Understanding Rainfall Intensity for a 2-Hour Unit Hydrograph

A unit hydrograph is a fundamental concept in hydrology used to represent the watershed's response to rainfall. It specifically depicts the relationship between effective rainfall (rainfall excess) and the resulting direct runoff hydrograph.

Key characteristics of a unit hydrograph are:

• It is based on a unit depth of effective rainfall over the catchment area. Typically, this unit depth is considered 1 cm or 1 inch.
• The duration of this effective rainfall is constant and defines the specific unit hydrograph (e.g., a 2-hour unit hydrograph corresponds to a 2-hour rainfall duration).
• The rainfall is assumed to occur uniformly in space and time over the drainage basin.

Calculating Rainfall Intensity

The question asks for the intensity of rainfall for a 2-hour unit hydrograph. The intensity is the rate at which the unit depth of rainfall occurs over the specified duration.

The formula to calculate rainfall intensity is:

\begin{equation} \text{Rainfall Intensity} = \frac{\text{Unit Depth of Rainfall}}{\text{Duration of Rainfall}} \end{equation}

For a standard unit hydrograph, the unit depth is 1 unit (let's assume 1 cm based on the options). The duration is given as 2 hours.

Plugging these values into the formula:

\begin{equation} \text{Rainfall Intensity} = \frac{1 \text{ cm}}{2 \text{ hours}} \end{equation}

\begin{equation} \text{Rainfall Intensity} = 0.5 \text{ cm/h} \end{equation}

Determining the Correct Option

Based on the calculation, the intensity of rainfall for a 2-hour unit hydrograph is 0.5 cm/h. Comparing this result with the given options:

• Option 1: 2.0 cm/h
• Option 2: 1.0 cm/h
• Option 3: 0.25 cm/h
• Option 4: 0.5 cm/h

The calculated intensity matches Option 4. Therefore, the correct intensity of rainfall for a 2-hour unit hydrograph is 0.5 cm/h. This calculation highlights how the definition of a unit hydrograph helps determine the characteristics of the effective rainfall event it represents.