How much streamflow is groundwater discharge? - a method for assessment

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How much streamflow is groundwater

discharge?

- A method for assessment

Vince Bidwell

Lincoln Environmental

-

Pukemanga Working Group

(

NIWA, GNS, LandcareResearch,

AgResearch, Lincoln Environmental

)

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Why do we want to know?

1. Water quality

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Why do we want to know?

2. Water quantity

• Groundwater discharge is the source of “baseflow” in streams

• Baseflow is vital for stream ecology and human use

• Temporal distribution of baseflow (e.g., recession rate) is

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Model of groundwater discharge

Recharge ≤ D_max Aquifer: conductivity k storativity S L B Groundwater discharge to surface water Surface drainage

Vadose zone response time T

2

SL kB

=

α

Soil-water balance model

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Two questions to be addressed

and two model parameters to answer them

1. What proportion of catchment drainage becomes groundwater

discharge?

• Defined by the maximum recharge rate D_max

• What is the temporal distribution of the groundwater

discharge?

• Defined by the parameter α of a dynamic groundwater model that maintains a set of water storage states

• These two parameters are optimised for best fit to the lower

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Example 1: Toenepi Catchment, Waikato

• Area = 15.1 km2

• Slope: flat (89%), rolling (10%), steep (1%)

• Elevation: 40 – 130 m (amsl)

• Land use: dairy pasture

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Toenepi Catchment: hourly data 24/11/02 – 23/11/03

Rainfall = 1081 mm; Streamflow = 273 mm 0.00 0.10 0.20 0.30 0.40 0.50 0.60

Nov 2002 Jan 2003 Mar 2003 May 2003 Jul 2003 Sep 2003

Fl

ow

(mm/

h)

Observed streamflow

Predicted groundwater discharge

74%

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Toenepi Catchment

Groundwater discharge dynamics

0.00 0.05 0.10 0.15 0.20 0.25

Jun 2003 Aug 2003 Oct 2003

Fl ow (mm/ h) Observed streamflow Predicted groundwater discharge

Rapid response _Complex

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Example 2: Pukemanga Catchment, Waikato

• Area = 3 ha

• Slope: hilly (17°- 20°) to steep (>30°)

• Elevation: 72 – 146 m (amsl)

• Land use: sheep pasture

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Pukemanga Catchment: hourly data 1/4/98 – 31/3/99

Rainfall = 1706 mm; Streamflow = 1013 mm; Area = 1.43/3.0 ha

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0

Apr 1998 Jun 1998 Aug 1998 Oct 1998 Dec 1998 Jan 1999

Fl ow (mm/ h) Observed streamflow Predicted groundwater discharge 62% D_max = 2.0 mm/h

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Pukemanga Catchment

Groundwater discharge dynamics

0.00 0.10 0.20 0.30 0.40 0.50

Apr 1998 Jun 1998 Aug 1998 Oct 1998 Dec 1998 Jan 1999

Fl ow (mm/ h) Observed streamflow Predicted groundwater discharge

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Pukemanga Stream catchment is 48% of topographical area: hillslope “stream function” model

Wetland origin of Pukemanga Stream Extent of groundwater system

For Pukemanga Stream

“Deep” groundwater system draining to river

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Sources of the other 38% of Pukemanga streamflow? - piezometric levels near the wetland (current research)

0.00 0.20 0.40 0.60 0.80 1.00

Apr 2004 May 2004 Jun 2004

Fl ow (mm/ h) -300 -250 -200 -150 -100 -50 0 50 P ie z ome tr ic l e v e l (c m be low gr ound) Observed streamflow

Predicted groundwater discharge Piezometric level ~1 m depth

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Comparison of baseflow recession in response to a unit pulse recharge

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0 10 20 30 40 50 60 70 80 90 100

Days since unit pulse recharge

Relat ive g ro u n d w at er d isch ar g e Toenepi - 1510 ha Pukemanga - 3 ha

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0.0001 0.001 0.01 0.1

0 10 20 30 40 50 60 70 80 90 100

Days since unit pulse recharge

Relat ive g ro u n d w at er d isch ar g e ( lo g ) Toenepi - 1510 ha Pukemanga - 3 ha

Comparison of baseflow recession (log scale)

α = 0.0096 /d α = 0.0278 /d kB α = SL2 __

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Conclusions about quantity and quality

• For both catchments, during the year of observations, more

than 60% of the total annual streamflow is groundwater discharge, from recharge at rates less than 2 mm/h

• Quality of this groundwater discharge would be determined by

leaching of the soil profile and exposure to geochemical processes in the aquifer

• Much of the remaining streamflow may be from drainage of

saturated near-surface zones, with quality determined mainly by leaching of the soil profile (current research)

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Conclusions about groundwater discharge dynamics

• Response of groundwater discharge to recharge entering the

groundwater surface is essentially instantaneous

• More of the streamflow during drainage events can be ascribed

to groundwater recharge than predicted by some alternative baseflow separation methods

• The complex behaviour of streamflow recession is described

by a single-parameter model, based on the theoretical

dynamics of groundwater flow, which preserves water storage states