CIVE1145 Rainfall Abstraction (Exam)
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What is interception? What is the limitation of using linear (Hortonian type) equation for its estimation? Does depression storage contribute to surface runoff?
- A process by which rainfall is abstracted prior to reaching the ground. Wetting of surface vegetation is typically the primary form of interception.
- Hortonian eq shows that there will be unlimited interception as precipitation increases – it can not be true. Depression storage does not contribute to surface runoff, it either infiltrates or evaporates following the rainfall.
What are the factors influencing infiltration in soil? Draw a net sketch to show the distribution of soil moisture during the downward movement of water
- Surface cover – vegetation tend to have significantly higher infiltration capacity
- Underlying soil – porosity, hydraulic conductivity, soil strata, current moisture content, etc
How is sorptivity (S) of the Phillip’s equation is estimated?
- For horizontal column of soil, soil suction is the only force drawing water into the dry soil, and Phillip’s eq reduces to F(t) = St^1/2
- So we can solve the eq for S when the cumulative infiltration F can be measured for a duration of t
Draw a net sketch to show the distribution of soil moisture in Green-Ampt method. What is effective porosity?
effective porosity is the interconnected pore volume or void space in a rock that contributes to fluid flow or permeability in a reservoir.
What is called ponding time? What was the assumption in deriving all the 3 infiltration equations? Is it going to provide the same infiltration rate after 30 minutes if ponding times are 10 and 40 minutes, respectively? If not, why?
- Ponding time tp is the elapsed time between the time rainfall begins and the time water begins to pond on to the soil surface
- Assumption that water is ponded to a small depth on the soil surface and so all the water the soil can infiltrate is available at the surface
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