| ||Crop water use and evapotranspiration | Water use for vegetative growth | Water use for reproductive growth | Effects of moisture stress on crops
Crop water use is the amount of water used by a crop for growth and cooling. Crop water use can be determined on a daily, weekly or growing season basis.
The amount of water used by a crop is affected by a number of factors including the availability of soil water, precipitation during the growing season, stages of crop growth, crop rooting depths and environmental factors including amount of solar radiation, humidity, temperature and wind. A prolonged water deficit will have a significant adverse effect on a crop.
Crop Water Use and Evapotranspiration
Crop water use is referred to as “evapotranspiration” or “ET.” Evapotranspiration is the combination of water evaporation (E) from soil and plant surfaces as well as water used by plants for growth and transpiration (T). Transpiration refers to the water lost to the atmosphere through the stomata, which are small pores on the surface of plant leaves, as the plants work to avoid heat stress.
Evaporation is usually only significant when the soil surface is moist or when the crop canopy is wet, which is typical after precipitation or irrigation events. After the top 2 to 4 cm of surface soil have dried, evaporation of water from soil is usually minimal.
Evaporation from the soil surface is also greatly reduced as the crop canopy closes to completely shade the soil surface. At full crop canopy, almost all the ET is from transpiration by the crop. The maximum ET rate occurs when soil water is not a limiting factor.
Crops use their root system to extract water from the soil. The rate and amount of water taken up by a crop is affected by the soil water content, stage of plant growth and effective rooting depth.
The average amounts of ET for alfalfa, barley, canola, corn, dry bean, flax, pea, potato, sugar beet, timothy and wheat are provided in Figure 1 for May to September, along with the approximate growing season water requirement. These moisture use curves and total crop water use values are based on optimum soil moisture conditions; therefore, water use can be quite variable depending on environmental conditions.
From Figure 1, annual crop water use is low at the beginning of the growing season and gradually increases as the crop develops through the various vegetative growth stages, peaks at reproductive growth and then gradually declines as the crop matures.
Water Use for Vegetative Growth
For annual crops such as wheat, barley or canola, a certain amount of moisture is needed to not only initiate germination, but to take the crop through the vegetative growth stages to the stage where seed can be produced.
For wheat, barley and canola, at least 100 mm (4 inches) and often closer to 125 mm (5 inches) of water are needed to get a crop from germination to the reproductive growth stage where it can produce grain. The amount of moisture needed during vegetative growth varies because crops do not need as much moisture for transpiration in a cool spring compared to a warm, dry spring.
All crops shown in Figure 1 are cool season crops, with the exception of corn (Figure 1d). For cool season crops, daytime high temperatures in the range of 20° C are ideal for growth as crops are able to use more of the available soil moisture for vegetative crop growth than for transpiration to keep cool.
Cereal crops at the tillering stage use approximately 2 to 3 mm/day of water, and at the stem elongation stage, they need about 3 to 5 mm/day of water. When temperatures are above 25° C, the moisture needed is about 5 mm/day.
On warm days at the stem elongation growth stage, a cereal crop will use about 20 to 35 mm of water in one week, depending on environmental conditions such as solar radiation, temperature, humidity and wind.
When cereal crops are at the heading stage, often by early July, water use is 7 to 8 mm/day under ideal conditions. This situation means that peak water use is substantial from mid-June to late July or early August for cereal crops grown in Alberta. If moisture is lacking during this period, significant yield reduction can occur.
Figure 1. Approximate daily water use and total growing season water use in millimetres (mm) for some commonly grown crops in Alberta. Average water use shown is when soil moisture is adequate throughout the growing season. Shaded area indicates variability in daily water use due to crop cultivar, plant density and environmental conditions.
Water Use for Reproductive Growth
Once a crop shifts from vegetative to reproductive growth, water use remains high. Cereal crops after heading and canola at the flowering growth stage will continue to use 7 to 8 mm/day of water from heading to flowering and to grain filling, under optimum growth conditions. As grain filling nears completion, crop water use declines and drops off rapidly as plants approach maturity.
Alberta research has shown that under good environmental conditions, for each 25 mm (1 inch) of water used, wheat produces 5 to 7 bushels/acre, barley produces 7 to 9 bushels/acre and canola produces 3.5 to 4 bushels/acre.
Effects of Moisture Stress on Crops
When a crop is in a moisture deficit condition during vegetative growth, the first effect is a reduction in the growth rate of leaves and stems. When soil moisture availability is limited, cell expansion and division within the plant slow down. The effect is that plants reduce the production of enzymes and proteins needed for growth.
As the soil moisture deficiency increases, plant roots cannot take up enough water to meet transpiration needs. Crops respond by closing their stomata. Plant leaves become less rigid, and leaves exhibit wilting in mid-day heat. As air temperatures cool and solar radiation decreases later in the day and into the evening, plants recover from wilting as stomata open to meet transpiration needs.
When cereal crops begin wilting, older leaves and tillers are aborted, and stem elongation is reduced. When oilseed crops wilt, plants respond by abortion of older leaves, reduced stem elongation and reduced branching, which will reduce crop yield potential.
If the moisture deficit becomes more advanced, wilting becomes more prolonged each day until plants reach a condition where recovery overnight does not occur, and plants completely senesce and die.
Alberta Agriculture and Rural Development
Dr. Ross H. McKenzie
Research Scientist – Agronomy
Agriculture Research Division
Dr. Shelley A. Woods
Research Scientist – Soil and Water
Irrigation and Farm Water Division
For more information
Alberta Ag-Info Centre
Call toll-free: 310-FARM (3276)
Source: Agdex 100/561-1. Revised November 2011.