Rainfall Losses

Question 1

Derive the Green and Ampt Infiltration equation

Answer 1

Refer to notes

Question 2

What are the assumptions that come with Green and Ampt

Answer 2

• Soil is homogeneous
• The infiltration creates a sharply defined infiltration front
• At the start of the infiltrations, the soil moisture is uniformly distributed 𝜃(𝑧,0) = 𝜃i
• Above infiltration front, the water content is uniformly distributed (near saturation) 𝜃(𝜃,t) = 𝜃s with 𝜃s being the water content above the wetting front

Question 3

What are the parameters required for Green and Ampt

Answer 3

• The Green and Method contains four parameters K, phi, 𝜃s and 𝜃i.
• The value of 𝜃i will strongly depend on the antecedent rainfall conditions. The moisture content above the wetting front is limited to the effective porosity of the soil. Due to the air entrapment, full saturation is unliley so the value of the hydraulic conductivity above the wetting front is often set to 0.66*Ks where Ks represent the conductivity at satuaration
• Phi can be calculated using the Brooks-Corey equation

Question 4

Explain the Mein and Larson Model

Answer 4

Refer to notes

Question 5

Discuss Infiltration

Answer 5

• Infiltration highly depends on the state and characteristics of the soil
• Infiltrability or the infiltration capacity of the soil, which is the rate at which water can infiltrate the soil with sufficient water supply and no pressure is key
• The states and characeristics of the soil which are important to consider are:
  > the soil type and compaction
  > hydraulic conductivity of the soil
  > initial moisture content
  > surface cover and soil structure
  > viscosity of water
• Infiltration rate decreases exponentially with. continuous and sufficient water supply due to the saturation of the upper soil zone

Question 6

Explain the initial losses

Answer 6

• Initial losses are abstractions at the initial stage of a rainfall storm
• They are the:
  > Interception losses that are caused by the interception of rainfall by vegetation or litter
  > Wetting losses which are the abstractions caused by adsorption on the receiving surface
  > Depression storage which accounts for rain that is trapped in small depressions at the surface

Question 7

Describe interception losses

Answer 7

• It is the rainfall that does not reach the soil
• After a certain amount of rainfall however, intercepted rainfall can enter the soil by throughfall or stemflow
• There is no additional interception when this occurs
• Water on the vegetation will eventually evaporate
• The amount of interception loss depends on rainfall intensity, wind (speed), vegetation and the season
• During small rainfall events, the rainfall is almost fully intercepted
• It’s important for water balance

Question 8

Describe wetting losses

Answer 8

It is the amount of water needed on the surface (without depressions) before runoff will occur.

Question 9

Describe depression storage

Answer 9

• It is the rainwater that is trapped in small depressions, that cannot participate in runoff and is removed by evaporation or infiltration
• It is mainly important for minor storms in pervious areas rather than intense storms in urban areas
• The depression storage has a high correlation with the surface slope

Question 10

Describe Evapotranspiration

Answer 10

• Potential evaporation is the evaporation that takes place at the surface of a large water volume and is the maximum evaporation which may occur at a given location
• Actual evaporation accounts for the limitations on the availability of the water for evaporation
• Transpiration is the physiological process by which water is taken from the soil moisture storage by vegetation roots, transported through the plant and evaporated by the leaf stomata
• Evapotranspiration is the water vapour produced from the watershed, taking the growth of plants into account.