Using your lawn as a Biological Tensiometer

Irrigation with a standard amount of water but at varying time intervals
Scheduling irrigation by lawn demand.
Ran Pauker
Update of earlier versions that appeared in Hebrew in:
Gan veNof (Garden and Landscape), May 1983, p. 521
Mayim ve Hashkaya (Water and Irrigation), 1998/9 No. 383, p. 36 (monthly publication of the Israel Water Works Association)

Observations and experiments on lawn irrigation have been conducted at Kibbutz Nir Oz since the late 1960s. The local soil is a deep sandy loess; annual potential evaporation amounts to 1,990 mm and average precipitation is 250 mm a year. Our data set has given us the opportunity to develop an accurate and water-efficient protocol for general use. A tensiometer is an instrument used for measuring the energy status of soil water which tells us about the ability of plant roots to extract that water. There are many mechanical tensiometers on the market but the simplest and, arguably, most accurate method is to use the plants themselves. Many lawn grasses are highly water responsive and easily recover from water stress. Thus, the lawn acts as a 'biological tensiometer'.

Basic Assumption
Since we are not interested in obtaining a grass sward with maximum growth, with the exception of sports fields, we can delay irrigation until the first signs of water stress. These are expressed as changes in the color of the grass from fresh to dull green and these signs disappear rapidly after irrigation.

Watering all the lawns at Nir Oz takes 22 dusk-to-dawn time periods. This is termed the 'irrigation cycle'. Each lawn is usually irrigated once every 25 to 35 days, depending on the condition of the grass (30 days on average), with a fixed application of 130 mm.

In principle, the first irrigation cycle after the rainy season is started when the first signs of water stress are detected. However, experience has shown that irrigation should be started on the first lawns before they show water stress to ensure that the last irrigation of the cycle will not be not unduly delayed. In the Autumn, watering finishes with the first rains; this accounts for the differences between lawns in terms of the number of irrigation cycles applied and for water wastage on those lawns that are the last in the irrigation cycle.

The depth of the active root zone is 1.5 m in all the species examined (Adar, 1979; Lipschitz, 1980). In dry years, when the boundary zone of moist soil (wetting front) is less than 1.5 m at the end of the winter, the time between irrigations must be reduced; accordingly, the time between irrigation cycles is shortened to 22 days. These conditions result in a continuous irrigation cycle.

Irrigation Coefficient (See Glossary)

The Irrigation Coefficient (IC) is determined as the ratio of plant transpired water to evaporation from a Class A evaporation pan (See Glossary) in the Nir Oz area, expressed as a percentage. IC values for the varieties tested range from 74% to 101% in the Nir Oz area (experiment conducted in June, Lipschitz 1

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