Soil moisture surplus is the condition that occurs when the soil is unable to hold any more water because it has reached its field capacity, and the amount of water coming into the soil (precipitation) is more than the amount of water leaving the soil (potential evapotranspiration).
Understanding Soil Moisture Surplus
Based on the provided reference, the surplus condition arises from a specific balance of water inputs and outputs relative to the soil's capacity:
- Precipitation Exceeds Potential Evapotranspiration: More water is falling as rain or snow than can be lost through evaporation from the soil surface and transpiration from plants.
- Soil is Saturated: The soil has already absorbed all the water it can hold, reaching its field capacity.
When both of these conditions are met, any additional precipitation has nowhere to go within the soil profile.
Conditions for Surplus
The reference outlines the key factors leading to a soil moisture surplus:
- Precipitation > Potential Evapotranspiration
- Soil Moisture Level = Field Capacity
This can be summarized:
| Factor | Condition Leading to Surplus |
|---|---|
| Precipitation | Greater than Potential Evapotranspiration |
| Potential Evapotranspiration | Less than Precipitation |
| Soil Moisture Level | At or above Field Capacity |
Consequences of Surplus Water
As stated in the reference, when soil moisture surplus occurs:
- Any additional water applied to the soil simply runs off.
- If this runoff flows into streams and rivers, it can potentially cause flooding.
Essentially, the soil is full, and the excess water becomes surface or subsurface runoff rather than being stored in the soil.
This condition is often associated with specific times of the year, referred to as the "surplus season," typically occurring when rainfall or snowmelt is high and plant water usage is low.
