Step 1: Understanding the Concept:
A unit hydrograph (UH) is a fundamental tool in hydrology used to predict the direct runoff response of a watershed (catchment) to a unit input of rainfall. It represents the hydrograph of 1 cm (or 1 inch) of direct runoff generated by a storm of a specified duration occurring uniformly over the catchment. The question asks what determines the characteristic shape of this UH.
Step 2: Detailed Explanation:
1. The unit hydrograph is essentially the "fingerprint" or "impulse response" of a particular watershed. It shows how that specific watershed collects and drains water.
2. The way a watershed drains water is determined by its physical properties. These are known as catchment characteristics.
3. Key catchment characteristics that influence the shape of the unit hydrograph include:
Size and Shape of the Catchment: A long, narrow catchment will have a flatter, more spread-out hydrograph than a circular one of the same area.
Slope: Steeper slopes lead to faster runoff and a more peaked, shorter-duration hydrograph.
Drainage Density: The number and length of streams per unit area. Higher density means more efficient drainage and a quicker, more peaked response.
Land Use and Cover: Urban areas with impervious surfaces generate runoff faster than forested or agricultural areas, leading to different hydrograph shapes.
Soil Type: Infiltration rates vary with soil type, affecting the amount and timing of runoff.
4. While the {magnitude} of the runoff hydrograph depends on the amount of rainfall, and the {timing} is related to the duration of the unit rainfall, the fundamental shape (the characteristic response) is dictated by the unchanging physical properties of the catchment itself.
5. Rainfall intensity and duration are inputs to the model; the UH is the catchment's response function. Wind speed has a negligible effect on the overall hydrograph shape.
Step 3: Final Answer:
The shape of a unit hydrograph is a function of the physical characteristics of the catchment. Therefore, option (B) is the correct answer.