simulating reduction of runoff sediment load by …simulating reduction of runoff sediment load by...
TRANSCRIPT
Simulating reduction of runoff sediment load by restoration of cultivated land to forest
in Jialingjiang catchment,the upper region of the Changjiang
退耕還林シナリオによる水・土砂流出抑制効果－嘉陵江流域を対象として－
MURAKAMI, ShogoNational Institute for Environmental Studies
Background• Reduction of forest area and increase of sediment yield
in the upper region of Changjiang River Catchment
⇒ decrease of water retention function, sedimentation in lake and reservoir（degradation of retarding function)，
sedimentation in middle and lower region （Raised bed river, decrease of flow conveyance）
↓One of main reasons which cause large flood frequently
• Sedimentation problem in TDM reservoir ⇒bad influence on flood control function (one of functions expected to TDM )
退耕還林project （restoration of cultivated land to forest ）↑
One of National Project for flood disaster protection“The 32-character plan”
Necessary to evaluate quantitatively effect of project scenario on reduction of rainfall runoff and sediment runoff
0
2 0 0 0
4 0 0 0
6 0 0 0
0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0
D o w n - S t r e a m
M i d d l e - S t r e a mU p p e r - S t r e a m
N a n j i n gW u h a nY i c h a n gC h a n g q i n g
E l e v a t i o n ( ｍ )
D i s t a n c e f r o m t h e R i v e r S o u r c e ( k m )
Lake Donting
Main Stream
Lake Poyang
69.7
38.2
62.0
3.41 211
27.9
25.5
22.3
14.2
3.630.466.78
2.55
36.816.9
17.2
4.15
561
620427
389
376
345Y
alon
gjia
ng
Min
jiang
Tuo
jiang
Jial
ingj
iang
Wuj
iang
Han
jiang
Lak
eD
ongt
ing
Lak
e Po
yang
JinshajiangTongtian
River
TuotuoRiver
SS Flux at Changjiang River in 1987（Unit：Million ton/year）
Main Stream
58.8
0
2 0 0 0
4 0 0 0
6 0 0 0
0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0
D o w n - S t r e a m
M i d d l e - S t r e a mU p p e r - S t r e a m
N a n j i n gW u h a nY i c h a n gC h a n g q i n g
E l e v a t i o n ( ｍ )
D i s t a n c e f r o m t h e R i v e r S o u r c e ( k m )
Lake Donting
Main Stream
Lake Poyang
69.7
38.2
62.0
3.41 211
27.9
25.5
22.3
14.2
3.630.466.78
2.55
36.816.9
17.2
4.15
561
620427
389
376
345Y
alon
gjia
ng
Min
jiang
Tuo
jiang
Jial
ingj
iang
Wuj
iang
Han
jiang
Lak
eD
ongt
ing
Lak
e Po
yang
JinshajiangTongtian
River
TuotuoRiver
SS Flux at Changjiang River in 1987（Unit：Million ton/year）
Main Stream
58.8
Sediment balance in Changjiang River
1910s 1930s 1980s
MODIS（2002.1.8 ）
sedimentation
1 month after closure by TGD bodytransparency before and after dam
→sedimentation
2003，7月17日
32 Characters of Environmental Policies for Management of Watershed in China
封山育林 Close mountain and plant trees退耕还林 Convert cultivated lands to forest平垸行洪 Make river-bed flat and control flooding退田还湖 Convert paddy fields to lakes加固堤防 Make dikes higher and stronger疏浚河湖 Make flows smooth in rivers and lakes以工代赈 Develop industries without government help移民建镇 Build new towns for migrations
川
Steep slope
Mild slope
Original farm
Forest
退耕还林 Project(restoration of cultivated land to forest )
cultivated land in steep slope
forest
Chinese government
Threshold slope angle：25°
Flat area
Evaluation of reduction of rainfall runoff and sediment runoff by restoration of cultivated land to forest
Purpose
1．Application and verification of watershed hydrologic model and
sediment runoff model based upon various kinds of landuse.
2．Investigation of reduction of rainfall runoff and sediment runoff “退耕還林” scenario by means of numerical simulation.
Jialingjiang Watershed
Three GorgesDam site
Taoyuan
Shanghai
Chongqing
Jinsha River
Changjiang River嘉陵江流域
三峡 Dam
桃源観測所
上海
重慶
金沙江
長江（揚子江）
Jialingjiang ： Main tributary in the Changjiang basin
・Agricultural land : 50% of the whole area・ Total sediment loads from Jialingjiang :
20 % of those from the upper Changjiang basin↓
Main source of sediment production in the Changjiang basin
嘉陵江流域
・Catchment area : ～160,000km2
• Core model ：distributed watershed hydrologic model (EPA, US) Hydrological Simulation Program – FORTRAN (HSPF)
⇒ watershed slope and river network ＋
• submodel：Paddy field Runoff Model,(PRM)⇒region of paddy field ( flat area)
Outline of Watershed hydrologic model
Precipitation, PR Evapotranspiration, ET
Irrigation, IR
O verflow , O F
D rainage,D R
Infiltration, IF
W Dm
H r
W D
α
unified
Outline of Paddy field Runoff Model
Structure of Runoff Model
Precipitation
Surfaceflow
Interflow
Groundwater
Interception
Evapotrans-piration
Paddy
Farm
Forest
Grass
Urban
Waste
River channel
Runoff process
Transport
process
Lumped process
Upper region Middle and Lower
region
Chongqing
NanjingWuhanYichang
Sub-watersheds for the calculation of water andsediment runoff from the headwater to Nanjing:
Total number 738 (556)
Land-cover data
In the model application, a lot of distributed paddy filed are lumped as one big paddy filed in each sub-watershed
Coupling a Watershed hydrologic model with a sediment runoff model in a watershed
(1) Sediment surface erosion model(2) Riverbank erosion(3) Bed-elevation variation model(4) Transportation of wash load(5) Channel hydraulic geometries model
Outline of Sediment Runoff Model
10-3 10-2 10-1 100 10110-4
10-3
10-2
10-1
100
101
102
qB*
q*I
*
Murakami Holy Sakanisi(sand) Sakanisi(kuroboku) Sibuya(kuroboku) Asiya(sand) Loch (sand)
Experimental verification of surface erosion model
35
*** )008.0(01.1 −= IqqB
10-1 100 101 102 10310-1
100
101
102
103
Perfect
agree
ment
Observed QB(kg)
Cal
cula
ted
QB(k
g)
Filed verification of surface erosion model
・Dahou experimental site (Poyang lake watershed)・Area 9000m2, slope angle θ =37.5(degree)
Total sediment yields from various land-use
:corresponding reach lengthiLi
γ：reduction rate due to protection works and countermeasuresπ = cropping and management factor
ε : projected area of vegetation cover per unit area
: respective land uses
[ ]
{ } [ ]cim
ciiii
i
ci
iBiB
xxobLqIIqA
xxobLqQ
≥⋅−−=
≥⋅=
∑
∑Pr)()()1(
Pr
*0γπε
Prob. = probability of further moving irrespective of
variation of condition: slope angle,
accumulated effect of protection works etc.
Riverbank erosion model ・Lack of detailed information about riverbank erosion
→steady supply of fine sands to riverbank．・supply from catchments slopes～ erosion near riverbank
BpdAq sWL Δ⋅⋅= 1
L ：fine sediment volume along eroded reach
A1d：sand diameter
ps ：sediment erosion rate
qWL：sediment yield per unit length
ΔB ：width of eroded riverbank
Sediment erosion expression from riverbank
3
*
*
*
*0
*
* )4.01(ττ
ττ
τc
cc
s Fp
−=
Cohesive fine sediment erosion rate ps
10-1
100
101
102
103
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
τ* /τ*c
w=300% w=200% w=100% w=60% w=24%
csp ** / τ
・Patheniades(1965,1970)
・Otubo(1985)
・Ashida & Sawai(1978)
Experimental verificatuion
Calculation conditionCalculation period;1987
Input weather data: ISLSCP (International Satellite Land Surface Climate Project) data
Initial condition: Calculation started after removing the effect of initial parameter sets using spin-up calculation (2 years)
Calculation time step:1 hour
重慶
嘉陵江
倍江渠江
Hydrological stationflow discharge, sediment concentration
↓Data for verification of proposed model
Landuse in the Jialingjiang watershed
Paddy field
Farm land
Forest area
Shrub and Bush
Grassland(high dens.)
Grassland（middle and low）
Riparian ara
Urban area
Industrial and miming
Saline land
Bare land
Rock land
Others
Water body
Ratio(%)
Farm ：38.6 Rock：0.5
Forest：20.2 W ater body：0.1
Paddy：12.8 Urban:0.1
Shrub and Bush：12.4 Riparian:0.04
G rass (m iddle & low) ：7.9 Bare:0.03
Grass (high)：6.8 Industrial:0.007
O thers：0.5 Saline：0.003
250m X 250m digital elevation model
Down scaling from daily rainfall intensity r24to hourly rainfall intensity r1
Sharman type expression based upon IDF-curves
1;24
24 =⎥⎦
⎤⎢⎣
⎡⋅= TTD
k
T
1;2424
24 =⎥⎦
⎤⎢⎣
⎡⋅= T
TR
rk
T
KTcTr =)(max
Conversion ： r24 to r1
Rainfall duration
Effect of conversion
c, k = empirical constants
kkmT
m
T
B
BqB
TT
DR
rqqr ⎟⎟
⎠
⎞⎜⎜⎝
⎛⋅⎟
⎠⎞
⎜⎝⎛=⋅⎟
⎟⎠
⎞⎜⎜⎝
⎛==
2424
2424/2424
1
0.00 0.25 0.50 0.75 1.000
5
10
15
20C
hang
e of
r 1/r 24,D
r and
r SE
k
r1/r24
Dr
rSE
parameter k ：large effect of estimation of sediment erosion k → large : larger effect than simple disaggregation
(24 uniform division) k value in Japan： 1/3<k<2/3，usually used value k=0.5
Effect of conversion from r24 to r1
krD −= 11 24
k
qBr 32
24=
kR
r−
= 124
1 24
m=5/3, T=1
TsukubaTsukuba
UrumqiBeijing
Singapore
Singapore
Yellow river
Changjiang river
MODIS dataReceiving and Analyzing Station
Ecological Monitoring Station
Mekong River
Taoyuan(桃源)
Estimated r1 agrees with the observed precipitation than uniform disaggregationr1=R24/24.
0 1000 2000 3000 4000 5000 60000
10
20
30
40
50r 1,R
24/2
4(m
m/h
r)
Elapsed hours
observed r1 estimated r1
0 1000 2000 3000 4000 5000 60000
10
20
30
40
50
Elsed hours
r 1,R24
/24(
mm
/hr)
observed r1
estimated r1
0 10 20 30 40 500
10
20
30
40
50
estimated precipitation (mm/h)
observed m axim um precipitation (m m /h）
based on proposed conversion uniform ly devided by 24
Evaluation of Hydrological Response Units (HRUs）
Landuse data(1:1,000,000)
+
River network
DEM (1:1,000,000)
+
Sub-wataershed(Total number: 29 )
overlayLumping
Urban area
Wasteland
Bush&
Shrub
Forest
Grassland
Farmland
Paddyfield
Runoff
River channel
HRUs
Main channelSub channel
Degital
Surface analysis
Evaluation of parameters for rainfall/sediment runoff processes according to kinds of landuse
Simulated results (1)：Rainfall runoff
Jan Mar May Jul Sep Nov0
2000400060008000
100001200014000
Jan Mar May Jul Sep Nov0
2000
4000
6000
8000
Jan Mar May Jul Sep Nov0
5000
10000
15000
20000
Jan Mar May Jul Sep Nov0
5000100001500020000250003000035000
Dai
ly a
vera
ge fl
owra
te (m
3 /s)
1987
Dai
ly a
vera
ge fl
owra
te (m
3 /s)
1987
Dai
ly a
vera
ge fl
owra
te (m
3 /s)
1987
O bservation Sim ulation
Dai
ly a
vera
ge fl
owra
te (m
3 /s)
1987
Good agreement between observation and simulation①
②
③
④
① ②
③ ④
Jan Mar May Jul Sep Nov0
10
20
30
40
50
60
monthly sediment load(109 kg)
1987 O bservaton Sim ulation
Jan Mar May Jul Sep Nov0
20
40
60
80
100
120
1987
Sediment fux(103 kg/s)
O bservation Sim ulation
Daily sediment flux
Simulated results (2)：Sediment runoff
Monthly sediment outflow
Better agreement :monthly sediment outflow→ macro-scopic erosion model
Policy of returning farmland to forest and its effects on prevention of runoff and sediment loads
10°以上15°以上
20°以上25°以上
10°以上15°以上
20°以上25°以上25°≧（0.6％ of total area） 20°≧（1.5％）
China Governmentpolicy Assuming the farmland
to be the mature forest, and simulating the rainfall runoff and sediment runoff
Simulation
Detection of farmland with steep slope value
over a threshold using GIS
Distribution of detected farmlands with steep slope over each threshold to return to forest area
15°≧（3.2％） 10°≧（6.3％）
Policy of returning farmland to forest and its effects on prevention of runoff and sediment loads
10°以上15°以上
20°以上25°以上
10°以上15°以上
20°以上25°以上25°≧（0.6％ of total area） 20°≧（1.5％）
China Governmentpolicy
Small effect on flood control
Maximum flood peak value only decreased 2.2% in the case of
threshold 10°in the whole catchment.
↓
Direct runoff easily occurs even in forest area due to thin soil surface
layer ?
Simulation
Detection of farmland with steep slope value over a threshold using GIS
Distribution of detected farmlands with steep slope over each threshold to return to forest area
15°≧（3.2％） 10°≧（6.3％）
Jan M ar M ay Jul Sep Nov0
5000
10000
15000
20000
25000
30000
Daily flow discharge (m
3/s)
1987
O riginal ≧25° ≧20° ≧15° ≧10°
7/17 7/18 7/19 7/200
5000
10000
15000
20000
25000
30000
Daily flow discharge(m
3 /s)
1987
O riginal ≧25° ≧20° ≧15° ≧10°
Extended figure of maximum peak flow discharge
Variation of daily flow discharge↓
small effect
Maximum effect : threshold slope 10°Decrease in peak discharge: 3.5%Decrease in flow depth: 0.5m
Effect of “退耕還林”scenario is small in case of runoff ?
Reduction of rainfall runoff Accumulated effect through the whole catchment
0
20
40
60
80
100
120
140
160
≦10°≦15°≦20°≦25°
Annual total sediment
production (109 kg)
Sedim ent prodcution on land slope River bank erosion
O riginal
Restoration of cultivated land in steep slope to forest →obvious reduction of sediment load
Reduction of sediment loadAccumulated effect through the whole catchment
Summary• To evaluate the effect of the restoration of cultivated land to
forest in the Jialingjiang basin on runoff and sediment loads, we used a spatially distributed catchment hydrologic model, HSPF, with improved sediment production processes and disaggregated hourly precipitation data from gauged daily precipitation data for 1987 as inputs, and confirmed its applicability as a tool for evaluating the runoff and sediment load processes.
• Through the simulations to evaluate the effect of the restoration of farmland to forest on sediment production in the Jialingjiangbasin, we found that restoration to forest clearly reduces sediment production even in the case of the minimum restoration at the slope threshold of 25°, although the effect on the reduction of peak flow rate in river channels was small. Moreover, the increase in restored forest area by lowering the slope thresholddecreased sediment production further.