Exercises: The hydrosphere

State how Earth's water is distributed (oceans, ice caps/glaciers, groundwater, surface water, and atmosphere) using approximate percentages, and calculate what fraction remains available as usable fresh water.

Observe the diagram of the hydrological cycle. Name the processes labelled with numbers 1 to 5 and explain what energy source drives the cycle.

What is an aquifer and how is it recharged? Explain the difference between an unconfined aquifer and a confined aquifer.

Explain what the salinity of seawater is, its average value ($35\ \text{g}$ of salts per litre) and name a process that increases it locally (intense evaporation, freezing) and another that decreases it (the mouth of a river, rainfall, melting ice).

What are tides and which celestial bodies cause them? Distinguish between spring tides and neap tides and explain why the tidal range is much greater in the Cantabrian Sea than in the Mediterranean.

Explain what evapotranspiration is and what role it plays in the water cycle, discussing why a forested basin returns more water to the atmosphere than a deforested one.

Distinguish between surface water, groundwater and soil (edaphic) water, and indicate one typical human use of each and what management problem each type poses.

What is runoff? Discuss why it is much greater in a city than in a forest with the same rainfall (impermeabilisation, sewers) and what consequences this has for urban flash floods and for aquifer recharge.

Explain what ocean thermohaline circulation is, which two properties of seawater drive it, and why the climate of north-western Europe is milder than that of other regions at the same latitude.

Describe how temperature and salinity vary with depth in the ocean (thermocline and halocline) and what consequence water stratification has for marine life.

Several aquifers along the Mediterranean coast suffer from salinisation due to overexploitation. Explain the mechanism by which extracting more water than is recharged leads to seawater entering the aquifer, and why the damage is so difficult to reverse.

The great surface currents form gyres in each ocean. Explain the role of the prevailing winds and of the Coriolis effect in their formation, and discuss why the currents of the western margin (the Gulf Stream, Kuroshio) are warm and narrow and those of the eastern margin (the Canaries, California) cold and wide.

Explain what happens during El Niño in the equatorial Pacific (weakening of the trade winds, warming of the waters off Peru) and one of its far-reaching consequences (collapse of the anchovy fishery, changes in the rainfall of half the world).

Mountain snow and glaciers act as "water towers". Explain how they regulate the regime of rivers (winter accumulation, meltwater that sustains summer flow) and discuss what changes in downstream irrigation when the snow dwindles or the glacier disappears.

The water footprint measures the total water used to produce a good. Calculate the difference between a ten-minute shower ($50\ \text{L}$) and consuming a kilo of rice (about $2\,500\ \text{L}$ of production water), and explain what the "virtual water" that travels with the food trade is.

Explain why the river basin, and not the municipality or the province, is the unit of water management in Spain (the basin authorities), and analyse which conflicts of use appear in a basin between irrigation, urban supply, energy and environmental flows.

A popular science article states: "Since global warming increases the planet's average temperature, it is impossible for northwestern Europe to experience colder winters or a harsher climate." a) Reasonably refute this statement by justifying the role of salinity, temperature, and density in the engine of the Atlantic Meridional Overturning Circulation (AMOC), and explain how accelerated melting in Greenland can slow it down. b) Predict what consequences this weakening would have for the climate of western Europe and for the oxygenation and primary productivity of the deep ocean in the North Atlantic.

An aquifer stores $500\ \text{hm}^3$ of water and its annual natural recharge is $20\ \text{hm}^3$, but irrigation extracts $35\ \text{hm}^3$ per year. Calculate its theoretical turnover time and annual deficit, and discuss how long this exploitation is sustainable.

In autumn, the sea surface of the Mediterranean frequently reaches anomalies of up to $26\ ^\circ\text{C}$ while pockets of cold air aloft ($-28\ ^\circ\text{C}$ at $9\,000\ \text{m}$) penetrate. a) Physically justify the instability generated by relating the vertical temperature gradient, the release of latent heat of condensation, and the role of the peninsular coastal topography in convective triggering. b) Explain why ocean warming intensifies the torrential nature of precipitation according to the relationship between temperature and the moisture-holding capacity of air. c) Refute the common idea that "every cut-off low (DANA) isolated over the Peninsula automatically causes catastrophic rainfall along the Mediterranean coast."

You are part of the technical committee responsible for evaluating a request for a transfer of $30\ \text{hm}^3$ from the headwaters of the Tagus to the Segura in a scenario where the donor reservoirs are at $27\,\%$ of their capacity. a) Technically justify whether you would authorize the transfer by prioritizing the principle of river basin unity and the guarantee of the ecological flow over the socioeconomic demands of the receiving irrigated land. b) Evaluate the alternatives available in the demanding basin (seawater desalination, tertiary wastewater reuse), analyzing their limitations in terms of energy cost, emissions, and brine discharge. c) Design an operating rule with objective indicators and quantitative thresholds that prevents discretionary decisions during drought periods.

The Mediterranean and the Spanish south-east concentrate the greatest water stress in the country: scarce rainfall, growing demand and overexploited aquifers. a) Design a management plan combining supply-side measures (desalination, reuse) and demand-side ones (efficient irrigation, crops suited to the climate, tariffs). b) Point out the limits of each measure (energy cost, brine, loss of competitiveness). c) Discuss what role the environmental flow should play in planning.

A high school student formulates the following hypothesis: "As the dissolution of $\text{CO}_2$ in seawater increases, more carbonic acid is produced, which provides more carbonate ions and makes it easier for corals and mollusks to build their $\text{CaCO}_3$ skeletons." a) Chemically refute their statement: deduce, from the carbonic acid equilibrium, why the increase in free $\text{H}^+$ protons does not increase the carbonate ion ($\text{CO}_3^{2-}$), but rather "sequesters" it, transforming it into bicarbonate ($\text{HCO}_3^-$). b) Predict the consequences of the decrease in the carbonate saturation state on mollusk larvae and organisms with aragonitic shells compared to those with calcite shells. c) Design a controlled laboratory experiment to measure the effect of different acidification levels on the calcification rate of a bivalve, identifying the independent variable, dependent variable, and controlled factors.

A reservoir receives an average inflow of $400\ \text{hm}^3$ per year, but the allocated uses add up to $450\ \text{hm}^3$ per year. a) Calculate the annual structural deficit. b) Explain why a reservoir does not "create" water but only shifts it in time, and what happens in a series of consecutive dry years. c) Analyse how demand should be planned taking variability into account (averages versus guaranteed years).

The Greenland ice sheet loses about $280\ \text{km}^3$ of ice per year, and the oceans have a surface area of about $360$ million $\text{km}^2$. a) Calculate the annual contribution of this melting to sea-level rise, in millimetres. b) If all the ice in Greenland melted, sea level would rise by about $7\ \text{m}$: estimate at that rate how many years it would take and discuss why the process could accelerate (feedbacks). c) Comment on the relevance of that annual figure compared with the total measured rise (about $3\ \text{mm/year}$).

Topic Challenge. Investigate how recent atmospheric dynamics have conditioned the water resources of your environment. You will prepare a brief technical report with management proposals.

1. Data collection: Access the AEMET website and record the monthly precipitation ($mm$) of the station closest to your locality over the last $6$ months. Then, consult the Hydrological Bulletin of MITECO or embalses.net for the filling percentage (for example, $48,5\,\%$) of the main reservoir of your basin on those same dates.

2. Processing and analysis: Design a combined graph (bars for precipitation and a line for water reserves). Analyze the observed correlation and explain the time lag between rainfall and storage, deducing the role of evaporation, surface runoff, and infiltration.

3. Reasoned proposal: Conclude your report by diagnosing how atmospheric action centers (low-pressure and high-pressure systems) have affected the regional hydrosphere, and propose two sustainable adaptation measures against water scarcity.

Evaluation criteria:

• Rigor in obtaining and graphically representing data from official sources (AEMET and MITECO).

• Coherent understanding of the causal interactions between atmospheric dynamics and water reserves.

• Feasibility and scientific foundation of the proposed management measures.