DROUGHT is a scary word for

Transcription

1 Irrigation DROUGHT is a scary word for farmers. Defined as a lack of precipitation over an extended period of time, drought can devastate crops. One way to offset the potential losses caused by drought is through irrigation. This unit takes a look at water management and irrigation practices. Objective: Describe water management and irrigation techniques. Key Terms: border strip irrigation center-pivot irrigation chemigation furrow irrigation ground truthing hand-move irrigation irrigation irrigation scheduling moisture sensor remote sensing ribbon test sap flow sensor soil moisture meter solid-set irrigation sprinkler irrigation subsurface irrigation surface irrigation tensiometer traveling-gun irrigation trickle or drip irrigation wheel-move irrigation wilting point Using Irrigation Irrigation is the artificial application of water to promote plant growth. It is an important part of soil moisture management in many locations. A number of irrigation benefits exist. For example, irrigation provides plants with water when adequate water would not otherwise be available for plant growth. In addition, irrigation promotes plant growth when fertilizer, growth regulators, and other materials are applied with the water. Irrigation is a way to dispose of waste water by land application, and it protects plants from extremely cold temperatures (e.g., preventing frost damage to fruit and vegetable crops). Irrigation also reduces dust from plant and field surfaces. Page 1

2 The effects of water deficiency include poor plant growth, stunted mature plants, lower crop yields, and loss of potential profit. Other adverse effects are death of plants, stress (which lowers disease and insect resistance), and loss of aesthetics due to dead plants. DETERMINING IRRIGATION NEEDS Knowing when to irrigate is important in soil moisture balance. Waiting until the plant shows signs of stress may be too late. Irrigation should be used before the wilting point is reached. The wilting point is when the plant cannot take in water as quickly as it is lost. When plants wilt, damage to production has already occurred. There are ways of determining the need for irrigation without damaging the plants. Soil moisture can be tested by a ribbon test, moisture sensor, sap flow sensor, tensiometer, soil moisture meter, remote sensing, or ground truthing. The ribbon test involves determining soil moisture content by feel. Small amounts of soil are rolled between the thumb and fingers. The feel and appearance of the soil are used to assess moisture content. Soil that is slightly sticky and holds together does not need irrigation, whereas soil that crumbles needs to be irrigated. A moisture sensor is an electronic instrument that is used to probe the soil and to provide a moisture content reading. Moisture sensors utilize a single probe. A sap flow sensor is a device that measures the movement of water (sap) inside the stem of a plant. The process is used on small herbaceous plants as well as on the branches and trunks of trees. The device clamps onto the stem of the plant. A tensiometer is a device that assesses moisture content by determining the pull of soil particles in the soil. Tensiometers are permanently placed in the soil and can be damaged by freezing weather. A soil moisture meter is a device that assesses moisture based on the flow of low-level electric current between its two probes. FIGURE 1. Leaves on this plant have wilted due to low soil moisture. FIGURE 2. A soil tensiometer is on the right and a soil moisture meter is on the left. Page 2

3 Remote sensing on soil moisture is collecting information without actually going into the field. Most remote sensing is done with satellites and airplanes. Ground truthing is used to verify the accuracy of remote sensing information. It involves actual field investigation. SCHEDULING IRRIGATION Experienced irrigators have developed their own procedures for scheduling applications. Irrigation scheduling is providing the right amount of water at the right time. Water should be available when the plant needs it. The greatest need for water for most crops is usually during the middle of the growing season. By using knowledge of scheduling practices, the most efficient use of water can be achieved. Scarce water supplies are more efficiently used through good scheduling. Water supplies should be increased before the peak need. Good scheduling prevents over irrigating and supplies water to the crop just before the time of highest demand. Always assess moisture in the soil before applying water. Most crops are not watered each day. Water needs are calculated, and the amount of water is applied to meet the need for a specific period. A few specialty crops have daily irrigation, which depends on the irrigation system used. Scheduling the use of water may involve gaining an allocation and time with the local water management district. IRRIGATION METHODS Irrigation has a long history in world agriculture, and irrigation water can be applied in a number of ways. Subsurface irrigation is watering from below. This type of irrigation uses capillary rise from a zone of saturated soil that is lower in the soil profile. The zone must be high enough that water can rise into the root zone, but not so high that it saturates the root zone. Water may be introduced into the soil profile through open ditches, mole drains, or pipe drains. Surface irrigation of fields involves flooding the soil surface with water released from canals or piping systems. Surface irrigation is most suitable for level or slightly sloping land of moderate permeability. When preparing land, fields are carefully leveled to the slight slope needed for water to flood the land. A system of canals uses gravity to carry water to the farm and among the fields. Border strip irrigation involves covering the entire soil surface of a field with a sheet of water. Each field is divided into smaller parts by the use of low dikes. Each of these sections is FIGURE 3. Furrow irrigation is best suited for row crops. Page 3

4 flooded, in turn, from a ditch or pipe running along the head of the field. Because of the large surface area of the water flooding the ground, evaporation causes some waste of water. Furrow irrigation distributes water through furrows, and the crops are planted in the ridge between two furrows. Furrows are best suited for row crops. Evaporation is less of a problem than in border strips because less surface area is exposed to the air. Sprinkler irrigation systems pump water under pressure through pipes to sprinklers that spray water out in a circular pattern. Sprinklers can be used where the soil is too permeable or too impermeable; they also can be used where the ground is not level. Sprinkler irrigation equipment can be used for other purposes in addition to watering crops. Chemigation is applying chemicals like fertilizers or herbicides while irrigating. It is used as a substitute for rainfall, for the activation of herbicides, or for frost control. Hand-move irrigation is the least expensive sprinkler system to install. However, this FIGURE 4. A sprinkler irrigation system watering a potato field. system is very labor intensive and consists of a lightweight aluminum pipe that can be moved from place to place by a single person. Solid-set irrigation uses the same equipment as hand-move irrigation, except that an entire field is set up at planting. The large number of pipes needed to supply all of the fields increases the cost of the additional initial equipment purchase, but it almost eliminates additional labor during the growing season since the pipes remain in place until harvest. Traveling-gun irrigation uses one large sprinkler mounted on a trailer that moves across a field. The sprinkler sends out a single large stream of water and also can be used to spray liquid manure and other slurries. Unfortunately, the gun is very liable to wind problems. Center-pivot irrigation has a central pivot point with the watering line elevated above the crop. As the system operates, the line slowly turns around the pivot point. This has the lowest labor requirement of any irrigation method. Wheel-move irrigation consists of a line of sprinklers mounted on wheels at both ends. The line of sprinklers slowly rolls down the field FIGURE 5. Center-pivot irrigation system. Page 4

5 until it reaches the end of the hose. The pattern of moisture that is distributed is rectangular and irrigates all parts of the field. Trickle or drip irrigation involves the use of plastic pipes on the ground running down a crop row with special emitters spaced along the pipe. The emitters drip water, at controlled rates, onto the soil surface near the plants. The system operates at low water volume and pressure. Problems occur with plugging of the emitters and variation in flow rates between emitters. EFFICIENT USE OF WATER FIGURE 6. Wheel-move irrigation system. Irrigation water is valuable and is becoming more valuable with the increasing demands on water resources. It should be used properly and not wasted. Using water more efficiently results in greater production. Some recommended practices follow. Use sprinkler irrigation during the cooler part of the day and when the wind is not blowing. More water is lost to evaporation in heat and when the wind is blowing. Monitor moisture in the root zone. Irrigate enough to have good root zone moisture. Stop adding water when the water has penetrated to the root zone. Adding more water will result in loss by percolation of the water outside the root zone. Keep the irrigation system in good condition. Leaks in pipes, canals, and other water structures should be prevented. Be sure all connections fit properly to avoid leaks. Apply water uniformly so that all areas of a field receive the appropriate amount. Depending on the irrigation method used, land forming may be needed to efficiently use the water. High places may not be irrigated adequately and low places may receive too much. Water needs vary within a field, and adjustments should be made accordingly in the rate of application. Apply only the amount of water that can be used. Avoid over irrigation where excess water runs from the field into nearby creeks. In addition to water waste, excess water from over irrigation may carry nutrients, sediment, or pesticides from the field. Summary: Irrigation is the artificial application of water to promote plant growth. Knowing when to irrigate is important in soil moisture balance. For example, irrigation should be used before the wilting point is reached. The wilting point is when the plant cannot take in water as quickly as it is lost. Soil moisture can be tested by a Page 5

6 ribbon test, moisture sensor, sap flow sensor, tensiometer, soil moisture meter, remote sensing, and ground truthing. Irrigation scheduling is providing the right amount of water at the right time because water should be available when the plant needs it. Some ways in which irrigation water can be applied are subsurface irrigation, surface irrigation, border strip irrigation, furrow irrigation, sprinkler irrigation, chemigation, hand-move irrigation, solid-set irrigation, traveling-gun irrigation, center-pivot irrigation, wheel-move irrigation, and trickle or drip irrigation. Irrigation water is valuable and is becoming more valuable with the increasing demands on water resources. As a result, water should be used properly and not wasted. Checking Your Knowledge: 1. What is irrigation? 2. How can soil moisture levels be tested? 3. What is irrigation scheduling? 4. What systems are used in irrigation? 5. What are some practices that promote the efficient use of water? Expanding Your Knowledge: Determine the soil moisture levels of different soil samples by using the ribbon test. Use a soil corer to extract soil from below the surface to test the moisture level. Practice judging when irrigation is needed to provide plants with the optimum amount of water. Web Links: Irrigation Irrigation Techniques Irrigation Water Use Water Efficiency Practices for Agricultural Irrigation Agricultural Career Profiles Page 6