Browsing by Author "Poesen, Jean"

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Animal-powered tillage erosion assessment in the southern Andes region of Ecuador
(2007-06-01) Dercon, Gerd; Govers, Gerard; Poesen, Jean; Sánchez, Henrry; Loaiza, George; Vandenbroeck, E.; Rombaut, K.; Deckers, Jozef
While water erosion has been the focus of past research in the Andes, former studies show that soil erosion could also be related to the methods used in cultivating the fields. The main objective of the present study was to assess (i) tillage erosion caused by the traditional animal-powered “yunta” or ard plough in the Andes and the factors controlling the process and (ii) the implications for soil conservation. Erosion rates were experimentally measured on 27 sites, having slopes from ca. 0% to 60% and soils ranging from Andosols to Cambisols, in the Andes region of Ecuador (Gima, Azuay). Different tillage methods were assessed: (i) tillage parallel to the contour lines (‘Paralelo’) and (ii) tillage at an angle with the contour lines. Statistical analysis points out that erosion caused by animal-powered tillage is gravity-driven. A strong correlation exists between slope and downslope displacement: furthermore, tillage depth and initial soil condition are important. For the ‘Paralelo’ tillage method the tillage transportation coefficient (k) is below 100 kg m− 1 Tillage Pass− 1, for the combined ‘Arado’–‘Cruzado’ tillage method k may exceed 300 kg m− 1. Tillage erosion is responsible for the reduction of the slope between the contour strips over a relatively short time period of 20 years, resulting in the formation of terraces and therefore the reduction of the water erosion risk. However, at the same time it may negatively affect soil quality.
Environmental factors controlling spatial variation in sediment yield in a central Andean mountain area
(2008-06-15) Molina, Armando; Govers, Gerard; Poesen, Jean; Van Hemelryck, Hendrik; De Bièvre, Bert; Vanacker, Veerle
A large spatial variability in sediment yield was observed from small streams in the Ecuadorian Andes. The objective of this study was to analyze the environmental factors controlling these variations in sediment yield in the Paute basin, Ecuador. Sediment yield data were calculated based on sediment volumes accumulated behind checkdams for 37 small catchments. Mean annual specific sediment yield (SSY) shows a large spatial variability and ranges between 26 and 15,100 Mg km−2 year−1 . Mean vegetation cover (C, fraction) in the catchment, i.e. the plant cover at or near the surface, exerts a first order control on sediment yield. The fractional vegetation cover alone explains 57% of the observed variance in ln(SSY). The negative exponential relation (SSY =a×e −b C) which was found between vegetation cover and sediment yield at the catchment scale (103 –109 m2 ), is very similar to the equations derived from splash, interrill and rill erosion experiments at the plot scale (1–103 m2 ). This affirms the general character of an exponential decrease of sediment yield with increasing vegetation cover at a wide range of spatial scales, provided the distribution of cover can be considered to be essentially random. Lithology also significantly affects the sediment yield, and explains an additional 23% of the observed variance in ln(SSY). Based on these two catchment parameters, a multiple regression model was built. This empirical regression model already explains more than 75% of the total variance in the mean annual sediment yield. These results highlight the large potential of revegetation programs for controlling sediment yield. They show that a slight increase in the overall fractional vegetation cover of degraded land is likely to have a large effect on sediment production and delivery. Moreover, they point to the importance of detailed surface vegetation data for predicting and modeling sediment production rates.
Impact of land use changes on the hydrological properties of volcanic ash soils in South Ecuador
(2002-06) Buytaert, Wouter; Deckers, Jozef; Dercon, Gerd; Bievre, Bert De; Poesen, Jean; Govers, Gerard
The effect of land use on the water retention capacity of Umbric Andosols in south Ecuador was studied. The objective was to acquire a better insight into the hydrological processes of the ecosystem and the role of the soil, in order to assess the impact of changing soil properties due to land use change on the hydrology of the high Andes region. Field data on the water retention capacity at wilting point of Umbric Andosols were collected for both cultivated field conditions and original bush vegetation. The pH in water and in NaF, texture, organic matter content and dry bulk density were measured to show which physicochemical soil characteristics are responsible for the water retention of the Umbric Andosols and for the irreversible loss in water retention due to air drying. Organic matter content appears to be very important and certainly more important than allophane clay content. Water retention of the organic litter layer was calculated to be 16 mm, this would be lost when vegetation was cleared and the land cultivated.
Linking hydrological, infinite slope stability and land-use change models through GIS for assessing the impact of deforestation on slope stability in high Andean watersheds
(2003-06-16) Vanacker, Veerle; Vanderschaeghe, Michiel; Govers, Gerard; Willems, Edith; Poesen, Jean; Deckers, Jozef; Bievre, Bert De
In the Ecuadorian Andes, episodic slope movements comprising shallow rotational and translational slides and rapid flows of debris and soil material are common. Consequently, not only considerable financial costs are experienced, but also major ecological and environmental problems arise in a larger geographical area. Sediment production by slope movement on hillslopes directly affects sediment transport and deposition in downstream rivers and dams and morphological changes in the stream channels. In developing countries world-wide, slope movement hazards are growing: increasing population pressure and economic development force more people to move to potentially hazardous areas, which are less suitable for agriculture and rangelands. This paper describes the methods used to determine the controlling factors of slope failure and to build upon the results of the statistical analysis a process-based slope stability model, which includes a dynamic soil wetness index using a simple subsurface flow model. The model provides a time-varying estimate of slope movement susceptibility, by linking land-use data with spatially varying hydrologic (soil conductivity, evapotranspiration, soil wetness) and soil strength properties. The slope stability model was applied to a high Andean watershed (Gordeleg Catchment, 250 ha, southern Ecuadorian Andes) and was validated by calculating the association coefficients between the slope movement susceptibility map of 2000 and the spatial pattern of active slope movements, as measured in the field with GPS. The proposed methodology allows assessment of the effects of past and future land-use change on slope stability. A realistic deforestation scenario was presented: past land-use change includes a gradual fragmentation and clear cut of the secondary forests, as observed over the last four decades (1963–2000), future land-use change is simulated based on a binary logistic deforestation model, whereby it was assumed that future land-use change would continue at the same rate and style as over the last 37 years (1963–2000).
Runoff generation in a degraded Andean ecosystem: Interaction of vegetation cover and land use
(2007-10-15) Molina, Armando; Govers, Gerard; Vanacker, Veerle; Poesen, Jean; Zeelmaekers, Edwin; Cisneros, Felipe
Tropical mountain regions are affected by rapid land use/-cover change, which may threaten their (eco-) hydrological functions. Although there is a growing interest in evaluating the effect of land use/-cover change on mountain hydrology, quantitative assessments of the impact of land use/- cover on hydrological processes are hampered by the lack of field measurements characterizing runoff generation processes. In this paper, we present results from field experiments of rainfall runoff mechanisms in the southern Ecuadorian Andes. A rainfall simulator was used to quantify the hydrological response of distinct land use/-cover types to intense rainfall (about 40 mm/h). The rainfall runoff experiments indicate that degraded and abandoned land generate surface runoff within a few minutes after the start of the rainfall event. These lands have a very rapid and sharp hillslope hydrological response, as Hortonian overland flow is the dominant runoff generation mechanism. In contrast, surface runoff on arable and rangelands is rare, as their soils are characterized by a high infiltration capacity (i.e. N29 mm/h). Our experiments provide evidence that runoff generation in degraded Andean ecosystems is mainly controlled by the surface vegetation cover and land management. When reducing the surface vegetation cover, the soil is increasingly affected by rapid hillslope runoff as the presence of large amounts of smectites in the outcropping soft rocks makes the material very prone to sealing and crusting, thereby enhancing runoff generation.
Spatial variability in crop response under contour hedgerow systems in the Andes region of Ecuador
(2006-03) Dercon, Gerd; Deckers, Jozef; Sánchez, Henrry; Ramírez, Marco; Vanegas, Raúl; Tacuri Espinoza, Víctor Eduardo; Loaiza, George; Govers, Gerard; Poesen, Jean
Land management practices on tropical soils have been studied extensively during the past decades. However, assessments of their impact on field-scale spatial variability in crop performance are rare. A case of practices affecting spatial crop response at field level in a systematic way is the contour hedgerow system, a widespread soil conservation technique on steep lands in the tropics. The objectives of this study were to assess: (i) spatial variability in crop response under contour hedgerow systems in the Andes and (ii) implications for management of steep land alley farming. In order to measure spatial variability in crop response, Triticum aestivum was sown in rows following the contours, placed every 25 cm along the slope direction. Grain yield was monitored on eight alleys, having slopes from 15 to 30%, a length of 4–8 m and soils ranging from Dystri-Vertic Cambisols to Haplic Phaeozem, in the Andes region of Ecuador. In order to relate crop response with soil fertility, soil sampling was carried out in bands following the contour. Top soil properties, such as organic carbon, NTotal, NO3−, P, exchangeable Aluminium, P fixation, exchangeable bases, CEC and texture, were analysed. The barrier strips of the contour hedgerow systems were composed of: (i) Pennisetum clandestinum and Lolium multiflorum or (ii) Phalaris tuberosa. Spatial variability in crop response was described by fourth-order polynomial equations, where position in the alley was the independent variable and plant characteristics were the dependent variables. Grain yield ranged from 0.8 to 4.0 t ha−1 (net area). Clear and significant patterns of spatial variability in crop response were found in all alleys, with R2 varying from 0.55 to 0.90. Crop response was strongly and positively correlated with NO3−, P, exchangeable bases or organic carbon, dependent on the management, age and soil conditions of the alleys. Tillage erosion and soil accumulation on respectively the upper and lower elevations of the alleys were the main causes for a tendency to a significantly increased crop response along the slope direction. However, according the type of grass used in the barrier strip, competition for soil fertility and soil moisture occurred and influenced in negative way crop productivity in the lower elevations of the alleys. An index is proposed to assess the effect of contour hedgerows on crop response. The present study shows that contour hedgerow systems cannot always be evaluated as completely positive. Soil fertility improvement on the upper part of the alleys and a better management of the barrier strip are suggested to enhance crop productivity of contour hedgerow systems.
Spatial variability in soil properties on slow-forming terraces in the Andes region of Ecuador
(2003-07) Dercon, Gerd; Deckers, Jozef; Poesen, Jean; Sánchez, Henrry; Vanegas, Raúl; Ramírez, Marco
Due to severe land degradation problems, soil conservation is a matter of major concern in the Ecuadorian Andes. Slow-forming terraces, a variation of hedgerow agro-forestry systems composed by contour grass barrier strips, can be considered as the most frequently used technique in Andean rural communities of Ecuador. However, due to shallow soils, terrace development often has the disadvantage causing gradients in soil properties from the upper to the lower elevations within the terrace. The main objectives of this study were to assess (i) spatial variability in soil properties on slow-forming terraces in the Andes and (ii) implications of variability for the management of this soil conservation technique. In order to measure spatial variability, soil sampling (0–15 cm) was carried out in bands following the contour, which were located every 1 m beginning at the highest point of elevation and including the lowest point on the terrace. Soil properties, such as pH(H2O), pH(NaF), organic carbon, total nitrogen, NO3−, P, K, exchangeable aluminium, P fixation, exchangeable bases, cationic exchange capacity, base saturation and texture, were monitored on eight terraces, having slopes from 15 to 30%, a length of 4–8 m and soils ranging from Cambisols to Phaeozems, in the Ecuadorian Andes (Gima, Azuay). Terraces, which were 2–4 years, were managed in a traditional or more intensified way (i.e. without or with the application of organic manure). Bufferstrips were composed by (i) Pennisetum clandestinum and Lolium multiflorum or (ii) Phalaris tuberosa. Spatial variability was analysed by stepwise multiple regression analysis, where position on the terrace was the independent variable and soil property was the dependent variable. Factor analysis was carried out in order to compare spatial variability patterns of the terraces and formulate management strategies. High spatial variability in soil properties was found. Although spatial variation was site dependent, it was clear that soil fertility increased from the upper to the lower part of the terraces. The present study shows that management strategy could influence the pattern and magnitude of spatial variation. Site-specific soil fertility improvement is suggested as means to combat variation caused by tillage erosion in slow-forming terraces.
Spatial variation of suspended sediment concentrations in a tropical Andean river system: The Paute River, southern Ecuador
(2007-06-01) Vanacker, Veerle; Molina, Armando; Govers, Gerard; Poesen, Jean; Deckers, Jozef
Tropical mountain ecosystems are sensitive to environmental change brought about by natural and anthropogenic processes. The steep topography, shallow soils and unsustainable land use practices following forest conversion often lead to enhanced rates of geomorphic activity including soil erosion, landsliding and fluvial activity. Rapid demographic growth and socio-economic development have accelerated environmental change over the past few decades. During the last 50 years, water erosion on agricultural fields caused by inappropriate agricultural practices was perceived as the major source of stream sediment in tropical Andean catchments. However, because of sporadic data on sediment production, -transport and -export in tropical Andean catchments, it is unclear as to whether geomorphic activity is mainly controlled by natural or anthropogenic factors. A research program has been carried out in the 5176 km2 Paute River catchment located in southern Ecuador to examine the variability in sediment loads in rivers. A monitoring program was set up in the central part of the watershed to extract information on the spatial variability in river sediment load in relation to catchment variables describing climate, lithology, topography and land use. Twenty-nine river sections were sampled during two successive rainy seasons. At each visit, suspended sediment samples were taken and instantaneous river discharge was measured. Geographic data related to topography, landscape morphology, rainfall, land cover and geology were collected. Watersheds that were affected by intense landslide erosion in the upstream area have a significantly higher weighted suspended sediment concentration than the remaining watersheds. There is some evidence to suggest that occurrence of these large, infrequent landslides is mainly related to topographic steepness, rather than to actual land use practices or human activities.
The impact of environmental change on the intensity and spatial pattern of water erosion in a semi-arid mountainous Andean environment
(2003-04-01) Vanacker, Veerle; Govers, Gerard; Poesen, Jean; Deckers, Jozef; Loaiza, George
Aerial photographs taken in 1976 and 1989 and a field survey in 1999 showed that land use in a 900-ha catchment in the southern part of the Ecuadorian Andes is highly dynamic. Over 23 years, ca. 83 ha of arable land was abandoned and ca. 70 ha was taken into agricultural production. Changes in land use were not spatially homogeneous. Parcels on unstable geologic formations and close to village centres were preferably set-aside. Land taken into cultivation was preferably located on gently sloping areas close to newly built sites and arterial roads. The area with bush vegetation increased by regeneration of natural vegetation on fields set-aside in the late 1970s and early 1980s. There was a complex interaction between water erosion and environmental change in the study area. Land taken into production was levelled for furrow irrigation: this led to a net reduction in the area susceptible to water erosion. However, one quarter of the area affected only by sheet and rill erosion 23 years ago has since become incised by deep gullies. This increase in gully density was related to inadequate construction and management of irrigation infrastructure, rather than to change in vegetation cover and/or soil erodibility caused by agricultural practices. This factor is often overlooked in studies of the effects of environmental change on geomorphologic processes.
Using sequential aerial photographs to detect land-use changes in the Austro Ecuatoriano
(2000) Poesen, Jean; Vanacker, Veerle; Govers, Gerard; Cisneros Espinoza, Felipe Eduardo ; Dercon, Gerd; Tacuri Espinoza, Víctor Eduardo
During the last decades, there is a boom of the use of aerial photographs and remote sensing images to collect quickly accurate digital information in remote areas. Due to an easier accessibility of aerial photographs and a significant decrease of their price as well as that of the image analysis and GIS software, these techniques are becoming increasingly popular. This article presents a methodology to use Geographic Information Systems for quantifying past land use changes from aerial photographs. An application of this technique to the Austro Ecuatoriano has demonstrated that such a methodology requires that the data are both spatially and temporally compatible. In order to obtain spatially compatible data, the air photos are geo-referenced using a set of ground control points (GCP's). Our analysis shows that, if certain conditions are satisfied, GCP's obtained from topographical maps are of sufficient quality to carry out the geo-referencing procedure with a very good precision. On the other hand, temporal compatibility appears more difficult to achieve : the scale (and probably the quality) of the air photos has a significant effect on the resulting land use classification. Consequently, a calibration of the various datasources is necessary in order to avoid bias in the final results.