Concept of hydrology:

Hydrology is a natural science that studies water on Earth’s surface and atmosphere, including its circulation, distribution, and movement in soil and rocks. It is a multidisciplinary field that examines the movement and distribution of surface and groundwater in both natural and man-made environments. Hydrologists analyse precipitation, evaporation, transpiration, infiltration, runoff, groundwater flow, and water storage in lakes, rivers, and aquifers. The field has practical applications in water resource management, flood forecasting, water quality monitoring, environmental protection, and climate change adaptation. Techniques like computer modelling and remote sensing are used to study the water cycle and its impacts.

Hydrological System Models

Abstraction of reality called model. A model is-

• An idealized representation of reality
• A description or analogy used to aid visualization and understanding.
• A system of postulates, data and inferences presented as a mathematical description of an entity or state.

Hydrological system models are mathematical representations that simulate the complex interactions of water movement within a given catchment, basin, or watershed. These models help us understand and predict various elements of the hydrological cycle, including:

• Precipitation
• Infiltration
• Runoff
• Evapotranspiration
• Groundwater flow
• Streamflow

 Why use hydrological models?

• Water Resource Management: Aid in planning, decision-making, and managing water resources for drinking, agriculture, hydropower, and ecosystem needs.
• Flood Assessment and Prediction: Help determine flood risks, develop warning systems, and design flood mitigation measures.
• Drought Analysis: Provide insight into drought conditions and their impact on water supplies.
• Climate Change Impact Studies: Explore the potential effects of climate change on water availability and flooding patterns.
• Environmental Assessment: Help understand how land-use changes or human activities might impact water quality and quantity.

Classification of models:

Hydrological model has two major parts–

1. Material model
2. Mathematical model.

Material models

Material models can be either ‘physical models’ or ‘analog models. It’s divided into two parts-

• Physical model is a tangible constructed representation of a portion of the natural world. A miniature, scaled version of a particular watershed and channel and a scaled sand tank model are the examples of physical models.
• Analog models use substances other than those in a real system. They use observations of one process to simulate a physically analogous natural process. An example of an analog model is an electric analog model in which the flow of electricity represents the flow of water.

Mathematical models

A mathematical model is a set of mathematical expressions and logical statements combined in order to simulate a natural system. It uses a governing equation thought to represent the physical processes that occur in the system, mathematical models range from a simple linear regression equation to highly complex partial differential equations. It has four major parts – analytical, numerical, empirical and theoretical models.

• Mathematical models can be solved analytically using calculus, which are known as ‘analytical models. This model may or may not involve computer; or they can be solved numerically using numerical techniques (possible for simple as well as complex cases), which are known as ‘numerical models. The use of computer is necessary for solving numerical models. The set of commands used to solve a mathematical model on a computer is called ‘computer program’, ‘computer code’, or simply ‘code.
• Mathematical models can also be classified as ‘empirical models’ or ‘theoretical models. Empirical models are based on input-output relationships and do not necessarily simulate the actual processes involved. Such models rely on observed input and output data, and simply relate the output to a given set of inputs through a structure which may be fully statistical. Empirical models are also known as ‘black-box models’ and they do not help in the physical understanding of processes involved. Examples of empirical models are regression models and artificial neural network (ann) models.
• Theoretical models rely on physical laws and theoretical principles. It is assumed that the hydrologic functions or relationships in a system are well understood and can be mathematically approximated directly from system characteristics. Thus, theoretical models use equations derived from basic physics (e.g., conservation of mass con, force balance, diffusion, etc.) To simulate flow, transport and storage. Examples are: ‘water balance models’, ‘analytical models’ and ‘numerical models. Theoretical models are called ‘white-box’ models or ‘grey-box’ models depending on whether model parameters and spatially-varying inputs are considered spatially distributed (white-box models) or lumped (grey-box models) for the model area (basin or sub-basin). Grey-box models are also known as ‘lumped or conceptual models’, whereas white-box models are also known as ‘physically based or process-based models.

Hydrological budget

Water is an important natural resource that sustains life on earth and contributes to the development of society. Interestingly, 97% of the earth’s water is held by the oceans, which cover 71% of the earth’s surface and have an average Depth of 3.8km. Frozen water accounts for 2%, while groundwater and fresh water from rivers and lakes account for 0.31% and 3%, respectively. The atmosphere holds only 0.3% of fresh water, which needs to be replaced 40 times every year. In India, rainfall is the primary source of water, and It varies from region to region.

A hydrologic budget, also known as a water budget or a water balance, is a quantitative analysis of the inputs, outputs, and changes in water storage in a defined area or system over a specified time period. It is used to track the movement of water within a specific region or system, such as a watershed or aquifer.

The hydrologic budget considers all sources of water, including precipitation, surface water, groundwater, and water diverted from outside sources. It also accounts for all losses of water, such as evapotranspiration, surface runoff, and groundwater discharge.

By examining the hydrologic budget, water managers and scientists can assess the availability and sustainability of water resources, as well as identify potential water shortages or surpluses. The hydrologic budget can also be used to evaluate the impacts of climate change, land use changes, and other factors on the water cycle in a particular region.

Global water budget:

input = output,

mass balance: input – output = change in balance;

input, output storage considers as budget or mass balance. Precipitation works as an input it divided into two ways as solid form (ice) and liquid from (rainfall). Its storage in soil and groundwater and evaporate through stream flow and groundwater outflow.

Hydrological measurement and data sources

Hydrology is a science built on observations and measurements. Hydrologic theories either have emerged from insights gained through analyzing data or have been confirmed through data that support the theory. The report Opportunities in the Hydrologic Sciences (NRC, 1991) recognizes the importance of data collection, distribution, and analysis by devoting a chapter to the issues concerned with this critical topic. The chapter presents compelling arguments for (1) the need to continue to utilize observational networks and experimental measurements, (2) an assessment of the status of hydrologic data collection, and (3) exploiting opportunities to improve hydrologic data. It seems redundant to repeat the arguments here. Nonetheless, it is possible to provide an assessment of whether concerns raised in the 1991 report have been heeded and whether opportunities have been seized that could provide new, innovative measurements for hydrological theory.

Hydrological measurements include measurement of the levels, bottom relief, depths and free surface of flows. It is used extensively in studying physical phenomena. Hydrological measurements are important for the interpretation of water quality data and for water resource management.

Hydrological measurements –

1. Surface runoff exits field through a flume, discharge based on a rating curve specific for the flume type and dimensions, as an example we can say in a field h type flume used to measure surface run off.
2. Subsurface flow exits the field through tile drains, discharge is measured by Endress and Hauser Prosonic flow meter, measurements are logged by datalogger.

Propeller based wind anemometer measure wind velocity and direction, temperature measured by thermometer, whereas relative humidity measure by red blub thermometer, evaporation and infiltration measured by evaporation pan and hydrograph.

Hydrological data

Hydrological data refers to any information related to the hydrological cycle, including the quantity, quality, and movement of water in the Earth’s system. This data is collected through a variety of methods, including field measurements, satellite and aerial imagery, and computer modelling.

Hydrological data refers to any information concerning the properties, distribution, and movement of water on Earth. It encompasses a wide range of variables, including both time series data (data collected regularly over time) and spatial data (data linked to geographic locations).

Types of Hydrological Data

1. Precipitation Data: Precipitation data include measurements of rainfall, snowfall, sleet, and hail. These measurements are typically collected using rain gauges, weather radar, and satellite-based remote sensing techniques. Precipitation data are essential for estimating water input into the hydrological system and for assessing rainfall patterns and trends over time.
2. Streamflow Data: Streamflow data comprises measurements of the quantity and timing of water flowing in rivers, streams, and other surface water bodies. Streamflow measurements are typically collected using stream gauges or flow metres installed at various locations along watercourses. Streamflow data are used for water resources management, flood forecasting, and assessing riverine ecosystem health.
3. Groundwater Data: Groundwater data include measurements of groundwater levels, groundwater flow rates, and groundwater quality parameters such as temperature, pH, and chemical composition. Groundwater data are collected from monitoring wells, piezometers, and aquifer tests and are used for assessing groundwater availability, recharge rates, and contamination risks.
4. Evapotranspiration Data: Evapotranspiration data refer to measurements of the process by which water is transferred from the Earth’s surface to the atmosphere through evaporation from soil and water surfaces and transpiration from plants. Evapotranspiration data are obtained using weather stations, remote sensing techniques, and field measurements and are used for water balance calculations and agricultural water management.
5. Soil Moisture Data: Soil moisture data include measurements of the moisture content of soil at various depths beneath the land surface. Soil moisture measurements are collected using soil moisture sensors, neutron probes, and remote sensing technologies. Soil moisture data are important for understanding soil water dynamics, assessing drought conditions, and optimizing irrigation practices.
6. Water Quality Data: Water quality data encompass measurements of various chemical, physical, and biological parameters in surface water and groundwater, including pH, dissolved oxygen, nutrients, heavy metals, and microbial contaminants. Water quality data are collected from water samples analysed in laboratories and in situ sensors deployed in water bodies. Water quality data are used for monitoring water pollution, assessing ecosystem health, and protecting human health.
7. Climate Data: Climate data include measurements of meteorological parameters such as temperature, humidity, wind speed, and solar radiation. Climate data are collected from weather stations, satellites, and climate models and are used for understanding climate variability and change, assessing the hydrological impacts of climate change, and developing climate adaptation strategies.

Why is hydrological data important?

1. Water Resource Management: Assessing water availability for drinking water, irrigation, industry, and hydropower.
2. Flood and Drought Forecasting: Developing prediction models and providing early warnings.
3. Infrastructure Design: Designing dams, irrigation canals, bridges, and other water infrastructure based on expected flow patterns and volumes.
4. Environmental Protection: Understanding and mitigating the impacts of human activities on rivers, lakes, and groundwater systems.
5. Climate Change Studies: Analyzing how climate change influences precipitation patterns, streamflow, and groundwater recharge.