William Stephens

Rainfall trends in the North East Arid Zone of Nigeria 1961–1990

Abstract Daily rainfall records for 1961–1990 for Nguru (12.53°N, 10.28°E, alt. 343 m), Potiskum (11.42°N, 11.02°E, alt. 415 m) and Maiduguri (11.51°N, 13.05°E, alt. 354 m) in Nigeria and Maine Soroa (13.13°N, 11.58°E, alt. 339 m) in Niger were analysed to describe any changes in season duration, rain-days per season and rainfall amount per rain-day. There was a consistent decrease in annual rainfall of 8 mm year −1 at all four stations. The majority of this reduction occurred in August or September. Although there was a delay in the onset of the rains at the two westernmost stations, the main reason for the decrease in rainfall was a reduction of 6–25 days in the number of rain-days during the rainy season. This increased the mean duration of dry spells during the rainy season by up to 1.5 days. No change in the average rainfall per rain-day was detected except at Maine Soroa where there was a slight reduction. The results are discussed in terms of their implications for sustainable agriculture in the region.

Crop-soil simulation models : applications in developing countries

Models as tools in research models as decision-support tools models as tools in education and training have crop models been useful? the way forward.

The potential for short rotation energy forestry on restored landfill caps

This review examines the potential for producing biomass on restored landfills using willow and poplar species in short rotation energy forestry. In southern England, the potential production may be about 20 t ha(-1) of dry stem wood annually. However, actual yields are likely to be constrained by detrimental soil conditions, including shallow depth, compaction, low water holding capacity and poor nutritional status. These factors will affect plant growth by causing drought, waterlogging, poor soil aeration and nutritional deficiencies. Practical solutions to these problems include the correct placement and handling of the agricultural cap material, soil amelioration using tillage and the addition of organic matter (such as sewage sludge), irrigation (possibly using landfill leachate), the installation of drainage and the application of inorganic fertilizers. The correct choice of species and clone, along with good site management are also essential if economically viable yields are to be obtained. Further investigations are required to determine the actual yields that can be obtained on landfill sites using a range of management inputs.

Applications of crop/soil simulation models in tropical agricultural systems☆

Abstract Crop simulation models have been used widely to describe systems and processes at the level of the genotype, the crop, the farming system, the region, and the global environment, but examples of how the use of such models has benefited poor people in developing countries are limited. There is, therefore, an urgent need to make the use of models in research more relevant to problems in the real world and to find effective means of disseminating the results from work using models to the potential beneficiaries. To achieve this, we believe that there must be a shift in the thinking of crop/soil modelers toward making people more center stage and toward a more problem-solving approach. This means researchers must think of the real problems faced by ordinary people in developing countries and construct and apply their models to contribute to solving these problems. For this to be effective, modelers need to define clearly who are the end users of their models and to enter into dialogue with these people so that the final product is tailored to their needs. There appear to be two opposite directions in which further crop modeling research can develop. On the one hand, in response to the rapidly expanding field of genomics, links between information at the gene level and performance at the phenotype level need to be established, and methodologies to do this must be developed. Such models will have the potential to contribute to enhancing the efficiency of crop improvement programs worldwide by providing more efficient ways of identifying and evaluating desirable characteristics for specific plant breeding goals. On the other hand, crop models need to be incorporated into higher order systems such as the whole farm, catchment, or region. Some progress has already been made in linking crop growth models with other physical process models to improve our understanding of how changes in agricultural systems influence overall environmental impacts. However, the role of people in these systems also needs to be made explicit so that the day-to-day decisions that they make to sustain and improve their livelihoods and the influence these decisions have on their environment and natural resource base can be taken into account.

Irrigating short rotation coppice with landfill leachate : constraints to productivity due to chloride

Abstract A pot-based experiment was conducted to determine the effect of different chloride concentrations (0–422 mmol l−1) on the growth of transplanted saplings of Salix viminalis (clone Q683). Chloride had a very rapid effect on the growth and development of the willow plants and on evapotranspiration (ETa). The degree of reduction in ETa of willows was directly related to chloride concentration and, in the short term, evaporative demand. Sustainable growth and development of willow is unlikely at chloride concentrations greater than 70 mmol l−1. Further investigations into the management of solute accumulation in the root zone are required in order to develop effective landfill leachate irrigation strategies for trees.

Effects of soil compaction and mechanical damage at harvest on growth and biomass production of short rotation coppice willow

The effects of soil compaction and mechanical damage to stools at harvesting on the growth and biomass production of short rotation coppice (SRC) of willow (Salix viminalis L.) were monitored on clay loam (CL) and sandy loam (SL) soils. Moderate compaction, more typical of current harvesting situations did not reduce biomass yields significantly. Even heavy compaction only reduced stem biomass production by about 12% overall; effects were statistically significant only in the first year of the experiment on sandy loam. Heavy compaction increased soil strength and bulk density down to 0.4 m depth and reduced soil available water and root growth locally. Soil loosening treatments designed to alleviate the effects of heavy compaction did not markedly improve the growth of willow on compacted plots. Hence the focus fell on harvesting. Extensive mechanical damage to stools caused a 9% and 21% reduction in stem dry mass on the clay loam and sandy loam soils as a result of fewer stems being produced. The particularly severe effect on the sandy loam soil probably resulted from a combination of dry conditions in the year of treatment, root damage and soil compaction under stools and might have been aggravated by the young age of the plants (1 year) at the time of treatment.

Seasonal and clonal differences in shoot extension rates and numbers in tea (Camellia sinensis).

Shoot extension rates and numbers recorded over an 18 month period in Tanzania from three contrasting clones were analysed to determine variation between and within seasons. Clonal differences in base temperatures for shoot extension ranged from 10.3 to 14.5°C, whilst variability in the response of shoot growth rates to temperature could be ascribed to the shoot selection technique employed. Considerable clonal variation in shoot population densities occurred, with maxima ranging from 200 to 1200 shoots m −2 . Total active shoot extension, the product of shoot growth rates and population densities, varied between 4 and 35 m m −2 week −1 , shoot numbers being the dominant component. The implications of these results are discussed in terms of shoot measurement techniques, clonal selection criteria, yield modelling and harvesting policies.

A Water Stress Index for Tea ( Camellia sinensis)

A simple Stress Time Index (STI) for predicting yield loss in the tea crop due to drought is proposed, based on the daily summation of the difference between the potential soil water deficit and a specified limiting value. Validation of the technique with results from a line-source irrigation experiment with a single clone in the Southern Highlands of Tanzania suggests that there is a linear relationship between STI and relative yield loss during the warm dry season. As a result of changes in the composition of the shoot population at each harvest the apparent critical deficit at which shoot growth is restricted increases from below 20 to 300 mm as the dry season progresses. The rate of yield loss with increasing STI also varies through the dry season for the same reason.

Responses of tea (Camellia sinensis) to irrigation and fertilizer. I. Yield

The yield responses of clonal tea (Clone 6/8) to irrigation and fertilizer were studied in a field experiment (based on the line-source technique) at a high altitude site (1840 m) in the Southern Highlands of Tanzania over a three-year period. In this area (latitude 8°33′S) the annual dry season can last up to six months with potential soil water deficits reaching 600 to 700 mm. In the third year of the experiments yields for the fully irrigated, well fertilized (375 kg N ha −1 ) treatments had reached 4.9 t ha −1 of made tea. These were reduced by about 2.9 kg ha −1 for each mm increase in the potential soil water deficit. For tea with little or no fertilizer applied the loss of yield was about 1.4 kg ha −1 mm −1 . These figures provide a basis for assessing the potential benefits from irrigation where other factors, such as large saturation deficits of the air, do not restrict shoot extension and yield. Yield responses to nitrogen (applied as N:P:K 20:10:10) were essentially linear up to a maximum of about 375 kg N ha −1 in the fully irrigated plots, and 300 kg N ha −1 for the unirrigated plots. Irrigation increased the proportion of crop harvested during the dry season, up to 45% in the fully irrigated treatments. The commercial implications of these results for ‘high’ and ‘low’ input producers are discussed.

Effects of Light, Temperature, Irrigation and Fertilizer on Photosynthetic Rate in Tea ( Camellia Sinensis )

Photosynthetic rates were monitored during the warm dry season in tea Clone 6/8 in a line-source irrigation × fertilizer experiment in the Southern Highlands of Tanzania. Irrigation and fertilizer increased photosynthetic rate both by enhancing photosynthetic rate per unit leaf area (A) in healthy leaves and by increasing the proportion of sunlight intercepted by photosynthetically efficient leaves. Irrigation-induced increases in A could be accounted for by increases in stomatal conductance (g) and associated reductions in leaf temperature. Fertilizer at an annual application rate of 225 kg N ha -1 caused increases in A associated with increases in g and improved responses to ambient CO 2 concentration and illuminance (photon flux density, PFD). However, a further increase in fertilizer application rate to 375 kg N ha -1 a -1 decreased A in spite of increasing g. Light-saturation of photosynthesis occurred only at the higher fertilizer application rate. In unfertilized tea or tea fertilized at the lower rate, A decreased at a PFD of between 1400 and 2000 μmol m -2 s -l . These results are discussed in terms of the relation between photosynthesis and yield in tea. Fotosintesis en el te