Due to more periods of prolonged drought, soils in a growing number of areas have become hydrophobic (water-repellent). The problem is that using large amounts of water when watering hydrophobic soils, is counterproductive because most of it will wash out. The solution: more frequent watering, using smaller amounts of water. The only way to manage this is through close monitoring.
Soils have pores. The size of these pores can vary, from large pores in sand to very small ones in clay. This is a useful property, because it allows the soil to provide trees and plants with moisture and oxygen.
However, if the soil dries out too far, there is so much air in the pores that they will have to be soaked for a while, until the soil is able to retain moisture again – and have it available for absorption by plants and trees. With dried-out soils, this process of re-moisturizing usually takes place during the winter time, when there is less evaporation and more rain.
By dumping large amounts of water at once, dried-out soil does not have enough time to absorb the water. This is comparable to a dry sponge or chamois that you try to push under water; it just floats back to the surface again. Only if you leave it there for a while, it gradually absorbs the water until all pores are filled.
The effect of a large amount of water at once, is very limited for the mid- and long term
Monitoring helps to better understand the ‘behavior’ of the soil and the effect of waterings. The steeper the peak on the moisture graph, the more water is washing out. By analyzing the sensor data and reviewing the graphs, it is possible to find the right amount of water to maintain the optimal average moisture percentage. Experience shows that this can save up to 70% of the amount of water. In a medium-sized municipality, this can quickly save several hundred thousand liters per growing season.
Smaller, more frequent waterings have a much longer-lasting effect
For Sportservice ede, sensors have been installed in Ede by Van de Haar Groep. Krinkels did the same for sports fields in the municipality of Winsum, and since this month in Maastricht.
The ConnectedGreen-system consists of smart sensors, a Cloud environment and an app for field owners and contractors. All tasks are recorded in a log. Via a dasbhoard, both parties can check the status of all projects. If something needs to be done, the relevant stakeholders will receive a notification.
Neil Claessen, Krinkels
René Voogt: ‘Especially for sports fields without a built-in irrigation system, it is important to monitor the moisture balance. Some municipal sports grounds are still irrigated by the clubs, using reels and pumps. It’s costing a lot of time to get these at the right place. So it is really handy to know when you have to be where with that reel; that saves a lot of unnecessary transport movements. Because the system gives an early warning for too low moisture levels, the grass won’t turn yellow. And when there is no shortage, no water is wasted. The mosture data are visible for both the field managers as the users.’
Installation of the sensors
Krinkels had already installed sensors under the lawn of the Museum square in Amsterdam. In the area of sports fields, the company had gained expirience three months earlier, with the installation of ConnectedGreen sensor in sports fields in Winsum.
‘The sports fields should not suffer from an outdoor water-use restriction’
Planner and estimator Neil Claessen from Krinkels in Heerlen has recently installed four sensors in three fields in the municipality of Maastricht: in two of the twelve fields at multi-functional sporting ground Geusselt and in one of the eight fields at multi-functional sporting ground West. The two fields at sporting ground Geusselt both have one sensor installed, the field at sporting ground West has got two sensors installed. Because the project is a pilot, Krinkels and the municipality of Maastricht want to figure out if one sensor per field is sufficient, or if two are needed. The sensors have a length of 15 centimetres, which would mean that the measurements are just under the root zone of the grass. That is why they are placed horizontally at a depth of around 10 centimetres. Voogt: ‘The sensor should not measure too deep; it should measure at root depth.’The sensors are placed at a recognizable spot on the field which is representative for the moisture level in the entire field. That would normally be in or around the center circle. If a second sensor is placed, this would be placed around the goal, because of the intense usage of that area.
Ad Boer, Van de Haar Groep
Claessen shows where he has installed the sensors under the grass: in the same line as the irrigation pipes in the fields. That is easy for the orientation of the maintenance crew. ‘We always stay clear of the irrigation pipes during maintenance work. Placing the sensors in the same line prevents them from being hit and the locations can easily be added to the irrigation map.’
Soil moisture measurement
Ad Boer, planner with Van de Haar Groep, has three years of experience with the ConnectedGreen sensors. In 2017, an innovation-minded colleague shared a brochure with information about soil moisture sensors. When Van de Haar Groep was awarded a four-year contrect by Sportservice Ede for maintenance of their sports fields, the sensors were installed immediatel; six in total, spread over four fields. Both Van de Haar Groep and Sportservice Ede have access to the Cloud environment to view the soil moisture data via computer or app.
Krinkels Heerlen also shares the moisture data with their client. Claessen: ‘The field managers of the municipality of Maastricht have the ultimate responsibility for the fields, so they have to be able to look over our shoulder. Also, since it’s a pilot, these are the people who will have to judge if there is enough added value in using these sensor data.’
Earlier, Van de Haar had also installed sensors at sports club Candia ’66, but these have already been removed. Club volunteers are very frequently present on the sports ground and they water regularly. Boer explains: ‘The volunteers over there are the ears and the eyes of the club; remote monitoring is not necessary for them. We offer the sensors as additional service to our clients and then evaluate for which clubs or sports ground they offer real added value.’
Interpretation of data
The ideal soil moisture percentage lies between 5 and 15 percent. These percentages can easily be viewed in the app. This way, Boer monitors all fields on a daily basis. Van de Haar Groep shares the soil moisture data with Sportservice Ede. And they forward the message to the clubs, if a field is showing to be too wet or too dry. There is one field that consistently shows abnormal values: ‘I think this is due to a sub-optimal place of the sensor; we will move it to another place. That can be caused by a hard underground layer or a wrong setting of water pressure or reach of the irrigation system.
Preferably, Boer wants the sensors to be a bit shorter, so they don’t measure as deep as they do now. ‘When the sensor is placed too deep, it measures below the root zone. That means there should be a customization of the sensor geometry for sports fields, That is something for the future.’
René Voogt, ConnectedGreen
Take action at the right moment
Claessen, of Krinkels: ‘More and more sports clubs have issues with shortage of volunteers, so it is important to work efficiently. Besides that, water should be used more efficiently too. The past years we have seen outdoor water-use restrictions in many places. The grass should not suffer from these challenges. The sensors measure the exact need for moisture of the grass plant, so in principle there is no shortage.
In Maastricht, both sports grounds are equipped with an automated irrigation system and watering is overseen by employees of the municipality. Claessen still thinks the sensors provide added value. ‘At these sports grounds, the usage of the fields is quite high and they are also rented out to host events. The pressure to provide good fields on which many people can set foot, is high. Watering is essential for a strong field. The sensors and app can be of help for field managers; remote monitoring helps them saving time and kilometres and watering is being done efficiently.’
In public spaces, more and more sensors are being installed. And there are many vendors diving into this topic. Things are evolving so rapidly, that Geonovum has even written a guidebook with a first draft of a ‘sensor data ordinance’ for all stakeholders involved.
Data driven operations and the Smart City are slowly but certainly becoming the new reality. Unfortunately, it is also reality that more than 90 percent of sensor data is not, or suboptimally used. This is because sensor data in itself does not mean anything. The data should be placed in a context and the information should be translated in understandable and actionable form for the different stakeholders. For that reason, the ConnectedGreen platform focuses fully on the right calibration and interpretation of sensors for landscaping, to make sure that the right information reaches the right person at the right time. Domain knowledge of trees, plants, soils and the weather is leveraged to help the users to calibrate and tune the system. This domain knowledge is crucial to make a system like this function in a practical way, and get the added value out of it.
Smart monitoring helps to work in a more sustainable way and save costs (less water, less project visits, less failing plants and trees). And through more transparency, collaboration between contractor and clients will improve. This means that all stakeholders will benefit. ConnectedGreen observes different ways of collaboration. Some landscapers deploy the system to monitor their own projects and work more efficiently. Others are also giving their clients access to the data, which enables better collaboration. And in some cases it’s the other way way around: a municipality buys the sensors to monitor newly planted trees and lets their contractor view the data.
An example of a landscaper that is successfully using the system is the project of replacement of trees in the ‘Hemsterhuis’ neighborhood in Amsterdam. Due to circumstances, the trees had to be planted out-of-season and in extreme heat and drought. ‘We have installed sensors to keep an eye on the soil moisture situation, to make sure we water the trees at the right moment and they do not get not enough – or too much water’, says Henk Werner from Pius Floris Boomverzorging Amsterdam. ‘Three weeks after planting, the trees had developed about ten centimeters of new branches. Before the winter, new roots will have grown and the trees will be fixed, which means that they will have a head start next spring.’
Of course it is important for governments to aggregate the ‘green’ sensor data on a higher level in the organization, and combine it with other (sensor) data. That is the only way to get an integrated view which is needed for developing new strategies and policies and create a ‘real’ Smart City. In a pilot project for the municipality of Houten, ConnectedGreen collaborates with partners Nazca IT and Boomtotaalzorg. Nazca IT has developed a Smart City platform, that combines data from many different sensors throughout the municipality. And ConnectedGreen is used by the municipal arborists, in close collaboration with the consultants of Boomtotaalzorg to let the newly planted ‘landmark’ Fagus Sylvatica succeed!
The Golden Watering Can for ‘Best Product’ went to René Voogt from ConnectedGreen. ConnectedGreen helps landscapers and governments to save on watering, project visits and failing plants/trees. The system operates with wirelees sensors which are placed invisibly at strategic locations in landscaping projects. The sensors are available in different lengths to make sure that soil moisture levels are measured at root depth.
Within ConnectedGreen, projects are defined (for example per landscaping project, street or neighborhood). These projects are then divided into different indication trees, planters or beds, fitted with one or more sensors. The sensors are calibrated by entering a plant/tree name and soil type. Data can be shared between contractors (landscaping companies) and clients (governments), including notifications and alerts. This goes for both (too) dry and (too) wet situations. Besides optimization of watering, ConnectedGreen offers insights which can help improving the quality and effectiveness of growing places.
The three finalists in the category ‘Best Product’: Sharell Hogervorst from Greenmax, René Voogt from ConnectedGreen and Henk Vlijm from Optigrün
The Golden Watering Can
This year for the first time, trade journals ‘De Hovenier’ and ‘Stad+Groen’ went on a search for professionals who have developed creative, practical solutions for everyday problems that are caused by climate change. All contestants were judged by a renowned jury of experts, consisting of: Hein van Iersel (editor-in-chief of ‘De Hovenier’ and ‘Stad+Groen’), Lodewijk Hoekstra (tv-host and founder of NL Greenlabel), Egbert Roozen (director VHG), Dick Oosthoek (director Stichting Groenkeur), Ben van Ooijen (CEO/owner of De Tuinen van Appeltern) and Mathieu Gremmen (member of dike board and loco-bailiff of ‘Waterschap Rivierenland’). After the announcement, the winners were congratulated by Lodewijk Hoekstra via a Vlog.
In November 2019, René Voogt from ConnectedGreen won the Golden Watering Can award at the Climate Expo in Utrecht. ConnectedGreen helps landscaping companies to save on water, project visits and failing plants/trees. In ConnectedGreen, ‘projects’ are defined (for example per landscaping project, street, or neighborhood) which are divided into indication trees, planters, or beds. The sensors are calibrated by entering the tree/plant name and soil type. Besides optimization of watering, ConnectedGreen provides insights which help to improve growing places.
With ConnectedGreen, Voogt is involved daily with questions about moisture supply for trees, directly after planting. Here below he answers five pressing questions about this topic.
Question 1: How can a planting place dry out while being watered frequently?
‘Particularly in more porous and sandy soil types, like tree sand or planting soil, the process of drying can go faster than the compensation from watering’, says Voogt. ‘In certain periods, this means that watering once a week is not enough, and that a downward trend becomes visible in the moisture levels. This can even lead to the moisture level dropping to near zero between two waterings. And the more dried out, the more difficult it is for a soil to absorb water, so this is an accelerating effect (hydrophobic soil).’ The solution according to Voogt: ‘More frequent watering, in smaller quantities. These more frequent waterings can be compensated in cooler, wetter periods, when less frequent watering is required. Another option is to add more organic and/or granular material in the planting place.’
Question 2: How is it possible that rain or watering is not noticed by a sensor?
‘The sensors measure the soil moisture percentage at root depth’, explains Voogt. ‘That is roughly between 15 and 60 centimeters deep, dependable of the plants. If the top layer dries out, we observe the phenomenon that (rain)water simply washes off the top layer and cannot penetrate to root depth. This effect is called “hydrophobic” soil. In that case, the soil literally repels water. The water flows away via “flow channels” and does not enter the ground. The problem here is that the speed of drying out can be much higher than the speed with which the soil can absorb water.’ The solution according to Voogt: ‘Open up the top layer (for example with a rake) before watering or create a tree planting system, for example with reverse drainage’.
Question 3: How is it possible that different sensors in the same project show different soil moisture values?
‘This question pops up frequently when sensors show different values in beds and tree planting places that are cultivated in the same way’, says Voogt. ‘Looking through all projects in our systems, we see it is more an exception when all beds or trees in a project show the same percentages. Here, several variables are in play:
-The interaction between the top layer, underground, cultivated soil and root ball. Especially the top layer and underground are very divers in our country. Also, the soil that is added to the project is not always exactly similar in every bed or with every tree.
-The shadow of buildings or other plants/trees plays a rol, besides orientation/vulnerability to the sun or wind.
-Sometimes, very local impermeable layers or horizontal water flows play a role.’
Question 4: How come that some growing places are wet in the top layers and dry at the bottom, while this is the opposite for other growing places?
‘Here too, environmental factors play a big role. Soil with a high amount of organic matter can hold moisture very well. Sometimes, there is more organic matter in the top layers and sometimes more in the lower layers. Besides that, the drainage capacity of the growing place is essential. If an impermeable layer or heavy clay is present, we often see the growing place ‘filling up’ like a bathtub. First the moisture levels rise in the lower levels, and then in the top levels too. We also see the opposite, when plants and trees are planted on a slope or very sandy underground and the water just flows away.’ The solution according to Voogt: ‘Impermeable layers or clay can be penetrated or dug up. In very porous soils it will help to add clay to the bottom of the planting hole.’
Question 5: Why is the soil moisture percentage a few days after a(big) watering sometimes lower than before?
This is a remarkably interesting phenomenon, that Voogt has encountered in a couple of occasions: ‘After watering, the moisture percentage rises really quickly, to then drop dramatically. It is widely known that excess water drains away if it is much more than the soil can hold/absorb. It seems that, in certain situations, the water that is already present, is ‘drawn down’ with the water that flushes out. We have seen this in a couple of places. There are several theoretical explanations, but additional research is needed to find out the real cause. The observed pattern seems to support the theory that it makes no sense to give (much) more water than the soil can hold (more than the field capacity). On many graphs in the summer period we observe these high peaks, followed by lower levels than before. The preliminary results with giving less water (30 to 50 percent of the original quantities) are indicating that the soil moisture percentages drop much less quickly, thus being better maintained.’
And as afterburner: why is it important to receive a signal if the growing place is too wet? Aren’t these sensors meant for watering in case of drought? Voogt: ‘After having hundreds of sensors for more than two years in the field, we know that it is evenly important to receive signals if it is wet, even in wintertime. There are several examples known of projects where many trees and plants have failed due to an abundance of water. This can have many causes which were not known in the design phase, like:
-The growing place or bed is lower than the rest of the area, so all water flows there
-There is an impermeable layer or heavy clay, which prevents excess water from draining
-The planting place is next to a (sloping) road, a square or path, which makes all water flow in one direction
-The soil contains too much (raw) organic matter, holding large quantities of water
Often the notification “too wet” comes as a surprise (we have seen more than one occasion of broken water pipes or valves). It is however extremely useful to know, because the cause can be investigated and taken away – for example with vertical or horizontal drains.’