(Aktu Btech) Engineering Hydrology Important Unit-1 Introduction to Hydrology

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Q1. What is meant by hydrological cycle ? How can the parameters of the cycle be written in an equation form ? Draw a neat diagram to illustrate your answer.

Ans. Hydrologic Cycle:

As depicted in Fig. the hydrologic cycle is the process of transferring moisture from the atmosphere to the earth in the form of precipitation, transporting precipitated water through streams and rivers to oceans and lakes, and evaporating water back into the atmosphere.

This cycle consists of the following processes: 

• 1. Evaporation and Transpiration (E): Water evaporates from the ocean’s, rivers’, and lakes’ surfaces, as well as from damp soil. The vapours are carried over the country by air as clouds. Transpiration is the process through which water is lost from the pores of plant leaves.Thus, the total evaporation (E), inclusive of the transpiration consists of: 
  • i. Surface evaporation. 
  • ii. Water surface evaporation. 
    • a. From river surface. 
    • b. From oceans. 
  • iii. Evaporation from plants and leaves (transpiration).
  • iv. Atmospheric evaporation. 
• 2. Precipitation (P): Precipitation may be defined as the fall of moisture from the atmosphere to the earth surface in any form. Precipitation may be of two forms:
  • i. Liquid precipitation: i.e., rainfall. 
  • ii. Frozen precipitation: This consists of:
    • a. Snow.                         
    • b. Hail. 
    • c. Sleet.                          
    • d. Freezing rain.
• 3. Runoff (R): The percentage of precipitation that does not evaporate is referred to as runoff. When moisture falls to the earth’s surface as precipitation, some of it evaporates off the water surface, soil, and vegetation, as well as through plant transpiration, leaving the remainder as runoff, which eventually drains to the ocean via surface or sub-surface streams. Thus runoff may be classified as follows:
  • i. Surface Runoff: Water moves across the land, first reaching streams and rivers, which eventually discharge the water into the sea.
  • ii. Inter-flow or Sub-Surface Runoff: A portion of the precipitation infiltrates the surface soil and, depending on the geology of the basins, runs as subsurface runoff to the streams and rivers.
  • iii. Groundwater Flow or Base Flow: It is the part of precipitation that percolates down after infiltration and joins the groundwater reservoir, which is eventually connected to the ocean.

Thus, the hydrologic cycle may be expressed by the following simplified equation:

Precipitation (P) = Evaporation (E) + Runoff (R) 

Q2. Define the hydrology and discuss critically the statement “Knowledge of hydrology is must for any water resource planning”. 

Ans. Hydrology: 

• 1. It is the science that studies the presence, circulation, distribution, and movement of water on Earth, including water in the atmosphere and beneath the earth’s surface.
• 2. It is concerned with water in streams and lakes, rainfall and snowfall, snow and ice on land, and water occuring beneath the earth’s surface in the pores of soil and rocks.

Knowledge of Hydrology: Any civil engineer, particularly those involved in the design, planning, or construction of irrigation facilities, bridges, highway culverts, or flood control works, must have a fundamental understanding of this science.

• 1. Maximum flows are projected at a spillway or a highway culvert in a city drainage system after various years.
• 2. During droughts, a minimum reservoir capacity will be sufficient to ensure adequate water for irrigation or municipal water delivery.

Q3. What do you understand by precipitation ? Explain various types of precipitation.

Ans. Precipitation: Precipitation is the term used by hydrologists to describe all types of moisture that fall from the clouds. 

Types of Precipitation: 

• 1. Cyclonic Precipitation: The lifting of air masses converging into the cyclone’s low pressure area causes cyclonic precipitation. There are two types of cyclonic precipitation: (a) frontal precipitation and (b) non-frontal precipitation.
• 2. Convective Precipitation:
  • i. Convective precipitation occurs as a result of the natural rise of warmer, lighter air in colder, denser surroundings.
  • ii. The temperature differential could be caused by unequal heating at the surface, unequal cooling at the top of the air layer, or mechanical lifting when air is pushed to pass over denser colder air masses.
  • iii. Convective precipitation is sporadic and varies in intensity from light showers to cloud bursts.
• 3. Orographic Precipitation: 
  • i. Orographic precipitation is caused by geographic barriers raising warm moisture-laden air masses (such as mountains).
  • ii. As it ascends, it comes into touch with cold air, resulting in precipitation.
  • iii. The zone on the other side of the mountain will be a rain shadow area where rainfall may not occur, as indicated in Fig. 
  • iv. All precipitation in the Himalayan region is due to the orographic ascent of air masses rich in moisture content due to their long journey over seas.

• 4. Precipitation Due to Turbulent Ascent: 
  • i. After crossing the ocean, the air mass is forced to rise due to increased friction on the earth’s surface.
  • ii. As a result of increased turbulence and friction, the air mass rises, eventually condensing and precipitating.

Q4. What is meant by probable maximum precipitation (PMP) over a basin ? Explain how PMP is estimated.  

Ans. A. Probable Maximum Precipitation: 

• 1. The precipitation resulting from the most essential meteorological combinations that are thought likely of occurring is known as the probable maximum precipitation (PMP) for a region.
• 2. It is the rainfall over a basin that would create the flood flow with almost no probability of being exceeded.
• 3. From the statistical studies, PMP can also be estimated from the following equation:

B. Maximum and Minimum Rainfall: The magnitudes of maximum rainfall and minimum rainfall within specified time period can be determined by the use of frequency formula given by Hazen:  

where, T = Recurrence interval within which the event is either equal to or greater than the specified amount. 

Procedure for Determination of Maximum Rainfall: 

• 1. Sort the rainfall data into descending order and assign a rank number (m) to each rainfall event, with the total number of events equal to N.
• 2. For each occurrence, compute the recurrence interval (T).
• 3. Draw a graph using the recurrence interval (T) as the abscissa and the matching rainfall as the ordinate. Calculate the maximum predicted rainfall magnitude from this graph for any desired value of T.

Procedure for Determination of Minimum Rainfall: 

• 1. Sort the rainfall data in ascending order and assign a rank number (m) to each rainfall event, with the total number of occurrences equal to.
• 2. For each occurrence, compute the recurrence interval (T).
• 3. Draw a graph using the recurrence interval (T) as the abscissa and the matching rainfall as the ordinate.
• 4. From this graph, calculate the estimated minimum rainfall magnitude for every chosen value of T.

Q5. Define evaporation. Discuss the factors that affect the evaporation from a water body. 

Ans. A. Evaporation: It is the transmission of heat energy that causes a liquid to transition to a gaseous state at the free surface, below the boiling point. 

B. Factors: Following are the factors that affect the evaporation from the water body :

• 1. Vapour Pressure: The rate of evaporation is proportional to the difference between the saturation vapour pressure at the water temperature, e_w and the actual pressure in the air, e_a Thus, 

• 2. Temperature: Other parameters remaining constant, the rate of evaporation increases as water temperature rises.
• 3. Wind: Wind helps to remove evaporated water vapour from the zone of evaporation, creating more space for evaporation.
• 4. Atmospheric Pressure: Other parameters remaining constant, a drop in barometric pressure, such as at high altitudes, enhances evaporation.
• 5. Soluble Salts: 
  • i. When a solute is dissolved in water, the solution’s vapour pressure is lower than that of pure water, resulting in a slower rate of evaporation.
  • ii. The proportion decrease in evaporation approximates the percentage increase in specific gravity.
• 6. Heat Storage in Water Bodies: 
  • i. Deep water bodies store more heat than shallow water bodies.
  • ii. A deep lake may store summer radiation energy and release it in winter, resulting in less summer evaporation and more winter evaporation than a shallow lake subjected to the same condition.
  • iii. Nevertheless, the effect of heat storage is primarily to adjust seasonal evaporation rates, with little effect on annual evaporation rates.

Q6. Discuss briefly the various abstractions that take place precipitation.

Ans. Abstractions from Precipitation: When precipitation takes place on land surface whole of it is not available as runoff because of losses that take place during or after the precipitation. Important losses consist of, 

i. Interception.                                  ii. Evaporation. 

iii. Transpiration.                              iv. Infiltration. 

v. Depression storage.                     vi. Watershed leakage.

Out of this evaporation transpiration and infiltration are the major losses. 

• 1. Interception: 
  • i. It can be described as the amount of precipitation water intercepted by vegetation, structures, and other objects on the land surface.
  • ii. Interception does not reach the land surface but instead evaporates back into the atmosphere.
  • iii. The intercepted precipitation may follow one of the three possible routes :
    • a. It may be kept as surface storage by vegetation and returned to the atmosphere by evaporation; this is known as interception loss.
    • b. It can drip off the plant leaves and join the ground surface or surface flow; this is referred water as throughfall.
    • c. Rainwater may run down the stem and along the leaves and branches to reach the ground surface. This is known as stemflow.
• 2. Depression Storage: 
  • i. A catchment region often features multiple shallow-depth depressions of different size and shape.
  • ii. When precipitation occurs, water flows towards these depressions and fills them before true overland flow or runoff into a stream occurs.
  • iii. Depression storage depends on a vast number of factors. The major factors are:
    • a. The type of soil. 
    • b. The condition of the surface reflecting the amount and nature of depression.
    • c. The slope of the catchment. 
    • d. The antecedent precipitation as a measure of the soil moisture.  
  • iv. The following relationship may be used for computing the depression storage, 

3. Watershed Leakage:

• i. Adjacent basins are separated by ridge lines, so that rainfall falling over a basin flows towards the drainage lines (i.e., streams) of the basin. 

• ii. Watershed leakage is described as the movement of water from one basin to another or from one basin to the sea via large faults, fissures, or other physical features.
• iii. These faults, fissures, and underground hydraulic conduits produced as a result of these faults, fissures, and underground hydraulic conduits transmit the discharge falling over a portion of the catchment.

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