Unit Hydrograph Theory - Part 1

Summary

This video explains the concept of **unit hydrographs**, which model a watershed's response to rainfall excess. Rainfall excess is the total rainfall minus abstractions like interception and infiltration. The video demonstrates how a unit hydrograph is created by plotting rainfall excess against time and the resulting direct runoff hydrograph. Using the **linear superposition principle**, the video shows how to calculate runoff for various rainfall depths. The simple yet powerful approach helps in understanding runoff behavior for different rainfall conditions, providing valuable insights for hydrological modeling.

Takeaways

• 😀 Unit hydrographs describe the response of a watershed to rainfall excess.
• 🌧️ Rainfall excess is the total rainfall minus abstractions like interception, depression storage, and initial infiltration.
• 📊 The unit hydrograph is graphically represented with time on the x-axis and rainfall excess rate on the y-axis.
• 🌊 Below the rainfall excess graph, a hydrograph shows the direct runoff with time on the x-axis and runoff on the y-axis.
• 💧 A block of rainfall excess represents a fixed duration (delta t) and a depth of one unit (e.g., 1 cm or 1 inch).
• 🔢 The rate of rainfall excess is calculated by dividing the depth by time (one unit divided by delta t).
• 📈 The result of one unit of rainfall excess is a direct runoff hydrograph, referred to as a delta t unit hydrograph (U(t)).
• 🔄 The principle of linear superposition allows us to scale the unit hydrograph for different rainfall depths.
• 📐 The shape of the runoff hydrograph remains consistent, and only the magnitude changes when applying the linear superposition principle.
• 📏 For a rainfall depth 'p', the direct runoff hydrograph (Q(t)) is determined by multiplying the unit hydrograph (U(t)) by 'p'.
• 🤔 This method helps calculate direct runoff hydrographs for rainfalls of any depth, allowing hydrologists to predict runoff based on rainfall excess.

Q & A

What is a unit hydrograph?

A unit hydrograph is a graphical representation that shows the direct runoff from a watershed as a result of one unit of rainfall excess, such as one inch or one centimeter of rainfall, uniformly distributed over a specified time period.

What does the x-axis and y-axis represent on a unit hydrograph?

In a unit hydrograph, the x-axis represents time, and the y-axis represents the rate of rainfall excess or direct runoff.

What is 'rainfall excess' and how is it calculated?

Rainfall excess is the amount of rainfall that contributes to runoff, calculated by subtracting abstractions (such as interception, depression storage, and initial infiltration) from the total rainfall.

What are some examples of 'abstractions' in hydrology?

Abstractions include interception (rainfall caught by vegetation or structures), depression storage (water retained in surface depressions), and initial infiltration (water that soaks into the ground before runoff begins).

What is the role of a 'block of rainfall' in creating a unit hydrograph?

A 'block of rainfall' represents a period of rainfall excess, where the rainfall depth is uniformly distributed over the watershed for a specific duration (Δt). This block is used to generate the corresponding unit hydrograph.

How is the rainfall rate calculated from a block of rainfall?

The rainfall rate is calculated by dividing the rainfall depth (one unit) by the duration (Δt), which gives the rainfall excess rate.

What is the purpose of using the linear superposition principle in hydrology?

The linear superposition principle allows for the generation of direct runoff hydrographs for different rainfall depths by scaling the unit hydrograph accordingly, assuming that runoff is proportional to the rainfall depth.

How does the linear superposition principle work in practice?

When applying the linear superposition principle, the shape of the hydrograph remains the same, but its magnitude is adjusted by multiplying the unit hydrograph by the depth of rainfall excess (p). This allows us to calculate the runoff for any rainfall depth.

What is the formula used to calculate the direct runoff hydrograph for a rainfall depth of 'p'?

The formula is: Q(t) = p * u(t), where Q(t) is the runoff hydrograph, p is the rainfall depth, and u(t) is the unit hydrograph.

Why is it important to understand unit hydrographs in hydrology?

Understanding unit hydrographs is essential because they provide a standardized way to predict runoff from a watershed based on rainfall data. This helps in flood forecasting, water resource management, and understanding watershed behavior during storms.