water and that too during the wet season only when the ground water table is above the stream bed and there is base flow contributing to the stream flow. Excepting for some occasional storm that can produce short duration flow, such streams remain dry for most of the dry season periods of a year.

Fig. 2. Temporal Variation of Discharge in Intermittent Streams

iii) Ephemeral streams do not have any contribution from the base flow. The annual hydrograph of such a stream shows series of short duration hydrographs indicating flash flows in response to the storm and the stream turning dry soon after the end of the storm. Such streams, generally found in arid zones and it do not have well defined channels.

Fig. 3 Temporal Variation of Discharge in Ephemeral Streams

Streams are also classified as effluent (streams receiving water from ground water storage) and influent (streams contributing water to the ground water storage) streams. Effluent streams are usually perennial while the influent streams generally remain dry during long periods of dry spell.

Graphical Representation of Stream Flow

The stream flow data are usually recorded in tabular form. For analyzing these data, one has to prepare graphical plots of the stream flow data such as hydrograph, flow-duration curve, flow-mass curve or simply mass curve etc. Hydrograph is a graphical plot between discharge (on y-axis) and the corresponding time (days or months or even hours).

Flow-Mass Curves

Flow-mass curve or runoff-mass curve or inflow mass curve or simply mass curve is cumulative flow volume ‘V’ versus time curve. The mass curve ordinate V (m³ or ha.m or cumec-day) at any time t (in days or weeks or months) is given as

where, t0 is the time at the beginning of the curve.

The mass curve is an integral (i.e., summation) curve of a given hydrograph. Also, slope of the mass curve at any point on the plot i.e., dV/dt equals the rate of stream flow (i.e., stream discharge) at that time. Mass curve is always a rising curve or horizontal (when there is no inflow or runoff added into the stream) and is a useful means by which one can calculate storage capacity of a reservoir to meet specified demand as well as safe yield of a reservoir of given capacity.

Fig. 4 Reservoir Capacity from Mass-Flow Curve

Slope of the cumulative demand curve (usually a line since the demand rate is generally constant) is the demand rate which is known. The reservoir is assumed to be full at the beginning of a dry period (i.e., when the withdrawal or demand rate exceeds the rate of inflow into the reservoir) such as A in Fig. 4. Draw line AD (i.e., demand line) such that it is tangential to the mass curve at A and has a slope of the demand rate, between A and B (where there is maximum difference between the demand line and the mass curve) the demand is larger than the inflow (supply) rate and the reservoir storage would deplete. Between B and D, the supply rate is higher than the demand rate and the reservoir would get refilled. The maximum difference in the ordinates of the demand line and mass curve between A and D (i.e., BC) represents the volume of water required as storage in the reservoir to meet the demand from the time the reservoir was full i.e., A in Fig. 4. If the mass curve is for a large time period, there may be more than one such duration of dry periods which obtain the storages required for those durations (EH and IL). The largest of these storages (BC, FG and JK) is the required storage capacity of the reservoir to be provided on the stream in order to meet the demand.

For determining the safe yield of (or maintainable demand by) a reservoir of given capacity one needs to draw tangents from the apex points (A, E and I of Fig. 4) such that the maximum difference between the tangent and the mass curve equals the given capacity of the reservoir. The slopes of these tangents equals to the safe yield for the relevant dry period. The smallest slope of these slopes is the firm dependable yield of the reservoir. It should be noted that a reservoir gets refilled only if the demand line intersects the mass curve. Non-intersection of the demand line with the mass curve indicates inflow which is insufficient to meet the given demand. Also, the vertical difference between points D and E represents the spilled volume of water over the spillway.

The losses from reservoir (such as due to evaporation and seepage into the ground or leakage) in a known duration can either be included in the demand rates or deducted from inflow rates. In practice, demand rates for irrigation, power generation or water supply vary with time. For such situations, mass curve of demand is superposed over the flow-mass curve with proper matching of time. If the reservoir is full at the first intersection of the two curves, the maximum intercept between the two curves represents the required storage capacity of the reservoir to meet the variable demand.

Flow-Duration Curve

Flow-duration curve (or discharge-frequency curve) of a stream is a graphical plot of stream discharge against the corresponding per cent of time the stream discharge was equaled or exceeded. The flow-duration curve describes the variability of the stream flow and is useful for

1. Determining dependable flow which information is required for planning of water resources and hydropower projects
2. Designing a drainage system
3. Flood control studies

For preparing a flow-duration curve, the stream flow data (individual values or range of values) are arranged in a descending order of stream discharges. If the number of such discharges is very large, one can use range of values as class intervals. Percentage probability Pp of any flow (or class value) magnitude Q being equaled or exceeded is given as

in which ‘m’ is the order number of the discharge (or class value) and N is the number of data points in the list. The discharge Q is plotted against Pp to get the yield flow-duration curve.

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Runoff

Runoff can be defined as the portion of the precipitation that makes its way towards rivers or oceans etc., as surface or subsurface flow. Portion which is not absorbed by the deep strata. Runoff occurs only when the rate of precipitation exceeds the rate at which water may infiltrate into the soil. Precipitation (or rainfall), after satisfying the requirements of evapotranspiration, interception, infiltration into the ground and detention storage, drains off or flows off from a catchment basin as an overland flow (or surface runoff which includes precipitation falling on the stream system too) into a stream channel. Some part of the infiltrating water moves laterally through the upper layers of the soil and returns to the ground surface as interflow or subsurface runoff at some place away from the point of infiltration into the soil. Part of the infiltrating water percolates deep into the ground and joins the ground water storage. When water table intersects the stream channels of the catchment basin, some ground water may reach the surface or join the stream as ground water runoff, also called base flow or dry-weather flow. Thus, the runoff from a catchment includes surface runoff, subsurface runoff and base flow.

The surface runoff starts soon after the precipitation and is the first to join the stream flow. Subsurface runoff is slower and joins the stream later. Depending upon the time taken by the subsurface runoff between the infiltration and joining the stream channel, it may be termed as prompt subsurface runoff or delayed subsurface runoff. The groundwater runoff is the slowest in joining the stream channel but, is responsible in maintaining low flows in the stream during dry season. Based on the time interval between the precipitation and runoff, the runoff is categorized as direct runoff (that enters the stream immediately after precipitation i.e., surface runoff, subsurface runoff) and base flow (i.e., ground water runoff). Runoff is the response of a catchment to the precipitation reflecting the combined effects of the nature of precipitation, other climatic characteristics of the region and the physiographic characteristics of the catchment basin.

Type, intensity, duration and areal distribution of precipitation over the catchment are the chief characteristics of the precipitation that affect the stream flow. Precipitation in the form of rainfall is quicker to appear as stream flow than when it is in the form of snow. For the surface runoff to start, the intensity of rainfall (or precipitation) must exceed the infiltration capacity of the soil which decreases with the increase in the duration of rainfall. It is obvious that a longer duration rainfall may produce higher runoff even if the intensity of rainfall is less but exceeding the infiltration capacity of the soil. Heavy rainfalls in the downstream region of the catchment will cause rapid rise in the stream levels and early peaking of the discharge. A rare occurrence of uniformly distributed rainfall may result in increased infiltration and therefore, increased subsurface runoff and base flow resulting in slow rise in levels and delayed peaking of the discharge. Likewise, antecedent higher soil moisture conditions at the time of precipitation would hasten the rise in the stream levels. Other climatic characteristics influencing the runoff are temperature, wind velocity and relative humidity. These characteristics affect the evapotranspiration and thus influence the availability of the precipitation for runoff.

Types of Runoff

1) Surface Runoff

Portion of rainfall (after all losses such as interception, infiltration, depression storage etc. are met) that enters streams immediately after occurring rainfall. After laps of few time, overland flow joins streams; sometime termed prompt runoff (as very quickly enters streams).

2) Subsurface Runoff

Amount of rainfall first enter into soil and then flows laterally towards stream without joining water table. It also takes little time to reach stream.

3) Base Flow

It is a delayed flow. Water that meets the groundwater table and join the stream or ocean. It is very slow movement and take months or years to reach streams.

Factors affecting runoff

• Climatic factors
• Rain and snow fall
• Duration of rainfall
• Rainfall distribution
• Direction of prevailing wind
• Other climatic factors like Temperature, wind velocity, relative humidity, annual rainfall etc.

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Interpretation of Precipitation Data

Precipitation data must be checked for the continuity and consistency before they are analysed for any significant purpose. This is essential when it is suspected that the gauge site (or its surroundings) might have changed appreciably during the period for which the average is being computed.

Estimation of Missing Data

The continuity of a record of precipitation data may have been broken with missing data due to several reasons such as damage (or fault) in a rain gauge during a certain period. The missing data is estimated using the rainfall data of the neighbouring rain gauge stations. The missing annual precipitation Px at a station x is related to the annual precipitation values, P1, P2, P3 ...... Pm and normal annual precipitation, N1, N2, N3 ...... Nm at the neighbouring M stations 1, 2, 3, … M respectively. The normal precipitation (for a particular duration) is the mean value of rainfall on a particular day or in a month or year over a specified 30-year period.

The 30-year normals are computed every decade. The term normal annual precipitation at any station is, therefore, the mean of annual precipitations at that station based on 30-year record.

The missing annual precipitation Px is simply given as

If the normal annual precipitations at various stations are within about 10% of the normal annual precipitation at station x i.e., Nx. Otherwise, one uses the normal ratio method which gives

This method works well when the precipitation regimes of the neighbouring stations and the station x are similar (or almost the same).

Multiple linear regression (amongst precipitation data of M stations and the station x, excluding the unknown missing data of station x and the concurrent (or corresponding) data of the neighbouring M stations) will yield an equation of the form

The regression method allows for some weighting of the stations and adjusts, to some extent, for departures from the assumption of the normal ratio method.

Test for Consistency of Precipitation Data

Changes in relevant conditions of a rain gauge (such as gauge location, exposure, instrumentation or observation techniques and surroundings) may cause a relative change in the precipitation catchment of the rain gauge. The consistency of the precipitation data of such rain gauges needs to be examined. Double-mass analysis, also termed double-mass curve technique, compares the accumulated annual or seasonal precipitation at a given station with the concurrent accumulated values of mean precipitation for a group of the surrounding stations (i.e., base stations). Since the past response is to be related to the present conditions, the data (accumulated precipitation of the station x, i.e., ΣPx and the accumulated values of the average of the group of the base stations, i.e., ΣPav) are usually assembled in reverse chronological order. Values of ΣPx are plotted against ΣPav for the concurrent time periods and is given in Fig. 1. A definite break in the slope of the resulting plot points to the inconsistency of the data indicating a change in the precipitation regime of the station x. The precipitation values at station x at and beyond the period of change is corrected using the relation,

Where,

     Pcx = corrected value of precipitation at station x at any time t

     Px = original recorded value of precipitation at station x at time t.

     Sc = corrected slope of the double-mass curve

     Sa = original slope of the curve

Fig. 1 Double-Mass Curve

Thus, the older records of station x have been corrected so as to be consistent with the new precipitation regime of the station x.

Presentation of Precipitation Data

Precipitation (or rainfall) data are presented as either a mass curve of rainfall (accumulated precipitation v/s time plotted in chronological order, Fig. 2) or a hyetograph (rainfall intensity v/s time). Mass curves of rainfall provide the information on the duration and magnitude of a storm. Intensities of rainfall at a given time can be estimated by measuring the slope of the curve at the specified time. The hyetograph derived from the mass curve is usually represented as a chart. The area of a hyetograph represents the total precipitation received during the period.

Depth – Area - Duration (DAD) Analysis

Depth-area-duration (DAD) curves are plots of accumulated average precipitation versus area for different durations of a storm period. Depth-area-duration analysis of a storm is performed to estimate the maximum amounts of precipitation for different durations and over different areas. A storm of certain duration over a specified basin area seldom results in uniform rainfall depth over the entire specified area. The difference between the maximum rainfall depth over an area P0 and its average rainfall depth (P-bar) for a given storm, i.e., (P0 – P-bar) increases with increase in the basin area and decreases with increase in the storm duration. The depth-area-duration curve is obtained as given in the following example figure.

Fig. 2 Example Figure for DAD Curves

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Average Depth of Precipitation Over an Area

The information on the average depth of precipitation (or rainfall) over a specified area on either the storm basis or seasonal basis or annual basis is often required in several types of hydrologic problems. The depth of rainfall measured by a rain gauge is valid for that rain gauge station and in its immediate vicinity. Over a large area like watershed (or catchment) of a stream, there will be several such stations and the average depth of rainfall over the entire area can be estimated by one of the following methods.

1) Arithmetic Mean Method

This is the simplest method in which average depth of rainfall is obtained by obtaining the sum of the depths of rainfall (say P1, P2, P3, P4 .... Pn) measured at stations 1, 2, 3…. n and dividing the sum by the total number of stations i.e. n. Thus, 

This method is suitable if the rain gauge stations are uniformly distributed over the entire area and the rainfall variation in the area is not large.

2) Theissen Polygon Method

The Theissen polygon method takes into account the non-uniform distribution of the gauges by assigning a weightage factor for each rain gauge. In this method, the enitre area is divided into number of triangular areas by joining adjacent rain gauge stations with straight lines, as shown in Fig. 1 (a and b). If a bisector is drawn on each of the lines joining adjacent rain gauge stations, there will be number of polygons and each polygon, within itself, will have only one rain gauge station. Assuming that rainfall Pi recorded at any station ‘i’ is representative rainfall of the area Ai of the polygon i within which rain gauge station is located, the weighted average depth of rainfall P-bar for the given area is given as

Here, Ai/ A is termed the weightage factor for i^th  rain gauge.

Fig. 1 Areal averaging of precipitation a) Rain gauge network, b) Theissen polygons, c) Isohyets

This method is, obviously, better than the arithmetic mean method since it assigns some weightage to all rain gauge stations on area basis. Also, the rain gauge stations outside the catchment can also be used effectively. Once the weightage factors for all the rain gauge stations are computed, the calculation of the average rainfall depth P-bar is relatively easy for a given network of stations.

While drawing Theissen polygons, one should first join all the outermost rain gauge stations. Thereafter, the remaining stations should be connected suitably to form quadrilaterals. The shorter diagonals of all these quadrilaterals are then drawn. The sides of all these triangles are then bisected and thus, Theissen polygons for all rain gauge stations are obtained.

3) Isohyetal Method

An isohyet is a contour of equal rainfall. Knowing the depths of rainfall at each rain gauge station of an area and assuming linear variation of rainfall between any two adjacent stations, one can draw a smooth curve passing through all points indicating the same value of rainfall, Fig. 1 (c). The area between two adjacent isohyets is measured with the help of a planimeter.

The average depth of rainfall P-bar for the entire area A is given as

Since this method considers actual spatial variation of rainfall, it is considered as the best method for computing average depth of rainfall.

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Measurement of Precipitation - Rain Gauges

Rainfall may be measured by a network of rain gauges which may either be of non-recording or recording type. Rainfall can be measured with a weather radar also.

Non-Recording Rain gauge

The non-recording rain gauge used in India is the Symon’s rain gauge (Fig. 1). It consists of a funnel with a circular rim of 12.7 cm diameter and a glass bottle as a receiver. The cylindrical metal casing is fixed vertically to the masonry foundation with the level rim 30.5 cm above the ground surface. The rain falling into the funnel is collected in the receiver and is measured in a special measuring glass graduated in mm of rainfall; when full it can measure 1.25 cm of rain. The rainfall is measured every day at 08.30 hours IST. The funnel shank is inserted in a bottle which receives the rainwater. The water collected in the bottle is measured by pouring it into a measuring cylinder which gives the depth of rainfall in mm.

Fig. 1 Symon’s Rain Gauge

During heavy rains, it must be measured three or four times in the day in which the receiver fill and overflow, but the last measurement should be at 08.30 hours IST and the sum total of all the measurements during the previous 24 hours entered as the rainfall of the day in the register. Usually, rainfall measurements are made at 08.30 hr IST and sometimes at 17.30 hr IST also. Thus the non-recording or the Symon’s rain gauge gives only the total depth of rainfall for the previous 24 hours (i.e., daily rainfall) and does not give the intensity and duration of rainfall during different time intervals of the day. It is often desirable to protect the gauge from being damaged by cattle and for this purpose a barbed wire fence may be erected around it.

Recording Rain Gauge

This is also called self-recording, automatic or integrating rain gauge. This type of rain gauge has an automatic mechanical arrangement consisting of a clockwork, a drum with a graph paper fixed around it and a pencil point, which draws the mass curve of rainfall. From this mass curve, the depth of rainfall in a given time, the rate or intensity of rainfall at any instant during a storm and time of onset and cessation of rainfall can be determined. The gauge is installed on a concrete or masonry platform 45 cm square in the observatory enclosure by the side of the ordinary rain gauge at a distance of 2-3 m from it. The gauge is so installed that the rim of the funnel is horizontal and at a height of exactly 75 cm above ground surface. The self-recording rain gauge is generally used in conjunction with an ordinary rain gauge exposed close by, for use as standard, by means of which the readings of the recording rain gauge can be checked and if necessary adjusted. There are three types of recording rain gauges—tipping bucket gauge, weighing gauge and float gauge.

1) Tipping Bucket Rain Gauge

This consists of a cylindrical receiver 30 cm diameter with a funnel inside (Fig. 2). Just below the funnel a pair of tipping buckets is pivoted such that when one of the bucket receives a rainfall of 0.25 mm it tips and empties into a tank below, while the other bucket takes its position and the process is repeated. The tipping of the bucket actuates on electric circuit which causes a pen to move on a chart wrapped round a drum which revolves by a clock mechanism. This type of rain gauge cannot record snowfall.

Fig. 2 Tipping Bucket Rain Gauge

2) Weighing Type Rain Gauge

In this type of rain-gauge, when a certain weight of rainfall is collected in a tank, which rests on a spring-lever balance, it makes a pen to move on a chart wrapped round a clock driven drum (Fig. 3). The rotation of the drum sets the time scale while the vertical motion of the pen records the cumulative precipitation. The record thus gives the accumulation of rainfall with time.

Fig. 3 Weighing Type Rain Gauge

3) Float Type Rain Gauge

In this type, as the rain is collected in a float chamber, the float moves up which makes a pen to move on a chart wrapped round a clock driven drum. When the float chamber fills up, the water siphons out automatically through a siphon tube kept in an interconnected siphon chamber. The float rises with the rise of water level in the chamber and its movement is recorded on a chart through a suitable mechanism. The clockwork revolves the drum once in 24 hours. The clock mechanism needs rewinding once in a week when the chart wrapped round the drum is also replaced. A siphon arrangement is also provided to empty the chamber quickly whenever it becomes full. The weighing and float type rain gauges can store a moderate snow fall which the operator can weigh or melt and record the equivalent depth of rain.

Fig. 4 Float Type Rain Gauge

Bureau of Indian Standards has laid down the following guidelines for selecting the site for rain gauges (IS : 4897-1968).

• The rain gauge shall be placed on a level ground, not upon a slope or a terrace and never upon a wall or roof.
• On no account, the rain gauge shall be placed on a slope such that the ground falls away steeply in the direction of the prevailing wind.
• The distance of the rain gauge from any object shall not be less than twice the height of the object above the rim of the gauge.
• Great care shall be taken at mountain and coast stations so that the gauges are not unduly exposed to the sweep of the wind. A belt of trees or a wall on the side of the prevailing wind at a distance exceeding twice its height shall form an efficient shelter.
• In hills where it is difficult to find a level space, the site for the rain gauge shall be chosen where it is best shielded from high winds and where the wind does not cause eddies.
• The location of the gauge should not be changed without taking suitable precautions. Description of the site and surroundings should be made a matter of record.

Radar Measurement of Precipitation

In regions of difficult and inaccessible terrains, precipitation can be measured (within about 10% accuracy of the rain gauge measurements) with the help of a radar (radio detecting and ranging). A radar transmits a pulse of electromagnetic waves as a beam in a direction depending upon the position of the movable antenna. The wave travelling at a speed of light is partially reflected by cloud or precipitation particles and returns to the radar where it is received by the same antenna. The display of the magnitude of energy of the returned wave on the radarscope (i.e., radar screen) is called an echo and its brightness is termed echo intensity. The duration between the transmission of the pulse and appearance of the echo on the radarscope is a measure of the distance (i.e., range) of the target from the radar.
Direction of the target with respect to the radar is decided by the orientation of the antenna at the time the target signal is received. The echo is seen in polar coordinates. If there is no target (i.e., cloud or precipitation particles), the screen is dimly illuminated. A small target would appear as a bright point whereas an extended target (such as a rain shower) would appear as a bright patch. The radarscope being divided as per the coordinate system; the position of the target can be estimated. By having a proper calibration between the echo intensity and rainfall (or its intensity), one can estimate the rainfall (or rainfall intensity).

Satellite Measurement of Precipitation

It is a common experience that gauge network for measuring precipitation in a large and inaccessible area (such as in desert and hilly regions) is generally inadequate and non-existent in oceans. The satellite observation is the only effective way for continuous monitoring of precipitation events over a large or inaccessible area. Use of the meterological satellites for weather and water balance studies is, therefore, continuously increasing.

In satellite measurements, the precipitation is estimated by correlating the satellite- derived data and observed rainfall data. These relationships can be developed for a part of electromagnetic spectrum using cloud life history or cloud indexing approach. The first approach uses data from geo-stationary satellites that produce data at every half an hour interval. The second approach, based on cloud classification, does not require a series of consecutive observations of the same cloud system. Microwave remote sensing techniques that can directly monitor the rainfall characteristics have great potential in rainfall measurement.

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Precipitation

Precipitation refers to all the moisture that comes to the earth from atmosphere. This may be in the form of rain, snow, sleet, fog, dew or hail. Precipitation can occur only when the air containing moisture is cooled sufficiently to condense a part of the moisture which is present in the atmosphere. The atmospheric air always contains moisture. Evaporation from the oceans is the major source (about 90%) of the atmospheric moisture for precipitation. Continental evaporation contributes only about 10% of the atmospheric moisture for precipitation. The atmosphere contains the moisture even on days of bright sun shine.

For the occurrence of precipitation, some mechanism is required to cool the atmospheric air sufficiently to bring it to (or near) saturation. This mechanism is provided by either convective systems (due to unequal radiative heating or cooling of the earth’s surface and atmosphere) or by orographic barriers (such as mountains due to which air gets lifted up and consequently undergoes cooling, condensation and precipitation) and results into, respectively, convective and orographic precipitations. Alternatively, the air lifted into the atmosphere may converge into a low pressure area (or cyclone) causing cyclonic precipitation. Artificially induced precipitation requires delivery of dry ice or silver iodide or some other cloud seeding agent into the clouds by aircraft or balloons.

Forms of Precipitation

1) Drizzle

A light steady rain in fine drops (0.5 mm) and intensity <1 mm/hr.

2) Rain

The condensed water vapour of the atmosphere falling in drops (>0.5 mm, maximum size - 6 mm) from the clouds.

3) Glaze

Freezing of drizzle or rain when they come in contact with cold objects.

4) Sleet

Frozen rain drops while falling through air at subfreezing temperature.

5) Snow

Ice crystals resulting from sublimation (i.e., water vapour condenses to ice).

6) Snow flakes

Ice crystals fused together.

7) Hail

Small lumps of ice (>5 mm in diameter) formed by alternate freezing and melting, when they are carried up and down in highly turbulent air currents.

8) Dew

Moisture condensed from the atmosphere in small drops upon cool surfaces.

9) Frost

A feathery deposit of ice formed on the ground or on the surface of exposed objects by dew or water vapour that has frozen.

10) Fog

A thin cloud of varying size formed at the surface of the earth by condensation of atmospheric vapour (interfering with visibility).

11) Mist

A very thin fog.

Classification of Rainfall Based on Rate

Table 1: Classification of Rainfall Based on Rate

Sl. No.	Type of Rainfall	Rate (mm/hr)
1	Light	upto 2.5
2	Moderate	2.5  - 7.5
3	Heavy	>7.5

Types of Precipitation

Adiabatic cooling resulting from the vertical transport of air mass is the primary cause of condensation and hence of precipitation. Depending on the conditions responsible for the vertical motion of the air mass, precipitation can be classified into the following four types.

1) Convective Precipitation

Convective precipitation results from the heating of the earth's surface. The warm ground heats the air over it. As the air warms, the air molecules begin to move further apart. With increased distance between molecules, the molecules are less densely packed. Thus, the air becomes “lighter” and rises rapidly into the atmosphere. As the air rises, it cools. Water vapour in the air condenses into clouds and precipitation.

Fig. 1 Convective Precipitation

2) Orographic Precipitation

Orographic precipitation results when warm moist air moving across the ocean is forced to rise by large mountains. As the air rises, it cools because a higher elevation results in cooler temperatures. Cold air cannot hold as much moisture as warm air. As air cools, the water vapour in the air condenses and water droplets form. Clouds forms and precipitation (rain or snow) occurs on the windward side of the mountain.

This air is now dry and rises over top the mountain. As the air moves back down the mountain, it collects moisture from the ground via evaporation. This side of the mountain is called the leeward side. It receives very little precipitation.

Fig. 2 Orographic Precipitation

3) Cyclonic Precipitation or Frontal Precipitation

Cyclonic or frontal precipitation results when the leading edge of a warm, moist air mass (warm front) meets a cool and dry air mass (cold front). The molecules in the cold air are more tightly packed together (i.e., more dense) and thus, the cold air is heavier than the warm air. The warmer air mass is forced up over the cool air.

As it rises, the warm air cools, the water vapour in the air condenses, clouds are formed and it result in precipitation. This type of system is called Frontal Precipitation because the moisture tends to occur along the front of the air mass. In another words, unequal heating of the earth’s surface creates low and high pressure regions. Movement of air masses from the high pressure regions to low pressure regions displaces the low pressure air upward to cool and cause precipitation. Cyclonic precipitation can be of two types.

Fig. 3 Cyclonic Precipitation

a) Frontal Precipitation

A frontal is called as the hot moist air mass boundary. This precipitation is caused by the expansion of air near the frontal surface.

b) Non-Frontal Precipitation

This is a cold moist air mass boundary that moves and results in precipitation.

There are two main types of cyclones namely Tropical Cyclone (also called hurricane or typhoon) of comparatively small diameter of 300-1500 km causing high wind velocity and heavy precipitation and the Extra-Tropical Cyclone of large diameter up to 3000 km causing wide spread frontal type precipitation.

When two air masses due to contrasting temperatures and densities clash with each other, condensation and precipitation occur at the surface of contact. This surface of contact is called a ‘front’ or ‘frontal surface’. If a cold air mass drives out a warm air mass’ it is called a ‘cold front’ and if a warm air mass replaces the retreating cold air mass, it is called a ‘warm front’. On the other hand, if the two air masses are drawn simultaneously towards a low pressure area, the front developed is stationary and is called a ‘stationary front’. Cold front causes intense precipitation on comparatively small areas, while the precipitation due to warm front is less intense but is spread over a comparatively larger area. Cold fronts move faster than warm fronts and usually overtake them, the frontal surfaces of cold and warm air sliding against each other. This phenomenon is called ‘occlusion’ and the resulting frontal surface is called an ‘occluded front’.

Characteristics of Precipitation in India

India receives more than 75% of its annual precipitation during the monsoon season (June to September). The monsoon (i.e., south-west monsoon) originates in the Indian Ocean and appears in the southern part of Kerala by the end of May or the beginning of June. Monsoon winds, then, advance and cover the entire country by mid-July. The monsoon season is not a period of continuous rainfall. The temporal and spatial variability of the magnitude of rainfall results into regions of droughts and floods. Assam and the north-eastern region are the heavy rainfall regions (with average annual rainfall ranging from 2000-4000 mm) and Uttar Pradesh, Haryana, Punjab, Rajasthan and Gujarat constitute low rainfall regions (with average annual rainfall less than about 1000 mm). Western Ghats receive about 2000-3000 mm of annual rainfall.

Around mid-December, the western disturbances cause moderate to heavy rain and snowfall (about 250 mm) in the Himalayas and Jammu and Kashmir and other northern regions of the country. Low pressure areas formed in the Bay of Bengal during this period cause some rainfall in the south-eastern parts of the country.

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Hydrology

Hydrology means the science of water. It is the science that deals with the occurrence, circulation and distribution of water on the earth and earth’s atmosphere. 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. In a general sense, hydrology is a very broad subject of an inter-disciplinary nature depending upon allied sciences, such as meteorology, geology, statistics, chemistry, physics and fluid mechanics. Hydrology also deals with estimation of water resources, study of precipitation, runoff, evapotranspiration and their interaction. It also involves the study of problems such as floods and droughts and strategies to combat them. Hydrology is basically an applied science.

Engineering hydrology deals with all these aspects which are pertinent to planning, design and operation of hydraulic engineering projects for the control and use of the available water. Hydrology finds its application in the design and operation of water resources projects to estimate the magnitudes of flood flows at different times of a year to decide reservoir capacity, spillway discharge, dimensions of hydraulic structures etc.

Hydrologic Cycle

Water occurs on the earth in all its three states, viz. liquid, solid and gaseous and in various degrees of motion. Evaporation of water from water bodies such as oceans and lakes, formation and movement of clouds, rain and snowfall, streamflow and groundwater movement are some examples of the dynamic aspects of water. The various aspects of water related to the earth can be explained in terms of a cycle known as the hydrologic cycle. Hydrologic cycle is the water transfer cycle, which occurs continuously in nature. The three important phases of the hydrologic cycle are

1. Evaporation and Evapotranspiration
2. Precipitation
3. Runoff

Schematic Diagram of Hydrologic Cycle

The total water of earth, excluding deep ground water, is in constant circulation from the earth (including oceans) to atmosphere and back to the earth and oceans. This cycle of water amongst earth, oceans and atmospheric systems is known as hydrologic cycle. The hydrologic cycle can be visualized to begin with the evaporation (due to solar heat) of water from the oceans, streams and lakes of the earth into the earth’s atmosphere. The water vapour, under suitable conditions, get condensed to form clouds moving with wind all over the earth’s surface and which, in turn, may result in precipitation (in the form of rain water, snow, hail, sleet etc.) over the oceans as well as the land surface of the earth. Part of the precipitation, even while falling, may evaporate back into the atmosphere. Another part of the precipitation may be intercepted by vegetation on the ground or other surfaces. The intercepted precipitation may either evaporate into the atmosphere or fall back on the earth’s surface.

The greater part of the precipitation falling on the earth’s surface is retained in the upper soil from where it may return to the atmosphere through evaporation and transpiration by plants find its way, over and through the soil surface as runoff, to stream channels and the runoff thus becoming stream flow. Yet another part of the precipitation may penetrate into the ground to become part of the ground water. The water of stream channels, under the influence of gravity, moves towards lower levels to ultimately meet the oceans. Water from ocean may also find its way into the adjoining aquifers. Part of the stream water also gets evaporated back into the atmosphere from the surface of the stream. The ground water also moves towards the lower levels to ultimately reach the oceans. The ground water, at times, is a source of stream flow. Further, it is a continuous recirculation cycle in the sense that there is neither a beginning nor an end or a pause.

Block Diagram of Hydrologic Cycle

The hydrologic cycle is a very complex phenomenon that has been taking place since the earth formed. It should also be noted that the hydrologic cycle is a continuous recirculation cycle with neither a beginning nor an end. Hydrologic system is defined as a structure or volume in space surrounded by a boundary that receives water and other inputs, operates on them internally and produces them as outputs. The global hydrologic cycle can be termed a hydrologic system containing three subsystems such as the atmospheric water system, the surface water system and the subsurface water system. Another example of the hydrologic system is storm-rainfall-runoff process on a watershed. Watershed (or drainage basin or catchment) is a topographic area that drains rain water falling on it into a surface stream and discharges surface stream flow through one particular location identified as watershed outlet or watershed mouth. Each path of the hydrologic cycle involves one or more of the following aspects.

1. Transportation of water
2. Temporary storage
3. Change of state

Hydrologic Systems

The hydrologic system was defined as a structure or a volume in space, surrounded by a boundary, that accepts water and other inputs, operates on them internally, and produces them as outputs. The structure (for surface or subsurface flow) or volume in space (for atmospheric moisture flow) is the totality of the flow paths through which the water may pass from the point it enters the system to the point it leaves. The boundary is a continuous surface defined in three dimensions enclosing the volume or structure. A working medium enters the system as input, interacts with the structure and other media, and leaves as output. Physical, chemical and biological processes operate on the working media within the system; the most common working media involved in hydrologic analysis are water, air and heat energy.

Global Hydrologic Cycle

The global hydrologic cycle can be represented as a system containing three subsystems: the atmospheric water system, die surface water system and the subsurface water system. Another example is the storm rainfall- runoff process on a watershed, which can be represented as a hydrologic system. The input is rainfall distributed in time and space over the watershed and the output is stream flow at the watershed outlet. The boundary is defined by the watershed divide and extends vertically upward and downward to horizontal planes.

Global Hydrologic Cycle

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Water Resources Engineering

Water is an essential ingredient for sustenance of life. The total quantity of water available on earth is estimated to be about 1400 million cubic kilometer, which is enough to cover the earth with a layer 3 km deep. However, 97.3% of this is saltwater in oceans, thereby leaving only 2.7% freshwater. Again, about 75% of the freshwater lies frozen in the Polar Regions and about 22.6% are groundwater- some of it very deep to be economically extractable. The surface freshwater is thus only about 0.007% of the total water occurring on earth. With increasing population, while the demand for water increases, anthropogenic pressures are rendering many water sources unfit for use because of the high level of pollution. Development and management of water resources is thus one of the important aspects of development at the present time. 

Water Resources Engineering is the science of designing, developing and managing projects and systems to protect and use water resources in a sustainable and efficient manner. This includes the design, construction and operation of water supply systems, flood control, water quality and water resources management. This engineering field is closely related to hydrology and the science of studying the properties of water and how it is distributed and used by humans. It can include the study of watersheds, rivers, lakes and coastal areas. 

Water Resources Engineering is a huge field which involves managing available water resources from the standpoint of both water quantity and water quality to meet the water needs of humanity and habitats at the local, regional, national or international level. It is the study and management of equipment, facilities and techniques that are used to manage and preserve life’s most plentiful resource - water. In addition to assessing how and the best ways to control water as it pertains to water-related activities – such as irrigation, waste disposal and canal development. Water resource engineers are also frequently involved in water management to ensure that it’s safe to drink both for humans, plants and animal usage. 

Managing water requires a sound understanding of water distribution systems such as rivers, canals, pipelines, culverts, ground water wells and water storage systems such as reservoirs, retention-detention ponds and aquifers. Water resources engineers must also have knowledge of various structures that are used to manage the conveyance of water such as sluice gates, emergency spillways and structures that are used to store water such as dams and dikes. In addition, water resources engineers must know techniques to assess future water demand as well as the quantity and quality of the available water resources in water bodies such as rivers, lakes and groundwater. Water resources engineers should also be familiar with the transport processes such as evaporation, transpiration, runoff and infiltration which are used by nature to move water globally.

History 

Water resources engineering has its roots in the ancient world, with evidence of its use in the Middle East, India and China since 3000 BC. Ancient civilizations used water engineering techniques to irrigate their agricultural land and store water for long-term use. In the middle ages, water engineering was used to build dams and canals for the purpose of flood control and agricultural irrigation. In the 16th and 17th centuries, water engineering began to be used for other purposes such as water supply and sewage systems, as well as providing a source of energy through the use of water wheels. In the 19th century, advances in engineering and technology have allowed engineers to design and build more complex water resource projects such as aqueducts and irrigation canals.

Water resources engineering has evolved over the past 9000 to 10,000 years as humans have developed the knowledge and techniques for building hydraulic structures to convey and store water. Early examples include irrigation networks built by the Egyptians and Mesopotamians and by the Hohokam in North America. The world’s oldest large dam was the Sadd-el-kafara dam built in Egypt between 2950 and 2690 B.C. The oldest known pressurized water distribution (approximately 2000 B.C.) was in the ancient city of Knossos on Crete. There are many examples of ancient water systems throughout the world.

The importance of water resources engineering includes the following: 

1. It is essential to managing our water resources, helps to ensure the availability of clean drinking water, efficient use of water for agricultural purposes and protect our water sources. 
2. It helps improve water quality to meet human and environmental needs. It can also help to reduce water pollution and to conserve water resources. 
3. It is also important for infrastructure development and management of water systems. This includes the construction of dams, reservoirs, canals, pipelines and other water-related infrastructure. 
4. It is essential for flood prevention and management, helps to identify possible areas of flooding, design and construction of dams and other flood control structures. 
5. It is important for agricultural production and also helps develop irrigation systems, drainage systems and erosion control systems that improve crop yields and reduce the risk of crop loss due to drought and other environmental factors. 
6. It is important for coastal zone management. They help to identify risk zones for coastal erosion and flooding and develop solutions to reduce these risks. 

Examples of water resources engineering projects include the following: 

• Construction of artificial reservoirs 
• Groundwater recharge 
• Drip irrigation systems 
• Desalination plants
• Flood Control Systems

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