Technical Abstract: Field water supply has been a major focus of agricultural research and management. The soil water balance is a widely used method of tracking soil water supply in a field. This approach provided some of the earliest information available about the amount of water required to produce a crop, the relationship between water use and plant production, and water stress impacts on plant water use, and remains an important approach to research and management today. The field soil water balance involves accounting for inputs of water to the system, such as precipitation and irrigation, as well as water leaving the system via evapotranspiration, runoff, and drainage below the root zone. Management practices can have a large impact on the magnitude of the components of the water balance, such as runoff and drainage, as well as patterns of evapotranspiration and partitioning of the water loss into soil evaporation and transpiration. In agriculture, increasing the amount of transpiration increases productivity of the system. Soil water balance approaches have been applied to irrigated and rainfed agriculture and are an integral part of all plant growth and natural resource models, so understanding the basic concepts and principles is important for sound water management.