The infiltration of water into soil has profound importance as a central component of the hydrologic cycle and as the means of replenishing soil water that sustains terrestrial life. Systematic quantitative study of infiltration began in the 19th century and has continued through to the present as a central topic of soils, soil physics, and hydrology. Two forces drive infiltration: gravity, and capillarity, which results from the interaction of air-water surface tension with the solid components of soil. There are also two primary ways water moves into and within the soil. One is diffuse flow, through the pores between individual soil grains, moving from one to the next and so on. The other is preferential flow, through elongated channels such as those left by worms and roots. Diffuse flow is slow and continues as long as there is a net driving force. Preferential flow is fast and occurs only when water is supplied at high intensity, as during irrigation, major rainstorms, or floods. Both types are important in infiltration. Especially considering that preferential flow does not yet have a fully accepted theory, this means that infiltration entails multiple processes, some of them poorly understood. The soil at a given location has a limit to how much water it can absorb—the infiltration capacity. The interplay between the mode and rate of water supply, infiltration capacity, and characteristics of the soil and surrounding terrain determines infiltration into the soil. Much effort has gone into developing means of measuring and predicting both infiltration capacity and the actual infiltration rate. Various methods are available, and research is needed to improve their accuracy and ease of use.
Infiltration of Water Into Soil
John Nimmo and Rose Shillito
Historical Development of the Global Water Cycle as a Science Framework
Richard G. Lawford and Sushel Unninayar
The global water cycle concept has its roots in the ancient understanding of nature. Indeed, the Greeks and Hebrews documented some of the most some important hydrological processes. Furthermore, Africa, Sri Lanka, and China all have archaeological evidence to show the sophisticated nature of water management that took place thousands of years ago. During the 20th century, a broader perspective was taken and the hydrological cycle was used to describe the terrestrial and freshwater component of the global water cycle. Data analysis systems and modeling protocols were developed to provide the information needed to efficiently manage water resources. These advances were helpful in defining the water in the soil and the movement of water between stores of water over land surfaces. Atmospheric inputs to these balances were also monitored, but the measurements were much more reliable over countries with dense networks of precipitation gauges and radiosonde observations. By the 1960s, early satellites began to provide images that gave a new perception of Earth processes, including a more complete realization that water cycle components and processes were continuous in space and could not be fully understood through analyses partitioned by geopolitical or topographical boundaries. In the 1970s, satellites delivered quantitative radiometric measurements that allowed for the estimation of a number of variables such as precipitation and soil moisture. In the United States, by the late 1970s, plans were made to launch the Earth System Science program, led by the National Aeronautics and Space Agency (NASA). The water component of this program integrated terrestrial and atmospheric components and provided linkages with the oceanic component so that a truly global perspective of the water cycle could be developed. At the same time, the role of regional and local hydrological processes within the integrated “global water cycle” began to be understood. Benefits of this approach were immediate. The connections between the water and energy cycles gave rise to the Global Energy and Water Cycle Experiment (GEWEX)1 as part of the World Climate Research Programme (WCRP). This integrated approach has improved our understanding of the coupled global water/energy system, leading to improved prediction models and more accurate assessments of climate variability and change. The global water cycle has also provided incentives and a framework for further improvements in the measurement of variables such as soil moisture, evapotranspiration, and precipitation. In the past two decades, groundwater has been added to the suite of water cycle variables that can be measured from space. New studies are testing innovative space-based technologies for high-resolution surface water level measurements. While many benefits have followed from the application of the global water cycle concept, its potential is still being developed. Increasingly, the global water cycle is assisting in understanding broad linkages with other global biogeochemical cycles, such as the nitrogen and carbon cycles. Applications of this concept to emerging program priorities, including the Sustainable Development Goals (SDGs) and the Water-Energy-Food (W-E-F) Nexus, are also yielding societal benefits.