Evaporation
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
Infiltration into the soil surface will depend on a number of factors, including:

• The type of soil - well structured and sandy soils will have much higher infiltration rates than heavy, poorly structured soils (e.g. clays).
• The intensity of the rainfall - if rainfall is heavy then the amount of water reaching the surface maybe greater than the amount of water that can be infiltrated. This means that the water either sits on the surface until it can infiltrate into the soil later once rainfall has stopped or has reduced in intensity, or be evaporated back into the atmosphere or it will runoff (overland flow).

As can be seen from the diagram ("The Hydrological Cycle") the water that passes into the soil can also do several things:

• It can be held in the soil (as "Soil Moisture Storage") where it can be utilised by plants and animals (with some being transpired back into the atmosphere via the plants.
• It can pass through passages in the soil (e.g. cracks, animal burrows, cavities created by decomposing plant roots) and pass out into lower areas as surface runoff. This is known as "Through-flow". Passage of water in this way can be very rapid, and it can be a powerful cause of erosion in soils that are easily dispersed (e.g. tunnel erosion).
• It can seep deeper into the underlying soil to an area known as the "Aeration Zone Storage". The water here can be utilised by plants during very dry seasons. Water from this zone can also pass out to the surface down-slope (into lower lying areas). This is known as "Interflow". Some water will also percolate down deeper into "Ground Water Storage". In areas where ground water levels meet the soil surface then water flows out (e.g. springs) reaching creeks, rivers, etc.  
  It is this generally slow, regular flow of water from groundwater areas that keeps permanent streams flowing in dry seasons. This is known as "Base Flow". Base Flow is one of two components to a streams flow, the other being "Storm Flow", which is the water after rain from Overland Flow, Through Flow and Interflow (see diagram). It is the often sudden surge of water after heavy rain that gives this component of stream flow its name. These components of storm flow can often be diverted or slowed down and used for providing water at a later date (e.g. heavy vegetation or mulching reducing the runoff rate, increasing the water-holding capacity of the soil so that more moisture is retained there, terracing, banking or in some other way forming slopes to catch the runoff or slow it down).
• It can percolate even deeper into deep storage (e.g. aquifers, major artesian basins). Water from here can flow out to the surface in much lower lying areas (e.g. in saline affected areas). This water can also be utilised through bores.

Effective Rainfall
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.