Lecture 11: Water Flow; Soils and the Hydrologic Cycle

Lecture 11: Water Flow; Soils and the Hydrologic Cycle

Water Flow in Soils

Types of Water Flow in Soil Saturated flow: Soil pores completely filled with water; controlled by the hydrostatic potential After rain or irrigation Flooded/wetland soils Groundwater Unsaturated flow: Large pores contain air, water flows through small pores; controlled by gravitational and matric potentials Dominant flow condition in soil Vapor movement: Transport of gaseous water Generally only important in arid soils Gravitational potential has greatest effect on flow during infiltration and drainage of soils

Saturated Flow in Soils Governed by Darcy s Law Q = AK sat (Δψ/L) Q is the discharge: Quantity of water per unit time Textbook uses Q/t A is area K sat is the saturated hydraulic conductivity Δψ/L is the hydraulic gradient

Movement of Rain or Irrigation Water Varies with Texture High K sat Low K sat Lateral movement is mostly via capillary action; slow infiltration allows time for capillary flow

Transition to Unsaturated Flow Saturated flow involves gravitational water, unsaturated flow involves capillary water

Unsaturated Flow Rate Depends on Matric Potential Q = AK unsat (Δψ/L) K unsat

Water Infiltration into Soil Rainfall or irrigation water lands on the soil surface It must infiltrate to become soil water Water initially pours into open macropores Infiltration rate then slows to that predicted by K sat as the soil swells upon wetting Macropores become smaller Infiltration is not governed by unsaturated flow, which occurs only in micropores

Water Infiltration into Soil Infiltrating water often produces a wetting front Plant roots dry out subsoils New water may eventually contact moist lower horizons

Texture Changes can Limit the Depth of Water Infiltration Grey B horizon (gleyed) indicates poor aeration Large pores underlying small pores causes water to be held up, trapped in the upper horizon, unable to drain into the lower, coarser soil horizon

Key Concepts in Water Flow Water flows from high pressure (or potential or height) to low pressure Water flow controlled by Darcy s Law Unsaturated flow is complex because water flow (Q) changes the water content (θ), which changes the hydraulic conductivity (K), which changes the flow rate (Q) Important to understand conceptually, but not practical to deal with unsaturated flow quantitatively in this course Infiltration occurs through the macropores

Soils and the Hydrologic Cycle

Soil Water Constitutes Only a Small Fraction of the Water on Earth

The Hydrologic Cycle

Processes Controlling Soil Moisture

Processes Controlling Soil Moisture Evaporation (E): Volatilization of water from land surface Transpiration (T): Volatilization of water from plant leaves These together are called Evapotranspiration (ET) Discharge (D): Removal of water from a watershed through surface or subsurface flow (Runoff + Percolation) Soil Storage (SS): Retention of water in soil Maximizing soil storage provides long-term water availability At the watershed scale, these processes balance water addition from Precipitation (P) P = ET + SS + D or D = P ET SS

Factors Affecting Soil Storage

Fate of Precipitation and Irrigation Water Interception: Precipitation intercepted by plant foliage that evaporates before reaching the soil Surface Runoff: Water that reaches the soil but does not infiltrate; enters streams and rivers Infiltration: Water that penetrates into the soil Drainage/Percolation: Infiltrated water that flows to depths below the root zone; generally enters the groundwater Retention of infiltrated water results in soil storage

Soil Structure and Vegetation Influences Infiltration and Runoff

Factors Controlling Infiltration Type of Vegetation: Plants and residue in grasslands and dense forests encourage infiltration Stem Flow: Plants, including trees, funnels rainwater down along their stems or trunks, localizing infiltration Soil Management Practices: Creating water storage areas on soil surface, planting cover crops, and minimizing soil compaction all encourage infiltration Natural Soil Properties: Soil structure and the presence of dense horizons affect infiltration

Soil Drainage Varies Across a Landscape

Evapotranspiration and Soil Water Cycling

Evapotranspiration In natural systems it is difficult to separately measure evaporation and transpiration Often grouped together as evapotranspiration (ET) Individual component affected by vegetation type

Diurnal Cycle of Evapotranspiration

Capillary Movement May Supply Plants with Moisture During Dry Conditions In period of dry conditions, the shallow soil has little available water Capillary action pulls water up from the water table Creates the capillary fringe Deep rooted plants can access this water

Soil Storage Infiltration Runoff + Drainage = Discharge Water Cycling in Soil-Plant- Atmosphere Systems 5-30% of rain is intercepted 0-30% becomes runoff 45-95% of water infiltrates 10-30% is lost as drainage 15-60% is released via transpiration 5-50% is released via evaporation