A Review of Regional and National Meteorological Networks offering soil moisture sensors and a review of the analytical methodology

Abstract

Soil moisture is a significant factor in Earth’s hydrological cycles that influences weather, drought, climate, and water resources on land and in water bodies. However, throughout most of the 20th century, soil moisture received less attention and was not included in many hydrological studies. In the 1978, J. W. Deardorff with the United States’ National Center for Atmospheric Research started to demonstrate the relationship between soil moisture and meteorologic conditions. Just two years later in 1980, G. C. Topp at the University of Toronto developed the Topp Equation - the first empirical calibration for soil moisture using time domain reflectometry (TDR). Additionally, that same year, M. T. van Genuchten published the van Genuchten Equation, which established a numerical relationship between soil moisture an unsaturated hydrologic head. Starting in the 1990s, the United States Department of Agriculture began using impedance-based soil moisture sensor technology to equip SNOTEL sites for water shed scale water supply forecasts. Since then, numerous large-scale regional meteorological networks incorporate soil moisture sensors, often referred to as ‘mesonets’, have emerged worldwide.   Soil water dynamics is complex often not well understood. Analytical methods using electromagnetic principles rely on the behavior and distribution electromagnetic energy in soil, making the operational theory of commercial sensors unclear at times. Soil moisture exhibits significant variabilities in space and time, as well as being influenced by hydrological and mineralogical properties of the soil. These factors give rise to several misconceptions about soil moisture monitoring.   This presentation discusses the growing importance of soil moisture as a critical parameter of the Earth’s hydrological cycle. This discussion also focuses on the objective and goals of North American soil moisture monitoring networks. Furthermore, the availability and emerging electromagnetic sensor technologies are reviewed. Lastly, calibration and validation soil sensors are also examined.