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• If soil moisture content is too little (below the Permanent Wilting Point, PWP) the soil is dry.  The plant cannot extract any more water and dies.
• Higher soil moisture implies greater evapotranspiration efficiency.  This reaches a maximum at Field Capacity (CAP) when the soil is wet and contains all the water it can hold against gravity.  Not all water drains through the soil and some moisture is retained within the soil pores and cavities.  The soil is said to be at Field Capacity when large soil pores are filled with both air and water while the smaller pores are still full of water.  These conditions are considered ideal for crop growth and plants flourish best. 
• As soil moisture increases beyond Field Capacity the large soil pores are increasingly filled with water.  However, the efficiency of plant evapotranspiration remains the same.
• The soil is said to be at Full Saturation (SAT) when all soil pores, large and small, are filled with water.   Flooding is possible as a result of additional precipitation. 

For each soil type and location there is a pre-defined value of the ability to hold moisture and this is used to assess the impact of model rainfall.  The HTESSEL system includes allowance for water capture by interception of precipitation and dew fall, and at the same time, there are infiltration and run-off schemes that take account of soil texture and the standard deviation of sub-grid scale orography.

Measurement of soil moisture

Soil moisture is a forecast variable in IFS.  It needs to be initialised at each analysis cycle but there are very few directly measured observations.  Soil surface (skin) moisture is derived from:

• the expected air temperature and moisture structure in the lowest 2 m together with energy fluxes (from HTESSEL) and an analysis of observed screen level (2 m) humidities.
• satellite soil moisture data from the ASCAT sensor on the MetOp satellites
• data from the Soil Moisture and Ocean Salinity satellite mission (SMOS) is used for operational monitoring (see Fig2.1.4.5-42).

The 2m temperature and humidity are diagnostic parameters of the model, so their analysis only has an indirect effect on atmosphere through the soil and snow variables. 

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Fig2.1.4.5-2: Measurements from the Soil Moisture and Ocean Salinity satellite mission (SMOS)  polar orbiter satellite data.  At L-band frequency (1.4 GHz) the surface emission is strongly related to soil moisture over continental surfaces. Surface radiation at this frequency is influenced by the vegetation layer (and hence soil moisture if the vegetation type is known), but proximity of lakes etc cause difficulties with interpretation.

Soil moisture charts

Fig2.1.4.5-23: Examples of Soil Moisture at T+00 and T+192 DT 00UTC 06 March 2023.  

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• Sandy shades: Soil moisture SM < Permanent wilting point PWP.  Living vegetation cannot be sustained. Values show soil moisture as a percentage of the permanent wilting point  valuepoint value.
• Yellow/Green shades: Permanent wilting point PWP < soil moisture SM < field capacity CAP.  Evapotranspiration efficiency in percent increases as soil moisture increases.
• Blue shades: Capacity CAP < Saturation SAT.  Soil moisture super-saturation. Dark blue (>90%) suggests flooding (in the model).

Note the change in soil moisture over France from ~60% of field capacity (greens) to above 60% of saturation (blues). This is largely due to rain exceeding evaporation in these areas during the forecast period.  Conversely, parts of northern Morocco,  northern northern Algeria and northern Tunisia have become a little drier.

See the current soil moisture chart.  Select "Layer 1 2 3" from the drop down menu for the average moisture in the top metre of the earth..

Rarely in moist areas there are some soil moisture plots (except over Europe) indicating the soil is exceptionally dry.

Grid point data is plotted for Europe.  Elsewhere, for (most) other parts of the world, soil moisture is interpolated from surrounding grids points.  Field capacity, saturation, wilting point etc. depend on the soil type so can consequently be affected.  Users should check nearby soil moisture before accepting misleading soil moisture actual and forecast data.

Contrasting examples of surface and soil water budgets

Surface water budget in a typical mid-latitude agricultural landscape reacting to high rainfall in the model.

Recent periods of persistent rain over Britain over the winter of 2023/24 increased the soil moisture content in the river valleys and countryside around Reading.  Soil water storage in all model soil layers had been consistently between 120% and 150% of field capacity but generally below saturation.  Nevertheless there were areas of standing water in low-lying areas.

• On 12 Feb: There was some minor depletion of water in levels 1 and 2 due to evaporation.  
• On 13 Feb: Rain caused an increase in the rate of storage in the already high water content in soil levels 1 and 2.  There is only a small change in the already high fraction of field capacity in these levels.
• On 14 Feb: Rain also showed a small rise in the rate of storage in level 1 but the fraction of field capacity remains constant.   Water storage from the rain is partially offset by evaporation.  

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Fig2.1.4.5-5: Example of surface and soil water budget.  DT12UTC 12 Feb 2024, VT12-14 Feb 20-24.  Temperate mid-latitudes.

Surface water budget in desert soil reacting to extreme rainfall in the model.

A tropical system moved over the Northern Territories, Australia depositing a period of significant rainfall.  

The coarse soil type allows the water to penetrate instead of creating runoff even after the heavy rain.  

• On 21 Jan: The fraction of field capacity in level 1 is virtually zero in the desert.  
• On 22 Jan: The very heavy rain caused significant rate of water penetrations into levels 1, 2 and 3.  The rate of water storage penetrates steadily down from level 1 through level 2 percolates down into level 3.  The total soil water storage within all layers rises through the day and approaches, but does not reach, field capacity.  However, this hides the indication of fraction of field capacity in level 1 and level 2 reaches saturation for a time and even soil level 3 approaches field capacity for a while.  All water is soaked up by the soil and the rainfall does not create an immediate increase in evaporation.

Recycling of moisture by evaporation often has an impact on maintaining cyclones over the dessert.

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Fig2.1.4.5-4: Example of surface and soil water budget.  DT00UTC 21 Jan 2024, VT21-23 Jan 20-24.  Desert areas.

Surface water budget in a dry desert

The model soil moisture charts sometimes show moisture layers below the surface in dry desert areas.  There is very little ground truth so there must be some uncertainty.

Soil moisture charts consistently give an indication of water below the surface in mid-Sahara (near 23N 7E).  This should not be relied on.  However, it may well be correct as the area is around the oasis town of Tamanrasset in Algeria.  There is indication of water in layers 2 and 3 in Arabia, locally as high as 60%, but there is little data to confirm this.

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 Fig2.1.4.5-6: Example of soil moisture in desert areas.  DT and VT 12UTC 07 Sep 2023.  In the area around Tamanrasset (Algeria) and much of Saudi Arabia, soil moisture charts show dry surface layers (level 1, orange) and about 20% moisture in lower layers (levels 2 and 3, green) and locally as high as 60% in Saudi Arabia.

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Fig2.1.4.5-7: Afilal Oasis lies within the area where the model indicates soil with moderate moisture content in level 2 (~25% of Field Capacity) and level 3 (~40% of Field Capacity).  Level 1 has water content below the wilting point) and remains so as there is no vegetation and roots to bring water upwards from lower layers.  Nevertheless, subterranean water is locally sufficient to reach the surface at Afilal Oasis as springs. Soil moisture is a mean over a grid square.  Local details and individual oases are unlikely to be captured.  Soil moisture and soil type is not necessarily representative of an individual location:Measurements from the Soil Moisture and Ocean Salinity satellite mission (SMOS)  polar orbiter satellite data.  At L-band frequency (1.4 GHz) the surface emission is strongly related to soil moisture over continental surfaces. Surface radiation at this frequency is influenced by the vegetation layer (and hence soil moisture if the vegetation type is known), but proximity of lakes etc cause difficulties with interpretation.

Considerations

• Actual soil characteristics can vary widely within a grid box.  Users and forecasters should take into account the peculiarities of a location when interpreting model output.
• The assigned average soil type for a grid box is not necessarily representative of an individual location.
• Runoff can be up to 30% of rainfall in complex orography or mountainous regions.
• Recycling of moisture by evaporation from surface often has an impact on maintaining cyclones over the dessert.
• Impacts of errors associated with soil moisture. 

Additional sources of information

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