26. Infiltration

Infiltration is defined as the movement of water from the surface into the soil profile. It begins when rainfall or irrigation water is applied to the land surface and continues until the soil becomes saturated. It is expressed in terms of:

• Infiltration rate (f): Rate at which water enters the soil, usually in mm/hr or cm/hr.
• Cumulative infiltration (F): Total volume of water that has infiltrated per unit area over a given time period.

Infiltration refers to the process of water moving from the ground surface into the soil. It starts when water (from rain or irrigation) lands on the soil and continues until the soil can absorb no more. We measure infiltration in two main ways: the infiltration rate, which indicates how quickly water enters the soil (usually measured in millimeters or centimeters per hour), and cumulative infiltration, which tells us the total amount of water that has entered the soil over a certain time period.

Imagine pouring a glass of water on a sponge. At first, the sponge absorbs the water quickly until it becomes saturated, and then it can’t hold any more. This is much like how soil absorbs rainfall until it reaches its capacity.

Infiltration capacity refers to the maximum rate at which a given soil can absorb rainfall. It varies with time and soil conditions and is represented by a curve that typically decreases with time.

• Initially high due to dry soil and large capillary suction.
• Gradually decreases as the soil becomes wet.
• Eventually reaches a nearly constant value (steady-state infiltration).

Infiltration capacity is the maximum speed at which soil can absorb water. This rate isn’t static; it changes over time and depends on various conditions. Initially, when the soil is dry and has strong capillary suction, it can absorb water quickly. As the soil gets wetter, its ability to take in more water slows down until it reaches a consistent rate known as steady-state infiltration.

Think of a sponge again: when it’s dry, it can soak up water very rapidly. But as it fills up with water, you’ll notice it takes longer for new water to get absorbed until it eventually won’t absorb any more water.

Infiltration is influenced by several interrelated factors:

1. Soil Characteristics:
   • Texture: Sandy soils have higher infiltration due to large pores.
   • Structure: Well-aggregated soils allow more rapid infiltration.
   • Porosity and permeability.
   • Presence of impervious layers.
2. Vegetative Cover:
   • Reduces runoff velocity.
   • Enhances soil porosity through root activity.
   • Provides organic matter improving soil structure.
3. Soil Moisture Content:
   • Dry soils have higher initial infiltration due to suction.
   • As moisture increases, suction decreases, lowering the infiltration rate.
4. Rainfall Intensity:
   • If rainfall intensity > infiltration capacity → runoff occurs.
   • High-intensity rainfall often leads to surface ponding.
5. Land Use and Land Management:
   • Urbanization reduces infiltration due to impervious surfaces.
   • Tillage, mulching, and compaction affect soil structure and infiltration.
6. Temperature:
   • Affects viscosity of water and biological activity.
   • Frozen soils reduce infiltration drastically.

Multiple factors can influence how well water infiltrates into the soil. For instance, the type of soil plays a significant role; sandy soils are better at absorbing water than clayey soils because of their larger pore sizes. Vegetation also helps, as plants slow down water flow and their roots can create channels in the soil that make it easier for water to seep in. Moreover, the drier the soil, the easier it is for it to absorb more water, but as the soil gets wetter, this absorption rate decreases. The intensity of rainfall can also apply pressure to the land; heavy rains may cause flooding if the water saturates the ground quickly. Lastly, how land is used—such as urban development—can drastically reduce water infiltration due to the prevalence of impervious surfaces like roads and buildings.

Consider a sponge in different scenarios: when it’s sitting on the ground with water around it (like flooded land), it can only take on more water until it's full. If you cover it with plastic (like a city with concrete roads), it can't absorb water at all. Additionally, putting plants around it will allow it to absorb much more water, just like soil with vegetation increases its absorption capacity.

Various field methods are used to measure infiltration rates:

1. Infiltrometer Methods: (a) Double Ring Infiltrometer:
   • Consists of two concentric rings.
   • Water is maintained in both rings; infiltration from the inner ring is measured.
   • Reduces lateral flow effects. (b) Single Ring Infiltrometer:
   • Simpler but less accurate due to lateral water movement.
2. Basin or Flooding Method:
   • A known quantity of water is applied to a bounded area.
   • Change in depth over time gives the infiltration rate.
3. Soil Moisture Accounting Method:
   • Based on changes in soil moisture profiles before and after rainfall.

To measure how much water infiltrates into soil, scientists use various methods. One method is the infiltrometer, which can be double or single ring types. The double ring infiltrometer uses two rings, allowing for more accurate measurements by minimizing side water movement. Another method is the basin or flooding method, where a specific amount of water is spread over an area, and the drop in water level over time indicates the infiltration rate. Additionally, soil moisture accounting methods look at changes in water content in the soil before and after it rains to calculate infiltration.

Measuring infiltration is akin to figuring out how much juice a sponge can hold. If you pour juice into a bowl (the basin method), you can see how quickly the sponge absorbs it. Using different bowls (double ring vs. single ring infiltrometers) helps you get accurate readings, just like using precise measuring cups would when filling different sized glasses.

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