ACS Distance Education UK
Actual rainfall is one of the major limiting factors determining what you grow in any particular site. Deficiencies in rainfall can in many cases be offset by irrigation from alternative sources of water, however if these sources are not available, or if water output is greater then input (compromising sustainability) then you need to choose a site that provides sufficient natural rainfall for your plants.
There are four major points to consider regarding rainfall:
Evaporation is the loss of water as water vapour. It increases as temperatures increase, humidity drops and winds increase. It can be measured by determining the amount of water evaporated from a free water surface exposed in a pan. In countries, such as Australia, where surface water storage is extremely important for agricultural purposes, evaporation is very significant. As with other climatic data, maps or tables of evaporation data are generally readily available.
Infiltration into the soil surface will depend on a number of factors, including:
As can be seen from the diagram ("The Hydrological Cycle") the water that passes into the soil can also do several things:
Perhaps the most important climatic parameter that determines the growing season at a particular site is 'Effective Rainfall'. This can be defined as the rainfall over a certain period (eg. month) minus the soil evaporation (equivalent to approximately one third of pan evaporation figures) during the same period. Positive figures indicate that soil moisture is increasing, or in other words the amount of rainfall received in that period exceeded the amount of water lost by evaporation. Negative figures indicate that evaporation has exceeded rainfall and that the soil is drying up. The number of months in succession in which rainfall exceeds evaporation (as long as temperature isn't a limiting factor) determines the growing season of a particular site.