Subsurface Flow of Groundwater
Groundwater is one of the most important sources of fresh water on Earth. After precipitation infiltrates and passes through the soil, it collects in aquifers and moves slowly beneath the Earth’s surface through the subsurface. This subsurface flow is an important part of the hydrologic cycle and plays a major role in the water supply of many arid and semi-arid regions. Despite its importance, subsurface flow still remains one of the least understood parts of the hydrologic cycle, and is especially difficult to measure and quantify.
Subsurface flow occurs when water moves through pores in the subsurface, such as in sand and other granular materials. It is driven by the hydraulic gradient, which is created by a difference in hydraulic head between two points in an aquifer. Every point in an aquifer has a different water pressure, and the difference in water pressure between two different points creates a driving force that causes subsurface flow. This subsurface flow is driven by gravity, and can occur both vertically (downward) and horizontally.
The rate at which subsurface flow occurs is affected by a variety of factors from the water chemistry to the subsurface materials it flows through. A few of the most important of these influences include porosity, permeability, ground cover, and vegetation. Porosity and permeability refer to the amount of open space between soil and rock particles and their ability to move water through those spaces, respectively. Ground cover, such as soils and sediments, affects the water’s ability to infiltrate and re-charge the aquifer. Vegetation can also affect subsurface flow by taking up water and releasing it through transpiration.
Due to the importance of subsurface flow in the hydrologic cycle, it is important to quantify and understand it better. However, due to the difficulty of accessing and measuring subsurface materials, most of what we know about subsurface flow comes from computer models of porous media. These models are used to simulate the subsurface flow and can be used to predict future conditions or answer a variety of research questions.
Groundwater is essential for life and subsurface flow is an important part of the water cycle that can have far-reaching effects. Despite this, it is still a poorly understood process and much work remains to be done to fully understand its dynamics. Computer models can help us to better quantify subsurface flow and make more accurate predictions, but monitoring and instrumentation of the subsurface are also needed to further our understanding of this important part of the hydrologic cycle.
