The mass of internal soil erosion adjacent to a geosynthetic drainage material due to precipitation and percolation was evaluated. Down-flow precipitation/percolation tests using a column diameter of 10 cm and height of 10 cm were conducted on sandy soils under several precipitation intensities. The experimental results showed that there are two patterns which can be differentiated in terms of the masses of soil losses; large soil erosion and small soil erosion. The mass of eroded soils for large soil erosion was approximately 5 times greater than that of small soil erosion. Large soil erosion occurred when sufficient precipitation/percolation intensity for the hydraulic conductivity of soil was continuously given, and is attributed to the seepage failure which occurred when the wetting-front reached the bottom of the column specimen and the condition for all the depth of the specimen reached saturation. In contrast, only preferential flow occurred either with insufficient precipitation/percolation intensity or with a limited period of precipitation with sufficient intensity resulting in small soil erosion. In small soil erosion, the value of the mass of soil erosion from the eroded portions at the bottom of the specimen was up to 90% of the losses. The formation of the eroded portions depends on the falling droplets. A theoretical method to estimate the magnitude of erosion is proposed based on the balance of weight of the droplet and the capillary force. The calculated and measured values of the mass of soil erosion adjacent to the geosynthetic material showed good agreement.