Question

Imagine a water molecule that is part of a groundwater system in an area of gently rolling hills in the eastern United States. Describe some possible paths the molecule might take through the hydrologic cycle if: a. it were pumped from the ground to irrigate a farm field. b. there was a long period of heavy rainfall. c. the water table in the vicinity of the molecule developed a steep cone of depression due to heavy pumping from a nearby well. Combine your understanding of the hydrologic cycle with your imagination and include possible short-term and long-term destinations and information as to how the molecule gets to these places via evaporation, transpiration, condensation, precipitation, infiltration, and runoff. Remember to consider possible interactions with streams, lakes, groundwater, the ocean, and the atmosphere.

Short answer

Water can be absorbed by plants, evaporate, or recharge groundwater; flow into rivers during rain; or move deeper underground near cones of depression.

Step-by-step solution

Hydrologic Cycle Overview

The hydrologic cycle describes how water moves through the environment via processes such as evaporation, condensation, precipitation, infiltration, and runoff. Water can reside in the atmosphere, surface water bodies like rivers and lakes, the ocean, and underground as groundwater.

Scenario A: Farm Irrigation

When the water molecule is pumped from the ground to irrigate a farm field, it can follow several paths. First, it might be absorbed by plants and be involved in transpiration, where it evaporates from plant leaves and returns to the atmosphere. Alternatively, it might return to the atmosphere through direct evaporation from the soil's surface. Some of the water may percolate deeper into the ground, recharging groundwater supplies.

Scenario B: Heavy Rainfall

During heavy rainfall, the water table can rise, increasing hydraulic pressure and creating surface runoff. The molecule might flow overland into rivers or lakes. If it infiltrates the soil, it recharges groundwater but may later reach streams as baseflow. Eventually, water evaporates from surfaces or through transpiration and condenses in the atmosphere, possibly returning as rain or snow.

Scenario C: Cone of Depression

If a steep cone of depression forms in the water table due to pumping, the water could flow towards this lower area. The molecule might be extracted by wells for human use or continue seeping deeper into the groundwater system if not intercepted. Over time, the water might migrate further underground or return to the surface through springs or seeps.

Key concepts

Groundwater Movement

Groundwater movement is a crucial aspect of the hydrologic cycle. It refers to the flow of water within the Earth's subsurface. This water moves through soil pores and rock fractures, influenced by gravity and mechanical pressure.

Several factors affect groundwater movement:

• Permeability: The ability of soil or rock to allow water to pass through. Higher permeability means water flows more freely.
• Gradient: The slope of the water table or aquifer affects how quickly water moves. Steeper gradients usually result in faster groundwater flow.
• Porosity: The amount of space within soil and rocks where water resides, impacting how much water can be stored.

Groundwater plays an essential role in recharging surface water bodies like lakes and rivers, especially during dry periods. This movement is continuous but often slow, allowing deep infiltration of water over time.

Evaporation and Transpiration

Evaporation and transpiration are key processes in the hydrologic cycle that return water to the atmosphere.

**Evaporation** is the transformation of water from liquid to gas. This primarily happens on the surface of oceans, lakes, and rivers.
**Transpiration** occurs in plants. Here, water absorbed by roots travels through plants and eventually evaporates from small openings in leaves, known as stomata.
Together, these processes are sometimes referred to as 'evapotranspiration.' They depend on several factors:

• Temperature: Higher temperatures usually increase evaporation rates.
• Wind: Wind can accelerate evaporation by displacing saturated air around water surfaces with drier air.
• Humidity: Lower humidity levels often enhance evaporation as air can hold more moisture.

Evapotranspiration is a vital balance within ecosystems, influencing water availability for various organisms.

Precipitation and Infiltration

Precipitation and infiltration are essential processes for replenishing Earth's water reservoirs.

**Precipitation** occurs when water vapor in the atmosphere condenses into droplets that fall as rain, snow, sleet, or hail. This is a primary way water returns to Earth from the atmosphere.
**Infiltration** is the process where precipitation soaks into the ground. It replenishes groundwater supplies and is influenced by soil characteristics:

• Soil texture: Sandy soils, with larger particles, allow faster infiltration compared to clayey or silty soils.
• Vegetation cover: Plant roots create channels that help water infiltrate.
• Soil saturation: Already saturated soils cannot absorb as much water, increasing runoff.

Infiltration ensures that water continues to feed aquifers, contributing to the persistence of water bodies.

Surface Water Interactions

Surface water interactions describe the exchanges between various water bodies, including rivers, lakes, and oceans, and their surroundings.

These interactions are vital for maintaining ecological balance and include processes like:

• Runoff: Water from precipitation and melting snow that flows over land into rivers and streams.
• Streamflow: The movement of water within a stream or river, a crucial component of the water cycle, affecting erosion and landscape shaping.
• Lake turnover: Temperature-driven mixing of lake water that redistributes nutrients and oxygen.

Surface water is a dynamic system, continually exchanging with groundwater through infiltration and discharge. These interactions help sustain aquatic habitats, support biodiversity, and provide water for human use.