Estimating Groundwater Availability at the Catchment Scale Using Streamflow Recession and Instream

Flow Requirements of Rivers in South Africa

Girma Y. Ebrahim Karen G. Villholth

IUGG 26th conference, Prague, Jun 22- July 2, 2015

Introduction

• Groundwater (GW) is a critical source of water for domestic consumption, agriculture, and industrial development in South Africa

• Besides that, GW is of vital importance for securing functioning and biodiversity of ecosystems

• As demand for water continues to increase there is potential for conflict between human and ecosystem needs

• Traditionally, GW allocation has ignored the requirements of GW-dependent ecosystems and made no provision for a water regime that might sustain them

Introduction…

• In the National Water Act of South Africa of 1998, the GW components of the Reserve are:– that part required to satisfy basic human needs

(the GW component of the BHN Reserve), and – that portion required to protect the aquatic

ecosystems (the GW component of the environmental Reserve) (Xu et al., 2003)

• Further allocation of GW to beneficial uses can only be granted once GW has been set aside for the Reserve

Objectives

• The overall aim of this study is to develop and test an integrated method for assessing allocable GW (for all uses) from stream flow recession and instream flow requirements

• Apply the method to selected quaternary catchments in South Africa

Assumptions

• Baseflow = groundwater contribution to the river

• Groundwater contribution to environmental requirements = Maintenance low flow requirements (MLIFR)

• Shallow aquifer acts as a linear reservoir

• Allocatable groundwater = annual baseflow - MLIFR

Steps in Method1. Select streamflow gauging stations and study sites

2. Determine drainage time scale K from recession flow

3. Determine the river baseflows through a baseflow separation method

4. Determine MLIFR using the desktop reserve model (DRM)

5. Determine the surplus baseflow (annual baselflow - MLIFR)

6. Convert the surplus baseflow to equivalent GW storage thickness, using the drainage time scale K (S=KQ)

7. Determine the upper limit of groundwater abstraction as the GW storage thickness equalled or exceeded 75% of the years from the storage thickness duration curve

Selection of study sites

• Quaternary catchments were selected using the following criteria Long-term (>30 years) daily stream flow data Long-term baseflow index (BFI) should be higher than

0.25 (based on map in Hughes et al. (2007)). BFI is a proportion of baseflow to total streamflow

Catchments should be relatively pristine Missing data should be less than 10%

• These criteria were quite strict (identifying only 1.7% of the stations in the database)

• The selected catchments varied in size from 23 to 697 km2, and annual rainfall from 526 to 1058 mm.

Selected quaternary catchments

Geological conditions of the selected sitesPercentage area of the quaternary catchment covered by certain lithology

Zone-3Zone-2Zone-1

Estimating drainage time scale from recession flow analysis

• Recession analysis is a well-known tool in hydrological analysis

• The Boussinesq equation describing flow in unconfined aquifers based on the hydraulic approach is shown in Brutsaert and Nieber (1977) to be expressed in general as a power law function:

• (1)• During low flow, b=1 and the flow recession is

approximated as an exponential decay function (linear reservoir) (Brutsaert and Lopez, 1998)

• S= KQ (2)Adopted from Rupp and Selker (2005)

Estimating drainage time scale from recession flow analysis

• From Eq. 1 and assuming a linear reservoir approximation:

(3)

• When daily values of -dQ/dt and Q are plotted on log-log graph, the cluster of the points is ‘enveloped’ by a lower line of slope equal to 1

• The envelope line represents the lowest recession rate (-dQ/dt) for a given Q, which is considered the baseflow condition

Extracted recession points from streamflow and lower envelope line

Extracted recession flow data points and the total streamflow

Lower envelope line

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Baseflow separation

• The recursive digital filter by Nathan and McMahon (1990) (4) Where qt is the filtered surface runoff at time step

t, Qt is the original streamflow at time t, and β is the filter parameter

• Baseflow bt is calculated as:

(5)

Baseflow separation and baseflow index

• The value of β ranges from 0.90 to 0.95, while a β value of 0.925 is recommended as optimal

• For rivers in South Africa, Smakhtin and Watkins (1997) recommended 0.995 as being suitable for daily baseflow separation

• We used β = 0.995

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Estimating environmental flow requirements using the Desktop Reserve Model (DRM)

• DRM uses two Index of flow variability • CVB=CV/BFI • DRM computes maintenance low instream flow

requirements (MLIFR) as a percentage MAR using the following relation (Hughes and Hannart, 2003)

• (6) Where LP1, LP2, LP3 and LP4 are parameters

which need to be determined for a particular level of ecological category [A-D]

Present ecological conditions of riversQuaternary catchment ID

River gauge ID

Present Ecological state

Quaternary catchment ID

River gauge ID

Present Ecological state

B81D B8H010 C K80C K8H001 C

A42D A4H008 B K80C K8H002 C

B73A B7H004 C K70B K7H001 B

B42F B4H005 C K60A K6H001 B

B11K B1H004 C K50A K5H002 B

C81F C8H005 C K40B K4H001 B

V60A V6H004 A K40C K4H002 B

U20D U2H006 B K30D K3H005 B

U20B U2H007 C K30A K3H003 C

U20A U2H013 B K20A K2H002 C

U70A U7H007 C

Zone-1 Zone-2 Zone-3

Analysis and ResultsDrainage time scale (K) and BFI

Drainage time scale (K) BFI

K ranges from 28 to 168 daysBFI ranges from 0.14 to 0.54

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Analysis and ResultsCorrelation of K with catchment attributes

Area (km2)

River length

(m)

Drainage density (km-1)

Slope (%)

Mean elevation BFI

Water level

(amsl)K

daysArea (km2) 1.00

River length (m) 0.91 1.00

Drainage density (km-1) 0.19 0.57 1.00Slope (%) -0.20 -0.06 0.20 1.00

Mean elevation 0.69 0.63 0.16 -0.34 1.00BFI 0.17 0.08 -0.08 -0.75 0.51 1.00

Water level (amsl) 0.50 0.43 0.11 -0.49 0.89 0.75 1.00K (days) 0.06 -0.11 -0.31 -0.80 0.47 0.92 0.69 1.00

Analysis and ResultsMLIFR and MAR

MLIFR ranges from 6-26% of MAR

Analysis and ResultsUpper limit of groundwater allocation

Annual groundwater storage thickness

Duration curves for groundwater storage thickness

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Variability b/n years is assumed to be manageable through buffering capacity of GW

Analysis and ResultsGroundwater storage thickness

Conclusions

• The methodology is based on simple assumptions and concepts, implying uncertainties. However, it is useful for first estimates in GW data-scarce regions

• The method accounts for inter-annual variability of stream flow

• In 17 out of 21 catchments assessed, opportunities exist to further utilise GW

• The method needs to be tested against current GW assessment and allocation tools and supplemented by GW monitoring data - for method refinement and for compliance and future impact assessment

References

1. Brutsaert, W., and Lopez, J. P., 1998, Basin-scale geohydrologic drought flow features of riparian aquifers in the southern Great Plains: Water Resources Research, v. 34, no. 2, p. 233-240.

2. Brutsaert, W., and Nieber, J. L., 1977, Regionalized drought flow hydrographs from a mature glaciated plateau: Water Resources Research, v. 13, no. 3, p. 637-643.

3. Hughes, D. A., and Hannart, P., 2003, A desktop model used to provide an initial estimate of the ecological instream flow requirements of rivers in South Africa: Journal of Hydrology, v. 270, no. 3, p. 167-181.

4. Hughes, D. A., Parsons, R., and Conrad, J. E., 2007, Quantification of the groundwater contribution to baseflow, Water Research Commission.

5. Nathan, R. J., and McMahon, T. A., 1990, Evaluation of automated techniques for base flow and recession analyses: Water Resources Research, v. 26, no. 7, p. 1465-1473.

6. Rupp, D. E., and Selker, J. S., 2005, Drainage of a horizontal Boussinesq aquifer with a power law hydraulic conductivity profile: Water resources research, v. 41, no. 11.

7. Smakhtin, V. U., and Watkins, D. A., 1997, Low-flow estimation in South Africa. Water Research Commission Report N 494/1/97, Vol 1; Vol 2: Appendices.

8. Xu, Y., Colvin, C., van Tonder, G., Hughes, S., Le Maitre, D., Zhang, J., Mafanya, T., and Braune, E., 2003, Towards the resource directed measures: groundwater component (Version 1.1): WRC Report.

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