INDUSTRY JOURNAL Vol . 8
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 freestanding 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 longduration 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 .
Dr Anthony Turton
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