What happens to water yield in the
Motueka catchment when land use
(and climate) change?
John Dymond & Robbie Andrew
Presented by Tim Davie
Manaaki Whenua – Landcare Research
The approach
• Using a water balance model developed as
an assessment of seasonal water balance,
spatially distributed through the catchment.
• Scenarios of land use and climate change
were modelled using “reasonable knowledge”
- estimates of previous & possible land cover
- extremes from current climate data
• Work is still in progress; preliminary results
presented here
Background on model
• Based on Wigmosta et al (1994) but with
several simplifications.
• Water balance throughout the catchment is
simulated using routines for:
•
•
•
•
•
Canopy interception
Infiltration
Evapotranspiration
Soil water movement
Channel routing (very simplistic)
• 25m pixels
• Daily timestep
Wigmosta et al (1994) Water Resources Research 30:1665-1679
Precipitation
Evapotranspiration
Rain
Interception
Store
Infiltration
Run-off
Exfiltration
Unsaturated Soil Store
from next
cell(s)
Saturated Soil Store
Ground water
flow to next
cell
Input variables & parameters
• Variable
• Daily rainfall
• Parameters
•
•
•
•
•
•
Landcover
Soil depth
“Permeability”
DEM (slope & flow direction)
Potential evaporation
Annual rainfall distribution
• PE used in relationship with actual Evap
• Annual rainfall used for spatial distribution of
daily rainfall (from 7 gauges)
Land cover
scenario 1
“Current”
Land cover
scenario 2
“Prehistoric”
Land use change scenarios
• Summer discharge during the 1990’s
Summer runoff
350
Current
Prehistoric
Max. Pines
250
200
150
100
50
0
90
/9
1
91
/9
2
92
/9
3
93
/9
4
94
/9
5
95
/9
6
96
/9
7
97
/9
8
98
/9
9
mm runoff
300
Summer
Land use change
• Summer discharge during the 1990’s
– Prehistoric most conservative with water
– Max. Pine and current very similar
– Transpiration largest factor
• 7 days Low flows
7 day low flows
Ln discharge
3
Current
Prehistoric
2.5
Max. Pines
2
1.5
1
0.5
0
0.5
1
1.5
Ln Recurrence interval
2
2.5
Land use change
• Summer discharge during the 1990’s
– Prehistoric most conservative with water
– Max. Pine and current very similar
– Transpiration largest factor
• 7 day low flows
– 5 year low
• Prehistoric
• Current land use
• Max. pine
• Dryness distributions
4.95 cumecs
4.03 cumecs
3.78 cumecs
Dryness in driest year
3
Current
2.5
% catchment
Prehistoric
2
Max Pines
1.5
1
0.5
0
1
11
21
31
41
51
61
Number of dry days
71
81
91
Dryness in wettest year
3
Current
% catchment
2.5
Prehistoric
2
Max Pines
1.5
1
0.5
0
1
11
21
31
41
51
61
Number of dry days
71
81
91
Summary of land use scenarios
• Prehistoric land cover appears the most
conservative with water
• Transpiration low
• Soil moisture staying high
• Maximum pine coverage pretty similar to
current
• Not that much extra pine being added
• Transpiration rates similar
• Soil moisture staying high
Land cover
scenario 1
“Current”
Land cover
scenario 3
“Max Pine”
Summary of land use scenarios
• Prehistoric land cover appears the most conservative
with water
• Transpiration low
• Soil moisture staying high
• Maximum pine coverage pretty similar to current
• Not that much extra pine being added
• Transpiration rates similar
• Soil moisture staying high
• But is it right?
• Doesn’t agree with small catchment studies
Compared to measured
300
Simulated
Measured
250
mm runoff
200
150
100
50
0
90/91
91/92
92/93
93/94
94/95
Summer
95/96
96/97
97/98
98/99
Compared to measured
3.5
Simulated
Ln discharge
3
Measured
5 year low
flows
2.5
2
1.5
Measured 9.12 cumecs
1
0.5
0
0.5
1
1.5
Ln Recurrence interval
2
2.5
Simulated 4.03 cumecs
Why are low flows so far out?
• Soil moisture stays high throughout the
catchment
• Therefore the trees transpire freely
throughout the summer
• Therefore transpiration becomes the major
loss (pines ≈ pasture)
• Would expect drying out and interception to
be more dominant (pines ≈ beech > pasture)
• Soil moisture and regolith functioning within
the model isn’t working correctly
Work currently being undertaken
• Trying to improve the soil moisture and
deeper regolith accounting
• Putting in a deeper regolith and allowing slow
water movement through it
• Testing this against known soil moisture
measurements (neutron probes - Donald
Creek & Waiwhero)
• Then rerunning land use change scenarios
• Running climate change scenarios
• wettest years on record
• driest on record