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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018 RECHARGE IN ARID AND SEMI-ARID REGIONS Case study: Nubian Sandstone Aquifer System in Northern Chad Marie-Louise Vogt

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Page 1: RECHARGE IN ARID AND SEMI-ARID REGIONS · 2018-09-13 · CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018 WHY TO STUDY RECHARGE • «The abstraction from a groundwater

CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

RECHARGE IN ARID AND SEMI-ARID REGIONS

Case study: Nubian Sandstone Aquifer System in Northern Chad

Marie-Louise Vogt

Page 2: RECHARGE IN ARID AND SEMI-ARID REGIONS · 2018-09-13 · CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018 WHY TO STUDY RECHARGE • «The abstraction from a groundwater

CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

WHAT IS RECHARGE

RECHARGEThe amount of water thateffectively reaches the water table

Recharge ≠ Infiltration

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

WHY TO STUDY RECHARGE

• «The abstraction from a groundwater reservoir should in the long term not belarger than the long-term average recharge»

• «Sustainable management with respect to quantity requires that abstraction islimited to a fraction of recharge in order to guarantee a minimum availability of water in the downstream» Kinzelbach et al. 2003

• « A common misperception has been that the development of a groundwatersystem is «safe» if the average annual rate of groundwater withdrawal doesnot exceed the average annual rate of natural recharge. […] safe developmentdepend instead on how much of the pumpage can be captured fromincreased recharge and decreased discharge» Alley and Leake, 2004

• Bank analogy

Page 4: RECHARGE IN ARID AND SEMI-ARID REGIONS · 2018-09-13 · CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018 WHY TO STUDY RECHARGE • «The abstraction from a groundwater

CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

WHY TO STUDY RECHARGE

In arid and semi-arid countries, groundwater constitute an important long-term

storage reservoir, often being the only perennial water resources available

Rocky desert in southwest Jordan

Taklamakan, China

Sonoran Desert after rain Kalahari, Botswana

Australian outbackAtacama, Chile

Page 5: RECHARGE IN ARID AND SEMI-ARID REGIONS · 2018-09-13 · CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018 WHY TO STUDY RECHARGE • «The abstraction from a groundwater

CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

CONSEQUENCES OF OVERPUMPING

• Large drawdowns- > increased pumping costs

• In soft strata, land subsidence

• Reversal of hydraulic gradient

• Sea water intrusion (upconing)

• Soil salinization

San Joaquin Valley, California: Photo: USGSS

W.Kinzelbach

NCGRT

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

NON-RENEWABLE RESOURCES

Non-Renewable Groundwater (Margat et al. 2006): «Groundwater resource available for extraction, of necessity over a finite period,

from the reserves of an aquifer which has a very low current rate of average annual renewal but a large storage capacity»

Fossil Groundwater (International Glossary of Hydrogeology, 1992) : « Water that infiltrated usually millennia ago and often under climatic conditions

different to the present, and that has been stored underground since that time »

Aquifer Overexploitation (Margat & Saad, 1984):« Prolonged (multi-annual) withdrawal of groundwater from an aquifer in

quantities exceeding its average annual replenishment, bringing about a persistent fall in groundwater levels and reduction of aquifer reserves with undesirable side effects »

Groudwater Mining (Margat & Saad, 1984):« Extraction of groundwater from an aquifer having predominantly non-renewable

resources with depletion of aquifer reserves »

Page 7: RECHARGE IN ARID AND SEMI-ARID REGIONS · 2018-09-13 · CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018 WHY TO STUDY RECHARGE • «The abstraction from a groundwater

CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

MAJOR AQUIFERS CONTAINING PREDOMINANTLY NON-

RENEWABLE GROUNDWATER RESOURCES

• North Western Sahara (Algeria, Libya, Tunisia): 1 Mkm2

• Murzuk basin (Algeria, Libya, Niger): 450’000 km2

• Iullemeden Multilayer Continental (Mali, Niger, Nigeria): 500’000 km2

• Chad basin (Niger, Nigeria, Chad, Sudan, Cameroon, Libya): 600’000 km2

• Various (Saudi Arabia, Bahrain, Qatar, UAE): 250’000 km2

• Great Artesian Basin (Australia): 1,7 Mkm2

Reference for «thisvolume»:http://unesdoc.unesco.org/images/0014/001469/146997E.pdf

Nubian Sandstone (Egypt, Libya, Sudan, Chad): 2.2 Mkm2

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

SOME DEFINITIONS - RECHARGE AND DISCHARGE

Lakes of Ounianga, N Chad

Page 9: RECHARGE IN ARID AND SEMI-ARID REGIONS · 2018-09-13 · CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018 WHY TO STUDY RECHARGE • «The abstraction from a groundwater

CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

SOME DEFINITIONS - UNSATURATED AND SATURATED ZONE

Unsaturated zone: (or: Vadose Zone, Zone of Aeration)The zone between the land surface and water table (which may include the capillary fringe). Water in this zone is generally under less than atmospheric pressure, and the voids may contain water, air or other gases.

Page 10: RECHARGE IN ARID AND SEMI-ARID REGIONS · 2018-09-13 · CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018 WHY TO STUDY RECHARGE • «The abstraction from a groundwater

CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

SOME DEFINITIONS - UNSATURATED AND SATURATED ZONE

Saturated zone: (or: Phreatic zone)The saturated zone is that part of the earth’s crust beneath the regional water table or piezometric surface in which all voids, large and small, are filled with water under pressure greater than atmospheric.

Page 11: RECHARGE IN ARID AND SEMI-ARID REGIONS · 2018-09-13 · CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018 WHY TO STUDY RECHARGE • «The abstraction from a groundwater

CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

SOME DEFINITIONS - UNSATURATED AND SATURATED ZONE

AquiferAn aquifer is a geological formation which has structures or textures that hold water or permit appreciable water movement through them.

Page 12: RECHARGE IN ARID AND SEMI-ARID REGIONS · 2018-09-13 · CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018 WHY TO STUDY RECHARGE • «The abstraction from a groundwater

CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

FUNCTION OF AN AQUIFER

An aquifer has three important functions, namely:

Storage - It stores water, underground, as a reservoir. The aquifer characteristic describing

the ability of an aquifer to store water is its porosity. The characteristic describing its

ability to release water under gravity drainage is called its specific yield.

Transmission - It transmits water like a pipeline. The relevant aquifer characteristics which

determine how easily water is transmitted in an aquifer are hydraulic conductivity and

transmissivity.

Mixing - It mixes water of different qualities. A poor quality water can be injected into an

aquifer at one point, mixed with the local groundwater and the mixture withdrawn at

another location as useable water.

Sedimentary aquifer(porous unconsolidated)

Primary porosity

Fractured rock aquifer(granite, volcanic, metamorphic)

Secondary porosity

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

QUICK EXERCIZE

Exercize 1

e = 30 mAquifer unconfinedSurface area of the aquifer 10 km2Specific yield (vol water/vol rock) is 12.5%

Calculate the volume of water that can beextracted if the drawdown was of 5 m across the aquifer

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

SOME DEFINITIONS – PISTON AND

PREFERENTIAL FLOW

• Piston flow: a wetting front of ϴw content of water, infiltrating uniformily into a fairlydry sediment of ϴi

• Preferential flow: fingers, heterogeneitiesin layers, porosities, landscape change

Displacement faults in sand near Socorro, New Mexico. The wet spots coincide with the faults. From J.M. Herrin (September 1997)

Unstable wetting patterns at 0.20 m depth in wettable sand after 403 mm precipitation; dark spots are wet. From Hendrickx and Dekker (1991)

Hendrickx and Flury (2001)

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

SOURCES OF RECHARGE

• Precipitation recharge

• River recharge, including perennial, seasonal

and ephemeral flows

• Irrigation losses, both from canals and fields

• Interaquifer flows

• Urban recharge

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

RECHARGE IN ARID AND SEMI-ARID CLIMATES

• Rainfall tends to be more variable in both space and time

• Small frequency of occurrence

• Short duration

• High intensities

-> Recharge time and space variability : when significant recharge results only from

infrequent large events, it is highly misleading to talk of mean annual recharge or of

recharge as a proportion of mean annual rainfall; empirical rainfall-recharge expressions

can only be applied to specific areas/basins with known hydrologic characteristics

(characteristic of the vadose zone, antecedent moisture, vegetation, precipitation

distribution within and between years, local topography, watershed shape)

• Arid zones are often in a delicate hydrological balance. 95% of precipitation is lost

through evaporation: virgis, interception, soils and evapotraspiration

-> Rates of precipitation and evaporation are very similar: the residual is very small and

with high uncertainties

• Recharge rates and distribution are highly sensitive to the soil cover type and the land

change use

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

MECHANISMS OF RECHARGE

Lucien Blandenier (2015)

In arid and semi-arid cliates determining diffuse precipitation is not straightforward:

size, temporal and spatial variability, direction of water balance fluxes are distinct than

in more humid climates

-> thick unsaturated zone

-> piston flow against preferential flow

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

RECHARGE FROM INTERMITTENT FLOW

Al-Bawada, Saudi Arabiahttp://pages.uoregon.edu/millerm/fan.html

• Recharge by intermittent streams• Recharge of alluvial fans• Hidden recharge

Petra hydrological simulationhttps://www.youtube.com/watch?v=uCak7721ox8

In arid climates, recharge from intermittent flow is the most important source of recharge

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

CRITERIAS TO SELECT A METHOD FOR ESTIMATING

RECHARGE

• Clearly state the objective (future water supplies, vulnerability of an aquifer to contamination) and time required (estimate of coming decades or millennia)

• Take care when defining the conceptual model: most common and serious type of error

• Different sources and different processes of groundwater recharge will guide the selection of the method (or methods)

• Take into account spatial and temporal variability: infrequent major recharge isdifferent than small and regular events; small amounts of precipitation my provokerecharge or not depending on the season when it occurs; a multiple temporal resolution is needed (day, 10-day, month, year, 10-years, etc.)

• In arid climates, rates of precipitation and evaporation are similar, the net amount istherefore small and with high uncertainties; to reduce uncertainties, use multiple techniques

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

METHODS FOR ESTIMATING RECHARGE

1. Darcyan methods

2. Direct measurements: lysimeters, TDR probes

3. Water balance methods: soil moisture budgets, water tablefluctuation method, river channel water balance, river baseflowmethod, spring or river flow recession curves

4. Tracer methods: artificial tracers, environmental tracers (modern recharge: chlorine method, tritium; paleorecharge: 14C, 36Cl, 18O, 2H)

Methods can be grouped into the following classes:

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

COMPARISON OF METHODS FOR ESTIMATING RECHARGE –

DIRECT METHODS

LysimetersA lysimeter is a device consisting of an in situ weighable soil column of a 1m2 or greater cross-sectional area. It measures the flux created by rainfall (diffuse) as the outflow by seepage at 1-2m depth. It is a measure of drainage and not recharge.• High quality data• Point-based• 5-10 years needed for equilibration• Characterizes diffuse recharge

Soil moisture budget by neutron probe (or TDR probes)Soil moisture budget by neutron probe (or TDR probes) relies on the fact that water molecules will scatter neutrons. The amount of scattering is proportional to the amount of water present, which essentially determines the vertical distribution of the soil’s water content. It is a direct measure of soil water content.

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

COMPARISON OF METHODS FOR ESTIMATING RECHARGE –

WATER BALANCE METHODS

• A water balance links recharge and

discharge to changes in storage

(conservation of mass). A balance can be

written as a mass or volume change in time.

• In the long-term average and under neglect

of surface runoff, interflow (in-out from

adjacent aquifers) and pumping, the

difference between precipitation and

evapotranspiration is an indication of

recharge.𝑷 − 𝑬𝑻 = 𝑹

• As these quantities are both of the same magnitude and inaccurately known, theirdifference is even more inaccurate -> use long time series (identify systematic errors)

• The time-step to be used for water balance methods is important. • Niono in Mali P = 567 mm and ET = 2432 mm per year -> no recharge! • But the second ten day of August P = 45 to 175 mm in 10 days and ET = 50 to 65

mm /10 days -> recharge in August

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

COMPARISON OF METHODS FOR ESTIMATING RECHARGE –

WATER BALANCE METHODS

Soil moisture budgets

For all fluxes involved in the soil water balance, some empirical formulae exist. The

calculation from standard data is simple. For example, evaporation can be

computed by Penman’s formula and others (FAO guidelines). The usual inputs for

such a model are meteorological data from a nearby weather station, including

precipitation, radiation budget, wind speed, relative humidity, air temperature, and

soil data such as the field capacity.

River channel water balance

If recharge is confined to seepage from a river channel, the observations necessary

could in principle be very simple. Measurements of river flow are easily carried out.

If flow is measured between two points along the river, the difference will at least

convey some information about seepage and give an upper bound for recharge. If

the total flow infiltrates, the method is especially interesting.

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

COMPARISON OF METHODS FOR ESTIMATING RECHARGE –

WATER BALANCE METHODS

Water table fluctuation method

A water table rise is the clearest indicator of recharge

if all abstractions remain unchanged and atmospheric

pressure effects can be ruled out. This is a

straightforward and simple method, especially when

a full cycle of water table variation is considered. If

the storage coefficient of an aquifer is known, the

spatially interpolated water table rise can be

converted into a volume of water.

Main disadvantage: the aquifer storage coefficient should be known

• usually unknown for confined aquifers

• estimates from pumping tests are local

• density of observation boreholes in a typical arid zone is small

• water table obtained from pumped boreholes is often unreliable

• unknown amounts of “hidden” inflow/outflow from/to adjacent basins

R(tj) = Sy* DH(tj)

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

COMPARISON OF METHODS FOR ESTIMATING RECHARGE –

WATER BALANCE METHODS

River baseflow method

This method is one of the few

integrative measures of

recharge. It estimates an

integral regional value instead

of a local result. But not

feasible for ephemeral rivers.

Spring or river flow recession curves

The outflow of a catchment after the passage of the surface water wave is parametrized by

an exponential curve. The typical time constant of outflow is determined. With some

estimates on the storage volume of the drained aquifer, the recession constants allow an

estimate of recharge. It is an integrative method, but applicable only when there are

springs or perennial rivers. The catchment area must be known (physical one, not

topographical one) as well as the storage coefficient /specific yield.

Main disadvantage: long-term observations are necessary to obtain reliable results. It is

often not applicable in arid zone.

RECESSION CURVE (FROM MEYBOOM, 1991, AGU)

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

COMPARISON OF METHODS FOR ESTIMATING RECHARGE –

TRACERS METHOD

• Artificial tracers: fluorescent dyes used to measure spring flow or stream flow-> method is straightforward, the movement of the tracer is directly related to the rate of water movement

• Historical tracers: historical events which have caused the deposition of high concentration of an element at the soil surface, the rate of movement of the high concentrated tracer’s front is used to infer recharge rates

• Environmental tracers: exist naturally in the landscape and their spatial pattern or overall mass balance is used to infer recharge rates

what kind of tracer could that be?

what kind of tracer could that be?

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

HISTORICAL TRACERS

Bomb Tracers : 3H, 36Cl, 14C

1940 1950 1960 1970 1990 2000YEAR

600

400

200

0

T.U

.

1980

200

100

0

10

9ato

ms m

-2

ClH

36

3

C14

250

150

200

pm

c

100

“Society” Tracers: CFC, SF6

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

TRACERS TIMESPAN

222Rn

3H/3He

85Kr

SF6

CFC-12

CFC-113

CFC-11

39Ar

14C

81Kr

36Cl

3H

-110

-310

310

510

710

110

Age (years)

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

ENVIRONMENTAL TRACERS – THE PRINCIPLE

If we know the change in concentration over time in input, then we can infer information about water flow from changes in concentration measured at some

other location.

Concentration can be measured …

• At a point in time and space

• Through space (along a flowline)

• Through time

Three basic philosophies:

• dating methods (profiles, along flow lines)

• signature methods as a basis for mixing calculations (stable isotopes)

• flux estimation methods (mass conservation of conservative tracers)

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

ENVIRONMENTAL TRACERS – THE PRINCIPLE

• Environmental tracers provide information on the physical processes

• Example: Computed salt and water content distributions for a fallow (unvegetated) loam soil during cycles of infiltration, redistribution of soil moisture, and evaporation.

• The movement of tracers in the unsaturated zone is governed by the long-term mean soil water fluxes. The pattern of tracer distribution may therefore not correspond to momentary piezometrichead distributions and recharge.

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

ENVIRONMENTAL TRACERS – DIFFICULTIES

• Assumption piston flow -> difficult to take into account preferential flow

• Complexity in the unsaturated: hydrodynamic dispersion and diffusion

• Estimation of porosity is critical

Example: Hypothetical tracer concentration-depth profiles in the unsaturated zone after pulse application of a tracer at land surface followed by infiltration of tracer-free water

Healy, Scanlon 2012

http://unesdoc.unesco.org/images/0014/001469/146997E.pdf

• When interpreting samples from wells which mix waters of different age (travel time), or in depressionalzones, a model is needed to interpret the measured values using an assumption on travel time distribution.

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

TRACER’S METHODS: TRITIUM APPLICATION

Example: • 3H vertical profiles were measured by Solomon (1996) in a sand aquifer

in Ontario. Average porosity: 0.35. • 3H peak was found at a depth of 8.7 m below the ground.• 3H peaked in 1963

Estimate the average linear velocity and the recharge rate (recharge rate = porosity times the linear velocity)

v= dz/dt = (-8.7m) / ( 1996-1963) = -0.26 m/a

Recharge rate: R = n v = 0.09m/a

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

• Chloride is a conservative tracer: it concentrates over time or through evaporation

• Chlorine is originated in …. • Together with rainfall data, and under the

assumption of negligible runoff, recharge can

be computed:

Subsurface distribution of chloride beneath

native vegetation at sites in (a) the Murray Basin,

Australia (b) the Chihuahuan Desert, West Texas Healy, Scanlon, 2012

TRACERS METHOD - CHLORIDE APPLICATION

• Unsaturated zone chlorine concentration depthprofiles can serve as archives of past climates

𝑅 =(𝑐𝑃 ∗ 𝑃 + 𝐷)

𝑐𝐵

𝑐𝑃 : chlorine concentration in rainfall𝑃: precipitation𝐷: dry deposition of chloride𝑐𝐵: chloride concentration belowthe zero upward flux plane

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

TRACERS METHOD - CHLORINE APPLICATION

• Environmental chloride has been used to estimate impacts of changing from natural ecosystems to agricultural ecosystems, on the drainage rates through the unsaturated zone

Southern High Plains, Texas

(Scanlon et al ., 2007 )

Murray Basin, Australia (reprinted from

Journal of Hydrology , Jolly et al . ( 1989 )

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

CASE STUDIES FROM CHAD

1. Monitoring run-off, accumulation and infiltration in the arid Ennedi mountains (Northern Chad), using remote sensing

archeï

2. Identifying modern recharge in the arid region of Northern Chad: an hydrochemical baseline study of the Nubian Sandstone Aquifer System

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7 8 9

Tota

l rai

nfa

ll m

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1-10july

10-20 july

20-31 july

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10-20august

20-31august

1-10sept

10-20sept

July 2014 August 2014 September 2014

𝑃 = 121 𝑚𝑚

Rainfall was estimated by using FEWS-NET data (FAO’s portal)

Case study 1

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

Case study 1

Hydrological dynamic can be monitored by remote sensing (LandSat8)NDWI index18 august 2014

3 september 2014

6 november 2014

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

Evapotranspiration was estimated by calculating a SurfaceEnergy Balance from LandSat8 images (30 m resolution)

18.08.2014 05.10.2014 22.11.2014

𝐸𝑇 = 88 𝑚𝑚

Case study 1

𝜆𝐸 = 𝑅𝑛 − 𝐺 − 𝐻

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

0

10

20

30

40

50

60

70

80

20.07.2014 09.08.2014 29.08.2014 18.09.2014 08.10.2014 28.10.2014 17.11.2014 07.12.2014

Cu

mu

late

dre

sid

ual

(mm

)

05

10152025303540

1-1

0 ju

ly 2

01

4

11

-20

july

20

14

21

-31

july

20

14

1-1

0 a

ugu

st 2

01

4

11

-20

au

gust

20

14

21

-31

au

gust

20

14

1-1

0 s

epte

mb

er2

01

4

11

-20

sep

tem

ber

20

14

rain

fall

(mm

)

𝑃 −𝐸𝑇 = 33 𝑚𝑚

0

10

20

30

40

ET (

mm

)

We calculate the residual

Case study 1

𝑃 = 121 𝑚𝑚 𝐸𝑇 = 88 𝑚𝑚

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

Case study 1

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

22.11.2013 11.01.2014 02.03.2014 21.04.2014 10.06.2014 30.07.2014 18.09.2014 07.11.2014 27.12.2014

chan

ge t

ota

l wat

er

sto

rage

(m

m)

GRACE

Change in Total Water Storage ∆TWS (GRACE satellite, resolution 110 km)

SnWS = snow-water storage

RESS = reservoir storage

SMS = soil moisture storage

GWS = groundwater storage

∆𝑇𝑊𝑆 = ∆𝑆𝑛𝑊𝑆 + ∆𝑅𝐸𝑆𝑆 + ∆𝑆𝑀𝑆 + ∆𝐺𝑊𝑆

Lakes of Ounianga, N Chad

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

Case study 2

Stable isotopes were measured and compared to the spatial distribution of rainfall (FEWS-NET data)

Ennedi

Tibesti

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

Case study 2Groundwater flow conceptual model based on water table elevation of 150 water points in an area of 80’000 km2. Use of hydrochemical and isotopic patterns to formulate hypothesis.

Origin of the Ounianga Lakes

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

HOW DO I GET STARTED

First step:

• Gather daily meteorological data for the area under consideration, if you can. Consider using remote sensing data.

• Calculate potential precipitation recharge for one or ten-day periods usingprecipitation and potential evapotranspiration rates

𝑅 = 𝑃 − 𝐸𝑇𝑝

-> will reveal the temporal variability and the minimum rate of potentialprecipitation recharge

Second step:

• Collate all information available of the geology, surface and groundwaterhydrology, climate, topography, pedology and vegetation

-> construct the conceptual model of the precipitation recharge mechanisms, taking into account localized recharge, occurrence of

preferential flow paths, characteristics of the unsaturated zone

Page 44: RECHARGE IN ARID AND SEMI-ARID REGIONS · 2018-09-13 · CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018 WHY TO STUDY RECHARGE • «The abstraction from a groundwater

CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

Third step:

• Select a method appropriate to the hydrogeological conditions and the final aim(e.g. water management)

• In arid and remote areas, coupling tracer’s methods, which generally do not necessitate repeated field investigations, and other indirect methods (water balance, darcyan methods, numerical modelling) holds much potential. It can beadapted to different timescales, therefore their application can be multiple.

Fourth step:

• Long-term monitoring programs

…AND THEN…

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

PROBLEMS OF DATA AVAILABILITY

• The scarcity of data is a great problem in arid zone modelling

• In some cases data are available but are not published

• Acquiring of such data might be of high cost and is often not seen as a priority

• This situation is unlikely to change

• But remote sensing data are helping filling the gap

• For morphological and hydrological analysis: SRTM: 80% of the land surface

mapped, 90% confidence interval of elevation: 16 m

• LandSat or other multispectral images (from 1972)

• Analysis of multi-components through a GIS interface

• Example from Chad: the ResEau project

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

REMOTE SENSING PRODUCTS

https://lta.cr.usgs.gov/get_data/

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

EXTERNAL WATER SOURCERECHARGE COMPLETE

Mixed Zone

InjectedWater Level

INJECTEDWATER

Higher water level and greatly

improved groundwater salinity

Mixed Zone

Progressive PumpingWater Level

INJECTEDWATER

Pumping gradually lowers groundwater

level to original water table level

INJECTED WATER or MIXEDWATER FOR IRRIGATION

Artificial storage and recovery

Watch the movie: https://www.youtube.com/watch?v=NUM9OAKjcyA

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CHYN-UNINE Recharge in Arid and Semi-Arid Regions 06.09.2018

THANK YOU FOR YOUR ATTENTION!

Marie-Louise Vogt

Centre d’Hydrogéologie et Géothermie

Université de Neuchâtel

Rue Emile-Argand 11

CH-2000 Neuchâtel

[email protected]

www.unine.ch