Risks to Murray-Darling Basin Water Resources Program
Project Summaries
Project code:
CP3
Project title:
The effects of climatic changes on plant physiological and catchment
ecohydrological processes in the high-rainfall catchments of the
Murray-Darling Basin: A scoping study.
Project timeline:
March – November 2010
Contractors:
CSIRO: Water for a Healthy Country Flagship
Report Authors:
Tim R. McVicar, Randall J. Donohue, Anthony P. O’Grady, and
Lingtao Li
Project objectives:
The objective of this project was to undertake a scoping study on the potential impacts of climate change
in the MDB on forest hydrology in the high yielding upper catchments, with a focus on plant physiology.
This would build on earlier work undertaken by Roderick and Farquhar, and would provide a synthesis of
information on the collective and cumulative impacts of climate, catchment processes and ecohydrology,
on Basin catchments and water resources. The researchers were required to have a detailed understanding
of plant physiology, including likely effects of climate change on evapotranspiration and tree water use,
and the ability to consider the timescale of potential change in catchment water yield, as well as spatial
scale from point-scale (individual tree) to landscape (sub-catchment) scales.
The project was to provide estimates of potential effects of different climate scenarios on catchment water
yields, based on current knowledge, as well as identifying the factors that are likely to most influence the
climate: water yield relationship. Recommendations were to be provided on further work needed to
clarify the water yield relationships and to provide greater confidence in forecasting climate change
effects. The work might also include the development of models based on climate change scenarios to
predict the likely impacts of climate change on forest evapotranspiration and water use, and hence
impacts on water yield within the specified catchments. This project was to also take account of the work
of two other MDBA-funded projects concerned with tree and forest water use, CP1 and CP2.
Methods:
The project team gathered available information in the form of a draft literature review. This was
forwarded to participants prior to their attendance at an expert workshop held to review and assess what is
currently known about plant physiology and forest hydrology under climate change, with a focus on
eucalypts and the high rainfall upper catchments of the MDB. Using the information from the workshop
and from follow-up discussions, a written report was presented to the MDBA outlining the state of
knowledge and identified knowledge gaps regarding plant physiology and forest hydrology under climate
variability.
The project workplan was:
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CP3 Project Summary
1. an assessment of previous relevant forest hydrology / plant physiology studies, with a focus on
Australian studies, placing the findings (where possible) in the Budyko energy-limited /waterlimited framework.
2. based on the synthesis of current knowledge determine knowledge gaps and develop a series of
key questions (formulated in consultation with MDBA) that need to be addressed to reduce these
gaps.
3. refine the synthesis of previous knowledge, and to assist in developing approaches to address the
previously identified key questions, organise a workshop with specialists invited from key
government and academic organisations. One approach to be discussed in the workshop is how
catchment functional characteristics can be determined from analysis of long-term datasets of
streamflow, remotely sensed vegetation information, meteorological surfaces and atmospheric
CO2 concentration.
4. provide an assessment of the likely impact, and where possible the potential magnitude of the
impact, that vegetation functioning non-stationarity will have on the Basin’s water resources.
5. provide advice on how vegetation functioning non-stationarity could be included in water
resource management models that underpin strategic water allocation decisions.
Summary of key results:
A key element of the project’s analytical framework is determining the climatological limit to
evapotranspiration. There are two options: (1) in wet places evapotranspiration is limited by the available
energy (energy-limited catchments); and (2) in dry places evapotranspiration is limited by available water
(water-limited catchments). This distinction is important, as catchments respond to changes in climatic
and ecohydrological conditions differently depending on the degree of energy or water limitation at
annual or sub-annual timescales. The project considered five key aspects of climate induced change and
their effects on vegetation water use, these were: changes in precipitation, potential evaporation, air
temperature, radiative regime, and atmospheric carbon dioxide concentrations. Results showed that while
air temperature is increasing, in the key runoff generating parts of the MDB evaporative demand is
decreasing as a result of the combined impact of changes in net radiation, near-surface wind-speed, and
vapor pressure deficit (related to relative humidity). The latter variables also influence evaporative
demand. In the south-eastern Australian mountain ranges, winter is the time when most precipitation is
received and when energy-limited conditions dominate, and hence is usually the period when much
streamflow is generated. For these areas, while there maybe less precipitation in future, increased
atmospheric CO2 concentration is likely to lead to an increase in the proportion of precipitation becoming
runoff from these energy-limited catchments.
The project also examined the likely impacts on vegetation functioning and water use due to changes in:
water availability, air temperature, radiative regime, and atmospheric CO2 concentration. Over the longterm, natural vegetation biomass and cover are in balance with the long-term water availability and its
variability, whereas over the shorter term, vegetation responds dynamically to a diverse array of
environmental signals, including water deficits and air temperature. Results showed that the potential
increased water use from forests due to higher air temperatures will mainly be offset by greater water use
efficiency due to increased atmospheric carbon dioxide concentrations, although this might be offset
somewhat by concomitant increases in biomass (including changes in leaf area and rooting depth). In
order to unpack these effects and to model their likely overall impact on water yield, the project
considered the MDB as comprising five zones as outlined in the following table (see next page).
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CP3 Project Summary
Zone
Description
The extremely high yield zone (EHYZ)
This zone encompasses the energy-limited locations (i.e., annual
average precipitation P is greater than annual average potential
evaporation).
The very high yield zone (VHYZ)
This zone encompasses all areas that contribute the upper 25% of
total MDB runoff and includes the EHYZ.
The southern high yield zone (sHYZ)
The HYZ encompasses all areas that contribute the upper 60% of
total MDB runoff. The southern HYZ contains those areas south of
the 32nd parallel of latitude and includes the EHYZ and the VHYZ.
The northern high yield zone (nHYZ)
The nHYZ encompasses all areas of the HYZ that are north of the
32nd parallel of latitude. This does not include the EHYZ or the
VHYZ.
The whole Murray-Darling Basin
Includes all of the above zones.
The sensitivity of runoff in each zone was modeled to changes in five key ecohydrological parameters
using analytical techniques applied at an annual time-step. The five ecohydrological drivers were:
1. annual precipitation (P; mm.y-1);
2. annual potential evaporation (using Penman’s formulation, Ep; mm.y-1);
3. catchment-average rooting depth (Zr; mm);
4. average rain depth (α; mm–one of the variables governing precipitation intensity);and,
5. atmospheric CO2 concentration (C; ppm).
The sensitivity of runoff in each zone to each parameter is shown in the table below.
Parameter and units
P in mm.y-1/mm.y-1
Ep in mm.y-1/mm.y-1
Zr in mm.y-1/mm
α in mm.y-1/mm
C in mm.y-1/ % ΔC
EHYZ
VHYZ
0.73
-0.42
-0.40
23.5
0.08
0.61
-0.29
-0.44
30.6
-0.16
sHYZ
0.43
-0.16
-0.38
34.1
-0.22
nHYZ
0.32
-0.09
-0.33
29.2
-0.19
All MDB
0.13
-0.03
-0.13
16.1
0.02
The figures show that the relative sensitivity of runoff to different ecohydrological factors varies widely
across the landscape. For example, in the energy-limited area of the MDB (the EHYZ), a 100 mm per
year precipitation increase is predicted to result in a 73 mm per year increase in runoff. In contrast, for
the nHYZ, a 100 mm per year precipitation increase is simulated to only provide a 32 mm per year
increase in runoff. If precipitation were to decrease by 100 mm per year for these two areas, then runoff
is predicted to decrease by 73 mm per year and 32 mm per year, respectively. An increase in
precipitation causes an increase in runoff in all zones, and an increase in potential evaporation results in a
decrease in runoff (hence these coefficients have a negative sign as might be expected).
To provide an indication of the relative importance of each variable to runoff (broadly reflective of water
yield) in each zone, the sensitivity coefficients were multiplied by their observed natural variability in
each yield-zone. The following table shows summaries for each zone of the estimated changes in runoff
in response to the observed level of variation in the five key ecohydrological variables. All units are
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CP3 Project Summary
mm.y-1. Numbers are given for a 12% increase in C (the observed change over the study period) and for
a 100% increase in C (estimate of mid-century levels). All figures are +/- (the variable could in future be
above or below its long-term average), except for changes in CO2 which is assumed to increase for the
foreseeable future.
Parameter
P
Ep
Zr
α
C +12%
+ 100%
EHYZ
253
46
124
31
+1
+8
VHYZ
190
22
97
38
-2
-16
sHYZ
102
12
68
34
-3
-22
nHYZ
56
7
36
26
-2
-19
All MDB
18
3
13
16
0
-2
As might be expected, the figures show that the variation in runoff due to the natural variability in annual
precipitation (P) has the largest influence on water resources in the MDB. This analysis also shows that
natural variation in annual rates of potential evaporation (Ep) is, at most, ~20% of that of precipitation
(see the EHYZ figures). Considering the natural variability of average rain depth (α) calculated from
daily gridded data over south-east Australia, this variable has minimal long-term impact on water
resource generation in the higher yield areas. However, it should be noted this is an assessment of longterm trends of annual data, and, of course, there are many isolated events that produce important quick
flow and base flow (via recharging aquifers) that are important to filling the major water supply dams.
Changes in rooting depth (Zr), caused by, for example bushfire, forestry activities (including both
clearing and re-growth) and re-afforestation (including plantations), will impact on the runoff generated
from the catchments. The relative importance of changes in rooting depth relative to variability of
precipitation impacting runoff generation varies from between 75% (in the nHYZ) to approximately 25%
(in the EHYZ). Of the five variables studied, change of rooting depth is the only variable that can be
directly altered by human activity via land management practices.
These figures show that all key variables, other than increasing atmospheric CO2 concentration, exhibit
natural variability that can either increase or decrease runoff generation. As atmospheric CO2
concentration is extremely likely to continue to increase, the project’s current data-driven hypothesis
suggests that this will result in increased runoff generation from the energy limited EHYZ, and decreased
runoff from the other zones due to changes in vegetation water use. In energy-limited catchments there
can be no increase in evaporation (as this is already limited by available energy), so a hydrological
response of increasing runoff is expected. In contrast, for the remaining water-limited zones, it is
expected that there will be an ecological response as vegetation will optimally adapt to use the increased
water availability. This ecological response is likely to invoke a hydrological response causing reductions
in runoff due to increased canopy interception and/or increased plant water use due to improved rooting
system of efficiency.
In considering these results in relation to long-term water resource management planning, it is important
to keep in mind that the data shown in the tables above are indicative only. Estimates of the sensitivity of
runoff to changes in key ecohydrological parameters are reliable to different levels. The current
assessment of their reliabilities is: precipitation and potential evaporation are deemed to be ‘very high’;
rain depth and rooting depth are ‘medium-high’; and the increasing atmospheric CO2 concentration
response is ‘low’. The lowest reliability model is one key area where future research should be focused if
this analysis is to be extended. Other issues that should be considered beyond this initial scoping study
include changes in the seasonality of precipitation and potential evaporation; reducing the time-step of the
modeling from steady-state so that changes in water storages (soil water and groundwater) can be
accounted for; and vegetation disturbances due to: (a) herbivory (b) mortality and (c) fire regime changes,
which are unlikely to occur in isolation.
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CP3 Project Summary
Comparisons with other projects:
Results from this project broadly match those of the related projects CP1 and CP2, and all three projects
discussed each others’ results prior to completing their final reports to the MDBA. Changes in land use
and vegetation type (through afforestation) would be expected to have a significant effect on catchment
water yields in energy-limited areas due to changed rooting depth, but little effect in water-limited
catchments as found in CP1. Even in the former, their overall impact on MDB water resource would be
related closely to the proportion of each catchment planted. Projects CP2 and CP3 both point to the
significant impact on catchment water yield following bushfires.
Knowledge gaps identified:
The project team recommended that in order to improve the reliability and value to water resource
planning and management of the research performed in this scoping study, further research be conducted
in the following areas:
1. Increase the certainty of the streamflow response modelling to increasing atmospheric CO2
concentration- this is of paramount importance. This is especially the case as CO2 -induced
changes in vegetation functioning are likely, changing the reliability of base flow generation in
times of drought from the higher water yielding areas. It is anticipated that this could be done via
analysis and modelling of time series gauged streamflow, climate surfaces and remotely sensed
databases. In concert, suitable physiological modelling activities should be pursued, as a multiple
lines of evidence approach will be needed to reconcile this impact.
2. Adapt the energy-limited / water-limited framework used here (i.e., the Budyko framework) to a
monthly time-step. This will mean that changes in water storages (soil water and groundwater)
will need to be accounted for. Using a monthly time-step also means that the impact of changes
in precipitation and potential evaporation seasonality on runoff generating processes will be
better captured.
3. Perform a meta-analysis of changes to the water balance from existing international FACE (free
air CO2 enrichment) experiments. In addition, opportunities to monitor ecohydrological changes
within an Australian ecosystem should be realised through a connection with a FACE experiment
being installed within an Eucalypt woodland by the University of Western Sydney.
4. Improve the understanding and characterisation of the dynamics of the high water yielding areas
of the MDB, and possibly more widely to include all southeast Australia (as similar dominating
weather systems and ecohydrological processes exist either side of the Dividing Range), via
analysis of the:
i.
storage capacity of the alpine and sub-alpine landscapes (including the important
sphagnum bogs);
ii.
likelihood of tree encroachment in the alpine and sub-alpine landscapes;
iii.
observed ecohydrological responses of montane forests to the climate dynamics
and weather extremes; and
iv.
observed variability and change of rain depth and (where possible) sub-daily
rainfall intensity
Implications for policy:
The major findings of this project show the relative importance to water yield of rainfall vs evaporation,
the effect of changes in rooting depth and rainfall intensity, and the potential impact of increasing CO2
concentration on vegetation water use efficiency. All, but especially rainfall, rooting depth and CO 2, are
likely to have a significant impact on the water yield from the energy-limited catchments of the MDB
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CP3 Project Summary
which are the major source of the Basin’s managed water resources. These factors will need to be
considered in planning the future capture and management of water in the MDB, and the respective roles
and opportunities in energy-limited and water-limited regions.
Recommended Communication Approach:
The Final Report should be made widely available in electronic format as it contains much valuable
discussion of the modelling used and of the detailed results obtained. The project team should be
encouraged to present their findings at workshops and conferences, and it would be useful to have them
present at a forum with the related projects CP1 and CP2, as there are clear synergies between the three
studies.
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CP3 Project Summary