Use of the K factor from the Universal Soil Loss Equation can show arable land in Palau

Site description

Palau forms part of the Micronesia region in the western Pacific Ocean. Palau's economy is mainly owing to tourism and the increase of tourists increases the consumption of agricultural products, but they had been afforded by imports. The agriculture in Palau is mainly taro cultivation at swamp by a traditional and environmentally friendly method. However recently, modern agriculture is rapidly grown and that has a large risk of causing erosion. There are open agricultural fields that were once utilized by the Japanese, prior to World War Two. The redevelopment of these fields is starting. Generally, 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°. As well as having steep slopes, the island is also subject to heavy rainfall. Most of the country (92%) was categorized within the most tolerable rank, having a T factor of 5 (more than 5 tons per acre per year) (Smith, 1983); however, the data was updated by USDA Natural Resources Conservation Service in 2009, and most of the land (80%) was categorized within the most fragile rank, having a T factor of 1. 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 field is one of the agricultural fields that were once utilized by the Japanese. The soil here is Oxisol—“Ngardmau-Bablethuap Complex”, which is characterized as a very gravelly loam with low organic matter content of between 1% and 4%, and (Smith, 1983). 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). The previous crop grown on the land was taro (Colocasia esculenta). The slope is 15.4° (13.4°–17.3°).

Treatments

We conducted the experiment from January to July 2019. The treatments were plants (with or without) × ridge (with or without) × 2 replications. We set these eight plots (2 × 10 m) randomly on the field (Table 1, Figure 1 and Figure 2). We tilled the field using a hand tractor on 22 January, leveled the field, 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. We cut weeds on 16 April, blew off the residue, removed the weed control fabric on 17 April (Figure 3), then tilled each plot using the hand tractor up and down so that the soil did not mix with the soil of the neighboring plots. The average thickness of the soil tilled was 16 cm. We made a 70 cm width of the monitoring areas in the center of the plots by ridges or wooden boards (for the no-ridge treatment). We transplanted sweet potatoes (Ipomoea batatas) at 70 cm intervals on 17 April (Figure 4). We dug trenches at the upper end of the fields to prevent rainwater inflow. We embanked the lower ends and added 1-m lengths of weed control fabric to trap any eroded soil. Fertilizer was not applied. Hand weeding was conducted on 21 May and 6 June.

Figure 1. Location of plots.

Green: No mulch treatment, Stripe: Ridge treatment.

Figure 2. Initial condition of the field.

Figure 3. Conditions before cultivation.

Figure 4. Initial conditions.

The order of the plots is 4, 7, 1, 5, 6, 8, 2, 3.

Determination

Following every heavy rainfall event, we collected any soil that had been eroded and accumulated on the weed control fabric (Figure 4). We collected precipitation data every 5 minutes via a weather station in the Palau Community College Research and Development Station (about 100 m from the experimental fields). The condition of the fields was recorded using an automatic camera.

Analysis

We identified rainfall events that caused severe erosion (more than 3 mm/10 min) (Onaga, 1969) and compared the amount of eroded soil of each event.

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.

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

The above equation was parameterized as follows.

E=210+89 log₁₀ I₃₀ 100 metric ton ha^–1

I₃₀ 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.

A = EI×K×LS×P×C metric ton ha^–1

K = 0.05

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

P = 1.00; vertical ridge

C =1.0; Tillage

EI = (E×I₃₀)/100

Plot area = 7 m²

Precipitation

The field site received regular rainfall, with total precipitation of 992 mm during the experimental period, from 17 April to 15 July (Figure 5). There were 46 days of erosive rainfall more than 3 mm per 10 min (Figure 6). 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). There was a highly erosive rainfall event on day 7 after planting (2 May). Following weeding, an erosive period, a heavy rainfall event of 17 mm per 10 min occurred on the next day after weeding took place. The second weeding was conducted after seven days of intensive rainfall, with a further erosive rainfall event of 7 mm per 10 min that occurred just after weeding took place. Thus, the rainfall conditions during the experimental period were expected to result in severe soil erosion.

Figure 5. Daily precipitation.

Figure 6. 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

There were 24 rainfall events that could have 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 7).

Figure 7. 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 (1 May). The surface of the soil was covered by small weeds on day 21 (8 May), the day of the first weeding. The vegetation coverage by visual inspection ranged from 15–85% on day 54 (10 Jun), after the second weeding. The vegetation coverage was 100% by day 89 (15 July) (Figure 8).

Figure 8. 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). Data are available under the terms of the Creative Commons Zero "No rights reserved" data waiver (CC0 1.0 Public domain dedication).