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To explain the meaning of the term ‘causality’ in geography we can paraphrase the words of the scientist Sir Isaac Newton: ‘for every action, there is a reaction’. Causality is the relationship between cause and effect. Across the subject, in both physical and human contexts, we recognise that a variety of processes result in change; these changes have impacts, which in turn have consequences. Such consequences may be beneficial or problematical – if the latter, they will need to be addressed. Either way the consequences will need to be managed effectively, although disagreement may arise over the best form of management which in turn may generate conflict (Figure 1).
Figure 1. The concept of causality
We can examine these ideas with reference to two geographical phenomena – one physical (overland flow) and one human (international migration).
Cause and effect – overland flow
It is well-known that torrential rain (a cause) leads to widespread flooding (an effect). Flooding has a number of consequences, and needs careful management. However, at a smaller scale the origin of overland flow (or runoff) has been the source of much theoretical study.
The classic theory is that of Robert Horton, an American hydraulic engineer. According to Horton’s theory, overland flow does not occur unless the rainfall intensity exceeds the infiltration capacity of the soil (Figure 2(a)). He saw the process developing as follows:
· infiltration capacity declines until rainfall intensity exceeds infiltration rate
· a thin layer of water forms on the soil surface and accumulates in small depressions which, when full, begin to overflow
· individual areas of flow join up, as rills, and in time overland flow arrives at the channel
· if the rainfall continues, widespread overland flow is generated across the drainage basin and the entire area contributes to the runoff of the river.
The weakness in Horton’s theory is that such ‘infiltration-excess overland flow’ does not occur everywhere, only regularly in dry un-vegetated areas. In the grassy plains and hills of the UK it rarely, if ever, occurs.
An alternative theory is that of John Hewlett, another American hydrologist. He noted a different sequence by which rainfall is converted into overland flow (Figure 2(b)). Since rainfall intensities are nearly always well below the infiltration capacity of the soil, rain water will enter the soil and rock and percolate vertically downwards. However, any reduction in permeability, especially at the soil–bedrock junction, will cause the water to deflect in a downslope direction. This means that soils will tend to be wetter at the base of a slope, nearer the stream channel, so this is the dynamic zone for the development of overland flow. The molecules of water going into the soil are not the same ones that come out into the channel. If the soil becomes totally saturated, then any extra water must flow over the soil surface. This is ‘saturation-excess’ overland flow.
This discussion illustrates the complexity of causality – the effect may be similar, but the causes are different.
Figure 2. Types of overland flow
Cause and effect – international migration
Now consider the factors that cause people to migrate. Many factors influence whether a person or a family will migrate. For most, these factors are deeply intertwined. The left hand side of Figure 3 shows the various ‘drivers’ that may cause someone to migrate. However, other socio-economic factors play a role too, including personal characteristics and the degree to which migration can be facilitated or hindered – the right hand side of Figure 3. Ultimately an individual decision has to be made – to migrate or to stay.
Figure 3. The drivers of migration
Why do so many West and North Africans risk their lives to reach Europe?
A contemporary example of international migration concerns the movement of people from the Sahel of Africa to Europe. For the people who migrate there are few alternatives. Desertification and swelling populations outstrip agricultural productivity in much of the Sahel. Resurgence of conflict in the Central African Republic, Mali, northern Nigeria, Somalia, South Sudan has continued to displace millions, pushing ever more young men and women north towards Europe. Every new outbreak of conflict adds to the pressure to move; camps in Mauritania and Burkina Faso are all overflowing with refugees from previous rounds of fighting. The current conflict (May 2023) in Sudan has already added to the movement of people from this region. It is not surprising that Europe beckons.
The decision to migrate may be influenced by a number of factors. Africa has the fastest population growth rate in the world and, although the continent is making great economic gains, it has broadly failed to translate these gains into sustainable livelihoods for its youth. Social and economic deprivation, conflict and crime in several countries throughout the continent cause many Africans to seek out new opportunities across the Mediterranean.
This journey is ‘facilitated’ by smugglers and criminal groups that provide a range of ‘services’ such as transportation, fraudulent identification, corruption of border officials and settlement services. Smugglers co‑ordinate with others to act as guides in the journey across the Sahara Desert. Although some smuggling networks are organised criminal structures, many are loosely-linked chains of individuals, which make it challenging for authorities to dismantle. In order to access these ‘services’ potential migrants need resources (money and contacts), which are not available to everyone. Often families pool finances to enable the most educated and/or willing person, with the greatest potential to send money home, to migrate.
The picture therefore is not straightforward - the ability to migrate is a function of mobility and resources (both financial and social). In other words, the people who are most vulnerable are not necessarily the ones most likely to migrate.
Conclusion
The principles of causality illustrate the inherent complexity in our subject. They demonstrate perhaps why we should avoid a strict compartmentalising of our work into Physical and Human sections. It may be worth reflecting on where else one of the key principles at the heart of our subject cuts across this traditional divide.
[Cover photo: Al Jazeera]