Water and Water Resources

Y1

What are the three common aspects shared by sediments and solutes?

Sources; Transfer/Transport; Deposition → Transformation

What are the five big issues with solutes?

1. Natural vs anthropogenic sources

2. Geographical and temporal variability in sources

3. The role of connectivity (direct and indirect)

4. Dilution

5. Accumulation and history effects

What is hydrological connectivity?

The water-mediated transfer within or between elements of the hydrological cycle — directly getting solutes into rivers, and indirectly controlling transformations along a flow path

What are the natural sources of solutes in rivers?

Sea salt (rain contains ~10–20 mg/L of sea salts from spray), fine dust blown from soil (mainly calcium, Ca²⁺), and weak carbonic acid from atmospheric CO₂ giving rain a natural equilibrium pH of ~5.7

Why is average solute concentration generally higher near the coast than inland?

Because coastal rainfall picks up more sea salt spray, increasing the dissolved load in precipitation before it enters the hydrological cycle

What causes temporal variability in natural solute concentrations?

Changes in airmass trajectories — sea salt, dust and other materials can be carried long distances. The 1986 Chernobyl fallout is an example of how far airborne material can travel and deposit

What is acid rain and what causes it?

Acid rain results from atmospheric pollution (high fossil fuel emissions) reacting with water vapour to form acidic precipitation. It leaches normally immobile metals (especially aluminium) from the soil, causing fish kills and ecological damage

Under what conditions is acid rain most damaging?

Where fossil fuel emissions are high AND rock weatherability is low — meaning there are low levels of base cations to neutralise the acidity

What is dilution in the context of solute dynamics?

When discharge increases, if the solute mass flux remains constant, the concentration decreases — the same amount of solute is spread through a larger volume of water

What is accumulation in the context of solute dynamics?

The build-up of pollutants and solutes over time. Pollutants can concentrate through the food chain (bioaccumulation) and mix to create a 'cocktail effect'

What is the current chemical health status of England's rivers?

0% of England's rivers are in good chemical health — concentrations of toxic chemicals exceed safe limits in every river in England

What is the current ecological health status of England's rivers?

85% of river stretches fall below good ecological standards; only 15% achieve good or above ecological health status; 23% are classed as poor or bad overall

What drives geographical variability in solute dynamics?

Chemical weathering rates, which vary as a function of precipitation, temperature, geology and soils

What drives temporal variability in solute entrainment?

Water availability — you need water for dissolution and entrainment (flushing), but increased discharge also causes dilution, creating a complex relationship between flow and solute concentration

What is flood risk and how is it calculated?

Flood risk = flood hazard × vulnerability and exposure (people and assets). Risk depends on both the physical hazard and the demographic/economic characteristics of exposed populations

How many people globally are exposed to a 1-in-100 year flood risk?

More than 1 in 5 people — approximately 1.8 billion people — are exposed to a 1-in-100 year flood risk, with exposure disproportionately higher in low-income households

What are the two main physical mechanisms for generating overland flow?

Infiltration-excess (Hortonian) overland flow — when rainfall intensity exceeds infiltration rate — and saturation overland flow (Dunne) — when the water table rises to the surface during prolonged rainfall

What is Darcy's Law and when does it apply?

Q = Ks × A × H. It applies in the saturated zone and states that discharge is a function of the saturated hydraulic conductivity (Ks), cross-sectional area (A), and pressure head (H)

What is Richards' equation and when does it apply?

A modification of Darcy's Law for the unsaturated zone, where both flux and porosity can change. It accounts for suction as well as pressure head but is difficult to solve numerically

What is macropore flow and why does it matter for flooding?

Rapid subsurface flow through large pores created by roots or animal burrows. It bypasses the soil matrix, dramatically speeding up drainage and contributing to rapid runoff responses

What is flow accumulation in a drainage network?

The process by which discharge increases downstream as water from a larger catchment area combines. It raises flood wave peaks and increases total discharge

What is flow attenuation and what causes it?

The reduction of flood wave peaks as water moves downstream. Caused by different timings of sub-catchment responses, transfer of water onto floodplains, and energy/momentum loss within the water wave

What is Manning's n and how does it relate to flooding?

An empirical roughness parameter relating flow depth to velocity. Higher roughness → slower flows → deeper water. Used to manage attenuation; rougher floodplains slow flood waves

What is the continuity equation for river discharge?

Q = width × depth × velocity. Because water is incompressible, if velocity decreases, depth must increase to conserve discharge — Q1 = Q2

What are the pros and cons of dredging rivers to reduce flooding?

Pros: improves drainage, navigation, and allows gravel extraction.

Cons: can increase flooding downstream, is short-lived (rivers return to graded state), costly, disturbs ecosystems, and causes bank destabilisation and increased erosion

How does urbanisation increase flood risk?

Impermeable surfaces increase runoff; drainage systems transfer water rapidly through cities; development on floodplains removes natural attenuation; infrastructure increases vulnerability and damage costs

What are the social impacts of flooding?

Immediate injury and illness; long-term psychosocial damage; gender inequality (women disproportionately affected); unequal access to support and insurance; ghettoisation of high flood-risk communities

What makes flooding a 'hybrid problem'?

It requires understanding of both physical processes (catchment hydrology, channel hydraulics) and social/cultural factors (land use, vulnerability, inequality) — these interact and feed back on each other, so neither alone is sufficient

What is the Hortonian (infiltration-excess) overland flow model?

Robert Horton (1933): overland flow occurs when rainfall intensity (i) exceeds the soil infiltration capacity (f). Water cannot enter the soil fast enough and runs off across the surface

What is the Variable Source Area (VSA) concept?

The idea that runoff-generating areas vary in time and space within a catchment. Different parts of the catchment saturate and generate runoff at different times during and between rainfall events

What is the mass balance equation for partitioning rainfall on a hillslope?

P − Et = Re (Precipitation − Evapotranspiration = Effective rainfall). This effective rainfall becomes available for runoff and infiltration

What is the 'trashline' in flood hydrology?

The line of debris (including plastics) left on riverbanks or vegetation that marks the maximum water level reached during a flood — used as a physical indicator of flood stage

What percentage of plastics are recycled globally, and what are the implications for rivers?

Only 9% of plastics are ever recycled. Rivers are the primary terrestrial conduit of plastics, delivering an estimated 0.8–2.7 million metric tonnes to coastal/marine environments annually

What did Hurley et al. (2018) find about microplastics in the River Irwell catchment?

Microplastic contamination was found at 39 of 40 sample sites. Hotspots exceeded 40,000 particles/kg of fine sediment, concentrated downstream of wastewater treatment plants. The 2015 Boxing Day flood flushed contamination, but rapid reaccumulation occurred within weeks

What are microplastics and what are their main sources in rivers?

Particles between 1 micrometre and 5 millimetres. Sources include degradation of larger plastic items, synthetic fibres, and plastic pellets (nurdles). They enter rivers via runoff, wind transport, and direct dumping

Why are microplastics problematic in UK river monitoring?

Because microplastics are part of the sediment load, the Environment Agency and Scottish Environment Protection Agency do not routinely analyse sediment properties as part of quality monitoring — leaving a significant data gap