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Summary1 2

INCONSISTENCIES BETWEEN PRECIPITATION AND STREAMFLOW DATA IN THE MEKONG RIVER BASIN, AND

THE NEED FOR REANALYSIS PRODUCTS IN SOUTHEAST ASIAMariza Costa-Cabral and Dennis P. Lettenmaier

Department of Civil and Environmental Engineering, University of Washington, Seattle WA 98195; dennisl@u.washington.edu

A31D-0935AGU Fall 2006

Acknowledgement: Financial support for this work came from the US National Science Foundation Carbon program, and in part from the Functional Value of Biodiversity Program, funded by the World Bank-Netherlands Partnership Program. We thank Jeffrey E. Richey for this support. We thank Nathalie Voisin for preparing the figures in panel 4. References:Goteti, G., and D.P.Lettenmaier (2001): Effects of streamflow regulation and landcover change on the hydrology of the Mekong river basin. Technical Report 169, Department of Civil and Environmental Engineering, University of Washington.Liang X, Lettenmaier DP, Wood EF, Burges SJ. 1994. A simple hydrologically based model of land surface, water, and energy fluxes for general circulation models. J Geophysical Research 99 (D7): 14415-14428.MRC (2003): State of the Basin Report: 2003. Mekong River Commission, Phnom Penh. 300 pages.Nijssen B, Lettenmaier DP, Liang X, Wetzel SW, Wood EF. 1997. Streamflow simulation for continental-scale river basins. Water Resources Research 33: 711-724. Nijssen B, O’Donnell GM, Lettenmaier DP, Lohmann D, Wood EF. 2001b. Predicting the discharge of global rivers. J Climate 14: 3307-3323.Shepard, D.S. (1984): Computer mapping: The SYMAP interpolation algorithm. In Spatial Statistics and Models, G.L. Gaile and C.J. Willmott (eds). D. Reidel Publishing Co.: Dordrecht, The Netherlands: 133-145.

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Inadequacy of Station Precipitation Data for Forcing Hydrologic Models

Inadequacy of Station Precipitation Data for Detecting Temporal Trends

In the Lower Mekong river basin, estimated precipitation totals for some tributary basins are lower, or barely higher, than observed streamflow (see graphs and the map below). Yet, runoff ratios in the region are generally below 50%, implying that precipitation must be at least double the streamflow.

Simulated streamflow for the east-central region, encompassing the eastern mountain ranges (highlighted in the figure) is lower than observed streamflow (calculated by subtracting Ubon and Nakhon Phanom observed flows from that at Stung Treng). It also shows a rising trend that is absent from the observations. The probable explanation is lack of areal coverage by the few stations in this high-relief region (see figures in panel 1).

4 ERA40 : A Diagnostic Tool but Resolution too Coarse for Modeling Stations and ERA40 estimates of mean annual precipitation are compared in the figure. ERA40 is roughly 60% higher than stations data over a large region that includes Burma, the Northern Highlands of Laos (i.e., latitude 17°N to 23°N) and the eastern part of Thailand’s Khorat Plateau. ERA40 is however roughly 30% lower over Southern Cambodia and the Vietnam Delta.

ER40’s native resolution is slightly coarser than 1°, too coarse for hydrologic models.

Mean Precipitation in March: (large difference)

Mean Precipitation in November:

(smaller difference)

River Network Soil Texture

ConclusionA high quality reanalysis, at a spatial resolution which ideally would be no coarser than 25 km (exceeding the ca. 32 km resolution of the North American Regional Reanalysis), and ideally would include data assimilation, would be fundamental for understanding runoff generation in the Mekong and other river basins in Southeast Asia.

Meteo Stations

The ability to predict the hydrologic effects of scenarios of land cover and climate change over large river basins, using macro-scale hydrologic models, is a crucial scientific challenge in the face of global change. We applied the VIC grid-based macroscale hydrologic model (Liang 1984; Nijssen et al. 1997, 2001) to the Mekong River basin (ca. 800,000 km2). For the baseline scenario, representing current conditions, we used the best land surface data available, and daily precipitation interpolated from the available meteorological stations, for 1979-2000. But in vast mountainous areas in the east-central region our streamflow simulations were compromised by unrealistically low precipitation data. In some sub-basins, precipitation totals were in fact lower than observed streamflow (panel 2).

Stations data must be interpolated to the model’s grid to be used for model forcing. But stations are too sparce (see figure) considering the basin’s high relief and monsoonal precipitation regime, leading to mis-representation of mountainous areas. Detection of temporal precipitation trends over a region that might help explain observed streamflow trends is also not possible (panel 3).

Comparison of the interpolated station data against ERA40 (panel 4) corroborates severe under-estimation by the stations. The difference relative to ERA40 is especially great over Burma and Laos’ Northern Highlands and Eastern Highlands – the regions where inconsistencies with streamflow had been found. While useful as a diagnostic tool, ERA40 data is too coarse to be used for model forcing (native resolution >1°).

Problem: Precipitation estimates are too low over vast mountainous regions:

Mean Annual Precipitationin 1979-2000:

Application of the VIC model to the Mekong

Long-term trends were detected in the streamflow records of the Mekong River basin (see figure). These trends may be due in part to a) precipitation trends; a) land cover and use changes; c) air temperature changes; d) measurement methods.

Unfortunately, the network of meteo stations (see panel 1 above) is too sparce to provide reliable areal precipitation estimates. Hence, the contribution of possible precipitation and temperature trends to the detected streamflow trends cannot be determined. An analysis of existing rainfall records since 1950 found no trends consistent with the streamflow trends (MRC, 2003).

-4%

-3%

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-1%

0%

1%

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4%

jan feb mar apr may jun jul aug sep oct nov dec

Ave

rag

e ch

ang

e in

str

eam

flo

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each

yea

r in

195

3-20

00

Chiang SaenVientiane minus Chiang SaenMukdahan minus VientianePakse minus MukdahanUbonYasothonStung Treng minus Pakse

In the annual time series of Mekong streamflows for individual months, we encounter significant trends predominantly in dry-season months November-May. The trend direction differs between regions.

Daily meteo data from these stations was interpolated into the model grid using the SYMAP algorithm (Shepard, 1984).

From Goteti and Lettenmaier (2001).

Topography

1 Tibetan Plateau2 Three Rivers area3 Lancang Basin4 Eastern Highlands5 Kontum Massif6 Bolovens Plateau7 Khorat Plateau8 Southern Uplands9 Phanh Hoei Range10 Lowlands11 Cardamom Range12 Chao Phraya River Plain13 Salween River Plain14 Mekong Delta15 Malay Peninsula16 Yunnan Plateau17 Sichuan Basin18 Northern Highlands

Stations and ERA40 data were interpolated to 0.25° and 0.5°, respectively, using the SYMAP algorithm (Shepard, 1984).

The largest differences between ERA40 and stations precipitation estimates are for the dry season months January-April, where ERA40 values are several-fold higher, over nearly all of the basin’s area (figures to the right).

0

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1979 1984 1989 1994 1999

Flo

w (

mm

/y)

Estimated annual precipitation

(significant rising trend)

VIC Simulated annual streamflow

(too low, and significant rising trend)

Observed mean annual streamflow

(no significant trend)

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1979 1984 1989 1994 1999

Flow

(mm

/y)

Observed annual Streamflow

(4 years available)

Estimated annual precipitation

(lower than observed streamflow!)

VIC Simulated annual streamflow

(mean: 1246 mm/y)

(1190 mm/y)

(mean: 387 mm/y)

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1979 1984 1989 1994 1999

Flow

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/y)

Nam Ngum Dam Catchment (ca. 8,300 km2)

Observed mean annual inflow to dam

Estimated annual precipitation(comparable to

observed streamflow!)

VIC Simulated annual inflow to dam

East-Central Region (ca. 170,300 km2)

We used the highest quality input data: We obtained good simulated streamflows for most of the basin:Streamgauges

Chiang Saen

Stung Treng

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1979 1984 1989 1994 1999

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m3s

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Ubon

01000200030004000500060007000

1979 1984 1989 1994 1999

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m3s

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Chiang Saen

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1979 1984 1989 1994 1999

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Observed Simulated

Chiang Saen

Ubon

Stung Treng

Good Results at most locations, e.g. these:

But low simulated streamflows towards the mainstem south:

Precipitation is sometimes lower than streamflow:

Ban Keng Done Catchment (ca. 13,000 km2)

Land Cover

(1190 mm/y)

East-Central Region

mm/daymm/day