1. Introduction
Soil erosion is one of the most serious environmental problems the world is facing today [
1]. In 2003, the global mean of annual soil erosion by water was estimated at 1150 t/km
2 [
1]. Soil erosion by water is by far the most important type of soil degradation, affecting about 1100 million ha worldwide (56% of the total area affected by human-induced soil degradation) [
1]. Almost 80% of the terrain affected by water erosion has a light to moderate degree of degradation. Among the major continents, Africa ranks second in the severity of soil erosion after Asia [
1]; deforestation and inappropriate cultivation practices are among the major causes of soil erosion. In Africa, the deforestation rate increased rapidly from 1100 km
2·a
−1 in the 1990s to 5400 km
2·a
−1 in the early 2000s [
2]. In most of Sub-Saharan Africa, more than 50% of the population heavily relies on local agriculture for food [
3], where poor land management usually leads to extreme soil erosion [
4]. Today, soil erosion is the major cause of land degradation in Sub-Saharan Africa with serious impacts on agricultural productivity [
5]. Numerous studies have highlighted that land conversion to agricultural land use stands as a catalyst for accelerated soil erosion rates [
6,
7,
8,
9,
10]; and if these agriculture-related erosion rates remain far beyond the rates of soil production, the global society will eventually be compelled to either adopt agricultural methods that sustain the soil or face increasing competition over a shrinking agricultural land base [
6].
With a population density of 470.6 per km
2 and an annual population growth rate of 2.7%, Rwanda is one of the most densely-populated countries in Africa (in comparison, the mean population densities of the world and Eastern Africa are 56.5 per km
2 and 59.2 per km
2, respectively) as of 1 July 2015 [
11]. The Rwandan economy is primarily based on rudimentary agriculture where about 83.4% of the population is dependent on subsistence agriculture [
12]. The rapid population increase caused a shortage of land resources, leading to an average of a 1.6% annual decrease in forest area from 1960–2000 [
13]. To cope with the demographic pressure, the government launched a large-scale national cropland conversion campaign that aimed to expand the irrigated cropland by 100,000 ha by 2020, among which 35,000 ha will be hillside irrigation, and 90% of the cropland is located on slopes of 5%–55% [
14]. With a hilly and mountainous relief, a fragile soil [
15] and a high average rainfall intensity of 1156 mm·a
−1 that concentrates in the wet season, the lands of Rwanda are highly susceptible to soil erosion [
16,
17]. According to a global assessment, Rwanda was among the 22 countries most seriously affected by soil degradation [
18]. Previous studies suggested that severe soil erosion in Rwanda, like in other East African countries, could be associated with unsustainable agricultural land management increased by high population densities [
19,
20,
21]. However, the magnitude and the spatial pattern of the agriculturally-induced soil erosion are still unclear at the national scale. This study aimed to assess the current cropland extent and the related soil erosion risks and to identify the lands that are unsuitable for cultivation in terms of soil conservation in Rwanda. Furthermore, the potential effects of the terracing cultivation method were evaluated as a conservation support practice to control soil erosion from croplands in Rwanda.
4. Discussion
As has been extensively reported, a galloping demography poses serious threats to environmental sustainability in Rwanda [
14]. The combination of the previous statistics of LCLU for three periods, namely 1990, 2000 and 2010 [
33], and the LCLU statistics obtained from the present study revealed that LCLU in Rwanda has been greatly changing in response to the demographic pressure during the past 25 years (
Figure 8). Just 25 years ago, forest covered about 44% of Rwanda, and the cropland only occupied 28% of the territory. Today, more than 56% of the country’s land areas have been converted to croplands to meet the food demands, at the expanse of massive deforestation. Especially, our results indicated a sharp increase of cropland area in the recent five years (2010–2015).
As the land resource became scarce, many of the croplands were established in the mountain areas that were unsuitable for cultivation, leading to extreme soil erosion that threatened the environment, such as soil and water resources, and the agriculture sustainability of Rwanda (
Figure 6b). Our modeling results confirmed previous findings that intensive farming on steep slopes of the Buberuka highlands has led to heavy soil loss in the north of Rwanda [
53]. In addition, we found that the west mountain ranges that stretch from the Congo Nile Ridge in the south to the Volcanic Range in the north also faced severe soil erosion due to unsuitable cropland cultivation. A field study in the Guacerique, Honduras, a watershed with similar mountainous terrain (elevation = 1420 m) and climate (precipitation = 1400 mm·a
−1), showed that traditional agricultural practices on the mountain slopes led to a soil erosion rate exceeding 300 t·ha
−1·a
−1 [
54]. The findings of this study are in agreement with the findings of previous research that estimated the erosion rate at around 300 t·ha
−1·a
−1 for other areas in the tropics [
47,
54]. Moreover, the extreme soil erosion rate of 1642 t·ha
−1·a
−1 observed in this study falls within the range of high erosion occurring in areas with very fragile soils, steep slopes and high rainfall intensity, as previously discussed by Ganasri et al. while estimating the soil erosion in Nethravathi basin located in the middle region of the Western Ghats, western India where the soil erosion rate was ranging from 0–1,907,287 t·ha
−1·a
−1 [
46]. This study also highlighted that croplands on steeper terrains are exposed to mean erosion rates much higher (421 t·ha
−1·a
−1) than the other LCLU types, as previously indicated by various research [
4,
18,
46,
47,
55]. Soil erosion is responsible for water pollution in Rwanda, for example the high turbidity (737 ± 571 Nephelometric Turbidity Units) of the Nyabarongo River of Rwanda where 62% of its pollution comes from the agricultural land use in its catchment [
56]. Soil erosion also reduced soil fertility and caused declining cropland productivity in Rwanda [
53]. In the world, soil erosion causes an estimated production loss of 10% for cereals, 5% for soybeans, 5% for pulses and 12% of roots and tubers [
57]. The consequence is an increasing demand for more land conversion to cropland to maintain food production, leading to an unsustainable loop that will quickly deplete the land resources of Rwanda.
Therefore, it is ultimately urgent to take actions to curb the severe soil erosion caused by the inappropriate cropland management in Rwanda. Returning the unsuitable cropland (as identified in
Figure 6b) to forest might effectively reduce soil erosion or else, other practical mechanisms should be assessed. Our soil erosion map clearly shows that the four forest reservations—the Nyungwe National Park, the Gishwati Forest Reserve, the Mukura Forest Reserve and the Volcanoes National Park (
Figure 3)—had very low soil erosion, in contrast to the high erosion rate of the rest of the areas in the west of Rwanda (
Figure 5). Our study, like the previous studies (e.g., [
38,
58]), indicated that a slope greater than 30% is associated with severe erosion and can be used as a threshold in determining the site of reforestation. According to our analysis, 26% of the total Rwandan croplands located on a slope >30% accounted for 60% of the total soil erosion in Rwanda. However, our assessment indicated that under the current agricultural management practice, the unsuitable cropland accounted for 24% of the cultivated area. Abandoning such a large fraction of cropland will create big challenges for the Rwanda government to meet the food demand from a galloping demography. One of the solutions would be the adoption of terraces. Properly-designed terraces and outlet channels collect surface runoff and convey it off the field at non-erosive velocities [
38]. Several studies suggested that bench (radical) terraces could effectively curb soil erosion risk in mountain areas [
59,
60].
Using the RUSLE model, we re-assessed the soil erosion rate in Rwanda under the terracing practice scenario (
Table 2) and found that the mean cropland erosion rate in Rwanda could be reduced by 79%, leading to a drastic decrease of national soil loss by 75%. Furthermore, the terracing could reduce the percentage of cropland area with an extreme erosion rate (i.e., >300 t·ha
−1·a
−1) from 24% down to 7.6% (
Figure 9). The remained unsuitable croplands would only occupy 4% of the national land area, and it will be very viable to return them to forest.
It should be made aware that many of the people cultivating these marginal croplands under extreme soil erosion are from the underprivileged stratum of the society. Directly depriving their land would exacerbate the poverty issues. If economically viable, the government and/or NGOs can buy the lands from these people and return them to tropical rainforest. The old landowners can be hired as forest managers to guarantee a quick reforestation to reduce soil loss as much possible. Otherwise, the government and/or NGO should take the responsibilities to help the farmers to develop soil conservative cultivation practices, such as, for example, applying Vetiver Eco-engineering Technology due to its economical soil erosion control, stabilization of steep slopes [
61] and the construction of erosion control dams to absorb and break down excess runoff from unusually intense storms in various parts of the watersheds [
62,
63,
64].
Rwandan farmers are aware of the benefits of bench terraces, in the sense that they are productive, i.e., having a positive effect on production (85.3%), effective for soil erosion control (90.6%) and that they increase soil fertility (56.2%) [
65]. Greater costs of conservation than expected benefits on food production, uncertainties of land fragmentation, land tenure and misguided efforts of earlier interventions in soil conservation, however, remain a hindrance to both the maintenance of existing terraces and the creation of new ones [
12,
66]. Bench terraces require interventions of the government and/or non-government organizations (NGOs), because they are not profitable when market prices are applied [
67]. For example, bench terracing investment costs were estimated at RWF 800,000 ha
−1 in Rwanda; the net present value (NPV) is negative (loss of RWF47,384); and the internal rate of return (IRR) (11%) is lower than the discount rate of 13%, during a period of 20 years [
66]. However, the benefits from soil erosion control are not limited to sustaining cropland productivity, but also include important social/ecological services, such as protecting water quality and preventing flooding. When the aggregated economic costs (in both provisional land services and non-provisional ecosystem services) of soil loss and degradation are taken into account, it usually makes sound economic sense to invest in programs that are effective in the control of widespread erosion. For instance, the total investment for U.S. erosion control was estimated at around $8.4 billion per year. Given that erosion causes about $44 billion in damages each year, it would seem that the investment is a small price to pay: For every $1 invested, $5.24 would be saved. This small investment would reduce U.S. agricultural soil loss by about 4 × 10
9 tons and help protect their water and soil resources, as well as current and future food supply [
68]. Since the social costs of soil erosion (i.e., losses in non-provisional ecosystem services) and offsite externalities could be higher than the costs of applying terracing, it makes sense for public policy to support the application of terracing [
67]. If appropriate economic incentives, such as maintaining high relative prices for crop products, high subsidies for fertilizers and the availability of rural credit at affordable rates, the farmers can adopt land management practices and farming systems (i.e., agroforestry, terracing, etc.) that promote considerably soil erosion control [
67].
It should be recognized that our modeling results only estimated the soil erosion risk of Rwanda rather than actual soil loss, which is very costly and can only be measured at the plot scale. Previous studies indicated variables, but acceptable accuracy of RUSLE in similar situations [
46,
47,
69,
70]. Furthermore, due to the local climate conditions and the deficit in field observations of actual soil erosion rates, it was difficult to optimize the parameters for the RUSLE model. For example, the C factor was usually calculated with Equation (9).
Previous studies indicated that seasonal variations of the Normalized Difference Vegetation Index (NDVI) could strongly influence the estimated soil erosion rate [
18,
71]. However, the heavy cloudiness problem in the tropical rainforests and mountain areas of the country [
18,
26] made it nearly impossible to retrieve the dynamic of NDVI and land cover change with satellite data for a whole year. Using the NDVI values from the dry season, when the cloudiness problem is less serious, would risk a biased C factor. Moreover, with the lack of field-observed soil erosion data, we were unable to estimate the empirical parameters (
a and
b) in the above Equation (9) for our study area. Therefore, the present study established the C factor map by attributing the C factor value according to LCLU categories (
Figure 2d) following the guide of Kim et al., 2005 [
47]. The approach has been approved by a number of similar studies, particularly in locations with limited data availability [
41,
46].
The outcomes of this work could be useful for projects related to the soil protection strategy of Rwanda, one of the most densely-populated countries of the world. Based on the latest land cover map, we found large areas of lands under severe soil erosion risks in the west mountain ranges (the Congo Ridge and the Volcanic Range), particularly in the Gakenke, Ngororero and Muhanga districts. The soil erosion risk maps developed in this study (
Figure 6) are useful for identifying hotspots of soil erosion in the country. Furthermore, our study indicated the terracing could substantially reduce the soil erosion in Rwanda.