1. Introduction to Hydrology, Water Resources Engineering, CE 4005 Same as CV 4005

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For 4th Semester Polytechnic CE Students
Written by Garima Kanwar | Blog: Rajasthan Polytechnic

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Subject: Water Resources Engineering, CE 4005 Same as CV 4005

Branch: Civil Engineering 🏗️
Semester: 4th Semester 📚

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1. Introduction to Hydrology 🌍💧

Hydrology is the study of water in the environment. It includes how water moves through the Earth's systems, like rivers, lakes, groundwater, and the atmosphere. It’s an important field of study to understand the water cycle, water availability, and managing water resources efficiently.

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1.1 Hydrology: Definition and Hydrological Cycle 🌦️🔄

Definition of Hydrology: Hydrology is the scientific study of the movement, distribution, and quality of water on Earth. This field helps us understand how water is distributed across the Earth, how it moves, and how it interacts with various components of the environment.

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Hydrological Cycle (Water Cycle): The hydrological cycle explains how water moves between land, atmosphere, and oceans. It is a continuous cycle that involves various stages:

1. Evaporation: Water from rivers, lakes, and oceans turns into water vapor due to heat from the sun 🌞.
2. Transpiration: Plants also release water vapor into the air through their leaves 🌱💧.
3. Condensation: The water vapor cools down and forms clouds ☁️.
4. Precipitation: Water falls from the clouds as rain, snow, sleet, or hail 🌧️❄️.
5. Infiltration/Percolation: Some of the water from precipitation seeps into the ground 🌍.
6. Runoff: The remaining water moves across the surface and returns to rivers, lakes, or oceans 💦.

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1.2 Rain Gauge ☔

A rain gauge is a device used to measure the amount of rainfall at a particular location. There are various types of rain gauges that help gather data on precipitation.

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1.2.1 Symons Rain Gauge 🌧️

The Symons Rain Gauge is one of the oldest types of rain gauges. It consists of a funnel that channels the rain into a measuring container. This container collects the rainwater, which is then measured.

• Structure: The gauge has a wide funnel that directs rain into a smaller collection container.
• How it Works: When it rains, the water is collected and the amount is measured at regular intervals.
• Advantages: Simple to use and maintain.
• Limitations: Manual reading; doesn’t give real-time data.

Example: If 100 mm of rain is collected in the container, it means 100 mm of rainfall occurred during the time it was recorded.

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1.2.2 Automatic Rain Gauge 🌦️

An automatic rain gauge is a more advanced version of a rain gauge that uses electronic sensors to detect and record rainfall.

• Structure: It has a collector funnel that directs rain into a container or sensor.
• How it Works: Rainfall is automatically measured by the sensor, and the data is sent to a computer system.
• Advantages: Real-time data, no need for manual readings, more accurate.
• Limitations: More expensive and requires maintenance.

Example: Automated systems in weather stations use these rain gauges to collect rainfall data in real-time, which is then used in weather forecasts.

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1.3 Methods of Calculating Average Rainfall 🌧️📊

Calculating the average rainfall for a region helps in understanding the water resources available in that area.

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1.3.1 Arithmetic Mean 📐

The arithmetic mean is the simplest method for calculating average rainfall. You simply add up the rainfall measurements from all rain gauges and divide it by the number of gauges used.

• Formula:

  $$\text{Average Rainfall} = \frac{\text{Sum of Rainfall Measurements}}{\text{Number of Stations}}$$

• Example:

  • Station 1: 50 mm
  • Station 2: 60 mm
  • Station 3: 70 mm
  • Average = (50 + 60 + 70) / 3 = 60 mm

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1.3.2 Isohyetal Method 🌍📏

The Isohyetal method uses contour lines, called isohyets, to represent equal rainfall amounts. These lines are drawn on a map, and the average rainfall is calculated by finding the area between the isohyets.

• How it Works:

  • First, you plot the rainfall data from various stations on a map.
  • Draw lines connecting points with the same amount of rainfall.
  • Then, calculate the area enclosed by the isohyets and determine the average rainfall.

• Example: If rainfall at two stations is 50 mm and 100 mm, an isohyet line would be drawn to represent the transition between these two values, and the average rainfall is calculated based on the area covered by these lines.

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1.4 Runoff 💦

Runoff refers to the water that flows over the surface of the Earth after it rains. Some of this water goes into rivers and lakes, while some may infiltrate into the soil.

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1.4.1 Factors Affecting Runoff 🌧️🌍

Several factors can influence the amount of runoff in an area:

1. Rainfall Intensity: Heavy rainfall leads to more runoff, as the ground may not be able to absorb all the water.
2. Soil Type: Sandy soil absorbs water better than clay, meaning less runoff in sandy areas.
3. Land Use: Urban areas with concrete or asphalt surfaces increase runoff because water cannot infiltrate.
4. Vegetation: Dense vegetation helps absorb water, reducing runoff.
5. Topography: Steep slopes encourage water to flow faster, leading to higher runoff.
6. Impervious Surfaces: Roads, buildings, etc., prevent water from soaking into the ground, increasing runoff.

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1.4.2 Computation of Runoff 💡

To compute runoff, hydrologists often use formulas like the Rational Method or SCS Method. The most common one is the Rational Method.

• Formula: $$Q = CiA$$Where:
  • Q = Runoff (cubic meters per second)
  • C = Runoff coefficient (depends on land use)
  • i = Rainfall intensity (mm/hour)
  • A = Area (hectares)

Example:

• If the area is 2 hectares, the rainfall intensity is 10 mm/hour, and the runoff coefficient is 0.5: $$Q = 0.5 \times 10 \times 2 = 10 \, \text{m}^3/\text{s}$$So, the runoff will be 10 cubic meters per second.

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Summary 🌍💧

Hydrology helps us understand water’s movement and behavior. By using tools like rain gauges and calculating runoff, we can better manage water resources and prepare for things like floods or droughts. Understanding average rainfall through methods like the Arithmetic Mean and Isohyetal method aids in water resource planning, while considering factors like soil type, rainfall, and vegetation can help explain the rate of runoff.

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