41.4. Movement of Water in Soil

Infiltration

• Entry of water into soil surface.
• Measured using infiltrometers (double-ring or rainfall simulators).
• Influenced by soil texture, structure, initial moisture content, surface conditions.

Infiltration is the process by which water enters the soil surface. This is the first step in the movement of water in soil. To measure how quickly water can enter the soil, scientists use devices known as infiltrometers. These can be double-ring infiltrometers or rainfall simulators designed to replicate natural rainfall conditions on a smaller scale. Factors that affect infiltration include the texture of the soil (whether it is sandy, clayey, or silty), the structure of the soil (the arrangement of soil particles), the amount of moisture already present in the soil, and the condition of the soil surface (if it's compacted or covered by vegetation).

Think of a sponge. When you pour water onto a dry sponge, it soaks up the water quickly. This is how infiltration works with soil—if the soil is dry, it will absorb water readily. However, if the sponge is already wet, it may not absorb much more water. Similarly, a compacted or crusty surface on the soil can prevent water from entering easily, just like a solid plastic surface wouldn’t absorb water.

Percolation

• Downward movement of water through soil profile.
• Occurs after saturation of upper layers.
• Important in groundwater recharge.

Percolation refers to the downward movement of water through the layers of soil. This process starts after the upper layers of soil become saturated, meaning they hold as much water as they can. Once there is excess water, gravity pulls it down through the soil profile, which is crucial for recharging underground water sources, or aquifers. Percolation helps distribute water evenly over different soil layers, allowing plants and organisms to access water at different depths.

Imagine a coffee filter filled with coffee grounds. When you pour hot water over it, the water seeps down through the coffee grounds and collects at the bottom pot. This is similar to how water percolates down through the various soil layers after it rains or when irrigation is applied. Just like the coffee filter collects the brewed coffee, the soil collects and stores the water, nourishing plants and replenishing groundwater.

Capillary Rise

• Upward movement of water due to surface tension.
• Important in root zone water availability in fine soils.

Capillary rise is the ability of water to move upward through tiny pores in the soil due to surface tension. This phenomenon is particularly significant in finer soils, such as clay, where the spaces between soil particles are very small. The water can 'climb' against the force of gravity to reach the root zones of plants. Capillary action is essential for plants to access water that is not only supplied by heavy rainfall or irrigation but also by water that is naturally present in the soil profile.

Think of how a paper towel absorbs juice when you dip one end into a spilled drink. The juice travels up the paper towel against gravity due to capillary action. In similar fashion, water rises in the small spaces between soil particles, making it accessible to plant roots. This is crucial during dry periods when plants need to find water stored in the soil.

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