# Application of RFID to Soil-Erosion Research
## 2 Materials and Methods  
The experiments were conducted on an experimental plot (Figure 1) using the large-scale rainfall simulator at the National Research Institute for Earth Science and Disaster Resilience (NIED), Tsukuba Japan. The simulator is able to provide rainfall intensities between 15 and 300 mm/h from nozzles located 16 m above the ground, enabling the rainfall to attain near terminal velocity. The plot (originally constructed for experiments in 2009, see Reference [8]) was modified for the first experiment by adding a 60-mm-deep surface layer of quartz gravel with a median diameter of 2.86 mm (see Figure 2). For this experiment, we manufactured artificial gravel particles each embedded with an RFID tag. The tags were made by TOPPAN FORMS Co., Ltd., Tokyo, Japan, and had a size of 2 mm \(\times\) 1.8 mm \(\times\) 0.4 mm. The tags were read by using R/W (TOPPAN FORMS Co., Ltd., Figure 3). To make the artificial gravel particles as similar as possible to the surface layer on the plot, they were coated in household-use cement, which is in use for making fishing lure, in order to make them roughly spherical in shape They were then coated in bronze powder to give them an overall density similar to  
Figure 1: The experimental plot shown under construction in 2009.  
that of quartz. Thus, the particles were comparable in size, shape and density to the surface layer on the plot. For ease of identification, they were further coated in fluorescent powder. The final particles were between 2.5 mm and 4.0 mm in diameter and had specific weights of 2.5 to 3.0. As each particle was made by hand the size and specific weights vary, but their overall characteristics make them comparable in size, shape and density to the surrounding gravel particles. Within the plot, 300 tagged particles were placed on the surface approximately 50 mm apart in three lines 5.5, 7.0 and 8.5 m from the upper boundary of the plot. The precise location of each particle was obtained using a Tajima laser total station (Type: TT-027, accuracy of distance: 1.5 mm + 2 ppm). The experiment consisted of applying rainfall to the plot for 45 min at an intensity of 137 mm/h. This duration and intensity were selected in order that infiltration would be exceeded, and the runoff would have time to reach equilibrium. During the experiment, runoff (and any included sediment) was sampled volumetrically at the outlet of the plot every two minutes in order to generate hydrographs and sedigraphs to be used in subsequent modelling. In each interval between these samples, sediment leaving the plot was collected in a sieve. After the experiment, the location of each of the tagged particles was determined using the same method as before, after which the particle was removed from the plot to read its RFID tag. Although most of the particles moved only a short distance during the experiment (see Section 3), a few were transported off the plot and were collected with the water and sediment samples. These particles were identified and the time they left the plot was recorded. Figure 2: The 1-m-wide plot used for the second experiment, showing also the surface layer of gravel used for both experiments. For the second experiment, we used an UHF tag manufactured by Hitachi Chemical Co., Tokyo, Japan, measuring 2.5 by 2.5 by 0.4 mm. By placing the tag within a booster antenna (Figure 4), remote detection becomes possible. Similar to the first experiment, we manufactured 100 artificial soil particles by encasing the tag and its antenna in a clear silicone resin (NisshinAssociates Co., Ltd., Tokyo, Japan), in order to make them roughly spherical in shape, and then coated the resin in bronze powder to give the particles an overall density similar to that of quartz particles. For ease of identification, they were further coated in fluorescent powder. The final surrogate soil particles had a median diameter of 6 mm and had specific weights of 2.5 to 3.0. Again, as each particle was made by hand, the size and specific weights vary.