October 5, 2005,The 4th IAHR Symposium on River, Coastal and Estuarine Morphodynamics
Field Observation and WEPP Field Observation and WEPP Application for Sediment Yield in Application for Sediment Yield in
an Agricultural Watershedan Agricultural Watershed
Kazutoshi Osawa & Syunsuke IkedaTokyo Institute of Technology, Tokyo, Japan.
Satoshi YamaguchiMinistry of Land, Infrastructure and Transport, Japan
Location of the Okinawa region
Okinawa region
Location of the Okinawa region in Japan
Tokyo
1000km
Ishigaki Island
1000km
INTRODUCTION
Recently many farm land reclamation projects are carried out.Land use is changed dramatically from forest to upland fields.
Red-soil runoff problems in Okinawa region
Soil erosion is accelerate heavily
Subtropical climate (high intensity of rainfall) Red soil (high erodibility)Steep slope ( ~14%)
Upland fields and bare lands become the main source of sediment
Introduction
Red soil runoff problems in Okinawa regionHeavy soil erosion occurs
Marine pollution Disruption of the oceanic ecosystems (coral, fishes, etc.)
Flow into channels and the rivers Very short river length (5-10km) Discharged into the sea directly
Dead corals in near of the mouth of the river
Corals barely live in Nagura Bay, Ishigaki Island
Corals in the unpollutedwater
Corals have been damaged by oversupply of sediment and nutrients from the river basin including agricultural zone.
Objectives
Soil erosion plot test was carried out to compare the amounts of sediment yield depending on the difference of agricultural management
Water Erosion Prediction Project (WEPP) model was applied to these test fields in predicting sediment yield at farmland and simulated sediment runoff in watershed
Introduction
Field plot test OutlinesSt-1: Non-cultivating (control) St-2: Spring sugarcane
by conventional tillage
St-3: Spring sugarcane with grass strip
St-4: Perennial sugarcane by zero-tillage farming
Canopy cover effect
Grass strip effect Zero-tillage effect
Temporal variations of measured parametersonly put the data during each rainfall event
Canopy cover effect 59 %
Zero-tillage effect (vs. non-cultivating) 94 % Zero-tillage effect (vs. spring sugarcane) 85 %
Grass strip effect 8 %
Sediment runoff reduction ratio
Amount of sediment yields
59%
94%
85%
8%
Water Erosion Prediction Project (WEPP)
Constructed by Nearing et al. in 1989 as hillslope erosion modelIn 1995, the model was expanded to the watershed scale Physically-based model Erosion, climate, hydrology, daily water balance, plant growth, residue decomposition, etc. Different from the USLE, the WEPP model was constructed for the purpose of estimating soil loss at every rainfall event Constructed as a post-USLE, however, little study has been done to apply the WEPP model to Japan
Plant growth
InfiltrationInfiltrationSoil conditionSoil condition conductivityconductivity erodibilityerodibility
Percolation
Overland flowOverland flow
Climate1. Climate
2. Overland flow
3. Water balance
4. Plant growth
5. Soil condition
6. Managements
7. Erosion
Managements Tillage,
Plant seedlings, Harvest, etc.
Evapo-transpiration
Hillslope components of WEPP model
Rill erosionRill erosion
Interrill erosionInterrill erosion
Sediment continuity equation
if DDdxdG G: sediment load, Di: interrill erosion rate, Df: rill erosion rate
WRFSDRIKD s
nozzlerrireii
Ki: interill erodibility,
Ie: effective rainfall intensity,σir: interrill runoff rate, SDRrr: sediment delivery ratioFnozzle: adjustment factor for sprinkler irrigation,Rs: rill spacing, W: rill width
Interill erosion
Tc: transport capacity of flow in the rill
τf: flow shear stress acting on soil particlesτc: critical shear stress of the soilKr: rill erodibility
β: raindrop-induced turbulence coefficientVf: effective fall velocityqir: flow discharge.
Rill erosion and deposition
Erosion process of WEPP model
isliscilridrigcicanibi CKCKCKCKCKCKKK
23
ftc τkT
scrrcbc CC
rscrlrrdrrbrrbr CKCKCKCKKK
GTqV
DTG
DTGTGKDTG
cr
ffc
fcfc
ccfrfcfc
0,
1,erosion
deposition
Watershed scale
HillslopeChannel
Impoundmentdeposition
transportation
erosion
Interrillerosion
Rill erosion
Plant
Management
Soil
Hillslope scale
Watershed components of WEPP model
transportationdeposition
erosion
WEPP model verification
Most of calculated results were agree with observed ones.Disagreements of discharge at St-4 would be attributed to the overestimate of the hydraulic conductivity. Differences of sediment runoff at St-1 can be attributed to the growth of weeds and the loss of fine and easily erodible sediment at the actual plot.If these conditions were expressed properly with the model, these gaps will be improved.
Simulated results by WEPP model
spring-perennial sugarcane cycle was more effective cropping method in view of sediment yield reduction than summer sugarcane.The measures of residue mulch and no-tillage planting at summer sugarcane field reduced sediment yield more effective than that at spring-perennial sugarcane field.
Outlines and land use of Kandabara basin
Farmlands occupied most of the basin. Sugarcane: 49% (Summer: 38%, Spring: 4%, Perennial: 6%)Pineapple: 2%, Paddy: 24%, Grassland: 14%
Calculated sediment yield and discharge
The feasible combinations of sediment yield reduction methods:
(1) shifting land use of summer sugarcane into spring-perennial sugarcane cycle
(2) mulching by residue of sugarcane
(3) installing the grass strip.
Sediment yield tended to be large at summer sugarcane fields or pineapple fields. As slope length or slope angle enlarged, sediment yield tended to be increased.
Calculated sediment yield reduction ratios
The reduction ratio was high at sugarcane fields with measure (1) and pine-apple fields with measure (2) and (3). Sediment discharge at outlet of the basin was decreased by 56% in comparison with present situation.
Conclusion
We have carried out multi-points observations at four test plots in sugarcane fields to compare the amounts of sediment yield depending on the difference of agricultural management at each plot. Zero-tillage perennial farming is found to reduce sediment effectively.
The WEPP model is effective to estimate the sediment yield at farmlands affected by various agricultural management conditions.
In the present application to the watershed, the authors carried out some case studies to choose proper combinations of sediment yield reduction methods. The calculation has shown that the sediment runoff can be reduced by 56% at the watershed if the combination is adequately chosen.
Field plot test Outlines
Amount of discharged sediment is calculated as the product of water discharge and sediment concentration.
g・
L-1m
g・
L-1m
g・
L-1
SS concentration
N
P
June 8, 2004
Nutrients yield