A GEOSTATISTICAL STUDY OF SOIL DATA FROM AN IRRIGATED VINEYARD NEAR WAIKERIE, SOUTH AUSTRALIA

Effective supply of water for irrigation requires that the capacity of the soil to hold water be measured. Field measurement of the depth an d texture of each soil layer in a profile allows calculation of the re adily available water for the site. The spatial variation of readily a vailable water is characterised by its semivariogram, calculated over the property under study, This function is used in a geostatistical an alysis to determine the average value of the variable over areas water ed by opening irrigation valves. The accuracy of such estimates is als o provided in the geostatistical procedure of kriging. A case study ap plied to an existing vineyard near Waikerie in South Australia deals w ith root zone readily available water and depth of topsoil, The proper ty has been sampled with a grid 75 m x 75 m. Irrigation valve areas ar e typically 1.3 ha. A 50% reduction in confidence intervals for the es timates occurs when the valve areas are estimated by kriging compared with those obtained using a simple average of the samples contained wi thin the area. Of interest is the anisotropy seen in the semivariogram s. Variation in the N-S direction is much more rapid than in the E-W. This factor is currently incorporated into the rectangular design of t he irrigation valve areas. It is suggested that it should also influen ce sampling design.