Optimizing crop water use in sparse stands of pearl millet

Most of current theory on crop water use-yield relations has been developed for intensively managed crops with well-developed canopies. In most agricu ltural systems of semiarid West Africa, low input levels and endemic enviro nmental stress predominate. In farmers' fields, leaf area index (LAI) of th e staple grain, pearl millet [Pennisetum glaucum (L.) R. Br.], may never re ach 1. In contrast to dense canopied crops, pearl millet yield is little co rrelated with ET. Reduced LAI decreases ET efficiency (kg dry matter mm(-1) evaporation from crop and soil surfaces) because evaporation (E) from the soil surface constitutes a large portion of ET. Additionally, atmospheric w ater vapor pressure deficit (e*-e) increases within sparse canopies due to sensible heat transfer from the soil surface, and small and irregular rough ness length of the canopy. Greater (e*-e) further decreases crop T efficien cy (kg dry matter mm(-1) transpiration) and therefore ET efficiency. Under low-input conditions, pearl millet ET efficiencies are roughly one-third of those obtained under intensive management, suggesting that T efficiency is also reduced by environmental stress, especially soil nutrient deficiency. Environmental stresses also cause poor root development, which results in reduced crop water supply, and increased resistance to water uptake. Optimi zing crop water use of sparse pearl millet stands will require some form of nutrient input. Other appropriate technologies include certain forms of in tercropping and agroforestry that have been traditionally practiced in part s of West Africa. These can improve soil nutrient availability, increase ef fective crop cover, and reduce canopy (e*-e).