Types of irrigation efficiencies

Irrigation efficiency is the ratio of the water beneficially used to the water supplied at a stage, expressed as a percentage. Types are as follows.

1. Water conveyance efficiency ($\eta_c$): the ratio of water delivered to the field (outlet) to water diverted at the canal head. It accounts for seepage and evaporation losses in canals.

$$\eta_c = \frac{W_f}{W_r}\times 100$$

2. Water application efficiency ($\eta_a$): the ratio of water stored in the root zone to water delivered to the field.

$$\eta_a = \frac{W_s}{W_f}\times 100$$

3. Water storage efficiency ($\eta_s$): the ratio of water stored in the root zone to the water needed to bring the root zone to field capacity.

$$\eta_s = \frac{W_s}{W_n}\times 100$$

4. Water distribution (uniformity) efficiency ($\eta_d$): how uniformly water is spread over the field.

$$\eta_d = \left(1-\frac{y}{d}\right)\times 100$$

where $d$ is the mean depth stored and $y$ the average numerical deviation from it. 5. Water use efficiency: crop yield per unit volume of water used (kg/m$^3$). 6. Consumptive use efficiency: consumptive use of crop water divided by water depleted from the root zone. 7. Overall (project) efficiency: $\eta_{o}=\eta_c\times\eta_a$ (also including reservoir/ distribution efficiencies); typically 25-40% in Nepal's surface systems.