David Seddon1, Japhet Kashaigili2, Richard G. Taylor1,

Mark Cuthbert1,3, Lucas Mihale4, Catherine Mwihumbo4,

Alan MacDonald5

1UCL Geography, UK; 2Sokoine University of Agriculture, Tanzania;3Cardiff University, UK; 4Ministry of Water, Tanzania;5British Geological Survey, UK

Focused groundwater recharge in a dryland

environment: hydrometric and isotopic evidence

from central Tanzania

EGU2020, Session HS8.3.5 Irrigation, soil hydrology and groundwater

management for resilient arid and semi-arid agroecosystems

Makutapora Basin

(2015-16 El Niño)

© David Seddon 2020. All rights reserved

Makutapora Wellfield, Tanzania

• supplies >50 000 m3 of safe water daily to the city

of Dodoma in central, semi-arid Tanzania

Taylor et al. (2013) Nature Climate Change 3: 374-378.

• recharge associated with extreme seasonal rainfall during El Niño events

© David Seddon 2020. All rights reserved

How does recharge to this

weathered crystalline rock

aquifer system occur?

• directly from rainfall?

• indirectly via leakage from

ephemeral streamflow?

© David Seddon 2020. All rights reserved

evidence from long-term monitoring

• groundwater recharge correlates

most strongly to the seasonal

duration of ephemeral stream flow

R2 = 0.85

DUWASA pumpage

Makutapora rainfall

Meia-Meia river stage

WRBWB

threshold? *

* see presentation of Zarate et al. in this session

© David Seddon 2020. All rights reserved

-0.3

0.0

0.3

0.6

0.9

1.2

Δhyd

raulic

head (

m)122/75

C5

89/75

0.0

0.5

1.0

1.5

2.0

2.5

Str

eam

sta

ge (

m)

Meia Meia

Chihanga

high-frequency monitoring

• groundwater levels

respond to ephemeral

streamflow, not localised

rainfall at Makutapora

0

20

40

60

80

100

01 January 2016 01 February 2016 01 March 2016

Pre

cip

ita

tio

n (

mm

)

0

2

4

6

8

Wellf

ield

pum

pin

g

(x10

4m

3·d

ay

-1)

© David Seddon 2020. All rights reserved

evidence from stable isotope ratios

• stable isotope ratios (“H:1H, 18O:16O) in groundwater

traced to heavy (> 70th percentile) rainfalls

isotopically

enriched

light rainfalls

isotopically

depleted

heavy

rainfalls

groundwaters (orange)

• evidence of

minor

evaporative

enrichment

consistent with

ephemeral

stream flow

local meteoric waterline

evaporation

line

© David Seddon 2020. All rights reserved

Conclusions:

• in the drylands of central Tanzania, groundwater

recharge to the wellfield supplying Tanzania’s capital

city occurs, in part, via ephemeral stream flows

(generated by heavy rainfalls) that leak to the

subsurface (focused recharge) – groundwater in

drylands can be critically connected to surface waters

• precise pathways remain uncertain and may involve

threshold dependent infiltration “windows” via faults

and stream reaches underlain by coarser, fluvial

sediments

• identification of focused recharge to the wellfield

raises the possibility of Managed Aquifer Recharge

strategies to amplify replenishment

© David Seddon 2020. All rights reserved

SUPPLEMENTARY SLIDES

© David Seddon 2020. All rights reserved

numerical modelling: MIKE-SHE

• conceptual model of groundwater recharge test using an

integrated surface water – groundwater model

© David Seddon 2020. All rights reserved

numerical model supports research findings

• model calibration to stream stage (very good NSE= 0.88)

and groundwater levels (satisfactory, NSE = 0.56)

• focused recharge accounts for ~ two-thirds of total

recharge recorded from 1955 to 2016

© David Seddon 2020. All rights reserved