Freshwater is naturally occurring water that has a low concentration of dissolved salts. This distinguishes it from saline water. It typically contains less than 1,000 milligrams of dissolved solids per liter.
Common sources of freshwater include precipitation like rain and snow, which replenish rivers, lakes, and streams. Glaciers and ice caps also hold significant amounts of freshwater. A substantial portion is stored underground as groundwater within aquifers.
Defining Soil Moisture
Soil moisture refers to the water held within the spaces between soil particles, known as pores. This water originates primarily from precipitation and can also be introduced through irrigation. The soil absorbs and retains this water.
Water is held in the soil through adhesion, where water molecules cling to soil particles, and cohesion, where water molecules attract each other. Different forms of soil water exist. Capillary water, held in smaller pores, is readily available for plant uptake, while gravitational water drains downwards through larger pores. Hygroscopic water is tightly bound to soil particles and generally unavailable to plants.
Understanding the Relationship Between Soil Moisture and Freshwater
In most terrestrial environments, soil moisture is a form of freshwater. This is because the water entering the soil, primarily from rain or snowmelt, is inherently freshwater with low salt content. As this precipitation infiltrates the ground, it becomes soil moisture, retaining its freshwater classification as long as its dissolved salt concentration remains low. The water molecules are simply held within the soil matrix rather than flowing freely in a river or lake.
However, there are specific contexts where soil moisture may not be considered freshwater due to increased salinity. In coastal regions, saltwater intrusion can occur, where saline ocean water seeps into the soil, raising its salt content. Similarly, irrigation with brackish water, which has a higher salt concentration than freshwater, can lead to salt accumulation in the soil over time. In arid and semi-arid regions, high evaporation rates can concentrate naturally occurring salts in the upper soil layers, making the soil water saline. In these cases, despite its origin, the elevated salt levels mean the soil moisture no longer fits the definition of freshwater.
Importance of Soil Moisture for Ecosystems
Soil moisture, as a primary source of freshwater for land-based life, is fundamental for sustaining ecosystems. It serves as the medium through which plants absorb essential nutrients from the soil, dissolved in the water. This water is also a direct reactant in photosynthesis, the process by which plants convert light energy into chemical energy.
Plants draw significant amounts of soil moisture through their roots for transpiration, a process that cools the plant and helps transport nutrients. Adequate soil moisture supports robust agricultural productivity, directly impacting crop yields and food security. Beyond individual plants, soil moisture plays a crucial role in the broader water cycle. It influences evaporation, runoff, and the recharge of groundwater reserves, thereby supporting diverse terrestrial habitats and the organisms within them.