Living Land

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sources. (Some treated effluents may actually have lower salin-

ity than alternative freshwater sources with salinity levels. For

example: wastewater in Israel that is derived from desalinated

seawater has lower salt levels than fresh water removed from

the Kinneret — Sea of Galilee.) Given the seasonal demand in

Israel’s Mediterranean climate, Israel stores effluents in reser-

voirs during the rainy months. The evaporation that takes place

during storage invariably leads to higher salinity levels.

Some 15 years ago, there were already Israeli experts who

questioned the wisdom of the country’s aggressive promo-

tion of effluent reuse. They argued that over time, using

wastewater for irrigation would lead to the accumulation of

sodium compounds in soils. While chloride is required in

very small quantities for photosynthesis and enzymatic reac-

tions, sodium, the other component of salt, makes little if any

contribution to plant health or yields. Even modest quantities

can be toxic to plants and cause damage to soil structure,

making it unproductive for agriculture. Indeed, conventional

fresh water used in irrigation can contain sodium at levels two

orders of magnitude higher than the plant needs to develop.

With time, signs of salinity damage from long-term effluent

usage in Israel were ubiquitous. A study in Israel by research-

ers in the Ministry of Agriculture’s Volcani Institute compared

yields in orchards on that been utilizing effluents via drip

irrigation for 10 years: avocado and citrus yields were 20-30

per cent lower than trees in the same orchards that had been

using fresh water. Soil damage from wastewater tends to be

concentrated in the upper soil layers. But again, the plants

themselves are affected: another recent analysis by leading

expert Alon Ben-Gal and Eran Raveh shows clearly that as

wastewater reuse in Israel has increased over the last 20 years

so have sodium concentrations in soil and crops.

Traditionally, Israeli farmers have overcome salinity prob-

lems by applying high irrigation rates to crops, leaching excess

salts out of the root zone to protect plant health. In an arid

region, farmers may use an additional 30-40 per cent more

water simply to manage salt levels in cultivated soils. This can

solve the plant’s problems but can contribute to contamina-

tion of the underlying aquifers. Recent experience in large

olive tree plantations in Israel’s Negev desert constitutes a

cautionary tale.

The olive trees relied on drip irrigation with water from

relatively saline, underlying aquifers. The salts were managed

by applying water for leaching. During the rare winter storm

of 30 mm or more, salts accumulating on the surface could

dissolve and be delivered directly into the root zone. Facing

perennially high evapotranspiration levels, the trees took in

large quantities of the salty water and immediately showed

signs of distress. Recently, when olive oil prices dropped, the

cost of such massive leaching became prohibitive and the

farmers reduced the magnitude of irrigation dramatically. It

did not take long before the tree production began to signifi-

cantly decline due to exposure to the salts.

The lesson is clear: in arid and semi-arid regions where

there is not sufficient precipitation to flush salts out of soils,

deficit irrigation will not work. This makes leaching impera-

tive but economically and environmentally problematic.

In a recent article, two leading Israeli agricultural research-

ers with Alon Ben-Gal and Eran Raveh wrote: “Israel’s policy

of lower prices for salty water and absolute utilization of

wastewater for irrigation without addressing salinity may

have been reckless. Leaching, necessary in agricultural water

management when using water containing salts, is of itself

unsustainable, as the water leaving the root zone contains not

Lysemeters measure the response of pepper plants to salinity at the Wyler

Department of Dryland Agriculture, Ben Gurion University

Lysimeters measure the response of olive trees to different irrigation regimes

at the Ministry of Agriculture’s Gilat research station

Image: Alon Tal

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