Investigation of runoff and sediment transport using rainfall simulation and erosion control by using mulching boards

概要

The investigations of the mechanisms of runoff and sediment discharge using a rainfall simulator were conducted using Andosols soil using Micro Portable Rainfall Runoff Simulator (MPRRS). The designed rainfall intensities for this study were set to be 50 and 70 mm/h with the slope gradients of 5% and 10%. For Andosols soil, increasing rainfall intensity and slope gradient caused increase in runoff discharges as well as sediment losses.
The high bulk density soil (Phusawan soil, Phetchburi province, Thailand) caused shorter runoff occurring time and greater amount of runoff and sediment discharge as compared to Andosols soil in Tokyo.

The surface flow and permeability characteristics of mulching boards using MPRRS rainfall simulator were examined. There were 2 kinds of mulching board, chipwood board and bagasse board, in these experiments. The 2 cm chipwood board had the highest percolation rate, hence it was chosen for the erosion control in the plot experiments.

Next, the evaluation of the effectiveness of mulching boards in erosion control was conducted in the field of Tokyo University of Agriculture and Technology, Koganei campus using Portable Rainfall Runoff Simulator. The field was prepared for 3 conditions, such as bare soil, 25% and 50% covering with 2 cm chipwood boards. The results showed that the runoff discharges and sediment losses was reduced greatly when the soils were covered by 2 cm chipwood boards. In conclusion, the application of mulching boards significantly reduced runoff occurring time, runoff discharges and soil losses from the experimental plots.

Introduction

The study of soil erosion mainly focuses on surface runoff experiment (Cerdan et al., 2010). Rainfall runoff occurs when the soil is saturated and rainfall rate exceeding infiltration rate. Erosion takes place when the raindrops break the bond between soil particles and displace them and surface flow carries displaced sediment.

To develop a soil erosion protection method, mulching has been successfully applied to reduce soil and water losses in different contexts, such as agricultural lands, fire-affected areas, rangelands and anthropic sites (Massimo et al., 2016). Mulching is referred to as the agronomic practice of leaving mulch on the soil surface for soil and water conservation and to favour plant growth (Jordán et al., 2010).

The main objectives of this research were to: 1) Investigate the mechanisms of runoff, sediment discharge and infiltration using rainfall simulators.

2) Investigate
runoff, sediment discharge and infiltration in Phusawan watershed in Phetchburi Province in Thailand. 3) Determine the surface flow and permeability characteristics of mulching boards by rainfall simulation and 4) Evaluate the effectiveness of mulching boards in erosion control.

Literature reviews
Soil erosion consists of four main steps. As rainfall occurs, the rain drops onto the soil surface and makes first contact with the soil. The detachment process takes place when soil particles disengage as the rain touches down on the soil. Afterwards, the soil particles are transported by rolling, splashing, or dragging and translocate to another place. Finally, the soil particle is deposited at some other place at a lower elevation (Zafirah et al., 2017).The main types of soil erosion caused by nature are from water and wind. Water erosion is the result from the removal of soil by flowing water. The soil particles are detached and transported by rainfall, runoff, melting snow or ice, and irrigation (Gilley, 2005).

The mulching materials can either be organic such as straw, shredded bark, wood chips or inorganic as plastic sheeting, volcanic rock (Onwona-Agyeman et al., 2007). The mulching boards had many benefits in the field experiments such as decreased the evaporation loss from soil, weed suppression, improved the nitrogen and carbon in soil, soil hydraulic conductivity, infiltration and increased the biomass yield (Onwona-Agyeman et al., 2007).

Materials and methods

The experiments were used 3 kinds of rainfall simulators. They are Micro Portable Rainfall and Runoff Simulator (MPRRS) for the experiments in the 1st – 3rd objective, Portable Rainfall and Runoff Simulator (PRRS) and Mini Portable Pressure Head Type Rainfall Simulator (MPPH) were used in the experiments of the 4th objective.

The investigation of runoff, sediment discharge and infiltration using the rainfall simulator used Andosols soil which brought from Sakaecho field, Tokyo University of Agriculture and Technology, Fuchu campus and Phusawan soil from Phetchburi province, Thailand. The designed rainfall intensity for this study was set to be 50 and 70 mm/h with the slope gradient of 5% and 10% for Andosol soil and rainfall intensity of 70 mm/h and slope gradient of 5% and 10% for Phusawan soil, respectively.

First, the designed rainfall intensities were manually calibrated by collected amount of water in the plastic tray in 2 minutes. The runoff and infiltration outflow samples were collected by using glass bottles placed under the runoff collector and infiltration outflow outlet of lysimeter, respectively. The time step for replacing the new bottles was every 10 minutes for 100 minutes. The weights of all samples were measured after sampling.

The runoff samples were filtered to separate the sediment from the runoff water by using the WhatmanⓇ glass microfiber filter φ 60 mm with pore size of 1.6 micrometer.

The mass of sediment in the runoff water samples were recorded.

The mulching board with best percolation rate from overflow and percolation experiments using MPRRS rainfall simulator were evaluated for the field experiments.

The field soil sample from Koganei field was transferred to examine the runoff, infiltration and sediment loss using MPPH rainfall simulator in laboratory at Tokyo University of Agriculture and Technology, Fuchu campus. The design covering areas in the field experiments were 0%, 25% and 50% of plot area. The mulching boards were laid on the end of each plot closed to the runoff collector. Both sides of the boards were attached to the border of the plot for preventing the side leakage. The designed rainfall intensity for this study was set to be 70 mm/h. First, the designed rainfall intensities were manually calibrated by collected amount of water on the plastic sheet from runoff collector at the end of the plot in 2 minutes. The replication was made for three plots and the field experiments were conducted 2 rainfall events. The samples were collected every 10 minutes after the runoff occurred with collecting amount of 1 litre continuously for 60 minutes. The runoff samples were filtered to separate the sediment from the runoff water by using the WhatmanⓇ glass microfiber filter φ 60 mm with pore size of 1.6 micrometer. The dry mass of sediment in the runoff water samples were recorded

Results and discussion

The experiments of runoff and infiltration showed the increasing of rainfall intensity was caused the increasing of both runoff discharge and infiltration. The slope gradient is also one of the important factors affecting the surface flow and infiltration outflow. In general, increasing runoff increased the runoff discharge and decreased the infiltration outflow. Under the same rainfall condition, surface flow could be drastically different depending on different slopes.

For the investigations of sediment, the results showed the higher rainfall intensity lead to increase in runoff and may bring about the formation of the soil crust, and the development of such soil crusts would reduce the infiltration (MU et al., 2015). Zhang et al., (2002) reported that the detachment rates of top soil were sensitive to discharge than slope gradient.

For the experiments on mulching boards, permeability was the decision factor in this experiments. The permeability of 2 cm chipwood boards was statistically different from others. It allowed about 60% of rainfall became percolation water. Then, the 2 cm chipwood board was chosen for mulching board material for soil erosion control and protection in the next plot experiment.

The average runoff discharge was decreased as the percentage of covering area by the mulching board increased. The average runoff discharges from all the treatments were significantly different from one another. The results in the 1st rainfall event were significantly decreased 27.0% and 44.5% when the 0% covering increased for the 25% and 50% covering, respectively. The results of average runoff discharge in the 2nd rainfall event were similar to the 1st rainfall event. The corresponding values were 35.9% and 62.5% when the 0% covering increased for the 25% and 50% covering, respectively.

The cumulative sediment losses in the 1st rainfall simulation were decreased by 89.8% and 96.5% of bare soil case for 25% and 50% covering, respectively. For the 2nd rainfall simulation, the cumulative sediment losses were decreased 89.7% and 99.6% of bare soil case for 25% and 50% covering, respectively.

Conclusions

From the series of experiments, following conclusions were derived. The laboratory experiments revealed that increasing rainfall intensities and slope gradients increased runoff discharges as well as sediment losses. Also, increasing slope gradients increased the runoff discharges and sediment losses from the soil in Phusawan watershed in Thailand. Among tested mulching board materials, the 2 cm thick chipwood board had the highest percolation rate and the lowest runoff discharge, hence it was chosen for the erosion control in plot experiments. Finally, the evaluation of the effectiveness of mulching boards in erosion control was conducted in the experimental field using Portable Rainfall Runoff Simulator. The experiments revealed that the application of 2 cm thick mulching board significantly reduced runoff discharges and soil losses from the experimental plots.