Locally measured USLE K factor expands sustainable agricultural land in Palau

Site description

Palau forms part of the Micronesia region in the western Pacific Ocean. Palau's economy is mainly due to tourism and the increase in tourists increases the consumption of agricultural products. Palau imports them, but it is preferable to produce them domestically. The agriculture in Palau is mainly taro cultivation at swamp by a traditional and environmentally friendly method. Before World War II, Japanese settlers developed agricultural land. In recent years, the redevelopment of these fields using modern farming methods has begun. Fields with inclines of more than 8° are unsuitable for growing crops, but most of the agricultural fields in Palau have slopes of more than 8°. The island is also subject to heavy rainfall (ca. 3300 mm – 3900 mm). USDA Natural Resources Conservation Service in 2009 categorized most of the land (80%) as the most fragile rank (T factor = 1). T Factor values range from 1 ton/acre/year for the most fragile soils, to 5 tons/acre/year for soils that can sustain more erosion without losing significant productive potential (USDA Natural Resources Conservation Service). A study estimated the risk of soil erosion from agricultural land was reported to be from 720 to 813 tons per ha per year (Gavenda et al., 2005).

The experiment was conducted at the Palau Community College Research and Development Station (N7.529694, E134.560522). The station is located in the interior of Babeldaob Island, the second largest of the Micronesian islands, and is surrounded by forest. The field is one of the agricultural fields that were once used by the Japanese settlers. The soil here is Oxisol (ferralsols)—“Ngardmau-Bablethuap Complex,” which is characterized as a very gravelly loam with low organic matter content of between 1% and 4%. The permeability is moderately rapid (15–50 cm/hr) and very well drained. The available water capacity is between 0.05 and 0.10 cm/cm (Smith, 1983).

Treatments

The experiment was conducted from January to July 2019. The slope is 15.4° (13.4°–17.3°). The previous crop grown on the land was taro (Colocasia esculenta). The treatments were plants (with or without) × ridge (with or without) × 2 replications. These eight plots (2 × 10 m) were randomly set on the field (Table 1, Figure 1). The field was tilled using a hand tractor on January 22, leveled, and covered half the plots with weed control fabric (polypropylene, 0.4-mm thick, 120 g m^–2; I-Agri Corp., Tsuchiura) on 28 January. Weeds were cut on April 16, blown off the residue, removed the weed control fabric on April 17 (Figure 2), then tilled each plot using the hand tractor up and down direction to avoid the soil moving to neighboring plots. The average thickness of the soil tilled was 16 cm. A 70 cm width of the monitoring areas was made in the center of the plots by ridges or wooden boards (for the no-ridge treatment). Sweet potatoes (Ipomoea batatas) were transplanted at 70 cm intervals on April 17 (Figure 3). Trenches were dug at the upper end of the fields to prevent rainwater inflow. The lower ends were embanked and added 1-m lengths of weed control fabric to trap any eroded soil. Fertilizer was not applied. Hand weeding was conducted on May 21 and June 6.

Figure 1. Location of plots.

Figure 2. Conditions before cultivation.

Figure 3. Treatments of plots and the initial conditions.

Green: No mulch treatment, Stripe: Ridge treatment.

Determination

Weed control fabric was set up at the lower end of the ridge and fixed them to the ground with several wire bents into a U-shape (Figure 3). At the bottom of the rows, The soil was raised to a height of about 20 cm to create a weir to prevent the soil from flowing out. Precipitation data were collected every 5 min via a weather station in the Palau Community College Research and Development Station (about 100 m from the experimental fields; Figure 1). The condition of the fields was recorded using an automatic camera.

Analysis

Rainfall events that might cause severe erosion (more than 3 mm/10 min) were identified (Onaga, 1969) and the amount of eroded soil of each event was compared.

The amount of eroded soil was predicted with the Universal Soil Loss Equation (USLE) equation (Wischmeier & Smith, 1978) by the following formula using Microsoft Excel (Changed the original formula to meters).

A = R K LS P C metric ton ha^–1 year^–1

Where A = computed soil loss per unit area, R = the rainfall and runoff factor, K = the soil erodibility factor, LS = the topographic factor, C = the cover and management factor, and P = the support practice factor.

Storm soil losses from cultivated fields are directly proportional to a rainstorm parameter defined as the EI, and the A of each storm can be obtained using EI instead of the R.

E = 210+89 log₁₀ I 100 metric ton ha^–1

I cm h^–1: maximum rainfall in 30 min multiplied to 60 min; rainfall less than 1.27 cm is omitted, and the maximum value is 7.62 cm.

Soil erosion in a rainfall event is the cumulative value.

R = EI/100 metric ton ha^–1

For the United States of America, for convenience, the average annual rainfall intensity for the region is prepared as a table. In the case of Palau, the frequency of rainfall is extremely high, so the integrated values during this test period are considered to be sufficiently accurate.

Finally, the above equation was parameterized for each rainfall event as follows.

A = EI K LS P C/100 metric ton ha^–1

K = 0.05 (USDA Natural Resources Conservation Service; estimated based on percentage of silt, sand, and organic matter and on soil structure and permeability)

LS = (10/20.0)^0.5・(68.19 sin² 15.4° + 4.75sin 15.4°+0.068)= 4.34

C =1.0; Tillage

P = 1.00; vertical ridge

Plot area = 7 m²

When the survey area is less than 10 m², the erosion rate is almost constant, but when the survey area exceeds 10 m², the erosion rate decreases linearly as the area increased (García-Ruiz et al., 2015). Accordingly, this experiment may overestimate soil erosion.

Precipitation

The field site received regular rainfall, with total precipitation of 992 mm during the experimental period, from 17 April to July 15 (Figure 4). The field had 24 days of erosive rainfall more than 3 mm per 10 min (Figure 4). The rainfall threshold where surface runoff occurs is 2–3 mm per 10 min on a 15° slope, although these values vary according to different soil characteristics (Onaga, 1969). Highly erosive rainfall events occurred on day 7 after planting (May 2). After weeding is an erosive period. A heavy rainfall event of 17 mm per 10 min occurred on the next day after the first weeding. The second weeding was conducted after seven days of intensive rainfall, and a further erosive rainfall event of 7 mm per 10 min occurred after weeding. Thus, the rainfall conditions during the experimental period were expected to result in severe soil erosion.

Figure 4. Daily precipitation and erosive rainfall events.

The blocks show a rainfall event of more than 3 mm/10 min and the amount of precipitation. The colors distinguish the events.

Soil loss prediction by USLE

The 24 rainfall events potentially caused erosion during the observation period (Table 2). The soil loss prediction for bare land conditions by USLE was 0.57 kg per plot on day 7 (the first rainfall event after transplanting) and 2.82 kg (after the first weeding).

Soil erosion

Despite the severe rainfall conditions, none of the plots had any erosion at all through the experimental period (Figure 5).

Figure 5. Zero erosion of the first rainfall event after transplanting.

The predicted erosion was 0.57 kg per plot; however, the actual erosion was 0 kg in all plots.

Vegetation coverage

Most of the soil surface was bare by day 14 (May 1). Small weeds covered the soil surface by on day 21 (May 8), the day of the first weeding. The vegetation coverage by visual inspection ranged from 15%–85% on day 54 (Jun 10), after the second weeding. The vegetation coverage was 100% by day 89 (July15) (Figure 6).

Figure 6. Vegetation coverage.

Top panel: day 14, Upper middle panel: day 21 (before the first weeding), Lower middle panel: day 54 (after second weeding), Bottom panel day 89.

Underlying data

Figshare: Precipitation of Palau, https://doi.org/10.6084/m9.figshare.11769909.v1 (Oda et al., 2020).

This project contains the following underlying data:

Data are available under the terms of the Creative Commons Zero "No rights reserved" data waiver (CC0 1.0 Public domain dedication).