Q.1. Define hydrology with water balance equation.
Q.2. Briefly describe the hydrological cycle with its components.
Q.3. Write down the scope of the hydrological study and its application in Agricultural Engineering field.
Q.4. Briefly describe the importance of hydrological study in the context of Nepal.

Q.1. Define hydrology with water balance equation.
Ans:- The word Hydrology is derived from two words 'hydro' and 'logos'.  Hydro means water and the 'logos' means study. Simply hydrology is defined as the study of water, In other words it may be defined as the science that deals with the occurrence of, circulation and distribution of water in the earth and earth's atmosphere. It is concerned with the water in streams and lakes rainfall and the snowfall. Snow and ice on the land water occurring below the ground surface in the pores of soil and rocks. In general sense, Hydrology is a very broad subject of the interdisciplinary nature drawing support from applied science, such as metrology, geology, statistics, chemistry, physics and fluid mechanics.
It also includes the chemical and physical properties and their reaction with the environment including their relations to living things. Hydrology is basically an applied science it also deal with 1). Estimation of the water resources, 2). study of the process such as precipitation, runoff, evapotranspiration  and their interaction and 3). Study of problem such as floods and droughts and strategies to combat them.
The quantity of water going through various paths of the hydrological cycle in a given system can be described by the continuity principle known as water budget equation or water balance equation or hydrologic equation. The water balance equation is the statement of the law of the conservation of mass. It is also called the continuity equation or conservation equation. It is the balance of input and output of the water within a given area taking into account not changes in storages. It can also be defined by the following relationship,

Mass input – Mass output = Change in storage,
If the density of the inflow and the outflow is same then,
Volume of inflow – volume of outflow = change in storage
 i.e. d(storage)/dt=inflow-outflows
Here the main source of inflow  is precipitation and outflow is tare runoff, evaporation, transpiration, interception etc. The relationship between rainfall and runoff is very complex phenomenon as the parameters involved in the relationship are not very accurately determined. General water budget equation in hydrology for time interval  dt is P-(R+G+E+T)=Δ S
Where p = precipitation, R=Surface runoff, G=net ground water, E= Evaporation, T=transpiration Δ  S = Change in storage,

Q.2. Briefly describe the hydrological cycle with its components.
Ans:- Water occurs on the earth in all its three states(Gas, Liquid, Solid) and in various degree of motion; Evaporation of water from water bodies such as oceans and lakes,  Formation and movement  of clouds, rain  and snowfall, stream flow and the groundwater movements are the
some example of dynamic aspect of water. The various activities of water related to the earth can be explained in terms of cycle is known as the hydrological cycle.

Thus in other words, hydrologic cycle is the water transfer cycle which occurs continuously in nature. It is center of focus of hydrology. The cycle has no beginning and no end and it's process may occurs continuously. Thus it is the process of transfer of moisture from the atmosphere to the earth in the form of precipitation, conveyance of the precipitated water by stream and river to ocean and lake evaporation of water back to the atmosphere. A convenient starting point to describe the cycle is in the oceans. Water in the oceans evaporation due to the heat energy provided by solar radiation. The eater vapour moves upward and forms clouds, while much of the cloud condense and fall back to the pecans as rain, a part of clouds is driven to the land areas by winds. There they condense and precipitated onto the lands mass as rain, snow, hail, sleets etc. A part of the precipitation may evaporate back to the atmosphere even while falling. Another part of the precipitation may be intercepted by vegetation, structure and other such surface modifications from which it may be either evaporated back to the atmosphere or move down to the ground surface.
The main components of hydrological cycles are
i) Precipitation, ii) Evaporation, iii) Transpiration, iv) Infiltration, v) Runoff
i. Precipitation (Storage on the land surface):-
Water in the oceans evaporates due to the heat energy provided by solar radiation. The water vapour moves upwards and forms clouds. While much of the clouds is driven to the land areas by winds. There they can condense and precipitate onto the land mass as rain, snow, hail, sleet etc. A part of precipitation may evaporate back to the atmosphere even while failing. Other part may be intercepted by vegetation, structures and other such surface modification from which it may be either evaporated back to atmosphere even while falling.
The term precipitation denotes all forms of water that reach the earth from the atmosphere. The usual forms are rainfall, snowfall, hail, frost and drew of all these only the first two contribute significant amounts of  water, rainfall being the predominant form of the precipitation using stream flow, especially the flood flow in a majority of the rivers in Nepal, unless otherwise stated the term rainfall is used synonymously to precipitation.

ii. Evaporation (Soil moisture storage):-
Evaporation is the process in which a liquid changes to the gaseous state at the free surface below the boiling point through the transfer of heat energy.
iii. Transpiration (Ground water storage interaction):-
Transpiration is the process by which water leaves the body of living plants and reach to the atmosphere as water is taken up by the plant root system and escapes through the leaves. Transpiration is essentially confined to day light hours and the rate of transpiration depends upon the growth period of the plants.
iv. Infiltration:-
Infiltration is the flow of water into the ground through the soil surface. The distribution of soil moisture takes place within the soil profile during the infiltration process, when water is applied at the surface of a soil, for moisture zone on the soil.
v. Runoff:-
It means the draining of the flowing off of precipitation from a catchment area through a surface channel. It thus represents the output from the catchment in a given unit of time. The portion of the precipitation which by a variety of paths above and below the surface of the earth reaches the stream channel is called runoff.

Q.3. Write down the scope of the hydrological study and its application in Agricultural Engineering field.
Ans:- Hydrology has the greatest application in the design and operation of water resources engineering project, such as those for
1)Irrigation  2)Water Supply 3)Flood control 4)Water Power 5)Navigation
It is applicable for
·         the correct assessment of flows in hydropower, irrigation, drainage and water supply projects.
·         To determine the maximum expected flow at dam, reservoir, spillway, bridges, culvert5s and city drainage system.
·         In determining the effect of river water level before and after completion of reservoir and other irrigation structure.
·         Estimation of total volume of water that may be available from drainage (basin) over a long period.
·         Hydrology is applicable for the control of erosion to minimize sedimentation of rivers.

It involves the collection of relevant data and analysis of the data by applying the principles and theories of hydrology seen solution to practical problem.
In all above (five) project, hydrological investigations for the proper assessment of the following factors are necessary.
1)      The capacity of the storage structure(such as reservoir)
2)      The magnitude of flow and quantity of flow to ensure 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 structure such as leaves, reservoir, barrage and bridges.
The hydrological study of a project necessarily precedes 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.
                       Hydraulic failure( overtopping and consequent failure of an earthen dam due to an inadequate spillway capacity, failure of bridge and culverts due to excess flood flow, inability of a large reservoir to fill up with water due to over estimation of stream flow etc) can be removed by hydrological study. Various phases of the hydrological cycle such as rainfall, runoff, evaporation and transpiration are all non uniformly distributed both in time and space. All hydrologic phenomenon 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 process and the historical data retaliating to the event are analyzed by statistical methods to obtain information on probability 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 economics and other criteria to be taken in a given project.

Q.4. Briefly describe the importance of hydrological study in the context of Nepal.
Ans:- In context of Nepal, Nepal just started for the hydrological development, Nepal is in the primitive phase. So for the broad hydrological development in Nepal, the study of hydrology is very important.

Nepal has the 83,000 MW electricity generation capacity but we cannot produce 3% yet. To develop the hydropower effectively, more and more hydrological structure are required, but for these the study and the application of hydrological study is required.

Many area of Nepal faces in hydrological problem such as flood in Terai, landslide in hilly area, snowfall in mountain area etc, so for the protection from these problem hydrological study plays a great important role.

About 65% of people of Nepal are faced in water deficiency problem in dry country even Nepal is the second richest country in water among the other countries. About 70% of the Agricultural lands in Nepal are in lack of irrigation for agricultural crops. These are dependent on the rainfall for irrigation.

For the proper management of water resources, and for the proper utilization of these water resources, development of hydrological structures is required but for this, hydrological study has great importance in Nepal.

hydrology1.pdfAssignment question are prepared by Lecturer. GUPTA (A teacher of Department of Agricultural Engineering)
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