How much streamflow is groundwater
discharge?
- A method for assessment
Vince Bidwell
Lincoln Environmental
-
Pukemanga Working Group
(
NIWA, GNS, LandcareResearch,
AgResearch, Lincoln Environmental
)
Why do we want to know?
1. Water quality
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
Model of groundwater discharge
Recharge ≤ Dmax Aquifer: conductivity k storativity S L B Groundwater discharge to surface water Surface drainageVadose zone response time T
2
SL kB
=
α
Soil-water balance model
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 Dmax
• 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
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
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 0.60Nov 2002 Jan 2003 Mar 2003 May 2003 Jul 2003 Sep 2003
Fl
ow
(mm/
h)
Observed streamflow
Predicted groundwater discharge
74%
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
Example 2: Pukemanga Catchment, Waikato
• Area = 3 ha
• Slope: hilly (17°- 20°) to steep (>30°)
• Elevation: 72 – 146 m (amsl)
• Land use: sheep pasture
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% Dmax = 2.0 mm/h
Pukemanga Catchment
Groundwater discharge dynamics0.00 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
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
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
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
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 __
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)
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