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The impacts of agricultural land use change on surface run-off and soil erosion
Agriculture has always played a role in the UK economy, going through many improvements in yield and outputs in alignment with technology advancements; such as the industrial Revolution where UK farms saw agricultural labour productivity levels a third higher than the French due to vast improvements in mechanisation (Floud and McCloskey, 1995). This has caused agricultural land use change over the years. Defra publishes a report annually outlining this change in the UK. They have devised a measure known as utilised agricultural area (UAA) which is “made up of arable and horticultural crops, uncropped arable land” excluding “woodland and other non-agricultural land” (DEFRA, 2018). The report illustrates the change over the last 4 years being minimal, with 17240 thousand ha in 2014 (71% total land use) increasing to 17361 thousand ha in 2018 (71% total land use). (DEFRA, 2018). However, there has been a slow decline in the UAA in the UK since the second world war (Mitchell, 1988), where UAA was 50% higher than the previous year (House of Commons, 2019). Despite this decline in the UAA there has been an increase in the intensity of farming practices.
Impacts on soil erosion and surface runoff
Soil erosion as defined by John Boardman is, “the detachment and transport of particles or aggregates of soil” (Boardman, 2013). Agriculture has impacted rates of soil erosion and surface runoff in the UK through ultimate issues such as; the common agricultural policy encouraging farmers to grow crops even when supply is lacking, farmers growing crops out of season, increase use of machinery especially ploughing and increasing populations requiring greater quantities of food. Additionally, immediate causes such as climatic patterns. (Boardman, 2013).
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The issue with the mechanisation of farms is twofold, issues arise in the ploughing of the crops and additionally in the machinery that is used to plough the fields and apply chemicals. The ploughing of the fields creates rills and gullies which increases the velocity of the water as it flows through the fields in turn increasing the rates of soil erosion (Shaxson et al. 1977). Additionally ploughing breaks up the top soil layer altering its structure, this leaves it more vulnerable to wind erosion and to splash erosion (Morgan, 2005). Secondly the machinery that is used is very heavy this compresses the top layer of organic matter, closing up both the micro and macro pores in the soil. This leads to the porosity and permeability of the soil to be reduced (Soane and Ouwerkerk, 2013). As a result, less water can infiltrate through the soils and there is an increase in the amount of surface run off (O’Connell et al., 2007). This increase in surface runoff leads to 2 major issues, depositional and erosional, the first is more sheet erosion where greater quantities of water will flow over the surface removing more sediment. This sediment has to be deposited which leads to a whole new set of issues such as muddy floods and eutrophication; this is where the phosphorus and nitrites caught up in the runoff enters water bodies and causes the promotion in growth of cyanobacteria this in turn has impacts on the ecology and water quality (Daniel, Sharpley and Lemunyon, 1998) this costs the economy £55m annually to remove (Pretty et al., 2001). Secondly, muddy floods which are formed from surface runoff with sediment suspended in it, can cause lots of damage to infrastructure such as blocking roads which costs £30m/yr to repair (Evans, 1996). This was seen in the rother valley in the period 2006-07 where in one case it travelled 1500m disrupting roads and flooding a row of houses (Boardman et al., 2009).
Hedge row decline has been a big issue in the UK since WW2, in 1946 there were an estimated 500,000 miles of hedgerow which has decreased to 236,000 in 1993 (Bevan, n.d.). The reason for this decline was to increase the amount of available land for farming as the UK had to become more self-sufficient additionally new larger machinery was developed which was not as manoeuvrable and there for larger areas with no obstacles became more effective spaces. However, removing these hedgerows has an influence on the local hydrological cycle. Hedgerows act as an important barrier during high intensity rain fall events, intercepting as much as 80% of the rainfall (Ghazavi et al., 2008). This reduces the velocity of the water moving through the cycle as rainfall is more likely to be infiltrated and transported via baseflow as opposed to surface runoff resulting in less erosion. Additionally, hedge rows reduce the amount of soil evaporation (Ghazavi et al., 2008) this means that the soils retain more moisture preventing them from crusting over and acting as an impermeable surface promoting surface runoff.
With increasing economic constraints on farmers, they are doing everything possible to increase their yields to try and further increase their profits. This means that they are planting winter cereal crop varieties, which have a higher yield. However, many farmers are planting these too late. This means that when the UK experiences the majority of its rainfall the crops are not established and don’t provide adequate protection to the soil. The lack of root networks binding the soil and allowing infiltration means that there will be large amounts of soil erosion, additionally at the point where the soil becomes fully saturated any additional rainfall will cause surface runoff, as there are no root networks to help with infiltration, further adding to the erosional problem (Boardman and Favis-Mortlock, 2013).
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