Wetland Loss as an Indicator of Subsoil Moisture Content

Overview

Wetland loss—defined as the reduction or degradation of wetland area and condition—is sometimes considered in ecological studies as a signal of underlying hydrological changes, including subsoil moisture dynamics. The relationship between wetland loss and subsoil moisture content is complex and is influenced by various factors, such as topography, soil type, climate, and land use.

The Relationship Between Wetland Loss and Subsoil Moisture

• Wetland Hydrology: Wetlands are sustained by hydrological inputs (precipitation, surface inflow, groundwater contributions). Soil moisture—specifically in the subsoil—is critical to wetland function because it supports water saturation and maintains anaerobic conditions essential for wetland biota1.

• Wetland Loss Drivers: Loss is frequently driven by drainage, land conversion, climate change, or hydrological alteration, all of which can reduce the subsoil's capacity to retain water2.

• Correlation With Subsoil Moisture: As wetland area diminishes, studies have shown that subsoil moisture content (measured as volumetric water content [VWC] or gravimetric percentages) tends to decrease, especially when root zones and subsoils are no longer receiving regular water recharge from surface inundation or groundwater34.

Wetland Loss as a Measurable Indicator

Sensitivity and Thresholds

• Moisture Content as an Indicator: Soil moisture at specific depths (such as 10cm) is commonly used to track wetland health and degradation. When moisture content drops below a site-specific threshold (commonly around 50% VWC), wetland function and extent sharply decline. This makes a decrease in wetland area or loss of hydrologic indicators a potential proxy for decreased subsoil moisture3.

• Degradation Stages: Soil moisture remains relatively stable until a critical threshold is crossed. After loss of regular inundation, subsoil moisture may sharply decline and plateau at a lower range, beyond which further wetland loss may not correspond to measurable differences in soil moisture at certain depths3.

Limitations

• Non-linearity: The degradation process and its reflection in subsoil moisture is not always linear. In some cases, further soil drying may not occur even as the visible wetland area continues to decrease, especially if soil or climate conditions limit further moisture loss3.

• Confounding Factors: Subsurface characteristics such as soil permeability affect the relationship. For instance, wetlands over high-permeability soils respond more rapidly to water table changes, whereas those above low-permeability soils show less interaction between surface water loss and groundwater or subsoil moisture changes56.

• Vegetative Cover: While wetland loss may signal reduced moisture, vegetative cover sometimes provides a more sensitive and earlier indicator of functional degradation than soil moisture alone3.

Methods for Monitoring

• Direct Measurement: Gravimetric and sensor-based approaches quantify moisture content at various soil depths within and around wetlands for objective tracking7.

• Remote Sensing: Satellite data (e.g., Sentinel-1, Sentinel-2) can distinguish between wetland and non-wetland areas based on surface soil moisture, helping inventory wetland loss and associate it with subsoil moisture changes over large landscapes8910.

• Hydrological Modeling: Models can help relate wetland area dynamics to subsoil moisture trends by incorporating rainfall, evaporation, infiltration, and groundwater dynamics611.

Summary Table: Interpreting Wetland Loss as an Indicator

Indicator	Strengths	Limitations
Wetland Area/Extent Loss	Clear visual/measurable changes; widely mapped	May lag behind actual subsoil moisture changes; confounded by non-hydrologic disturbances32
Subsoil Moisture Content	Direct hydrological metric; well-correlated in early stages of wetland loss	Plateaus at high levels of degradation3; requires repeated measurement
Vegetative Cover	Responds rapidly to hydrological stress	Not always directly correlated with subsoil moisture3
Remote Sensing Indicators	Landscape-level monitoring; strong predictive models	Limited in subsoil specificity; requires ground-truthing8910

Conclusion

Wetland loss is broadly indicative of underlying reductions in subsoil moisture, particularly in the early and moderate stages of degradation. However, its sensitivity diminishes at extreme loss levels and can be confounded by other ecological changes. For robust assessment, wetland loss should be monitored with complementary measurements of soil moisture and, where possible, vegetation indices and remote sensing data38102.

1. https://www.nature.com/scitable/knowledge/library/ecology-of-wetland-ecosystems-water-substrate-and-17059765/
2. https://www.undrr.org/understanding-disaster-risk/terminology/hips/en0016
3. https://d-nb.info/1239794959/34
4. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2022.824267/full
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7. https://eos.com/blog/soil-moisture/
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10. https://www.mdpi.com/2072-4292/10/12/1979
11. https://www.ijeab.com/upload_document/issue_files/5IJEAB-10920227-Soilmoisture.pdf
12. https://www.sciencedirect.com/science/article/pii/S1470160X21010578
13. https://www.sciencedirect.com/science/article/pii/S1470160X22005982
14. https://www.mdpi.com/2072-4292/11/23/2834
15. https://wetlandassessment.epa.gov/webreport/
16. https://www.nrcs.usda.gov/publications/ceap-wetland-2020-SoilWaterBalanceImpactsWetlandEffectiveness.pdf
17. https://wetlandsalberta.ca/wetland-loss/
18. https://www.gjesm.net/article_706487_c0b93f89a2677421bf051e7c23e38a31.pdf
19. https://www.sciencedirect.com/science/article/pii/S2468584423000363
20. https://www.sciencedirect.com/science/article/pii/S2095633924000480