Modeling and monitoring vadose zone processes across multiple scales is a fundamental component of many environmental and natural resource issues including nonpoint source (NPS) pollution, watershed management, and nutrient management, to mention just a few. In this special section in Vadose Zone Journal we present a collection of papers reflecting current trends in modeling and monitoring vadose zone processes from field to landscape scales. The objectives of this introductory paper are to set the stage for the special issue by providing background information, by showing the interrelationship of the papers, and by identifying the significant contribution(s) of each paper. The spectrum of topics covered includes (i) issues of scale, (ii) spatial analysis of model error, (iii) modeling of NPS pollutants and hillslope stability, (iv) the use of estimation and conditioning tools such as upscaling, pedotransfer functions, and generalized likelihood uncertainty estimation, (v) data assimilation in conjunction with flow modeling and passive microwave remote sensing to estimate moisture distribution, (vi) effective hydraulic parameters across spatial scales, (vii) spatiotemporal stability of soil properties (e.g., Cl, B, and NO3–N transport; salinity; and soil physical and hydraulic properties), and (viii) nested sampling to determine spatial patterns. A commonality among the papers, whether for modeling or monitoring vadose zone processes, is the question of how to address complex issues of spatial and/or temporal variability at the scale of interest. Future research will likely involve inverse modeling, the use of multiple sensors to monitor at various scales, and continued applications of pedotransfer functions, upscaling and downscaling, and hierarchy of scales.

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