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.

What is surface runoff and which terrain factors (gradient, vegetation, permeability) increase or reduce 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.

In a drainage basin of $150\ \text{km}^2$, the historical average annual water balance was: precipitation $P = 750\ \text{mm/year}$, actual evapotranspiration $ETR = 450\ \text{mm/year}$, effective infiltration into the aquifer $I = 150\ \text{mm/year}$, and surface runoff $Es = 150\ \text{mm/year}$. Over the last three decades, $35\ \%$ of the basin's surface area has been sealed by industrial estates and housing developments.

1. a) State the water balance equation ($P = ETR + Es + I \pm \Delta S$) and predict qualitatively and semi-quantitatively how the terms $Es$ and $I$ will change. Picture and explain how the flood hydrograph of the main river will change (basin response time and peak flow) during a flash flood event of $80\ \text{mm}$ in $4\ \text{h}$.
2. b) Climate change projections predict a $15\ \%$ decrease in annual precipitation and an increase in average temperature of $+1,8\ ^\circ\text{C}$ (increase in potential evapotranspiration). Explain why a $15\ \%$ reduction in rainfall does not cause an equivalent $15\ \%$ drop in streamflow, but rather a proportionally much greater decrease in available runoff.
3. c) Design two actions based on Sustainable Drainage Systems (SuDS) that can be integrated into the urban fabric of the basin to restore the original infiltration values and mitigate flood risk.

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

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

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

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

According to Darcy's law, groundwater flows faster through gravels than through clays. Explain that law qualitatively (hydraulic gradient and permeability) and its practical consequence when choosing where to drill a well.

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

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

An unconfined coastal aquifer made of permeable sands supplies a tourist municipality and $400\ \text{ha}$ of irrigated land. Historically, the water table in a piezometer located $600\ \text{m}$ from the coastline remained stable at $+2,5\ \text{m}$ above sea level. After five years of summer overexploitation, the water level in said piezometer has dropped to $+0,5\ \text{m}$, with an electrical conductivity of $5\,200\ \mu\text{S/cm}$ recorded in agricultural abstractions.

1. a) Applying the Ghyben-Herzberg relationship ($\text{freshwater density} = 1\,000\ \text{kg/m}^3$ and $\text{seawater density} = 1\,025\ \text{kg/m}^3$), calculate the theoretical depth below sea level at which the freshwater-saltwater interface was located before and after the water table decline. Explain what hydrogeological process the marine intrusion cone has undergone.
2. b) Faced with salinization, a water user association proposes deepening the existing boreholes by an additional $25\ \text{m}$ to "look for cleaner water at the bottom of the aquifer". Justify technically why this measure would accelerate the ruin of the aquifer.
3. c) Design a combined hydrological strategy to halt the saline wedge and recover groundwater quality, evaluating the effectiveness of implementing a hydraulic barrier via deep injection of reclaimed water versus relocating abstractions inland.

In the aeolian mantle and marshes of Doñana, the Almonte-Marismas aquifer system sustains peridunal lagoon complexes of high ecological value and discharges freshwater into the tributary streams that feed the marsh.

1. a) Explain the hydrogeological functioning of a peridunal lagoon (such as Santa Olalla) as a discharge wetland connected to the water table, and predict how the hydrological dynamics of the basin respond when massive pumping for red berry cultivation creates cones of depression in the vicinity, even if water is not pumped directly from the lagoon.
2. b) In the hydrogeological monitoring network, it is observed that several streams have shifted their regime from gaining (effluent) to losing (influent) reaches, and that former artesian wells on the edge of the marsh have ceased to flow spontaneously. Interpret both facts in terms of hydraulic gradients and piezometric pressures in the confined and semi-confined levels.
3. c) Faced with the degradation of the wetland, two proposals clash: constructing a surface water transfer from another basin to maintain the current irrigated area versus shutting down illegal wells and purchasing agricultural estates for their renaturalisation. Argue which of the two solutions guarantees long-term sustainability based on ecosystem resilience and the ecological limits of the resource.

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 administration urgently authorizes the opening of "drought wells" in the headwater aquifers to save irrigation 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.

Put the stages of a drinking water treatment plant in order: filtration, abstraction, 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 kilo 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 Ministry 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 agricultural union: "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 administration financed the comprehensive modernization to drip irrigation, achieving a theoretical reduction in plot consumption of $30\ \%$. However, after the works, 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 surface area) 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 concession and water rights revision plan that allows reaching quantitative balance in the basin, justifying what environmental and socioeconomic criteria should guide the reductions.

A river monitoring station tracks the physical-chemical and biological water quality at four points along a river during low-flow conditions:

• Point 1 (forested headwater): $\text{OD} = 9,4\ \text{mg/L}$; $\text{DBO}_5 = 1,2\ \text{mg/L}$; $\text{DQO} = 3,0\ \text{mg/L}$; $\text{NO}_3^- = 3,5\ \text{mg/L}$; Conductivity $= 180\ \mu\text{S/cm}$; Fecal coliforms $= 15\ \text{CFU}/100\ \text{mL}$.

• Point 2 (downstream of an urban discharge without adequate treatment): $\text{OD} = 1,8\ \text{mg/L}$; $\text{DBO}_5 = 70\ \text{mg/L}$; $\text{DQO} = 115\ \text{mg/L}$; $\text{NO}_3^- = 14\ \text{mg/L}$; Conductivity $= 820\ \mu\text{S/cm}$; Fecal coliforms $= 95\,000\ \text{CFU}/100\ \text{mL}$.

• Point 3 ($20\ \text{km}$ downstream, after an extensive intensive irrigated floodplain with mineral fertilization): $\text{OD} = 6,0\ \text{mg/L}$; $\text{DBO}_5 = 6\ \text{mg/L}$; $\text{DQO} = 16\ \text{mg/L}$; $\text{NO}_3^- = 72\ \text{mg/L}$; Conductivity $= 2\,200\ \mu\text{S/cm}$; Fecal coliforms $= 350\ \text{CFU}/100\ \text{mL}$.

• Point 4 (downstream of a petrochemical and industrial estate with prior physical-chemical treatment): $\text{OD} = 4,2\ \text{mg/L}$; $\text{DBO}_5 = 10\ \text{mg/L}$; $\text{DQO} = 160\ \text{mg/L}$; $\text{NO}_3^- = 11\ \text{mg/L}$; Conductivity $= 3\,100\ \mu\text{S/cm}$; Fecal coliforms $= 200\ \text{CFU}/100\ \text{mL}$.

1. a) Interpret the biodegradability of the discharges by comparing the $\text{DBO}_5/\text{DQO}$ ratio at points 2 and 4. Justify what causes the critical drop in dissolved oxygen at point 2 using the conceptual model of river self-purification.
2. b) Diagnose the origin of the pollution at point 3 by analyzing the nitrate and conductivity values. Assess the consequences on the aquatic macroinvertebrate community and explain whether this water is suitable for abstraction for human consumption without prior denitrification.
3. c) Design the necessary treatment lines in the wastewater treatment plant responsible for the discharge at P2, and two structural corrective measures in the agricultural practices at P3 to restore the parameters within ecological limits.

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

• Urban water supply for a conurbation 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 Water Framework Directive and the Water Act.
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.