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 accelerated 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 DDdx

dG G: sediment load, Di: interrill erosion rate, Df: rill erosion rate

W

RFSDRIKD 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 velocity

qir: flow discharge.

Rill erosion and deposition

Erosion process of WEPP model

isliscilridrigcicanibi CKCKCKCKCKCKKK

23

ftc τkT

scrrcbc CC

rscrlrrdrrbrrbr CKCKCKCKKK

GTq

VDTG

DTG

T

GKDTG

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-1

mg

・L

-1m

g・

L-1

SS concentration

N

P

June 8, 2004

Nutrients yield