High yields are achieved by irrigating at the proper time. When furrow irrigating there is a trade off between the time to irrigate and the level of irrigation efficiency obtained. The soil is a reservoir that contains available water that the plant can deplete. The level of water that can be depleted before the plants show a large amount of stress is when the readily available water has been depleted. The available water is the water held in the soil between field capacity and permanent wilting point. The amount of water that can be depleted is call the Management Allowed Depletion (MAD) which varies from 30 percent of the available water to 60 percent.
An irrigation scheduling model calculates a water budget that keep track of the available moisture and recommends an irrigation at the time the depletion level is reached. As the roots increase in length into the soil profile, the amount of available water in inches that can be depleted before an irrigation occurs increases. However, the rate of withdraw also increases as the root depth increases and the plant cover increases. The rate of withdraw also increases as the climate changes with increasing temperature and solar radiation. See the extension publication from Colorado State University for a complete explanation of the water budget approach [pdf requiring Adobe Acrobat].
The water budget approach determines the depletion level for the whole root zone. If a tensiometer is used to schedule irrigation's, then it must be placed in the portion of the root zone that represent the same depletion level as the average depletion level calculated using the water budged approach. The general depletion levels are: (1) 40 percent of the water used by the crop is extracted from the first quarter of the root zone (2) 30 percent is extracted from the second quarter (3) 20 percent is extracted from the third quarter and (4) 10 percent is taken from the last quarter of the root zone. Consequently, a tensiometer needs to be placed between the second and third quarter of the root zone or at 63 percent of the depth of the roots in order for it to be placed in the depth of the root zone representing the average extraction level. As an example, if the root zone is 36 inches the tensiometer should be placed at 23 inches, or two feet. The irrigation should occur when the selected MAD is reached. The tensiometer reading is different for different soils to reach the same MAD. In order to convert from the selected MAD to a tensiometer reading use figure 1.
To determine the MAD for different crops see the table 1 by Sanders 1993 VEGETABLE CROP IRRIGATION, table 2 by AlKaisi and Broner CROP WATER USE and CRITICAL GROWTH STAGES or figure 2 (Doorenbos and Kassam 1986). Sanders presents the level of Available Soil Moisture (ASM) that must be in the soil at the time of irrigation. The MAD is equal to 1 - ASM. Irrigating when the soil moisture has decreased below the MAD causes the crop to be under water stress and decreases the evapotranspiration and yield. The depth of the roots for selected crops is also given by Sanders 1993. VEGETABLE CROP IRRIGATION The rooting depth is highly dependent on local conditions and should be determined for a site by using a shovel and digging up the plants at selected times during the growing season. Generally, the plants will reach their maximum root depth when plant cover reaches a maximum. The tensiometer needs to be lowered as the root depth increases, or two or three tensiometers need to be installed and the tensiometer used to schedule irrigation's should be the one closes to 63 percent of the root zone depth at the time of scheduling irrigation's.
As can be observed from figure 1, tensiometers work best in sandy or sandy loam soil because they break suction and quit working when the soil tension exceeds 80 cbars.
An example of how to use a tensiometer follows: Soil type sandy loam Crop onions MAD 30 percent Rooting depth 12 to 18 inches select 12 Depth of tensiometer .63*12= 8 inches after cover is 70-80 percent before then use 4 inches Tensiometer reading at irrigation using figure 1 is 20 cbar.
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