Regional soil erosion assessment based on a sample survey and geostatistics

Soil erosion is one of the most significant environmental problems in China. From 2010 to 2012, the fourth national census for soil erosion sampled 32 364 PSUs (Primary Sampling Units, small watersheds) with the areas of 0.2–3 km2. Land use and soil erosion controlling factors including rainfall erosivity, soil erodibility, slope length, slope steepness, biological practice, engineering practice, and tillage practice for the PSUs were surveyed, and the soil loss rate for each land use in the PSUs was estimated using an empirical model, the Chinese Soil Loss Equation (CSLE). Though the information collected from the sample units can be aggregated to estimate soil erosion conditions on a large scale; the problem of estimating soil erosion condition on a regional scale has not been addressed well. The aim of this study is to introduce a new model-based regional soil erosion assessment method combining a sample survey and geostatistics. We compared seven spatial interpolation models based on the bivariate penalized spline over triangulation (BPST) method to generate a regional soil erosion assessment from the PSUs. Shaanxi Province (3116 PSUs) in China was selected for the comparison and assessment as it is one of the areas with the most serious erosion problem. Ten-fold cross-validation based on the PSU data showed the model assisted by the land use, rainfall erosivity factor (R), soil erodibility factor (K), slope steepness factor (S), and slope length factor (L) derived from a 1 : 10 000 topography map is the best one, with the model efficiency coefficient (ME) being 0.75 and the MSE being 55.8 % of that for the model assisted by the land use alone. Among four erosion factors as the covariates, the S factor contributed the most information, followed by K and L factors, and R factor made almost no contribution to the spatial estimation of soil loss. The LS factor derived from 30 or 90 m Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) data worsened the estimation when used as the covariates for the interpolation of soil loss. Due to the unavailability of a 1 : 10 000 topography map for the entire area in this study, the model assisted by the land use, R, and K factors, with a resolution of 250 m, was used to generate the regional assessment of the soil erosion for Shaanxi Province. It demonstrated that 54.3 % of total land in Shaanxi Province had annual soil loss equal to or greater than 5 t ha−1 yr−1. High (20–40 t ha−1 yr−1), severe (40–80 t ha−1 yr−1), and extreme ( \u3e  80 t ha−1 yr−1) erosion occupied 14.0 % of the total land. The dry land and irrigated land, forest, shrubland, and grassland in Shaanxi Province had mean soil loss rates of 21.77, 3.51, 10.00, and 7.27 t ha−1 yr−1, respectively. Annual soil loss was about 207.3 Mt in Shaanxi Province, with 68.9 % of soil loss originating from the farmlands and grasslands in Yan\u27an and Yulin districts in the northern Loess Plateau region and Ankang and Hanzhong districts in the southern Qingba mountainous region. This methodology provides a more accurate regional soil erosion assessment and can help policymakers to take effective measures to mediate soil erosion risks

The effect of leaf litter cover on surface runoff and soil erosion in Northern China.

The role of leaf litter in hydrological processes and soil erosion of forest ecosystems is poorly understood. A field experiment was conducted under simulated rainfall in runoff plots with a slope of 10%. Two common types of litter in North China (from Quercus variabilis, representing broadleaf litter, and Pinus tabulaeformis, representing needle leaf litter), four amounts of litter, and five rainfall intensities were tested. Results revealed that the litter reduced runoff and delayed the beginning of runoff, but significantly reduced soil loss (p<0.05). Average runoff yield was 29.5% and 31.3% less than bare-soil plot, and for Q. variabilis and P. tabulaeformis, respectively, and average sediment yield was 85.1% and 79.9% lower. Rainfall intensity significantly affected runoff (R = 0.99, p<0.05), and the efficiency in runoff reduction by litter decreased considerably. Runoff yield and the runoff coefficient increased dramatically by 72.9 and 5.4 times, respectively. The period of time before runoff appeared decreased approximately 96.7% when rainfall intensity increased from 5.7 to 75.6 mm h-1. Broadleaf and needle leaf litter showed similarly relevant effects on runoff and soil erosion control, since no significant differences (p≤0.05) were observed in runoff and sediment variables between two litter-covered plots. In contrast, litter mass was probably not a main factor in determining runoff and sediment because a significant correlation was found only with sediment in Q. variabilis litter plot. Finally, runoff yield was significantly correlated (p<0.05) with sediment yield. These results suggest that the protective role of leaf litter in runoff and erosion processes was crucial, and both rainfall intensity and litter characteristics had an impact on these processes

Study on hydrological functions of litter layers in North China.

Canopy interception, throughfall, stemflow, and runoff have received considerable attention during the study of water balance and hydrological processes in forested ecosystems. Past research has either neglected or underestimated the role of hydrological functions of litter layers, although some studies have considered the impact of various characteristics of rainfall and litter on litter interception. Based on both simulated rainfall and litter conditions in North China, the effect of litter mass, rainfall intensity and litter type on the maximum water storage capacity of litter (S) and litter interception storage capacity (C) were investigated under five simulated rainfall intensities and four litter masses for two litter types. The results indicated: 1) the S values increased linearly with litter mass, and the S values of broadleaf litter were on average 2.65 times larger than the S values of needle leaf litter; 2) rainfall intensity rather than litter mass determined the maximum interception storage capacity (Cmax ); Cmax increased linearly with increasing rainfall intensity; by contrast, the minimum interception storage capacity (Cmin ) showed a linear relationship with litter mass, but a poor correlation with rainfall intensity; 3) litter type impacted Cmax and Cmin ; the values of Cmax and Cmin for broadleaf litter were larger than those of needle leaf litter, which indicated that broadleaf litter could intercepte and store more water than needle leaf litter; 4) a gap existed between Cmax and Cmin , indicating that litter played a significant role by allowing rainwater to infiltrate or to produce runoff rather than intercepting it and allowing it to evaporate after the rainfall event; 5) Cmin was always less than S at the same litter mass, which should be considered in future interception predictions. Vegetation and precipitation characteristics played important roles in hydrological characteristics

Quantifying Characteristics of Fine Roots Contributing to Water Flow Process in Rocky Mountainous Area Using Dye Tracer Experiment and Monolith Method

Root channel serve as one major type of pathway for water flow in forest soil. In this paper, we combined a dye tracer experiment and a monolith method to distinguish the fine roots (0 < d ≤ 5 mm) that served as preferential water flow pathways in rocky mountainous areas. The characteristics of these types of roots among different soil layer and different root diameter were discussed. In this modified method, two typical forest species in Mount JiuFeng were selected. One was broad-leaved forest (BF) with Quercus variabilis, and the other one was needle-leaved forest (NF) afforested with Platycladus orientalis (L.) Franco. Monoliths were taken from horizontal profiles along with the soil depth after the dye tracer experiment was conducted. Fine roots contributing to water flow processes were defined as being dyed by the colour of the tracer and selected from the soil. Our results showed that not all of the fine roots can contribute to water flow, even in the soil layer of 0 to 10 cm. In the soil layer of 0 to 30 cm, the proportion of fine roots with 0 < d ≤ 1 mm made more than approximately 50% contributions to water flow in every soil layer. Meanwhile, the share of stained fine roots with a diameter of 1 < d ≤ 5 mm had an increased trend along soil depth, which was more clear in NF than in BF

Soil properties (0–40 cm deep) in experimental and control plots.

<p>Soil properties (0–40 cm deep) in experimental and control plots.</p

Relationship between litter mass and sediment concentration for bare-soil plots (open diamonds), <i>Quercus variabilis</i> litter (closed diamonds), and <i>Pinus tabulaeformis</i> litter (open triangles) for four litter masses.

<p>Relationship between litter mass and sediment concentration for bare-soil plots (open diamonds), <i>Quercus variabilis</i> litter (closed diamonds), and <i>Pinus tabulaeformis</i> litter (open triangles) for four litter masses.</p

Sediment yield and sediment concentration data corresponding to each treatment and simulated rainfall event.

<p>Sediment yield and sediment concentration data corresponding to each treatment and simulated rainfall event.</p

Relationships between litter mass and <i>C_max</i> of <i>Q.variabilis</i> (A), <i>A</i>. <i>truncatum</i> (B), <i>P. tabulaeformis</i> (C), <i>P. orientalis</i> (D).

<p>These values were obtained using five rainfall intensities. To clarify the relationship between litter mass and <i>C_max</i> (or <i>C_min</i>), rainfall intensities are not shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0070328#pone-0070328-g003" target="_blank">Figure 3</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0070328#pone-0070328-g005" target="_blank">5</a>.</p