A general characterization of moisture conditions is achieved through the concept of length of growing period (LGP), i.e. the period during the year when both moisture availability and temperature are conducive to crop growth. Thus, in a formal sense, LGP refers to the number of days within the period of temperatures above 5°C when moisture conditions are considered adequate. Under rain-fed conditions, the begin of the LGP is linked to the start of the rainy season. For establishing crops, 0.4 - 0.5 times the level of reference evapotranspiration is considered sufficient to meet water requirements of dryland crops (FAO 1978-81a; 1979; 1992a). This layer is a result of the Agro-ecological Zones (AEZ) methodology for land productivity assessments and is part of the Global Agro-ecological Zoning Database.

The Food and Agriculture Organization of the United Nations (FAO) with the collaboration of the International Institute for Applied Systems Analysis (IIASA), has developed a system that enables rational land-use planning on the basis of an inventory of land resources and evaluation of biophysical limitations and potentials. This is referred to as the Agro-ecological Zones (AEZ) methodology.