Expl Agric. (2003), volume 39, pp. 293–306 C  2003 Cambridge University Press DOI: 10.1017/S0014479703001297 Printed in the United Kingdom DEVELOPING IMPROVED DRYLAND CROPPING SYSTEMS FOR MAIZE IN SEMI-ARID TANZANIA. PART II. USE OF A MODEL TO EXTRAPOLATE AND ADD VALUE TO EXPERIMENTAL RESULTS By J. W. GOWING†, M. D. B. YOUNG†§, N. HATIBU‡, H. F. MAHOO‡, F. RWEHUMBIZA‡ and O. B. MZIRAI‡ †Centre for Land Use and Water Resources Research, University of Newcastle upon Tyne, UK and ‡Soil-Water Management Research Group, Sokoine University of Agriculture, Tanzania (Accepted 20 December 2002) SUMMARY Dryland agriculture is critically important to food security and rural livelihoods in Tanzania, but crop production is seriously constrained throughout the semi-arid lowlands by the rainfall regime. A major challenge is to develop improved cropping systems to alleviate the moisture constraint. Experimental evidence indicates that adoption of rainwater harvesting systems can bring benefits, but the restricted spatial and temporal extent of the experimental work leads to difficulties in extrapolation. This paper shows how the PARCHED-THIRST model can add value to the experimental results and provide important insights into their transferability. The model is seen as an aid to researchers, planners and extensionists in interpreting experimental results and designing locally appropriate interventions. Simulation based on 30 years of daily meteorological data provides an opportunity for temporal extrapolation. The long-term simulation allows an objective assessment of the risks and benefits associated with alternative rainwater harvesting systems. Simulation for different soils and modified rainfall regimes permits objective analysis of spatial transferability of experimental results to any other site for which rainwater harvesting interventions might be considered. It is shown that macrocatchment rainwater harvesting reduces drought risk within the target area, but may bring a serious risk of erosion due to excessively high flow rates. The overall assessment of the twin-track approach (experimentation + simulation) is that rainwater harvesting has potential for increasing productivity and sustainability of maize cropping systems in semi-arid Tanzania provided that the innovations are properly matched to the site-specific environmental conditions. INTRODUCTION In the extensive semi-arid lands of Tanzania, agriculture and the livelihoods that depend thereon, are greatly affected by the unreliable and highly variable rainfall regime. Any attempt to improve agriculture must therefore tackle the moisture constraint. However, knowledge of appropriate techniques is poor. A significant knowledge gap exists between two practices that previously have received great attention. On one hand, widespread concern about land degradation has led to a focus on soil erosion control. On the other hand, efforts to exploit water resources have led to a focus on irrigation. § Corresponding author: M. D. B. Young. Centre for Educational Software, The Open University, Milton Keynes, MK7 6AA, UK. Tel: 01908 659937. Fax: 01280 817457. Email: m.d.b.young@open.ac.uk