Grip on evaporation of our blue gold

Liduin Bos-Burgering (PhD) is a Hydrogeology and drought expert at Deltares. Her curiosity about 'evaporation' brought her on board of WUNDER. Through her network, she was asked to help write a research proposal on this topic. Liduin is now doing full-time PhD research at TU Delft under the Deltares banner and can focus entirely on evaporation. There is growing interest from Deltares to learn more about this complex subject now that we are experiencing more frequent periods of drought

Liduin: "The evaporation study is taking place on the high sandy soils at the Herenboeren in Apeldoorn. The question is how to get a better understanding of water consumption for agriculture, preferably worked out in a model. The Herenboeren is a cooperative farm employing a single farmer. It will be interesting to see how our research with accompanying insights will then eventually land later - from researcher to farmer to manager."

Insights into drought water demand

"In WUNDER, we mainly look at Eco-hydrology combined with other hydrological aspects within a farm. Evaporation is one branch of this. We are specifically studying what role this plays under dry conditions. This issue has repeatedly surfaced in the Netherlands. Hydrological models often fail to accurately predict water demand under dry conditions, as well as the permitted irrigation levels during such periods.

Especially for water managers in the area, it is important to understand the total water demand as well as the total water availability to better manage irrigation bans. From Deltares, together with another PhD candidate, I am researching how we can contribute to improving hydrological modelling and which processes we should include in this," says Liduin.

" My work largely consists of performing various measurements. I measure different components of the water balance with the main aim of getting a better grasp of evaporation. Applying theory to real-world practice is a key aspect of Dutch water management, and we actively work on that translation."

Measuring=planning

"Fieldwork is super complex and there is a lot of planning that goes into it, which is also often adjusted as we go! It is important to get the technical details right about your equipment in the field. It requires a detailed schedule on paper beforehand on how you are going to do this." Liduin explains that everything depends on what we aim to measure and what knowledge we seek. "What I primarily want to understand is the extent of the difference compared to current measurement methods, and whether there is a noticeable gap between the theoretical evaporation value and the measurable evaporation value during prolonged dry periods. We know they both have a margin of uncertainty."

Different sensors

Various sensors are deployed in the field to collect data, each differing in complexity and measurement methodology. Their results will soon all count towards the conclusions. Liduin: "Actual evaporation, for example, can be measured by combining data from two types of sensors that measure wind speed and the amount of moisture in the air. Simply put, a combination of these sensors then gives the so-called evaporation flux - a rapid change in the amount of moisture in the air. This is the most complicated sensor. Next to it is a similar, but much simpler and less expensive sensor that does pretty much the same thing."

Measuring in soil and air

"We also measure with fibre-optic cables in the soil and in the air. Soil temperature is measured with a laser, which combined with soil properties is a measure for moisture in the soil.

In the atmosphere, we can measure temperature differences and the so-called heat flux. This is then a measure of drought that can be linked to evaporation. There is also a weather station with which theoretical evaporation values can be determined. The observed rainfall aids in determining changes in soil moisture levels.

In addition, we use traditional soil moisture sensors and ERT (which stands for "Electrical Resistivity Tomography”) to determine soil resistivity with electrical pulses. Combined with weather data, you can also derive soil moisture from this. I am proud that in the end we managed to apply all the measuring techniques I had in mind within our budgets!"

The impact of drought in the Netherlands could become large-scale. We irrigate a lot but what effects does that have and what does it do to adjacent land? And what happens in terms of evaporation? With this research, we will get more insight on a small scale into the uncertainties and how big the shortcomings might be in hydrological modelling.

Liduin Bos-Burgering

Overestimation of water availability

"In current hydrological models, we assume that there is always enough water. And that means you can just keep evaporating! But when it gets dry, you no longer have full water availability and then your evaporation goes down. If your model doesn't take that into account, you get what's called an overestimate of your water availability. So here is a gap between the model and reality. With this research, we hope to further close this gap."

Tips from the field: dare to learn and be critical

"With fieldwork, things always go wrong. Therefore, flexibility is important because something can always go differently. This can be very simple or practical like a sensor not working or data not being transferred somewhere in the link, a clamp not working or an assembly problem, to big questions where you suddenly need to intervene or adjust, causing delays elsewhere.

Learning from (senior) colleagues with a lot of experience in the field is also important. For example, I learned from them that your planning can never be detailed enough.

Testing is key to success in the field. We have now been able to test all the sensors properly and have our data analysis and workflow well developed. Because there are so many data flows, it is a challenge to execute and reconcile all the different steps at the right time. This overview is something we must keep a constant eye on.

Making regular data downloads and keeping your own control when standing in front of important research of your own, is important. Daring to be critical is also an art. And staying authentic!"

What are your hopes for WUNDER?

"I hope that I can contribute to an improved and more conscious understanding of evaporation in the Netherlands. What touches me is that our blue gold is declining worldwide. Until recently, this was not an issue in the Netherlands. Here, of course, we know very well how to deal with water. But especially under dry conditions, we see that it is becoming more acute and that here too - especially in the growing season - water is becoming scarcer. I find it extremely interesting to be part of rethinking to a picture of having enough to the picture of realising that we do not have enough. And there are complex issues involved. Also great to mention - our paper on decision support tools and their use within drought management in the Netherlands - in specific the role of hydrological models - has just been accepted."