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SPARSE CANOPY SYMPOSIUM INTRODUCTION

Optimizing Crop Water Use in Sparse Stands of Pearl Millet

Texas Agricultural Experiment Station, 6500 Amarillo Blvd. West, Amarillo, TX 79106 USA

w-payne{at}tamu.edu

Most of current theory on crop water use–yield relations has been developed for intensively managed crops with well-developed canopies. In most agricultural systems of semiarid West Africa, low input levels and endemic environmental stress predominate. In farmers' fields, leaf area index (LAI) of the staple grain, pearl millet [Pennisetum glaucum (L.) R. Br.], may never reach 1. In contrast to dense canopied crops, pearl millet yield is little correlated 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 water vapor pressure deficit (e*-e) increases within sparse canopies due to sensible heat transfer from the soil surface, and small and irregular roughness length of the canopy. Greater (e*-e) further decreases crop T efficiency (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. Optimizing crop water use of sparse pearl millet stands will require some form of nutrient input. Other appropriate technologies include certain forms of intercropping and agroforestry that have been traditionally practiced in parts of West Africa. These can improve soil nutrient availability, increase effective crop cover, and reduce canopy (e*-e).

Abbreviations: LAI, leaf area index • E, evaporation from the soil surface • ET, crop transpiration + E • e*-e, atmospheric water vapor pressure deficit • ET efficiency, kg dry matter/ET • T efficiency, kg dry matter/T • z₀, crop roughness length • Y, crop yield • m, crop water use coefficient