The Hydrologic Cycle More than 97% of all Earth’s water is in the oceans. Only 1 % of Earth’s water is available to us as water vapor, groundwater, and freshwater.
The Hydrologic Cycle Earth’s waters are constantly circulating. The driving forces are: • Heat from the Sun • Force of gravity
The Hydrologic Cycle The hydrologic cycle is the set of processes that controls the circulation of water on Earth. Processes involved in the hydrologic cycle: • Evaporation • Precipitation • Infiltration • Runoff
The Hydrologic Cycle Water that goes from the ocean back to the ocean makes a complete loop in the hydrologic cycle. The journey is not always direct. • Water can flow as streams, rivers, and groundwater • Water can also be frozen in ice caps and glaciers
Groundwater Water beneath the ground exists as groundwater and soil moisture. Groundwater occurs in the saturated zone—water has filled all pore spaces. Soil moisture is above the saturated zone in the unsaturated zone—pores filled with water and air. The water table is the boundary between these two zones.
Groundwater The depth of the water table varies with precipitation and climate. • Zero in marshes and swamps, hundreds of meters in some deserts. • At perennial lakes and streams, the water table is above the land surface. • The water table tends to rise and fall with the surface topography.
Groundwater Factors that influence storage and movement of groundwater: • Porosity: ratio of open space in soil, sediment, or rock to total volume of solids plus voids—the amount of open space underground. • Greater porosity equals more potential to store greater amounts of groundwater. • Particle size, shape, and sorting influence porosity. • Soil with rounded particles of similar size has higher porosity than soil with various sizes.
Groundwater Permeability • Degree to which groundwater can flow through a porous material—higher permeability, greater potential for fluid flow. • Sediment packing and connectedness of pores influences permeability. • Hydraulic conductivity—a measure of permeability—tells us the degree to which the material can transmit water.
Groundwater Aquifers are reservoirs of groundwater. Aquifers generally have high porosity and high permeability. Aquifers underlie the land surface in many areas; they are a vital source of fresh water. It is important to keep this vital source of fresh water clean and contaminant free.
Groundwater A perched water table occurs when discontinuous, low-permeability layers in an unconfined aquifer intercept percolating water above the water table.
Groundwater Geologists can often use springs to locate faults, because a spring can indicate that there are cracks or breaks in the rock.
Groundwater The elevation of a water table above a particular location—usually sea level—is called the hydraulic head.
Groundwater Darcy’s law: Groundwater flow rate = hydraulic conductivity cross-sectional area hydraulic gradient
The Work of Groundwater Flowing groundwater can alter and change features at the surface: • Land subsidence • Caves and caverns • Sinkholes
The Work of Groundwater Land Subsidence: • Extreme groundwater withdrawal by pumping from wells can result in lowering of the land—land subsidence. • Land subsidence is especially prevalent in areas underlain by aquifers made of sandy sediments and interbedded clays. The clays leak water to the sand, then when water is pumped out, the clays shrink and compact, causing subsidence.
The Work of Groundwater Caverns and caves • The dissolving action of groundwater “eats away” at rock—limestone in particular. • Rainwater chemically reacts with CO2 in the air and soil, producing carbonic acid. The acidified water seeps into rock (especially limestone), partially dissolving it.
The Work of Groundwater Groundwater has carved out magnificent caves and caverns (a cavern is a large cave).
The Work of Groundwater Karst regions are characterized by soft rolling hills or sharp, rugged surfaces. Karst regions are areas where sinkholes, caves, and caverns define the land surface.
The Work of Groundwater Sinkholes are funnel-shaped cavities in the ground that are open to the sky; they are formed in a manner similar to caves. They can also be formed from conditions of drought and the over- withdrawal of groundwater.
Surface Water and Drainage Systems Surface water includes streams, rivers, lakes, and reservoirs. Infiltration of water is controlled by: • Intensity and duration of precipitation • Prior wetness condition of the soil • Soil type • Slope of the land • Nature of the vegetative cover
The Work of Surface Water Flowing surface water sculpts and shapes Earth’s surface: • Erosion—erosive sculpting action carves the landscape • Deposition—shapes the land as sediment is deposited
The Work of Surface Water Running water shapes Earth’s surface in two opposing ways: fast water transports sediment, slow water deposits sediment. Streamflow—two types of flow; stream speed • Laminar flow—slow and gentle • Turbulent flow—fast and rapid Factors that determine velocity: • Gradient, or slope • Channel characteristics (shape and size) • Discharge—volume of water moving past a given point in a certain amount of time
The Work of Surface Water Average stream speed = discharge / cross-sectional area Stream speed is usually not constant along the length of a stream. As the stream moves downslope, the gradient decreases and the channel widens. Discharge usually increases as tributaries add water.
The Work of Surface Water Stream erosion: • Loosely consolidated particles are lifted by abrasion and dissolution. Stronger currents lift particles more effectively: • Stronger currents have “higher” energy • Lift and transport more and bigger particles • Turbulent versus laminar flow
The Work of Surface Water The land area that contributes water to a stream is called the drainage basin. Drainage basins are separated by drainage divides. The largest drainage divides are continental divides.
The Work of Surface Water Stream water carries substances that chemically weather and erode rock. Abrasion occurs when sediments and particles scour a channel. Hydraulic action erodes and moves great quantities of sediment and rock.
The Work of Surface Water Streams transport great amounts of sediment from one location to another. Laminar flows can lift and carry only the very smallest and lightest particles. A turbulent flow can move and carry a range of particle sizes—it moves particles downstream mainly by lifting them into the flow or by rolling and sliding them along the channel bottom. The smaller, finer particles remain suspended to make the water murky.
The Work of Surface Water Stream channels in high mountain areas cut into underlying rock. Fast-moving rapids and beautiful waterfalls are characteristic of V-shaped mountain stream valleys.
The Work of Surface Water Stream speed plays a role in erosion and deposition. Water speed varies within a channel. It is slower along the stream bed and greater near the surface. Maximum flow speed occurs mid-channel.
The Work of Surface Water Meandering streams create a wide belt of almost flat land: a floodplain. When a flood occurs, sediment is deposited in the floodplain. Large, coarse sediment creates natural levees.
The Work of Surface Water A delta is where a flowing stream meets a standing body of water. The flow slows down and the stream dumps sediment. The result is a fan-shaped deposit of new land.
Glaciers and Glaciation Glaciers are powerful agents of erosion. A glacier is like a plow as it scrapes and plucks up rock and sediment. Glaciers are also powerful agents of deposition. A glacier is like a sled as it carries its heavy load to distant places.
Glaciers and Glaciation A glacier is an accumulation of snow and ice thick enough to move under its own weight. • Two types of glaciers: • Alpine • Continental
Glaciers and Glaciation Alpine glaciers develop in mountainous areas, generally confined to individual valleys. • Cascades, Rockies, Andes, Himalayas • Erosional landforms: cirque, arête, horn, hanging valley, U-shaped valley
The Work of Glaciers When a glacier’s ice mass becomes thick enough—about 50 meters—the pressure of the overlying material causes the base of the ice to move plastically—the entire mass shifts. Also, meltwater at the base of the glacier creates basal sliding.
The Work of Glaciers When glacial ice melts, it drops a poorly sorted, heterogeneous load of boulders, pebbles, sand, and clay. A wide range of particle sizes is the hallmark that differentiates glacial sediment from the much-better-sorted material deposited by streams and winds.
The Work of Glaciers The mass of a glacier changes over time. As snow falls, accumulation makes the glacier grow. As ice melts, sublimates, or breaks off, ablation occurs.
The Work of Air Wind blows everywhere, but its impact on sculpting the land is minor. Impact is greatest where: • Strong winds blow frequently • Vegetation is sparse or absent • Plant roots keep particles together • Plants deflect wind and shelter particles • Surface particles are small • Small particles are more easily lifted and transported
Runoff • Water that can`t be absorbed and it is moving in the surface • Runoff is helping to get more water in the aquifers, but also helps to the erosion of the land