The hydrological cycle has fascinated humanity for millennia, with a diversity of ancient civilisations known to manipulate river systems for agricultural purposes, and adapt – although not always successfully – to periodic floods and droughts. It was not until seventieth century, however, that hydrology (literally, the study of water) was born as a quantitative science, with some of the first ‘hydrologists’ taking measurements of both rainfall and river flows in catchments in France, and showing that the former was sufficient to cause the latter. Since that time, continued research and developments in land, ocean and space-based observing systems has meant that we now have a good idea about the state and movement of water throughout the globe, and the principal factors driving that movement.
We now know that the hydrological cycle exhibits a remarkable consistency and predictability in its behaviour. The vast majority (over 85%) of the world’s evaporation is sourced from the oceans, and this water is transported via relatively stable atmospheric circulation pathways so that it arrives preferentially in some regions such as the tropics, and bypasses others, such as the world’s deserts. Some regions experience rainfall distributed evenly throughout the year, whereas others have distinct dry and wet seasons. In the ‘Mediterranean’ climate zones such as in southern Europe, California, southern Africa, and parts of southern Australia it rains preferentially in winter, while closer to the tropics preference is given to summer rainfall.
It is this consistency which has dictated where humans will live, and what civilisations will grow and thrive. It is not for nothing that many of the world’s highest population densities occur in ‘hydrologically stable’ zones where water is both plentiful and predictable. Floodplains in particular have attracted humans since the advent of agriculture, as these regions not only have some of the most fertile soils (thanks to the power of water in eroding and transporting silts from further upstream) but also the flows in rivers in the lower reaches of catchments have the important feature of being more stable (and thus more predictable) than further upstream.
Of course other factors – most importantly technology and trade – also play an important role, and developments in both have allowed great metropolises to spring up in deserts and other marginal zones. Because of our capacity to transport food and other resources over great distances, desalinate sea water and genetically modify crops to grow on increasingly arid lands, we are becoming less dependent on, and less connected to, the vagaries of the hydrological cycle. Nevertheless humanity has always shared a deep and fundamental relationship with water, implicitly influencing more of our behaviour than we realise.
Because of this, deviations from the usual behaviour of the cycle can surprise us, and sometimes these deviations can lead to catastrophe. Droughts can cause agricultural failure, economic losses and – in countries without the financial wherewithal to acquire food from elsewhere – famine. Floods, too, can create disaster, often causing widespread damage to property and infrastructure, displacement of people and contributing to the spread of water-borne disease. With the emergence of human-induced climate change pushing the hydrological cycle outside of the envelope of its natural historical fluctuations, and with continued global population growth forcing people to move into ever more marginal land, dealing with the hydrological extremes of floods and droughts will only become more challenging.
This blog will try to cover many of the issues described here, focusing on my interest in the extremes of the hydrological cycle. After all, it is these extremes which have the most influence on our behaviour, attract the most publicity, and can also on occasion evoke the most distorted media commentary. I would love to hear your thoughts, ideas and suggestions. I will try to post something regularly, but, like the hydrological cycle, I cannot guarantee the absence of the occasional drought.