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9.3.2: Soil Moisture Seasons

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    The Four Soil Moisture Seasons

    Four soil moisture seasons can be defined by the soil moisture conditions.

    Line graph showing annual soil moisture seasons: recharge, surplus, utilization, and deficit for Rockford, Illinois.
    Figure \(\PageIndex{1}\): Soil Moisture Seasons for Rockford, Illinois ( CC BY-SA 4.0; Courtesy Michael Ritter in The Physical Environment).

    Figure \(\PageIndex{1}\) is a monthly water-balance graph that compares precipitation (P), potential evapotranspiration (PE), and actual evapotranspiration (AE) over the course of a year and highlights periods of surplus, recharge, utilization, and deficit.

    The horizontal axis shows the months from January through December. The vertical axis represents the amount of water, ranging from approximately 0 to 150 units.

    Three lines are shown:

    • P (precipitation) represents incoming water from rainfall and snowfall.
    • PE (potential evapotranspiration) represents the atmospheric demand for water.
    • AE (actual evapotranspiration) represents the amount of water actually transferred to the atmosphere.

    Surplus - From January through May, precipitation exceeds potential evapotranspiration, and the soil is already at or near field capacity. The blue shaded area labeled Surplus represents excess water that cannot be stored in the soil and is therefore available for runoff, streamflow, or groundwater recharge.

    Utilization  - From approximately June through August, potential evapotranspiration exceeds precipitation. The tan shaded area labeled Utilization indicates that plants and evaporation are using water stored in the soil to satisfy the excess demand. Soil moisture storage decreases during this period.

    Deficit - During late summer, particularly around August, the graph shows a red shaded area labeled Deficit. This occurs after soil moisture storage has been exhausted. At this point, potential evapotranspiration remains greater than the available water supply, and plants experience moisture stress because water demand exceeds water availability.

    Recharge - During autumn, approximately October through November, precipitation once again exceeds evapotranspiration. The green shaded area labeled Recharge represents water being added back into soil moisture storage. Before surplus can occur again, the depleted soil moisture reservoir must first be replenished.

    Annual cycle - The graph illustrates a repeating sequence:

    1. Winter and spring: Surplus water accumulates because precipitation exceeds water demand.
    2. Early summer: Soil moisture is utilized to meet increasing water demand.
    3. Late summer: Soil moisture deficit develops when stored water is depleted.
    4. Autumn: Soil moisture is recharged as precipitation again exceeds evapotranspiration.
    5. Winter: Surplus conditions return after the soil reaches field capacity.

    The figure demonstrates how water availability changes throughout the year as precipitation and evapotranspiration fluctuate. It shows the seasonal progression from water surplus, to soil moisture utilization, to water deficit, and finally to soil moisture recharge. This type of graph helps explain irrigation needs, drought stress, groundwater recharge, and seasonal water-resource availability in a region.

    Recharge

    The recharge season is a time when water is added to soil moisture storage (+ΔST). The recharge period occurs when precipitation exceeds potential evapotranspiration but the soil has yet to reach its field capacity. Figure \(\PageIndex{1}\) shows September to mid November as the recharge season. 

    Surplus

    The surplus season occurs when precipitation exceeds potential evapotranspiration and the soil has reached its field capacity. Any additional water applied to the soil runs off. If this water runs off into nearby streams and rivers it could cause flooding. Thus, the intensity (amount) and duration (length of season) of surplus can be used to predict the severity of potential flooding. According to Figure \(\PageIndex{1}\), mid November to May is the surplus season. 

    Utilization

    The utilization season is a time when water is withdrawn from soil moisture storage (-ΔST). The utilization period occurs when potential evapotranspiration exceeds precipitation but soil storage has yet to reach 0 (dry soil). As shown in Figure \(\PageIndex{1}\), May to August is the season for utilization. 

    Deficit

    Figure \(\PageIndex{1}\) shows that the deficit season occurs when potential evapotranspiration exceeds precipitation and soil storage has reached 0. August to September is the deficit season.  This is a time when there is essentially no water for plants. Farmers then tap ground water reserves or water in nearby streams and lakes to irrigate their crops. Thus, the intensity (amount) and duration (length of season) of deficit can be used to predict the need for irrigation water.

    Whether a place experiences all four seasons depends on the climate and soil properties. Wet climate and those places with soils having high field capacities are less likely to experience a deficit period. Likewise the duration and intensity of any season will be determined by the climate and soil properties. Given equal amounts of precipitation, coarse textured soils will generate runoff faster than fine textured soils and may experience more intense surplus

    This page titled 9.3.2: Soil Moisture Seasons was last modified on Fri, 19 Jun 2026 21:59:12 GMT and is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Michael E. Ritter (The Physical Environment) via source content that was edited to the style and standards of the LibreTexts platform.