The hydrosphere

Classify these bodies of water as saltwater or freshwater: the Atlantic Ocean, a glacier, a lake, a river, the Mediterranean Sea, and water from an aquifer.

Order these water reserves of the planet from largest to smallest amount: rivers and lakes, oceans, ice and glaciers, groundwater.

In a debate on sustainability, a municipal representative states: "Given that 71 % of the Earth's surface is covered by water, the fear of water scarcity is unfounded: water is a virtually inexhaustible resource."\na) Using data from the hydrosphere (97.5 % of the planet's water is salt water and only 2.5 % is fresh water; and of that fresh water, 69 % is frozen in glaciers and barely 0.3 % is found in accessible surface rivers and lakes), calculate what exact percentage of the planet's total water is represented by liquid surface fresh water.\nb) Based on your numerical calculation, reasonably refute the representative's statement and predict the hydrological and ecological consequences of managing a region's water resources under the assumption that available water is limitless.

Imagine that all the water on the planet equals a volume of 100 L: 97.5 L are salt water; and of the 2.5 L of fresh water, 1.7 L are frozen, 0.76 L are groundwater, and only 0.04 L circulate through surface rivers and lakes.\nAn agricultural cooperative in an arid region plans to double its irrigated crops by relying exclusively on deep groundwater boreholes, claiming that "groundwater represents almost twenty times more volume than rivers and lakes, so its depletion is impossible."\na) Calculate what percentage of the planet's total water is represented by surface waters (0.04 L) and what percentage by groundwater (0.76 L), and compare them.\nb) Refute the cooperative's argument: justify why the overexploitation of an aquifer can end up drying out connected surface rivers and wetlands, and predict the risks arising from the low natural recharge rate of groundwater.

Name the three states in which water is found in nature and give an example of each.

Define what an aquifer is and explain why it is an important freshwater reserve for people.

Where does the water from a spring come from? Relate it to rainfall, infiltration, and aquifers.

At the top of a permeable mountain, an accidental spill of a biodegradable, water-soluble dye occurs. Weeks later, dyed water is detected in an agricultural well in the valley and, months later, at the mouth of a river in the ocean, without the dye having been observed in the surface torrents on the hillside.\na) Reasonably reconstruct the path followed by the dyed water from the summit to the sea, naming and explaining the processes of the water cycle involved (infiltration, groundwater flow, spring emergence or discharge, and surface runoff).\nb) Explain through which physical processes of the water cycle that same seawater will be able to return to the mountain peak, and justify whether the dye would travel with it in water vapor and precipitation.

The diagram shows the water cycle with five numbered processes. Name the processes labelled 1 to 5 and briefly describe what happens in each.

What is the hydrological cycle? Name its main processes and explain why we say that the Earth's water is always the same.

State which process of the water cycle each sentence describes: water vapour turns into droplets and forms clouds; seawater passes into the atmosphere as vapour; water falls onto the surface as rain or snow.

What is condensation? Explain why the glass of a cold window or the bathroom mirror fogs up.

Explain the role played by the Sun as the "engine" of the water cycle.

Distinguish between surface runoff and infiltration, and state how ground slope and vegetation cover influence each of them.

Explain what evapotranspiration is and what role plants play in the water cycle.

Clouds travel with the wind. Explain why water that evaporated days earlier in the ocean can rain down in the interior of a continent.

Describe the journey of water from a glacier that melts in spring until it returns to the sea, naming the cycle processes involved at each stage.

Two villages receive the same annual rainfall, but one sits on impermeable clays and the other on permeable limestone rock. Compare runoff and aquifer recharge in both cases and reason which one will be at greater risk of flooding.

Seawater is not suitable for drinking or for watering most crops. Explain which process makes it possible to obtain fresh water from it (desalination) and cite one drawback of that procedure.

After the construction of a housing development with paved streets and impermeable roofs on a forested hillside, technicians find that during heavy rainfall, it takes only 30 minutes for water to reach the river instead of the usual 4 hours, and that the peak flow rate has tripled, flooding the town located downstream.\na) Justify this hydrological change by analyzing how paving the ground alters infiltration, evapotranspiration, and surface runoff.\nb) To prevent further flooding, a company proposes channeling the river with concrete in a straight line to the river mouth. Critically evaluate this proposal by predicting what will happen in areas located even further downstream, and design two alternative measures based on sustainable drainage or environmental restoration to promote aquifer recharge.

In a coastal municipality with a sharp increase in tourist population in summer and intensive irrigation, farmers notice that the leaves of their crops show salt burns and that well water has turned brackish. Piezometers indicate that the water table has dropped several meters below sea level.\na) Diagnose the hydrogeological phenomenon that has occurred and justify why the drop in the freshwater water table disrupts the equilibrium and allows the intrusion of seawater toward the mainland.\nb) An irrigators' association proposes drilling wells twice as deep to "search for clean fresh water deeper down." Predict the outcome of this action and design two viable measures (technical or managerial) to halt the intrusion and restore the aquifer.

The construction of a large dam in the middle course of a river guarantees the hydroelectric and drinking water supply to a large city. However, in the delta located 90 km downstream, farmers report that the sea is invading the rice paddies and the delta's coastline is retreating several metres per year.\na) Analyse how the reservoir's large water surface alters the local hydrological cycle (evaporation and environmental humidity) compared to the conditions that existed with the natural river.\nb) Justify the direct cause-and-effect relationship between the retention of water and sediments in the dam and the phenomenon of delta retreat and salinisation, and propose a water management measure to mitigate this impact downstream.

Two neighbouring river basins (A and B) with the same soil type and slope experience an identical torrential storm (75 L/m² in two hours). Basin A keeps its headwater forest intact, while in basin B the forest was completely cleared to create pasture.\nThe hydrograph of river B shows a sudden and violent flash flood just 40 minutes after the rain begins, while river A records a gentle, delayed, and progressive increase in streamflow over two days.\na) Justify the difference observed between both rivers by analysing, step by step, the role played by vegetation and its roots in interception, infiltration, evapotranspiration, and surface runoff.\nb) Predict how deforestation in basin B will affect the loss of fertile soil (erosion) and river discharge during the dry summer months compared to basin A.

A mountaineer fills his canteen from a high-mountain stream that originates directly from snowmelt and states: 'The water is crystal clear and comes from snowmelt and rain; since the hydrological cycle is a perfect natural distillation and filtration system, there is no need to treat it: it is 100% drinkable water.'\na) Analyse which stages of the water cycle effectively act as natural mechanisms of physical purification and refute the mountaineer's conclusion by justifying why crystal-clear surface water can contain biological or chemical contaminants hazardous to health.\nb) If the mountaineer only has a tightly woven cloth handkerchief and a portable gas camping stove, design a two-stage procedure to treat the water before drinking it, justifying what type of impurities each stage removes and why neither is sufficient on its own.