Lecture7 vegetation, landuse and runoff generation (1)

• Less evapotranspiration may affect high base flow • Reducing infiltration capacity due to soil compaction increase runoff.. Effects of forest roads on storm flow generation and sedi

VFU, 23-September-2014

Principles of

Watershed management

Lecture #7 Vegetation, landuse, and runoff generation

Dr Bui Xuan Dung- Department of Environment Management

Watershed morphometry and

discharge

• What is the primary difference between these two basins?

• How much potential hydrograph differed

Drainage Density

• The drainage density is a measure of the total length

of well defined channels that drain the watershed

(sometimes measured as the blue lines representing

the streams on a topographic map)

• Drainage density is defined as the sum of the lengths

of all of the channels (km or miles) divided by the total

watershed area (km2 or ft2) This ratio can be

determined from topographical maps

Drainage Density

• Drainage density affects the response of the

watershed to rainfall

• High densities usually allow fast runoff removal

Therefore, greater peaks and hydrographs with

shorter durations are expected for watersheds with

higher drainage densities

• The effect of drainage density on runoff volume is

associated with the time during which the runoff

remains in the watershed

• Low densities allow for long

residence times; therefore,

abstraction mechanisms have

more time to remove water

• Drainage density can be increased by urbanization Because of the faster response facilitated by high densities, the frequency distribution of streamflow approximately follows that of precipitation events

Effects of land use on hydrograph

shape

Urbanization effects

Vegetation Management and water

yield

• What are the real and anticipated effects of

vegetation removal on the amount and distribution of

precipitation?

• What degree can water yields be manipulated by

altering vegetation yield?

• Can vegetation be manipulated to complement water

resources management objectives?

• What extent of seasonal stream flow patterns altered

by changing vegetation cover?

Forest Types

acre to several hundred acres

been cut

Forest Management

• Clear-Cutting - cutting all trees in a stand at

one time

• Shelterwood cutting – cutting dead/undesirable

trees - Always have young trees

Approaches, cont

• Seed-tree Cutting - removes all but a few with good genetics and high seed production which will be used to promote regeneration

• Selective Cutting - Individual trees are cut

Thinning of small, poorly formed trees, or trees of specific sizes

Landuse activity and flooding

• Removal vegetation decrease interception

loss

• Less evapotranspiration may affect high

base flow

• Reducing infiltration capacity due to soil

compaction increase runoff

• Increasing the soil erosion decreases

capacity of channel for flow

Effects of forest roads on storm flow

generation and sediment transport in

mountainous catchments

五味 高志

throughfall stemflow

subsurface flow

precipitation evapotranspiration

precipitation

roads overland flow

subsurface flow

evapotranspiration

Road hydrology

Hortonian overland flow (HOF)

intercepted subsurface flow (ISSF)

Does ISSF-derived erosion and solute export need to

be considered?

If so, road design needs to

be better planned in relation with soil depth and subsurface flow pathway

Cutslope seepage

Bedrock

Subsurface flow

………

Hortonian overland flow

林道のり面からの水

岩盤

地表面流

地下水流・土壌中水流 林道面からの排水

Drainage discharge Effects of forest road on flow generation and soil erosion （Hillslope scale)

林道が水・土砂流出に与える影響 （斜面スケール）

Hortonian overland flow Cutslope

interception

135  E

130  E

35  N

40  N

Stream Experimental plot Forest road plot

Forest road plots

林道面

等高線

Convex shape

Planar shape

Concave shape

V-notch weirs pipe

water shut plate

gutter

water gauge Road runoff monitoring

50

100

Rainfall (mm)

0

25

50

谷

y = 0.5x 尾根

y = 0.3x

平行

y = 0.5x

0

50

100

150

総降雨量 (mm)

kr-1

kr-3

線形 (kr-1)

線形 (kr-3)

2 4

2011.5.28 - 30 Total rainfall : 62.2 mm Max 1hour rainfall: 6 mm

0 60 120

1 2 0 0 1 1 2

ff （%

kr-1 kr-3

2011.10.5 - 6 Total rainfall: 64.4 mm Max 1hour rainfall: 7.6 mm

kr-1 kr-3

0 0.5 1 1.5 2 2.5

0

1

2

3

4

API = 0

n = 12, 17, 12

0 < API < 0.1

n = 5, 7, 5

API > 0.1

n = 7, 9, 7

kr-1 kr-2 kr-3 (5.5)

Soil

Bedrock/

Restrictive Layer Travelway

Cutslope Interception

of SSF

Effects of roads on runoff:

Intercepting subsurface flow (SSF)

Interception of SSF is greatest when cutslope height > soil depth

Primary Controls on Road Surface Erosion

1 Amount of road surface runoff, which depends on:

- Precipitation;

- Road surface area;

2 Hydraulics of overland flow, which depends on:

- Amount of runoff;

- Road segment slope (energy);

- Road surface area x segment slope (A*S) often used to predict road

surface erosion;

3 Erodibility of the road surface = complex function of:

- Soil texture;

- Surface cover (vegetation, rocks, litter);

- Pattern of flow on the road surface (e.g., rilling);

- Time since construction or maintenance;

- Age of road;

- Amount and type of traffic

Base flow

Road and skid trail

Overland, Quick flow

Utilization of machine

area in road increased peak flow from 50 to 100%

attributed to the hydrological connectivity of overland flow via skid trails and roads

Effects of harvest operation

Definition of hydrological connectivity

Stream channel

Hydrological connection

No hydrological

connection

Road, skid

trail runoff

12

True or false?

• The forest lower air temperature inside and above it?

• Forest increase the abundance and frequency of precipitation?

• The destruction of forest affect the climate?

• In hill and mountain country, forests conserve water for streamflow?

• Forest road affect high runoff?

True and false?

• Forest prevent erosion?

• Forest regulate the flow of spring?

• The total discharge the large rivers depends on

forest

• Forests tend to equalize throughout year and by

making the low stage higher and the high stages

lower

• Forest cannot prevent floods production by

exceptional precipitation, but they can mitigate

their destructiveness

Difficulties for evaluation of effect of forest removal

1970 1980 1990 2000

0

1e+7

2e+7

3e+7

4e+7

Year vs Total

Year vs Platation forest

40000

0

10000

20000

All forest

Plantation forest

Ratio of plantation： 44%

30000

All land in Japan

林業センサス累計統計書より

Cypress plantation： 26%

Forest plantation in Japan

Background

Infiltration capacity of dense Japanese cypress forest with sparse understory vegetation tends to be low

Hortonian overland flow and related soil erosion is one of the major concerns in the forest management

of headwater catchments in Japan

rain

Infiltration

Forest plantation and runoff generation

Natural Forest

Plantation

Subsurface and

groundwater flow

Bare soil surface

Low infiltration Overland flow

M3

M5

No understory vegetation

M6

M4 Dense Hinoki with fern cove M７

M1: 4.9ha M8

Deciduous forest

M2 Managed Hinoki

Outline of Mie Site

M5 M4 M2

Kyoto

Mie

Field Infiltration Measurement

0

100

200

300

Cover class

+ 2 3 4 5

8 x 25 m hillslope scale

Small plot

Large plot

Monitoring Hortonian Overland Flow in Multiple Scales

0.5 x 2.0 m plot scale

Catchment scale

Small plot with a stem

12/4/2004

12:00

12/5/2004

0:00

12/5/2004 12:00

0

2

4

0

2

4

0

2

4

Runoff coefficient：

0.28

Runoff coefficient：

0.41

Runoff coefficient:

0.28

Plot1(M2)

Plot3 (M5)

Plot4 (M8）

Observed overland flow in the small plots

•Significant overland flow occurred from the beginning of the storm event

Infiltration

Runoff

Overland flow

Before thinning After thinning

Changes in hydrological processes due to thinning

Decrease interception loss

Increase throughfall