Tanya M. Doody

Impacts of tree plantations on groundwater in south-eastern Australia

In some regions dependent on groundwater, such as the lower south-east of South Australia in the Green Triangle, deep-rooted, woody vegetation might have undesirable hydrological impacts by competing for finite, good-quality groundwater resources. In other regions, such as the Riverina in south-central New South Wales, where rising watertables and associated salinisation is threatening the viability of agriculture, woody vegetation might have beneficial hydrological impacts. In response to a growing need to better understand the impacts of tree plantations on groundwater, annual evapotranspiration and transpiration were measured at 21 plantation sites in the Green Triangle and the Riverina. Sources of tree water uptake from rainfall and groundwater were determined by measurements of evapotranspiration and soil water over periods of 2-5 years. In the Green Triangle, under a combination of permeable soil over groundwater of low salinity (<2000 mg L −1 ) at 6-m depth or less, in a highly transmissive aquifer, annual evapotranspiration at eight research sites in Pinus radiata D.Don and Eucalyptus globulus Labill. plantations averaged 1090 mm year −1 (range 847-1343 mm year −1 ), compared with mean annual precipitation of 630 mm year −1 . These plantation sites used groundwater at a mean annual rate of 435 mm year −1 (range 108-670 mm year −1 ). At eight other plantation sites that had greater depth to the watertable or a root- impeding layer, annual evapotranspiration was equal to, or slightly less than, annual rainfall (mean 623 mm year −1 , range 540-795 mm year −1 ). In the Riverina, where groundwater was always present within 3 m of the surface, Eucalyptus grandis Hill ex Maiden trees at three sites with medium or heavy clay, alkaline, sodic, saline subsoils used little or no groundwater, whereas E. grandis and Corymbia maculata (Hook.) K.D.Hill and L.A.S.Johnson trees at a site with a neutral sandy soil and groundwater of low salinity used 380 and 730 mm year −1 of groundwater (respectively 41 and 53% of total annual evapotranspiration). We conclude that commonly grown Eucalyptus species and P. radiata are able to use groundwater under a combination of light- or medium-textured soil and shallow depth to a low-salinity watertable.

Quantifying water savings from willow removal in Australian streams

Willows (Salix Spp.), while not endemic to Australia, form dense stands in many stream locations. Australia has been experiencing a long-term drought and potential water extraction by willows is considered a significant problem, although little global scientific evidence exists to support such concerns. The extent of willow occupation in Australian streams has been deemed large enough to warrant investigation of their evapotranspiration rates and quantification of potential water savings from willow removal. Willows situated in-stream (permanent water) and on stream banks (semi-permanent water) were monitored over three summers from August 2005 to May 2008 employing heat pulse velocity sap flux sensors and field measurement of water balance components. A comparative study of native riparian River Red Gum trees was also undertaken. Differences in transpiration flux rates between willows with permanent and semi-permanent access to water were substantial, with peak transpiration of 15.2 mm day(-1) and 2.3 mm day(-1) respectively. Water balance calculations over the three year period indicate that an average potential net water saving of 5.5 ML year(-1)ha(-1) of crown projected area is achievable by removal of in-stream willows with permanent access to water. On stream banks, replacement of willows with native riparian vegetation will have no net impact on site water balances. Results also indicate that under the influence of natural environmental events such as drought, heat stress and willow sawfly infestation, evapotranspiration rates from in-stream willows remain greater than that from open water. These results will have important implications in environmental management of willows and in future water resource allocation and planning in Australia.

Estimating Riparian and Agricultural Actual Evapotranspiration by Reference Evapotranspiration and MODIS Enhanced Vegetation Index

Abstract: Dryland river basins frequently support both irrigated agriculture and riparian vegetation and remote sensing methods are needed to monitor water use by both crops and natural vegetation in irrigation districts. We developed an algorithm for estimating actual evapotranspiration (ET a ) based on the Enhanced Vegetation Index (EVI) from the Moderate Resolution Imaging Spectrometer (MODIS) sensor on the EOS-1 Terra satellite and locally-derived measurements of reference crop ET (ET o ). The algorithm was calibrated with five years of ET a data from three eddy covariance flux towers set in riparian plant associations on the upper San Pedro River, Arizona, supplemented with ET a data for alfalfa and cotton from the literature. The algorithm was based on an equation of the form ET a = ET o [a(1 − e −bEVI ) − c], where the term (1 − e −bEVI ) is derived from the Beer-Lambert Law to express light absorption by a canopy, with EVI replacing leaf area index as an estimate of the density of light-absorbing units. The resulting algorithm capably predicted ET

Inundation of a floodplain lake woodlands system: nutritional profiling and benefit to mature Eucalyptus largiflorens (Black Box) trees

River management continues to challenge riparian systems worldwide, with climate change impacts and anthropogenic extractions escalating. The Murray–Darling basin (MDB) in Australia is critical to agricultural production and habitat provision to maintain biodiversity. Concern for the condition of native trees and biota in the MDB has led to substantial research investment to increase ecosystem function understanding and improve floodplain and wetland management. This field study offers new insights into tree nutrition and physiology as interpreted against the plant-soil-environment dynamics of recent flooding. Black Box (Eucalyptus largiflorens (Myrtaceae) is the only key native riverine MDB tree restricted to that region; and appears stressed at the far reaches of certain significant floodplain ecosystems. Here, nutritional and ecophysiological comparisons were made between Black Box trees that had just been inundated, and those nearby that had not. Leaf stomatal conductance, transpiration, total soil aluminium (Al) concentration, soil pH, and soil conductivity were different between inundated and dry sites. Soil moisture increased due to inundation, thus reducing tree water stress across the three study locations. Changes in leaf chemistry were not detected at the very early stages of flooding examined in this study. An increase in soil acidity due to inundation may also enhance bioavailability of nutrients to trees. New insight into immediate plant benefits gained from this study suggests further investigation is warranted to elucidate the influence of flood and drought on nutrient balance and how future wetland management can benefit from a more holistic understanding of plant-soil-environment dynamics.

Re-framing the decision context over trade-offs among ecosystem services and wellbeing in a major river basin where water resources are highly contested

Water resources and water-related ecosystem services are vital to social–ecological systems, yet in many parts of the world water as a finite resource is revealed by its unsustainable and inequitable use. Increased threats to water security and supply of ecosystem services arise due to increasing and contested demand and declining supply due to climate change and other stressors. Trade-off decisions need to be made between competing sectors of food production, hydropower generation and environmental needs: the water–food–energy–environment nexus. New approaches are needed to address how water resources and ecosystem service benefits are shared among competing interests. One approach involves changes to decision contexts, shaped by the values, rules and knowledge which decision makers draw upon when considering options. By changing decision contexts, new opportunities become available. Here, we describe Nexus Webs; a knowledge framework designed to promote collaborative exploration of synergies and trade-offs and enable changes in decision contexts for water use. As part of the process of shifting this framework from concept to operation, we apply Nexus Webs to contrasting water use scenarios in the Pangani Basin (Tanzania and Kenya), where water is over-allocated and highly contested. Under each scenario, we detail linkages between different water uses and their effects on assets (ecosystems, biodiversity and built infrastructure), the effects on assets for the supply of ecosystem services and how these affect livelihoods and wellbeing. We outline how Nexus Webs can be developed and used to change the decision context to consider options for more socially inclusive and equitable use of water resources.

The use of historical environmental monitoring data to test predictions on cross-scale ecological responses to alterations in river flows

Abstract Determination of ecological responses to river flows is fundamental to understanding how flow-dependent ecosystems have been altered by regulation, water diversions and climate change, and how to effect river restoration. Knowledge of ecohydrological relationships can support water management and policy, but this is not always the case. Management rules have tended to be developed ahead of scientific knowledge. The lag between practice and knowledge could be addressed by using historical monitoring data on ecological responses to changes in flows to determine significant empirical ecohydrological relationships, as an adjunct to investigating responses prospectively. This possibility was explored in the Murray–Darling Basin, Australia. We assessed 359 data sets collected during monitoring programs across the basin. Of these, only 32 (9%) were considered useful, based on a match between the scale at which sampling was done and ecological responses are likely to occur, and used to test flow–ecology predictions for phytoplankton, macroinvertebrates, fishes, waterbirds, floodplain trees, basin-scale vegetation and estuarine biota. We found relationships between flow and ecological responses were likely to be more strongly supported for large, long-lived, widespread biota (waterbirds, basin-scale vegetation, native fishes), than for more narrowly distributed (e.g. estuarine fishes) or smaller, short-lived organisms (e.g. phytoplankton, macroinvertebrates). This pattern is attributed to a mismatch between the design of monitoring programs and the response time frames of individual biota and processes, and to the use of local river discharge as a primary predictor variable when, for many biotic groups, other predictors need to be considered.

The River Murray-Darling Basin: Ecosystem Response to Drought and Climate Change

The River Murray-Darling Basin is one of Australia’s largest river basins, and contains highly valued water-dependent ecosystems, including 16 Ramsar-listed wetlands. Through the impact of drought and over-allocation (69 % of the basin’s water is abstracted for irrigation, industrial, and domestic use), these ecosystems are now widely considered to be severely degraded. Future climate scenarios suggest a drier and more variable climate with continued and intensified drought periods. Future water-sharing policies are under consideration to address this degradation by changing the balance between consumptive and environmental water, including the security of environmental water. This chapter outlines the challenges involved in managing ecosystem adaption to a drier climate while maintaining key ecosystem assets. We conclude that it is unlikely that it will ever be possible to return to an ecosystem like what existed pre-irrigation development. While this past ecosystem state has often been used as benchmark in ecological assessment, the great scientific challenge now is to provide rigorous assessment that allows those setting policy to gain a better sense of what is ecologically possible and socially desirable within constraints of water diversion and climate futures that we now face.