IMPACTS OF LANDUSE CHANGE ON SEDIMENT TRANSPORT IN THE YALI RESERVOIR CATCHMENTTarek Ketelsen, Luke Taylor, Mai Ky Vinh, Truong Hong

CHALLENGE PROGRAM ON WATER & FOOD IN THE MEKONG: Optimizing the management of hydropower cascades at the catchment level (MK3)

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UPSTREAM

RESERVOIR

DOWNSTREAMWater use options of reservoir for catchment benefits1. Irrigation potential2. Flood control3. Fish passage requirements4. E-flow

Watershed communities/

water user sectors

Land management options for reservoir benefits1. Soil conservation 2. Enhancing reservoir life

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Yali Reservoir, Vietnam• What is the sediment

inflow to Yali?• How do historic

sediment loads vary with changes in flow/rainfall and land use?

• What are the implications for reservoir life?

Trung Nghia (TN) sub -catchment

Kon Tum (KT) sub -catchment

Catchment suspended sediment yield (SSY)

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Global: (Vanoni, 1975)10 – 20,000 t/km²/yr

Mekong: (Kummu, 2010)18 – 489 t/km²/yr

3S Basin (Kummu, 2010)220 – 240 t/km²/yr

Yali (KT & TN observation data 1990 – 2009)79 – 1,051 t/km²/yr

Proportion of catchment erosion which ends up in the waterways

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Large variability in Yali catchment SSY

2009: Ketsana1996: Wukong

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Long term trends in SSY

Annual increment in SSY in 2005-09 is ~ double the 90-97 rate

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Drivers of trends in SSY

Source: IWMI, 2002

A. Changes in rainfall/flow regime

B. Changes in land use

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A. Long term trends in Hydrology

High variability but no clear increasing trend

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B. Trends in land use

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Trung Nghia (TN) sub -catchment

Kon Tum (KT) sub -catchment

1. PECC1 official design eqn.– Based on long time series of

discharge at KT

2. PECC1 empirical catchment SSY relationships – based on long time series discharge

data (1960s-1990s) at KT and TN– Scaled based on ungauged area

3. Observed sediment loads KT, TN (1990-2009)– Scaled based on ungauged area

Estimating Yali sediment inflow

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Estimated Yali sediment inflowSuspended sediment yield (Mt/yr)

coarse sediment yield (Mt/yr)

Suspended Load (MCM)

Bed load (MCM)

SL (tot) Yali (MCM)

SL Yali (Catchmnent ) Ave 2.258 0.452 1.829 0.29 2.12

SL Yali (Catchmnent ) Max 6.695 1.339 5.426 0.86 6.29 SL Yali (Catchmnent ) Min 0.712 0.142 0.577 0.09 0.67

SL Yali (Empirical) 2.030 0.406 1.645 0.26 1.91 SL Yali (Design Calc.) 2.139 0.428 1.733 0.27 2.01

SL Yali (basin SSY) 2.423 0.485 1.964 0.31 2.28

Note:(1) Bed load was estimated based on PECC1 estimate:

Bed load = 0.2* suspended load

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Yali Reservoir trapping efficiency• Brune’s method (1953) based on residence

time of reservoir• Applied previously in the Mekong • Sesan TE: 0.851 – 0.95 (Kummu, 2010)• Yali: 0.83

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Yali annual sediment deposition dynamics

SL(in) = 2 mcmSL(res) = 1.67 mcm

SL(out) = 0.33 mcm

SL(res)05-09 = 3.13 mcm

SL(res)90-97 = 1.38 mcm

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Next (final!) steps1. Explore sensitivity of Yali sediment inflow to land use change

using the Revised Universal Soil Loss Equation– Maximum catchment erosion (aggressive deforestation scenario)– Minimised catchment erosion (aggressive afforestation scenario)

2. Estimate changes in reservoir half life based on change upland management

Thank you for your attention

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Sediment delivery ratio• Sediment transport cycle more

complex and slower than water cycle

• Signification proportion stored in catchment:– Hillslopes– Gullies– Microtopographic features– Human structures (check dams,

reservoirs)• SSY = <10% of sediment

production in large catchments (>100km²) (Annandale, 2003)

No. Reservoir name

Main crop under irrigation

Number of Reservoirs

Reservoir volume (m3)

Total Irrigated area (ha)

I. Kon Tum city Total Rice 10 10,554,000 1759II. Dak Gley district

Total Rice 5 1,188,000 198

III. Tu Mo Rong district

Total Rice 1 750,000 125

IV. Sa Thay district

Total Rice 8 2,496,000 416

V. Ngoc Hoi district

Total Rice 6 3,138,000 523

VI. Kon Plong district

Total Rice 1 90,000 15

VII. Kon Ray Total Rice 4 648,000 108VIII. Dak Ha district

Total Rice & Coffee 19 19,875,000 7236

VIII. Dak To district

Total Rice 15 3,048,000 508

TOTAL 69 41,787,000 ~0.5%% of long term mean annual inflow to Yali