The water balance is an accounting of the inputs and outputs of water. The water balance of a place, whether it be an agricultural field, region, or continent, can be determined by calculating the input and output of water. The major input of water is from precipitation and output is evapotranspiration. The geographer C. W. Thornthwaite (1899-1963) pioneered the water balance approach to water resource analysis. He and his team used the water-balance methodology to assess water needs for irrigation and other water-related issues.

The Water Balance

To understand water-balance concept, we need to start with its various components:

Precipitation (P). Precipitation in the form of rain, snow, sleet, hail, etc.  makes up the primarily supply of water to the surface. In some very dry locations, supplies of water can be added by dew and fog. 

Actual evapotranspiration (AE). Evaporation is the phase change from a liquid to a gas releasing water away from a wet surface into the air above. Similarly, transpiration is represents a phase change when water is released into the air by plants. Evapotranspiration is the combination of water released to the atmosphere by evaporation and transpiration. Actual evapotranspiration is the amount of water delivered to the air from these two processes. Actual evapotranspiration is an output of water that is dependent on moisture available at the surface, temperature and humidity. Think of actual evapotranspiration as water use, that is, water is actually evaporating and transpiring given the environmental conditions of a place. Actual evapotranspiration increases as temperatures increase, so long as there is water to evaporate and for plants to transpire. The amount of evapotranspiration also depends on how much water is available, which depends on the field capacity of soils.

Potential evapotranspiration (PE). Potential evapotranspiration is the amount of water that would be evaporated under an optimal set of conditions, among which is an unlimited supply of water. Think of potential evapotranspiration of water need or demand. In other words, it would be the water demand for evaporation and transpiration given the local environmental conditions. One of the most important factors that determines water demand is solar radiation. As energy input increases the demand for water, especially from plants increases. Regardless if there is, or isn't, any water in the soil, a plant still demands water. If it doesn't have access to water, the plant will likely wither and die.

Soil Moisture Storage (ST). Soil moisture storage refers to the amount of water held in the soil at any particular time. The amount of water in the soil depends soil properties like soil texture and organic matter content. The maximum amount of water the soil can hold is called the field capacity. Fine grain soils have large field capacities than coarse grain soils. Thus, more water is available for actual evapotranspiration than fine soils than coarse (sandy), soils. Soil moisture storage falls between 0 and the field capacity.

Change in Soil Moisture Storage (DST). The change in soil moisture storage is the amount of water that is being added to or removed from what is stored. The change in soil moisture storage falls between 0 and the field capacity.

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