Climate and water cycles

We live on the planet of water, and without it there would be no life, so it is an essential resource that we must take care of. The amount of water in its different forms (liquid, gaseous and solid) does not vary over time, and it could be said to be relatively abundant.

However, it should not be forgotten that only 1% is freshwater; found in lakes, rivers, and other readily available surface forms.

_{Figure 1: Only 2.4% of water on Earth is freshwater, and less than 1% of freshwater is easily accessible to living things.}

Fortunately, humans have been able to make more water available for cultivation through the application of well-drilling technology, desalination*, or rainwater harvesting. However, it is crucial to manage water for our crops in the most efficient and reasoned way possible, as it is increasingly becoming a scarce commodity. Not because there is less of it, but because of climate changes, we indirectly influence the distribution of water on earth.

_{*Desalination: the removal of salts and minerals from a target substance, such as soil or water.}

Understanding the water cycle

There are, in principle, 5 phases of the water cycle.

1. Beginning with evaporation, which is driven by the sun heating water surfaces such as oceans, seas, rivers, lakes, etc., resulting in a change from a liquid to a gaseous state. Likewise, the transpiration of plants involves the emission of water vapour into the atmosphere, which, when added to the evaporation of water from the soil, results in the combination of both processes, known as evapotranspiration.
2. This is followed by condensation of water vapour that rises and meets cooler layers of the atmosphere. Clouds or fog is formed, which may be carried by air currents, eventually turning to liquid form, discharging the water as precipitation.
3. Precipitation reaches the earth’s surface.
4. Much of that water evaporates quickly.
5. The remaining water can take several courses. First, it can infiltrate into the soil and be retained in aggregates, and then be used by crops and other living things. It can also percolate* (vi) to deeper layers, recharging aquifers, or surface runoff can occur due to slopes and terrain features, to eventually re-enter rivers, lakes, lagoons, seas and oceans. This phenomenon generally occurs when there is no more “room” for water in the soil, or when the infiltration capacity of the soil is less than precipitation. In these cases, erosion and unwanted soil loss can occur.

_{*Percolation: the process by which water moves downward through the soil under gravitational forces.}

_{Figure 2: Water from the land and oceans enters the atmosphere by evaporation or sublimation, where it condenses into clouds and falls as rain or snow. Precipitated water may enter freshwater bodies or infiltrate the soil. The cycle is complete when surface or groundwater reenters the ocean. Source: Modification of work by John M.Evans and Howard Periman, USGS.}

Climate change and impact on the water cycle

Changes in climate will affect both the quality and quantity of water available to humans and the environment. Extreme weather events such as droughts, floods, melting glaciers, sea level rise and storms are intensifying with severe consequences. There is therefore a clear need to adapt, to develop climate resilience.

In that sense, farmers are an important part of this adaptation process, as much of the freshwater consumed is used for agricultural production, whether plant or animal. Apart from being more efficient in water use, agricultural practices that improve water capture and retention capacity in the soil, and avoid practices such as intensive tillage, which result in loss of soil and fertility, should be pursued.

The use of vegetation cover to adapt

Although it may seem counter-intuitive, the use of groundcover can help us in this adaptation process. Although it may be an element that competes with the crop, if properly managed, it can even improve crop productivity.

Broadly speaking, the use of plant cover will reduce runoff and soil erosion (and therefore fertility) by improving the infiltration capacity of the soil. This is important to take into account in situations of increasingly frequent heavy rainfall. In addition, the use of groundcover generally leads to an increase in soil organic matter, thus improving soil structure, water retention capacity, and potentially soil fertility.

To summarise, the benefits of using cover crops on soil properties, crop management and the environment are numerous and well documented through experience and scientific literature. However, canopy management must consider the potential detriment of excessive competition with the crop for water and nutrients, and adapt to circumstances of low water availability and high atmospheric demand.

Agriculture is an economic activity and to be sustainable over time it must be profitable. In this sense, it is essential to adapt management strategies to different situations.

The competition of a canopy can be regulated in different ways, basically by regulating the space it occupies, the length of its active period and the intensity of its activity. Regulation must be planned specifically for each region, each crop and for the weather conditions that occur each year. Control of the intensity of canopy activity and competition is based on limiting leaf area development and thus water consumption through transpiration, either by selecting species that produce little biomass, or by control through mowing or weeding.

If the need for intervention on the canopy is high in terms of frequency of clearing, this could lead to a situation that is of little economic interest in terms of CO₂ and other emissions. On the other hand, the lower the canopy development, the lower the benefits in terms of increased organic matter and carbon sequestration, although they can fulfil other functions such as limiting erosion or improving soil structure.

The use of groundcover is just one of many techniques that can be used to mitigate the effects of climate change on the water cycle. There are other techniques such as keyline design, agroforestry, or direct rainwater harvesting in ponds that will be discussed in the next steps.

In any case, part of the success lies again in understanding the soil not only as physical support for plants, but as a very important element of the agro-ecosystem that must be respected, and promote what has been formed naturally over time.