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112 results on '"Vlek, P.L.G."'

24. Mapping energy balance fluxes and root zone soil moisture in the White Volta Basin using optical imagery

Author

Hendrickx, J.M.H., Hong, S., Friesen, J., Compaore, H., Van de Giesen, N.C., Rodgers, C., and Vlek, P.L.G.

Subjects
latent heat flux, SEBALnm, evapotranspiration, sensible heat flux
Abstract

Accurate information on the distribution of sensible and latent heat fluxes as well as soil moisture is critical for evaluation of background characteristics. Since these fluxes are subject to rapid changes in time and space, it is nearly impossible to determine their spatial and temporal distributions over large areas from ground measurements alone. Therefore, prediction from remote sensing images is very attractive as it enables extensive area coverage and a high repetition rate. In this study, the Surface Energy Balance Algorithm for Land as implemented at New Mexico Tech (SEBALNM) is used to estimate sensible and latent heat fluxes in the White Volta Basin of Ghana, West Africa. The objectives are (i) to demonstrate a SEBALNM application in a part of the world were ground measurements are very scarce and (ii) to compare evapotranspiration (ET) maps obtained from Landsat and MODIS imagery, respectively. The results of this study demonstrate that SEBALNM can be applied for mapping sensible and latent heat fluxes as well as soil moisture over areas where few or no ground measurements are available using common satellite products (Landsat and MODIS).

Published
2006

25. Biodiversity in rubber agroforests, carbon emissions, and rural livelihoods: An agent-based model of land-use dynamics in lowland Sumatra

Author

Villamor, G.B., Le, Q.B., Djanibekov, U., van Noordwijk, M., Vlek, P.L.G., Villamor, G.B., Le, Q.B., Djanibekov, U., van Noordwijk, M., and Vlek, P.L.G.

Abstract

Rubber agroforests in the mostly deforested lowlands of Sumatra, Indonesia are threatened by conversion into monoculture rubber or oil palm plantations. We applied an agent-based model to explore the potential effectiveness of a payment for ecosystem services (PES) design through a biodiversity rich rubber eco-certification scheme. We integrated conditionality, where compliance with biodiversity performance indicators is prerequisite for awarding incentives. We compared a PES policy scenario to ‘business-as-usual’ and ‘subsidized land use change’ scenarios to explore potential trade-offs between ecosystem services delivery and rural income. Results indicated that a rubber agroforest eco-certification scheme could reduce carbon emissions and species loss better than alternative scenarios. However, the suggested premiums were too low to compete with income from other land uses. Nevertheless, integrating our understanding of household agent behavior through a spatially explicit and agent-specific assessment of the trade-offs can help refine the design of conservation initiatives such as PES.

Published
2014

27. Effect of Zai Soil and Water Conservation Technique on Water Balance and the Fate of Nitrate from Organic Amendments Applied: A Case of Degraded-Crusted Soils in Niger

Author

UCL - SST/ELI/ELIE - Environmental Sciences, Fatondji, Dougbedji, Martius, C., Vlek, P.L.G., Bielders, Charles, Bationo, A., UCL - SST/ELI/ELIE - Environmental Sciences, Fatondji, Dougbedji, Martius, C., Vlek, P.L.G., Bielders, Charles, and Bationo, A.

Abstract

Experiments were conducted on degraded crusted soils to study water status and nitrogen release in the soil during the dry seasons of 1999 at ICRISAT research station and on-farm during the rainy seasons of 1999 and 2000 in Niger. Zai is a technology applied on degraded crusted soil, which creates conditions for runoff water harvesting in small pits. The harvested water accumulates in the soil and constitutes a reservoir for plants. The organic amendment applied in the Zai pits releases nutrients for the plants. Soil water status was monitored through weekly measurement with neutron probe; access tubes were installed for the purpose. Nutrient leaching was measured as soil samples were collected three times throughout the cropping season. A rapid progress of the wetting front during the cropping period was observed. It was below 125 cm in the Zai-treated plots 26 days after the rain started versus 60 cm in the non-treated plots. Applying cattle manure leads to shallower water profile due to increased biomass production. Total nitrate content increased throughout the profile compared to the initial status, suggesting possible loss below the plant rooting system due to drainage, which was less pronounced when cattle manure was applied. This study shows that the system improves soil water status allowing plants to escape from dry spells. However, at the same time it can lead to loss of nutrients, particularly nitrogen

Published
2012

33. Mapping energy balance fluxes and root zone soil moisture in the White Volta Basin using optical imagery

Author

Hendrickx, J.M.H. (author), Hong, S. (author), Friesen, J. (author), Compaore, H. (author), Van de Giesen, N.C. (author), Rodgers, C. (author), Vlek, P.L.G. (author), Hendrickx, J.M.H. (author), Hong, S. (author), Friesen, J. (author), Compaore, H. (author), Van de Giesen, N.C. (author), Rodgers, C. (author), and Vlek, P.L.G. (author)

Abstract

Accurate information on the distribution of sensible and latent heat fluxes as well as soil moisture is critical for evaluation of background characteristics. Since these fluxes are subject to rapid changes in time and space, it is nearly impossible to determine their spatial and temporal distributions over large areas from ground measurements alone. Therefore, prediction from remote sensing images is very attractive as it enables extensive area coverage and a high repetition rate. In this study, the Surface Energy Balance Algorithm for Land as implemented at New Mexico Tech (SEBALNM) is used to estimate sensible and latent heat fluxes in the White Volta Basin of Ghana, West Africa. The objectives are (i) to demonstrate a SEBALNM application in a part of the world were ground measurements are very scarce and (ii) to compare evapotranspiration (ET) maps obtained from Landsat and MODIS imagery, respectively. The results of this study demonstrate that SEBALNM can be applied for mapping sensible and latent heat fluxes as well as soil moisture over areas where few or no ground measurements are available using common satellite products (Landsat and MODIS)., Water Resources Section, Civil Engineering and Geosciences

Published
2006

40. Effect of planting technique and amendment type on pearl millet yield, nutrient uptake, and water use on degraded land in Niger.

Author

Bationo, Andre, Waswa, Boaz, Kihara, Job, Kimetu, Joseph, Fatondji, D., Martius, C., Bielders, C.L., Vlek, P.L.G., Bationo, A., and Gerard, B.

Abstract

Due to increased population pressure and limited availability of fertile land, farmers on desert fringes increasingly rely on marginal land for agricultural production, which they have learned to rehabilitate with different technologies for soils and water conservation. One such method is the indigenous zai technique used in the Sahel. It combines water harvesting and targeted application of organic amendments by the use of small pits dug into the hardened soil. To study the resource use efficiency of this technique, experiments were conducted 1999-2000, on-station at ICRISAT in Niger, and on-farm at two locations on degraded lands. On-station, the effect of application rate of millet straw and cattle manure on millet dry matter production was studied. On-farm, the effects of organic amendment type (millet straw and cattle manure, at the rate of 300 g per plant) and water harvesting (with and without water harvesting) on millet grain yield, dry matter production, and water use were studied. First, the comparison of zai vs. flat planting, both unamended, resulted in a 3- to 4-fold (in one case, even 19- fold) increase in grain yield on-farm in both years, which points to the yield effects of improved water harvesting in the zai alone. Zai improved the water use efficiency by a factor of about 2. The yields increased further with the application of organic amendments. Manure resulted in 2-68 times better grain yields than no amendment and 2-7 times better grain yields than millet straw (higher on the more degraded soils). Millet dry matter produced per unit of manure N or K was higher than that of millet straw, a tendency that was similar for all rates of application. Zai improved nutrient uptake in the range of 43-64% for N, 50-87% for P and 58-66% for K. Zai increased grain yield produced per unit N (8 vs. 5 kg kg-1) and K (10 vs. 6 kg kg-1) compared to flat; so is the effect of cattle manure compared to millet straw (9 vs. 4 kg kg-1, and 14 vs. 3 kg kg-1), respectively, Therefore zai shows a good potential for increasing agronomic efficiency and nutrient use efficiency. Increasing the rate of cattle manure application from 1 to 3 t ha-1 increased the yield by 115% TDM, but increasing the manure application rate further from 3 to 5 t ha-1 only gave an additional 12% yield increase, which shows that optimum application rates are around 3t ha-1. [ABSTRACT FROM AUTHOR]

Published
2007
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43. Application of the caesium-137 technique to soil degradation studies in the Southwestern Highlands of Ethiopia.

Author

Mekuria, A., Vlek, P.L.G., and Denich, M.

Subjects
SOIL degradation, UPLANDS, MASS budget (Geophysics), SOIL erosion, CESIUM
Abstract

ABSTRACT Soil degradation is a serious problem in the central and northern Highlands of Ethiopia. It has been so for several decades as a result of over exploitation and mismanagement. Relocation of a portion of the population from these regions to the relatively less populated Southwestern Highlands has taken place for decades to try to address the problem. However, such mass resettlements have caused severe soil degradation problems in many destination areas in the Southwestern Highlands. The aim of this study was to assess the problem of soil degradation using the caesium-137 isotope and to test its value for erosion study in the region. The adapted USLE was applied to compare results from the caesium-137 isotope studies. Along a deforestation continuum, fields cultivated for various years were studied for erosion. From a reference grazing land plot, total caesium-137 fallout of 2026 ± 176 Bq m−2 with a CV of 24·6 per cent was recorded showing the presence of sufficient fallout to apply the technique. Erosion in cultivated fields was estimated against this reference using conversion models. Results from the Proportional Model |−13·9 ± 2·7|and the adapted USLE |12·3 ± 2·6| were not significantly different ( p < 0·05), meaning the technique provides reliable results. A positive relationship was observed between severity of erosion and time of cultivation after forest clearing ( R2 = 0·78). The mean annual loss of soil from cultivated land, 14·9 ± 2·9 t ha−1 y−1, is already beyond the tolerable threshold and might exacerbate further clearing of forests for cultivation if the land is not properly managed. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Published
2012
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44. Effects of Solution Chemistry and Environmental Conditions on Ammonia Volatilization Losses From Aqueous Systems

Author

Vlek, P.L.G. and Stumpe, J. M.

Abstract

Laboratory studies were conducted to explain the wide variation in reported estimates of ammonia volatilization losses from N‐fertilized paddy fields. The ammonia volatilization capacity of a system was found to be equivalent to its alkalinity [NH3(aq), HCO32‐]. In solutions lacking alkalinity, loss of (NH4)2SO4was limited, whereas loss of (NH4)2CO3was essentially complete. Ammonia loss from solution is best described as a consecutive reaction with opposing step. Ammonia volatilization per se followed first‐order reaction kinetics. The rate of ammonia volatilization is thus directly related to the concentration of aqueous ammonia and therefore to the concentration of ammoniacal N and pH. The rate of ammonia volatilization was severely restricted by limiting the movement of air above the water, as is often the case in the laboratory and field studies reported to date. Ammonia volatilization was enhanced by water turbulence and increased exponentially with temperature from almost nil at 0°C to approximately 20 mg N/100 cm2/5 hours at 46°C.

Published
1978
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45. Ammonia Volatilization from Urea and Urea Phosphates in Calcareous Soils

Author

Stumpe, J. M., Vlek, P.L.G., and Lindsay, W. L.

Abstract

Ammonia volatilization from urea is a considerable problem if fertilizer is surface applied, whether by choice (minimum tillage) or by lack of implements, which is common in the tropics. Urea phosphate [CO(NH2)2·H3PO4] was tested in several experiments as a means of reducing ammonia volatilization losses. Incubation experiments using 15N‐labeled urea were conducted to evaluate ammonia loss from urea, urea phosphate, and urea‐urea phosphate mixtures (N content = 460, 190, 370, and 350 g kg−1, respectively), applied at rates up to 100 kg N ha−1to a variety of soils using a range of initial moisture contents. Evolved ammonia was measured by collection in an external acid trap. Ammonia volatilization losses from surface‐applied urea ranged from 10% to 30% of that applied, dependent on soil type. Initial soil moisture affected the timing of N loss, but not the total amount. Addition of urea with phosphate (190 g N kg−1) slightly decreased ammonia loss from 17.0% to 12.2% of the applied N on a highly calcareous (225 g CaCO3kg−1) Mollisols. On the other hand, NH3losses from a slightly calcareous (20 g CaCO3kg−1) Vertisols were 10.5% and 3.1%, and from a slightly calcareous Entisols (2.6 g CaCO3kg−1) were 14.4% and 4.4% of the applied N for urea and urea phospate, respectively. The relative savings of urea‐N were much higher in low CaCO3soils (approximately 70%) than in the highly calcareous soil (approximately 30%). Mixtures of urea and urea phosphate were substantially less effective in reducing ammonia volatilization with relative reductions ranging from 0% to 20%. Detailed studies of the mobility and reactions of urea phosphate in soils revealed that rapid precipitation of calcium phosphates near the placement site and the diffusion of urea away from the placement site were responsible for the poor performance of granular urea phosphate in highly calcareous soil. It is concluded that urea‐urea phosphate mixtures should probably not be recommended as a means of reducing ammonia volatilization on highly calcareous soils, though its use on noncalcareous soils and soils low in CaCO3merits further testing.

Published
1984
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