Productivity and water use of an irrigated high country pasture

Abstract

Pasture production and irrigation water-use efficiencies were measured for 3 irrigation regimes on a Mackenzie shallow, stony soil in the Mackenzie Basin near Omarama. Irrigating at 25% was the most efficient of three irrigation frequencies maintained over the 198 1-1986 period (irrigated at O%, 25%, and 50% plant-available water capacity). Less frequent irrigation resulted in lower pasture yields due to increased water stress, while more frequent watering resulted in a 55% drainage loss of irrigation water below the root zone. At 1.16 t DIvUha per irrigation, the increase in pasture yield with irrigation at 25% was higher than most increases achieved in existing irrigation schemes in Central Otago and mid Canterbury. Over 6 years, this treatment required 4-7 irrigations annually and, on average, produced 6.0 t DM/ha more than did dryland pasture. The water-use efficiencies achieved in the border-strip irrigation system used were generally greater than expected for highly permeable stony soils where large drainage losses of irrigation water below the root zone are common. This was the result of reduced soil water transmission rates after compaction by heavy machinery during border strip* preparation. That effect of compaction was persistent, and had not diminished 9 years after initial pasture and irrigation development, as structural improvement of this soil is slow, even under irrigation. Keywords: high country pasture, irrigation, Mackenzie Basin