Project 054120. Response of the River Murray floodplain to … · 2011. 3. 28. · water (flooding / fresh groundwater) to survive dry periods. Response of the River Murray floodplain

December 2010

Project 054120. Response of the River Murray

floodplain to flooding and groundwater management Final Report to the Centre for Natural Resource Management

Kate L. Holland, Tanya M. Doody, Kerryn L. McEwan and Ian D. Jolly

Water for a Healthy Country Flagship Report series ISSN: 1835-095X

Australia is founding its future on science and innovation. Its national science agency, CSIRO, is a powerhouse of ideas, technologies and skills.

CSIRO initiated the National Research Flagships to address Australia’s major research challenges and opportunities. They apply large scale, long term, multidisciplinary science and aim for widespread adoption of solutions. The Flagship Collaboration Fund supports the best and brightest researchers to address these complex challenges through partnerships between CSIRO, universities, research agencies and industry.

The Water for a Healthy Country Flagship aims to achieve a tenfold increase in the economic, social and environmental benefits from water by 2025. The work contained in this report is a collaboration between CSIRO and the Department of Water, Land and Biodiversity Conservation (DWLBC).

For more information about Water for a Healthy Country Flagship or the National Research Flagship Initiative visit www.csiro.au/org/HealthyCountry.html

Citation: Holland KL, Doody TM, McEwan KL, Jolly ID, 2010. Project 054120. Response of the River Murray floodplain to flooding and groundwater management. Final Report to the Centre for Natural Resource Management. CSIRO: Water for a Healthy Country National Research Flagship. 14 pp.

Copyright and Disclaimer

© 2010 CSIRO To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO.

Important Disclaimer:

CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it.

Cover Photograph:

From Kate Holland File: Chowilla_redgum_creek.jpg Description: Trees lining edges of Monoman Island Horseshoe wetland on the Chowilla floodplain. Photographer: Kate Holland © 2003 CSIRO

Response of the River Murray floodplain to flooding and groundwater management Page iii

CONTENTS Acknowledgments ....................................................................................................... iv

Executive summary...................................................................................................... v

1. Introduction ......................................................................................................... 1

2. Project summary ................................................................................................. 3 2.1. Effectiveness of artificial watering at Chowilla.......................................................... 3 2.2. Bookpurnong Floodplain Pilot Project....................................................................... 6

2.2.1. Ecological benefits of flooding and groundwater management .............................7 2.2.2. Ecophysiology of the major floodplain tree species ...............................................8

2.3. Overall project conclusions..................................................................................... 10 2.4. Further application of research results ................................................................... 11

3. Reporting against CNRM project milestones ................................................. 11

4. Project publications .......................................................................................... 12

References .................................................................................................................. 13

LIST OF FIGURES Figure 1. Conceptual model of surface water-groundwater interactions in lower River Murray floodplain wetlands illustrating the location of important groundwater discharge pathways in the floodplain. Not to scale. ..................................................................................................... 1 Figure 2. Location of the Monoman Island Horseshoe wetland on the Chowilla floodplain in South Australia. The three piezometer and vegetation transects and control site are shown in red............................................................................................................................................ 3 Figure 3. Wetland and riparian zone water balance over three years. The arrows show wetland inflows, wetland evaporation and recharge, riparian zone inflows and evapotranspiration. Not to scale. ............................................................................................. 5 Figure 4 Location of the piezometers (B01-B12, B25), groundwater production bores (SIS and LM bore) and experimental transects at Bookpurnong. The inset map shows the location of the Bookpurnong floodplain in the South Australian Riverland............................................ 7 Figure 5 Groundwater salinity (μS cm-1) between December 2005 and December 2008. ...... 8 Figure 6. E. largiflorens water potentials at B08 (groundwater freshening) and at B11 (control) between December 2005 and December 2008. Symbols show individual tree water potentials, lines represent the mean water potential for the period before and after groundwater freshening occurred in September 2006. Note more negative water potentials represent increased tree water stress. .................................................................................... 8 Figure 7. Box plots of (a) predawn and (b) midday water potentials taken on eight field trips between December 2005 and March 2008 at Transects 1 – 4. Data is separated by tree species and groundwater salinity. The boxes show the 25th and 75th percentile of the mean, the error bars show the 10th and 90th percentiles of the mean, the median is shown by the solid line and the mean by the dashed line.............................................................................. 9 Figure 8 Daily transpiration of E. largiflorens, A. stenophylla and E. camaldulensis at the high and low salinity plots on Transect 3 between September 2007 and December 2008. Daily rainfall and pan evaporation rates at Loxton are shown for reference (BoM, 2009).............. 10

LIST OF TABLES Table 1 Extent of bank recharge (m from the wetland) estimated using five different indicators. ................................................................................................................................ 4 Table 2 Water balance for the wetland and riparian zones. .................................................... 4

Response of the River Murray floodplain to flooding and groundwater management Page iv

ACKNOWLEDGMENTS The authors are grateful for the support of this research by (i) the South Australian Centre for Natural Resource Management Project ‘Response of the River Murray Floodplain to Flooding and Groundwater Management’ funded by the National Action Plan for Salinity and Water Quality, and (ii) the Bookpurnong Floodplain Pilot Project, a Murray Darling Basin Commission – The Living Murray Initiative and South Australian Department of Water, Land and Biodiversity Conservation funded project.

Effectiveness of artificial watering at Chowilla

The assistance of Jane Roots, Brenton Erdmann, Judy Goode, Todd Goodman, Megan Lefournour, Glen Drogemuller, Claire Roberts, Michelle Philp, Ian Schneider, Jenny Watling, Molly Whalen, John Hutson and staff from South Australia Water Corporation during the project is gratefully acknowledged. Comments from Bill Young, Matthew Colloff, Russell Crosbie and Nick Souter greatly improved the paper.

Bookpurnong floodplain pilot project

The authors acknowledge the assistance and advice of Glen Drogemuller, Phil Strachan, Steve Tyerman, Wendy Sullivan, Craig Simmons, Richard Benyon, Alison Charles, Steve Clark, Mel White, Volmer Berens, Nick Souter and the Bookpurnong Floodplain Pilot Project Steering Committee. The report was improved by the review and comments of Russell Crosbie and Jacquie England.

Groundwater freshening and riparian vegetation water balance

The authors acknowledge the assistance and advice of Glen Drogemuller, Phil Strachan, Steve Tyerman, Steve Clark, Mel White, Volmer Berens, Nick Souter and the Bookpurnong Floodplain Pilot Project Steering Committee. The paper was improved by the review and valuable comments of Sébastien Lamontagne, Ian Overton, Tivi Theiveyanathan, Matthew Colloff and two anonymous reviewers.

Final report to the Centre for Natural Resource Management

This final report was improved by the review and comments of Glen Walker, Russell Crosbie, Tony Smith and Sébastien Lamontagne.

Response of the River Murray floodplain to flooding and groundwater management Page v

EXECUTIVE SUMMARY In most regions of the world, floodplains and their wetlands support higher biodiversity and productivity than surrounding upland areas. River regulation has significant negative environmental impacts on floodplain biota by changing the extent, duration and timing of inundation. This reduces the rate at which salt is leached from the plant root zone, causing a reduction in soil water availability and riparian vegetation health. Raised river levels associated with river regulation also affect surface water – groundwater interactions in floodplains, causing water tables to rise closer to the surface. This also increases salt accumulation rates in floodplain soils. Floodplain hydrology is also affected by irrigated agriculture, which leads to increased subsurface drainage. This increases the inflow of saline groundwater to the river valley and floodplain leading to shallow floodplain water tables and often seepage at the break in slope of the river valley.

These changes to floodplain hydrology have lead to a decline in the health of the dominant riparian tree species in the lower River Murray (black box, Eucalyptus largiflorens; river red gum, E. camaldulensis; and river cooba, Acacia stenophylla). A recent survey of the lower River Murray floodplain found that less than a quarter of all trees were classified as healthy. Regional, State and Basin managers have responded to this decline in vegetation health by setting improved flow regimes and reduced impacts of saline groundwater as key floodplain management targets for lower River Murray floodplains and wetlands. However, it is unclear how the floodplain aquifer and tree communities will respond to improved flooding regimes and groundwater management. This report describes outcomes from CNRM Project 054120: ‘Response of the River Murray floodplain to flooding and groundwater management’.

Effectiveness of artificial watering at Chowilla

Artificial watering, where river water is pumped into a dry wetland fringed by riparian vegetation, has been used as an emergency management action to save riparian vegetation communities in severe decline. The rationale behind this remediation strategy is that bank recharge can replenish soil water and groundwater stores in the riparian zone along the edge of rivers, creeks and backwaters. This study assessed the response of both the floodplain aquifer and E. camaldulensis and E. largiflorens trees to artificial watering. The spatial extent and degree of groundwater freshening around the wetland was greatest at sites with relatively large bank and aquifer hydraulic conductivities. E. camaldulensis and E. largiflorens plant water potentials increased by two to five fold (i.e. decrease in plant water stress) and E. camaldulensis tree water use increased by three- to six-fold over the three to four months after artificial watering. Artificial watering is an effective management option to preserve significant ecological assets during periods of low flow. The wetland water balance estimated that recharge from the wetland would be evaporated and transpired within three years, indicating that regular artificial watering would be required during extended periods of low flow to maintain high value sites. This time period is consistent with estimates of natural flood return periods for E. camaldulensis growing in this region and of the wetland in this study (six floods in ten years). To our knowledge, this is the first study to quantify the extent of lateral recharge and tree response to artificial watering in a saline, semi-arid floodplain wetland.

Bookpurnong Floodplain Pilot Project

The Bookpurnong Floodplain Pilot Project was designed to test some of the assumptions regarding the ecological benefits of a salt interception scheme (SIS) in conjunction with manipulated environmental flows for floodplains of the lower River Murray. Field measurements of floodplain tree, soil and groundwater conditions were used to investigate the ecological response to improved flooding regimes and groundwater management. The experimental design consisted of four transects of three piezometers each that were aligned perpendicular to the river. The different treatments were as follows:

Transect 1: groundwater lowering by groundwater production bores;

Transect 2: groundwater lowering by groundwater production bores and artificial flooding;

Transect 3: groundwater freshening by a groundwater production bore; and

Response of the River Murray floodplain to flooding and groundwater management Page vi

Transect 4: control.

At Transect 1 and 2, SIS groundwater production bores were used to lower the floodplain water table. Natural flood runner creeks at Transect 2 were artificially flooded by pumping River Murray water over small earthen banks into the creeks. A groundwater production bore located at the edge of the riparian zone (~200 m from the river) at Transect 3 was used to freshen the groundwater by creating a hydraulic gradient to draw fresh river water into the aquifer. Transect 4, the control transect was established where there was minimal groundwater lowering and no surface water flooding to account for climatic and river flow variations over the three years of measurement.

As a result of this experiment, we demonstrated that:

1) groundwater lowering reduced groundwater salinity less than 10 m from the river, but did not significantly decrease tree water stress;

2) groundwater lowering and artificial flooding temporarily reduced groundwater salinity and tree water stress, but did not significantly decrease long term tree water stress; and

3) groundwater freshening extended over 150 m from the river, significantly decreasing long term tree water stress

Detailed ecophysiological measurements during the experiment showed that the three floodplain tree species adopt different water use strategies at high and low groundwater salinity sites. At low groundwater salinity sites, A. stenophylla, E. camaldulensis and E. largiflorens were able to maintain higher rates of transpiration in response to higher water availability. In contrast, at high groundwater salinity sites A. stenophylla and E. largiflorens were able to tolerate the highly saline conditions by reducing transpiration rates. E. camaldulensis was dead or absent at high groundwater salinity sites. This represents a trade-off between tree water use and drought/salinity tolerance. The empirical dataset of vegetation response to floodplain management collected in this project will be used to improve model predictions of vegetation response to improved environmental flows and groundwater management.

This project has demonstrated the importance of managing surface water and groundwater together to maximise environmental benefits in the lower River Murray. Artificial watering can benefit the majority of E. camaldulensis communities growing within ~50 m of a water body. Similarly, groundwater production bores that lower water tables and draw fresh River Murray water into alluvial aquifers, can benefit all trees within ~150 m of a permanent water body. However, water table lowering alone does not improve tree health because it does not remove stored salts. The project showed that riparian trees need a long term source of fresh water (flooding / fresh groundwater) to survive dry periods.

Response of the River Murray floodplain to flooding and groundwater management Page 1

1. INTRODUCTION In most regions of the world, floodplains and their wetlands support higher biodiversity and productivity than surrounding highland areas (Tockner and Stanford, 2002). The highly variable nature of flows in arid and semi-arid regions has led to regulation of rivers and streams by weirs and storages (Jolly, 1996). River regulation has significant negative environmental impacts on floodplain biota by changing the extent, duration and timing of inundation (e.g. Walker, 1985; Rood and Mahoney, 1990; Bren, 1992, Walker and Thoms, 1993; Rood et al., 1995; Kingsford, 2000; Bunn and Arthington, 2002, George et al., 2005).

River regulation can also affect surface water – groundwater interactions in floodplain wetlands. River regulation can cause water tables to rise closer to the surface, increasing salt accumulation rates in floodplain soils (Jolly et al., 1993). Reductions in the frequency, duration and extent of flooding mean that the leaching of salt from the plant root zone is reduced, causing a reduction in soil water availability and riparian vegetation health (Jolly et al., 1993; Busch and Smith, 1995). In areas where rivers lose water to groundwater, reductions in the frequency, duration and extent of flooding can result in lowering of water tables beneath floodplains. If the groundwater is relatively fresh, then lowering of water tables results in a reduction of water availability (e.g. Busch et al., 1992; Stromberg et al., 1992; Stromberg et al., 1996; Scott et al., 1999; Le Maitre et al., 1999; Naumburg et al., 2005).

Many floodplains and wetlands in semi-arid environments experience natural inflows of saline regional groundwater (Jolly et al. 2008). In the lower River Murray, groundwater inflows have increased by a factor of three as a result of the development of over 63,000 ha of irrigated horticulture (predominantly citrus, grapes, vegetables, stone fruits and nut crops) immediately adjacent to the river and its floodplain (Holland et al. 2005). Increased rates of recharge occur in these irrigated areas, which has lead to the formation of groundwater mounds with elevations of up to 20 m above the river and floodplain water tables (Figure 1). Subsurface drainage from the groundwater mounds toward the floodplain cause increases in the inflow of saline groundwater to the river valley. When combined with the effects of river regulation, the increased groundwater inflows to the river valley lead to raised water tables beneath the floodplain and, in some areas, seepage at the break in slope of the river valley.

Figure 1. Conceptual model of surface water-groundwater interactions in lower River Murray floodplain wetlands illustrating the location of important groundwater discharge pathways in the floodplain. Not to scale.


Changes to the hydrology of the lower River Murray floodplains have lead to a decline in the health of the dominant riparian tree species (black box, Eucalyptus largiflorens; river red gum, E. camaldulensis; and river cooba, Acacia stenophylla). A recent survey of the lower River Murray floodplain found that less than a quarter of all trees were classified as healthy (DEH 2004). Two floodplain and wetland areas in the lower River Murray have been listed under the Ramsar Convention (1987) for the protection of wetlands of international importance. This includes the Chowilla floodplain, which is included as a Living Murray icon site by the Murray Darling Basin (MDB) Authority. Regional, State and Basin managers have responded to this decline in vegetation health by setting improved flow regimes and reduced impacts of saline groundwater on lower River Murray floodplains and wetlands as key floodplain management targets (e.g. MDBC 1998, DWR 2001, RMCWMB 2002, INRMG 2003, MDBC 2003).

In the case of the major riparian over storey species of the lower River Murray floodplain, it is unclear whether:

1. riparian trees will respond to improved flooding regimes and lowering of saline groundwater because of large stores of salt in the floodplain soils;

2. groundwater salinity can be reduced by flood recharge and groundwater pumping to provide an alternative water source during the long periods between floods and heavy rainfall; and

3. vegetation have the physiological capability to recover after long periods of salt and drought stress.

This report summarises the outcomes of two field experiments designed to measure the response of the floodplain aquifer and tree communities to improved flooding regimes and groundwater management. The field experiments were conducted on the Chowilla and Bookpurnong floodplains in the lower River Murray in South Australia between 2004 and 2008. The overall aim of CNRM Project 054120 was to improve current understanding of the response of the River Murray floodplain to flooding and groundwater management. Improved knowledge of the system response will enable River Murray managers to target flow enhancements and groundwater management practices toward maximising health improvements of the terrestrial floodplain vegetation.

There were four main components to the project. The first task was to:

1. Review the data collected from the previous Chowilla watering projects and other studies to further current understanding of vegetation responses to artificial watering (Holland et al., 2009a).

Then, at the Bookpurnong Floodplain Pilot Project Site (Figure 4) the remaining tasks were to:

2. Determine the ecological benefits of groundwater management and artificial flooding (Holland et al., 2009b).

3. Improve existing knowledge of the ecophysiology of the major floodplain tree species, in particular A. stenophylla for which no prior knowledge is available (Doody et al., 2009); and

4. Provide an empirical basis for the application of vegetation health modelling tools for Chowilla and other floodplains as a basis for quantifying floodplain health benefits from groundwater and flow management (Holland et al., 2009a,b; Doody et al., 2009).

The findings from each of these components and the overall conclusions from the project are summarised in this report. Further detail is contained in the published papers and technical reports listed in Section 4.


2. PROJECT SUMMARY

2.1. Effectiveness of artificial watering at Chowilla Artificial watering, where river water is pumped into a dry wetland fringed by riparian vegetation, has been used as an emergency management action to save riparian vegetation communities in severe decline. The rationale behind this remediation strategy is that bank recharge can replenish soil water and groundwater stores in the riparian zone along the edge of rivers, creeks and backwaters. The study assessed the response of both the floodplain aquifer and E. camaldulensis and E. largiflorens trees to the artificial watering of the Monoman Island Horseshoe wetland on the Chowilla floodplain in March 2004.

The specific aims of the study were to:

1. determine if the extent of lateral recharge from artificial watering was controlled by the hydraulic properties of the wetland bank and underlying aquifer;

2. measure if the trees responded to watering by monitoring the changes in tree water potentials and water use; and

3. estimate the volume of water recharged to the floodplain aquifer by filling the wetland.

Figure 2. Location of the Monoman Island Horseshoe wetland on the Chowilla floodplain in South Australia. The three piezometer and vegetation transects and control site are shown in red.


The spatial extent and degree of groundwater freshening around the wetland that was caused by artificial watering was greatest at sites with relatively large bank (0.70 – 6.03 m d-1) and aquifer (0.21 – 1.15 m d-1) hydraulic conductivities. The large spatial variability of wetland bed conductance and its control of the extent and degree of bank recharge observed in this study suggested that good estimates of wetland bed conductance should be made when modelling surface water– groundwater fluxes in wetlands, rather than using it as a calibration variable.

Artificial watering resulted in a two to five fold increase in E. camaldulensis and E. largiflorens plant water potential (i.e. decrease in plant water stress) and a three- to six-fold increase in E. camaldulensis tree water use over the three to four months following watering, which is comparable to that observed after natural floods. The extent of bank recharge was estimated based on changes in

1. groundwater salinity and isotopic composition along piezometers transects perpendicular to the wetland edge, and

2. vegetation response of trees at known distances from the wetland edge.

Artificial watering improved conditions in the riparian zone up to ~50 m from the edge of the wetland (Table 1).

Table 1 Extent of bank recharge (m from the wetland) estimated using five different indicators.

Groundwater Vegetation Salinity 18O (‰) Water potential Water use 18O (‰) Transect 1 28 m 14 m 26 m 13 m 26 m Transect 2 2 m 7 m 12 m 1 m 12 m Transect 3 43 m 43 m 48 m 48 m 31 m

In order to determine the effectiveness of this artificial watering project, it is necessary to estimate for how long the fresh groundwater will persist. A water balance for the wetland was estimated for the three years after flooding from estimated of daily rainfall, wetland area, wetland volume and wetland bed conductivity. Total inflows to the wetland from rainfall and pumped river water totalled 63,000 m3 (Figure 3). Two thirds (67%) of the inflow was lost as evaporation from the wetland water surface and exposed wetland bed in the first year after the wetland was inundated. About one fifth of inflows (19%) were stored in the unsaturated zone between the wetland base and the water table. The remaining 9,000 m3 (14%) of the wetland inflow was recharged laterally to groundwater in the riparian zone. The riparian zone also received an additional 24,000 m3 of rainfall over the three years. Riparian zone inflows were discharged over the three years as evaporation through the soil profile (9%) and transpiration by vegetation (91%).

Table 2 Water balance for the wetland and riparian zones.

Wetland zone Riparian zone

Surface area 40,000 m2 Surface area 40,000 m2

Volume at 19.1 mAHD 55,000 m3 Lateral recharge from wetland 9,000 m3

Rainfall 22,400 m3 Rainfall 24,000 m3

Surface water evaporation 31,000 m3 Soil evaporation 3,000 m3

Bed evaporation 11,000 m3 Transpiration 30,000 m3

Bed recharge 12,000 m3 Lateral recharge to riparian zone 9,000 m3


Wetland inflowsPumped 55,000 m3

Rainfall 22,400 m3

EvaporationSurface water 31,000 m3

Wetland bed 11,000 m3

Riparian inflowsRecharge9,000 m3

Rainfall 24,000 m3

EvapotranspirationSoils 3,000 m3

Vegetation30,000 m3

RechargeWetland bed 12,000 m3

Riparian zone 9,000 m3

Figure 3. Wetland and riparian zone water balance over three years. The arrows show wetland inflows, wetland evaporation and recharge, riparian zone inflows and evapotranspiration. Not to scale.

In summary, artificial watering by filling floodplain wetlands can be an effective management option to preserve significant ecological assets during periods of low flow. The water balance indicated that the additional recharge would be evaporated and transpired within three years. Thus, regular artificial watering would be required during extended periods of low flow to maintain high value sites. This conclusion is consistent with the estimates of natural flood return periods for E. camaldulensis growing in this region (Jolly et al., 1993; Roberts and Marston, 2000; Overton and Jolly, 2004) and of the studied wetland (six floods in ten years). To our knowledge, this is the first study to quantify the extent of lateral recharge and tree response to artificial watering in a saline, semi-arid floodplain wetland.

Further detail is contained in:

Holland KL, Charles AH, Jolly ID, Overton IC, Gehrig S, Simmons CT. (2009). Effectiveness of Artificial Watering of a Semi-Arid Saline Wetland for Managing Riparian Vegetation Health. Hydrological Processes 23: 3474-3484.


2.2. Bookpurnong Floodplain Pilot Project The Bookpurnong Floodplain Pilot Project was designed to test some of the assumptions regarding the ecological benefits of a salt interception scheme (SIS) in conjunction with manipulated environmental flows for floodplains of the lower River Murray. The Bookpurnong SIS consists of seven highland and 16 floodplain groundwater productions bores, including six bores on the Bookpurnong floodplain. Each bore yields 2-3 L s-1 to reduce the hydraulic gradient that drives regional saline groundwater towards the River Murray and improve river water quality. Field measurements of tree ecophysiology, soil water availability, groundwater depth and salinity were used to investigate the ecophysiological response of the major terrestrial floodplain vegetation species (E. largiflorens, E. camaldulensis and A. stenophylla) to improved flooding regimes and lowering of saline groundwater.

The experimental design consisted of four transects of three piezometers each that were aligned perpendicular to the river. Changes in aquifer and vegetation conditions near the river (10-20 m from the river), in the middle of the riparian zone (70-110 m from the river) and at the distal edge of the riparian zone (130-190 m from the river) (Figure 4) were measured between December 2005 and December 2008. The different treatments were as follows:

Transect 1: groundwater lowering by groundwater production bores;

Transect 2: groundwater lowering by groundwater production bores and artificial flooding;

Transect 3: groundwater freshening by a groundwater production bore; and

Transect 4: control.

At Transect 1 and 2, SIS groundwater production bores were used to lower the floodplain water table. Natural flood runner creeks at Transect 2 were flooded by pumping River Murray water over small earthen banks into the creeks. A groundwater production bore located at the edge of the riparian zone (~200 m from the river) at Transect 3 was used to freshen the groundwater by creating a hydraulic gradient to draw fresh river water into the aquifer. Transect 4, the control transect was established further from the groundwater production bores where there was minimal groundwater lowering and no surface water flooding to account for climatic and river flow variations over the three years of measurement.

Groundwater depth and salinity were used to detect changes in floodplain aquifer conditions. Vegetation response was detected using Randomised Intervention Analysis (RIA, Carpenter et al. 1989) of predawn leaf water potentials (PD, MPa), a measure of tree water stress. Predawn leaf water potentials at the experimental (Transects 1, 2 and 3) and reference (Transect 4) transects were compared before (December 2005 – September 2006) and after (November 2006 – December 2008) the experimental treatments. At each of the 12 piezometers, three trees of each species (E. largiflorens, E. camaldulensis and A. stenophylla) were selected for detailed ecophysiological study. Note that the three tree species were not present at all 12 piezometers. These measurements were also designed to improve our understanding of the ecophysiology of the major floodplain tree species, in particular, A. stenophylla, for which no prior knowledge was available.


Figure 4 Location of the piezometers (B01-B12, B25), groundwater production bores (SIS and LM bore) and experimental transects at Bookpurnong. The inset map shows the location of the Bookpurnong floodplain in the South Australian Riverland.

2.2.1. Ecological benefits of flooding and groundwater management

The floodplain water table along Transect 1 was lowered by up to 0.7 m below river level. The hydraulic gradient created by the SIS bores reduced groundwater salinities in the piezometer nearest the river (<10 m), but did not extend further into the riparian zone. Despite the observed groundwater lowering and freshening, there was a significant (p=0.032) decrease in E. largiflorens water potentials (i.e. increase in tree water stress) near the river relative to trees at B10 at the control transect. Four A. stenophylla trees along Transect 1 died during the project. In conclusion, water table lowering alone does not reduce tree water stress.

The SIS groundwater production bores lowered the water table along Transect 2 by up to 0.7 m below river level. Artificial flooding caused a temporary reduction in groundwater salinity that persisted for less than six months. A temporary, but not statistically significant increase in tree water potentials (i.e. decrease in tree water stress) was observed in response to artificial flooding. Two E. camaldulensis trees died along Transect 2 during the project. Water table lowering and artificial flooding temporarily reduced groundwater salinity and tree water stress, but did not reduce long term tree water stress.

The groundwater production bore at Transect 3 lowered groundwater levels by up to 1.0 m. This hydraulic gradient drew fresh river water into the aquifer, which reduced groundwater salinities in all piezometers within ~150 m of the river within two years (Error! Reference source not found.). A significant (p=0.012) increase in E. largiflorens water potentials (i.e. decrease in tree water stress) (Figure 6) was observed at B08, where groundwater salinity decreased from ~55,000 S cm-1 to less than 1,000 S cm-1. Tree water source measurements showed that E. largiflorens and A. stenophylla trees at B08 started using groundwater in response to the groundwater freshening. An increase in the proportion of all tree species with >25% canopy cover was also recorded along Transect 3. Groundwater freshening by induced recharge significantly decreased long term tree water stress and improved tree health.


Transect 3

Dec-0

5

Mar

-06

Jun-

06

Sep-0

6

Dec-0

6

Mar

-07

Jun-

07

Sep-0

7

Dec-0

7

Mar

-08

Jun-

08

Sep-0

8

Dec-0

8

Mar

-09

0

20000

40000

60000

Transect 1

Gro

un

dwa

ter

salin

ity ( S

cm

-1)

0

20000

40000

60000 Transect 2

B01

B03B02

B04

B06

B05

Transect 4

Dec-0

5

Mar

-06

Jun-

06

Sep-0

6

Dec-0

6

Mar

-07

Jun-

07

Sep-0

7

Dec-0

7

Mar

-08

Jun-

08

Sep-0

8

Dec-0

8

Mar

-09

B07

B25

B09B08 B10

B12

B11

Figure 5 Groundwater salinity (μS cm-1) between December 2005 and December 2008.

Groundwater freshening

-4

-3

-2

-1

0

Control

Dec-0

5

Mar

-06

Jun-

06

Sep-0

6

Dec-0

6

Mar

-07

Jun-

07

Sep-0

7

Dec-0

7

Mar

-08

Jun-

08

Sep-0

8

Dec-0

8

pr

edaw

n (M

Pa)

-4

-3

-2

-1

0

Figure 6. E. largiflorens water potentials at B08 (groundwater freshening) and at B11 (control) between December 2005 and December 2008. Symbols show individual tree water potentials, lines represent the mean water potential for the period before and after groundwater freshening occurred in September 2006. Note more negative water potentials represent increased tree water stress.

2.2.2. Ecophysiology of the major floodplain tree species

The tree ecophysiology measurements showed that the three tree species are adapted to floodplain conditions, with a gradient in drought and salinity tolerance from E. camaldulensis to A. stenophylla to E. largiflorens. Measurements of predawn and midday water potentials were significantly different between tree species (E. largiflorens, A. stenophylla and E. camaldulensis) and between sites with low and high groundwater salinity (Figure 7). Groundwater salinity >10,000 S cm-1 was classified as high salinity. E. largiflorens trees had


significantly more negative predawn and midday water potentials than E. camaldulensis and A. stenophylla trees at the low and the high salinity sites. E. camaldulensis trees were able to create a significantly larger midday water potential gradient than A. stenophylla.

E. larg

iflore

ns

A. ste

noph

ylla

E. cam

aldule

nsis

E. larg

iflore

ns

A. ste

noph

ylla

p

red

aw

n (

MP

a)

-5

-4

-3

-2

-1

0Low salinity High salinity

E. larg

iflore

ns

A. ste

noph

ylla

E. cam

aldule

nsis

E. larg

iflore

ns

A. ste

noph

ylla

m

idd

ay

(MP

a)

-5

-4

-3

-2

-1

0Low salinity High salinity(a) (b)

Figure 7. Box plots of (a) predawn and (b) midday water potentials taken on eight field trips between December 2005 and March 2008 at Transects 1 – 4. Data is separated by tree species and groundwater salinity. The boxes show the 25th and 75th percentile of the mean, the error bars show the 10th and 90th percentiles of the mean, the median is shown by the solid line and the mean by the dashed line.

At low groundwater salinity sites, all three tree species were able to maintain relatively high rates of transpiration in response to the relatively high water availability (Figure 8). E. camaldulensis trees were restricted to low groundwater salinity sites. At high salinity sites, E. largiflorens and A. stenophylla trees maintained low to moderate sapflow rates by using water from all available sources, including the majority of the soil profile, groundwater, surface water and moist surface soils. In contrast, E. camaldulensis trees were limited to using water from the capillary fringe above the water table and low salinity groundwater during dry periods. This was supplemented with water from surface water bodies and moist surface soils when available. The observed tree water use strategies highlight the importance of maintaining riparian groundwater salinities within the water potential thresholds of the tree species to enable these trees to survive dry periods.


High salinity

Tra

nspi

ratio

n (m

m)

0.0

0.2

0.4

0.6

0.8

Loxton 024024

Sep-0

7

Oct-07

Nov-0

7

Dec-0

7

Jan-

08

Feb-0

8

Mar

-08

Apr-0

8

May

-08

Jun-

08

Jul-0

8

Aug-0

8

Sep-0

8

Oct-08

Nov-0

8

Dec-0

8

Rai

nfal

l (m

m)

Eva

pora

tion

(mm

)

0

5

10

15

20

25 Rainfall Evaporation

Low salinity

0.0

0.2

0.4

0.6

0.8

E. largiflorensA. stenophyllaE. camaldulensis

Figure 8 Daily transpiration of E. largiflorens, A. stenophylla and E. camaldulensis at the high and low salinity plots on Transect 3 between September 2007 and December 2008. Daily rainfall and pan evaporation rates at Loxton are shown for reference (BoM, 2009).

2.3. Overall project conclusions In the lower River Murray floodplain, surface water and groundwater need to be managed in combination to maximise environmental benefits. The overall project has demonstrated the importance of bank recharge for maintaining riparian vegetation communities through artificial watering and groundwater freshening. Artificial watering can benefit the majority of E. camaldulensis communities growing within ~50 m of a water body. Similarly, groundwater production bores that lower the water table and draw fresh River Murray watering into the alluvial aquifer, thereby reducing groundwater salinities, can benefit all trees within ~150 m of a permanent water body. However, water table lowering alone does not improve tree health as it reduces further salinisation of the soils and groundwater, but does not remove stored salts. The project showed that riparian trees need a long term source of fresh water (flooding/ fresh groundwater) to survive dry periods.

The detailed ecophysiological measurements showed that the three floodplain tree species adopt different water use strategies across a salinity gradient. At low groundwater salinity sites, all three tree species were able to maintain higher rates of transpiration in response to higher water availability. In contrast, at high groundwater salinity sites A. stenophylla and E. largiflorens were able to tolerate the highly saline conditions by reducing transpiration rates. This represents a trade off between tree water use and drought/salinity tolerance. The project has provided an empirical basis to improve the prediction of the response of the River Murray floodplain to floodplain rehabilitation, particularly flooding and groundwater management by floodplain vegetation health models.


2.4. Further application of research results The results of this project are transferrable to other floodplain areas where riparian vegetation adjoins a water body and where the underlying shallow saline groundwater can be freshened by bank recharge using artificial watering or aquifer pumping. These conditions occur along the River Murray in NSW and Victoria downstream of the confluence with the Darling River, and along parts of the Darling River in NSW. The riparian vegetation in these areas are under similar stresses to those in South Australia and will respond similarly to bank recharge and groundwater freshening. The results of this project are of particular relevance to the design and operation of SIS in the Murray Darling Basin, where riparian vegetation could benefit from groundwater freshening. Other areas where the findings of this project may be relevant include the Darling River, Lake Eyre Basin and Upper South East wetlands, where riparian vegetation overlies saline groundwater. Whilst the differing geomorphological, hydrogeological and climatic conditions in these regions prevent direct application of the results from the lower River Murray, the underlying surface water – groundwater interactions are likely to be similar.

3. Reporting against CNRM project milestones 1) Literature review and data summary on vegetation responses to artificial flooding

and groundwater management, focusing on the previous Chowilla watering projects to further our understanding of vegetation responses to watering. A journal paper on vegetation responses to artificial flooding and groundwater management using data from previous Chowilla watering projects was published in the international journal Hydrological Processes in November 2009.

2) International conference papers and presentations on the literature review and initial field results. The aims and objectives of the project were communicated at HydroEco 2006 in September 2006 in Carlsbad, Czech Republic and at the 10th Murray-Darling Basin Groundwater Workshop held in Canberra in September 2006. Results of the field investigations were presented at the XXXV IAH Congress, Groundwater and Ecosystems 2007 in Lisbon, Portugal in September 2007 and at the 2nd International Salinity Forum in Adelaide in April 2008. The results of the comparative ecophysiological studies were presented at an International workshop on ecohydrology and ecophysiology of plants in water-limited environments at the University of Western Australia in September 2008.

3) CSIRO Technical Report on the interpretation of the plant/soil/ groundwater field data from Sites B and D and the ecological benefits of groundwater management and manipulated surface water / environmental flows. A CSIRO Water for a Healthy Country National Research Flagship Report was published on the field work undertaken during this project. This report contained a record of all measurements taken, interpretation of the data and discussion of the main conclusions from the field investigations.

4) Journal paper on the comparative ecophysiology of River Cooba, one of the major floodplain tree species for which no prior ecophysiological knowledge is available. A paper on the response of the riparian vegetation to groundwater freshening was published in the international journal Hydrological Processes in November 2009.

5) Paper on improving vegetation health modelling tools using empirical data to predict the ecological benefits of surface water groundwater management. This output could not be completed to a satisfactory level within the timeframe of the project. Instead, more time was committed to the completion of the Technical Report (Output 3), the journal paper on the comparative ecophysiology of River Cooba (Output 4) and the Final Report summarising the detailed scientific findings of the project (Output 6).

6) Final report summarising the detailed scientific findings of the project. This objective was met with publication of a CSIRO Water for a Healthy Country National Research Flagship Report (this report) summarising the review of the effectiveness of artificial watering for managing riparian vegetation health, the ecological benefits of groundwater management and environmental flows for managing riparian vegetation health and improvements in our knowledge of the comparative ecophysiology of A. stenophylla.


4. Project publications Journal Papers

Holland KL, Charles AH, Jolly ID, Overton IC, Gehrig S, Simmons CT. (2009). Effectiveness of Artificial Watering of a Semi-Arid Saline Wetland for Managing Riparian Vegetation Health. Hydrological Processes 23: 3474-3484.

Doody TM, Holland KL, Benyon RG, Jolly ID. (2009). Effect of groundwater freshening on riparian vegetation water balance. Hydrological Processes 23: 3485-3499.

Conference Papers

Berens V, White MG, Souter NJ, Jolly ID, Holland KL, McEwan KL, Hatch MA, Fitzpatrick A, Munday TJ (2007) Surface water, groundwater and ecological interactions along the River Murray. A pilot study of management initiatives at the Bookpurnong Floodplain, South Australia. XXXV IAH Congress, Groundwater and Ecosystems 2007, Lisbon, Portugal.

Holland KL, Doody TM, McEwan KL, Jolly ID (2008) Comparative ecophysiology of Acacia stenophylla growing on the floodplains of the lower River Murray in south eastern Australia. In 'International workshop on ecohydrology and ecophysiology of plants in water-limited environments', University of Western Australia pp 38.

Holland KL, Jolly ID, Doody TM, McEwan KL (2010) Response of riparian vegetation to flooding and groundwater management in a saline, semi-arid environment. In ‘Groundwater 2010’, Canberra, October 31 – November 4, 2010, p 176.

Holland KL, Jolly ID, McEwan KL, Doody TM, White MG, Berens V, Souter NJ (2008) The ‘Bookpurnong Experiment’: will groundwater management and flooding improve the health of the floodplain vegetation? In '2nd International Salinity Forum', Adelaide, Australia

Jolly ID, Holland KL, McEwan KL, Overton IC, White MG, Berens V, Mensforth LJ (2006) The ‘Bookpurnong Experiment’: will groundwater management and flooding improve the health of the floodplain vegetation? Poster paper for HydroEco 2006 Conference, Sept 11-14 2006, Carlsbad, Czech Republic.

White MG, Berens V, Souter NJ, Holland KL, McEwan KL, Jolly ID (2006) Vegetation and groundwater interactions on the Bookpurnong floodplain, South Australia. In 'Proceedings of the 10th Murray-Darling Basin Groundwater Workshop', Canberra.

Reports

Holland KL, Doody TM, McEwan KL, Jolly ID (2009) Project 054120. Response of the River Murray floodplain to flooding and groundwater management. Final Report to the Centre for Natural Resource Management. CSIRO: Water for a Healthy Country National Research Flagship Report. pp 12.

Holland KL, Doody TM, McEwan KL, Jolly ID, White MG, Berens V, Souter NJ (2009) Response of the River Murray floodplain to flooding and groundwater management: Field investigations. CSIRO Water for a Healthy Country National Research Flagship Report. pp 65.

Charles AH (2006) Controls on the extent of lateral recharge from managed flooding and its impact on lower River Murray floodplain vegetation health. Honours Thesis, Flinders University of South Australia submitted November 2006.

Hassam MG (2007) Understorey vegetation response to artificial flooding frequencies and other environmental factors at temporary wetlands found in the Chowilla floodplain. Honours Thesis, Flinders University of South Australia submitted May 2007.


REFERENCES Berens V, White M, and Souter N, 2009, Bookpurnong Living Murray Pilot Project: A trial of

three floodplain water management techniques to improve vegetation condition, DWLBC Report 2009/21

Bren LJ. 1992. Tree invasion of an intermittent wetland in relation to changes in the flooding frequency of the River Murray, Australia. Australian Journal of Ecology 17: 395-408.

Bunn SE, Arthington AH. 2002. Basic principles and ecological consequences of altered flow regimes for aquatic biodiversity. Environmental Management 30: 492-507.

Bureau of Meteorology (2009) 'Climate averages for Loxton Research Centre (024024).' Available from URL: http://www.bom.gov.au/climate/averages/

Busch DE, Ingraham NL, Smith SD. 1992. Water uptake in woody riparian phreatophytes of the southwestern United States: a stable isotope study. Ecological Applications 2: 450-459.

Busch DE, Smith SD. 1995. Mechanisms associated with decline of woody species in riparian ecosystems of the southwestern U.S. Ecological Monographs 65: 347-370.

DWR. 2001. South Australian River Murray Salinity Strategy 2001 - 2015. Department for Water Resources, Government of South Australia: South Australia.

Doody TM, Holland KL, Benyon RG, Jolly ID. 2009. Effect of groundwater freshening on riparian vegetation water balance. Hydrological Processes 23: 3485-3499.

George AK, Walker KF, Lewis MM. 2005. Population status of eucalypt trees on the River Murray floodplain, South Australia. River Research and Applications 21: 271-282.

Holland KL, Charles AH, Jolly ID, Overton IC, Gehrig S, Simmons CT. 2009a. Effectiveness of Artificial Watering of a Semi-Arid Saline Wetland for Managing Riparian Vegetation Health. Hydrological Processes 23: 3474-3484.

Holland KL, Doody TM, McEwan KL, Jolly ID, White M, Berens V, Souter N. 2009b. Response of the River Murray floodplain to flooding and groundwater management: Field investigations. CSIRO Water for a Healthy Country National Research Flagship Report. pp 65.

INRMG. 2003. Integrated Natural Resource Management Plan for the South Australian Murray-Darling Basin. The Integrated Natural Resource Management Group for the South Australian Murray Darling Basin Incorporated: Murray Bridge, South Australia, Australia.

Jolly ID. 1996. The effects of river management on the hydrology and hydroecology of arid and semi-arid floodplains. In Floodplain Processes, MG Anderson, DE Walling, PD Bates (eds) John Wiley & Sons Ltd.: Chichester, New York; 577-609.

Jolly ID, McEwan KL, Holland KL. 2008. A review of groundwater-surface water interactions in arid/semi-arid wetlands and the consequences of salinity for wetland ecology. Ecohydrology 1: 43-58.

Jolly ID, Walker GR, Thorburn PJ. 1993. Salt accumulation in semi-arid floodplain soils with implications for forest health. Journal of Hydrology 150: 589-614.

Kingsford RT. 2000. Ecological impacts of dams, water diversions and river management on floodplain wetlands in Australia. Austral Ecology 25: 109-127.

Le Maitre DC, Scott DF, Colvin C. 1999. A review of information on interactions between vegetation and groundwater. Water SA 25: 137-152.

MDBC. 1998. Murray Darling Basin Commission Floodplain Wetlands Management Strategy. Murray-Darling Basin Commission, Canberra. 62 p.


MDBC. 2003. Murray Darling Basin Commission Floodplain Management Strategy. Murray-Darling Basin Commission, Canberra. 28 p.

Naumburg E, Mata-Gonzalez R, Hunter RG, McLendon T, Matrin DW. 2005. Phreatophytic vegetation and groundwater fluctuations: A review of current research and application of ecosystem response modeling with an emphasis on Great Basin vegetation. Environmental Management 35: 726-740.

Overton IC, Jolly ID. 2004. Integrated Studies of Floodplain Vegetation Health, Saline Groundwater and Flooding on the Chowilla floodplain South Australia. CSIRO Land and Water Technical Report 20/04: Adelaide.

Ramsar Convention. 1987. UNESCO Ramsar convention on wetlands of international importance. Ramsar, Iran.

RMCWMB. 2002. Water Allocation Plan for the River Murray Prescribed Watercourse. River Murray Catchment Water Management Board: Berri, South Australia, Australia.

Roberts J, Marston F. 2000. Water regime of wetland and floodplain plants in the Murray-Darling Basin: A source book of ecological knowledge. CSIRO Land and Water: Canberra, Australia.

Rood SB, Mahoney JM, Reid DE, Zilm L. 1995. Instream flows and the decline of riparian cottonwoods along the St-Mary River, Alberta. Canadian Journal of Botany-Revue Canadienne De Botanique 73: 1250-1260.

Rood SB, Mahoney JM. 1990. Collapse of riparian poplar forest downstream from dams in western prairies: Probable causes and prospects for mitigation. Environmental Management 14: 451-464.

Scott ML, Shatfroth PB, Auble GT. 1999. Responses of riparian cottonwoods to alluvial water table declines. Environmental Management 23: 347-358.

Stromberg JC, Tiller R, Richter B. 1996. Effects of groundwater decline on riparian vegetation of semiarid regions: the San Pedro, Arizona. Ecological Applications 6: 113-31.

Stromberg JC, Tress JA, Wilkins SD, Clark SD. 1992. Response of velvet mesquite to groundwater decline. Journal of Arid Environments 23: 45-58.

Tockner K, Stanford JA. 2002. Riverine flood plains: present state and future trends. Environmental Conservation 29: 308-330.

Walker KF. 1985. A review of the ecological effects of river regulation in Australia. Hydrobiologia 125: 111-129.

Walker KF, Thoms MC. 1993. Environmental effects of flow regulation on the lower River Murray, Australia. Regulated Rivers: Research and Management 8: 103-119.

Documents

Project 054120. Response of the River Murray floodplain to … · 2011. 3. 28. · water (flooding / fresh groundwater) to survive dry periods. Response of the River Murray floodplain