Trees and Rain

Is deforestation increasing flooding events around the globe?

An article in the Buenos Aires Herald sparked my interest in writing this post. I have touched upon the effect deforestation has had on flooding, very briefly, in my previous blog post and I have decided to dedicate an entire post to quite a topical area of interest. The floods that have struck South America (Argentina, Paraguay and Brazil) occurred as a result of heavy rain brought on by an El Nino event. Interestingly, the countries that were most effected by this rainfall were the countries that had experienced the greatest amount of deforestation in recent years due to increased demand for Soya.

Citizens in Cobija (Bolivia) wade through flood waters that have destroyed their homes (Al Jazeera)

How does deforestation cause/influence flooding?

The relationship between trees and flood events is one that appears to be very simple in that trees reduce the impact of flood events but in reality it is much more complicated. In simple terms trees reduce runoff to a river in several ways:

• Intercept rainwater by capturing it in leaves
• Evaporation/transpiration
• Leaves reduce raindrop impact meaning there is less soil erosion. 
• Tree roots absorb water droplets from the soil.
• Tree roots hold the soil together reducing sediment movement and preventing landslides. 

With floods the most common natural disaster in tropical areas, most of the academic research has been focused on South East Asia and the Amazon. Tan-Soo et al. (2014) who explored flooding in Malaysia came to the same conclusion as the article in the Buenos Aires Herald. They concluded that the conversion of tropical forests to oil palm plantations increased the number of days flooding occurred, but only during the wettest months of the year. A similar study found the same to be occurring in Amazonian Peru where the height of annual flood waters had increased over the last decade in correlation with deforestation (Gentry and Lopez-Parodi 1980). However, there has been criticisms of this study (Nordin et al. 1982) because it drew conclusions that were not supported in the evidence/data. It assumed river stage correlated directly with river discharge which as Nordin et al. stated was not always the case. Furthermore, Gentry and Lopez-Parodi dismissed an increase in flood heights due to precipitation despite analyzing the years with unusually high precipitation. 

Clark (1987) is critical of research studies into land use change (mainly that of converting deforestation into farmland) suggesting that these studies are limited because they only ever focus on 1 or 2 catchments and that this is not representative of all catchments. This is indeed true, the two studies discussed already only focused on 1 catchment in a tropical region. Clark does make an interesting point that  the knowledge that trees can reduce flooding has been around for centuries - in 14th century Italy it was suggested that afforestation in upland areas could combat flooding - so why are we continually clearing forests and making the same mistakes?

Lets throw in some statistics - a 10% decrease in forest cover increased flood frequency between 4-28% and increased flood duration by 4-8% (Bradshaw et al. 2007). 

It is widely agreed that on a local scale trees mitigate medium scale flood events but on a large scale flood event this is not the case  (Tan-Soo et al. 2014; The Economist 2005). The Economist offers a very different take on flooding compared to the previous articles and blames economics as the cause of flooding rather than widespread forest clearing in South East Asia. Towns and cities are situated on hazardous areas such as flood plains because of the significant economic benefits of situated near a river or the coast - transportation, trade, sanitation, food source, employment etc. Despite looking at the cause of flooding in economic terms, the article says measuring flooding in economic loss rather than geographical extent of the water gives the impression that the floods are more severe than they actually are. 

It is a really interesting article and a great read - it can he found here

Deforestation on a floodplain (source)

Flooding within a river basin is influenced by other factors such as hydrology, as well as deforestation, but the focus of this post has been to explore flooding as a result of deforestation. It is a controversial topic with which different people have different opinions. It has further helped understand the overarching question of this blog - is deforestation a necessary evil? - by exploring some very negative impacts of deforestation. Flooding demonstrates how deforestation erodes human life and the economies of developing nations and illustrates the need for greater forest protection policies in areas that are already prone to flooding. 

Case Study: A History of Deforestation in the United Kingdom

As the jingle of Santa's sleigh approaches the inspiration for this post came from the dead tree standing next to me covered in baubles and tinsel. This blog post has examined deforestation on various corners of the globe - the Amazon, Borneo, America and Africa - but is yet to explore the deforestation of trees in the UK. This is what this blog post will do - it will explore deforestation in Britain.

A History of Deforestation

Woodland Cover in England (Forestry Policy Statement)

Long ago Britain was covered in a primeval forest of mighty oaks and sharp pines that stretched from North to South. The significant clearing of trees started during the Mesolithic and Neolithic periods of our history which can be shown in the pollen records (Brown 2002).  It was initially thought that deforestation was the result of conversion of woodland to agricultural land (Edwards and Hirons 1984) - similar to what we see in Africa and the Amazon today. However, archaeological evidence is contradictory to the pollen evidence that supported this hypothesis. The decline in tree pollen ad increase in grass pollen at this time was caused by purposive deforestation - a concept that embodies a theory of planned alteration to the environment. In this case the creation of clearing within the forests for hunting.

A post online offers a brief account of when deforestation occurred for different regions around the UK with most woodland (particularly that in the South East and East Anglia) clearing during the Bronze Age or the Roman Period. An article published in Science Letter in 2003 whilst brief showed the link between flooding and climate change  but also showed the importance of land use change in flooding. Flood risk has increased over the last 4000 years since the Bronze Age because woodland has been turned into farmland. In natural river basins, trees intercept rainwater and slows the runoff process or returns the water back into the atmosphere via transpiration. When deforestation occurs, this runoff increases creating a shorter lag time and a larger peak meaning it is more likely for the river to burst its banks. This was demonstrated in 2004 in the Cornish village of Boscastle in which land use change within the basin increased runoff (although it was a combination of many factors that resulted in the flash flood).

As the graph above demonstrates that was a rapid reduction in forest cover following and during the industrial revolution and during WWI. However, forest cover increases after the 1920s following the Acland Report in 1918 and the formation of the Forestry Commission in 1935 that implemented the formation of forest parks and afforestation techniques by 1935 (Smout et al. 2007*).

 In recent years, with a greater awareness of the environment, forest cover has increased under various government and non-government initiatives. Levy and Milne (2004) provide an intriguing synthesis into recent deforestation rates. Deforestation rates must be reported under the Kyoto Protocol with 500 hectares of unlicensed deforestation in the UK in recent years (since 1990). The article makes an interesting point that variations in techniques for estimating deforestation since 1990 yield different results. The results range from 1000 hectares to 7000 hectares. The article settles on the value of 1375 hectares since 1990.