The Impact of Climate Change on a Humid, Equatorial Catchment in Uganda.

Lucinda Mileham, Dr Richard Taylor, Dr Martin ToddDepartment of Geography – University College London

• Africa has experienced a mean continental 20th Century warming of 0.7 °C

• The periods of greatest warming are 1910-1930 and post 1970, with the five warmest years on record occurring since 1988.

• decrease in runoff of 17 % in major river basins

• Predicted temperature increase of between 2-6 °C by 2100

• Accompanied by evaporative increases of 19-27 % by 2080

• Changes in the seasonality and intensity of future precipitation

Changing Climate

Why is it important in Uganda

• Dependence on rainfall-fed agriculture

• Reliance on localised (untreated) sources of water

• Groundwater is the only reliable source of potable water

• Current population of 25.8 million is estimated to more than double by 2025

• To evaluate the ability of a RCM to reproduce the current (1960-1990) climate at scales appropriate for hydrological modelling.

• To develop a soil-moisture balance model for groundwater modelling operating at a scale which allows coupling with a RCM.

• To quantify the impacts of climate change (2070 to 2100) on catchment-scale terrestrial water resources in Uganda.

Specific objectives

Main Objective

To quantify the impacts of climate change on groundwater recharge and surface runoff in Uganda, East Africa.

PRECISWhat is PRECIS? (Providing Regional Climates for Impacts Studies) A simple-to-use PC-based RCM, has been developed by the Hadley Centre (UK) specifically to address the need for countries to make regional-scale climate predictions.

• Model resolution 25 or 50 km²

• Daily time-step

• Boundary conditions - A three member ensemble of central experiments (1960-1990)

- ECMWF ERA 40 reanalysis experiment 1957-2001

- A three member ensemble of SRES A2 scenario experiments (2070-2100)

- A single SRES B2 scenario experiment 2070-2100

• User defined emission scenarios, and specification of output levels

• Limited user manipulation – land use, inland water features and topography can be changed but model parameterizations cannot be altered.

Catchment Scale River Mitano catchment.

• 2, 098 km 2

• High relief and incised drainage reflect a runoff-dominated regime

• Land use - Agriculture (79 %), Grassland (17 %) wetland (3 %), small areas of forest and plantation

Study Area / Method

Regional scale - PRECIS Mean Climatology

• Bi-modal precipitation regime

• Good representation of the temporal distribution of precipitation

• Overestimation of precipitation between December-March

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PRECIS-ERA PRCIS-GCM VasCLIMO

UDEL CRU GPCC V3 0.5 PRECIP 1960-1990

GPCP PRECIP 1979-now CMAP (1979-now) TRMM

PRECIS CRU VasCLIMO

GPCC GPCP TRMM

• <10 % error in mean annual precipitation

• Better representation of Sep-Nov rainy season

• Magnitude remains poorly resolved in Jan-Mar

• Poor representation of peak Precipitation in first rains

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PRECIS CRU Station VasCLIMO GPCC GCPC CMAP TRMM

Catchment-scale validation of precipitation

• Increased precipitation July-December• Shift in timing of peak seasons• Mean annual precipitation increase of 14 %• 3.5 °C increase in Temperature• 53 % increase in PET

Future PRECIS Change (2070-2100) cont …

Soil moisture balance model (SMBM)• Simulates changes in soil moisture (‘green water’) and provides estimates of rainfall-fed, groundwater recharge and runoff (infiltration excess)

• Can be run as a lumped parameter model OR as a semi-distributed model by running for different soil, slope and vegetation characteristics.

• Critically, can be run using gridded RCM or downscaled GCM data.

• Validated against discharge data from the River Mitano gauging station

• Poor performance is due to lag responses

• Stormflow is significantly better represented than baseflow due to its shorter lag response

• Should only be used to represent baseflow and stormflow on annual and longer timescales

• SMBM reproduces well the mean annual recharge and runoff

SMBM calibration (1965 – 1979)

DJF MAM

JJA SON

Mean monthly delta factors

• Difference between 1960-1990 and 2070-2100 modelled precipitation.

• All seasons exhibit small increases in precipitation across Uganda

• Delta factors 0.9 to 1.8

• Mean monthly factor

• Applied to daily data

• 14 % increase in Recharge

• 84 % increase in Runoff.

Effect on Hydrology

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GRIDDED STAT RECHARGE

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Runoff

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DELTA RUNOFF

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Changes in Daily Precipitation Distribution

• Reduction in small precipitation events (<10 mm)

• Increase in large precipitation events (>10 mm)

• Variable results for extreme precipitation events

•The distribution of precipitation is key for modelling of recharge and runoff

Monthly Delta factors fail to account for changes in the daily precipitation distribution

The transformation method –

matches future precipitation

and historical precipitation

distributions.

• 66 % increase in Recharge• 123 % increase in Runoff

Fails to account for changes

in occurrence

Impacts on hydrology

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TRANSFORMED RECHARGE

GRIDDED STAT RECHARGE

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Runoff

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TRANSFORMED RUNOFF

GRIDDED STAT RUNOFF

• PRECIS does a reasonable job at representing the climate of east Africa

• Observational uncertainty is large

• SMBM represents well the mean annual catchment recharge and runoff

• Recharge and runoff are sensitive to the distribution of precipitation

• Increase in the magnitude and intensity of precipitation

• Large increases in evapotranspiration and temperature

• Increases in surface runoff and groundwater recharge under future climates

Conclusion