The local climate is one of the main things that influences how much water will need to be applied to maintain good plant growth. The plant water requirement includes the water lost by transpiration and evaporation. Transpiration and evaporation are the two principle ways in which water in the soil is depleted. Transpiration is the process by which plants consume water. Evaporation is the direct loss of water from the soil surface.
Water loss can also occur with runoff and percolation below the level of root absorption.
Evapotranspiration rate is often used to indicate the total loss due to evaporation and transpiration. The Environment Above and Below the Ground. While this does not include all of the variables, it is a practical solution for design and is considered to be reliable for estimating water requirements. ET rates, commonly used in landscape irrigation, include general use tables with monthly rates for a city, region or type of climate. Listed rates for a regional area are based on average weather conditions using historical data.