Runoff Generation

Question 1

Objective:

Answer 1

 In a wider sense: Separation of precipitation into several flow components RO, RI und RB

 In a narrower sense: Which part of total rainfall P becomes direct runoff (RD=RO+RI)? This part is called effective rainfall Peff (or excess rainfall). The rest of P is called losses or abstractions.

Question 2

Methods

Answer 2

1. Simple abstraction methods (e.g. runoff coefficient, Φ-index) → HYDRO I
2. Empirical time dependent functions (e.g. Horton) → HYDRO I
3. Physically based infiltration and soil water balance models (Richard’s equation, Green-Ampt, etc.) → here

Question 3

Richards Equation

Answer 3

 Solution of Richards equation usually by numerical methods

 Main problem is the determination of k(Θ) and ψ(Θ), given the poor data availability and the high variability for soil properties

 For estimation of k(Θ) and ψ(Θ) see van Genuchten (1980) and Mualem (1976)

Question 4

Green-Ampt Method: basic assumptions

Answer 4

 Vertical water movement in form of a sharp saturated wetting front (“piston flow”) based on Darcy equation

 Homogeneous soil, constant initial soil moisture over depth, rain rate PI > ks and ponding of water on surface

Question 5

Two-Stage model

Answer 5

 Further development of the original Green-Ampt method for practical hydrological use

 No ponding of water on surface; considering time variable rainfall rates also with PI < ks and applicable for layered soils

Question 6

Complexity:

Answer 6

• Macropores
• Heterogeneity of soils*1
• Variability of soil moisture*1
• Variability of rainfall