Sediment sources, cells and budgets.

The idea

In hot desert systems, sediment sources include the mechanical weathering of bedrock, fluvial inputs from episodic flash floods in wadis, and aeolian (wind-blown) material. A sediment cell represents a largely self-contained system where sediment is cycled, often bounded by topographic features like mountain ranges enclosing a desert basin (bolson). The sediment budget is the balance between sediment inputs (addition), stores (such as sand dunes or alluvial fans), and outputs (removal, like dust storms exporting material). If inputs exceed outputs, a positive budget occurs, leading to net deposition and the growth of landforms like ergs. Conversely, a negative budget results in net erosion, exposing deflation hollows or desert pavements.

What earns the marks

• Identify primary sediment sources in deserts, including insolation weathering, fluvial transport from wadis, and aeolian deposition.
• Define a sediment cell as a functional system with defined boundaries, typically topographically closed basins in desert environments.
• Explain the components of a sediment budget using the systems approach: inputs, transfers, stores, and outputs.
• Relate a positive sediment budget to the formation of depositional landforms (e.g., barchan dunes) and a negative budget to erosional features (e.g., deflation hollows).

Common mistakes and how to put them right

• Confusing sediment cells with atmospheric circulation cells (like the Hadley cell).

  Put it right: Clearly define sediment cells as terrestrial systems bounded by topography where sediment is sourced, transferred, and stored.

• Assuming desert sediment is exclusively sourced by wind.

  Put it right: Acknowledge that fluvial processes (flash floods) and in situ weathering are major, often dominant, sediment sources.

• Stating that a sediment budget is always in equilibrium.

  Put it right: Explain that budgets can be positive (net deposition) or negative (net erosion) depending on environmental changes.

Exam tips

• Use the systems framework (inputs, outputs, stores, transfers) to structure answers about sediment budgets.
• Provide a specific example of a desert sediment sink, such as an erg (sand sea) or a playa lake, when discussing positive budgets.
• Link changes in the sediment budget to long-term climate shifts, such as transitions from pluvial to arid periods.