Unit 5 — Dryland and aeolian geomorphology

How can episodic water and wind build deserts and dust records?

Drylands are commonly described as places dominated by wind, but episodic runoff and sediment-laden floods can accomplish major geomorphic work even where annual rainfall is low. Sparsely vegetated surfaces, patchy protective crusts and variable sediment availability modify wind entrainment thresholds. Wind transport includes suspension, saltation and surface creep; the size and migration of a dune depend on sediment supply, wind regime and topographic setting. A dune crest therefore cannot be used as a direct wind-speed instrument. Desert pavements, alluvial fans, playas and loess deposits record different combinations of sediment sorting, deposition and exposure. A playa may fill during rare storms and then concentrate evaporites; those chemical layers may subsequently be deflated. Students must use landform assemblages and stratigraphy rather than a single isolated dune shape to reconstruct environment. Human disturbance of protective vegetation and soil crust can alter dust emissions over periods much shorter than the formation timescale of the underlying basin.

Learning outcomes and scale discipline

By the end of this unit, students should be able to describe at least five independent landform or sediment observations, demonstrate one dimensionally consistent quantitative calculation, design a comparative measurement, and distinguish an interpretation from the observations used to support it. Practice viewing the same landform in a map, a section and a material description. Keep an explicit column for phenomena that are absent, unobserved or only assumed. At the finest scale, a few grain contacts may govern transport; at larger scales, inherited valley geometry and event histories control the response. Switching scale is legitimate only after specifying which variables remain meaningful. At the start of the field exercise, make a drawing of dune crests, fan lobes, loess exposures and playa salt chemistry; list which parts would be measured directly and which reconstructed from other evidence.

The unit's measurement choices should include sediment source, wind regime, storm rainfall and surface crust; the strongest report also checks source fingerprints and storm-event timing. Even a carefully derived rate does not establish its formative mechanism unless the sedimentary or stratigraphic evidence agrees. Interpret changes in the context of the process and observation interval, and use at least two sources of independent evidence. Most useful scientific uncertainties are not just numerical instrument errors: researchers must also consider incorrect boundaries, missing events, biased sample sites, mistaken ages and incomplete constitutive laws.

Process mechanism and scale

Grains move when fluid stresses overcome resisting forces; availability, crust, vegetation and moisture alter the effective threshold. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Dust collectors, wind records, surface moisture measurements and disturbed crust patches constrain sources. Record sediment source, wind regime, storm rainfall and surface crust. In the diagram of dune crests, fan lobes, loess exposures and playa salt chemistry, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

A dust haze can be transported from far away rather than emitted from the observed local surface. Compare upwind and downwind sampling during a documented wind event and examine mineral signatures. Consider dust provenance attributed to a local surface without trajectory evidence. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Simplified wind transport proxy

Given. Wind friction velocity .45 m/s above a threshold .25 m/s; proxy proportional to excess speed cubed. Calculation: (0.45 − 0.25)³. Result: 0.008 (m/s)³. Work through the units before using the output to discuss Earth-surface history. A small increase above threshold produces a nonlinear proxy change. This is not calibrated sediment discharge and has no field-use coefficient. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document surface crust; independently check wind direction. Assess sensitivity to storm-runoff episodes and verify provenance. Record each raw observation, units, uncertainty and assumptions before model fitting.

The threshold velocity is not a universal constant across soils or turbulence regimes. Treat dust source attribution as a trajectory and composition problem, not visual guesswork. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for wind entrainment and dust emission without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

Sand transport across a dune creates erosion on the windward face and deposition near the lee slope; dune shape integrates wind regime and sediment supply. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Repeat crest surveys, cross bedding, grain sorting and satellite time series reveal active migration. Record sediment source, wind regime, storm rainfall and surface crust. In the diagram of dune crests, fan lobes, loess exposures and playa salt chemistry, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

A stationary crest can mask internal sediment flux if opposing winds balance net displacement. Measure crest movement relative to fixed benchmarks and compute velocity over the known interval. Consider dust provenance attributed to a local surface without trajectory evidence. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Dune crest migration

Given. Crest shifts 22 m downwind over 11 years. Calculation: 22 / 11. Result: 2 m/yr. Work through the units before using the output to discuss Earth-surface history. Migration can be irregular under seasonally reversing winds. A shifting crest alone does not quantify sand flux. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document surface crust; independently check wind direction. Assess sensitivity to storm-runoff episodes and verify provenance. Record each raw observation, units, uncertainty and assumptions before model fitting.

Migration scaling by flux divided by dune height ignores three-dimensional leakage and variable supply. Use simplified migration formulas for comparison, not site-specific forecasts. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for dunes, ripples and migration without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

Short intense floods and debris flows distribute sediment from confined mountain valleys across unconfined basin margins, producing distributary lobes. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Lobe boundaries, channel avulsions, matrix support, sorting and surface varnish constrain active and abandoned sectors. Record sediment source, wind regime, storm rainfall and surface crust. In the diagram of dune crests, fan lobes, loess exposures and playa salt chemistry, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

A fan surface may contain deposits from both streamflow and debris flows within a single storm history. Map facies and channel patterns along apex-to-toe transects before assigning one dominant transport style. Consider dust provenance attributed to a local surface without trajectory evidence. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Alluvial fan gradient

Given. Fan elevation falls 160 m over 4,000 m horizontal distance. Calculation: 100 × 160 / 4000. Result: 4 %. Work through the units before using the output to discuss Earth-surface history. An individual channel gradient can differ from the whole fan surface. A gradient cannot uniquely identify debris flow versus streamflow. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document surface crust; independently check wind direction. Assess sensitivity to storm-runoff episodes and verify provenance. Record each raw observation, units, uncertainty and assumptions before model fitting.

Flood recurrence statistics cannot be inferred reliably from a single preserved fan deposit. Integrate sediment facies with upstream catchment and storm information. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for alluvial fans and ephemeral channels without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

Closed basins collect intermittent runoff; evaporation precipitates salts, while deflation and lag formation can armor nearby surfaces. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Mud cracks, graded lake beds, evaporite crusts and pavement clast fabrics constrain environmental episodes. Record sediment source, wind regime, storm rainfall and surface crust. In the diagram of dune crests, fan lobes, loess exposures and playa salt chemistry, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

Surface salinity can result from irrigation return flow rather than an ancient lake stage. Core a short basin transect and analyze salts, grain size and sedimentary structures. Consider dust provenance attributed to a local surface without trajectory evidence. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Salt concentration mass balance

Given. A brine volume of 2 million L holds dissolved salts at 3 g/L. Calculation: 2000000 × 3 / 1000. Result: 6,000 kg. Work through the units before using the output to discuss Earth-surface history. Evaporation concentrates solute without necessarily increasing total dissolved mass. Precipitation and mineral reactions must be considered. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document surface crust; independently check wind direction. Assess sensitivity to storm-runoff episodes and verify provenance. Record each raw observation, units, uncertainty and assumptions before model fitting.

Evaporite assemblages are affected by groundwater and post-depositional dissolution. Distinguish depositional record from later diagenesis and modern salinization. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for playas, desert pavements and evaporites without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

Wind-deposited silt blankets can record dust transport, soil formation and climate changes across broad intervals. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Grain-size shifts, palaeosols, luminescence dates and magnetic susceptibility supply related age and process clues. Record sediment source, wind regime, storm rainfall and surface crust. In the diagram of dune crests, fan lobes, loess exposures and playa salt chemistry, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

A silt bed can be overbank alluvium instead of loess if sedimentary structure is ignored. Log contacts and soil horizons, sample provenance and assess the depositional setting outside the vertical section. Consider dust provenance attributed to a local surface without trajectory evidence. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Loess sediment accumulation

Given. A 1.4 m loess layer accumulated across an idealized 20,000 years. Calculation: 1.4 / 20000 × 1000. Result: 0.07 mm/yr. Work through the units before using the output to discuss Earth-surface history. Palaeosols may represent intervals of little net accumulation. Compaction means thickness change is not original deposited volume. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document surface crust; independently check wind direction. Assess sensitivity to storm-runoff episodes and verify provenance. Record each raw observation, units, uncertainty and assumptions before model fitting.

Luminescence ages require sufficient resetting and palaeosols may represent time-averaged processes. Pair chronology with sedimentology before reconstructing a regional wind regime. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for loess and dryland environmental archives without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

In a dry basin, compare an alluvial fan and dune field using a synthetic wind-frequency rose and sediment grain-size table. Trace the potential origin of dust to playa, farmland and old fan deposits. Design a two-sided sampling campaign and explain how salt crusting could alter emissions during one season without changing the underlying landform.

Protocol and evidence allocation

Write a project proposal that names the study question, a defined sampling region, hypotheses H1 and H2 and three observations that could discriminate them. Build an input table with at least twelve synthetic measurements; record a unit, identifier, uncertainty class and data source for every row. Design one transect, one temporal repeat survey and one test using a different kind of evidence. State how the field team would avoid confirmation bias: preregister predicted outcomes, assign a second mapper to identify landforms independently and blind the initial data processing to the favored model. Any unmeasured quantity must be marked as an assumption. Base the arithmetic on the numerical cases in the unit, but do not present the invented data as actual measurements.

In the analysis, start with a mass or elevation balance rather than a purely visual explanation. If the balance does not close, label the residual and list processes that may lie outside the boundary, including sediment stored in a channel, transport at an unmonitored exit and error in estimated density or surface area. Separate interpretation of rate from reconstruction of age and chronology. An apparently consistent average rate can conceal long inactive periods punctuated by large storms, landslides or channel switching. Explore a parameter range wide enough to expose this sensitivity. Explain what observation would tell you whether the residual is an actual physical loss or a measurement artifact.

Reporting template and peer review

The final report should contain: a title that describes an inquiry rather than a conclusion; data source and acquisition notes; a map or section with scale; a table of raw observations; a short derivation with units; alternative interpretations; uncertainty and detection limits; and an honest conclusion identifying what the exercise cannot show. A peer reviewer should trace one plotted value back to an original input without assistance. Report uncertainty in both physical data and causal attribution. Describe how sampling locations could bias the result, and identify one independent validation datum that was not used when choosing the model. Treat a good fit as a starting point for further testing, not proof of a unique historical trajectory.

Assessment prompts and rubric

Interpretation. Provide two competing process histories consistent with the invented evidence, predict one differentiating field measurement, and identify which uncertainties could reverse the preferred interpretation. Quantitative reasoning. Recompute the selected unit case in SI units, vary two inputs independently, and interpret signs, magnitudes and nonlinearities. Evidence ethics. Identify all fabricated datasets and explain why they must never be described as real hazard, glacial or coastal monitoring records. Review standard. Full credit requires correct units, reproducible methods, a mass or geometry balance, explicit chronology limits and an alternative interpretation; numerical correctness alone is insufficient.

Return to the guiding question: How can episodic water and wind build deserts and dust records? Make a process diagram contrasting an observable pattern with a process that could produce it. Now add a second process capable of leaving a similar pattern. Underline measurements that would distinguish the two. Ask whether the lesson's calculations predict the type of change measured by those instruments or only provide a scale for discussing it. The emphasis throughout geomorphology is not on naming spectacular landforms but on reconstructing defensible histories, with uncertainties, from repeatable measurements and physical consistency.