13.2 Surface Water, Groundwater, and the Oceans

Water Is the Connector Among the Spheres

Competency 016 asks teachers to understand "the form and function of surface and subsurface water." Water is where the geosphere, atmosphere, biosphere, and hydrosphere interact most visibly, so items in this space often ask you to trace a consequence across systems: pumping an aquifer changes spring flow, which changes a river ecosystem, which changes a bay's salinity.

Watersheds and Surface Drainage

A watershed (drainage basin) is all the land that drains to a single outlet. A divide is the ridge of high ground separating adjacent watersheds; the Continental Divide sends water either to the Pacific or toward the Gulf of Mexico. Texas is organized into major river basins including the Trinity, which drains the Dallas-Fort Worth region to Galveston Bay, and the Colorado, Brazos, Nueces, and Rio Grande basins.

The teaching consequence students should reach on their own: everything within a watershed eventually reaches its river. Fertilizer on a lawn, oil on a parking lot, and sediment from a construction site are all nonpoint source pollution — diffuse inputs with no single discharge pipe — and they are far harder to regulate than point sources such as an outfall.

Stream processes shape the land as water moves:

Velocity is the master variable: doubling stream velocity dramatically increases the size of particle a stream can carry, which is why a flash flood moves boulders that the same creek cannot budge in July. A meander grows because water moves fastest on the outside of a bend (eroding a cut bank) and slowest on the inside (depositing a point bar); eventually the neck is cut off and an oxbow lake is left behind.

Groundwater

Infiltration carries precipitation into the soil, through the unsaturated (vadose) zone, and down to the saturated zone. The top of the saturated zone is the water table, which is not flat — it broadly mirrors surface topography and rises after wet periods.

Two rock properties control groundwater behavior and are frequently confused:

• Porosity — the percentage of total volume that is open pore space. Well-sorted sand and gravel have high porosity.
• Permeability — how readily water flows through connected pores. This depends on pore size and connection, not just pore volume.

Clay is the standard counterexample: it has high porosity but very low permeability, because its pores are tiny and poorly connected. That combination is precisely what makes a clay layer an aquitard — a confining layer that water cannot easily cross.

Karst terrain develops where slightly acidic groundwater dissolves limestone, producing caves, sinkholes, disappearing streams, and highly permeable fractured rock. The Edwards Aquifer is a karst system, which is both an asset (enormous transmissivity and spring flow) and a vulnerability: surface contamination reaches the aquifer quickly with little natural filtration, so recharge-zone land use is regulated.

Recharge and overdraft. An aquifer's sustainable yield equals its recharge rate. Where pumping exceeds recharge, water levels decline, springs weaken, wells must be deepened, and in some settings the land surface subsides as dewatered sediments compact. This links directly to the resource principle developed in Domain I: renewability depends on the ratio of use to renewal.

The Oceans

Oceans hold about 97% of Earth's water. Of the remaining fresh water, roughly two-thirds is locked in glaciers and ice caps and most of the rest is groundwater, leaving well under 1% of all water as accessible fresh surface water — the number that makes the water-conservation argument concrete for students.

Salinity averages about 35 parts per thousand (‰), meaning 35 g of dissolved salts per kilogram of seawater, dominated by sodium and chloride ions. Salinity rises where evaporation is high or ice forms (leaving salt behind) and falls where rivers, rainfall, or melting ice add fresh water.

Seawater is layered by density:

• Surface (mixed) layer — warmed and stirred by wind, roughly uniform temperature.
• Thermocline — the zone of rapid temperature decrease with depth.
• Deep layer — cold, dark, and dense, near 2-4 °C almost everywhere.

Currents move in two systems. Surface currents are driven by prevailing winds and deflected into large rotating gyres; the Gulf Stream carries warm water northward along the U.S. East Coast and moderates northwestern European climate. Deep currents form the thermohaline circulation, driven by density: cold, salty water sinks in polar regions and flows along the ocean floor, eventually upwelling elsewhere. Upwelling brings nutrient-rich deep water to the surface and supports some of the world's most productive fisheries.

Estuaries are where rivers meet the sea, producing brackish water with salinity between fresh and marine values. Galveston Bay and the Nueces Estuary are Texas examples. Estuaries are nurseries for shrimp, crabs, and juvenile fish, and their health depends on adequate freshwater inflow — which is why upstream reservoir and pumping decisions are also coastal ecosystem decisions.