What is a unit area hydrograph and how is it computed?

What is a unit area hydrograph and how is it computed?

• A. The hydrograph for 1 unit area; computed by normalizing discharge by drainage area and scaling to other basins.
• B. A hydrograph that shows rainfall intensity over time for a unit area.
• C. A plot of soil moisture content over time.
• D. A hydrograph of discharge scaled to rainfall depth.

Answer: (A)

A unit area hydrograph is the discharge response of a watershed to a unit amount of effective rainfall distributed over the entire basin, expressed per unit area so it can be applied to other basins and rainfall amounts. To compute it, take a measured hydrograph from a storm with known drainage area A and effective rainfall depth E. Normalize the discharge by the drainage area to get discharge per unit area, and then divide by the rainfall depth to convert to a response to unit rainfall. The resulting shape is the hydrograph for 1 unit of rainfall over 1 unit area. For other basins or rainfall depths, scale by the new area and depth and use convolution with the rainfall hyetograph to obtain the corresponding discharge hydrograph. This differs from a rainfall intensity over time (that’s a rainfall hyetograph) and from soil moisture trends, which are unrelated to the outlet discharge response. The idea of simply scaling discharge by rainfall depth without accounting for area is incomplete, since unit area normalization is essential for transferring the unit hydrograph between basins.

A unit area hydrograph is the discharge response of a watershed to a unit amount of effective rainfall distributed over the entire basin, expressed per unit area so it can be applied to other basins and rainfall amounts. To compute it, take a measured hydrograph from a storm with known drainage area A and effective rainfall depth E. Normalize the discharge by the drainage area to get discharge per unit area, and then divide by the rainfall depth to convert to a response to unit rainfall. The resulting shape is the hydrograph for 1 unit of rainfall over 1 unit area. For other basins or rainfall depths, scale by the new area and depth and use convolution with the rainfall hyetograph to obtain the corresponding discharge hydrograph.

This differs from a rainfall intensity over time (that’s a rainfall hyetograph) and from soil moisture trends, which are unrelated to the outlet discharge response. The idea of simply scaling discharge by rainfall depth without accounting for area is incomplete, since unit area normalization is essential for transferring the unit hydrograph between basins.