SOIL MOISTURE AND ITS INFLUENCE ON PASTURE PRODUCTION IN THE WAIKATO

Abstract

The occurrence and distribution of soil moisture deficit days in the Waikato were determined for 43 years (1930-73) by calculating day-to-day changes in soil moisture for four soil types. The calculations were based on actual daily rainfall data and an estimate of mean daily potential evapotranspiration. Seasonal distributions of the number of deficit-days are given. On three soil types with 87, 102 and 117 mm of available soil moisture there were more than 20 such days in at least 15 of the 43 years. January to April inclusive are the "high-risk months". A significant negative correlation was obtained between the number of deficit-days and pasture production from a non-irrigated, "rate-of-growth" trial, such that 20 deficit-days gave a 21% reduction in expected annual pasture production (below that expected with no deficit-days).