A geostatistical study of soil data from an irrigated vineyard near Waikerie, South Australia

Abstract

Effective supply of water for irrigation requires that the capacity of the soil to hold water be measured. Field measurement of the depth and texture of each soil layer in a profile allows calculation of the readily available water for the site. The spatial variation of readily available water is characterised by its semivariogram, calculated over the property under study. This function is used in a geostatistical analysis to determine the average value of the variable over areas watered by opening irrigation valves. The accuracy of such estimates is also provided in the geostatistical procedure of kriging. A case study applied to an existing vineyard near Waikerie in South Australia deals with root zone readily available water and depth of topsoil. The property has been sampled with a grid 75 m x 75 m. Irrigation valve areas are typically 1.3 ha. A 50% reduction in confidence intervals for the estimates occurs when the valve areas are estimated by kriging compared with those obtained using a simple average of the samples contained within the area. Of interest is the anisotropy seen in the semivariograms. 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 the irrigation valve areas. It is suggested that it should also influence sampling design.