TOPIC 2: Rainwater management

TOPIC 2: Rainwater management

In this topic you learn about why careful rainwater management plays a major role in settled areas. On the one hand the water is needed during dry and hot periods, f.i. for watering the plants and cooling the microclimate. On the other hand, due to heavy rain falls more flooding occurs, which needs to be answered with adaptation measures.

Rainwater management

Heavy rain falls or extreme drought are impacts of climate change that are going to increase in the future. Depending on the local conditions, both extreme weather events will severely affect the water household.

Therefore, rainwater management is needed, so as the valuable rain water does not flow off superficially into the sewage system but is stored locally, also in community gardens.

Excursus: What is heavy rain?

The so called heavy rain is occurring when during a very short period of time an excessive amount of rain falls. By an amount of 15-40l/m² per hour or 20-60l/m² within 6 hours we call it heavy rain.

source: Deutscher Wetterdienst, , date: 07.07.2023

As the soil can’t take up (receive) that high amount of water in that short time, it runs off superficially into the sewer. For that reason it is lost for the groundwater and process of transpiration. In infavourable cases the humus erodes, which can lead to massive damages on infrastructure and buildings. Also the agricultural production is harmed thereby.

Source: Pixabay

The role of climate change

Due to climate heating more unsealed and green areas are needed (areas of parks and meadows, trees and shrubs) in settled regions, as they are able to release the stored water to the ambient air by transpiration. Especially on heat days the cooling-effect through evaporated water plays an important role so as to counteract the overheating in cities – the so called urban heating islands.

By the process of evapotranspiration of water a greater part of the solar energy is stored as latent energy (non-palpable). That means that the energy for evaporation is extracted from the ambient air, which is then heated less and has even a cooling effect on our habitat.

Grafik: Source: BGMR Landschaftsarchitekten GmbH 2023

Sealed vs. leaching capable areas:

Because of the increasing drain of land resources for settling, business, industrial areas and  traffic the ground is sealed partly or totally. Soil is filled with air and watertight materials, which means the functions for certain habitats (link to soil functions) is either destroyed or severely damaged.

Picture: Urban (left) and natural (right) water household (Gößner, Optigrün international AG, pressbox, 2023)

Impacts of sealing

Rain water can’t be stored by the soil or leached locally anymore. Instead it flows directly into the sewage system and further to superficial stretches of water/rivers, where the amount of water can lead to overloading of the system or flooding. The water is not available for the local hydrological cycle and can’t be evaporated. Moreover, agricultural land is lost for the production of food by sealing. The same is true for opportunities for recreation for humans and animals.

Did you know: Leaching capable areas play a role for the
Water household:

➤ Leaching
➤ Reduced surface flow
➤ Relief of the sewage system
➤ Avoidance of local flooding
➤ Recharge of groundwater

➤ Cooling of microclimate by evaporation-coldness
➤ Reducing the heating of dark, sealed grounds (Albedo-effect)

What can community gardens contribute?

They can increase the possibilities for water storage and leaching areas by:

  • Providing planted green grounds and soil-like substrates
  • Damp biotope
  • Other Utilizable areas designed leachable
  • Store water artificially (in tanks, canister)

Percolation of Precipitation Water

Besides green grounds also soaking hollows are able to retard water in the case of heavy rain occurrence. They help to absorb and percolate the water locally, so as to hinder the surface flow of water.  Where percolation hollows should be placed depends on the ground plan and the topology of the garden. Moreover it is necessary to grow wet and dry-tolerant plants. It makes sense to get support from a professional planner.  

Generally overhead hollows play an important role for the ecology and microclimate of a settled region. Technically the soaking hollow is a greened, (mostly) shallow depression in the terrain, in which the water is dammed up and retained temporarily.

Due to the donation of rain water, the plants in the hollow are better supplied with water than the even-growing plants, without having problems with waterlogging. The more shallow the percolation-field is, the less careful you have to choose the plants. If the leaking hollow is shaped steeper, you have to choose close-growing or spur-driving plants.


CCCAct Sheet Nr.36 2022, 17.4.2023

Gößner (2020), Optigrün international AG, pressbox

Angelika Eppel-Hotz: Bepflanzte Sickermulden bieten Mehrwert. Magazin Dega Galabau 03/2019.

APCC (Hrsg.). (2014). Österreichischer Sachstandsbericht Klimawandel 2014 (AAR14). Austrian Panel on Climate Change (APCC). Verlag der Österreichischen Akademie der Wissenschaften.

Boden erleben, Land Oberösterreich: BODEN_erLEBEN.pdf, 25. 05. 2023