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The purpose of irrigation is to supplement rainfall to the extent that the plants are not deprived of water at any time during the course of their development. It is not possible to determine the extent to which rainfall fails to supply the needs of plants for water without knowing their water requirements. Increased costs of production have necessitated the improvement of the more hap hazard methods used in estimating irrigation requirement: i.e. the appearance of the soil surface, the feel of the soil a few inches below the surface, the application of water by the calendar or whenever a predetermined quantity of rain was not received in a set period. These procedures should be replaced by a more precise method of estimating actual water requirements of the crop. Since weather factors affect growth and water requirements, methods have been developed which estimate irrigation needs from meteorological data. These methods are referred to as soil moisture budgets. A soil moisture budget is calculated by subtracting estimated evapotranspiration from daily rainfall or irrigation. This amount is then subtracted (or added when rain or irrigation exceeds evapotranspiration) from the moisture present in the soil to give the new soil moisture content. In budgeting water for irrigation, this process is continued on a daily basis until a predetermined soil moisture level is reached. At this time an amount of water equal to the deficit is applied to return soil moisture to field capacity.
the development of an irrigation budget
Kerr, H. A. and J. M. Murray 1968. THE DEVELOPMENT OF AN IRRIGATION BUDGET. Canadian Agricultural Engineering 10(1):12-16.
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