Influence of Near-Surface Vegetation on the Values and Variability of Soil Water Content

Shallowly rooted vegetation can have a significant influence on the water content of near-surface soils. In precision agriculture applications, shallowly rooted crop cover is often used to modify the natural soil water content to produce optimal moisture conditions for the primary crop. In this study, the influence of crop cover on shallow soil moisture at a 3-acre California vineyard with heterogeneous soil textures was investigated using estimates of soil water content obtained from ground penetrating radar (GPR) measurements. The vineyard was planted with grapevines in rows separated by 1.2 m, and shallowly rooted crop cover (zorrow fescue grass) was planted in alternate rows between the grapevines. The shallow soil water content was estimated across the vineyard under varying conditions of precipitation and irrigation and at two depths. Water content estimates were produced from very high-resolution GPR data collected along selected rows, providing a very detailed data set (~20,000 measurements) of the water content in rows with and without crop cover. Analysis of these data showed that crop cover can significantly alter both the absolute values and the variability of the shallow soil water content, especially for data collected under drier conditions. Comparison of measurements collected at two different depths also showed that the effects of crop cover varied with depth, with the greatest influence exerted in the very near-surface soils. Rows planted with crop cover typically had a lower average water content than rows without crop cover, except under very wet conditions. The variability of the near-surface water content was investigated by calculating variograms for rows with and without crop cover and for all rows considered together. These variograms showed that rows with crop cover typically had lower variability than rows without crop cover, and that the influence of crop cover on soil water content variability was also dependent on the absolute value of the soil water content.