Passage II

In mountainous regions, a terrace is made by first building a low rock wall, then creating an area of level soil behind the wall (see Figure 1). Rainfall erodes the soil from terraces at different rates depending on whether a terrace is being farmed or is being used for animal grazing.

A study done in June of one year considered 86 terraces in the same 100 km² mountainous region, all at approximately the same altitude. Each terrace was assigned to 1 of 3 categories: farmed, lightly grazed, or heavily grazed. Also studied were talus slopes, which are inclined areas composed of rock fragments and small amounts of soil.

For each terrace or talus slope, erosion was measured in 5 different locations. At each location, a 1,385 cm² area was isolated, and simulated rainfall was applied evenly to the area at a rate of 75 mm/hr for 45 min. The time to runoff (time elapsed between the beginning of rainfall and the beginning of runoff) was recorded (in sec).

All runoff from each isolated area was collected over the 45 min to determine the runoff rate (in mm/hr), the soil concentration in the runoff (in g/L); and the soil erosion rate (in g/m²/hr). The wetting front was also determined at the end of 45 min. The wetting front was the depth (in cm) to which rainwater had infiltrated the soil. The results, averaged for each of the 4 types of surfaces, are shown in Table 1.

Figure and table adapted from Teodoro Lasanta et al., “Marginal lands and Erosion in Terraced Fields in the Mediterranean Mountains.” ©2001 by the International Mountain Society and the United Nations University.