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SOIL, WATER AND PLANT RELATIONSHIPS
PLANT EVAPOTRANSPIRATION
Water is extracted from the soil by evaporation at the soil and plant surfaces (crop transpira- tion). The combination of the two is termed "crop evapotranspiration", or ETc. (It has also been termed "consumptive use" or "crop water use".)
ETc is affected by many factors. ETc will vary with the type of plant and growth stage. Some plants just use less water than others. And obviously, a seedling is going to extract less water than a full grown plant.
ETc varies with the climate. Up to a certain point, increasing temperature and wind will increase ETc. (There is usually a maximum ETc rate for any plant, beyond which it just stops transpiring due to stress.) Increasing humidity and cloud cover will decrease ETc.
ETc varies with the amount of soil moisture in the effective root zone. Remember that the soil's water-holding forces increase as the soil moisture decreases. Thus, as the plant uses up soil moisture, it becomes harder and harder to extract more. Past a certain point, which depends on the plant, the soil's texture and structure, and the root zone salinity, the ETc rate will decrease. Below-normal ETc rates will place stress on the crop. Depending on the crop and growth stage, more or less stress is desirable. Knowing the acceptable level of stress and knowing what level of soil moisture will cause this stress is an important function of modern crop management. The Reference chapter of this Handbook contains information sheets for all major crops describing critical growth stages, ETc rates throughout the season, and desirable water management.
The standard for measurement of ETc is inches of water use per day or "inches/day". You may see ETc described in terms of inches of water use per season if someone is talking of seasonal ETc. Table SWP-3 contains approximate ETc's of common crops. The reference section of this handbook contains full information on ETc for most common crops grown in the District.
TABLE SWP-3. Approximate ETc at maturity of some crops at Five Points (mid-July)
| CROP |
ETc |
| Alfalfa | .12 - .30 |
| Almond | .27 |
| Corn | .35 |
| Cotton | .28 |
| Melon | .20 |
| Onion | .20 |
| Tomato | .30 |
| Sugar beet | .30 |
INTO AND THROUGH THE SOIL
During an irrigation or rain, water soaks into the soil at a rate dependent on the soil type/structure, the soil/water chemistry, and the current soil moisture. Usually, the soil's "infiltration rate" will decrease with time during an irrigation or rain. This is graphically illustrated in Figure SWP-3.
FIGURE SWP-3: Soil Infiltration Curves, Rate (in/hr) vs. Time.
Figure SWP-3 is a graph of the infiltration rate at the head of a furrow during an irrigation. At the left the irrigation has just started (elapsed time = 0) and the infiltration rate is high. The water is taking in water very quickly. But 20 hours into the irrigation, the infiltration rate has dropped dramatically.
Water movement into and through soil is very much influenced by the soil/water chemistry and soil structure. A "sodic" soil will usually have an imbalance in salts that reduces the permeability of the soil. More is said about salts and managing their effects in the Salts and Drainage section.
Water moves through the soil due to a combination of gravitational and matric forces. Water always tends to move down. However, as described previously, soil water-holding forces are higher in areas with low soil moisture. Thus, water will also tend to move from soil with high soil moisture to areas of less. This is why water can move up into the root zone from a shallow ground water table. As the plant's root system extracts water in the immediate vicinity of the root, water from the wetter surrounding soil will move towards the drier area around the root. As the soil moisture goes down, less and less water will move towards the root and that water surrounding the root is held tighter. Thus, the root can extract less and less water and stress is put on the plant as the soil dries.
Last updated September 2000