DRAINMAX-Green
Positive effect on the microclimate with the capillary subsurface irrigation system for urban and agricultural irrigation
As part of the BMBF-funded AgRain R&D project, a capillary subsurface irrigation system was developed and patented using the INTEWA DRAINMAX® Tunnel Rigole. The system decentrally stores rainwater from unsealed (seepage) or sealed surfaces. Subsequently, the collected water rises upwards by means of capillary force. Originally developed for the irrigation of wet rice in Burkina-Faso, new tests show that the system can also be used in German cities, as a water supply for urban greening (including urban farming) but also as a large-scale system for ambient cooling. Thus, the system provides answers to several pressing problems of the consequences of climate change: food security, local heat islands, water storage and rainwater management.
Successful irrigation trials show: halved water consumption with twice the yield
New trials in the drought summer of 2022 have confirmed the effectiveness of the system for irrigation in vegetable cultivation - also called "urban farming" in an urban context. At the Aachen site, the AgRain demonstrator with tunnel and the reference system without tunnel (see Fig. 2) were planted in early summer according to the planting plan (Fig. 3)
The reference system was irrigated manually to take account of the prevailing irrigation type in Burkina Faso. In practice, this was 5 to 6 litres/m² on hot summer days. For the example month of August, this results in a water consumption of 290 litres in the tunnel system, which is less than 50 % of the water consumption of the reference system, which consumed 600 litres.
A possible explanation for this is provided by the comparison of the soil moisture levels: It is noticeable that in the tunnel system the amplitude of the soil moisture in the main root zone (20 cm) is less pronounced with a maximum of -50 kPa. This means that the plants are exposed to less water stress during the day.
The analysis of the soil moisture data in the reference system, on the other hand, shows that the soil becomes saturated in the main root zone after the watering process, but the irrigation water hardly penetrates into deeper soil layers. Thus, even on hot days, extreme dryness (> 800 kPa) reappears in the main root zone within 24 hours.
At the same time, the tunnel system was less affected by mildew and cabbage flies, which were very hard on the surface-cast system.
Types of use: Irrigation and evaporation
The water is stored in the DRAINMAX tunnel and then transported upwards by the capillary rising action. In the process, it is proportionally absorbed by the roots of the plants and used for photosynthesis (6 H2 O + 6 CO2 = 6 O2 + C H O6126 ) or for cell pressure (turgor) and thus the stability of the plant. Water that is not absorbed by the plant continues to rise and evaporates at the surface. This causes a cooling of the environment of 670 kWh/m³ (evaporative cooling: for the phase transition from liquid to gaseous, energy is needed that is extracted from the environment and thus cools it). According to literature, optimal water supply can lead to a cooling of the ambient temperature of 1.5 - 7°C. (Gkatsopoulos 2017, Sudimac et al. 2019, Umweltbundesamt 2014) In the current tests, among other things, it was investigated whether the tunnel would "evaporate empty" in the absence of a capillary-breaking layer (pebbles, gravel, or similar). However, this was not the case. The higher yields in the tunnel system rather indicate a sufficient or optimal water supply for the plants. However, an exact measurement of evaporation is only possible in appropriate experimental boxes.
Apart from urban farming, i.e. the production of biomass suitable for consumption, the use of the capillary irrigation system is also possible as large-scale evaporative cooling for green spaces in an urban context. When using the system, the environment is cooled by evapotranspiration, which includes evaporation from soil and water surfaces on the one hand and from the surface of plants on the other (transpiration).
DRAINMAX-Green: The advantages at a glance
Compared to surface-irrigated green spaces, the system as a combined system for irrigation and evaporative cooling offers municipalities and their residents the following advantages:
- Optimal watering promotes plant growth so that the plant can develop more or larger leaves. This increases the surface area available for transpiration. More water can evaporate and the environment is cooled more effectively. The quality of life of the residents is improved and, if the system is used over a large area, the local rainfall frequency can even increase.
- Due to the underground installation of the system, no areas are consumed as footprints. Existing public green spaces become a "natural air conditioning system" by using the small water cycle in the context of evaporation and thus receive a higher added value and appreciation.
- Due to its design, the system replicates the natural precipitation cycle. The infiltration capacity of unsealed soil is increased by moistening "from below" and thus the near-surface runoff is reduced. This reduces flooding and sewer overload.
- Natural resources are conserved because the system does not require energy for capillary rise. However, the capillary rise is limited, depending on the soil type (pore size and shape). The installation depth of the system should therefore be selected according to the type of use (irrigation vs. evaporation).
- Irrigation is done with collected rain (seepage) water and the drinking water often used for irrigation can be "saved". Groundwater supplies are thus conserved or even replenished using seepage water from the overflow.
The use of DRAINMAX-Green as a subsurface irrigation system thus limits the pressing problems resulting from climatic changes in the long term. The demonstration system in Aachen is, by means of urban evaporation and agricultural irrigation, the first step towards a sustainable green future.
Author: Dipl. Ing. Oliver Ringelstein, INTEWA GmbH