Our challenge is to harvest rainfall to condition the soil so that plants can grow without depending on pumps, taps or drip water.  The proposal in this landscape gardens project is to grow only with harvested rainwater.      

Water Harvesting
Water harvesting is the collection of runoff for productive purposes.  Water harvesting is a directly productive form of soil and water conservation.  If the available rain can be concentrated on a smaller area, reasonable yields will be received, plant growth will be improved, and there will be softly seasonal rainfall fluctuation.

Micro-catchments for rainwater harvesting to grow fruits is a runoff harvested system from ground surfaces, which is sometimes referred to as a “Within-Field Catchment System”.Runoff stored in the soil profile increases soil fertility and water holding capacity, which prolongs soil moisture. This is possible because we will add a considerable amount of organic matter: composted soils and composted mulch in ongoing seasonal periods. The best way to hold water on your site is through developing the conditions to hold water in the soil.

In our Landscape Gardens Project, we are exploring the adaptation of contour bands and contour Keyline (Yeomans system) in combination with semi-circular bands of a Micro-catchment system.

contour keyline and semi-circular Micro-catchment

Edible Landscapes Garden Project specifics:

Nerang annual rainfall is 1370mm.
Edible Landscape SitesMean annual rainfall = 1370 mm/year (1.37m)Surface area of catchment = 1148 m2 Run-off coefficient = 0.2 (ground catchment -soil on slopes less than 10° = 0.0-0.3)Mean rainwater supply = 314 m3 (314,550 Litres)

  • Slope: special consideration is needed as the landscape slope is greater than 10°
  • January to March: there is a mean rainfall of 540mm in 33 days during the period.
  • July to September: there is a 138mm mean rainfall in only 14 days during the period.
  • Nerang annual rainfall is 1370mm.

Calculate potential supply of rainwater from catchment area
Mean rainwater supply in m3 = Mean annual rainfall in mm/year (Need to convert this value in ‘m’)  X  Surface area of catchment in m2 X Run-off coefficient

Edible Landscape Sites
Mean annual rainfall =1370 mm/year (1.37m)
Surface area of catchment =1148 m2
Run-off coefficient =0.2 (ground catchment -soil on slopes less than 10° = 0.0-0.3)
Mean rainwater supply =314 m3 (314,550 Litres)

Edible Landscape site catchment capacity.

Cultivate Area
the soil in the cultivated area should be a deep, fertile loam. Loams is a medium texture soils, which are best suited for plant growth in terms of nutrient supply, biological activity and nutrient and water holding capacities. A good soil structure is associated with loamy soil and a relatively high content of organic matter.  The application of organic material such as composted soil and mulch is helpful in improving the structure.

The depth of soil is particularly important. Deep soils have the capacity to store the harvested runoff as well as providing a greater amount of total nutrients for plant growth. Soils of less than one metre deep are poorly suited. The landscape gardens site on average has less than 0.5 of a metre-deep soil. Therefore, the project needs to add soil.
We are planning to top-up the garden area with 30cm-thick composted soil in the beginning, and add in every growing season another 10 to 15cm of compost, conditioned soil to maintain fertility levels.

Infiltration Rate
The infiltration rate of loamy soil typically is 12.5 mm/hour.  The soils of the cultivated area should be sufficiently permeable to allow adequate moisture to the plants root zone without causing waterlogging problems. A very low infiltration rate can be detrimental because of the possibility of waterlogging. The threshold rainfall can exceed 12mm in soils with a high infiltration capacity. In this case, rainfall of less than 12mm/hour will not produce runoff.  

Available Water Capacity
The capacity of soils to hold, and to release adequate levels of moisture to plants is vital. It is the depth of water readily available to plants after a soil has been thoroughly wetted to “field capacity”. The available water capacity for loamy soil varies from 100-200 mm/metre. In soils with a high available water capacity (200 mm/metre), there is no need for infiltration pits to depths greater than 40 cm.

Design Model for Catchment: Cultivated Area Ration
Each Water Harvest system consists of a catchment (collection) and a cultivated (concentration) area. Trees are almost exclusively grown in micro-catchment systems where it is difficult to determine which proportion of the total area is exploited by the root zone, bearing in mind the different stages of root development over the years before a seedling has grown into a mature tree. As a rule of thumb, it can be assumed that the area to be exploited by the root system is equal to the area of the canopy of the tree.In view of the above, it is therefore considered sufficient to estimate only the total size of the micro-catchment, that is the cultivated area and the infiltration pit together.

It is a  formula to design micro-catchment for fruit tree, which we have simplify in the form of a recipe.
  • Canopy area (= root system area)
  • Tree annual water requirement
  • Estimated annual lower percentage (%) of the annual rainfall potential (more or less 70 % of annual rainfall -this is known as
    design rainfall, --planning for dry years)
  • Soil annual runoff coefficient (the % rainfall not infiltrate in the soil)
  • 0.5 efficiency factor (merged for error)
recipe to estimate the micro-catchment size for a Loquat tree in Nerang.
  1. tree’s canopy area (7m2) multiply x The result of:
  2. Tree annual water requirement, minus (-) design rainfall
    (1000 - 959 = 41)
  3. Divide by the resulted of:Design rainfall, by (x) the annual runoff coefficient), by (x) efficiency factor
    (959 x 0.15 x 0.5)= 72
  • 41/72=0.57
  • MC = 7x 0.57
Total Micro-Catchment size = 4m2 (2.25m diameter)

We are learning here, we are testing theory. We are prepared for trial and error, the experience will teach us how it works. However, we are open to hear from experienced growers. Any advice is welcome!

Activity participants 9 June 2018


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