Irrigation made easy part 7:
Scheduling according to weather conditions I
f a farmer has access to a local weather station, or weather station data, calculated Penman-Montieth ETo (Evapotranspiration) values can be obtained. Penman figures are calculated estimates of the quantity of water that would be used by short grass. The values are calculated from data on temperature, wind run, evaporation and sunlight hours. Actual crop water usage for any crop will be directly proportional to Penman values. Example: Cabbage need 75% of the Penman ETo from day 14 to day 70. Irrigation can then be adjusted to supply this calculated water requirement only. Scheduling using tensiometers A tensiometer directly measures how hard plant roots suck to try and remove water from a sealed glass tube. The tensiometer comprises of a glass tube that is filled with water, a pressure gauge that measures suction (negative) pressure, a porous ceramic block and a small reservoir of water. The meter is placed in the field with the ceramic block in the centre of the root zone. A second tensiometer or wetting front detector can be installed below the root depth to indicate if any over-irrigation has occurred.
Tensiometer
Reservoir of water Pressure gauge Perspiglass tube Ceramic block
Soil, water and air
Placement of a tensiometer. The water reservoir must be filled on a regular basis, and the cap sealed. The tube section of the tensiometer must always be filled with water. Suction pressure is read off the gauge provided and provides a direct measurement of how hard roots around the ceramic block are sucking
Image: grofit-ag.com
to draw up water from the soil. The plant will always first use water that it can suck up easiest. If the soil profile is full, the plant will not have to work hard to get water and can concentrate energy on growth and crop production. If the soil is dry and the plant must suck hard to get water, it will start to wilt and may die. If the gauge reads (-) 50 to 60 kPa, then approximately 50% of the soil water has been used by the plant. In other words, the roots belong to a well organised trade union. If they have to work very much harder to get water, they will suck water at a slower rate and the plant will stop growing.
will be irrigated on that day is read. The duration of irrigation for the day is read off a scheduling chart that was prepared prior to planting of the crop. (Two examples are shown, for tensiometer readings of 30 and 50 kPa). Note: A separate scheduling chart is needed for each crop and soil type. It can clearly be seen that if the tensiometer reads 30 kPa, the farmer should irrigate for a period of 4 hours. Similarly, if a reading of 50 kPa is obtained, the stand time should be increased to 9 hours. Longer periods of irrigation will waste water and energy, and fertilisers will tend to be washed away below the root zone.
Next month we shall look at the maintenance of the tensiometer. This series is published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc. agric.za for more information.
Consider the following example: Maize is planted in a deep, well-drained soil that holds 120 mm/m water. The roots will extend to a depth of 1 m. A sprinkler irrigation system is designed to work on a 7-day cycle (see Pictures 2 and 3). Sprinklers deliver water at an effective 6 mm per hour. A tensiometer has been installed at each stand position of the first sprinkler in the block. Each day the tensiometer that represents the area that Picture 3: A tensiometer in a maize field test.
ProAgri Botswana / Namibia / Zimbabwe 14
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