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 is pumped from the ground to irrigate a farm field. b. There is a long period of heavy rainfall. c. The water table in the vicinity of the molecule develops 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 about 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

The molecule can experience evaporation, transpiration, runoff, or groundwater recharge, affected by irrigation, rainfall, and pumping. It might eventually re-evaporate, join surface streams, or travel to the ocean.

Step-by-step solution

Understanding Pumping Water for Irrigation

When the water molecule is pumped to irrigate a farm field, it is likely to be spread onto the soil surface. Once there, the molecule might evaporate into the atmosphere due to sunlight and heat, or it may be absorbed by plants through roots and later released back into the atmosphere as vapor through a process called transpiration.

Considering Heavy Rainfall Impact

In a scenario with heavy rainfall, the water molecule may infiltrate further into the soil, recharging groundwater. Alternatively, if the soil is saturated or the rainfall is too intense, it could end up running off the land surface into streams or bodies of water, eventually reaching lakes or the ocean, via rivers.

Exploring Effects of a Cone of Depression

A steep cone of depression caused by heavy pumping creates a local drop in the water table. This might draw the water molecule closer to the well, increasing its chances of being extracted again. Alternatively, if not pumped, the molecule can continue to flow with groundwater, potentially reaching streams or the base flow of rivers.

Pathways via Hydrologic Processes

Throughout these scenarios, the molecule can undergo several hydrologic processes. Through evaporation, it moves to the atmosphere, and through condensation, it forms clouds, leading to precipitation, where the cycle repeats. The molecule's journey may also include frequent interactions between ground and surface water, evidenced by exchanges in lakes, streams, and eventual ocean entry.

Short-term and Long-term Outcomes of Molecule's Journey

Short-term, the molecule could be involved in cycles of evaporation and precipitation within the local watershed. Long-term, it might travel to distant locations. After precipitation, it may enter another region's groundwater or surface water, participate in multiple evaporation-condensation cycles, and possibly reach the ocean.

Key concepts

Groundwater

Groundwater is a crucial component of the hydrologic cycle and refers to water stored beneath the Earth's surface, in the tiny spaces between rocks and soil. This vast reservoir serves as a critical source of drinking water and sustains agriculture. Groundwater flows slowly through aquifers—geological formations made of water-bearing permeable material, such as gravel or sand.

Key Functions:

• Provides water for irrigation and consumption
• Supports ecosystems by maintaining steady water supply to rivers and streams during dry periods
• Acts as a buffer against droughts, supplying water when surface sources dry up

When a water molecule is part of the groundwater, it may eventually be pumped for various uses, such as irrigating fields or supplying households. If not extracted, it continues its journey through the groundwater system, with potential long-term travel to streams or base flow of larger rivers, contributing to hydrological balance.

Evaporation

Evaporation is the process where water turns from a liquid to a gas or vapor. It is a key mechanism in the hydrologic cycle, moving water from bodies such as lakes, rivers, and oceans to the atmosphere. This transformation occurs when heat from the sun energizes water molecules, giving them the power to break free from liquid surfaces.

Important Highlights:

• Plays a critical role in the atmospheric water cycle
• Facilitates cooling as it absorbs heat during the transformation
• Prepares water for precipitation as it forms clouds through further processes like condensation

For a molecule that has been pumped onto a field, evaporation might occur if the direct sunlight and heat from the surroundings are strong. This process converts the molecule into vapor, sending it skyward, where it joins others to eventually form clouds, showcasing evaporation's role in water recycling within nature.

Precipitation

Precipitation occurs when water in the atmosphere falls to the Earth's surface. It is a pivotal step in the hydrologic cycle wherein moisture returning from the atmosphere replenishes water in rivers, lakes, and oceans, as well as groundwater reserves through infiltration.

Typical Forms of Precipitation Include:

• Rain
• Snow
• Sleet
• Hail

This process can significantly affect the path of a water molecule. If heavy rainfall occurs, a molecule originally in the ground or atmosphere might infiltrate the soil, thereby recharging groundwater. Conversely, excessive precipitation can lead to runoff, moving the molecule into nearby streams or lakes, further intertwining its journey with other components of the hydrologic cycle.

Infiltration

Infiltration refers to the process by which water on the ground surface enters the soil. This is a vital mechanism within the hydrologic cycle as it replenishes groundwater, ensuring long-term water availability for various uses, from agriculture to urban supply.

Factors Influencing Infiltration:

• Soil type and structure
• Land cover (vegetation enhances rates)
• Rainfall intensity and duration

Once in the soil, the water molecule embarks on a slower journey, seeping deeper into aquifers that can transport it far afield. In the event of a cone of depression—where increased extraction lowers the water table nearby—infiltration's role becomes even more crucial, as it seeks to maintain balance within the subsurface water system and prevent over-depletion.