Irrigation—Principles & Practices
Part 1 – 210 | Unit 1.5
B. Soil Moisture, Plant Stress, and Crop Productivity
- Yield may be reduced due to water stress
a) Water-stress-sensitive stages of crop development (prioritized); see also Appendix 5,
Critical Periods for Soil Water Stress by Crop
i. Flowering
ii. Yield formation/fruit set
iii. Early vegetative growth/seedling stage
iv. Fruit ripening
- General signs of water stress
Plants can show some water stress and still recover—however, extreme lack of water will
cause permanent wilting (see below) and death. Signs of water stress include:
a) Graying leaves: A change in leaf color from a vibrant green to a dull gray-green or bluish
color
b) Loss of sheen: Plant leaves change from glossy to dull in appearance
c) Insect damage: The presence of cabbage aphids on Brassica family crops (broccoli,
cabbage, kohlrabi, etc.) often indicates dry conditions
d) Damage to the root system: Upon closer examination, plants that look dry even after
watering often have root damage, e.g., from symphylans, and can’t take up sufficient
water
e) Red or purple leaf color: Can indicate dry conditions, saturated conditions (anaerobic),
or root damage
f) Development of small spines on the leaf margins or increased spinyness on stems: This
condition is especially likely to occur in lettuce and related species such as endive that
experience water stress
g) Wilting: Pay attention to the time of the day. If plants wilt early in the cool of the day,
this can be a sign that they need water. Some wilting in the mid-day heat (e.g., zucchini,
winter squash) is a plant-protective strategy to reduce transpiration losses.
h) Slower than expected growth: This can be detected over time with a practiced eye
- Water stress increases crops’ susceptibility to pests and pathogens
Crops repeatedly subjected to water stress will be less resistant to both pest and
pathogens
- Permanent wilting point
Permanent wilting point is defined as the point at which soil moisture is too low for the
plant to take up water against the pull of gravity. Crop plants reaching permanent wilting
point often do not grow well thereafter, are non-productive, or die.
C. Factors Influencing Frequency and Volume of Irrigation
A number of factors, from climate and soil type to stage of crop maturity, must be considered
in determining when and how much to irrigate. Factors include:
- Climate
a) Air temperature: Increased air temperatures will increase the rate of evapotranspiration
(ET)
b) Precipitation: In areas of regular summer rainfall, where precipitation exceeds ET,
irrigation is seldom required. Irrigation demands are based on ET rates. Where ET
exceeds precipitation, irrigation is required.
c) Humidity: Increased humidity will decrease the rate of ET
d) Wind: High wind speeds increase ET
Lecture 2: Irrigation Scheduling & Delivery Systems