LESSON 2a: WEATHER EFFECTS ON CROPS

ESTIMATING WATER SRESS OF A CROP

When all the water that a plant can draw off is taken away, we say that the field is dried to the wilting point, or the permanent wilting point of the crop. Then the plants can get no more water. That may be only half of the water actually in the soil profile, but nevertheless it is the plant-available water. The capacity for most Iowa soils is on the order of 10 inches in the top 5 feet, or 2 inches of water in each foot of the soil.

The amount of water in the soil may be used to evaluate the effect of soil water on the growth and the development of the plant. In an article by Shaw (1982), formerly of Iowa State University, a soil moisture and moisture stress prediction model is given for Iowa (Fig. 2.8). In the top 2 feet of soil, there is some water that is not available to the plant; it might be 1 inch of water unavailable to the plant (Fig. 2.8).

Fig. 2.8 Water contained in a soil profile at field capacity (Shaw 1982).

What is meant by an inch of water? It is just like an inch of rain. If enough rain fell into a flat pan to the depth of an inch, that is an inch of water. There is about an inch of water not available to the plant. In the topsoil (top foot or 30 cm) there could be 2 or 2.25 inches of water that is not plant available.

Study Question 2.3
If one inch of rain fell, how many gallons of water fell on an acre of land?

Another source of possibly another 1 or 2 inches of water per foot available to the plant may be the water table. This is the area where all of the pores of the soil are filled with water causing the soil to be saturated. A small capillary fringe exists between the water table and the height that water can rise in a capillary. Here the air spaces are partially filled or over-filled in the soil, depending on the size and the contacts of the air spaces. This capillary water rising from the water table may be available to plant roots.

The profile of the water in the soil could look something like Fig. 2.8. Roots withdraw the plant-available moisture, depending on where the roots are and the water demand into the atmosphere. If soil is completely loaded with water and the plant is healthy in mid-July, the plant would be using 80% of the water that would evaporate from the pan (Fig. 2.3). This is the potential amount a plant could use.

When the soil is at less than field capacity, less water is available for plant use and may affect the crops' actual water usage. Fig. 2.9 illustrates the reduction in evapotranspiration caused by limited soil moisture.

As an example, let's assume the soil is at 50% of field capacity (50% available), the crop will use almost the full amount as if the soil were at field capacity under low or medium demand conditions. On a day with very high demand, such as a hot, windy day, the amount used will drop off by 5-10%. If only the soil is at 25% of field capacity (0.5" water per foot of soil) on a high demand day, the amount of water used by the crop may be only half the potential. If the day is cloudy, humid, and calm, the plant may use around 95% of the water if the soil were filled with moisture.

Fig. 2.9 Evapotranspiration reduction in a corn crop caused by limited soil moisture under different demand conditions prior to silking.

On a high demand day, the plant would of course use a great deal of water. On a high demand day a fourth to a third inch of water evaporates from an open pan. If the soil is down to one-fourth of its water-holding capacity, the plant would just not be able to withdraw that much water. The use would be about half. So it would use, instead of 0.8, 0.4 of the pan evaporation. As the soils dry, the plant uses less and less until with a dried out soil the plant cannot use any water at all.

Study Question 2.4
If a day is a high demand day (very hot and windy) and the soil moisture is at 30% of capacity, what is the percentage of evapotranspiration that would be expected from a plant.

Study Question 2.5
How much transpiration would occur if there is a high demand day with the same conditions as in Study Question 2.4?

If agricultural plants are getting all of the water that they can use, it is assumed that they will grow at their potential rate. Perhaps this is a dangerous assumption. It assumes that water is the only limiting factor, or the only variable limiting factor. We say that the plant would be gaining dry matter at its potential rate when it uses the potential amount of water.

Should a plant not be able to use the full amount of water that it potentially could, something will happen. If the atmosphere is demanding a quarter of an inch of water, but the soil cannot provide that much to the roots of the plant, then the plant will either wilt, because of greater demand (call for water from the leaves) than the roots can provide, or the plant will by some mechanism prevent the water from being used.

One of the typical mechanisms is for the leaves to adjust. Leaves that might be horizontal under ideal conditions may become vertical as they come under water stress. Soybean leaflets may fold together and change their angle to the sun and change the surface area exposed to the atmosphere and thereby reduce the demand for water.

Most agricultural plants have a stomate aperture mechanism. A plant may have open stomates during a condition of ample water, and the stomates (or holes in leaves) allow carbon dioxide in and water and oxygen out (gas exchange through that pore). Most agricultural plants, if they come under stress, have a pore system which becomes very restricted if not completely closed. This serves as a water control mechanism (Fig. 2.10).

Fig. 2.10 Stomatal adjustment to restrict water loss when the plant is under stress.

Not all plants on the earth have this capacity to close their stomates under stress. About one-third of the natural growing plants do not demonstrate this capacity under imposed stress. The plant would just wilt if it could not have all the water available to it that it needed rather than be able to close the stomates and remain unwilted.

Agricultural plants do have a stomate-closing mechanism, so that water stress results in closed stomates and in restriction in the uptake of carbon dioxide, which results in diminished growth or dry matter increase for the plant. Therefore, we can evaluate the potential yield by knowing how much stress that plant has been under. That is, what percent of the time were the stomates closed? And from that we can know the reduction in yield, or at least have some idea concerning it. Based on that philosophy, we use a relationship called a "stress index". The stress index is derived from the ratio of actual water use to potential water use adjusted to the sensitivity of the crop to stress at various developmental stages.

The stress index is given in Dr. Shaw's (1982) paper as a formula and a straight-line relationship relating the ratio of how much water the plant would ideally use as compared to how much it did use based on availability of soil water to reduced yield.

The yield is expressed as kilograms/hectare (2.11). Observed results show that as the plant experiences stress, the yield decreases almost linearly. There could be some variability at the lower stress levels. Occasionally harvest may be larger than expected because of timing or because of errors in the system. Insect pests may also cause scatter. But generally the system works fairly well.

Fig. 2.11 Yield result as affected by weighted stress (Shaw 1982).

The amount of stress across the state does vary. The stress for the entire growing season in the eastern half of Iowa would normally be about 10 units, and typically in northwest Iowa the stress would be twice that (Fig. 2.12). Some years there would be no stress, and some years there would be very high stress. If the stress value reaches 50 or above, crop yields are greatly reduced. Some years, 1936 for example, in some parts of the state, the stress reached a value of 100 and the yield fell to a value of 0. Generally we assume that it is a serious situation for the crop or a crop failure if the stress value gets 50 or above, and 0 yield if it reaches 100.  (Note: Stress is based on lack of soil moisture. Earlier we discussed stress based on atmospheric conditions only. These are often related, but not always.)

Fig. 2.12 Average moisture stress days during the growing season in Iowa (Shaw 1982).

We can make a calculation of crop yield. For example, the yield in bushels to the acre (Bu/A) would equal some constant (potential yield, in this case given in kilograms per hectare), if we had no stress (9682 kilograms per hectare in this case), less the slope of the stress sensitivity line, times the stress experienced, and all of this divided by 62.71 (conversion of kg/ha to Bu/A); that would give us bushels to the acre.

With this model we have been able to evaluate quite accurately the effect of weather on yield through a growing season.