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Causes and controlling factors of Valley bottom Gullies
Amare, Selamawit ; Keesstra, Saskia ; Ploeg, Martine van der; Langendoen, Eddy ; Steenhuis, Tammo ; Tilahun, Seifu - \ 2019
Land 8 (2019)9. - ISSN 2073-445X
Badlands - Erosion - Landscape restoration - Runoff - Sediment - Soil saturation - Valley bottom
Valley bottomland provides diverse agricultural and ecosystem benefits. Due to concentrated flow paths, they are more vulnerable to gully erosion than hillslope areas. The objective of this review was to show what caused valley bottoms gullies and to present deficiencies in existing rehabilitation measures. From the literature review, we found the following general trends: watershed characteristics determine location of valley bottom gullies; an increase in water transported from the watershed initiates the formation of gullies; the rate of change of the valley bottom gullies, once initiated, depends on the amount of rainfall and the soil and bedrock properties. Especially in humid climates, the presence of subsurface flow greatly enhances bank slippage and advancement of gully heads. Valley bottom gully reclamation measures are generally effective in arid and semi-arid areas with the limited subsurface flow and deep groundwater tables, whereas, for (sub) humid regions, similar remedial actions are not successful as they do not account for the effects of subsurface flows. To ensure effective implementation of rehabilitation measures, especially for humid regions, an integrated landscape approach that accounts for the combined subsurface and surface drainage is needed.
Soil nutrient maps of Sub-Saharan Africa : assessment of soil nutrient content at 250 m spatial resolution using machine learning
Hengl, Tomislav ; Leenaars, Johan G.B. ; Shepherd, Keith D. ; Walsh, Markus G. ; Heuvelink, Gerard B.M. ; Mamo, Tekalign ; Tilahun, Helina ; Berkhout, Ezra ; Cooper, Matthew ; Fegraus, Eric ; Wheeler, Ichsani ; Kwabena, Nketia A. - \ 2017
Nutrient Cycling in Agroecosystems 109 (2017)1. - ISSN 1385-1314 - p. 77 - 102.
Africa - Machine learning - Macro-nutrients - Micro-nutrients - Random forest - Soil nutrient map - Spatial prediction
Spatial predictions of soil macro and micro-nutrient content across Sub-Saharan Africa at 250 m spatial resolution and for 0–30 cm depth interval are presented. Predictions were produced for 15 target nutrients: organic carbon (C) and total (organic) nitrogen (N), total phosphorus (P), and extractable—phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), sodium (Na), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), aluminum (Al) and boron (B). Model training was performed using soil samples from ca. 59,000 locations (a compilation of soil samples from the AfSIS, EthioSIS, One Acre Fund, VitalSigns and legacy soil data) and an extensive stack of remote sensing covariates in addition to landform, lithologic and land cover maps. An ensemble model was then created for each nutrient from two machine learning algorithms—random forest and gradient boosting, as implemented in R packages ranger and xgboost—and then used to generate predictions in a fully-optimized computing system. Cross-validation revealed that apart from S, P and B, significant models can be produced for most targeted nutrients (R-square between 40–85%). Further comparison with OFRA field trial database shows that soil nutrients are indeed critical for agricultural development, with Mn, Zn, Al, B and Na, appearing as the most important nutrients for predicting crop yield. A limiting factor for mapping nutrients using the existing point data in Africa appears to be (1) the high spatial clustering of sampling locations, and (2) missing more detailed parent material/geological maps. Logical steps towards improving prediction accuracies include: further collection of input (training) point samples, further harmonization of measurement methods, addition of more detailed covariates specific to Africa, and implementation of a full spatio-temporal statistical modeling framework.
Characterization of degraded soils in the humid Ethiopian highlands
Tebebu, Tigist Y. ; Bayabil, Haimanote K. ; Stoof, C.R. ; Giri, Shree K. ; Gessess, A.A. ; Tilahun, Seifu A. ; Steenhuis, Tammo S. - \ 2017
Land Degradation and Development 28 (2017)7. - ISSN 1085-3278 - p. 1891 - 1901.
Hard pan is a major cause of land degradation that affects agricultural productivity in developing countries. However, relatively little is known about the interaction of land degradation and hardpans. The objective of this study was, therefore, to investigate soil degradation and the formation of hardpans in crop/livestock mixed rainfed agriculture systems and to assess how changes in soil properties are related to the conversion of land from forest to agriculture. Two watersheds (Anjeni and Debre Mewi) were selected in the humid Ethiopian highlands. For both watersheds, 0-45 cm soil penetration resistance (SPR, n = 180) and soil physical properties (particle size, SOM, pH, base ions, CEC, silica content, bulk density and moisture content) were determined at 15 cm depth increments for three land uses: cultivated, pasture and forest. SPR of agricultural fields was significantly greater than that of forest lands. Dense layers with a critical SPR threshold of ≥ 2000 kPa were observed in the cultivated and pasture lands starting at a depth of 15-30 cm but did not occur in the undisturbed forest land. Compared with the original forest soils, agricultural fields were: lower in organic matter, CEC, and exchangeable base cations; more acidic; had a higher bulk density and more fine particles (clay and silt); and contained less soluble silica. Overall, our findings suggest that soil physical and chemical properties in agricultural lands deteriorated, causing disintegration of soil aggregates resulting in greater sediment concentration in infiltration water that clogged up macro-pores, thereby disconnecting deep flow paths found in original forest soils.
Morphological dynamics of gully systems in the subhumid Ethiopian Highlands : the Debre Mawi watershed
Zegeye, Assefa D. ; Langendoen, Eddy J. ; Stoof, C.R. ; Tilahun, Seifu A. ; Dagnew, Dessalegn C. ; Zimale, Fasikaw A. ; Guzman, C.D. ; Yitaferu, B. ; Steenhuis, T.S. - \ 2016
SOIL 2 (2016). - ISSN 2199-3971 - p. 443 - 458.
Gully expansion in the Ethiopian Highlands dissects vital agricultural lands with the eroded materials adversely impacting downstream resources, for example as they accumulate in reservoirs. While gully expansion and rehabilitation have been more extensively researched in the semiarid region of Ethiopia, few studies have been conducted in the (sub)humid region. For that reason, we assessed the severity of gully erosion by measuring the expansion of 13 selected permanent gullies in the subhumid Debre Mawi watershed, 30 km south of Lake Tana, Ethiopia. In addition, the rate of expansion of the entire drainage network in the watershed was determined using 0.5 m resolution aerial imagery from flights in 2005 and 2013. About 0.6 Mt (or 127 t ha−1 yr−1) of soil was lost during this period due to actively expanding gullies. The net gully area in the entire watershed increased more than 4-fold from 4.5 ha in 2005 to 20.4 ha in 2013 (> 3 % of the watershed area), indicating the growing severity of gully erosion and hence land degradation in the watershed. Soil losses were caused by upslope migrating gully heads through a combination of gully head collapse and removal of the failed material by runoff. Collapse of gully banks and retreat of headcuts was most severe in locations where elevated groundwater tables saturated gully heads and banks, destabilizing the soils by decreasing the shear strength. Elevated groundwater tables were therefore the most important cause of gully expansion. Additional factors that strongly relate to bank collapse were the height of the gully head and the size of the drainage area. Soil physical properties (e.g., texture and bulk density) only had minor effects. Conservation practices that address factors controlling erosion are the most effective in protecting gully expansion. These consist of lowering water table and regrading the gully head and sidewalls to reduce the occurrence of gravity-induced mass failures. Planting suitable vegetation on the regraded gully slopes will in addition decrease the risk of bank failure by reducing pore-water pressures and reinforcing the soil. Finally, best management practices that decrease runoff from the catchment will reduce the amount of gully-related sediment loss.
Citizen science in hydrology and waterresources: opportunities for knowledge generation, ecosystem service management, and sustainable development
Buytaert, W. ; Zulkafi, Z. ; Grainger, S. ; Acosta, L. ; Alemie, T.C. ; Bastiaensen, J. ; Bièvre, B. de; Bhusal, J. ; Clark, J. ; Dewulf, A.R.P.J. ; Foggin, M. ; Hannah, D.M. ; Hergarten, C. ; Isaeva, A. ; Karpouzoglou, T.D. ; Pandeya, B. ; Paudel, D. ; Sharma, K. ; Steenhuis, T.S. ; Tilahun, S. ; Hecken, G. van; Zhumanova, M. - \ 2014
Frontiers in Earth Science 2 (2014). - ISSN 2296-6463 - 21
klimaatverandering - gegevensverwerking - gegevensanalyse - burgers - publieke participatie - climatic change - data processing - data analysis - citizens - public participation
The participation of the general public in the research design, data collection and interpretation process together with scientists is often referred to as citizen science. While citizen science itself has existed since the start of scientific practice, developments in sensing technology, data processing and visualization, and communication of ideas and results, are creating a wide range of new opportunities for public participation in scientific research. This paper reviews the state of citizen science in a hydrological context and explores the potential of citizen science to complement more traditional ways of scientific data collection and knowledge generation for hydrological sciences and water resources management. Although hydrological data collection often involves advanced technology, the advent of robust, cheap, and low-maintenance sensing equipment provides unprecedented opportunities for data collection in a citizen science context. These data have a significant potential to create new hydrological knowledge, especially in relation to the characterization of process heterogeneity, remote regions, and human impacts on the water cycle. However, the nature and quality of data collected in citizen science experiments is potentially very different from those of traditional monitoring networks. This poses challenges in terms of their processing, interpretation, and use, especially with regard to assimilation of traditional knowledge, the quantification of uncertainties, and their role in decision support. It also requires care in designing citizen science projects such that the generated data complement optimally other available knowledge. Lastly, using 4 case studies from remote mountain regions we reflect on the challenges and opportunities in the integration of hydrologically-oriented citizen science in water resources management, the role of scientific knowledge in the decision-making process, and the potential contestation to established community institutions posed by co-generation of new knowledge.
|Analysing crop-loss in a bean rust pathosystem.␆II. Severity-damage relations.
Assefa, H. ; Zadoks, J.C. ; Tilahun, A. - \ 1998
Pest Management Journal of Ethiopia 1 (1998). - p. 1 - 13.