Let's have a look at very simple but interesting topic in watershed hydrology i.e. hydrologic cycle and its fundamentals.

Hydrology is a branch of water science that studies the occurrence, circulation, and distribution of water on the surface of the earth and in the atmosphere. Water is a necessity for human life as well as plant and animal survival. Hydrology can be used to assess and improve a region's water potential in this way. Below is a list of the importance of hydrology:

1. It provides an estimate of the river basin's water resource potential.

2. It is possible to compute reliable yields for huge irrigation projects and hydroelectric power plants.

3. It disseminates information on the likelihood of floods, their pattern, and magnitude.

4. Rainfall-based flood forecasting is also possible.

5. It provides erosion management standards and compares flood control strategies for any river basin.



Hydrology is a broad foundation science that is related to a variety of fields and involves a variety of facts and hypotheses that are distinct from those found in other fields. The challenges encountered in hydrology, in general, govern the scope and application of hydrology.

For measurement and publication of hydrologic data, as well as analysis and interpretation of data for the development of relationships, the collection and systematic presentation of hydrologic data, as well as accurate analysis and interpretation of data, can be evaluated.



  • Hydrologic data:

It refers to the data on available water and its quality both in time and space. The main types of hydrologic data are given below:

1. Historical data: This type of information is also known as chronological data. Historic data has unique characteristics that, if not recorded at the appropriate time, will be lost forever. Runoff, river discharge, rainfall depth, and other historical data are examples.

2. Field data: these are data that are recorded based on a requirement or necessity. In general, field data includes groundwater depth, sediment load, and river flow, among other things.

3. Laboratory and field data: Hydrology field and laboratory data are generated in the same way as hydraulic engineering data. These data are only to be used in fundamental or applied research.




  • Hydrologic cycle:

It is defined as the water transfer cycle in which water is carried from the ocean or sea to the atmosphere, then to the ground, and then back to the sea. It ensures that the amount of water on the earth's surface and in the atmosphere is always in balance. It's vital to remember that the earth's total water resources are constant, and the sun provides energy for the hydrologic cycle. The three phases of the hydrologic cycle are as follows:

  1. Evaporation and transpiration: These are the two processes by which water or moisture from a water body or soil surface is continuously removed under the influence of temperature. Evapotranspiration loss is the result of their combined action or water loss. The availability of moisture in the atmosphere is determined by these two processes. Evaporation from the oceans is thought to account for around 80% of total precipitation loss in India.


  1. Precipitation: It occurs as a result of the lifting of moisture into the atmosphere, which leads to the development of clouds at a certain altitude. Clouds release water in the form of precipitation, which is then circulated to the oceans, rivers, and inland bodies, among other places’



  1. Runoff: During precipitation, the vegetative foliage, dry surfaces of structures, the ground surface, and other surfaces intercept a portion of the precipitated water. Interception loss occurs when a signal is intercepted and then lost to the atmosphere. The remaining precipitation is distributed to streams, rivers, and other bodies of water. It's known as surface runoff, and it eventually ends up in the sea. Sub-surface runoff occurs when a portion of rain or runoff falls into the soil instead of flowing into rivers, oceans, or the sea.

The above three phases of the hydrologic cycle are always in function under influence of solar energy and complete an unending chain.

For inland areas, the hydrologic balance equation can be expressed as:

                              P= R+E

Where P= precipitation and E= losses through evapotranspiration



Precipitation, evaporation, runoff, and other components of the hydrologic cycle can be regarded as a dynamic system. The components of the overall cycle are further divided into subsystems. It consists of three types of water: atmospheric, surface, and subsurface. Precipitation, evaporation interception, and transpiration are all part of the atmospheric water system. Overland flow, surface runoff, subsurface groundwater outflow, and runoff to streams, oceans, and other bodies of water are all part of the surface water system. Infiltration, groundwater recharge, subsurface flow, and groundwater flow are all part of the subsystem water system.



A process is a term used to describe any occurrence that changes over time. The hydrologic process is the progression of hydrologic phenomena over time. The hydrologic processes are broadly classified as follows:

  1. Deterministic process: When the change in the occurrence of related variables is neglected and the process is considered to follow a clear law of certainty but not the law of probability, the process is said to be deterministic.


  1. Stochastic process: The sub-divisions of the stochastic process are as follows:
    1. Chance dependent process
    2. Time-dependent process: The order in which the variables appear is regarded in this procedure, and the variables are either pure random or non-pure random. Furthermore, the variable profitability distribution may or may not change over time.


      1. Pure random process: The variables in time series are independent of one another in stochastic processes, resulting in a random sequence.
  • Stationary process
  • Non-stationary process
      1. Non-pure random process: In this method, the time series variables are interdependent and form a random sequence. The deterministic and random components of non-random stochastic processes are combined to form non-random stochastic processes.
  • Stationary process
  • Non-stationary process


  1. Probabilistic process:
    1. Time independent process: The order in which variables appear is ignored, and the probability of their appearance is assumed to follow a definite probability distribution in which random variables are treated as pure random.


In reality, all hydrologic processes are stochastic in nature. The stalker stick processes are a family of random variables that can be a function of time or other factors in mathematics. In terms of quantification, the stochastic process can be discrete or continuous, and it can be sampled continuously, discretely, or uniformly.


Watershed as a hydrologic system

A watershed is the area of land that drains into a river or stream at a specific point. The line dividing the land area draining towards a certain stream is referred to as a watershed split. The watershed's system border is drawn by vertically projecting the watershed split above and downward to the horizontal plane. The rainfall that falls across the watershed's surface is the watershed's intake. The runoff, which is streamflow that is concentrated at the watershed's outlet, is the watershed's output.

Because all of the hydrologic cycle's associated processes occur within the watershed's division line, the watershed is also considered a hydrological entity.



It is based on the law of conservation of mass.

                        I = O + S

Where I = inflow

           O = outflow

           S = storage

The amount of total inflow into an area over a given period is always equal to the sum of outflow and change in storage volume made by that area, according to this equation. It enables the drainage basin's available water to be predicted.

The Water Budget Equation: The continuity principle, also known as the water budget equation for hydrologic equation, can be used to describe the quantities of water flowing through several specific channels of the hydrologic cycle in a particular system. The water budget equation mathematically describes a watershed's hydrologic cycle:


where Q is streamflow or discharge, P is precipitation, ET is evapotranspiration, ΔS is the change in soil moisture storage (i.e., water present in all types of soil pores), and ΔGW is the change in groundwater aquifer storage.



Because it is the only source of precipitation, atmospheric moisture is critical for maintaining the hydrologic cycle. Evaporation loss from land and water bodies is also affected. Atmospheric moisture refers to the presence of water vapor in the atmosphere, which changes spatially based on altitude. The temperature at which air contains the most water vapor at a given temperature is known as saturation, while the temperature at which air is saturated if chilled at constant pressure is known as dew point temperature.

Factors affecting relative humidity:

1. Altitude: As altitude rises, so does relative humidity, and vice versa.

2. Vegetation: It raises relative humidity by supplying moisture directly to the air moving over it. This is why, in comparison to farmed land, a band of barren land contains significantly less humidity.

3. Orographic effect: The largest amount of moisture in the atmosphere is found up to an elevation of 8000 metres above sea level. In terms of the orographic influence on relative humidity, it is stronger in the mountainous region than in-plane land at the same elevation.

4. Land and water mass: The presence of water bodies on the land surface has a significant impact on relative humidity. When all other conditions are held constant, humidity is found to be higher above the ocean surface than on land. As a result, the moisture content over the ocean is generally Li greater than some distance away from its inland parts.

5. Seasonal fluctuation in moisture content: this refers to the seasonal variation in moisture content.




As a result, the conclusion states that this article covers every basic idea linked to the hydrologic cycle of watershed hydrology. It explains the importance, scope, and applications of watershed hydrology, as well as many other fundamental concepts. How water travels across terrestrial habitats and produces groundwater and surface water is governed by hydrologic processes. The water budget equation is a basic mathematical tool for describing how water availability changes as inputs or outputs of the hydrologic cycle vary, either in relative terms or as calculated quantities. Basic watershed features, such as the percentages of various land uses, soil compaction and infiltration capacities, and streamflow sources, can also give useful information for managing and safeguarding water resources.

Frequently Asked Questions

Ans: Hydrology is a branch of water science that studies the occurrence, circulation, and distribution of water on the surface of the earth and in the atmosphere. Water is a necessity for human life as well as plant and animal survival. Hydrology can be used to assess and improve a region's water potential in this way. Below is a list of the importance of hydrology view more..
Ans: Precipitation is a general term for a type of atmospheric water that originates from a cloud and falls to the ground. In other words, precipitation is the dropping of water from the atmosphere on the earth in the form of rain, snow, mist, and other forms that are helpful to humans, and it is measured in terms of depth on a horizontal land surface. view more..
Ans: Precipitation is characterized as the depth of water that collects on the surface of the land during its occurrence. It's usually measured in mm. The term "rain gauge" refers to an instrument that is used to measure precipitation. view more..
Ans: The commonly used techniques for presentation of rainfall data, suitable for interpretation and analysis in different aspects are given: Mass curve Hyetograph Point rainfall. view more..

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