Within rapidly developing economies there is increasing demand for better planning for the use of resources for agricultural production. Planned agricultural development depends upon a variety of resources and conditions including soils, climate, land use history, management and cultural practices. Other factors can be more accurately defined providing one of these can be singled out and correctly evaluated. This paper deals principally with the evaluation of moisture available from the atmosphere and its effect upon crop production and crop yields. The methodology can, however, also be used for irrigation planning and for irrigation scheduling. Definitions are given for actual evapotranspiration, potential evapotranspiration, dependable precipitation, moisture availability index and moisture deficit. Moisture adequacies are related to crop growth and yields. Methods for estimating evapotranspiration are presented. A classification of moisture adequacies or deficits is proposed.