3.1 Soil Water Content
When the United States National Aeronautics and Space Administration (NASA) was building the Phoenix Mars Lander to explore the surface of Mars, the project leaders contacted soil physicists to create a special sensor for the rover’s robotic arm (Fig. 3‑1). The purpose of that sensor was to test for the presence of water in the Martian soil. The fascinating story of the resulting partnership between NASA and soil scientists is recounted in this video. The NASA scientists knew that finding water on Mars was key to answering questions about the possibility of life on that hostile planet. Water is the life-blood of the soil. Almost all terrestrial life and almost all soil biological, chemical, and physical processes are influenced by the water content of the soil.
3.1.1 Soil Water Content Terminology
As you continue to study soil physical properties and processes, you will need to know and use the following key terms related to soil water content.
- Gravimetric water content (θg) is the mass of water per unit mass of oven-dry soil particles. It has dimensions of mass over mass and may be written as a unitless decimal, as a percentage, or as a decimal with units of kg kg-1 or g g-1. We will use the latter in this book. For mineral soils it ranges from approximately 0 to 0.60 g g-1. Organic soils may have higher values.
- Volumetric water content (θ) is the volume of water per unit volume of soil. It has dimensions of volume over volume and may be written as a unitless decimal, as a percentage, or as a decimal with units of m3 m-3 or cm3 cm-3. We will use the latter in this book. For mineral soils it ranges from approximately 0 to 0.60 cm3 cm-3. Organic soils may have higher values. When gravimetric water content is known, the volumetric water content can be calculated using the soil bulk density (ρb) and the density of water (ρw). This relationship is further explained in this video.
(Eq. 3-1)
Soil water storage (S) is the equivalent depth of soil water contained in a soil layer of a specified thickness (Δz). It has dimensions of length and may be written as cm or mm. If the volumetric water content of the layer is known, then the soil water storage is calculated by S = θ × Δz.
3.1.2 Soil Water Content Measurement Techniques
Knowing the amount of water contained in the soil is important, not only in the search for life on Mars, but also for understanding and managing life on Earth. Researchers in disciplines such as soil science, hydrology, ecology, agronomy, meteorology, and civil engineering often need accurate measurements of soil water content; as do farmers, horticulturalists, turf managers, and government agencies responsible for managing natural resources. Since around the end of the World War II, researchers have been continually developing new methods for measuring soil water content to meet the needs of these diverse user groups [1]. We will briefly consider a few of those measurement techniques here.
The simplest and