**2. Materials and methods**

Three soils, Piarco sandy loam, Maracas clay loam and Talparo clay (Table 1) were used for the study. They were the same soils used in the earlier studies by Ekwue et al. (2009) and Ekwue and Harrilal (2010). Air-dry soil samples were ground to pass a 5 mm sieve. Particle size distribution (Table 1) was performed using the hydrometer method (Lambe, 1951). Organic matter content in the samples was measured using the Walkley and Black (1934) method. Organic matter content in the samples was increased by adding air-dry sphagnum peat moss (with 0.15 t m-3 air-dry density) at rates of 5%, and 10%, air-dry mass basis.


\* Classification according to the Soil Taxonomy System (Soil Survey Staff, 1999).

\*\* All values are means of three replicates

Table 1. Classification, organic matter, and the particle size distribution (%) of the soils

Raindrop erosion was measured with a soil erosion assessment facility whose soil test bed (Figure 1) was fully described by Ekwue et al. (2009). The difference is that this has now been added a rainfall simulator (Figure 2) designed using the original design of Tossel et al. (1990). The simulator utilized three continuous spray full jet nozzles (6.35 mm diameter) placed along the length of a 2 m high, ½ inch diameter P.V.C. frame mounted onto the frame of the test bed. The intensity of the simulated rainfall from each nozzle was 90 mm hr-1 with a Christiansen (1942) coefficient of uniformity of 89%, median drop size of 2.03 mm and a kinetic energy of 29.38 J m-2 mm-1. The rainfall intensity was chosen as a compromise between what is expected in temperate and tropical climates. During the erosion test period, the simulator frame was covered with a transparent material to limit the effect of wind on the raindrops falling to the test bed.

reduced soil transport since it is known to reduce inter-aggregate stability and soil strength which affects the soil erosion process. Ekwue and Harrilal (2010) followed it up by studying the effect of peat on wash erosion by raindrop impact and observed that peat decreased wash erosion by reducing soil bulk density, increasing infiltration rates and decreasing runoff. The effect of peat incorporation on the overall soil erosion process is therefore now clearly understood. Soil erosion by raindrops is also known to be affected by rainfall duration but it is not clear how this relationship is affected by other parameters that affect the soil erosion process including peat content, soil compaction and soil type. This paper reports the results of an interaction experiment set up to examine the relative effects of peat content, soil type, rainfall duration and compaction efforts on raindrop erosion. The aim is to further increase the general understanding of how peat affects the

Three soils, Piarco sandy loam, Maracas clay loam and Talparo clay (Table 1) were used for the study. They were the same soils used in the earlier studies by Ekwue et al. (2009) and Ekwue and Harrilal (2010). Air-dry soil samples were ground to pass a 5 mm sieve. Particle size distribution (Table 1) was performed using the hydrometer method (Lambe, 1951). Organic matter content in the samples was measured using the Walkley and Black (1934) method. Organic matter content in the samples was increased by adding air-dry sphagnum

peat moss (with 0.15 t m-3 air-dry density) at rates of 5%, and 10%, air-dry mass basis.

\* Classification according to the Soil Taxonomy System (Soil Survey Staff, 1999).

Table 1. Classification, organic matter, and the particle size distribution (%) of the soils

Raindrop erosion was measured with a soil erosion assessment facility whose soil test bed (Figure 1) was fully described by Ekwue et al. (2009). The difference is that this has now been added a rainfall simulator (Figure 2) designed using the original design of Tossel et al. (1990). The simulator utilized three continuous spray full jet nozzles (6.35 mm diameter) placed along the length of a 2 m high, ½ inch diameter P.V.C. frame mounted onto the frame of the test bed. The intensity of the simulated rainfall from each nozzle was 90 mm hr-1 with a Christiansen (1942) coefficient of uniformity of 89%, median drop size of 2.03 mm and a kinetic energy of 29.38 J m-2 mm-1. The rainfall intensity was chosen as a compromise between what is expected in temperate and tropical climates. During the erosion test period, the simulator frame was covered with a transparent material to limit the effect of wind on

Organic

Soil Content (0.06-0.002) (0.06-0.002) (<0.002) Series Classification\* (%) mm mm mm Piarco Aquoxic Tropudults 1.7\*\* 64.9 17.0 18.1 Maracas Orthoxic Tropudults 4.7 44.7 24.7 30.6 Talparo Aquentic Chromuderts 2.7 25.4 28.3 46.3

Matter Sand Silt Clay

soil erosion process.

**2. Materials and methods** 

\*\* All values are means of three replicates

the raindrops falling to the test bed.

The apparatus measures erosion on surfaces with slopes varying from 0% to 30%. The soil tray has a flexible drainage hose added to the bottom end throughout the length of it. Gravel was placed at the bottom of the soil tray to a depth of 8 cm before putting the soil to be tested, such that water that infiltrated through the soil first passed through the layer of gravel, which acted as a filter, and ensured that clean water flowed down the drain preventing the siltation of the drain pipes. During testing, the eroded soil and overflow water (runoff) flowed into the soil collection pan. Here soil settled under its own weight. From this compartment, the water flowed through a drainpipe and into a drain where the runoff was measured. Sediments were collected from the collection tray after the tests and oven dried to determine the mass of soil eroded.

For each test, soil was added to the soil tray to a depth of 2 cm. This is the depth of soil that is normally involved in the soil erosion process. Soil was then compacted at three levels (100 kPa, 150 kPa and 185 kPa). The three compaction levels were obtained using a 3.6 kg roller 2, 3, and 4 times each followed by a 5.8 kg roller 3, 4 and 5 times respectively. The aim was to produce a compacted soil similar to field conditions and to determine the effect of these levels of compaction on soil erosion. Bulk density and penetration resistance achieved after soil preparation were measured using a hand pushed spring-type Proctor penetrometer (ASTM, 1985). Erosion by simulated rainfall was assessed using a factorial experiment involving the three soils with the three peat contents, and exposed to four rainfall durations (5, 10, 20 and 30 min) with two replications giving a total of 216 tests. The slope gradient was fixed at 9% which is prevalent in agricultural soils in Trinidad (Gumbs, 1987). Analysis of variance of soil erosion values was performed using the MINITAB computer software.

Fig. 1. The soil bed of the erosion facility

Soil Loss-Rainfall Duration Relations as Affected

**3.1 Factors affecting runoff and soil loss** 

Peat content (%)

aValues are means of two replicates.

interactions and the main effects will be described below.

rainfall durations

Compaction effort, 100 kPa

**3. Results and discussion** 

soil.

Soil

by Peat Content, Soil Type and Compaction Effort 183

Table 2 shows the value of soil loss for the three soils. Soil loss decreased with increasing peat contents for all combinations of soil type, rainfall duration and compaction effort. Soil loss also decreased with increasing compaction effort and increased with increasing rainfall duration. Soil loss was consistently highest in the sandy soil, intermediate in the clay soil and lowest in the clay loam soil. Table 3 shows that cumulative runoff increased with increasing rainfall duration and compaction levels but decreased with increasing peat content. Runoff was highest in the clay, followed by clay loam and lowest in the sandy loam

Rainfall duration (min) Rainfall duration

Piarco sandy 0 0.48 0.85 2.10 3.39 0.49 0.82 1.85 3.30 0.42 0.85 1.66 2.39 loam 5 0.45 0.81 1.84 2.95 0.45 0.79 1.66 2.76 0.38 0.75 1.51 2.03 10 0.42 0.79 1.60 2.74 0.42 0.75 1.44 2.53 0.34 0.68 1.30 1.78 Maracas 0 0.40 0.81 1.47 2.51 0.40 0.72 1.16 2.01 0.31 0.63 1.19 1.62 clay loam 5 0.36 0.68 1.30 2.11 0.32 0.66 1.10 1.88 0.25 0.49 1.02 1.37 10 0.28 0.563 1.17 1.87 0.24 0.50 0.851 1.43 0.17 0.39 0.81 1.21 Talparo 0 0.46 0.81 1.97 2.86 0.46 0.78 1.88 2.81 0.39 0.74 1.49 2.29 clay 5 0.42 0.78 1.77 2.67 0.40 0.71 1.54 2.64 0.35 0.69 1.39 1.91

Table 2. Soil lossa (kg) for three soils with and without peat compacted and exposed to four

Table 4 summarizes the mean values of cumulative runoff and soil loss for the different experimental factors. Mean runoff increased with increasing clay content, compaction effort and rainfall duration but decreased with increasing peat content. For soil loss, mean values of soil loss varied from 1.36 kg in the sandy soil to 0.95 kg in the clay loam soil. The analysis of variance (Table 5) showed that the main effects of soil type, peat content, compaction effort and rainfall duration were all significant (P = 0.001) for the two parameters as depicted by the 'F' values. Rainfall duration was the most important factor that affected the two parameters. In addition, the most significant interaction that affected the two parameters was that between soil type and rainfall duration which was significant at 0.1% level. This was followed by the interactions between compaction effort and rainfall duration and between peat content and rainfall duration in that order for the two parameters. These

Compaction effort, 150 kPa

(min)

5 10 20 30 5 10 20 30 5 10 20 30

10 0.39 0.76 1.52 2.43 0.36 0.66 1.21 2.21 0.28 0.58 1.16 1.67

Compaction effort, 185 kPa

> Rainfall duration(min)

Fig. 2. The rainfall simulator with the soil bed
