Centre for Environmental Management, University of the Free State

These are anomalies that seem to defy logic, until we examine the processes in greater detail. Today we will be focusing on the notion of a green drought. This is a condition when rainfall coexists with sub-optimal growth conditions for plants and other living organisms. In a green drought it looks as if there is enough water – maybe even an abundance – but there is also clear evidence of water-induced stress in the system. Let us unpack this to gain a deeper understanding of how it works.

As a point of departure, the reader needs to understand that all water begins as precipitation – as either dew, fog, rain or ice. This is when water changes state from either vapour or solid to liquid form. This results in a volume of water being released at a given moment in time. Two things happen at this moment of release, because it is about how much water is present, and the period of time over which the release of water is taking place. The two dimensions are therefore volume and time. How much is there, and for how long is it there? Most people think only of the first part by focusing on the volume, ignoring the second part, which is about the duration of availability.

Why is the duration of availability important?

Simply put, a unit of water that is released suddenly is of less value than a unit of water that is released slowly, and for very good reasons. Water released suddenly can be called a flood, which moves with high energy across the landscape. It is that high energy that causes damage to the built environment, and erosion in farmers’ fields. Water released slowly has lower energy in it, so instead of flowing across the surface of the landscape, it infiltrates into the soil in a controlled and gentle way. This process is of vital importance for water security because slow infiltration provides three important benefits. Firstly, it saturates the shallow upper layer of the soil, which is where the roots of most plants are found. It is also the layer of soil in which trillions of microorganisms exist. It is these colonies of microorganisms that create soil fertility because they metabolise chemical elements and convert them to nutrients needed for plants to grow. In hydrological terminology we refer to this as soil water, typically found as a thin film adhering to the soil particles, leaving tiny spaces that become habitat for the microorganisms referred to above.

Secondly, it penetrates the deeper layers of soil, beyond which no roots grow. This deeper layer is of great importance because it is the transition zone between surface water and groundwater. Technically, this portion of the soil profile is known as the vadose zone in the terminology of geohydrology. It is characterised by the relative absence of oxygen, but also the absence of free-standing water. In other words, it is damp to the touch, but is not saturated, so when squeezed it will not yield free-flowing water. The importance of this zone in the context of a green drought is that only slow-duration release of water onto the surface has the time available to pass through to deeper aquifers. This is defined mathematically by a balance between the upward force of capillary attraction and the downward force of gravity. A balance of these forces creates the lateral movement of water in this portion of the soil profile. Technically this is calculated by geohydrologists and soil scientists using the Richards Equation, the exact details of which are irrelevant to the reader.

Thirdly, only once the soil water has been drawn by gravity through the multiple obstacles in its way does it finally enter the third zone, which is saturated. This is called phreatic water, because it is freely available, and permanently present, but subject to long-term pulses defined by rainfall events. Stated simply, the more long-duration rainfall events we have, the greater is the time available for water to slowly percolate down through the root zone, into the vadose zone, and finally into the saturated zone. The interface between the saturated and unsaturated zone is called the capillary fringe and it defines the water table. Water in the saturated zone is modelled mathematically by means of Darcy’s Law, which is used to calculate the flow and potential yield of a given borehole or well.

From this simple explanation we can see that rainfall in and of itself is not enough to break a drought. To do that, the rainfall must be persistent for a defined period. It is the volume multiplied by the time that creates flow. A big volume in a small time has high energy, so it is destructive as a violent flood, without penetrating the soil profile. A small volume over a longer duration is capable of deeper penetration, and because it has lower energy levels, this type of rain is not destructive. In some communities this sort of rain goes by colloquial names such as ‘geelperske reën’ (yellow peach rain), which is slow and gentle, often lasting for a week or more. Green drought occurs when localised water stress persists, typically in the root zone of the soil profile. This manifests as wilting of plants, despite recent rains, often triggering a decision to irrigate a garden, which in turn can overload a municipal water reticulation system if many people do this simultaneously. We saw this recently in parts of South Africa that were experiencing heavy rain accompanied by high temperatures, which caused a spike in demand at the same time that load shedding was reducing the capacity to pump and process water from rivers.

This is a simple explanation of a green drought, but there is a more important issue that the reader also needs to understand. The upper layer of soil is relatively shallow, which is why it is vulnerable under high temperatures. Not only does this heat cause water to evaporate faster, but it also increases the temperature of the soil surface. It is this elevated temperature that becomes destructive to soil fertility, because the trillions of microorganisms that inhabit the shallow topsoil are unable to survive temperatures above a certain threshold. Cultivated soil exposed directly to the sun can reach temperatures in excess of 40°C, which stresses living microorganisms and destroys fertility. This is why mulch on the soil in gardens and cultivated fields is so important. A layer of mulch slows down the flow of water, increasing the retention time as explained above, but also providing enough shade to lower the surface temperature. It must be remembered that closer to the surface, oxygen is most freely available to microorganisms, but higher temperature also poses a greater risk to the survival of bacteria, fungi and archaea that recycle nutrients.

Climate change is altering our weather events, so we are seeing heavier rain over shorter duration, with less retention time to penetrate the root zone and ultimately flow into deeper aquifers. Our best solution is to understand that soil is habitat for trillions of microscopic living organisms that perform useful functions, so mulching is both useful and will increasingly be necessary in the management of estates. Slow water is low-energy water that has the greatest benefit to humans. Angry water is not our friend and needs to be tamed through human intervention if we are to limit damage and mitigate the impact of green drought.