---
title: "Interception and depression storage losses"
subject: "Engineering Hydrology"
chapter: 3
key: "semester_5/engineering_hydrology:interception-depression-storage"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/interception-depression-storage
site: BCT Engineering
---

# Interception and depression storage losses

Engineering Hydrology · Chapter 3: Hydrological Losses

- Asked in: 2073 Shrawan, 2+2 marks
- Video: [Interception and Depression Storage](https://www.youtube.com/watch?v=7LbwD37GBg0) — John Lloyd Mariano
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/interception-depression-storage
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-3

## Question

Explain interception and depression storage losses. How are these losses estimated during hydrological analysis?

## Short answer

Interception is rain held on vegetation and evaporated; depression storage is water trapped in surface hollows. Interception is estimated via canopy measurements or empirical formulas (e.g., Horton), while depression storage is often modeled as an exponential function of rainfall excess or lumped into initial abstraction.

## Answer

#### Interception loss
When rain falls on vegetation, part of it is held on leaves, stems and branches, and later evaporates without reaching the ground. This is **interception**. It depends on the type and density of vegetation, season (leaf cover), rainfall intensity and wind. It is about 10-20% of annual rainfall in forests and may be 100% for a light shower.

#### Depression storage
Rain that reaches the ground fills small hollows, puddles and ditches. This water cannot flow as runoff; it is lost by evaporation and infiltration. It depends on the slope, soil, land use and the rainfall excess. Typical values: 5 mm for sand, 2.5 mm on slopes, up to 10-15 mm in flat farmland.

#### Estimation in hydrological analysis
- **Interception:**
  - Measured as the difference between rainfall above the canopy and the throughfall plus stemflow below it.
  - Empirical form (Horton): $S_i = a + bP^n$ with $a$, $b$, $n$ as vegetation constants.
  - In design, 1-3 mm is often lumped as initial loss.
- **Depression storage** (Horton):

$$
V_d = S_d\left(1 - e^{-kP_e}\right)
$$

  where $V_d$ is the storage filled, $S_d$ the maximum storage capacity, $P_e$ the rainfall excess and $k$ a constant. Alternatively, the initial abstraction $I_a$ (the sum of interception, depression storage and early infiltration) is subtracted from the storm rainfall (SCS: $I_a = 0.2S$).

## Check yourself

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

1. What is interception loss?
   A. Water filling hollows in the ground
   B. Water entering soil pores
   C. Rain held on vegetation that evaporates without reaching the ground
   D. Water leaking to another basin
   Answer: C. Interception is rain retained on leaves, stems and branches and later evaporated. (Key point: Interception)

2. Depression storage water is lost by
   A. Runoff to the stream
   B. Throughfall
   C. Stemflow
   D. Evaporation and infiltration
   Answer: D. Water held in hollows cannot run off; it evaporates or infiltrates. (Key point: Depression storage)

3. Interception is measured experimentally as
   A. Soil moisture change
   B. Rainfall above the canopy minus (throughfall + stemflow)
   C. Throughfall plus stemflow
   D. Rainfall minus runoff
   Answer: B. What does not reach the ground as throughfall or stemflow is intercepted. (Key point: Estimating interception)

4. In the SCS approach the initial abstraction is taken as
   A. Ia = 0.8S
   B. Ia = 2S
   C. Ia = S + P
   D. Ia = 0.2S
   Answer: D. Ia, the sum of interception, depression storage and early infiltration, is 0.2S. (Key point: Initial abstraction)

## Related in Engineering Hydrology

- [Hydrological losses from precipitation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Ahydrological-losses) — Engineering Hydrology, chapter 3
- [Hydrological losses statement, factors affecting evaporation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Alosses-not-actual-evap-factors) — Engineering Hydrology, chapter 3
- [Direct runoff hydrograph analysis, 4250-acre basin](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Atrapezoid-dro-4250-acre) — Engineering Hydrology, chapter 5

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