Runoff concentration

Question 1

Objective:

Answer 1

 In a wider sense: Transformation of generated runoff components RO, RI and RB, from location of genesis into discharge in the river

 In a narrower sense: Transformation of the effective rainfall Peff in direct runoff QD; estimation of the direct hydrograph

Question 2

Methods:

Answer 2

1. System hydrological methods e.g. unit hydrograph → see Chap. 5.3.1 and HYDRO I
2. Conceptual models e.g. linear reservoir, Clark-Model, timearea method, etc. → see Chap. 5.3.2/5.3.3
3. Physically based models: 3-D Richards equation, groundwater models, etc. → see courses about Groundwater

Question 3

System hydrological methods:

Assumptions when applying system hydrologic concepts

Answer 3

1) Proportionality
2) Superposition principle
3) Time invariance

Question 4

System hydrological methods: Synthesis: Discrete convolution using the unit hydrograph (UH):

Answer 4

The UH is the direct runoff hydrograph of a watershed resulting from 1 mm effective rainfall occurring uniformly over the drainage area at a constant rate for a specific duration.

Question 5

System hydrological methods: Analysis:

Answer 5

 Empirical identification of the unit hydrograph (UH) is possible by analysing observed rainfall runoff events → black-box-method

 Analytical identification of the UH (or the pulse response function) can be reached based on different hydrological model concepts, e.g. the linear reservoir (see later and HYDRO I).

Question 6

System hydrological methods: Analysis: (2)

Procedure black-box-method:

Answer 6

1) Selection of rainfall-runoff event(s), definition of Δt
2) Separation of base flow: Q - QB = QD
3) Calculation of effective rainfall: P → Peff (see Chap. 5.2)
4) Derivation of the system operator φ from QD and Peff

Question 7

Model concepts: Overview

Answer 7

 Simulation of complex hydrological processes using simple conceptual models or model concepts;

 First step to consider physical properties in a simple way (no “black-box” anymore → may be called “grey box”)

Question 8

Time-area method (method of isochrones):

Answer 8

 Transformation of effective rainfall Peff into direct runoff QD based on linear translation

 The main idea is to construct Isochrones = lines of equal travel time to the outlet Tt for a catchment

 The travel time for QD depends on topography, soil properties, roughness, etc.

 Compared to the rational method (Flutplan) the isochrones are not parallel because of variable flow velocities

 The generated runoff of the areas between the isochrones can be routed to the outlet assuming linear translation

 For construction of the isochrones empirical equations to calculate flow velocities (e.g. Manning-Stickler) or travel times (e.g. Kirpich) can be utilized

Question 9

Application of system hydrological standard functions on Time-Area-Method

Answer 9

1. Step response function h(t):
   is non-linear in general with stepwise linear segments for contributing areas ΔA between the isochrones
2. Pulse response function g(Δt,t):
   is constructed from the difference between two step response functions shifted by Δt
3. Impulse response function g(t) = time-area-histogram:
   Response of a catchment using the time-area method to a unit impulse input δ(t) in form of a impulse response function g(t)

Question 10

Clark-Model: parameter estimation

Answer 10

• k from model calibration or hydrograph separation
• gI(t) directly from basin properties or from regionalisation e.g. gI(t)=f(Tc) using Tc from topography or from calibration