Factbox 5.
Infiltration And Surface Runoff

By Professor Nils Nykvist

The soil’s uptake of water is affected by the form in which precipitation reaches the ground surface.

With a nice, quiet rain, water is sucked up into the fine pores and spreads by capillary action further down into deeper layers. During a heavy rain or after snowmelt, however, the largest pores are filled first. In a coarse, homogeneous soil such as gravel or coarse sand, the water drains quickly through the upper soil layers down to the groundwater.

In fine-grained soils with good structure there are fine as well as large pores. The structure that is affected by the biological activity when litter decomposes deteriorates with depth. The relatively rapid water movement into the soil is therefore reduced, and water begins to be sucked into the finer pores.

In clay soils with poor structure, the amount of water that can infiltrate into the ground is often less than the amount of rainfall during the same time, and the water can remain standing on the ground, unless it evaporates or runs off the surface.

On slopes, the water either runs off the soil surface or is infiltrated into the soil and moved downwards to the groundwater, towards lower levels of the terrain. Even if the water running off from the soil surface can be utilized by plants further down the valley, it depletes the soil on the slopes by moving away the most nutrient-rich soil particles.

Dense vegetation increases infiltration and reduces the surface runoff of water by forming organic substances that glue together clay particles into larger aggregates.

In addition, plants and their litter on the ground provide good protection from rain-drops that would otherwise shatter the aggregates and release the individual clay particles that can clog the larger pores. Even rocks and boulders provide good protection from rain-drops.

To decrease the surface runoff, it is of great importance that agriculture and forestry are conducted in such a way that the soil is not compacted or left open more than necessary, leading to a poor use of water.

A common way to reduce runoff is to build various forms of terraces or level bunds. To prevent erosion on the terraces, it is important to start building them on the upper part of the slopes, where the water force is not so great. In many cases, simple vegetation-covered embankments are enough to reduce the force of the flowing water.

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