Irrigation Scheduling Part 3 - Taking in account irrigation efficiency
When water is applied to an orchard, loss of some amount of water should be expected. Growers must consider this loss of water when calculating the actual amount of water being applied. This amount will vary on the irrigation system used, soil and climatic conditions, and water management conditions.
Water applied to a field can be lost by run-off, percolation below the root zone, sprinkler spray evaporation and off-target drift. Efforts to minimize these inefficiencies should be employed, and could include longer irrigation sets at lower rates, the construction of tailwater return systems, and/or sub-surface installed systems.
Irrigation efficiency (Ea) is defined as the percentage of applied water that is held in the root zone. Mathematically, it would look as follows:
Ea = water stored/water applied.
Irrigation methods commonly used within the almond orchard include surface irrigation systems such as flood and furrow and pressurized systems that include sprinkler, drip, and micro-sprinkler. As we can see in table 1, irrigation efficiency differs with each system. With surface systems, fast movement of the water from the head end to the tail of the field end usually results in higher irrigation efficiency. Within pressurized systems, distribution depends on design parameters including spacing, nozzle type and size, riser height and operating presure. Keep in mind that maintenance and filtration affect pressurized systems more than surface based systems. Pressurized systems, however, are usually more efficient due to the reduction of run-off and water loss through deep percolation.
Table 1: Application efficiency typical of various irrigation systems.
| System | Ea (%) |
| Basin/Flood | 65 - 80 |
| Furrow | 65 - 75 |
| Solid Set Sprinkler | 75-85 |
| Microsprinkler | 85-90 |
| Drip | 90-95 |
When calculation orchard water use, irrigation efficiency must be accounted or under-irrigation will occur. Below are some examples that show the different water application needs due to difference in irrigation efficiency.
General Information:
Location: San Joaquin Valley
Soil: Loamy Sand
Rooting Depth: 5 feet
Available Water-Holding Capacity (AWC): 0.6 in/ft
Depletion of AWC before irrigation: 50%
Irrigate when AWC reaches 1.5 inches
| Flood Irrigation | Microsprinkler | Drip Irrigation |
| Ea = .70 | Ea = .85 | Ea = .90 |
| Weekly ET = 1.5 inches | Weekly ET = 1.5 inches | Weekly Et = 1.5 inches |
| Water applied: 2.14 inches |
Water applied: 1.76 inches |
Water applied: 1.67 inches |
| Difference from drip: + 0.47 inches |
Difference from drip: + 0.09 inches |
Over the course of the crop year, there may be 15 or more inches of water needed to fully irrigate a flood irrigated orchard in comparison to a drip irrigated orchard. If unaccounted, this lack of water will lead to water stress and probably crop loss.
Source: Goldhamer, David. UC Almond Production Manual: Chapter 25 Irrigation Scheduling. UC ANR Publication 3364.