Deficit (D) A soil moisture deficit occurs when the demand for water exceeds that which is actually available . In other words, deficits occur when potential evapotranspiration exceeds actual evapotranspiration (PE>AE). Recalling that PE is water demand and AE is actual water use (which depends on how much water is really available), if we demand more than we have available we will experience a deficit. But, deficits only occur when the soil is completely dried out. That is, soil moisture storage (ST) must be 0. By knowing the amount of deficit, one can determine how much water is needed from irrigation sources.
Surplus (S) Surplus water occurs when P exceeds PE and the soil is at its field capacity (saturated). That is, we have more water than we actually need to use given the environmental conditions at a place. The surplus water cannot be added to the soil because the soil is at its field capacity so it runs off the surface. Surplus runoff often ends up in nearby streams causing stream discharge to increase. A knowledge of surplus runoff can help forecast potential flooding of nearby streams.
The best way to understand how the water balance works is to actually calculate a soil water budget. We'll use Rockford, Illinois which is located in the humid continental climate of northern Illinois. Rockford lies on the northern edge of the prairie and mixes with deciduous forest. This vegetation has been nearly completely replaced with agriculture. A knowledge of soil moisture status is important to the agricultural economy of this region that produces mostly corn and soy beans.
To work through the budget, we'll take each month (column) one at a time. It's important to work column by column as we're assessing the moisture status in a given month and one month's value may be determined by what happened in the previous month.