*5.1.1 Rainfall-based estimation*

The difference between rainfall and evaporation indicates the soil water deficit. To quantify the water stress, one should have potential evapotranspiration of the crop. Water deficit for a period can be calculated by the given formula.

$$\mathbf{D} = \mathbf{R} + \mathbf{P} - \mathbf{P}\mathbf{E}$$

where D is the water deficit factor, R is the theoretical reserve, P is the rainfall, and PE is the potential evaporation.
