---
title: "Processes in Clark's method"
subject: "Engineering Hydrology"
chapter: 7
key: "semester_5/engineering_hydrology:clark-processes"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/clark-processes
site: BCT Engineering
---

# Processes in Clark's method

Engineering Hydrology · Chapter 7: Flow Routing

- Asked in: 2078 Bhadra, 3 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/clark-processes
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-7

## Question

What are the processes that Clark's method considers in the transformation of excess precipitation to runoff? How are they achieved?

## Short answer

Translation (movement) is represented by time-area diagram from isochrones; attenuation (storage) is modeled by a single linear reservoir S=KQ at the outlet. Inflow from each strip is routed through the reservoir to obtain IUH.

## Answer

Clark's method considers two processes in transforming excess precipitation into direct runoff.

#### 1. Translation (movement)

Water from each part of the catchment travels to the outlet without change of shape, only delayed by the travel time. This is represented by the **time-area diagram** (isochrones). The area between two isochrones contributes a flow $I = 2.78\,A_i/\Delta t$ for 1 cm of excess rain.

How it is achieved: the basin is divided by isochrones at interval $\Delta t$ up to the time of concentration; the strip areas give the translation hydrograph (inflow ordinates).

#### 2. Attenuation (storage)

Storage in the channels, soil surface, and depressions reduces the peaks and spreads the flow in time. This is represented by a **single linear reservoir** at the outlet, $S = KQ$.

How it is achieved: the translation hydrograph is routed through the linear reservoir using $O_2 = C_0(I_1 + I_2) + C_2 O_1$, with $C_0 = \Delta t/(2K + \Delta t)$ and $C_2 = (2K - \Delta t)/(2K + \Delta t)$. The result is the IUH, and averaging two consecutive ordinates gives the unit hydrograph of duration $\Delta t$.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which two processes does Clark's method consider?
   A. Infiltration and evaporation
   B. Translation and attenuation (storage)
   C. Runoff and baseflow
   D. Interception and transpiration
   Answer: B. Excess rain is translated to the outlet then attenuated by storage. (Key point: Two processes)

2. How is translation achieved?
   A. By the time-area diagram from isochrones, I = 2.78 A_i/dt
   B. By a linear reservoir routing
   C. By the Muskingum method
   D. By a rain gauge network
   Answer: A. Isochrone strip areas give the translation inflow ordinates. (Key point: Translation)

3. How is attenuation achieved?
   A. Using the rational formula
   B. By summing strip areas
   C. By a Gumbel distribution
   D. Routing through a single linear reservoir S = KQ
   Answer: D. O2 = C0(I1+I2) + C2 O1 gives the IUH. (Key point: Attenuation)

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