5.3.7: Conveyance Efficiency

Water can also be lost in delivering the water from its origin to the irrigation system. Losses are most significant for unlined canals, field laterals, or ditch systems that convey water over long distances through permeable soils. Water can be lost due to seepage from the canal or other conduit, by evaporation from exposed water surfaces, and by evapotranspiration from phreatophytes along the conveyance system. Water can also be lost because of operational problems in moving water through complex delivery systems. If an irrigator originally requested water delivery but later decided not to take the full supply, some water might “spill” from the system. Alternatively, a few irrigators might request water, but the canal may not be able to deliver water with such small flows. Thus, excess flow would be required to supply the requested amount. The conveyance efficiency (Ec) is used to describe the ability of the delivery system to deliver the requested amount. The Ec is defined as the amount of water delivered to the irrigated area and applied divided by the total amount of water supplied or diverted from the supply (either reservoirs, rivers, or groundwater):

\(E_c = 100\% \left( \frac{d_u}{d_s} \right)\) (5.19)

where: E_c = conveyance efficiency (%),

d_a = gross depth of irrigation water applied, and

d_s = depth of water diverted from the source.

The conveyance efficiency can be reported as either a decimal fraction or a percentage.

Measuring water losses in canals and other delivery systems is difficult and expensive, and for most management purposes, the \(E_c\) can be estimated. Several efficiency terms have been used depending on where the delivery system is located. Doorenbos and Pruitt (1977) divide the efficiency of an irrigation project into three components: supply conveyance efficiency (\(E_c\)), field canal efficiency (\(E_b\)), and field application efficiency (\(E_a\)). Conveyance efficiency (\(E_c\)) and field canal efficiency are sometimes combined and called the distribution efficiency (\(E_d\)), where \(E_d = E_c \times E_b\). The combination of the field canal and application efficiencies is often called the farm efficiency (\(E_f\)), where \(E_f = E_a \times E_b\). Field application efficiency can be estimated from the methods described earlier in this section (e.g., Equation 5.11).

Factors affecting \(E_c\) include: the size of the irrigated area, type of schedule used to deliver water, types of crops, canal lining material, and the capabilities of the water supplies. The field canal conveyance efficiency is primarily affected by the method and control of operation, the type of soils, the canal transects, the length of the canal, and the size of the irrigated block and fields. The farm efficiency is very dependent on the operation of the supply system relative to the supply required on the farm. Doorenbos and Pruitt (1977) present approximate efficiencies for various conditions as summarized in Table 5.2.

A procedure used in the USDA-SCS Washington State Irrigation Guide (1985) can also be used to estimate seepage losses. The method gives a range of expected seepage losses depending on the type of material lining the delivery system and the amount of fines in the material (Figure 5.8). In addition to these guidelines, the following losses may be expected:

• Ditch side vegetation: 0.5 to 1.0% loss per mile

• Buried pipeline: 0.01 to 0.15 ft³/ft²/d depending on the age and type of pipe.

An example calculation of the season water loss from an earthen ditch follows.

Table 5.2. Conveyance, field, and distribution efficiencies for various types of systems (adapted from Doorenbos and Pruitt, 1977).

Project Characteristic Conveyance Efficiency
Continuous supply with no substantial change in flow	90 %
Rotational supply for projects with 7,000 to 15,000 ac and rotational areas of 150 to 800 ac and effective management	80 %
Rotational supply for large projects (> 25,000 ac) and small projects (< 2,500 ac) with problematic communication and less effective management:
• based on predetermined delivery schedules	70 %
• based on arranged delivery schedules	65 %
Field Size and Canal Characteristics Field Canal Efficiency
Irrigated blocks bigger than 50 ac with
• unlined canals	80%
• lined canals or pipelines	90%
Irrigated blocks smaller than 50 ac with
For rotational delivery systems with management and communication adequacies of
• adequate	65 %
• sufficient	55 %
• insufficient	40 %
• poor	30 %

Figure 5.8. Method to estimate seepage losses from irrigation delivery systems (adapted from USDA-SCS, 1985).

Example 5.6

An unlined field ditch is 1,320 ft long, transports 2.5 cfs with a flow contact area (wetted perimeter) of 2.5 ft² per ft of length for 180 d/yr. The ditch traverses through loam soil.

Find: Total conveyance loss in ac-ft/yr

Solution:

Figure 5.8 shows the seepage loss of a loam soil to be about 1.4 ft³/ft²/d

Seepage loss = \(\frac{\text{Flow Area} \times \text{Length} \times \text{Seepage Loss Rate} \times \text{Length of Irrigation}}{43,560 \text{ ft}^2/\text{ac}}\)

Seepage loss = \(\frac{(2.5 \text{ ft}^2/\text{ft})(1,320 \text{ ft})(1.4 \text{ ft}^3/\text{ft}^2/\text{d})(180 \text{ d})}{43,560 \text{ ft}^2/\text{ac}} = 19 \text{ ac-ft}\)

Assuming vegetation loss at 1% of the total flow for the period per mile, then:

Vegetative loss = \(\left[\left(\frac{1\%}{100\%}\right)(2.5 \text{ cfs})\left(\frac{1,320 \text{ ft}}{5,280 \text{ ft}}\right)\right] \times \left(\frac{1 \text{ ac-in/h}}{1 \text{ cfs}}\right) \times \left(\frac{24 \text{ h}}{1 \text{ d}}\right) = 0.15 \text{ ac-in/d}\)

0.15 ac-in/d × \(\left(\frac{1 \text{ ft}}{12 \text{ in}}\right)\) × 180 d/yr = 2.3 ac-ft

Total conveyance loss = seepage loss + vegetation loss = 19 + 2.3 = 21.3 ac-ft/yr