Conservation agriculture and smallholder farming in Africa: The heretics' view
Giller, K.E. ; Witter, E. ; Corbeels, M. ; Tittonell, P.A. - \ 2009
Field Crops Research 114 (2009)1. - ISSN 0378-4290 - p. 23 - 34.
soil carbon sequestration - sub-saharan africa - biological nitrogen-fixation - semiarid west-africa - tillage systems - organic-matter - sustainable agriculture - cropping systems - southwestern nigeria - water conservation
Conservation agriculture is claimed to be a panacea for the problems of poor agricultural productivity and soil degradation in sub-Saharan Africa (SSA). It is actively promoted by international research and development organisations, with such strong advocacy that critical debate is stifled. Claims for the potential of CA in Africa are based on widespread adoption in the Americas, where the effects of tillage were replaced by heavy dependence on herbicides and fertilizers. CA is said to increase yields, to reduce labour requirements, improve soil fertility and reduce erosion. Yet empirical evidence is not clear and consistent on many of these points nor is it always clear which of the principles of CA contribute to the desired effects. Although cases can be found where such claims are supported there are equally convincing scientific reports that contradict these claims. Concerns include decreased yields often observed with CA, increased labour requirements when herbicides are not used, an important gender shift of the labour burden to women and a lack of mulch due to poor productivity and due to the priority given to feeding of livestock with crop residues. Despite the publicity claiming widespread adoption of CA, the available evidence suggests virtually no uptake of CA in most SSA countries, with only small groups of adopters in South Africa, Ghana and Zambia. We conclude that there is an urgent need for critical assessment under which ecological and socio-economic conditions CA is best suited for smallholder farming in SSA. Critical constraints to adoption appear to be competing uses for crop residues, increased labour demand for weeding, and lack of access to, and use of external inputs
Heavy metals and soil microbes
Giller, K.E. ; Witter, E. ; McGrath, S. - \ 2009
Soil Biology and Biochemistry 41 (2009)10. - ISSN 0038-0717 - p. 2031 - 2037.
leguminosarum-biovar-trifolii - biotic ligand model - term field experiment - sewage-sludge - contaminated soils - agricultural soils - nitrogen-fixation - past applications - zinc concentrations - copper toxicity
The discovery in the early 1980s that soil microorganisms, and in particular the symbiotic bacteria Rhizobium, were highly sensitive to heavy metals initiated a new line of research. This has given us important insights into a range of topics: ecotoxicology, bioavailability of heavy metals, the role of soil biodiversity, and the existence of ‘keystone’ organisms. Concurrently, and particularly in Europe, the research led to new approaches to the protection of soils from pollution that take into account the many effects on soil microorganisms. To date these key findings have largely been ignored in the USA, although our results caused considerable controversy there. In the past decade there have been many advances in the ecotoxicological assessment of metals and their effects on soil organisms but major gaps in knowledge and theory remain with regard to how microorganisms are exposed and respond to metals in soils. In this brief review we emphasise the need for long-term experiments and basic research to forge this understanding and improve environmental protection policies.
Element balances as a tool for sustainable nutrient management: a critical appraisal of their merits and limitations within an agronomic and environmental context
Öborn, I. ; Edwards, A.C. ; Witter, E. ; Oenema, O. ; Ivarsson, K. ; Withers, P.J.A. ; Nilsson, S.I. ; Richert Stinzing, A. - \ 2003
European Journal of Agronomy 20 (2003)1-2. - ISSN 1161-0301 - p. 211 - 225.
dairy farming system - water-quality - nitrogen - phosphorus - fertilization - perspective - losses - model - land - uk
Element balances are widely used and are incorporated within national action programs to combat nutrient emissions from agriculture to the environment. They rely on data that are readily available at farm-gate and field level, and the information generated is easy to communicate to farmers and policy makers. This may have contributed to high expectations on element balances as a tool for optimising agricultural nutrient use efficiency and thereby reducing nutrient losses. Element balances are potentially useful as a screening tool across Europe provided the methods to calculate them are standardised, their limitations and usefulness defined and appropriate target values are established by which they can be compared. It is difficult to establish straightforward relationships between nutrient management, surplus, losses and environmental impact. Simple farm-gate and field balances need to be complemented by a better understanding of the processes regulating nutrient dynamics, and their spatial and temporal variability. Hence, agronomic and environmental reference or target values need to be established for different production systems, geographical areas and elements. Proper instruments and tools as well as training and educational programmes have to be developed for a successful implementation
|Genetic mapping of quantitative trait loci affecting susceptibility to Marek's disease virus induced tumors in F2 intercross chickens.
Vallejo, R.L. ; Ba Con, L.D. ; Liu, H.C. ; Witter, R.L. ; Groenen, M.A.M. ; Hillel, J. ; Cheng, H.H. - \ 1998
Genetics 148 (1998). - ISSN 0016-6731 - p. 349 - 360.
Neerslagspreiding en aanvoerreductie in het regionale afvalwatertransportstelsel van het Hoogheemraadschap West-Brabant
Dircke, P.T.M. ; Witter, J.V. ; Stricker, J.N.M. - \ 1995
Wageningen : Landbouwuniversiteit Wageningen (Rapport / Landbouwuniversiteit, Vakgroep Waterhuishouding 49) - 134
afvalwater - hydrologie - modellen - regen - afvalwaterbehandeling - oppervlakkige afvoer - nederland - noord-brabant - waste water - analogues - hydrology - models - waste water treatment - relationships - water treatment - noord-brabant
|Optimal integrated water management: a quantitative approach.
Witter, V.J. ; Engelse, R. den; Bogardi, J.J. - \ 1993
In: Proc. IWRA Int. Conf. Environmentally sound water resources utilization. Vol.1, Tawatchai Tiugsauchali (ed.). Bangkok, Thailand (1993) I-112 - I-124
Reliability of point counts of pedological properties on thin sections.
Finke, P.A. ; Mucher, H.J. ; Witter, J.V. - \ 1991
Soil Science 151 (1991). - ISSN 0038-075X - p. 249 - 253.
|Ruimtelijke statistiek van bodem en water.
Witter, J.V. ; Eijnsbergen, A.C. van - \ 1991
In: Covariantiestructuur van kaarten - p. 105 - 128.
Mogelijkheid van reductie van de hydraulische capaciteit van regionale RWZI's.
Witter, J.V. ; Dircke, P. ; Stricker, H. - \ 1989
H2O : tijdschrift voor watervoorziening en afvalwaterbehandeling 22 (1989). - ISSN 0166-8439 - p. 714 - 720.
analogen - hydrologie - modellen - regen - relaties - rioolafvalwaterverwijdering - afvalwaterbehandeling - waterzuivering - zuiveringsinstallaties - analogues - hydrology - models - rain - relationships - sewage effluent disposal - waste water treatment - water treatment - purification plants
|Statistische analyse met betrekking tot maxima.
Montfort, M.A.J. van; Witter, J.V. - \ 1988
Unknown Publisher - 35 p.
The generalized Pareto distribution applied to rainfall depths.
Montfort, M.A.J. van; Witter, J.V. - \ 1986
Hydrological Sciences Journal 31 (1986)2. - ISSN 0262-6667 - p. 151 - 162.
hydrologie - wiskunde - regen - statistiek - geostatistiek - hydrology - mathematics - rain - statistics - geostatistics
|Rainfall field modelling applied to a design problem.
Witter, J.V. - \ 1986
In: Proc. Chapman Conference on Rainfall Fields. Caracas (1986)
|Modelling total sewage water discharge to a regional treatment plant.
Witter, J.V. ; Stricker, H. - \ 1986
In: Urban storm water quality and effects upon receiving waters : international conference, Wageningen - The Netherlands, (October, 1986) - p. 327 - 329.
In the Netherlands, sewage water is often treated on a regional basis. In case of combined systems that are spread within a large region of several hundreds of square kilometers, reduction of the hydraulic capacity of the regional treatment plant seems possible, because of space-time variations in rainfall.
Heterogeneity of Dutch rainfall
Witter, J.V. - \ 1985
Netherlands Journal of Agricultural Science 33 (1985)1. - ISSN 0028-2928 - p. 72 - 75.
De berekening van instationaire stroming in open waterlopenstelsels met een boomstructuur
Baalen, D.J.N. van; Witter, J.V. - \ 1985
Wageningen : Landbouwuniversiteit Wageningen (Nota / Landbouwuniversiteit, Vakgroep Hydraulica en Afvoerhydrologie 66) - 40
kanalen - kanalen, klein - stroming - doorstroommeters - vloeistoffen (fluids) - hydraulica - hydrodynamica - vloeistoffen (liquids) - wiskunde - meting - rivieren - rivierafvoer - waterlopen - turbulentie - water - waterwegen - wind - numerieke methoden - canals - channels - flow - flow meters - fluids - hydraulics - hydrodynamics - liquids - mathematics - measurement - rivers - stream flow - streams - turbulence - water - waterways - wind - numerical methods
|Neerslagonderzoek in relatie tot de dimensionering van de rioolwaterzuiveringsinstallatie te Bath
Scholma, L. ; Witter, J.V. - \ 1984
Waterschapsbelangen : tijdschrift voor waterschapsbestuur en waterschapsbeheer 69 (1984)5. - ISSN 0043-1486 - p. 143 - 149.
neerslag - afvoer - oppervlakkige afvoer - stedelijke gebieden - afvalwaterbehandeling - waterzuivering - zeeuwse eilanden - precipitation - discharge - runoff - urban areas - waste water treatment - water treatment - zeeuwse eilanden
Ingegaan wordt op de vraag of de hydraulische capaciteit van de zuiveringsinstallatie Bath kleiner kan zijn dan de som van de capaciteiten van de ondergemalen. Dit stoelt op het gegeven, dat bij grote stroomgebieden de deelgebieden niet allen gelijktijdig maximaal afvoeren
Heterogeneity of Dutch rainfall
Witter, J.V. - \ 1984
Landbouwhogeschool Wageningen. Promotor(en): L.C.A. Corsten, co-promotor(en): D.A. Kraijenhoff van de Leur. - Wageningen : Witter - 204
neerslag - duur - hydrologie - meteorologische waarnemingen - weersgegevens - precipitation - duration - hydrology - meteorological observations - weather data
Rainfall data for the Netherlands have been used in this study to investigate aspects of heterogeneity of rainfall, in particular local differences in rainfall levels, time trends in rainfall, and local differences in rainfall trend. The possible effect of urbanization and industrialization on the distribution of rainfall has also been studied. Consideration has been given to whether local differences in rainfall justify a partition of the Netherlands into regions. Finally, the degree of areal reduction which is possible in hydrological design because of variation of rainfall in time and space has been investigated.A statistical analysis of these aspects is useful because they frequently appear in the hydrological literature. The statistical analysis presented in this thesis uses hydrological concepts, such as the statistical areal reduction factor, and attention is focused on moderately low return period events. Only rainfall levels and trends in rainfall have been investigated and not more complicated aspects, such as, trends in the variance of rainfall. Further, rainfall variations in time and space have been analysed separately.Estimates of the levels of the rainfall characteristics used in the investigation of homogeneity in time and space are given in Section 2.2. These are annual frequencies of exceedance during the summer or the winter period of a certain threshold value and the total annual rainfall (Tables 2.1 and 2.2). The expected daily rainfall has also been estimated for return periods in excess of half a year (Table 2.4, Figure 2.2). Time trends in rainfall averaged over the Netherlands have been estimated. For the period 1951-1979, the time trend is negative for the summer period; and for the period 1906-1979, the time trend is positive for the winter period (Table 2.5). Time trends in rainfall series were found to be related to the occurrence of circulation types (Figure 2.4).In Section 2.4 local differences in these rainfall characteristics have been investigated using the kriging method that gives the
best linear unbiased predictor. As may be expected, there are local differences, both in rainfall level (Figure 2.7), and in time trends in the rainfall series which were reduced by the annual mean (Figure 2.9). Many of the rainfall series investigated exhibit inhomogeneities (Table 2.7). Two possible causes of these inhomogeneities, changes in the frequency of occurrence of circulation types and anthropogenic activities, such as urbanization and industrialization, are discussed in Section 2.6.A possible partition of the Netherlands into regions is investigated by using rainfall data for the period 1951-1979. Earlier studies on the geographical distribution of certain rainfall characteristics in the Netherlands are presented in Section 2.5.1. The model to test the statistical significance of the partitions used in this study is presented in Section 2.5.2.One of the proposed partitions, an a posteriori partition based on mean annual rainfall (Figure 2.11D), is in agreement with the resulting spatial patterns of the levels of the rainfall characteristics considered (Table 2.10). Also, the level of hourly rainfall was found to be related to mean annual rainfall (Figure 2.18A), but, with a simple urban runoff model with a time-step of one hour, no differences were found between the number and quantity of overflow for 12 rainfall stations, classified according to this partition (Section 2.5.3).These partitions into regions are not satisfactory for rainfall trends, except for an a posteriori partition based on time trends for the period 1951-1979 (Figure 2.12). But both the geographical distribution of trends and the degree of trend in some long-term rainfall records are not in agreement with this partition. Apparently, the changes in rainfall pattern are recent. Because the partition is based on trends in reduced rainfall series (reduced by the annual mean), the changes are also local. Thus on the basis of data used in this study, it was not possible to devise a satisfactory partition of the Netherlands for rainfall trends. With regard to rainfall level it is suffice to assume that the design rainfall at a given location is proportionate to the mean summer or winter rainfall; therefore, a partition of the Netherlands into regions is not necessary. This has already been suggested in Buishand and Velds (1980).The influence of urbanization and industrialization on precipitation (urban effects) has been investigated by using the method of Lowry (1977), which allows for changes in frequency of occurrence of circulation types. In Section 2.6, this method is discussed and the findings of other studies on the occurrence, causes, and magnitude of urban effects are presented. In Section 2.6.1, the occurrence of urban effects is discussed, for instance, on the basis of changes in mean daily rainfall for 32 rainfall stations between the industrialized and urbanized period (1956-1979) and the non-industrialized period (1932-1955), with a stratification of days according to season and circulation type (according to Hess, 1977), see Figure 2.22. Although the results were sometimes inconclusive and not always in. accordance with the hypothesis of an urban effect, there are indications of urban effects for the zonal circulation type and for three of the meridional circulation types (Tables 2.16 and 2.17; Figure 2.22). Moderate rainfalls were also found to be affected (Table 2.17, where a threshold value for daily rainfall of 15 mm has been considered), and urban effects in the summer period increase with rainfall depth.In Chapter 3 consideration is given to the degree of areal reduction which is possible in hydrological design because. of variations of rainfall in time and space. Use has been made of the IRF-0 kriging theory, and semi-variograms were estimated by the multi-realization approach. The applicability of the IRF-0 theory to predict the mean areal rainfall is discussed in section 3.2.1. Contrary to what had been expected, in a substantial number of cases the estimated order of the intrinsic random function differs from zero (Tabel 3.1). Further research is needed on the structure identification, both on the statistical aspects (estimation of the order k of the intrinsic random function and of the coefficients of the generalized covariance model) and on the physical aspects (semi-variogram or generalized covariance model to be expected under certain assumptions regarding rainfall). The variation in semi-variogram estimates for individual rainfall events was found to be large (Figure 3.3). In Section 3.2.2, the kriging predictor of areal rainfall is compared with the more commonly used arithmetic mean and Thiessen predictor. All three predictors yield similar results (Table 3.5), but the kriging predictor is more efficient (Table 3.4).Methods to estimate the statistical areal reduction factor (ARF) are presented in Section 3.3.1. With the methods proposed in USWB (1957-1960), NERC (1975), Bell (1976), and Rodríguez-Iturbe and Mejía (1974) and Buishand (1977c), the areal reduction factor for daily rainfall (ARF 24 ) has been estimated for three areas each of about 1000 km 2in the Netherlands, for the summer period, the winter period, and the complete year. In Section 3.3.3, the variance of ARF 24 is estimated. All four estimators of ARF 24 were found to produce similar results (Tabel 3.12), and the three areas considered do not clearly differ with respect to ARF 24 . These estimates of ARF 24 are somewhat lower than those of USWB (1957-1960) for the United States and those of NERC (1975) for the United Kingdom (Figure 3.22), and they are in reasonable agreement with earlier estimates of ARF 24 for the Netherlands (Table 3.14). For small areas, ARF 24 is underestimated by the method which uses the marginal distribution of point rainfall and the fitted correlationdistance function. This is also evidenced by the higher ARF 24 values in Kraijenhoff (1963). ARF 24 depends heavily on season and return period (Table 3.7). Averaged over the three areas, the maximum areal rainfall occurs in the winter period in 33% of the years considered.In Section 3.4 ARF for hourly rainfall (ARF 1 ) is estimated. As a function of areal size and return period, ARF 1 has been estimated for the summer and the winter period (Figure 3.28) and for the complete year (Figure 3.21). These ARF 1 estimates are somewhat lower than those of USWB (1957-1960) and NERC (1975) (Table 3.18), probably because few hourly rainfall data were available for this study. Especially the correlation-distance function for hourly rainfalls could not be estimated very satisfactorily.The storm-centred areal reduction factor (SRF) is discussed in Section 3.5. Models for SRF based on a literature survey of minimum-rainfall curves are presented in Table 3.19. For equal areal size, SRF values from network data are generally lower than ARF values (Figure 3.29). The smaller the areal size and the shorter the period for which rainfall totals are considered, the closer SRF and ARF values.In this study, rainfall variations in time and space have been analysed separately. Because of this simplification of the problem, the results presented in Chapter 3 may be of less relevance to practical design issues related to areal rainfall. Areal reduction is partly caused by spatial differences in rainfall patterns in time. This aspect of areal reduction is not taken into account, when time aggregates of rainfall over a measurement interval are considered, and rainfall depths over consecutive intervals are assumed to be independent. For this reason, the degree of areal reduction applicable to regional transport systems of sewerage water cannot be determined by using the statistical areal reduction factor.When rainfall variations in time and space are analysed as being interdependent, the need for knowledge and understanding of meteorology increases because the rainfall events described have first to be classified. Further, instead of the univariate statistical methods as used almost exclusively in this study, multivariate methods are required. However, at present, data from a dense network of rainfall recorders, necessary for such an investigation, are not available for the Netherlands.Further research on the causes of homogeneities in rainfall series is necessary. Although this study of homogeneity has been restricted to rainfall records of good and even quality, many rainfall series are statistically inhomogeneous, and local differences in trend often seem inexplicable. To explain this, meteorological knowledge and knowledge of the station history of rainfall series used is essential.
Reconnaissance soil studies to determine optimum survey scales and mapping legend for soil moisture research in the Hupselse Beek Hydrological Catchement Oost-Gelderland
Burrough, P.A. ; Oerlemans, G. ; Stoffelsen, G. ; Witter, G. van - \ 1983
Wageningen : Landbouwuniversiteit Wageningen (Publication / Agricultural University, Department of Hydraulics and Catchment Hydrology 77) - 47
horizonten - landevaluatie - bodemgeschiktheid - bodemkarteringen - bodemwater - bodemwatergehalte - nederland - gelderland - achterhoek - horizons - land evaluation - soil suitability - soil surveys - soil water - soil water content - netherlands - gelderland - achterhoek
|Reconnaissance soil studies to determine optimum survey scales and mapping legend for soil moisture research in the Hupselse Beek hydrological catchment
Burrough, P.A. ; Oerlemans, G. ; Stoffelsen, G. ; Witter, J.V. - \ 1983
Unknown Publisher - 44 p.
|De gebiedsreductiefactorin het hydrologisch onterp ten gevolge van verschillen in tijd en plaats van de nerslag
Witter, J.V. - \ 1983