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Soil Moisture Regimes

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Soil moisture regimes are defined based on the watertable level and the presence or absence of available water (water that can be used by plants).  All moisture regimes, except aquic, are based on regional climate.  Aquic moisture regimes are based on the length of the period that the soil was saturated.

Soil moisture regimes are used as a soil classification criterion because they affect soil genesis (formation), affect the use and management of soils, and can be used to group soils with similar properties and morphology.

The soil moisture regime classes include:

Aquic (or Perudic) moisture regime:  Saturated with water long enough to cause oxygen depletion.

The aquic (L. aqua, water) moisture regime is a reducing regime in a soil that is virtually free of dissolved oxygen because it is saturated by water. Some soils are saturated with water at times while dissolved oxygen is present, either because the water is moving or because the environment is unfavorable for micro-organisms (e.g., if the temperature is less than 1° C); such a regime is not considered aquic.

It is not known how long a soil must be saturated before it is said to have an aquic moisture regime, but the duration must be at least a few days, because it is implicit in the concept that dissolved oxygen is virtually absent. Because dissolved oxygen is removed from ground water by respiration of micro-organisms, roots, and soil fauna, it is also implicit in the concept that the soil temperature is above biologic zero for some time while the soil is saturated. Biologic zero is defined as 5° C in this taxonomy. In some of the very cold regions of the world, however, biological activity occurs at temperatures below 5° C.

Very commonly, the level of ground water fluctuates with the seasons; it is highest in the rainy season or in fall, winter, or spring if cold weather virtually stops evapotranspiration. There are soils, however, in which the ground water is always at or very close to the surface. Examples are soils in tidal marshes or in closed, landlocked depressions fed by perennial streams. Such soils are considered to have a peraquic moisture regime.

Udic moisture regime:  Humid or subhumid climate.

The udic (L. udus, humid) moisture regime is one in which the soil moisture control section is not dry in any part for as long as 90 cumulative days in normal years. If the mean annual soil temperature is lower than 22° C and if the mean winter and mean summer soil temperatures at a depth of 50 cm from the soil surface differ by 6° C or more, the soil moisture control section, in normal years, is dry in all parts for less than 45 consecutive days in the 4 months following the summer solstice. In addition, the udic moisture regime requires, except for short periods, a three-phase system, solid-liquid-gas, in part or all of the soil moisture control section when the soil temperature is above 5° C.

The udic moisture regime is common to the soils of humid climates that have well distributed rainfall; have enough rain in summer so that the amount of stored moisture plus rainfall is approximately equal to, or exceeds, the amount of evapotranspiration; or have adequate winter rains to recharge the soils and cool, foggy summers, as in coastal areas. Water moves downward through the soils at some time in normal years.

In climates where precipitation exceeds evapotranspiration in all months of normal years, the moisture tension rarely reaches 100 kPa in the soil moisture control section, although there are occasional brief periods when some stored moisture is used. The water moves through the soil in all months when it is not frozen. Such an extremely wet moisture regime is called perudic (L. per, throughout in time, and L. udus, humid). In the names of most taxa, the formative element “ud” is used to indicate either a udic or a perudic regime; the formative element “per” is used in selected taxa.

Ustic moisture regime:  Semiarid climate.

The ustic (L. ustus, burnt; implying dryness) moisture regime is intermediate between the aridic regime and the udic regime. Its concept is one of moisture that is limited but is present at a time when conditions are suitable for plant growth. The concept of the ustic moisture regime is not applied to soils that have permafrost or a cryic soil temperature regime (defined below). If the mean annual soil temperature is 22° C or higher or if the mean summer and winter soil temperatures differ by less than 6° C at a depth of 50 cm below the soil surface, the soil moisture control section in areas of the ustic moisture regime is dry in some or all parts for 90 or more cumulative days in normal years. It is moist, however, in some part either for more than 180 cumulative days per year or for 90 or more consecutive days.

If the mean annual soil temperature is lower than 22° C and if the mean summer and winter soil temperatures differ by 6° C or more at a depth of 50 cm from the soil surface, the soil moisture control section in areas of the ustic moisture regime is dry in some or all parts for 90 or more cumulative days in normal years, but it is not dry in all parts for more than half of the cumulative days when the soil temperature at a depth of 50 cm is higher than 5° C. If in normal years the moisture control section is moist in all parts for 45 or more consecutive days in the 4 months following the winter solstice, the moisture control section is dry in all parts for less than 45 consecutive days in the 4 months following the summer solstice.

In tropical and subtropical regions that have a monsoon climate with either one or two dry seasons, summer and winter seasons have little meaning. In those regions the moisture regime is ustic if there is at least one rainy season of 3 months or more. In temperate regions of subhumid or semiarid climates, the rainy seasons are usually spring and summer or spring and fall, but never winter. Native plants are mostly annuals or plants that have a dormant period while the soil is dry. 

Aridic (or Torric) moisture regime: Arid climate.

These terms are used for the same moisture regime but in different categories of the taxonomy. In the aridic (torric) moisture regime, the moisture control section is, in normal years:

  1. Dry in all parts for more than half of the cumulative days per year when the soil temperature at a depth of 50 cm from the soil surface is above 5° C; and
  2. Moist in some or all parts for less than 90 consecutive days when the soil temperature at a depth of 50 cm is above 8° C.

Soils that have an aridic (torric) moisture regime normally occur in areas of arid climates. A few are in areas of semiarid climates and either have physical properties that keep them dry, such as a crusty surface that virtually precludes the infiltration of water, or are on steep slopes where runoff is high. There is little or no leaching in this moisture regime, and soluble salts accumulate in the soils if there is a source.

The limits set for soil temperature exclude from these moisture regimes soils in the very cold and dry polar regions and in areas at high elevations. Such soils are considered to have anhydrous conditions (defined earlier).

Xeric moisture regime :  Mediterranean climate (moist, cool winters and dry, warm summers)

The xeric (Gr. xeros, dry) moisture regime is the typical moisture regime in areas of Mediterranean climates, where winters are moist and cool and summers are warm and dry. The moisture, which falls during the winter, when potential evapotranspiration is at a minimum, is particularly effective for leaching. In areas of a xeric moisture regime, the soil moisture control section, in normal years, is dry in all parts for 45 or more consecutive days in the 4 months following the summer solstice and moist in all parts for 45 or more consecutive days in the 4 months following the winter solstice. Also, in normal years, the moisture control section is moist in some part for more than half of the cumulative days per year when the soil temperature at a depth of 50 cm from the soil surface is higher than 6° C or for 90 or more consecutive days when the soil temperature at a depth of 50 cm is higher than 8° C. The mean annual soil temperature is lower than 22° C, and the mean summer and mean winter soil temperatures differ by 6° C or more either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact if shallower.

Management considerations vary based on different moisture regimes.  Soils with an aridic (torris) moisture regime require irrigation to be used for crops. Soils with a ustic moisture regime can grow rain-fed crops, but moisture will be limited during some of the growing season. Soils with a udic moisture regime have sufficient moisture for crops.  Crops may be grown in the udic moisture regime without irrigation, but irrigation is needed for crops in most years in an ustic moisture regime.  Soils with an aquic (perudic) moisture regime need artificial drainage for most cropping practices.

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