Water in the Subsurface

Water in the Subsurface

• Water infiltrates through percolation, slowly passing through porous materials.
• Soil-moisture zone provides subsurface water for plant roots.
• Bound water in soil impacts plant water availability.
• 76% of precipitation infiltrates the subsurface; 85% returns to the atmosphere through evaporation or transpiration.
• Gravitational water percolates deeper during soil saturation.
• Water table marks the upper limit of the saturated zone, fully filled with water.
• Base flow occurs where the water table intersects stream channels, contributing to stream flow.
• Streams and groundwater flow into oceans, sustaining the hydrologic cycle.
• Some streams feed into closed lake basins, leading to evaporation or underground seepage.
• Groundwater flows slowly toward the sea, often intersecting the surface.

Water Budgets and Resource Analysis

• A water budget can be established for any area on Earth's surface.
• Evaluates the input and distribution of precipitation and evapotranspiration.
• Factors include evaporation, transpiration, surface runoff, and soil moisture storage.
• Timeframes for water budgets can range from minutes to years.
• Functions similarly to a financial budget, balancing inputs (precipitation) and outputs (evaporation, transpiration, runoff).
• Soil-moisture acts as a savings account, allowing for temporary storage and withdrawal of water.
• Water budget deficits or surpluses can affect crop yields.
• Thornthwaite developed methodologies to address irrigation and water use challenges based on climate influence on water supply and demand.

Groundwater Resources

• Groundwater supplies 80% of global irrigation and drinking water.
• Groundwater pollution threatens water quality and food security.
• Overconsumption leads to depletion beyond natural replenishment.
• 50% of the U.S. population relies on groundwater for freshwater access.
• In some states, groundwater meets 85% of water needs; rural areas may rely entirely on it.
• Groundwater withdrawal in Canada and the U.S. rose over 150% from 1950 to 2000.

The Groundwater Environment

• Precipitation is the primary groundwater source, moving down as gravitational water.
• Water travels through the unsaturated zone, where some pore spaces hold air.
• The saturated zone stores water completely filling soil pore spaces, bounded by an impermeable layer.
• The upper limit of the saturation zone is called the water table, which directs groundwater flow.

Aquifers and Wells

• Aquifers are permeable subsurface materials allowing groundwater flow.
• Unconfined aquifers have permeable layers above and impermeable rock below.
• Confined aquifers are surrounded entirely by impermeable layers.
• Aquitards are low-permeability layers that cannot conduct usable water.
• Humans extract groundwater through wells drilled until the water table is reached.

Components of the Water Budget

• Soil Water Change Rate Formula:
  ∆S = P - ET
  where ∆S is soil moisture change, P is precipitation, ET is evapotranspiration, and ∆t indicates time change.

Water Supply: Precipitation

• Precipitation occurs in various forms, including rain, snow, and hail.
• Rain gauges measure rainfall and snowfall depth, weight, or volume.
• Wind shields are used to measure precipitation accurately by minimizing losses from angular raindrops.

Water Supply: Precipitation Measurements Worldwide

• Over 100,000 locations monitor global precipitation levels.
• Maps display global annual precipitation averages.

Water Demand: Potential Evapotranspiration

• Evapotranspiration represents actual water loss to the atmosphere.
• Potential evapotranspiration (PE) indicates water loss under ideal moisture conditions.
• A dry bowl analogy illustrates that if replenished, total demand equals PE; if not, the deficit forms.
• Actual evapotranspiration (AE) equals PE minus any deficit.

Water Demand: Potential Evapotranspiration Measurement Tools

• Evaporation pans or evaporimeters measure evapotranspiration, with automatic water replacement.
• Lysimeters are used to isolate soil and measure moisture.
• Rain gauges can record precipitation next to lysimeters.

Thornthwaite's Methodology

• Thornthwaite's indirect method estimates PE for midlatitude locations using mean air temperature and day length.

Explore the critical concepts of subsurface water management, including infiltration, percolation, and the significance of the soil-moisture zone. Understand how soil composition affects plant water availability and the role of precipitation in subsurface water dynamics. This quiz provides a comprehensive overview of water's journey beneath the surface.