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Environmental effects on the variability of herbage production and nitrogen uptake of an ungrazed annual Mediterranean sward were studied with a well-tested simulation model. Meteorological data for a 21 year period were from a semi-arid site in the northern Negev of Israel and the observed daily rainfall was multiplied by 0.5, 1.0, 1.5 and 2.0 to simulate different rainfall zones.

Rainfall regime, nutrient level and soil depth accounted for 83-91% of the variation in the standard deviation of herbage production (SD(h)) and of nitrogen uptake (SD(n)), respectively, while rainfall and nutrient level alone accounted for more than 95% of the variation in the coefficient of variation (CV) of both herbage production (CV(h)) and nitrogen uptake (CV(n)). Distribution of the accountable variation in SD(h) was divided among the three factors (24% rainfall, 40% nutrient availability and 18% soil depth), while the variation in CV(h) was the result predominantly of rainfall (93%), to a much smaller extent to nutrient availability (3%) and to a negligible degree to soil depth (0.1%).

Greater nutrient availability tended to increase both SD(h) and CV(h). SD(h) in relation to rainfall was highest in the intermediate semi-arid zone, but relatively low in both the drier and wetter zones. On the whole, CV(h) and CV(n) decreased with higher rainfall, even though the CV of rainfall was held constant. CV(h) and CV(n) sharply distinguished between the zone with less than 264 mm rainfall and higher rainfall areas. This, indeed, is the approximate dividing line between relatively reliable conditions for arable agriculture and submarginal conditions in the region.

It is shown that the lower stability of herbage production, expressed as CV(h), in drier regions is not so much a consequence of greater rainfall CV as of lower absolute amounts of rainfall. Under more humid conditions, greater stability (i.e. lower CV(h)) can be a result of nutrient deficiency, and not only a consequence of lower rainfall CV.