Loess soil's ability to retain water is significantly influenced by its unique fine-grained composition and, critically, by its burial depth. As loess soil is subjected to increasing burial depth, its soil–water retention characteristics undergo distinct changes, impacting how much water it can hold and release.
Loess is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Its characteristic properties, such as high porosity and a relatively uniform particle size distribution (often dominated by silt), contribute to its water retention capabilities.
Water retention in soil refers to the amount of water the soil can hold against the force of gravity and how it releases that water. This is crucial for plant growth, groundwater recharge, and understanding hydrological processes. The soil–water retention curve graphically illustrates this relationship between soil matric potential (or suction) and volumetric water content.
Impact of Burial Depth on Loess Soil's Water Retention
Experimental results demonstrate that increasing burial depth significantly alters the soil–water retention curve of loess–paleosol sequences. This change has direct implications for several key water retention parameters:
Key Changes with Increasing Burial Depth:
- Saturated Volumetric Water Content: This refers to the maximum amount of water the soil can hold when all its pore spaces are completely filled. As burial depth increases, the saturated volumetric water content gradually decreases. This suggests that deeper loess soil may hold less total water at full saturation, possibly due to compaction reducing total pore space.
- Water Loss Rate: This indicates how quickly water drains or evaporates from the soil. With greater burial depth, the water loss rate gradually decreases. This implies that deeper loess soil tends to release its water more slowly, which could be beneficial for sustaining moisture over longer periods.
- Air Entry Value: The air entry value is the amount of suction (negative pressure) required before air begins to enter the largest pores of a saturated soil. As burial depth increases, the air entry value increases. An elevated air entry value means that deeper loess soil can retain water under higher suction before desaturation begins, indicating smaller, more interconnected pores or increased capillary forces.
- Residual Water Content: This is the amount of water that remains in the soil even after most of the gravitational and capillary water has drained out, held tightly by soil particles. The residual water content increases with greater burial depth. This suggests that deeper loess soil retains a larger proportion of tightly bound water, which may not be readily available for plant uptake but contributes to the soil's overall water holding capacity under dry conditions.
Summary of Effects:
The table below summarizes how increasing burial depth influences loess soil's water retention properties:
| Property | Change with Increasing Burial Depth | Implication for Water Retention |
|---|---|---|
| Saturated Volumetric Water Content | Decreases | Reduced maximum water-holding capacity at saturation |
| Water Loss Rate | Decreases | Slower release/drainage of water |
| Air Entry Value | Increases | Greater resistance to desaturation; holds water more tightly |
| Residual Water Content | Increases | More water tightly bound, even in dry conditions |
In essence, while deeper loess may hold less total water when fully saturated, it appears to release water more slowly and retain a greater amount of water under high suction, indicating a different hydraulic behavior influenced by compaction and altered pore structures at depth.
