How To calculate water balance

Hidrology ,

The main objective of an irrigation strategy is to maintain the level of soil water in the root zone of plants within desired limits, for example between 60 and 90% of field capacity, so they do not become stressed.

Expressions to use Water in the soil layer can be expressed as: Depth (mm water/mm soil depth). Volume (cm3 water/cm3 soil). A percentage of the layer (mm water/100mm soil or cm3 water/100cm3 soil).

Note:

A percentage figure is useful in the calculations of the amount of water in a soil layer. For example: if the water holding capacity (field capacity) of a layer is 12% and the layer is 150mm then the amount of water held at field capacity is (.12 x 150) = 18mm.
To understand this concept, think of a savings analogy where the strategy is to maintain a bank balance within desired limits. The new level of money in the account is the current level in the account plus the deposits and minus the withdrawals.

Keep in mind, the simplified soil water equation above is misleading because it mixes levels (water) with rates (rain, irrigation, evaporation, transpiration) and is, therefore, algebraically and dimensionally incorrect.
In fact, to be correct (i.e. algebraically and dimensionally) the equation needs to specify the rates of change:

• Rain (R, mm/day)
• Irrigation (I, mm/day)
• Evaporation (E, mm/day)
• Transpiration (T, mm/day)

Note also that the rates of evaporation and transpiration are usually combined as a rate of EvapoTranspiration (ET) and estimated from evaporation from a free film of water such as a Class A pan or Piché atmometer.

Time (DT) - the duration of the rates of additions and losses - also needs to be taken into account. If dealing with daily changes in the Water Balance then DT = 1 day and R, I, E and T must be daily rates. This is why 'precipitation rates' are talked about for rainfall and irrigation. The irrigation rate may be 5 mm/hour but if it is the only addition for the day then it becomes 5 mm/day.

The formula is:
Water (new) = Water (present) + [(R + I) - (ET)]