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Part of the water applied to the soil surface drains below the rootzone and feeds deeper soil layers which are permanently saturated; the top of the saturated layer is called groundwater table or sometimes just water table (see Fig. 40).

Fig. 40. The groundwater table

2.5.1 Depth of the groundwater table

The depth of the groundwater table varies greatly from place to place, mainly due to changes in topography of the area (see Fig. 41).

Fig. 41. Variations in depth of the groundwater table

In one particular place or field, the depth of the groundwater table may vary in time.

Following heavy rainfall or irrigation, the groundwater table rises. It may even reach and saturate the rootzone. If prolonged, this situation can be disastrous for crops which cannot resist “wet feet” for a long period. Where the groundwater table appears at the surface, it is called an open groundwater table. This is the case in swampy areas.

The groundwater table can also be very deep and distant from the rootzone, for example following a prolonged dry period. To keep the rootzone moist, irrigation is then necessary.

2.5.2 Perched groundwater table

A perched groundwater layer can be found on top of an impermeable layer rather close to the surface (20 to 100 cm). It covers usually a limited area. The top of the perched water layer is called the perched groundwater table.

The impermeable layer separates the perched groundwater layer from the more deeply located groundwater table (see Fig. 42).

Fig. 42. A perched groundwater table

Soil with an impermeable layer not far below the rootzone should be irrigated with precaution, because in the case of over irrigation (too much irrigation), the perched water table may rise rapidly.

2.5.3 Capillary rise

So far, it has been explained that water can move downward, as well as horizontally (or laterally). In addition, water can move upward.

If a piece of tissue is dipped in water (Fig. 43), the water is sucked upward by the tissue.

Fig. 43. Upward movement of water or capillary rise

The same process happens with a groundwater table and the soil above it. The groundwater can be sucked upward by the soil through very small pores that are called capillars. This process is called capillary rise.

In fine textured soil (clay), the upward movement of water is slow but covers a long distance. On the other hand, in coarse textured soil (sand), the upward movement of the water is quick but covers only a short distance.

Soil texture	Capillary rise (in cm)
coarse (sand)	20 to 50 cm
medium	50 to 80 cm
fine (clay)	more than 80 cm up to several metres

2.6 Soil erosion by water

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2.6.1 Sheet erosion
2.6.2 Gully erosion

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Erosion is the transport of soil from one place to another. Climatic factors such as wind and rain can cause erosion, but also under irrigation it may occur.

Over a short period, the process of erosion is almost invisible. However, it can be continuous and the whole fertile top layer of a field may disappear within a few years.

Soil erosion by water depends on:

– the slope: steep, sloping fields are more exposed to erosion;
– the soil structure: light soils are more sensitive to erosion;
– the volume or rate of flow of surface runoff water: larger or rapid flows induce more erosion.

Erosion is usually heaviest during the early part of irrigation, especially when irrigating on slopes. The dry surface soil, sometimes loosened by cultivation, is easily removed by flowing water. After the first irrigation, the soil is moist and settles down, so erosion is reduced. Newly irrigated areas are more sensitive to erosion, especially in their early stages.

There are two main types of erosion caused by water: sheet erosion and gully erosion. They are often combined.

2.6.1 Sheet erosion

Sheet erosion is the even removal of a very thin layer or “sheet” of topsoil from sloping land. It occurs over large areas of land and causes most of the soil losses (see Fig. 44).

Fig. 44. Sheet erosion

The signs of sheet erosion are:

– only a thin layer of topsoil; or the subsoil is partly exposed; sometimes even parent rock is exposed;

– quite large amounts of coarse sand, gravel and pebbles in the arable layer, the finer material has been removed;

– exposure of the roots;

– deposit of eroded material at the foot of the slope.

2.6.2 Gully erosion

Gully erosion is defined as the removal of soil by a concentrated water flow, large enough to form channels or gullies.

These gullies carry water during heavy rain or irrigation and gradually become wider and deeper (see Fig. 45).

Fig. 45. Gully erosion

The signs of gully erosion on an irrigated field are:

– irregular changes in the shape and length of the furrows;
– accumulation of eroded material at the bottom of the furrows;
– exposure of plant roots.