SGES 1302 INTRODUCTION TO EARTH SYSTEM LECTURE 12: Surface Water & Groundwater

Surface Water Surface water moves materials produced by weathering and shapes the surface of Earth. Running water is an agent of erosion, carrying sediments in streams and rivers and depositing them downstream. Earth’s water supply is recycled in a continuous process called the hydrologic cycle (last lecture). Although surface water constitutes a small proportion of the Earth’s water, it plays a critical role in the hydrologic cycle. Surface water mainly resides in streams, freshwater lakes and wetlands.

Surface Water – runoff Water flowing downslope along Earth’s surface is called runoff. Runoff might reach a stream, river, or lake, it might evaporate, or it might accumulate as puddles in small depressions and infiltrate the ground. Water that infiltrates Earth’s surface becomes groundwater. A number of conditions determine whether water on Earth’s surface will infiltrate the ground or become runoff. Soil composition (porosity, saturation, etc.) - the physical and chemical composition of soil affects its water-holding capacity. Rate of precipitation vs rate of infiltration Vegetation - Soils that contain vegetation allow more water to enter the ground Slope – the steeper the slope, the faster the water flows. Water standing on the surface of Earth eventually evaporates, flows away, or slowly enters the groundwater.

Surface Water – stream system Precipitation that does not enter the ground usually runs off the surface quickly. Some surface water flows in thin sheets and eventually collects in small channels, which are the physical areas where streams flow. A stream’s flow is maintained by 2 water sources: precipitation / runoff, and groundwater (base flow). Baseflow enables a stream to continue to flow between storms or during drought. All streams flow downslope to lower elevations. Some streams receive water from an network of other streams are called tributaries. All of the land area whose water drains into a stream system is called the system’s watershed. A divide is a high land area that separates one watershed from another. In a watershed, the water flows away from the divide, as this is the high point of the watershed.

Surface Water – stream load The material that a stream carries is known as stream load. Stream load is carried in three ways. Suspension is the method of transport for all particles small enough to be held up by the turbulence of a stream’s moving water. Bed load - Sediment that is too large or heavy to be held up by turbulent water is transported by roll or push along the bed of the stream. Solution is the method of transport for materials that are dissolved in a stream’s water. The ability of a stream to transport material (carrying capacity) depends on both the velocity and discharge (the volume of water passing a given point per unit of time).

Surface Water – flood The amount of water being transported in a particular stream at any given time varies with weather conditions. Sometimes, more water pours into a stream than the banks of the stream channel can hold. A flood occurs when water spills over the sides of a stream’s banks onto the adjacent land. The broad, flat area that extends out from a stream’s bank and is covered by excess water during times of flooding is known as the stream’s floodplain. Floodwater carries along with it a great amount of sediment eroded from Earth’s surface and the sides of the stream channel. As floodwater recedes and its volume and speed decrease, the water drops its sediment load onto the stream’s floodplain. Floodplains develop highly fertile soils as more sediment is deposited with each subsequent flood.

Groundwater - porosity Precipitation and infiltration contribute to groundwater, which is stored in underground reservoirs (aquifers) until it surfaces as a spring or is drawn from a well. Groundwater reservoirs provide water to streams and wetlands wherever the water table intersects the surface of the ground. Groundwater resides in the pore spaces and voids in the soil and rock. The greater the porosity, the easier water can flow through the material and the more water it can hold. Subsurface materials have porosities ranging from 2 % to > 50% The porosity of well-sorted sand is ~30 percent, however, in poorly sorted sediment, smaller particles occupy some of the pore spaces and reduce the overall porosity of the sediment. Porosity is low in igneous and metamorphic rocks. Groundwater is stored in voids along fractures and faults in these rocks.

Groundwater - distribution The region below Earth’s surface in which groundwater completely fills all the pores of a material is called the zone of saturation. The upper boundary of the zone of saturation is the water table. In the zone of aeration, which is above the water table, materials are moist, but because they are not saturated with water, air occupies much of the pores. Capillary water is water that is drawn upward through capillary action above the water table and is held in the pore spaces of rocks and sediment because of surface tension.

Groundwater - movement Groundwater flows downhill in the direction of the slope of the water table. Usually, this downhill movement is slow because the water has to flow through numerous tiny pores in the subsurface material. The tendency of a material to let water pass through it is its permeability. Materials with large, connected pores, such as sand and gravel, have high permeability and permit relatively high flow velocities. Groundwater flows through permeable sediment and rock, called aquifers. Fine-grained materials have low permeabilities because their pores are small. These materials are said to be impermeable. Impermeable layers, called aquicludes, are barriers to groundwater flow.

Groundwater - spring Groundwater moves slowly but continuously through aquifers and eventually returns to Earth’s surface. Groundwater emerges wherever the water table intersects Earth’s surface to form springs. A spring forms where a permeable layer and impermeable layer come together A layer of impermeable rock or clay can create a perched water table. Springs can result where groundwater emerges from a perched water table. Some springs form where a fault has brought together two different types of bedrock. Karst springs form where groundwater weathers through limestone bedrock, and water in the underground caverns emerges.

Groundwater – hot spring & geysers The temperature of groundwater that is discharged through a spring is generally the average annual temperature of the region in which it is located because it is insulated from the fluctuation of air temperature. Springs that discharge water that is >6°C warmer than the average annual temperature are called hot springs These hot springs emerge from aquifers that descend to tremendous depths in Earth’s crust and through which deep, hot water rises. Geysers are explosive hot springs, usually occur in volcanic areas. In a geyser, water is heated past its boiling point, causing it to vaporize. The resulting water vapor builds up tremendous pressure that fuels the eruptions.

Groundwater supply Wells are holes dug or drilled into the ground to reach an aquifer. There are two main types of wells: ordinary wells and artesian wells. As water is drawn out of a well, it is replaced by surrounding water in the aquifer. Overpumping occurs when water is drawn out of the well at a rate that is faster than that at which it is replaced. In coastal areas, overpumping of wells can cause the underlying salt water to rise into the wells and contaminate the freshwater aquifer. Another problem caused by the excessive withdrawal of groundwater is ground subsidence. Pollutants can enter the ground above the water table, but they eventually infiltrate to the water table to cause groundwater pollution.