Modelling the future of environmental water with climate change
Environmental water allocated to rivers to mimic natural flows has long been recognised by governments across the world as being important for stimulating healthy freshwater ecosystems.
The Murray–Darling Basin Plan is a 13-billion-dollar investment directed at sustainable water management and use in south-eastern Australia, and includes allocations of environmental water.
The term “environmental water” can have many meanings, but generally it refers to water that is actively managed to benefit the environment. In the Murray–Darling Basin, it has delivered some observable benefits, with native fish populations spawning more frequently in some parts of the river system and with healthier streambank vegetation.
Managing water flows in a changing climate
However, our current access to environmental water and the benefits it delivers cannot necessarily be expected in the future. This is the focus of a collaborative research project lead by the University of Melbourne, in partnership with the Department of Environment Land Water and Planning, Bureau of Meteorology and the Victorian Environmental Water Holder.
“Many of our environmental-water programs have been developed without considering how climate change might affect the viability of their objectives,” said Andrew John, a PhD student on the project.
In his PhD research, John seeks to understand the outcomes that environmental water can deliver in a changing climate and what this means for water managers. Given that Australia has one of the most variable climate and hydrological systems in the world, this is no easy task.
Most of Australia’s environmental watering programs typically depend on setting up a set of hydrologic reference conditions, based on past natural or pristine environments. These reference conditions can provide a useful benchmark for comparison or targets to aim for.
“But there are several problems with this under climate change,” said John. “It ignores how the environment will respond to a wider range of changes and it makes a lot of assumptions about how the future will unfold.
“We now have a much more managed landscape and it is likely not possible to go back to the pristine. The more things change, the more we can’t return to a vision of the past, and the harder it is to manage water to try and retain this vision.
“This means people need to think about what they want, and what their values are going forward.”
Models need to be future-focused, not based on past scenarios or data
John worked with other researchers to conduct a systematic review of the scientific literature to understand how different hydrological and ecological methods were being used to assess the effects of climate change on freshwater ecosystems.
They found a sample of 61 modelling studies across the globe that were relevant to freshwater ecosystems and that had well-defined records of their methodologies.
“Of these studies, more than three-quarters based their ecological responses only on historical data rather than looking at what might dynamically happen in the future,” John said. “The methods used in these studies are usually rapid to implement and might be useful when the rivers are unregulated by dams, weirs and other structures, and there is a high level of confidence in climate-change projections.
“But given the complexity and connected nature of ecosystems, and a very high level of uncertainty about future rainfall and stream flows, we really need more dynamic approaches.”
Less than 10 per cent of the studies reviewed took an integrated approach to consider how the range of possible climate futures interacts with natural variability, including the sequence of droughts and floods, and how the environment might dynamically respond to these changes.
John said that many models are based on “stationarity”, which means they look at the past to predict the future.
“This is great when things don’t change, but when they do you are moving from a fixed moment in history to one that is dynamic. Using relationships and practices that have happened in the past won’t always be relevant for the future.”
In one scenario, if the data about past habitat requirements are used to assess climate-change impacts on a vulnerable species of native fish, the ecosystem dynamics that can result as the climate changes will be ignored.
For example, if the climate changes in a way that increases the distribution of a pest fish species such as carp, native species such as golden perch may be affected by carp’s tendency to remove aquatic plants and stir up the mud in the stream. This means parts of the ecosystem can be affected even if the direct climate-based changes to hydrology do not present a threat.
“We need more process-based modelling rather than relying only on statistical modelling,” said John. “If you can simulate the more novel ecological conditions that might happen under climate change, you’ll get a better picture of future scenarios and how effective management responses may be.”
Different models can lead to a different understanding of climate change risk
In one example, John and colleagues used two different kinds of models to look at the response of a river red gum forest to flows under climate change.
The first model simply looked at whether flows were large enough to flood important habitat and compared this to the average historical frequency. If this was close, the forest was assumed to remain in a similar condition under climate change.
“But this did not tell us how the river red gum forest actually responded or was still suffering from the previous year’s drought,” John said.
“We then used a simple dynamic model to simulate flows changing over time and project the condition of the forest.”
This model took into account that the existing condition of the forest in one year will affect its health in the next, which simulates processes of recovery and decline.
“We found that the sequence of flows really matters to the health of the forest,” said John. “If there were evenly spaced high flow pulses, then there were likely good outcomes for the forest.
“But if the flows were spread out and separated by periods of drought, the forest was likely to be under stress.”
The simple dynamic model suggested there was a much larger risk from climate change to the health of the river red gum forest compared to the results from the first model, which just looked at whether flows were large enough.
“We need models that track condition over time to see how river red gums and other species respond, otherwise we can misrepresent the risks from climate change,” said John.
Need to integrate climatology, hydrology and ecology
Models need to link climatology, hydrology and ecology to achieve a multidisciplinary understanding of the possible effects of climate change.
“But our current way of looking at climate-change impacts tends to focus on just one discipline. This means you don’t get the vast amount of information and expertise you need to make decisions for a range of objectives,” John explains.
He urges better use of multidisciplinary expertise in future studies, which will help to more comprehensively assess and understand climate-change impacts.
“Ecohydrologists [those who study the interactions between water and ecology] have some great integrated tools for this purpose, but we need a more concerted effort to bring these into practice,” he said.
Using a simple model to stress test climate and environmental changes
One way of looking at the interactions between hydrology and ecology in more detail is to use complicated models, but these models can significantly increase computing time, especially when trying to include all the uncertainties.
Instead, simple models that focus on the main processes mean researchers can rapidly undertake many simulations each with different scenarios, and can quickly assess the impacts of various risks.
John has built a simple model of the Goulburn River to test a wide array of climate and environmental changes.
“We want to find out what the freshwater ecosystems are vulnerable to in terms of climatic changes, and how likely this is to happen,” he explains. “We start with the vulnerabilities first. For example, what happens when there is not enough water to meet the needs of people and the environment several years in a row, and what kinds of changes cause this?
“This allows us to understand what our system is vulnerable to, which may be outside the range of projections from current global climate models but is still possible.
“At what point can you just not deliver your environmental water? At what point do you have to make difficult trade-off decisions? For example, if there’s not enough water to meet the needs of all the important fish species, do we concentrate on those with higher conservation values?”
Climate-change impacts, especially on rainfall and steam flow, are highly uncertain. By modelling what happens when specific stresses are applied to a river system, water managers can better understand what can make a particular ecosystem vulnerable and what actions are needed to avoid it getting to this state.
Building a fast model for water managers of Victoria’s Goulburn River
The project is funded through an Australian Research Council Linkage Project and involves partnering with industry.
“Having a Linkage Project like this is a great format for getting government, industry and research together,” John said. “We meet on a regular basis with the partners providing us with data and checking our modelling assumptions. The outputs of our research can help inform their planning priorities.
“This provides a great opportunity for us to influence policy and practice and for our research to be better informed.”
John and the research team have built a custom water-resource model for the Goulburn River that aims to consider both hydrological and ecological outcomes from climate-change impacts.
“We have built a model which is system-wide but focused on environmental outcomes rather than only water accounting,” said John.
“By simplifying it down to the most important processes we have also made this model fast to run so it can answer many questions around uncertainty. It can be run millions of times to get answers in a week, which could take years of computation for traditional models even on a supercomputer.”
The model is ready to be run with a million different scenarios of how the climate might change. For example, along with long-term changes in average rainfall and temperature, it can also look at changes in climate variability or the characteristics of drivers of persistent drought such as El Niño.
“It will help answer questions about what the Goulburn River’s ecology is vulnerable to and how robust our current environmental watering programs are into the future,” said John. “Exploring the model outputs with the industry partners will then help identify possible responses for a more robust environmental water program.”
For further information: Avril Horne, The University of Melbourne