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Introducing: Sally Thompson

Ladies and gentlemen, it gives me great pleasure to introduce you to our next Conversation with a Scientist visitor: Sally Thompson.

Sally is an Assistant Professor in the Department of Civil and Environmental Engineering at the University of California Berkeley, where she researches and teaches hydrology. In particular, she studies ecohydrology – the relationships between biological life and the water cycle – with a focus on arid and semi-arid places. She’s interested in things like why plants grow in patterns across the landscape, how plant physiology influences the movement of water around the land, and the long-term sustainability of our water resources.

It’s been an honour to call Sally my friend since we were undergraduate students together at the University of Western Australia (side note: she was valedictorian of our graduating class and is one of the smartest people I’ve ever met). To balance her insane intellectual abilities, here are three fun facts about Sal: she’s spent time working in India and Vietnam, she makes a wonderful Christmas pudding, and she can bellydance like nobody’s business. (Have you demonstrated for your students yet, Sal?)

Sally (middle) with her husband Nic and friend Latasha at SEEDS in Durham, North Carolina (the community garden they used to volunteer at).

Sally (middle) with her husband Nic and friend Latasha at SEEDS in Durham, North Carolina (the community garden they used to volunteer at).

Without further ado, I’ll let my friend tell you about the work she does and why it’s so important. Enjoy!


Across the world’s land surface, 60% of the water that falls on the land disappears up a tree. That’s the global average. If you move to dry areas, that number becomes more like 95-99%. So if you’re worried about water resources in water-constrained systems, you’d really better worry about the role of vegetation and evaporation because it’s the dominant user of water in these landscapes.


For my undergraduate thesis, I worked on dryland salinity, which is a land degradation process that occurs in areas where the water table is too saline (i.e. salty) and poisonous for plants that have roots tapping that water table to use that water. In Western Australia, it has become a problem due to clearing of the native vegetation, which was really good at using the majority of the rain water that came in. Because the water coming in as rain was being used by native plants, it wasn’t recharging the groundwater and the salt levels stayed really quite deep below the surface.

Having cleared that native vegetation for agriculture, crops don’t use that water anywhere near as efficiently as the native vegetation does and they’re only in the ground for half of the year, so we started getting more water leaking from the surface down to the water table. Over a period of 15-20 years, the saline water table started creeping closer to the surface and started to impact the surface of the land, which is where the roots of most plants are. It started to poison the surface of the landscape and salinise surface water bodies and generally make a big mess of things.

Sally (foreground) installing environmental monitoring sensors with undergraduate students at Blue Oak Ranch Reserve, California.

Sally (foreground, in hat) installing environmental monitoring sensors with undergraduate students at Blue Oak Ranch Reserve, California.

One of the solutions that’s been proposed to try to deal with this is to go to areas that are still not saline but where a lot of recharge (leaking of rain water down to the water table) is occurring and to put native vegetation back in to basically mine out the groundwater, and bring those saline water tables to a lower level.

What my undergraduate project was about was saying, where would you do this? What’s a strategic place to do it? What are the parameters of how you would do that? Is it different if you plant a row of trees versus a square patch of trees? How long would their effects persist if you harvested them? And so on.

What got me really excited about this is it was really landscape-scale engineering: we were trying to manipulate the entire water cycle over these huge areas, and our tool of choice was native vegetation. I thought that was really, really cool. I’ve been thinking about plants and space and manipulation of water cycles ever since.


Mentors played quite a big role for me, a lot at younger ages. When I was in high school, I was in this really weird training program for people to get into the Australian Science Olympiads. The University of Western Australia (UWA) had supported folks to do that in chemistry, and a wonderful guy named Peter Simpson ran workshops on chemistry. He gave everyone a test at the end and took those who showed promise to do chemistry on Saturday mornings. It was tremendously enjoyable because we had this really charismatic and thoughtful committed teacher who was explaining how shit worked. There was a while there where I viewed everything in the world through a lens of atomic theory.

Sally (middle) in full costume with bellydance gals, Annette and Teresa.

Sally (middle) in full costume with bellydance gals, Annette and Teresa.

Bob Bucat in the UWA chemistry department was a tremendous mentor who got me into research. Working with Bob was wonderful – he’s so supportive, constructive and creative. He would engage with you about thinking laterally about new questions. Bob has continued to be a tremendous mentor – he wrote me many a letter of support for graduate school, and I catch up with him whenever I’m back in Perth.


Throughout the western United States, and in fact most of the United States since European folks arrived, we’ve tended to suppress fires. If a fire has started due to lightning strikes or unattended campfires or whatever, we tend to zoom on in and try to control that fire and put it out as fast as possible.

The result of that management is that in the watersheds we’re looking at, rather than having a fire about once every 6 years – which is what the record from tree cores that record fire scars tell us happened – we now have watersheds where there hasn’t been a fire in 150 years. That can set up the conditions for a really catastrophic wildfire, like the Rim fire we saw recently near Yosemite National Park in California, and it also changes the character of the forest.

My graduate student, Gabrielle, is working in the Illilouette Creek Basin in Yosemite, which is a watershed in the Sierra Nevada mountains where the way that the forest is controlled by fire has been fundamentally altered. Up until 1973, it hadn’t had a big fire in about 100 years. Then in 1973, the managers of Yosemite decided to just let the watershed burn when it burns. So since then, this area has reverted to having a fire about once every 6 years.

Sally in a dried up tank (historically part of the cascading tank irrigation systems) outside Bangalore, India.

Sally in a dried up tank (historically part of the cascading tank irrigation systems) outside Bangalore, India.

Those fires actually start to become quite small because they self-limit each other – they reduce the standing biomass. If a fire is burning away in dense forest and it hits a place where there was a fire fairly recently, it can’t propagate in the same way. So you start getting more frequent but much smaller burns.


We’re really diverse in what we do, in that we try to do theoretical work that’s well grounded in observations.

For instance, Gabrielle’s really interested in what changing the fire management plan might be doing to the water yield from the Illilouette Creek Basin, because of course the other thing that these Sierra Nevada watersheds are important for is supplying 80% of California’s water.

So she’s working with a bunch of fire ecologists who know the watershed well, measuring surface soil moisture and trying to compare what’s going on in sites that have burned versus sites that haven’t burned. There’s this very interesting observation that dry lodgepine forest will go through a burn and will regenerate with wetland vegetation. So she’s trying to make the preliminary measurements that confirm, yes, this change in vegetation is something that we can associate with a change in surface moisture content.

Then she will go to an aerial photography record, which is the long term record that we have at this site, showing the vegetation condition before the burns and about 10 years after the burns started, and then about 20 years after that. She’ll be using these observations of vegetation as a way to try to reconstruct the wetness of this watershed. Then she’ll be linking that to streamflow measurements and ultimately to hydrological models of what we think the watershed should be doing.

Sally (left) kayaking with her brother-in-law and the sea otters at Elkhorn Slough, California.

Sally (left) kayaking with her brother-in-law and the sea otters at Elkhorn Slough, California.

So she’s going from the field, through to different kinds of observations, satellite, flow-based observations as well, and then ultimately bringing that to a theoretical tool. I think most of the projects we do try to span that kind of link between something that’s grounded in observation – whether we make those observations or whether we use existing observations – and then using that to inform prediction or deeper understanding of a process.


You have no excuse for being bored. You can work towards things that you find intellectually stimulating and pursue curiosity, which is a tremendous privilege. You don’t get to do that in many jobs. Also the people you can collaborate with are ridiculous. I’m blown away by the people I get to call colleague and collaborate with – members of the National Academy, Macarthur fellows, people who are so far out of my league and yet care enough about mentoring young people and working collaboratively that they just come and have lunch with you.


Most of the water that we’re thinking about is lurking below the surface and we can’t measure it. I’m not necessarily talking about groundwater aquifers, but just the water that’s sitting in the soil, that’s sitting in unsaturated but fractured rock: the water that is the water in between being rainfall and turning into streamflow or transpiration.

The more that we look at this water, it almost seems like the less we understand it. We’ve got just so much spatial variability and so much that we can’t observe in that subsurface environment that it’s just a really horribly under-determined problem. There are too many plausible explanations that can link the things we can observe.

It’s not my part of this problem – I’m not a measurement devising person – but being able to measure this subsurface water is probably the biggest limitation that faces the field and probably where the biggest breakthroughs could really take things forward.


I honestly don’t know. I’m expecting a baby and questions about bringing that kid up as Aussie versus American are going to become really pressing before too many years pass, and I don’t know what we’re going to end up concluding. Assuming we stay here I’d like to get tenure, but really I’d just like to continue to work at this interface of the important processes that link water and ecological systems. I want to answer questions such as: where do I go to build the most important nature preserve if the climate’s going belly up? Are we hopeless in face of large trends? We’ve got such a long way to go before we can integrate that picture that there’s room for a great diversity of methods. I think I just want to be a cog in the big science machine.


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