Developing a digital soil mapping protocol for southern Africa using case studies

[摘要] English: Although there is an increasing need for spatial soil information, traditional methods of soil survey are too cumbersome and expensive to supply in that need. Digital soil mapping (DSM) methods can fulfil that need. Internationally, DSM is moving from the research to the production phase. As soil-landscape interaction and availability of data varies between locations, local DSM research is needed to make its application practical. This research aims to produce a working DSM protocol which can be used for mapping large areas of land in southern Africa. The protocol must meet soil surveyors where they are at, being easy enough to follow, while also allowing for the creation of products needed by industry. To keep the link with industry's needs, a case study approach was followed. Four case studies were done in succession, with the protocol being improved with every case study. The case studies cover an array of challenges faced by soil surveyors. In the first case study a baseline protocol was created when two land types near Madadeni were disaggregated in a series of soil maps. With each map, more information was incorporated when creating the map. For Map 1 only the land type inventory and terrain analysis were used. A reconnaissance field visit with the land type surveyor was added for the second map. Field work and a simplified soil association legend proved to improve the map accuracy for Maps 3 and 4, which were created using 30% and 60% of the observations points as training data respectively. The accuracy of the maps increased when more information was utilized. Map 1 reached an accuracy of 35%, while Map 4 achieved a commendable accuracy of 67%. Principles which emerged was that field work is critical to DSM, more data input improves the output and that simplifying the map legend improves the accuracy of the map. An unrealistic demand for a soil survey of 37 000 ha of land in the Tete Province, northern Mozambique, possibly infested with land mines, in 8 working days by two persons, created an opportunity to apply the soil-land inference model (SoLIM) as a digital soil mapping tool. Dividing the area into smaller areas where unique soil distribution rules would apply (homogeneous areas, HA's) was introduced. A free survey was conducted along the available roads of the area. The final soil map for 15 000 ha had an accuracy of 69%. A principle which emerged was that inaccessible areas can be mapped, provided that they occur within surveyed HA's. Near Namarroi, Mozambique, the potential of DSM soil survey methods to rapidly produce land suitability maps for a large area with acceptable accuracy was evaluated. Conditioned Latin hypercube sampling (cLHS) was introduced to determine field observation positions. SoLIM was used to run an inference with soil terrain rules derived from conceptual soil distribution patterns. A restriction of the expert knowledge based approach was found in that only six soil map units (SMU's) could be determined per HA. The map achieved an overall accuracy of 80%. Land suitability maps were created based on the soil class map. In the Kruger National Park a soil map was used to create and extrapolate 2-dimensional conceptual hydrological response models (CHRM's) to a 3-dimensional landscape. This is a very good example on how value could be added to a soil map. An error matrix convincingly identified problem areas in the map where future work could focus to improve the soil map. The current data indicates that at least 28 soil observations are necessary to create a soil map to an acceptable standard. When minimum observation criteria are met, observation density is irrelevant. The cLHS method to pre-determine observation positions improved the usability of observations. Although more research is needed to accurately determine the minimum observation criteria, an observation strategy is suggested. A 15 step protocol is produced with which it was shown that soil surveyors could produce a variety of maps in diverse situations. The protocol relies on the expert knowledge of the soil surveyor, combined with field observations. It has the advantages that fewer observations are necessary, map accuracy assessment is possible, problem areas are identified and under certain conditions unsurveyed areas can also be mapped. On the down side, there is a limitation of six SMU's per HA. Further research needs to be done to determine the minimum criteria for soil observations, and soil distribution relationships between soil and remotely sensed covariates.

[发布机构] University of the Free State