Meteorological

e.g.

• 15 days without rain
• 50% of average annual rain
• 1 in 10 year rainfall

This omits consideration of effects on plants

<Columbus rainfall data>

Biological

• Days of plant-limiting water deficit
• Loss of farm production

Sociological

• Impact on farm income, Impact on rural communities
• Often as a basis for determining government aid

Community e.g.

a) different species in sunny slopes (tall fescue, annual legumes) vs shady slopes (birdsfoot trefoil, moss)

b) Tall grass prairie - modium rainfall (Big blue stem, switchgrass, indiangrass); Short grass prairie - low rainfall (Blue grama, needle-and-thread)

Columbus OH 21 June 2002. vigorous spring growth

Same plots - 25 July 2002
left=ryegrass
right=alfalfa, orchardgrass and chicory

Morphological

• Leaf loss
• Smaller leaves
• Reduced shoot growth
• Greater absolute and/or relative root growth
• Senescence and death rates

Anatomical and physiological

• Concentration by dehydration
• Osmotic adjustment
• Proline accumulation
• ABA increase
• C3 vs C4 metabolism
• Cell wall elasticity

Biochemical and molecular

• Altered enzyme activity (alpha-amylase) is the result of molecular control
• Heat shock proteins
• Multiple gene responses
• GMO for water soluble carbohydrate production

Pasture perenniality
Dilemma whether or not to continue growth into drought

Defoliation (cutting or grazing)
Close grazing usually occurs during drought, but impairs recovery from drought

Dry/severe environments
Forages are often religated to the most difficult environments (e.g. rangelands)

Endophyte
greater plant persistence in drought, osmotic adjustment, photosynthesis, leaf survival

Quality is often severly reduced by drought
Green:dead ratio, Loss of legumes
Increased fiber, reduced protein, Anti-quality

Limited use since forages are usually lower value

Options

• Flood irrigation (low cost, low efficiency)
• Spray irrigation
• Line irrigation
• Sub-surface irrigation (high set-up cost)
• Center pivot irrigation