Upgrading a 1/20,000 soil map with an apparent electrical conductivity survey

Despite their shortcomings, choropleth soil maps remain the most widespread source of information on soil resources. Since most nationwide soil surveys were conducted in the second half of the previous century, a need for upgrading emerges. We evaluated the potential of detailed observations made by a mobile, non-invasive proximal soil sensor to upgrade a part of the 1/20,000 choropleth soil map of Belgium. This study was conducted on a 14 ha area which had been mapped twice in the 1950s: first, during the national soil survey yielding a 1/20,000 soil map, and second, during a detailed investigation resulting in a 1/5000 map. The first map failed to identify the presence of a Tertiary clay substratum at variable depths, while the second map indicated this substratum to be present within 1.2 m below the soil surface for about a third of the area. A recent survey with the EM38DD soil sensor provided 9192 measurements of the apparent electrical conductivity (EC_a) within the study area. The depth of the substratum (D_ts) was noted at 60 calibration locations by augering and the relationship between EC_a and D_ts was modelled by an exponential curve with an R² of 0.80. This allowed the detailed mapping of D_ts by regression kriging. The predictions were validated using 46 independent observations of D_ts indicating a reasonable average error of 0.24 m and a very good correlation coefficient between observed and predicted values of 0.94. A map accuracy assessment indicated that even after classification, the D_ts classes were better predicted by the sensor data than the 1/5000 map which was based on many more auger observations. Finally an upgraded 1/20,000 soil map was presented, illustrating the potential of combining existing soil maps with proximal soil sensing technology.     

Modeling within field variation of the compaction layer in a paddy rice field using a proximal soil sensing system

A key characteristic of flooded paddy fields is the plough pan. This is a sub‐soil layer of greater compaction and bulk density, which restricts water losses through percolation. However, the thickness of this compacted layer can be inconsistent, with consequences such as variable percolation and leaching losses of nutrients, which therefore requires precision management of soil water. Our objective was to evaluate a methodology to model the thickness of the compacted soil layer using a non‐invasive electromagnetic induction sensor (EM38‐MK2). A 2.7 ha alluvial non‐saline paddy rice field was measured with a proximal soil sensing system using the EM38‐MK2 and the apparent electrical conductivity (EC_a) of the wet paddy soil was recorded at a high‐resolution (1.0 × 0.5 m). Soil bulk density (n = 10) was measured using undisturbed soil cores, which covered locations with large and small EC_a values. At the same locations (within 1 m²) the depth of the different soil layers was determined by penetrometer. Then a fitting procedure was used to model the EC_a – depth response functions of the EM38‐MK2, which involved solving a system of non‐linear equations and a R² value of 0.89 was found. These predictions were evaluated using independent observations (n = 18) where a Pearson correlation coefficient of 0.87 with an RMSEE value of 0.03 m was found. The EC_a measurements allowed the detail estimation of the compacted layer thickness. The link between water percolation losses and thickness of the compacted layer was confirmed by independent observations with an inverse relationship having a Pearson correlation coefficient of 0.89. This rapid, non‐invasive and cost‐effective technique offers new opportunities to measure differences in the thickness of compacted layers in water‐saturated soils. This has potential for site‐specific soil management in paddy rice fields.     

基于磁感式土壤表观电导率空间变异性的插值方法比较

针对目前黄河三角洲地区存在的土壤盐渍障碍问题,以该地区典型地块为研究对象,运用磁感大地电导率仪(EM38),结合GIS与地统计学研究了不同样点密度下土壤表观电导率的空间变异特征,确定了最佳的空间插值模式,并采用偏差指数对各分布图层的空间相似性进行了评价。结果表明：不同样点密度下的土壤表观电导率均呈中等变异强度,并服从对数正态分布;各样点密度下的土壤表观电导率均表现为强空间相关性,其空间变异主要表现在小于10 m的田间尺度上,且对于预测精度,泛克立格>普通克立格>反距离权重>局部多项式。偏差指数法分析表明,各插值方法分布图的空间相似性均随样点密度的降低而下降;对于相同的样点密度减小比例下的空间信息保留度,泛克立格>普通克立格>反距离权重>局部多项式,采用泛克立格法可以在保证预测精度的基础上合理降低样点密度。该研究为黄河三角洲地区盐渍土地磁感式田间数据采集的合理密度确定以及优化空间插值模式选取提供了理论依据与技术参考。     

Soil landscapes,land cover change and erosion features of the Central Plateau region of Tigrai,Ethiopia: Photo-monitoring with an interval of 30 years

Human land use of the Tigray landscape (north Ethiopia) can be traced back for at least 3000 years and is recognizably very complex, but in the past half-century there have been multiple narratives on environmental change in the Northern Ethiopian Highlands in which statements such as “the forest and soil resources in Tigray are dwindling at unprecedented rates” are common. In an attempt to provide an objective assessment, we made a semi-quantitative analysis of observed changes in the environment of the central Tigray plateau, between 1975 and 2006, and its impact on soil erosion. The first part of this period saw strong degradation, caused by a combination of drought, impoverishment, poor land husbandry and war; but over the whole period intense rehabilitation activities have been high on the agenda. To study these changes, two sets of 51 landscape photographs have been used. The older photo-set was taken in 1975 by R.N. Munro during the Tigrai Rural Development Study; locations were revisited in 2006 by J. Nyssen and colleagues, when a new set of photographs was made at the same locations and with the same aspect. Based on longstanding experience in soil erosion and landscape analysis worldwide and in Ethiopia, the time-lapsed photographs were rated for visible erosion, land cover and protective measures. We present a quantitative evaluation of the change of soil loss by sheet and rill erosion, involving the Universal Soil Loss Equation (USLE) and particularly the changes in the C (cover) and the P (management) factors. This allowed assessing soil loss in 2006 as a percentage of the 1975 situation. Both the landscape and land unit analysis show that the situation for natural resources has improved (and locally strongly improved) since 1974. The rehabilitation is due both to improved vegetation cover and to physical conservation structures. The USLE application indicates that in terms of a whole landscape the current average soil loss would be at around 68% of its 1975 rate. Exceptionally, degradation is still ongoing around Desa'a forest and some other remnant forests, and conservation should be strongly implemented too in these forests. On average, gullies have expanded slightly since 1975, but these incisions appear to have originated in the drought years of the 1980s. This photo-monitoring analysis invalidates hypotheses on (a) irreversibility of land degradation in Tigray; and (b) futility of Soil and Water Conservation (SWC) programmes. The study demonstrates that (a) land management has become an inherent part of the farming system in Tigray, and (b) that the authorities and NGOs are on the right track when promoting SWC.