Models of below-ground interactions: their validity, applicability and beneficiaries

This paper discusses various models that incorporate simultaneous below-ground interactions between different plant species. These models include: SCUAF (soil changes under agroforestry); WANULCAS (water, nutrient, light capture in agroforestry systems); HYPAR (a combination of the Hybrid tree model); HYCAS (simulates competition of light, water, nitrogen and phosphorus); COMP8 (simulates nutrient uptake by both competing and single root systems); WIMISA (simulates crop growth as influenced by trees growing as windbreaks); APSIM (agricultural production systems simulator); CROPSYS (predict the performance of multiple cropping systems); ALMANAC (agricultural land management alternatives with numerical assessment criteria); and GAPS (general-purpose atmosphere-plant-soil simulator). The validity, reliability and applicability of these ten models are determined by their limitations, ability to reflect conditions in farmers' field, long-term processes and spatial variability. The relevance of simulation models to subsistence farmers and/or smallholders and target groups (researchers, consultants, educators, policy makers, and extension staff) are discussed. Applicability of the models to particular problems, uptake of models by end-users, and translation of this use of models into a measurable impact are the phases by which models will have an impact on the process of improving the livelihoods of farmers

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