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Impact Of Compaction On Runoff And Flood Risk Biology Essay

As modern farming equipment gets heavier and intensive farming is being more widely adopted, compaction becomes more of a problem. It increases bulk density, decreases porosity and hydraulic conductivity, reduces time to ponding, increases runoff and as a consequence increases flood risk. This study looked at compacted and uncompacted agricultural land on two different soil types, sandy loam and loamy sand. The effect of compaction on these two soil types was investigated by measuring bulk density, shear strength, porosity, saturated hydraulic conductivity, ponding limit, total pore area and average pore size. Generally, the results showed that higher levels of compaction led to higher amounts of runoff and therefore increased flood risk. The compacted soil showed results which would lead to increased runoff such as reduced time to ponding and lower porosity. Sandy loam was found to be more susceptible to compaction as it contained a higher portion of clay. This susceptibility was illustrated by a greater difference between compacted and uncompacted sandy loam when compared to loamy sand. On the other hand, loamy sand was better at resisting compaction as it showed smaller differences between its compacted and uncompacted results. This means that soil with a high clay content will compact more readily, leading to increased runoff and a higher flood risk.

Introduction

Soil compaction can be defined as a degradation of the soil structure caused by a force applied to the soil that exceeds its strength. Compaction is commonly cause by heavy machinery used in farming operations (Arvidsson et al, 2001; Koch at al, 2008; Sweeny et al, 2006). This leads to increased bulk density, decreased porosity, a reduction in macropores, decreased permeability, reduced time to ponding and consequentially, increased runoff. This increase in runoff causes an increased risk of flooding. This risk is exacerbated by urban land encroaching on rural land due to urbanisation, which is fuelled by the increasing population. This means infiltration is further reduced as more land becomes impermeable.

The global population increase has meant that food security is progressively becoming a more significant problem and has fuelled a need for more intensive farming to cope with rising food demand. Normal tillage practices, although initially helping to reduce topsoil compaction, can increase subsoil compaction below the plough layer. Kukal and Aggarwal (2003) found that switching to a more intensive form of puddling to produce rice-wheat increased bulk density from 1.63 to 1.67gcm-3, which then led to a decrease in yield as root penetration and nutrient acquisition becomes more difficult.

Bulk density is highest under wheel tracks where maximum pressure is applied to the soil. Compaction is highest here and consequentially porosity, infiltration and ponding limit are vastly decreased and so runoff increases. As the UK receives high amounts of rainfall it is important that infiltration rate is high to reduce runoff and in turn, flood risk. However, wet soil can cause wheel slip which can facilitate the formation of a surface crust. This is especially true for clay soil and can cause a large decrease in infiltration. However, much of the time it is unavoidable to undertake farming operations, like harvesting, on wet soil if the crop is not to be potentially lost to pests or adverse weather (Radford et al, 2007). Compaction of a wet soil can be more severe than compaction of a dry soil (Lipiec and Hatano, 2003), which Radford et al (2007) found can further increase soil strength and reduce grain yield. Increased runoff causes higher rates of erosion. Consequentially, more surface nutrients are eroded, heightening the risk of aquatic pollution. Additionally, soils with a higher clay content have a naturally lower bulk density than soils with a higher sand content. However, Bryant et al (2007) found that coarse textured soils like sandy soils are better able to resist compaction and therefore aid infiltration due to their large surface area.

There are various methods to either prevent or alleviate compaction. To prevent the impact of compaction a common method used is controlled traffic. This technique involves confining farm machinery to certain tracks in order to reduce the amount of land being affected (Sarmah et al, 1996). Another practice is the addition of organic matter to the soil, which helps to decrease bulk density (Bhagat et al, 1994) by improving soil stability through increased organic bonding between particles (Soane, 1990). Another advantage is that organic matter provides addition nutrients which as a consequence, can increase crop yield (Bhagat et al, 1994). Prevention of compaction is ideal as it is difficult to correct (Van Quang and Jansson, 2008). However, if compaction has already occurred then there are some methods used to alleviate it. A common method is tillage, which breaks up soil aggregates, decreases bulk density and penetration resistance, and increases permeability (Sweeny et al, 2006). However, tillage is usually done by heavy machinery that only reach down as far as the plough layer. This leads to subsoil compaction which is only alleviated through techniques such as subsoiling or deep ploughing. However, the absence of tillage would delay amelioration (Radford et al, 2007). Indeed, Sidhu and Duiker (2006) found that deep tillage performed after compaction of a silt loam increased the crop yield by 17% in 1 year. Soil can recover naturally through physical and biological processes (Logsdon and Karlen, 2004) such as freeze-thaw cycles or earthworm activity, but this can take a number of years. Additionally, compaction by heavy machinery has been found to reduce macrofauna and earthworm activity, especially on wet soil (Radford et al, 2001).

There has been much investigation into compaction in the past, especially when related to crop yield and tillage, as the effects can be detrimental, especially to soil health if compaction is not prevented or controlled. However, not a great deal of the research has been focused on the effect of compaction by agricultural machinery on runoff and especially flood risk. Flooding can lead to a loss of crops, damage to buildings and equipment and waterlogged fields that take time to recover. Flood risk has increased in recent years and the full effect of compaction on this has yet to be assessed. Therefore, this is where the study will be focused. Compaction on agricultural land of two differing soil types (sandy loam and loamy sand) will be assessed. The main hypothesis of this study is: compaction of soil caused by agricultural practices leads to significantly enhanced flood risk. The sub-hypothesis is: a coarse textured soil is more likely to resist compaction than a fine textured soil.

Materials and Methods

Sampling

The fieldwork was undertaken at two sites on the University of Nottingham farm; Sutton Bonnington and Bunny. Sutton Bonnington has sandy loam soil from the Dunnington Heath Series and Bunny has loamy sand from the Newport Series. At each site compacted and uncompacted areas were identified visually and sampled randomly in triplicate.

To take bulk density, bulk density tins were pushed into a random area of ground in the appropriate locale, lifted out using a trowel and lids placed on. The tin was labelled and placed in a labelled plastic bag. This was repeated 2 times for each soil type so that in total 12 samples were taken. To take saturated hydraulic conductivity and ponding limit, samples were taken with hydraulic conductivity tubes. The tube was pushed into the ground until approximately three quarters full and pulled out. The tube was labelled, wrapped in cling film to keep it intact and placed in a labelled plastic bag. This was repeated 2 times for each treatment. Soil strength and resistance was to be measured in the field using a penetrometer. However, on the day the penetrometer failed to work, so instead small soil columns were taken to be analysed in the X-ray Computed Tomography (CT) scanner. These samples were taken in the same way as the saturated hydraulic conductivity samples. Shear strength was measured in the field using a shear vane. It was pushed into the ground, twisted until soil strength was exceeded and a reading was taken. 3 readings were taken at each repeat so that in total 36 readings were taken. All samples were placed in a cold store at the University of Nottingham to prevent them drying out.

Labwork

Bulk Density

Bulk density was derived in a similar method used by Koch at al (2008). The tins were weighed without lids and placed in the oven at 105oC for approximately 48 hours in order to ensure they were fully dried. They were then taken out of the oven and reweighed. The soil was then removed from the tin and the tin weighed on its own. The tin’s dimensions were taken to calculate volume. Porosity (ε) was calculated from the bulk density results using the following equation ε = 1 - where Pb= bulk density (gcm-3) and Ps= particle density (a constant = 2.65gcm-3).

Saturated Hydraulic Conductivity

Before the saturated hydraulic conductivity measurements took place, the columns had to be prepared. Gauze was attached to the bottom of each tube and placed in a container of water. The water level came above the top of the soil column but not over the top of the tube. These were left for 24 hours in order to ensure fully saturation.

To measure the saturated hydraulic conductivity, constant head apparatus was set up. Firstly, the dimensions of the soil column were measured and recorded. Then the column was clamped into place with a beaker underneath to catch the water. A hydraulic head of 2cm was set up and allowed to achieve a constant flow rate. The column was timed for 5 minutes and the amount of water drained in this time was collected, measured and recorded. This was repeated for all 12 columns.

Ponding Limit

A rainfall simulator was set up to measure ponding limit. Ponding limit was found at 7 different flow rates ranging from 4cm3min-1 to 27.3cm3min-1. Once the correct flow rate was identified (by attaching tubes of differing flow rates to the rainfall simulator), the soil column was clamped into position and was timed until the water level covered the surface of the soil. The time was recorded and repeated for all soil columns at all 7 flow rates.

X-ray Computed Tomography

The soil columns were scanned using the X-ray Computed Tomography (CT) scanner at the University of Nottingham. The x-ray voltage was set at 80kV and the current at 300µA. The voxel size was 64µm and the image averaging was 3 in order to speed up scanning time. The column was placed vertically in the scanner and rotated 3600 whilst being scanned. Each image took 125msec to capture and approximately 1440 images were taken at a resolution of 480 x 450 pixels. The images were then reconstructed in order for them to be analysed. The analysis was done using ImageJ. An appropriate sample of images was taken from the middle of the column. Initially, automatic thresholding was used to estimate pore space. However, this overestimated the pore space and gave inaccurate results. The image analysis was re-done with manual thresholding in order to obtain more suitable results. Measurements of total pore space, average pore size and area fraction (porosity) were taken.

Once all the results were obtained, Genstat was used to perform Analysis of Variance (ANOVA) on all the results. Anything with a P value <0.05 is deemed a significant result.

Results and Discussion

Shear Strength

Table 1 and figure 1 both show that the highest shear strength of 68.8kPa was compacted sandy loam compared to compacted loamy sand which had a shear strength of 58kPa. Ekwue and Harrilal (2009) found that soil with an increased clay content will have a higher soil strength. This is expected as clay soil is more susceptible to compaction, especially in wet conditions which will consequently lead to a higher soil strength. In uncompacted soil loamy sand is found to have a higher shear strength of 26.1kPa compared to 21.0kPa for uncompacted sandy loam, as sandy soils have a naturally higher bulk density and therefore strength. According to ANOVA (table 1) these results were statistically significant (P value = 0.027).

Table 1 – ANOVA (P values) and mean shear strength results. Significant P values (<0.05) highlighted in green.

Mean (kPa) (+ standard deviation)

P Value

Soil Type

Loamy Sand

Sandy Loam

0.341

42.1 (3.51)

44.9 (15.24)

Compaction

Compacted

Uncompacted

<0.001

63.4 (14.25)

23.6 (4.50)

Soil Type and Compaction

Compacted

Uncompacted

0.027

Loamy Sand

58.0 (4.51)

26.1 (2.51)

Sandy Loam

68.8 (23.99)

21.0 (6.49)

Figure 1 – Graph showing the shear strength with standard error of compacted and uncompacted sandy loam and loamy sand.

ANOVA (table 1) also showed that compaction had a statistically significant result (P value = <0.001). Overall, compacted soil has much higher shear strength than uncompacted soil. This is expected as soil strength increases with higher compaction due to increased bulk density. The greatest difference in shear strength was 47.8kPa between compacted and uncompacted sandy loam, whereas loamy sand had a difference of 31.9kPa. This suggests the sandy loam is more susceptible to the effects of compaction, which is expected because sandy loam contains more clay and as the soil was wet on the day of sampling it would be expected to be more vulnerable to compaction. The higher shear strength found in compacted soil indicates a higher bulk density that leads to reduced infiltration and higher runoff, which in turn increases the likelihood of flooding during high rainfall events.

Bulk Density

As seen in table 2, compacted sandy loam has the highest bulk density of 1.32gcm-3. This is anticipated because sandy loam has more clay and therefore is more susceptible to compaction. This is also illustrated by the difference in bulk density between compacted and uncompacted soil. Loamy sand has a difference of 0.04gcm-3 whereas sandy loam has a difference of 0.5gcm-3.

Table 2 – ANOVA (P values) and mean bulk density results. Significant P values (<0.05) highlighted in green.

Mean (gcm-3) (+ standard deviation)

P Value

Soil Type

Loamy Sand

Sandy Loam

0.016

1.24 (0.06)

1.07 (0.29)

Compaction

Compacted

Uncompacted

0.002

1.29 (0.08)

1.02 (0.23)

Soil Type and Compaction

Compacted

Uncompacted

0.004

Loamy Sand

1.26 (0.06)

1.22 (0.07)

Sandy Loam

1.32 (0.09)

0.82 (0.08)

Compacted soil had a bulk density of 1.29gcm-3 which was statistically significantly (P value = 0.002) higher than uncompacted soil which had a bulk density of 1.02gcm-3 (table 2). This is confirmed by findings from Sweeny et al (2006), who found that bulk density was 10% higher in wheel tracks, which are known to be a source of high compaction. McQueen and Shepherd (2002) also found that compaction by wheel traffic increased bulk density significantly at a depth of 0-10cm. Koch at al (2008) found that bulk density in wheel tracks peaked at 1.6gcm-3 for a soil with a high clay content, and according to Rowell (1994) compacted clay soil has a bulk density >1.4gcm-3 and a compacted sandy soil has a bulk density over >1.7gcm-3. However, the highest compaction value obtained was 1.42gcm-3 for sandy loam which indicates that the agricultural land investigated in this study was not very compacted. In addition, the lowest value obtained was 0.78gcm-3 (table 2) which is loose for a sandy loam as Rowell (1994) stated that typical bulk density for a sandy soil lies between 1.4-1.7gcm-3. This could have been due to recent tillage operations or high organic matter content. This theory is backed up by Ekwue and Harrilal (2009) who found that bulk density along with penetration resistance were both reduced with increasing peat levels, which contains a large proportion of organic matter. This unusually low value for uncompacted sandy loamy explains why loamy sand has a statistically significant (P value = 0.016) higher bulk density of 1.24gcm-3 than sandy loam which has a bulk density of 1.07gcm-3 (table 2). This particularly low bulk density is not good for plant growth as it offers little stability to the plant as roots will have difficulty anchoring to the soil. Additionally, drainage would greatly increase, reducing the amount of water available to the plants. Due to the loose soil structure erosion would increase as well causing losses in nutrients and further decreasing plant growth. Therefore, as well as having a soil which is uncompacted, it is also important for the bulk density not to be too low to ensure optimum crop yield and reduced flood risk. A very low bulk density, although reducing runoff, would cause a significant increase in throughflow which reduces lag time and increases flood risk at the outlet.

Figure 2 – Graph to show the bulk density and standard error of compacted and uncompacted sandy loam and loamy sand.

Porosity

These porosity results were calculated from bulk density in a similar method used by Koch at al (2008). As seen in figure 3 and table 3, uncompacted sandy loam has the highest porosity. Indeed, overall sandy loam has a statistically significant (P value = 0.003) higher porosity (table 3). This is to be expected because sandy loam has a higher clay content and clay has a naturally higher porosity. It can also be seen that the difference in porosity between compacted and uncompacted soil is greater in sandy loam compared to loamy sand by a factor of 10. This correlates with the fact that clay soil is more susceptible to compaction, especially when wet. As the U.K. receives high rainfall, it is highly likely that compaction occurred when the soil had a high moisture content.

Table 3 – ANOVA (P values) and mean porosity results. Significant P values (<0.05) highlighted in green.

Mean (+ standard deviation)

P Value

Soil Type

Loamy Sand

Sandy Loam

0.003

0.53 (0.02)

0.60 (0.11)

Compaction

Compacted

Uncompacted

<0.001

0.51 (0.03)

0.62 (0.09)

Soil Type and Compaction

Compacted

Uncompacted

<0.001

Loamy Sand

0.52 (0.02)

0.54 (0.03)

Sandy Loam

0.50 (0.03)

0.69 (0.03)

The uncompacted soil has a statistically significant (P value = <0.001) higher porosity of 0.62 than the compacted soil with a value of 0.51 (table 3), which is to be expected as compacted soil has higher bulk density and therefore less pore space. This reduction in pore space leads to decreased time to ponding and therefore increased runoff.

Figure 3 – Graph showing the porosity and standard error of compacted and uncompacted sandy loam and loamy sand.

Saturated Hydraulic Conductivity (Ksat)

Compacted soil was significantly lower in terms of Ksat than uncompacted soil. Compacted soil had a value of 5.7x10-5cms-1 compared with uncompacted soil with a Ksat of 9.5x10-4cms-1 (table 4). This is expected because compacted soil tends to have increased bulk density and reduced porosity, which leads to a decreased Ksat and so less time to pond and more runoff. In a study done by Zhang et al (2006) on a silty loam in China the same result was found; a statistically significant (P < 0.05) reduction in Ksat on compacted soil (table 4). In addition, a study of a vertisol by Sarmah et al (1996) found that infiltration was 1.6 times faster in uncompacted bed areas compared to compacted wheeled areas, which would directly link to a faster Ksat.

Table 4 – ANOVA (P values) and mean saturated hydraulic conductivity (Ksat) results. Significant P values (<0.05) highlighted in green.

Mean (cms-1)

P Value

Soil Type

Loamy Sand

Sandy Loam

0.194

3.35x10-4 (4x10-4)

6.74x10-4 (8.3x10-4)

Compaction

Compacted

Uncompacted

0.008

5.7x10-5 (2.5x10-5)

9.51x10-4 (6.7x10-4)

Soil Type and Compaction

Compacted

Uncompacted

0.234

Loamy Sand

4.1x10-5 (1.4x10-5)

6.29x10-4 (3.7x10-4)

Sandy Loam

7.3x10-5 (2.4x10-5)

1.27x10-3 (8.1x10-4)

In a study by Ekwue and Harrilal (2009) it was found that infiltration, and therefore Ksat, was fastest in sandy loam compared to clay loam and slowest in clay soil. This contradicts this study’s findings. Figure 4 illustrates that although these results were not statistically significant, the loamy sand did have a slower Ksat of 3.6x10-4cms-1 than the sandy loam, which had a Ksat of 6.7x104cms-1. This could be due to the level of compaction or the moisture content of the soil when it was compacted. Alternatively it could be because the clay content in the loamy sand was higher than initially thought since the field that the loamy sand samples were taken from had a textural gradient, running from clay loam to loamy sand.

Figure 4 – A graph to show saturated hydraulic conductivity (Ksat) with standard error of compacted and uncompacted sandy loam and loamy sand.

Ponding Limit

Generally, as illustrated well in figure 5, as flow rate reduces the time to ponding increases and so does the significance of the results. At the highest flow rate of 27.3cm3 min-1, sandy loam took a statistically significantly (P value = 0.017) longer time to pond than loamy sand (table 5). However, in a study of 3 different soil types by Ekwue and Harrilal (2009) it was found that clay soil maintained a higher volume of runoff compared to clay loam and sandy loam, which suggests that the time to ponding is quickest on clay soil and slowest on sandy soil. Contrary to what this study found, the Ekwue and Harilal (2009) results are more conducive to expectations because loamy sand has a higher sand content which results in a higher number of macropores which are more conducive to aiding flow (Lipiec and Hatano, 2003) resulting in a higher Ksat. The inconsistency of these results could have been due to cultivation of a wet soil causing wheel slip or increased damage to the soil structure caused by the wet conditions. Alternatively, it is possible the loamy sand has a higher proportion of clay than was expected, as explained in section 4.4.

Table 5 – ANOVA (P values) and mean ponding limit results at 27.3cm3 min-1. Significant P values (<0.05) highlighted in green.

Mean (min:sec) (+ standard deviation)

P Value

Soil Type

Loamy Sand

Sandy Loam

0.017

01:31 (00:30)

02:25 (00:21)

Compaction

Compacted

Uncompacted

0.306

01:49 (00:24)

02:07 (00:27)

Soil Type and Compaction

Compacted

Uncompacted

0.413

Loamy Sand

01:14 (00:26)

01:47 (00:35)

Sandy Loam

02:23 (00:22)

02:27 (00:19)

Table 6 – ANOVA (P values) and mean ponding limit results at 4cm3 min-1. Significant P values (<0.05) highlighted in green.

Mean (min:sec) (+ standard deviation)

P Value

Soil Type

Loamy Sand

Sandy Loam

0.001

10:11 (02:18)

17:44 (01:18)

Compaction

Compacted

Uncompacted

0.008

11:24 (01:57)

16:31 (01:40)

Soil Type and Compaction

Compacted

Uncompacted

0.663

Loamy Sand

07:20 (01:17)

13:03 (03:19)

Sandy Loam

15:29 (02:36)

20:00 (00:00)

As flow rate reduces to 4cm3 min-1 the differences between compaction levels also becomes statistically significant (P value = 0.008), as seen in table 6. However, as the flow rate gets lower the standard deviation increases as well. Overall, uncompacted soil takes longer to pond than compacted, which would be expected as uncompacted soil has a lower porosity and Ksat. Further to this, in a study by Verbist et al (2007) of a silt loam with a hardpan it was concluded that saturated excess runoff induced by the subsoil compaction was a major contributor to ponding, surface runoff and loss of soil. They calculated that the risk of ponding and runoff was 50% higher when topsoil was saturated. Increased ponding and runoff also has the effect of increasing soil loss through erosion, as found by Basher and Ross (2001), who observed runoff eroding the base and edge of wheel tracks. The increase in ponding limit on compacted soil directly leads to increased runoff and inevitably higher rates of erosion. This causes an increased risk of aquatic pollution and a decrease in plant nutrients from the soil which can decrease crop yield, or cause an increased expense to the farm to invest in more fertiliser which (if compaction is not alleviated) would further add to pollution as runoff is still high.

Figure 5 – Graph showing the ponding limit and standard error of compacted and uncompacted sandy loam and loamy sand.

X-ray Computed Tomography

Total Pore Area

There was a statistically significant result (P value = 0.008) between compacted and uncompacted soil (table 7). Compacted soil has a total pore area of 40.6mm2 whereas uncompacted soil has a total pore area of 77.4mm2. This is because compacted soil has a higher bulk density and therefore will have a reduced pore space. This was also found by Koch at al (2008) who observed a significant decrease in pore volume in wheeled areas. Another study by Sarmah et al (1996) found that uncompacted bed areas have double the number of macropores and triple the number of micropores than compacted wheeled areas.

Mean (mm2) (+ standard deviation)

P Value

Soil Type

Loamy Sand

Sandy Loam

0.195

52.1 (6.4)

65.9 (5.7)

Compaction

Compacted

Uncompacted

0.008

40.6 (3.6)

77.4 (8.5)

Soil Type and Compaction

Compacted

Uncompacted

0.003

Loamy Sand

56.9 (5.2)

47.3 (7.6)

Sandy Loam

24.3 (2.0)

107.5 (9.4)

Table 7 – ANOVA (P values) and mean total pore area results. Significant P values (<0.05) highlighted in green

Uncompacted sandy loam has a statistically significant (P value = 0.003) higher total pore area than uncompacted loamy sand (table 7 and figure 6). This is as expected in uncompacted soil as sandy loam has a higher clay content and therefore a naturally higher bulk density and total pore area. The decrease in total pore area seen in compacted soil means a reduction in porosity and Ksat and therefore an increase in runoff.

Figure 6 – Graph showing the mean total pore space of compacted and uncompacted sandy loam and loamy sand.

Average Pore Size

Mean (mm) (+ standard deviation)

P Value

Soil Type

Loamy Sand

Sandy Loam

0.116

0.020 (0.004)

0.068 (0.05)

Compaction

Compacted

Uncompacted

0.068

0.015 (0.002)

0.073 (0.05)

Soil Type and Compaction

Compacted

Uncompacted

0.089

Loamy Sand

0.017 (0.002)

0.022 (0.006)

Sandy Loam

0.012 (0.001)

0.123 (0.1)

Table 8 – ANOVA (P values) and mean average pore size results. Significant P values (<0.05) highlighted in green

ANOVA showed that there were no statistically significant results for average pore size. However, as can be seen in table 8, uncompacted soil did show a higher average pore size of 0.073mm than compacted soil which had an average pore size of 0.015mm. Zhang et al (2006) confirms this. They found that in their compacted treatment, pore volume and diameter was significantly decreased in the surface and subsurface layer of their silty loam soil. Similarly, Lipiec and Hatano (2003) found that compaction induced by heavy machinery reduced the volume of macropores which actively assist the flow of water through soil. Indeed, Frey et al (2009) found a 59% decrease in macropores in compacted soil. A smaller pore size leads to a decreased Ksat and as a result, excess runoff.

Porosity

Mean (%) (+ standard deviation)

P Value

Soil Type

Loamy Sand

Sandy Loam

0.296

11.3 (1.9)

13.7 (2.3)

Compaction

Compacted

Uncompacted

0.012

8.7 (1.8)

16.2 (2.5)

Soil Type and Compaction

Compacted

Uncompacted

0.004

Loamy Sand

12.4 (1.6)

10.2 (2.2)

Sandy Loam

5.1 (1.9)

22.3 (2.7)

Table 9 – ANOVA (P values) and mean porosity results. Significant P values (<0.05) highlighted in green

These porosity results were calculated from CT scans. As with most other results, there was a statistically significant difference (P value = 0.012) between compacted and uncompacted soil. Compacted soil has a porosity of 8.7% whereas uncompacted soil has a porosity of 16.2%. This is most likely because compacted soil has a higher bulk density and therefore will have a reduced porosity. This coincides with the findings of Frey et al (2009) who found that on their 3 sites, porosity varied between 15-21% when comparing compacted soil to uncompacted soil. This decrease in porosity was found to be caused by wheel tracks made by heavy machinery which resulted in increased ponding and runoff in the wheel tracks.

Figure 8 – Graph showing mean porosity of compacted and uncompacted sandy loam and loamy sand.

Uncompacted sandy loam has a statistically significant (P value = 0.004) higher porosity than uncompacted loamy sand due to its higher clay content and so naturally lower bulk density and hence porosity. However, in compacted soil loamy sand has a higher porosity. This indicates that it is better able to resist compaction due to its higher sand content. Bryant et al (2007) also found that sandier soils were better able to resist compaction.

Overall, it is clear that the compacted soil had high shear strength, high bulk density, low porosity, low Ksat, longer time to pond, reduced total pore area and smaller average pore size when compared with uncompacted soil. All these factors contribute to increased runoff which in turn leads to increased risk of flooding. This proves the main hypothesis, compaction of soil caused by agricultural practices leads to significantly enhanced flood risk, to be correct. Compacted sandy loam had the highest shear strength and bulk density which indicates its higher susceptibility to compaction when compared to loamy sand. This is due to its higher clay content as clay is known to be more susceptible to compaction (Bryant et al, 2007). This is also illustrated by the bigger increase in shear strength and bulk density between uncompacted and compacted sandy loam compared to loamy sand. On the other hand, loamy sand shows that a coarse textured soil is better able to resist compaction due to smaller changes between compacted and uncompacted soil and general lower bulk density and shear strength and slower ponding limit. This proves the sub-hypothesis; a coarse textured soil is more likely to resist compaction than a fine textured soil, to be correct. This means that on agricultural land with a high clay content more care needs to be taken when preventing compaction, as it not only reduces crop yield because more energy is spent on root penetration and obtaining nutrients is more difficult, but also causes crop losses if flooding does occur. The land would not only flood more easily, but water drainage would take longer which further adds to the likelihood of yield reduction and crop mortality. If flood water takes longer to drain it might become unavoidable to conduct machinery operations on wet soil leading to further compaction. Furthermore, the increased erosion caused by additional runoff will lead to nutrient depletion and so a further reduction in crop yield might occur if the nutrient loss is not compensated for.

Conclusion

The results have shown that the compacted sandy loam had the highest soil strength and bulk density, the lowest porosity and total pore area and slowest Ksat indicating that it is more susceptible to compaction. This is backed up by the difference in all results between compacted and uncompacted soil being greater in sandy loam. Therefore the land will flood quicker, increasing the amount of runoff and subsequently increasing topsoil erosion and nutrient loss. On the other hand, compacted loamy sand was better at resisting compaction which is demonstrated by its lower bulk density and soil strength, larger total pore area, greater porosity and a slower Ksat. This indicates that loamy sand would decrease flood risk and aid water drainage.

These results have indicated that farmers need to be aware of the flood risk associated with compaction, not just the effect it has on crop yield. Flooding can have a detrimental effect on crop yield as well as reducing the structural quality of the soil. Local authorities also need to be aware of the impact that compaction can have on the flood risk in their local areas through increased runoff and decreased lag time.

Further research should be done on subsoil compaction and plough pan impacts on runoff and flood risk as a more compacted subsoil can lead to increased throughflow and decreased time to topsoil saturation. The increase in throughflow would lead to a decrease in lag time and so increase the likelihood of flooding. More research should be focused on how severe a risk this could pose as subsoil compaction below the plough layer is not often alleviated by farmers as it does not directly affect plant growth.


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