Soil Science Exam 2- Soil Hydrology

Water Balance Equation

P = ET + SS + D

p = precipitation

ET = evapotranspiration

SS = soil storage

D = Discharge (runoff + leaching)

What constitutes Evapotranspiration?

- Transpiration: Water released via stomata

- Evaporation: All other vapor losses (leaf surfaces, soil surfaces, impervious surfaces)

Potential Evapotranspiration

- Conceptually, PET is the maximum amount of ET that can occur at a well-watered site under prevailing vegetative and weather conditions

- practically, it is estimated as a percentage of evaporation measured from free water surface (i.e. it is lower value)

What factors affect PET?

- Temperature

- Wind Speed

- Relative Humidity

NOT soil moisture conditions because PET already assumes a soil is at FC (Well watered site = FC)

What factors affect actual ET?

- Soil and vegetation

- Weather (temp, wind, humidity)

- Human (impervious surfaces)

Soil water deficit equation

Deificit = PET - ET

- An index of moisture status for plant growth

- As deficit increases, plant growth decreases

- deficit increases as soil water content decreases

What affects discharge?

Discharge = runoff + leaching

- Soil and vegetation (texture/structure, current moisture content, surface cover, frozen soil)

- weather (rain intensity/distribution)

- Human (impervious surfaces)

Effect of soil disturbance

- Compaction/Loss of structure

- Loss of macropores open at the soil surface

- Loss of macroporosity in general

- possible loss of surface cover which acts to slow water movement at the soil surface

Effect of impervious surfaces

- limits both shallow and deep infiltration

- increases runoff

- Decreases evapotranspiration by decreasing transpiration

Mesures to increase infiltration, reduce, runoff, and reduce evaporation

- Conservation tillage

- Cover crop and earthworm burrows

- living pavement

- impoundment ponds

No-till without herbicides

A) Crimper-roller kills and flattens cover crop

B) - high residue cultivator in action

- Dead cover crop -> controls weeds

- Any weeds controlled by cultivator

C) Exposed high-residue cultivator

T/F : Water is pumped from plant roots to shoots

FALSE! Water is PULLED through the plant

Soil-plant-atmosphere continuum (SPAC)

Decreasing water potentials as you move from the top of the tree to the roots. As water is transpired, because of its cohesive nature, it tugs at the water column in the tree and pulls up some water from the soil. A continuous connection.

- Therefore, soil adjacent to roots is typically drier than in bulk soil

- however, following rainfall or sprinkler irrigation, stem flow can concentrate water at the base of the plants, temporarily increasing water potentials near roots.

Water use efficiency

Plant biomass produced per unit water consumed

- increased plant vigot can increase WUE

3 irrigation methods

Furrow

sprinkler

drip (Microirrigration)

T/F : Drip irrigration is generally most expensive, but also has the highest field water efficiency

TRUE: FWE is around 80-90% for drip irrigation

Why is over irrigating bad?

- Raises water table

- transports nutrients and pesticides to groundwater

Enhanced soil drainage

- Ditch Drainage

- Tile Drainage

- Buried "tile lines" made of perforated plastic pipe act very much like ditches, but have 2 advantages. They do not present an obstacle for equipment movement and they do not take land out of production.

Pros of artificial drainage

- enhanced plant growth

- improved access

- reduced plant disease

- more rapid soil warming

- lower greenhouse gas emissions

- removal of excess salts and capillary fringe effects

- less frost heaving

Cons of artificial drainage

- loss of wildlife habitat

- loss of nutrient uptake by wetlands

- increased leaching of nitrates, etc. to groundwater

- loss of soil organic matter

- Flooding problems