The semi-arid zones of the world are fragile ecosystems which are being sub­ stantially modified by the activities of mankind. Increasing human populations have resulted in greater demands on semi-arid zones for providing human susten­ ance and the possibility that this may enhance desertification is a grave concern. These zones are harsh habitats for humans. The famines that resulted from drought during the late 1960's and the 1970's in the African Sahel illustrated the unreliability of present agricultural systems in this zone. Large fluctuations in ag­ ricultural production have occurred in semi-arid zones of Australia, North Ameri­ ca, and the Soviet Union due to periodic droughts, even though considerable ag­ ricultural technology has been devoted to agricultural development in these zones. The challenge to mankind is to manage these different semi-arid zones so that pro­ ductivity is increased and stabilized, and environmental deterioration is decreased. Irrigation can be used to increase and stabilize agricultural production in semi-arid zones as discussed in Volume 5 of this series, Arid Zone Irrigation. The present volume, Agriculture in Semi-Arid Environments, focuses on dryland farming in semi-arid zones, and is relevant to the large areas of the world where rainfall is limiting and where water is not available for irrigation. This volume is designed to assist agricultural development in these areas and consists of reviews and analyses of available information by scientists working in Africa, Australia, and at the U ni­ versity of California.

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1 Ancient Agricultural Systems in Dry Regions.- 1.1 Introduction.- 1.2 The Beginnings of Agriculture.- 1.3 Ancient Agricultural Systems in Dry Regions of the Old World.- 1.3.1 The Near East.- 1.3.1.1 The Mesopotamian Heartland.- 1.3.1.2 Other Near Eastern Agriculture.- 1.3.2 The African Continent.- 1.3.2.1 Egypt.- 1.3.2.2 Other Dry Regions of Africa.- 1.3.3 The Indian Subcontinent.- 1.3.4 The Soviet Union and China.- 1.3.4.1 The Soviet Union.- 1.3.4.2 China.- 1.4 Ancient Agricultural Systems in Dry Regions of the New World.- 1.4.1 Mesoamerica.- 1.4.1.1 The Valley of Oaxaca.- 1.4.1.2 The Tehuacán Valley.- 1.4.1.3 The Basin of Mexico.- 1.4.2 The North American Southwest.- 1.4.2.1 The Hohokam.- 1.4.2.2 Lower Colorado River Tribes.- 1.4.2.3 Owens Valley Paiute.- 1.4.2.4 Anasazi.- 1.4.2.5 Casas Grandes.- 1.4.3 South America.- 1.5 Conclusions.- References.- 2 Development of Present Dryland Farming Systems.- 2.1 Introduction.- 2.2 Geographic Aspects of the World's Semi-Arid Tropics.- 2.3 Dryland Farming Systems in the Semi-Arid Tropics.- 2.3.1 Australia.- 2.3.2 India.- 2.3.3 Brazil.- 2.3.4 Southern Africa.- 2.3.5 Sahel Region.- 2.4 Geographic Aspects of the World's Semi-Arid Mid-Latitude Lands (Steppe).- 2.5 Dryland Farming Systems in the Mid-Latitude Steppes.- 2.5.1 North America.- 2.5.2 Eurasia.- 2.6 Perception of the Semi-Arid Environment.- 2.7 The Future of Dryland Farming Systems.- References.- 3 Semi-Arid Climates: Their Definition and Distribution.- 3.1 Introduction.- 3.2 Meteorological Factors Contributing to Aridity and Humidity.- 3.3 Defining Semi-Arid Regions.- 3.3.1 Moisture Balance Over the Year as a Whole.- 3.3.2 Seasonal Considerations in Moisture Balance.- 3.3.3 Attributes of the Bailey Moisture Index.- 3.3.4 Precipitation Variability in Semi-Arid Regions.- 3.4 Thermal Regions of Semi-Arid Zones.- 3.5 Conclusions.- References.- 4 Agroclimatology Applied to Water Management in the Sudanian and Sahelian Zones of Africa.- 4.1 Introduction.- 4.2 Climatology and the Hydrologic Budget of Cropping Systems.- 4.2.1 Evaporative Demand of the Atmosphere.- 4.2.1.1 Reference Crop Evapotranspiration.- 4.2.1.2 Pan Evaporation.- 4.2.1.3 Piche Evaporimeter.- 4.2.1.4 Calculating Evaporative Demand from Weather Data.- 4.2.1.5 Relations Between Evaporative Demand and Rainfall.- 4.2.2 Actual Crop Water Requirements.- 4.2.2.1 Methods of Measurement.- 4.2.2.2 Crop Water Requirements.- 4.2.3 Rainfall Efficiency.- 4.2.3.1 The First Useful Rainfall.- 4.2.3.2 The End of the Useful Rainy Season.- 4.2.3.3 Determining the Adequacy of Rainfall and Soil Moisture Reserves to Satisfy Crop Water Needs During the Growing Season.- 4.3 Application of Agroclimatology to Agriculture in Senegal.- 4.3.1 Cultural Practices.- 4.3.2 Varietal Development.- 4.3.3 Regional Water Management.- 4.4 Future Strategies.- References.- 5 Microbiological and Biochemical Aspects of Semi-Arid Agricultural Soils.- 5.1 Introduction.- 5.2 Microbial Ecology and Soil Physical Properties.- 5.2.1 Water.- 5.2.2 Aeration.- 5.2.3 Temperature.- 5.2.4 pH.- 5.3 Organic Matter.- 5.3.1 Decomposition.- 5.3.2 Synthesis.- 5.3.3 Aggregation.- 5.4 Mineral Nutrition of Plants.- 5.4.1 Phosphorus.- 5.4.2 Mycorrhizae.- 5.4.3 Chelation of Iron and Other Metals.- 5.4.4 The Nitrogen Triangle.- 5.5 Future Directions for Research.- References.- 6 Crop Adaptation to Semi-Arid Environments.- 6.1 Introduction.- 6.2 Water as a Limiting Factor.- 6.3 Adaptive Attributes for Semi-Arid Environments.- 6.3.1 Phenology and Mechanisms for Escaping Drought.- 6.3.2 Drought Avoidance.- 6.3.3 Tolerance to Drought.- 6.3.4 Heat Resistance.- 6.3.5 Acclimation to Drought and Heat.- 6.3.6 Water-Use Efficiency.- 6.4 Developing Cultivars for Semi-Arid Environments.- 6.4.1 Interactions Between Genotype and Environment.- 6.4.2 Selecting for Yield in Semi-Arid Environments.- 6.4.3 Selecting for Indices of Adaptation.- 6.4.4 Genetic Resources Available.- 6.4.4.1 Barley and Wheat.- 6.4.4.2 Millet and Sorghum.- 6.4.4.3 Maize.- 6.4.4.4 Cowpeas and Beans.- 6.5 Future Strategies for Research.- References.- 7 Water Transport Through Soil, Plant, and Atmosphere.- 7.1 Introduction.- 7.2 Components of the Soil-Plant-Atmosphere Continuum.- 7.2.1 Surface Water Hydrology.- 7.2.2 Soil Water Movement.- 7.2.3 Evapotranspiration.- 7.2.3.1 Potential Evapotranspiration.- 7.2.3.2 Evapotranspiration with Incomplete Crop Cover.- 7.2.3.3 Stress-Limited Evapotranspiration.- 7.2.4 Plant Water Uptake and Plant Response.- 7.2.4.1 Water Movement Through the Plant.- 7.2.4.2 Behavior of Plant Roots.- 7.2.4.3 Plant Response to Water Deficits.- 7.3 Models of the Soil-Plant-Atmosphere Continuum.- 7.3.1 Introduction and Classification.- 7.3.2 Descriptions of Individual Models.- 7.3.3 Application to Dryland Farming Systems.- 7.4 Conclusions.- References.- 8 Crop Management in Semi-Arid Environments.- 8.1 Introduction.- 8.2 Management Systems.- 8.2.1 Subsistence Cropping.- 8.2.2 Commercial Crop Production.- 8.3 Cultural Systems.- 8.3.1 Annual Cropping.- 8.3.2 Alternate Crop-Fallow System.- 8.3.3 Ley Farming.- 8.3.4 Shifting Cultivation.- 8.3.5 Mound Culture.- 8.3.6 Recession Farming.- 8.3.7 Mulch Farming.- 8.4 Cropping Methods.- 8.4.1 Sole Cropping.- 8.4.2 Mixed Cropping.- 8.4.3 Intercropping.- 8.4.4 Relay Cropping.- 8.5 Crop Selection.- 8.5.1 Choice of Species.- 8.5.2 Choice of Cultivar.- 8.5.3 Seed Sources.- 8.6 Planting Techniques.- 8.6.1 Time of Planting.- 8.6.2 Planting Depth.- 8.6.3 Seeding Rates.- 8.7 Crop Fertilization.- 8.8 Crop Rotation.- 8.9 Vertebrate Pest Control.- 8.10 Harvesting.- 8.11 Future Directions of Research.- References.- 9 Soil Management in Semi-Arid Environments.- 9.1 Introduction.- 9.2 Differing Responses of Soils to Tillage.- 9.3 Objectives of Soil Management.- 9.4 Tillage and Soil Water.- 9.5 Soil Fertility.- 9.6 Tillage and the Soil Microenvironment.- 9.7 Erosion Control.- References.- 10 Erosion and Its Control in Semi-Arid Regions.- 10.1 Introduction.- 10.2 Erosion by Water.- 10.2.1 Factors that Influence Erosion by Water.- 10.2.1.1 Rainfall.- 10.2.1.2 Temperature.- 10.2.1.3 Slope.- 10.2.1.4 Soil.- 10.2.1.5 Plant Cover.- 10.2.1.6 Universal Soil-Loss Equation.- 10.2.2 Extent of Erosion by Water for Different Farming Systems.- 10.3 Erosion by Wind.- 10.3.1 Factors that Influence Erosion by Wind.- 10.3.1.1 Wind.- 10.3.1.2 Surface.- 10.3.1.3 Soil.- 10.3.1.4 Wind Erosion Equation.- 10.3.2…