Field Water Balance
The water balance of a field is an itemized statement of all gains, losses, and changes of storage of water occurring in a given field within specified boundaries during a specified period of time. The task of monitoring and controlling the field water balance is vital to the efficient management of water and soil. A knowledge of the water balance is necessary to evaluate the possible methods to minimize loss and to maximize utilization of water which is so often the limiting factor in crop production.
Gains of water in the field are generally due to precipitation and irrigation. Occasionally, there may be gains due to accumulation of runoff from higher tracts of land, or to capillary rise from below (especially where a water table is present at some shallow depth). Losses of water include surface run-off from the field, deep percolation out of the root zone (drainage), evaporation from the soil surface, and transpiration from the crop canopy. The change in storage of water in the field can occur in the soil as well as in the plants. The total change in storage must equal the difference between the sum of all grains and the sum of all losses.
Accordingly, the water balance equation may be stated as follows: (Gains) - (Losses) = (Change in storage)
In which, P is precipitation, I= irrigation, R runoff from the field, D downward drainage out of the root zone, E evaporation from the soil, T transpiration by the crop canopy, A S the change in soil water content of the root zone, and A V the change in plant water content. All of these quantities are usually expressed in terms of water depth per unit of land area (ha-cm) or units of depth (cm).
An important consideration in water balance studies is the period, or time interval, for which the balance is made. Too short a period might be impractical, while too long a period might mask the occurrence of shortterm critical stages. At such critical stages as flowering and fruit-set even temporary imbalance in crop-water status can be have a lasting effect. It is necessary to ensure that the crop water balance is maintained positive continuously during the growing season.
Measurements of field water balance is usually conducted by the use of weighing or floating lysimeters. However, they cannot be used when the mixed nature of the species composition, the spatial distribution of the vegetation, the characteristics of the root systems, the size of plants, or other factors make it impossible to simulate the natural environment in the lysimeter itself. In such cases,, determination of soil water storage at different points in the community provide the only means for evaluating the water balance.