In urban planning, on-site collection of water run-off has three functions: to avoid burdening the drainage system, to provide moisture in the region of the plant root system, and to replenish groundwater reserves and prevent runoff from flowing into the sea, from which fresh water can only be recovered by desalination. In landscape and garden design in arid countries like Israel, run-off harvesting is an important way of enriching the soil with water, thereby making moisture available to garden plants throughout the summer and reducing irrigation outlays.
Measurements conducted at Nir-Oz indicate that the depth of the effective root system in ten varieties and species of lawn grass grown in Israel is 1.5 m [1]. Root systems of shrubs and trees go down to a depth of 6-17 m (Ran Pauker, unpublished). These data are in line with findings obtained by Itzik Moshe at the Keren Kayemet and Nir Atsmon at the Volcani Agricultural Research Organization (unpublished), by Yisrael Gev [2], as well as by M.C. Drew [3] and Walter Phillips [4].
Owing to the aridity of the climate (rainfall of 250 mm confined to winter, coupled with an average annual evaporation potential of 2000 mm), in years when winter rains are scanty the shortfall in precipitation has to be compensated for by supplementary irrigation, which can amount to as much as 40% of the normal irrigation [5]. For gardens with deep soil, these data justify instituting run-off harvesting and distribution as a means of reducing auxiliary irrigation in summer.
Indeed, in our conditions at Nir-Oz, the only economical way of storing water is in the soil. Fortunately we have deep soil, making it possible to store most of the run-off in the garden soil. Run-off water is directed to the plants in three ways. First, gutters are removed from buildings so that the run-off immediately irrigates the adjacent flowerbeds. Second, paths and roads are raised so that they act as dams; this is their most important function but they also allow water to flow off them and percolate into the ground on either side. Third, a system of small ditches and dams is created, whereby any remaining water is directed to the next part of the garden.
We have found that a lawn foundation consisting of sand down to a depth of 20 cm prevents soil compaction, promotes a deeper root system, prevents run-off from the lawn, allows more water to percolate into the soil and reduces annual irrigation requirements by 15%. Since 2001 we have adopted this composition, which has proved to be suitable for all garden plants, as the foundation of preference. Given appropriate planning of the surface contours of the garden, run-off collects in the immediate neighbourhood, percolates into the ground, and is distributed in the soil throughout the garden.
The residential neighbourhood of Kibbutz Nir-Oz comprises 60 dunams (6 hectares) of buildings, roads and other impermeable features. Trapping all the run-off from this area, given that the average precipitation is 250 mm annually, could add 15,000 cubic meters of water on average every year, or about 23% of the garden's water budget. Starting from the winter of 2001/2, we were able to direct all the run-off to the garden, including run-off from the farming and industrial areas (structures, chicken coops, cowsheds, factory), which amounts to a further 17,000 cubic meters.
Water can also be harvested from structures with metal roofing, which collect enough dew to enable cultivation of adjacent rows of plants without irrigation, and from air-conditioner discharge.
These practices contribute to increasing the reserves of water accessible to plants in the soil. Even more importantly, they help to replenish the groundwater instead
of allowing water to run off uselessly into the sea
Since the garden at Nir-Oz – which was designed by the landscape architect Hayyim Kahanovich – is an ‘ordinary' garden that has been undergoing transformation into a water-wise garden over the last 30 years, our exploitation of run-off is still incomplete. We believe that the most important thing of all is to be aware of waterwise gardening and to think creatively about ways of implementing it.
References
1. Pauker R. Water-wise landscaping, Water and Irrigation, 7th International Conference, Tel-Aviv, Israel, 1996. p. 308-315.
2. Gev Y. The influence of forestation on water flow and groundwater replenishment in sand dunes in a desert climate, Internal publication of Tel Aviv University, 1997.
3. Drew M.C. Root development and activities, “Arid-land ecosystems: structure, functioning and management”, 1979 vol. 1.
4. Phillips W.S. Depth of roots in soil. Ecology, 1963, 44:2, p.424.
5. Pauker R. Increase the irrigation in dry spells. Gan veNof 2001, 9/01, p. 40-41.
