Making every drop of rainwater count
Farmer's Weekly|November 20, 2020
In this first of a two-part series, Dr Louis du Pisani, an independent specialist agricultural consultant, explains the basic components of the water cycle and how these apply to the growth of grass on the veld.
Dr Louis du Pisani
Drought is a recurring phenomenon in South Africa. Although low rainfall is the main cause of drought, the effectiveness of rainfall also plays an important role. Farmers have no control over the amount of rainfall they receive, but they can control how effectively it waters their crops or the veld. To better understand why, one needs to have a broad understanding of the water cycle.

THE BASICS

Let’s start with the precipitation part of the cycle (see illustration). Precipitation is water that falls to earth under gravity when atmospheric water vapour condenses to form clouds. The main forms of precipitation are rain, drizzle, sleet, snow, ice pellets, graupel (also called soft hail) and hail. Precipitation can have two fates: it can be so light that it evaporates before it reaches the ground (these so-called virga are seen as wispy streaks of rain under a cloud) or it can reach the ground. The precipitation that falls on the ground either infiltrates into the soil or flows over the surface, ending up in rivers and streams (called run-off). Run-off is lost to the livestock farmer, as it does not contribute to veld production even though it can be measured in the rain gauge.

The water that infiltrates into the soil will do one of three things: percolate deep into the subsoil and replenish the groundwater; evaporate directly from the bare soil surface back into the atmosphere; or be absorbed by plants and transpire back into the atmosphere.

The water that evaporates from the soil surface or percolates into the underground water supply is also lost to the livestock farmer, as it is not absorbed by plants and does not contribute to veld production. The water that transpires from the plant into the atmosphere is the only portion of the precipitation that contributes to grass production and is thus the effective portion of the rain received.

In the final part of the water cycle, the water that has evaporated from open water sources, plants or bare soil condenses to form clouds, which then again produce precipitation.

To repeat: only that portion of the rain that infiltrates into the soil and is then transpired by grazing plants leads to forage production. The rest is effectively lost (to the livestock farmer at least).

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