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Into Thin Air: A Smarter Way to Water Crops, with A.J. Purdy

How can we measure water when it disappears into thin air?

On this episode of What About Water? we’re looking at evapotranspiration, or “ET” for short. It’s the combination of water evaporating from the soil, combined with the measure of water transpiring through crops’ leaves. Accounting for this water loss helps farmers know exactly how much water they should apply across their fields, and new agricultural technologies and satellites are making it much easier.

Jay sits down with California State University at Monterrey Bay Senior Research Scientist – and former student – A.J. Purdy, whose doctoral thesis looked at the advancement and applications of satellite-derived ET algorithms.

We also hear what this looks like in real life, with Brett Baker, a sixth-generation California pear farmer and lawyer. With the ever-present risk of flood on his family’s land in the Sacramento-San Joaquin River Delta, Baker explains how OpenET is helping farmers like him and his father take better measurements of consumptive use.

Robyn Grimm, Interim Director of OpenET, talks about how this open-source platform is making big waves.

Guest Bios

AJ PurdyA.J. Purdy

Dr. A.J. Purdy is a senior research scientist at California State University at Monterey Bay (CSUMB). He is optimistic about the power of science to impart positive change in one’s local community and the world. At CSUMB he is developing new applications of satellite data to support water managers, agricultural producers, and land managers in California.Prior to CSUMB, Purdy served as an assistant professor at University of San Francisco (2019-2022) and a postdoctoral fellow at NASA’s Jet Propulsion Laboratory (2018-2019). His research experience includes using data from multiple NASA satellite missions (Landsat, MODIS, SMAP, GRACE, and ECOSTRESS) to advance science related hydrology and water resources.

Purdy holds a PhD in Earth System Science from UC Irvine and a MS in Watershed Science and Policy from CSUMB. His PhD dissertation focused on the advancement and applications of satellite-derived evapotranspiration algorithms. Outside of the office, he enjoys spending time surfing, camping, and adventuring with his family.

Brett BakerBrett Baker

Brett Baker grew up on the meandering banks of Steamboat Slough in his family’s Pear Orchard on Sutter Island in the Sacramento-San Joaquin River Delta. Brett completed his undergraduate studies in Wildlife, Fish and Conservation Biology at the University of California, Davis in 2007. Brett’s first job out of college was as a Heritage and Wild Trout Biologist for the then California Department of Fish and Game. He served as a Water and Agricultural Policy analyst for California’s then Lieutenant Governor, John Garamendi. In 2010 Brett started Baker Advocacy and Consulting, Inc. to help his family, friends and neighbors comply with the newly enacted Delta Reform Act, which required the measurement and reporting of water diversion and use in the tidal Sacramento-San Joaquin Bay Delta for the first time in state history. Brett became a father in 2012 and enrolled in Law School in 2016. Brett passed the California State Bar examination and was duly licensed as an Attorney at Law in December of 2019. He currently practices Law in Stockton, California at the Nomellini, Grilli & McDaniel Law Office, and assists in the administration of the Central Delta Water Agency. He is now a proud father of three beautiful children: Clara Belle (10), Evelia Joy (8) and George (1.5).

Robyn GrimmRobyn Grimm

Robyn Grimm is director, climate resilient water systems, at the Environmental Defense Fund and the Interim Director of OpenET. For the past four years, Dr. Grimm has been working with partners at NASA, the Desert Research Institute, Google Earth Engine, a dozen other organizations and more than 100 stakeholders on the development of an online open platform for estimating consumptive water use, called OpenET. With the October 2021 launch of OpenET, Dr. Grimm is now working with partners on driving adoption of the platform and expansion to additional geographies. The platform provides critical information for the sustainable management of groundwater basins, the development of healthy water trading programs, and other land and water management practices that increase the sustainability of water resources and related economies and ecosystems. Dr. Grimm previously worked at the consulting firm CH2M-Hill, where she was a water resources policy analyst and advised the California Department of Water Resources on flood planning. Dr. Grimm holds a doctorate degree in hydrologic sciences from the University of California, Davis, and has contributed to a number of flood control and water management studies in California.

Dive Deeper

  • For irrigated agriculture, ET is a measure of the water used to grow food and is the biggest share of water use in most arid environments around the world. (OpenET)
  • More than 99.9% of the water used by an irrigated crop or turf is drawn through the roots and transpires through the leaves. Only a small amount (0.1%) of the water taken up by plants is actually used to produce plant tissue. (CSU)
  • Knowing how much water is transferred to the air allows farmers to better calculate crop water requirements, helping them use water more efficiently and better plan irrigation. This makes evapotranspiration a crucial measurement for farmers and other water resource managers, especially in the western United States where most of the water goes to irrigate crops. (NASA)
  • If you can predict evapotranspiration rates, you will be able to estimate the water demands of the crop. This may help you to determine whether or not to irrigate, for example. (NCSU)

 

Photo Credit

  • A.J. Purdy – Submitted
  • Brett Baker – Submitted
  • Robyn Grimm – Submitted

Full Transcript

Brett Baker
I truly believe the cliché that all farmers are essentially environmentalists, in that we have a great deal of connection and concern and care about our land and the resources that we rely on to make productive use of it. We can’t store water here. We’re not trying to use more water. We’re not trying to you know, we’re trying to just continue going about business as we have, making productive use of our land for the last 150 years.

Jay Famiglietti
Protecting farmland and producing the food we need mean getting better at using water. In California, the days of abundant, fresh water are long gone. Today, every drop counts because when too much water disappears into thin air, so do farmers’ livelihoods.

Jay Famiglietti
I’m Jay Famiglietti. On this episode of What About Water? we’re looking at evapotranspiration. Water leaving the soil, water leaving plants, literally vaporizing back into the air. And we’re looking at the way new technology measures that water loss so farmers can grow more from less. Farmers like Brett Baker.

Brett Baker
I’m a sixth generation pear farmer from the Sacramento-San Joaquin River Delta, smack dab in the middle of California. My family settled that piece of dirt in the 1850s, and we’ve been making productive use of it growing crops, mainly pears, for the last hundred and fifty years. And my father still manages and operates that orchard today. We have 30 farmed acres, which makes us one of the smallest, if not the smallest pear grower in the state of California.

Brett Baker
Having a close connection to that land and just growing and realizing how deep our roots as a family ran in that dirt was kind of what always was my my guiding light in my decision making.

Brett Baker
I started out as a business major, and I changed to wildlife, fish and conservation biology. Much to the chagrin of my father. I remember coming home and telling him and his response was, “Oh my God, you’re going to be a tree hugger.” And I said, “Don’t worry, Dad. I’ll always be a pear tree hugger.” At some point in time, I decided that I was going to pursue a law degree, so when I went to law school, people would ask me, why, why are you here?

Brett Baker
You know, I would say, “Well, I want to be a farmer.” “And why? What are you doing here?” Well, obviously, to farm in the State of California. The delta in general, is poorly understood, I think, and poorly described to a lot of people because it is so unique and it is so different from the rest of the state. When I’m traveling, whether it be South America or Europe or wherever you are and they ask you where you’re from, you say California and they nod their head.

Brett Baker
You know, they know where California is, sure. And then you tell them you live on an island in the middle of California. They look at you cross-eyed. You know, they’re like: “you mean like you’re surrounded by water and all four sides?” and like, “oh, yeah”. The ever present threat of flood is something that every resident of the delta concerns themselves with.

Brett Baker
I’ve seen the river height crest about the same height as our levee. Like I could stand on top of the levee and spit in the water and that’s a pretty humbling event to see high flows like that. Those levees don’t just protect the people living behind them. They do protect the water supply system for the State of California that provides water to 25 million people.

Brett Baker
Now, all 25 million of those people don’t get all of their water out of the delta. It’s a bit of a misnomer. But our levee system did a lot of things. It helped control the wild evaporation of the water. This was a swamp and overflowed land at one point. And so there was a great deal of uncontrolled evaporative losses.

Brett Baker
The evaporative transpiration, the measurement of consumptive use is really the relevant measure for water use in the delta. It’s necessary to know because the state operates the two largest diversions in the delta the state and federal government do. They determine how much water they’re going to extract or export out of the delta based on a poor calculation of consumptive use, an admittedly inaccurate model, an admittedly inaccurate algorithm.

Brett Baker
They know it to be inaccurate. There’s always a reason to pay attention to your water use as a farmer. It’s quite possibly one of the most critical management tools you have. But measuring water, generally speaking, is is an art more than it is a science.

Jay Famiglietti
That’s Brett Baker, a pear farmer in California. My next guest says if farmers like him can find the right tools, they can see how much water soaks into their crops, and they won’t have to guess how much of their water evaporates. A.J. Purdy is a senior research scientist at California State University, Monterey Bay, and he’s been studying this for years.

Jay Famiglietti
Welcome to What About Water? A.J.

A.J. Purdy
Thanks for having me. It’s good to see you.

Jay Famiglietti
I need to let our listeners know that you were actually one of my star graduate students. So it’s no surprise that you’re trying to solve the problem that farmers around the world wrestle with. You could have studied anything. What drew you to this?

A.J. Purdy
Yeah. No, I was really fortunate to be part of your team. Down to UC Irvine did my Ph.D..

Jay Famiglietti
You have to say that.

A.J. Purdy
Yeah. No. So, you know, it’s like I grew up in Silicon Valley. I grew up in San Jose, but I went to college in San Diego. And so you think those two areas are pretty far removed from a lot of agriculture in California. But to get between the two places, you drive straight down the center of the Central Valley on I-5.

A.J. Purdy
And it was really over those years that I started to see and conceptualize and really more so understand the footprint that agriculture has in the state. And it’s not necessarily just in terms of water, but also the prominence in terms of industry as well. During my master’s degree, I was fortunate to link up with a team at Cal State Monterey Bay, where I happen to be now, as well as a as a research scientist.

A.J. Purdy
But it was really during that time that, you know, I was able to work and install these sensors to measure water cycling on farm fields. And, you know, it was through those experiences that I really understood the power of science as that objective means to to reach out and work with farmers to better understand, you know, how much water they’re using, where those those blind spots in terms of, you know, getting useful information and then also coming away from that experience, understanding that there’s still quite a big gap between the science type data that we gather and collect and the information that’s useful to make actionable decisions. Then, fortunately along the way, I was able to join your lab and learn the the prominence of evapotranspiration, not just on a farm field but in the earth system as well and in global water cycling.

Jay Famiglietti
Well, it’s amazing, A.J., that, you know, California has a huge impact on you if you’re environmentally aware, you know, like you are and you do start taking that drive up and down the five or the 99 in the Central Valley. And you see the massive expanse of the agriculture. And you also see that there’s like no rivers in sight. So I think you realize, okay, this is a place that doesn’t have a lot of water.

Jay Famiglietti
But I want to get back to, you know, because you mentioned evapotranspiration. We mentioned it a couple of times. Can you explain what exactly is “evapotranspiration”?

A.J. Purdy
Besides being a mouthful, the word “evapotranspiration” is… it’s that flux of water from the land surface to the atmosphere. And so it’s the combination of evaporation from, you know, ponded water on the surface, evaporation from soil, and then transpiration through plants as well.

Jay Famiglietti
Okay. So going through going through the roots, right in out through the leaves and-

A.J. Purdy
Exactly. And, you know, it’s like one of the interesting things about evapotranspiration as well is that we can use satellites to observe this in an independent way. Then we can get at the actual amount of evapotranspiration. The amount of energy that it takes to convert water from liquid to vapor is quite a bit. And so when you think of the surface energy balance, just like you would think of, you know, a water balance, there are inputs in their outputs. For the energy balance, we have inputs from the sun.

A.J. Purdy
Some of that imagery goes to the ground surface, some of that heats the air, and then the rest of that energy goes towards converting water from liquid to vapor in the process of evapotranspiration.

Jay Famiglietti
Okay. So then it sounds to me like if there’s more water on the ground, then there- more of that incoming energy from the sun would be used to evaporate the water, to vaporize the water, than it would be to heat the ground or to heat the air.

A.J. Purdy
Correct. Yeah. Now, that’s as simple as that.

Jay Famiglietti
Thank you. So how does remote sensing of evapotranspiration actually work? Like, what are we measuring?

A.J. Purdy
Yeah. You know, satellites are designed to capture certain wavelengths of energy, whether it’s reflected energy from the earth or emitted energy from the earth as well. And so certain wavelengths they kind of indicate whether or not plants are, you know, green and healthy. And then other wavelengths may better represent which areas are cooler, which areas are hotter. And so using that combined information, we can start to model which areas are having higher rates of evapotranspiration, which areas are having lower rates?

A.J. Purdy
Yeah.

Jay Famiglietti
So as an aside, like what would we have to do if we wanted to measure it directly? How could we do that? Just throw a garbage bag out there and capture some more? Like, what would we do?

A.J. Purdy
If you go back in the hydrology textbooks, there are a few ways in which you can measure it directly. One of the more straightforward ways is to wait. And so people invested money in, you know, put these weighing lysimeters in the ground. And so you think of it as like a scale beneath the surface where you have a container above it. So long as you measure how much water you’re pouring in, you can measure that decrease in weight as water evaporates.

A.J. Purdy
Some of the other ways are using these things called eddy covariance towers. And it’s through those towers that you’re actually measuring more of that that turbulent energy exchange. And so there’s these really fine tuned, and they’re called sonic anemometers, that measure the three dimensional direction of wind. And so, in conjunction with measuring how much water vapor is in the air, you can actually capture and measure how much water is “evapotranspiring” from the area in which the wind is coming.

A.J. Purdy
So it’s really those two two ways that help us build confidence in the satellite-based approaches and ground those to to what we’re actually seeing on this on Earth.

Jay Famiglietti
It’s really hard for a farmer to know how much water is being lost in a field. So now you have technology that can help us quantify evapotranspiration. What does a farmer do with that? Because. Is that really going to help the farmer on the ground? Like, what does that do for a farmer?

A.J. Purdy
Scientifically speaking, you know, it’s it’s useful for us to close the water budget, the water balance for a basin. But for a farmer, farmers can see which areas of their fields are producing an optimal rates or suboptimal rates.

Jay Famiglietti
So are we at the point yet where a farmer or, you know, people like us could do this and share it with farmers? Do a water budget or a water balance, check the sums of the inflows and outflows. And so where the rainfall or irrigation would be an input — evapotranspiration would be an output, rate of loss. Are we at the point where we can do that and sort of do this like budget so that farmers know how much water basically is in the root zone of a field?

Jay Famiglietti
So they know how much water they need to add or don’t need to add?

A.J. Purdy
We are. Some of the more recent efforts — I know that you’ve been able to talk to some of the team at OpenET — in terms of making this data available, summarizing it at the field scales and integrating that precipitation information, not just the total amount of precipitation, but it’s really that effective precipitation — how much of that water sticks around, so that you can get at how much water you actually need to apply.

A.J. Purdy
Summarizing that information is really critical to the utility of ET models to support water consumptive use estimates.

Jay Famiglietti
So back at the farm level, what can someone like Brett do to control and manage water loss?

A.J. Purdy
The evapotranspiration data that’s available, you know, it gets at that consumptive use. And so what that means is it’s like ET water that goes into the atmosphere and leaves the system. But when you’re making irrigation choices, you can irrigate so that you’re kind of replacing that deficit. That way you’re optimally applying the exact amount of water you need to to support crops.

A.J. Purdy
One of the areas in which this type of irrigation management is beneficial, not just in terms of the quantity of water, it’s actually really beneficial for the quality of water as well. You’re not flushing as many fertilizers down into the groundwater or you’re not flushing them into nearby streams. And so from that standpoint, it’s not just trying to fine tune how much water you’re applying to support healthy crops, but it’s also, you know, preserving resources.

A.J. Purdy
You’re not overpumping groundwater if you’re applying the exact amount that you need. It’s a really a way to hopefully optimize, you know, how we’re managing water quantity and quality, not just for one farm field, but the system that it’s within.

Jay Famiglietti
So do you see promise for like technology helping us have super smart farms and being really, really efficient? And at the same time, you know, stewards of the environment?

A.J. Purdy
Yeah, I think we’re headed in that direction. Right. It’s not just, you know, through satellite based methods. There’s a lot of effort in the agricultural technology community industry as well. You know, when you think of the amount of water in a canopy, if you have more perennial crops like, say, like orchards or vineyards, you can actually install sensors on a select number of trees or vines to track how much water potential exists within those those canopies themselves.

A.J. Purdy
So it’s through, I think, a combination of satellite observations or maybe drone observations in the future to get at the spatial variability of what’s going on within your farm boundary. But the reality is that technology on the farm hasn’t necessarily caught up with all of those gizmos. One of the things going back to the utility of ET data, you know, how much water is leaving your farm.

A.J. Purdy
You can tell the farmer, hey, it transpired five millimeters yesterday, per unit area across your farm. But at the end of the day, what matters is how long you’re turning on and off your irrigation system. And so making those translations, really, at the end of the day, it’s the efficiency of irrigation. And, you know, that time dimension, that’s that’s going to make this data even more widely adopted or used.

Jay Famiglietti
You know, from my experience, I think we are, but I agree with you, it’s a long way to go. And it’s very expensive. So it’s one thing for us to, you know, hang out in our, you know, computer labs and in our in our research offices and think about all the great things that we can do in that can be done.

Jay Famiglietti
And it’s another for them to be implemented on the farm. And, you know, just kind of reflecting while you’re talking A.J., about like how important this hydrologic variable is of evapotranspiration. Like absolutely critical for food production and for, you know, supplying moisture for precipitation is a big part of the water budget. But, you know, most people give it zero thought.

Jay Famiglietti
What’s what’s that all about?

A.J. Purdy
I guess I don’t know.

Jay Famiglietti
Yeah.

A.J. Purdy
I think, you know, it’s the one thing I’ll kind of circle back to is that, you know, I think there’s a growing appreciation for this type of work. Yeah. And I think you’re seeing that through the openness of a lot of farming partners to work with teams to allow us to, you know, measure ET in in a field and use that to help inform and update and optimize some of the models that we use to measure those fluxes with satellites.

A.J. Purdy
Hopefully in the future, it’s even even more openness and appreciation. But, you know.

Jay Famiglietti
Well, let’s follow up on that, that openness and getting the data out there. I know you talk a lot about open science. So what does that mean?

A.J. Purdy
Open science is increasing the transparency it’s allowing for reproduce ability. It’s making it more accessible, forming an inclusive community. And all of those things are really needed, not just for science to advance at a more accelerated rate, but I think it’s also highly relevant for applications of science as well, that transparency. So, you know, your partners understand exactly what you’re doing, the reproducibility, so that you’re not just applying it for one farm field, you can apply it to many.

A.J. Purdy
The accessibility. It’s like publishing in open access journals, being able to lower the barriers towards scientific data use. Like when I started the Ph.D. program at Irvine in 2012, getting satellite data was a pain in the butt. And, you know, it still is for, frankly, for a lot of data that’s out there. But what we’ve seen over the past ten years is a huge leap in terms of making the data much more accessible.

A.J. Purdy
And that includes, you know, anything from not having to download on your computer and, you know, install software for a model. Now you can go to a website and click on a field boundary and get that data in real time.

Jay Famiglietti
Pretty amazing. Yeah, I think that’s pretty amazing. So are you feeling optimistic? You’re feeling hopeful about technology, not only remote sensing, but the things that we’re seeing become more available with open science?

A.J. Purdy
Absolutely. The arrows pointing in the right direction for sure. I think a lot of the more recent efforts, not just, you know, through the ET community and efforts like OpenET, but Google Earth engines really lowered the barriers and made it possible to do science on really large scales. I think even NASA’s recognized the value in open science and put together an initiative that you’re seeing that through things like the Earth Information System in which, you know, you’re you’re modeling things but it’s really that accessibility of going to a website, again, kind of being able to interact with these really sophisticated models in a much simpler way.

Jay Famiglietti
Love your enthusiasm, A.J., and your perspective on this. Thanks so much for joining us today.

A.J. Purdy
Thanks for having me.

Jay Famiglietti
A.J. Purdy is a senior research scientist at California State University, Monterey Bay.

Jay Famiglietti
To effectively manage water, the data for evapotranspiration has to be easily accessible. That’s where OpenET comes in. To find out what that will look like, I spoke with Robyn Grimm. She’s the Interim Director of OpenET and she’s the Director for the Environmental Defense Fund’s Water Program.

Robyn Grimm
The intent is that it helps a huge variety of water management solutions and innovations that are otherwise really difficult to implement at scale. We also have a use case where folks are taking this data in to a water accounting platform, right? So there’s folks who are trying to do that at the district scale, the local scale, and this makes that possible.

Robyn Grimm
It’s also possible for policymakers to understand and track supplies in near real time at sort of larger watershed scale applications. We’ve called it filling one of the largest data gaps in water management. And indeed I think it has been I even think that this data can be useful in helping to fill in pictures about groundwater recharge, for example, in some areas where that information isn’t as well understood. OpenET is an online data platform that makes evapotranspiration data available across the western 17 states at the field scale.

Robyn Grimm
So you can go online to OpenET and you can type an address in the search bar or you can zoom in using the zoom to any place in the western U.S. and understand with visuals and downloadable data what evapotranspiration has looked like in that particular place for the last five years. Farmers and water managers right across the West and other arid regions of the globe really are dealing with this incredibly challenging situation in which they are being asked to do more and more with less and less in terms of the water supply.

Robyn Grimm
Some farmers in California are seeing up to 100% cuts in their surface water allocations during the severe parts of the drought. And so one of the things that’s really surprising then is that they’re being asked to so precisely and so carefully manage this scarce and ever scarcer resource without really having a broadly accessible way to understand how much water is actually being consumed by crops and natural vegetation as it grows across the landscape, which is one of the most critical pieces of information for managing water.

Robyn Grimm
And so OpenET came about because that gap was there and it was really critical to fill it.

Jay Famiglietti
That’s Robyn Grimm I caught up with her earlier this fall during World Water Week in Stockholm. She’s worked with NASA, the Desert Research Institute, Google and other groups to build OpenET. This open source platform is something Brett Baker is using.

Brett Baker
We’re thrilled with this concept of OpenET because we think, number one, it’s relevant data. Number two, it’s uniform. Number three, it’s near real time because when we were reporting prior, we were reporting water under the bridge. We were doing last year’s stuff. This is going to be informative to the current water year. The timeliness I think is is probably one of the most advantageous things, you know, going for OpenET is that within six weeks they can turn around the imagery and give you the consumptive use figure.

Brett Baker
So that’s for us that is promising for our water users because it helps us comply with the regulation. I’m hopeful that it will allow the operators and the decision makers that operate the reservoirs in the state projects and the pumps in the delta to to make more informed decisions and have better outcomes. And we’re hopeful that delta water quality will be maintained to an extent that it is more beneficial for the environment and for our agricultural users.

Jay Famiglietti
That was Brett Baker, a sixth generation California pear farmer. As you heard, Baker is eager to collaborate with OpenET. If you’ve been listening to the episode and you have questions about something you hear on the show, send them to ideas@whataboutwater.org.

Jay Famiglietti
We record and produce this podcast on Treaty Six Territory, The Homeland of the First Nations and Métis People.

Jay Famiglietti
“What About Water?” is a collaboration between The Walrus Lab and the Global Institute for Water Security at the University of Saskatchewan.

Jay Famiglietti
This podcast is a production of Cascade Communications. Our audio engineer is Wayne Giesbrecht. Our producer is Erin Stephens. Our fact checker is Taisha Garby. Our GIWS crew is Mark Ferguson, Shawn Ahmed, Fred Reibin, Andrea Rowe and Jesse Witow. I’m Jay Famiglietti. Thanks for listening.