Basic Irrigation Operation
To irrigate successfully, the farmer needs to be concerned with water distribution uniformity and irrigation efficiency. Since the cost of energy used to run irrigation pumps is one of the highest single expenses in agriculture, the farmer tries to irrigate the land efficiently in order to save money. This chapter helps to understand how this is best achieved.
Achieving an effective and efficient irrigation
Irrigation is both an art and a science. Research has provided many concepts and methods for measuring the various processes involved in irrigation. However, knowledge of the field and crop, along with the grower’s experience in interpreting this science, will remain of utmost importance in achieving effective, efficient irrigations.
Irrigation efficiency does no good if it is not effective in producing a profitable crop. Effective, efficient irrigation is a result of
knowing when to irrigate, how much to irrigate, and how to irrigate.
• When – an agronomic decision based on how you want to manipulate your crop.
•How much – – depends on the soil moisture depletion in the effective root zone. This is the amount of water needed to take the soil moisture reservoir back to field capacity or other desired level.
•How – this is not just about knowing how to set a siphon, or connect a sprinkler pump. It is also knowing how to apply water evenly to a field while controlling the total amount applied.
An effective, efficient irrigation produces a profitable crop while making the best use of available water supplies and creating a minimal impact on water quality. In doing so it must also minimise energy use and save money.
Distribution Uniformity and Irrigation Efficiency
There are two measures of irrigation performance – distribution uniformity and irrigation efficiency.
Distribution Uniformity (DU) is a measure of how evenly water soaks into the ground across a field during the irrigation.
If eight inches of water soaks into the ground in one part of the field and only four inches into another part of the field, that is poor distribution uniformity. Distribution uniformity is expressed as a percentage between 0 and 100%. Although 100% DU (the same amount of water soaking in throughout the field) is theoretically possible, it is virtually impossible to attain in actual practice.
Irrigation Efficiency (IE) is the ratio of the volume of irrigation water which is beneficially used to the volume of irrigation water applied.
Beneficial uses may include crop evapotranspiration, deep percolation needed for leaching for salt control, crop cooling, and as an aid in certain cultural operations. Differences in specific mathematical definitions of IE are due primarily to the physical boundaries of the measurement (a farm, an irrigation district,
an irrigation project, or a watershed) and whether it is for an individual irrigation or an entire season.
Irrigation efficiency is also expressed as a percentage between 0 and 100%. An IE of 100% is not theoretically attainable due to immediate evaporation losses during irrigations. However, it could easily be close to 95% IE if a crop is under-watered. In this case, assuming no deep percolation, all water applied and not immediately evaporated would be used by the crop.
The term irrigation efficiency should not be confused with the term water use efficiency (WUE). WUE is generally a measure of yield per unit water applied.
Relationships Between DU and IE
There are two important relationships between DU and IE: deep percolation and under-irrigation. The illustrations below show a profile view of two adjacent sprinklers in a field and the root zone under them. The horizontal, dashed line in the figures depicts the depth of the actual soil water depletion at irrigation.
This is the amount of water that the grower would be trying to soak into the soil to satisfy crop water use requirements. The blue shading depicts the actual depth of water infiltrated during the irrigation.
Deep percolation is indicated whenever the actual depth of irrigation (blue water level) is below the soil water depletion line (the horizontal, dashed line). Conversely, under-irrigation is indicated whenever the actual depth of irrigation is above the soil water deficit line.
These two illustrations demonstrate the first relationship, that there must be good distribution uniformity before there can be good irrigation efficiency if the crop is to be sufficiently watered.
In the illustration to the left, the farmer has irrigated to sufficiently water the entire field. The poor DU, indicated by the uneven blue water level, has resulted in excessive deep percolation, meaning that much more water is infiltrated between the sprinklers than next to the sprinklers. It is important that leaching must be uniform across the field over a number of years to prevent areas of excessive salt accumulation.
To the right, the farmer has acted to prevent excessive deep percolation by shortening set times. Now part of the field remains under-irrigated. Under-irrigation usually results in high irrigation efficiency because most water applied is stored in the root zone, available for plant use. However, under-irrigation is usually not an effective way of growing since the resulting water stress on the crop in some parts of the field will usually decrease yields. Also, there is a need for some deep percolation for leaching to maintain a salt balance.
Key concepts for planning an irrigation
Please see the Glossary for further definitions.
Application rate (AR): the equivalent depth of water applied to a given area per hour by the system, usually measured in mm/hour.
Daily crop water use (evapotranspiration – ET): : the net amount of water extracted from the soil daily by the crop and surface evaporation from the soil.
Distribution uniformity (DU): a measure of how evenly water is applied across the field during irrigation.
Effective root zone: the depth of soil in which you are actively managing the crop.
Field capacity: the maximum amount of water the soil will hold.
Frequency: how often you irrigate: high frequency vs. low frequency.
Irrigation efficiency (IE): a measure of how much water that is pumped and applied to the field is beneficially used.Net water needed versus gross water applied: net water is what you need to replace in the field. Gross water is how much you have to pump in order to accomplish this goal.
Soil moisture depletions (SMD): the net amount of water that you need to replace in the root zone of the crop.
Soil probe: a long piece of 9.525 mm steel bar, usually tipped by a ball bearing, with a handle. The probe is pressed into wetted soil to judge how deep water has penetrated. It can be used during an irrigation to indicate when enough water has soaked into the ground. It can also be used to judge the uniformity of irrigation. If 2-3 days after an irrigation the probe can be pushed into the soil to a depth of 1.2 meters at the top of a furrow, and only to 0.61 meters at
the bottom of the same furrow, this is an indication of poor distribution uniformity.
These two illustrations demonstrate the second relationship, that good distribution uniformity is no guarantee of good
irrigation efficiency.
The illustration to the left depicts a good irrigation. There was a high DU as indicated by the flatter blue water level. Approximately the right amount of water was applied. There is little deep percolation (enough for salt control) and the entire field is wetted sufficiently. It is assumed that surface runoff was minimal or collected for reuse.
To the right an irrigation is shown with the same high DU as in the first illustration. However, twice as much water as needed was applied, resulting in low irrigation efficiency. A practical example of this situation is the farmer who is using a welldesigned and maintained micro-irrigation system. The hardware provides good DU and the potential for high IE. However, if the farmer runs the system twice as long as necessary, that potential is not realised.
Irrigation scheduling
Wherever data is being collected or provided in the field, the evapotranspiration rate at that particular piece of ground can be determined, meaning the amount of water needed to replenish at the root zone can be calculated. This data is obtained from weather stations in the field, from weather data providers, soil moisture sensors installed in fields for soil moisture levels at different depths, pH monitoring, soil temperature, solar radiation, information for insect and fungus control (humidity levels), and rainfall, all based on the crop being grown.
This information is available to farmers today, and the farmers can then define when and how much to irrigate. We are increasingly seeing the automation of this process and/or the collection of the data to be reviewed by the irrigator or farmer. The final decision always remains with the farmer and very
few, if any, systems are fully automated due to the high cost of failure. Smart controllers are still new to the irrigation industry. Their use will increase as farmers learn how to use them as a tool, as is the case in the landscape and turf industry.
The landscape and turf industry has less of a financial loss if the plants and turf are not irrigated properly, whereas the farmer has a much greater chance of loss if crops are not properly irrigated.
Where the irrigation scheduling of microirrigation on crops such as strawberries on soils with very little water holding capacity, such as sandy soils, it may be necessary for the farmer to calculate the crop ET hourly.
Scheduling irrigation is not only dependent on the above. Power is generally less expensive off-peak, meaning that the irrigation system and pumping system may be designed and built to deliver the desired water during off peak hours. Flow may be higher than necessary for the 12 hours (for example) that power costs less.Irrigation scheduling must be done to apply water in the limited time frame to meet the least energy costs – as is already the case in municipal water supply and wastewater handling – and this is now becoming more commonplace for farmers.
Achieving an effective and efficient irrigation
Irrigation is both an art and a science. Research has provided many concepts and methods for measuring the various processes involved in irrigation. However, knowledge of the field and crop, along with the grower’s experience in interpreting this science, will remain of utmost importance in achieving effective, efficient irrigations.
Irrigation efficiency does no good if it is not effective in producing a profitable crop. Effective, efficient irrigation is a result of
knowing when to irrigate, how much to irrigate, and how to irrigate.
• When – an agronomic decision based on how you want to manipulate your crop.
•How much – – depends on the soil moisture depletion in the effective root zone. This is the amount of water needed to take the soil moisture reservoir back to field capacity or other desired level.
•How – this is not just about knowing how to set a siphon, or connect a sprinkler pump. It is also knowing how to apply water evenly to a field while controlling the total amount applied.
An effective, efficient irrigation produces a profitable crop while making the best use of available water supplies and creating a minimal impact on water quality. In doing so it must also minimise energy use and save money.
Distribution Uniformity and Irrigation Efficiency
There are two measures of irrigation performance – distribution uniformity and irrigation efficiency.
Distribution Uniformity (DU) is a measure of how evenly water soaks into the ground across a field during the irrigation.
If eight inches of water soaks into the ground in one part of the field and only four inches into another part of the field, that is poor distribution uniformity. Distribution uniformity is expressed as a percentage between 0 and 100%. Although 100% DU (the same amount of water soaking in throughout the field) is theoretically possible, it is virtually impossible to attain in actual practice.
Irrigation Efficiency (IE) is the ratio of the volume of irrigation water which is beneficially used to the volume of irrigation water applied.
Beneficial uses may include crop evapotranspiration, deep percolation needed for leaching for salt control, crop cooling, and as an aid in certain cultural operations. Differences in specific mathematical definitions of IE are due primarily to the physical boundaries of the measurement (a farm, an irrigation district,
an irrigation project, or a watershed) and whether it is for an individual irrigation or an entire season.
Irrigation efficiency is also expressed as a percentage between 0 and 100%. An IE of 100% is not theoretically attainable due to immediate evaporation losses during irrigations. However, it could easily be close to 95% IE if a crop is under-watered. In this case, assuming no deep percolation, all water applied and not immediately evaporated would be used by the crop.
The term irrigation efficiency should not be confused with the term water use efficiency (WUE). WUE is generally a measure of yield per unit water applied.
Relationships Between DU and IE
There are two important relationships between DU and IE: deep percolation and under-irrigation. The illustrations below show a profile view of two adjacent sprinklers in a field and the root zone under them. The horizontal, dashed line in the figures depicts the depth of the actual soil water depletion at irrigation.
This is the amount of water that the grower would be trying to soak into the soil to satisfy crop water use requirements. The blue shading depicts the actual depth of water infiltrated during the irrigation.
Deep percolation is indicated whenever the actual depth of irrigation (blue water level) is below the soil water depletion line (the horizontal, dashed line). Conversely, under-irrigation is indicated whenever the actual depth of irrigation is above the soil water deficit line.
These two illustrations demonstrate the first relationship, that there must be good distribution uniformity before there can be good irrigation efficiency if the crop is to be sufficiently watered.
In the illustration to the left, the farmer has irrigated to sufficiently water the entire field. The poor DU, indicated by the uneven blue water level, has resulted in excessive deep percolation, meaning that much more water is infiltrated between the sprinklers than next to the sprinklers. It is important that leaching must be uniform across the field over a number of years to prevent areas of excessive salt accumulation.
To the right, the farmer has acted to prevent excessive deep percolation by shortening set times. Now part of the field remains under-irrigated. Under-irrigation usually results in high irrigation efficiency because most water applied is stored in the root zone, available for plant use. However, under-irrigation is usually not an effective way of growing since the resulting water stress on the crop in some parts of the field will usually decrease yields. Also, there is a need for some deep percolation for leaching to maintain a salt balance.
Please see the Glossary for further definitions.
Application rate (AR): the equivalent depth of water applied to a given area per hour by the system, usually measured in mm/hour.
Daily crop water use (evapotranspiration – ET): : the net amount of water extracted from the soil daily by the crop and surface evaporation from the soil.
Distribution uniformity (DU): a measure of how evenly water is applied across the field during irrigation.
Effective root zone: the depth of soil in which you are actively managing the crop.
Field capacity: the maximum amount of water the soil will hold.
Frequency: how often you irrigate: high frequency vs. low frequency.
Irrigation efficiency (IE): a measure of how much water that is pumped and applied to the field is beneficially used.Net water needed versus gross water applied: net water is what you need to replace in the field. Gross water is how much you have to pump in order to accomplish this goal.
Soil moisture depletions (SMD): the net amount of water that you need to replace in the root zone of the crop.
Soil probe: a long piece of 9.525 mm steel bar, usually tipped by a ball bearing, with a handle. The probe is pressed into wetted soil to judge how deep water has penetrated. It can be used during an irrigation to indicate when enough water has soaked into the ground. It can also be used to judge the uniformity of irrigation. If 2-3 days after an irrigation the probe can be pushed into the soil to a depth of 1.2 meters at the top of a furrow, and only to 0.61 meters at
the bottom of the same furrow, this is an indication of poor distribution uniformity.
These two illustrations demonstrate the second relationship, that good distribution uniformity is no guarantee of good
irrigation efficiency.
The illustration to the left depicts a good irrigation. There was a high DU as indicated by the flatter blue water level. Approximately the right amount of water was applied. There is little deep percolation (enough for salt control) and the entire field is wetted sufficiently. It is assumed that surface runoff was minimal or collected for reuse.
To the right an irrigation is shown with the same high DU as in the first illustration. However, twice as much water as needed was applied, resulting in low irrigation efficiency. A practical example of this situation is the farmer who is using a welldesigned and maintained micro-irrigation system. The hardware provides good DU and the potential for high IE. However, if the farmer runs the system twice as long as necessary, that potential is not realised.
Improved irrigation system hardware or management may result in higher distribution uniformity and improve the potential for higher irrigation efficiency. It then follows that distribution uniformity is the first concern when improving irrigation system performance. However, actually achieving high irrigation efficiency ultimately depends on two factors - knowing how much water is needed and controlling the amount of water applied to match that need.
Irrigation scheduling
Wherever data is being collected or provided in the field, the evapotranspiration rate at that particular piece of ground can be determined, meaning the amount of water needed to replenish at the root zone can be calculated. This data is obtained from weather stations in the field, from weather data providers, soil moisture sensors installed in fields for soil moisture levels at different depths, pH monitoring, soil temperature, solar radiation, information for insect and fungus control (humidity levels), and rainfall, all based on the crop being grown.
This information is available to farmers today, and the farmers can then define when and how much to irrigate. We are increasingly seeing the automation of this process and/or the collection of the data to be reviewed by the irrigator or farmer. The final decision always remains with the farmer and very
few, if any, systems are fully automated due to the high cost of failure. Smart controllers are still new to the irrigation industry. Their use will increase as farmers learn how to use them as a tool, as is the case in the landscape and turf industry.
The landscape and turf industry has less of a financial loss if the plants and turf are not irrigated properly, whereas the farmer has a much greater chance of loss if crops are not properly irrigated.
Where the irrigation scheduling of microirrigation on crops such as strawberries on soils with very little water holding capacity, such as sandy soils, it may be necessary for the farmer to calculate the crop ET hourly.
Scheduling irrigation is not only dependent on the above. Power is generally less expensive off-peak, meaning that the irrigation system and pumping system may be designed and built to deliver the desired water during off peak hours. Flow may be higher than necessary for the 12 hours (for example) that power costs less.Irrigation scheduling must be done to apply water in the limited time frame to meet the least energy costs – as is already the case in municipal water supply and wastewater handling – and this is now becoming more commonplace for farmers.
