In this article we would like to explain the formulas and the procedure to follow to obtain the amount of water that a green roof or green roof will need.
It is essential to know this data, on the one hand to choose the irrigation system for our green roof with greater security, and on the other hand, to design an irrigation schedule accordingly.
Importance of irrigation in green roofs
Irrigation in green roofs plays a fundamental role in the survival and proper development of vegetation. When considering the elements necessary for effective irrigation in these spaces, it is crucial to take into account several factors that impact on its effectiveness.
Elements to consider for irrigation
- Local climate and weather conditions
- The plant species used in the canopy
- The type of soil present in the area
- The efficiency and precision of the irrigation system in place.
Irrigation requirements of a vegetation cover
Calculation of irrigation requirements
In order to determine the irrigation needs of a green roof, it is essential to calculate the appropriate amount of water that the plants require to grow optimally. This calculation is made taking into account crop evapotranspiration and effective precipitation, which are key elements in the water management of these environments.
The irrigation needs of vegetation depend mainly on the climate, the characteristics of the plant species, the type of soil and the efficiency of the irrigation system used.
In the 1970s,the Food and Agriculture Organisation of the United Nations (FAO) developed a methodology for calculating the water and irrigation needs of crops that can be perfectly extrapolated to green roof projects. This methodology uses the following formula to determine as accurately as possible the irrigation needs of green roofs:
Nnr = ETc – Pe
Where:
- Nnr: net irrigation needs, units l/m2 x day (= mm/day)
- ETc: crop evapotranspiration, units l/m2 x day (= mm/day)
- Pe: effective precipitation, units l/m2 x day (=mm/day)
But… what do these concepts refer to? Let’s take a closer look.
Crop evapotranspiration.
This is the loss of water in the form of vapour due to evaporation from the soil and transpiration of the vegetation. The latter process is the passage of water absorbed by the roots to the atmosphere, in the form of water vapour, through the stomata of the leaves.
In order to calculate the evapotranspiration of our species, under the conditions in which they are found, we must resort to the following formula:
ETc =ET0 x Kc
Where:
- ET0: Evapotranspiration of the reference crop, units l/m2 x day (= mm/day)
- Kc: Crop coefficient
The reference crop evapotranspiration is defined as the typical evapotranspiration of a grass with a height between 8 and 15 cm; grown in a large field; actively growing and healthy; with the substrate fully covered and therefore shaded; and with optimal irrigation water quantity. The cultivation coefficient is a dimensionless corrective value, which depends on the type of vegetation and the growth stage. The growth stage influences the height of the plant and the degree of coverage, among other things. FAO established 4 growth stages, initial, development, mid-season and maturity. There are tabulated Kc values depending on the type of crop, e.g. by Villalobos et al., 2002, adapted from Doorenbos and Pruitt, 1977, and Allen et al., 1998.
Effective precipitation
Effective or efficient precipitation is the fraction of total precipitation used to meet the water needs of the crop; it is therefore excluded:
- Deep infiltration
- Surface runoff
- Evaporation from the soil surface
Once we know the net irrigation needs of our roof, we will have to consider the type of system to choose, looking for the most efficient one so that the greatest amount of water supplied reaches the vegetation. There are different irrigation systems for green roofs and when choosing them, we must weigh up their advantages and disadvantages. An in-ground irrigation system, for example, will have lower surface evaporation losses but will not be accessible, having the advantage that it cannot be manipulated by any user, but for maintenance it will need to be dug up.
Irrigation systems for green roofs
Irrigation systems are essential to ensure an adequate water supply to vegetation on green roofs. There are different options available, each with its own characteristics and benefits.
Drip irrigation system
The drip irrigation system is an efficient and accurate option that delivers water directly to the roots of the plants. Some advantages of this system include:
- Reduced water wastage
- Precise control of the amount of water delivered
- Reduced risk of fungal diseases
Sprinkler irrigation system
The sprinkler irrigation system is a common alternative that uses nozzles to spray water on vegetation. Some important features of this system are:
- Uniform coverage of large areas
- Adaptability to different soil types
- Possibility of automation for convenience
The choice between drip or sprinkler irrigation will depend on factors such as the amount of vegetation, the local climate and the preferences of the canopy designer. It is crucial to select the most suitable system to ensure an optimal and sustainable water supply over time.
Maintenance of the green roof irrigation system
Proper maintenance of the irrigation system in green roofs is essential to ensure the optimal development of the vegetation and to guarantee its health over time.
Periodic analysis and adjustments
- Regular monitoring of the irrigation system to detect possible malfunctions or failures that may affect the water supply to the plants.
- Review irrigation scheduling on a regular basis to ensure that the water needs of the vegetation are being met at each stage of growth.
- Check the condition of water emitters in drip irrigation systems, cleaning and replacing them if necessary to ensure uniform irrigation.
Strategies to ensure long-term success
- Implement an irrigation monitoring system to remotely control the operation and efficiency of the system, adjusting water quantities according to the actual needs of the plants.
- Apply fertilisers in addition to irrigation to ensure that plants receive the necessary nutrients for optimal development.
- Carry out water efficiency tests to evaluate the performance of the irrigation system and make the necessary improvements to minimise water losses and guarantee a sustainable use of this resource.
