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In agronomic problems the sampling procedure may create some confusion and bias in the analysis. Geostatisrics provides a method for the analysis of the spatial and temporal properties in a data set and a method of interpolation between selected points. This paper describes the theory of geostatistics and its application to selected agronomic problems. Geostatistics considers a set of data collected in either space or time at discrete intervals. These samples may be correlated with each other to provide some unique information about the parameters which would not be detected in the classical statistical methods. Through the application of geosraristics to this type of problem, we can estimate the spatial or temporal dependence of samples and from this knowledge arrive at an estimation of the sampling procedures or structure at a field. The application of these techniques is shown for air temperature, surface temperature, yield, clay content, and fertilizer content in various fields and reveals the versatility of the techniques. Geostatistics also allows for the evaluation of the dependence between two parameters in either time or space. From this information it is possible to develop sampling procedures which would allow the more costly or time consuming variable to be sampled less frequently and estimated from the other variable by the method of kriging. This report summarizes all of these techniques and provides several different examples of their utilization. Examples of the computer code are provided for the reader wishing to apply these techniques.

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This paper gives a review of our current conceptual understanding of the basic processes of water flow and chemical transport in the untsaturated (vadose) zone and of various deterministic mathematical models that are being used to describe these processes. During the past few decades, tremendous effort has been directed toward unravelling the complexities of various interactive physical, chemical, and microbiological mechanisms affecting unsaturated flow and transport, with contributions being made by soil scientists, geochemists, hydrologists, soil microbiologists, and others. Unfortunately, segmented, disciplinary research has contributed to a lack of experimental and theoretical understanding of the vadose zone, which, in turn, has precluded the accurate prediction and management of flow and contaminant transport through it. Thus a more unified and interdisciplinary approach is needed that considers the most pertinent physical, chemical, and biological processes operative in the unsaturated zone. Challenges for both fundamental and applied researchers to reveal the intricacies of the zone and to integrate these with currently known concepts are numerous, as is the urgency for progress inasmuch as our soil and ground water resources are increasingly subjected to the dangers of long‐term pollution. Specific research areas in need of future investigation are outlined.

Spatial variability of 1,280 field‐measured infiltration rates on Yolo loam (fine‐silty, mixed, nonacid, thermic Typic Xerorthents) was studied using geostatistical concepts. The measurements were made at the nodes of an irregular grid consisting of 160 rows and eight columns. Sample spacing within columns was 1 m. Columns were spaced irregularly at 1, 5, 15, and 19 m.A variogram for the 1,280 measurements was used to krige 800 additional values within five more columns and to draw a contour map of the area.The large number of measured values made it possible to calculate the minimum number of samples necessary to reproduce the infiltration rate measurements with a fairly large approximation. As a result, it could be concluded that a minimum of 128 samples was enough to obtain nearly the same information as with 1,280 samples. A suggestion on how to use autocorrelograms in sampling schemes is presented.