Water resources

What approximate percentage of Earth's water is salt water? Where is most fresh water concentrated?

Distinguish between surface water, groundwater, and atmospheric water, and give an example of human use of each.

What is the water cycle? Name its main processes and explain which energy drives it.

The graph shows the distribution of water on Earth. River and lake water (marked with '?') represents $1.2\ \%$ of fresh water. Calculate what percentage of total planetary water this represents and comment on what this figure implies for managing the resource.

What is an aquifer and what rock properties (porosity, permeability) make it possible?

Distinguish between an unconfined aquifer and a confined aquifer. Which of the two can give artesian wells and why?

Agricultural nitrates can take decades to disappear from an aquifer even if fertilization is reduced. Explain the concept of residence time of groundwater and its implications for the recovery of polluted aquifers.

What is the water table and which factors, such as recharge by rainfall or pumping, make it rise or fall throughout the year?

What is a spring and what relationship does it have with an aquifer? Explain why some springs only flow in the wet season.

What is artificial aquifer recharge and which techniques are used (infiltration ponds, injection wells)? In which situations of overexploitation or saltwater intrusion is it especially useful?

A Mediterranean hydrographic basin experiences a continuous dry cycle over five years. The evolution recorded by the river basin authority is as follows:

• Year 1: Rainfall decreases by $30\ \%$ compared to the average; reservoirs are at $65\ \%$ and the rainfed crop yield remains acceptable.

• Year 2: Rainfall scarcity persists; soil moisture drops drastically and $80\ \%$ of the rainfed cereal crop is lost, although the main river continues to flow thanks to groundwater discharges from a karst aquifer.

• Year 3: Reservoirs drop below $20\ \%$, springs and fountains dry up, and the state government urgently authorizes the opening of "drought wells" in the headwater aquifers to save irrigated farming.

• Years 4 and 5: Rainfall normalizes ($100\ \%$ of the average), but piezometric levels do not recover, springs remain dry, and several wells experience ground subsidence problems.

1. a) Chronologically identify in which years meteorological drought, agricultural drought, and hydrological drought manifest, justifying the time lag and inertia that exist between them.
2. b) Explain from a hydrogeological perspective why the aquifers were able to buffer the streamflow in Year 2, but suffered hysteresis and mechanical damage (such as the compaction of aquitard formations and irreversible loss of porosity) after the intensive exploitation of Year 3.
3. c) Design a technical protocol for activating drought wells: establish three rigorous quantitative indicators (piezometric thresholds, water electrical conductivity, and environmental base flow) that must be strictly met to immediately suspend emergency pumping.

What is surface runoff and which terrain factors (slope, vegetation, permeability) increase or reduce it?

Put the stages of a drinking water treatment plant in order: filtration, intake, settling, chlorination, and coagulation-flocculation. Explain what is removed in each stage.

A wastewater treatment plant with secondary treatment uses activated sludge with microorganisms. Explain what this biological treatment consists of and which parameter (BOD) is used to verify its effectiveness.

What is the water footprint of a product? Compare that of wheat, avocado and a kilogram of beef, and explain what debate is raised by exporting "water-rich" products from dry regions.

Reverse osmosis desalination plants supply a large part of south-eastern Spain. Explain the principle of the process and assess its two great costs: energy consumption and brine management.

A Mediterranean coastal region with intensive early-season agriculture and mass tourism requests an annual water transfer of $150\ \text{hm}^3$ from an Atlantic basin whose reservoirs are at $48\ \%$ of their capacity. As a technical alternative, the state water agency proposes the construction of two reverse osmosis desalination plants powered by an annexed solar photovoltaic plant.

1. a) Analyze the hydrogeological and ecological impacts that the continuous withdrawal of flows would cause in the lower reach and the mouth of the donor basin, considering the seasonal variability of the Atlantic river regime.
2. b) Evaluate the technical and environmental feasibility of desalination as an alternative in the receiving area: predict two specific environmental impacts of desalination plants (brine discharge management and energy balance) and justify why the unit cost of desalinated water usually generates opposition among irrigators compared to transferred water.
3. c) Refute with ecological and hydrological arguments the statement by the farmers' association: "River water that flows into the sea is a lost and wasted resource if it is not transferred."

A semi-arid inland basin has renewable water resources of $60\ \text{hm}^3/\text{year}$, compared to a licensed demand of $75\ \text{hm}^3/\text{year}$, of which $80\ \%$ corresponds to traditional flood irrigation. To tackle the deficit, the state water agency financed the comprehensive modernization to drip irrigation, achieving a theoretical reduction in plot consumption of $30\ \%$. However, after the work, the overexploitation of the underlying aquifer has worsened.

1. a) Explain the "efficiency paradox" (rebound effect or Jevons paradox) applied to irrigation: justify how the reduction of irrigation return flows to the aquifer and farmers' decisions (switching to more profitable crops and expanding irrigated acreage) contribute to the worsening of the overall water balance.
2. b) The regional wastewater treatment plant (WWTP) produces $6\ \text{hm}^3/\text{year}$ of secondary-treated effluent, with an electrical conductivity of $2\,900\ \mu\text{S/cm}$ and $28\ \text{mg/L}$ of nitrates. Evaluate the agronomic (phytotoxicity, soil salinization) and environmental risks of its direct use in irrigation, and deduce what tertiary treatments or blending schemes are mandatory before its application.
3. c) Design the main guidelines of a water rights and permit revision plan that allows reaching quantitative balance in the basin, justifying what environmental and socioeconomic criteria should guide the reductions.

In a river basin with regulating reservoirs, stored reserves at the beginning of the water year are only $180\ \text{hm}^3$ due to an exceptional drought. The annual water-right demands under normal conditions are:

• Urban water supply for a metropolitan area of $1.2$ million inhabitants: $80\ \text{hm}^3$.

• Irrigation of perennial woody crops (irrigated olive groves and citrus fruits): $90\ \text{hm}^3$.

• Irrigation of annual herbaceous crops (corn and rice): $70\ \text{hm}^3$.

• Cooling of a thermal power plant and chemical industry: $20\ \text{hm}^3$.

• Environmental reserve flow in the main river: $40\ \text{hm}^3/\text{year}$ (continuous $1.27\ \text{m}^3/\text{s}$ to prevent the degradation of the estuary).

1. a) The irrigators' association demands the complete suspension of the environmental flow during the drought, arguing that "the law places agriculture ahead of fish, and the river can always recover later." Technically rebut this statement by explaining why the environmental flow is not just another consumptive use, but a prior restriction that conditions all other uses according to the Clean Water Act and state water law.
2. b) Design a justified proposal for allocating and restricting the $180\ \text{hm}^3$ among the different sectors, applying the legal order of priority of uses and justifying the reduction percentage assigned to each.
3. c) Justify agronomically and economically why, within the agricultural sector, the water volume reduction must be applied asymmetrically, prioritizing survival irrigation for woody crops over annual herbaceous crops.