Hydrological Cycle and Water Budget Equation and World Water Balance
What is Hydrology?
Hydrology: Hydrology is the branch of science that deals with the occurrence, circulation and distribution of water of the earth and earth's atmosphere.
This subject is further classified in two parts:
Concerned mainly with academic aspects
Concerned with engineering application of study
Purpose of Engineering Hydrology
- Assess water resources.
- Study the various activities like runoff, precipitation, evapotranspiration.
- Study the challenges like droughts and floods and pan accordingly.
- The water on earth exists in solid, liquid and gaseous states.
- Formation and movement of clouds, rain and snowfall, evaporation, 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 hydrological cycle.
- To begin discussion on hydrological cycle, start with the oceans. Because of the heat energy from the sun, water in the oceans evaporate.
- The water vapour move upwards and forms clouds. While much of the clouds condense and fall back to the oceans as rain, a part of clouds is driven to the land areas by the winds. There they condense and precipitate onto the land mass as rain, snow, hail, sleet etc. A part of evaporation may evaporate back to the atmosphere even while falling.
- Another part may be intercepted by vegetation structures and other such surface modifications from which it may either evaporate back to atmosphere or move down to the ground surface.
- A portion of water that reaches the ground enters the earth's surface through infiltration, enhances the moisture content of soil and reaches the groundwater body.
- The transpiration process due to vegetation leads to the loss of some portion of groundwater to atmosphere. After meeting the needs of ground water the infiltrated water moves down the natural slope over the surface and through reach the ocean. and rivers to reach the ocean.
- Vegetation sends a portion of water from under the surface of the ground back to the atmosphere through the process of transpiration.
- The portion of the precipitation which by various paths above and below the surface of the earth reaches the stream channel is called runoff.
- Runoff after entering streams, it becomes streamflow.
- Thus hydrological cycle can also be defined as the study of movement of water in a cyclic manner.
- The sequence of events as above is a simplistic picture of a very complex cycle that has been taking place since the formation of the earth.
- It is seen that the hydrological cycle is a very vast and complicated cycle in which there are a large number of paths of varying timescales.
- Further, it is a continuous recirculating cycle in the sense that there is neither a beginning nor an end or a pause.
- Each path of the hydrologic cycle involves one or more of the following aspects:
- Transportation of water.
- Temporary storage and
- Change of state.
- The main component of hydrological cycle can be broadly classified as 'transportation (flow) component' and 'storage component.
- The quantities of water going through various individual paths of the hydrological cycle in a given system can be described by the continuity principle known as water budget equation or hydrologic equation.
- It is important to note that the total water resources of the earth are constant and the sun is the source of energy for the hydrological cycle.
- Recognition of the various processes such as evaporation, precipitation and groundwater flow helps one to study the science of hydrology in a systematic way.
- Storage component - Storage on the land surface (Depression storage, Ponds, Lakes, Reservoirs etc.). Soil moisture storage & ground water storage.
Residence Time: The average duration taken by a particle of water to pass from one phase to another phase is called residence time.
Sublimation and Deposition: It is a process, in which a solid particle gets converted into gaseous form without changing into liquid. (e.g. when camphor is heated it directly changes to gas without converting into liquid.)
water budget equation
Catchment Area: The area of land draining into a stream or a water course at a given location is known as catchment area. It is also known as drainage area and drainage basin.
Ridge Line/Divide Line: The catchment area is separated from its neighbouring area by a line called ridge. In USA, it is called divide and in UK, is called watershed.
- The areal extent of a catchment is obtained by tracing the ridge on a topographical map to delineate the catchment and measuring the area by a planimeter.
- Thus the catchment area affords a logical and convenient unit to study various aspects relating to the hydrology and water resources of a region.
- Further, it is probably the single most important drainage characteristic used in hydrological analysis and design.
Water Budget Equation: The continuity equation for water in its various phase is written as:
Mass inflow - Mass outflow = Change in mass storage
If the density of inflow, outflow and storage volumes are the same:
Vi - Vo = ΔS
Vi = inflow volume of water
Vo = outflow volume of water
ΔS = change in the storage of the water volume.
All the terms in the hydrological cycle cannot be known to the same degree of accuracy, so an expression for the water budget of a catchment for a time interval At is written as:
P- R- G-E-T = ΔS
P = Precipitation
R = Surface runoff
G = Net groundwater flow out of the catchment
E = Evaporation
T = Transpiration
AS = Change in storage
The total storage 'S' consists of three components: = Ss + Ssm + Sg
Ss = Surface water storage
Ssm = Water in storage as soil moisture
Sg = water in storage as ground water
Thus, ΔS = ΔSs + ΔSsm + ΔSg
In terms of rainfall - runoff relationship water budget equation can be represented as:
R = P - L
L = Losses = water not available to runoff due to infiltration, evaporation, transpiration and surface storage
world water balance
- The total quantity of water in the world is estimated to be about 1386 million cubic kilometres (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 24.4M km3 is contained in frozen state as ice in the polar regions and on mountain tops and glaciers.
- Residence time could be calculated 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.
- The volume of water in the rivers of the world = 0.00212 M km3
- The average flow rate of water in global rivers = 44700 km3/year
- Hence, residence time of global rivers, T = 2120144700 = 0.0474 year = 17.3 days.
- Similarly, the residence time for other phases of the hydrological cycle can be calculated. It will be found that the value of residence time, varies from phase to phase. In a general sense, shorter the residence time, greater is the difficulty in predicting the behaviour of that phase of that hydrologic cycle.
- Annual water balance studies of the sub-areas of the world indicate interesting facts.
- 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%.
- Approximately, 96.5% of all the water on earth is stored in the oceans, 1.69% is stored in underground and about 1.74% is frozen and stored in ice cups and glaciers 99.93% of water is present in these three locations.
- Only a minute fraction of Earth's total water supply comprises of the water found in river, lakes, the soil, and the atmosphere.
|Volume 103 Km3
Total Water (%)
|Residence Time (Years)
|Ice Caps and Glaciers
|0.25 (2-3 Months)
|0.022 (8 Days)
|0.05 (20 Days)
|1. Area (m km2)
|2. Precipitation (Km3/year)
|3. Evaporation (Km3/year)
|4. Runoff to Ocean
a) Rivers (Km3/year)
b) Groundwater (Mm/year)
c) Total Runoff (Km3/year)
- 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.
- The percentage of earth covered by oceans is about 71%.
- The percentage of total quantity of water in the world that is saline is about 97%.
- The percentage of total quantity of fresh water in the world available in the liquid form is about 3%.
application of hydrology in engineering
Hydrology finds its greatest application in the design and operation of water resource engineering projects, such as:
In all these projects, hydrological investigations for the proper assessment of the following factors are necessary:
- The capacity of storage structures such as reservoirs.
- The magnitude of flood flows to enable safe disposal of the excess flow.
- The minimum flow and quantity of flow available at various seasons.
- The interaction of the flood wave and hydraulic structure, such as levees, reservoirs, barrage & bridges.
- a) The hydrological study of a project should necessarily precede structural and other detailed design studies.
- b) It involves the collection of relevant data and analysis of the data by applying the principle and theories of hydrology to seek solutions to practical problems
- c) 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 spillway capacity.
- ii) Failure of bridges and culverts due to excess flood flow and inability of a large reservoir to fill up with water due to overestimation of the stream flow. Such failure, often called hydrologic failures.
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- Hydrology means the science of water. It is the science that deals with the occurrence, circulation and distribution of water of the earth and earth's atmosphere.
- The various aspects of water related to the earth can be explained in terms of a cycle known as the hydrological cycle.
- This whole cycle of movement of water from one form to another in the atmosphere is known as hydrological cycle.
- The average duration of a particle of water to pass from one phase to another phase is called residence time.
- Sublimation and deposition is a process in which in which a solid particle gets converted into gaseous form without changing into liquid. (e.g. when camphor is heated, it directly changes to gas without converting into liquid.)
- The area of land draining into a stream or a water course at a given location is known as catchment area. It is also known as drainage area and drainage basin.
- The catchment area is separated from its neighbouring area by a line called ridge. In USA, it is called divide and in UK, is called watershed.
- Water budget equation: The continuity equation for water in its various phases is written as:
- Mass inflow - Mass outflow = Change in mass storage
- All the terms in the hydrological cycle cannot be known to the same degree of accuracy. So, an expression for the water budget of a catchment for a time interval At is written as:
- P - R- G-E-T = ΔS
- P = Precipitation
- R = Surface Runoff
- G = Net groundwater flow out of the catchment
- E = Evaporation
- T = Transpiration
- ΔS = Change in storage
- Hydrological Cycle
- Residence Time