2.4 Selection and Evaluation of Performance Indicators
The performance of irrigation schemes was evaluated based on various indicators. Various internal and external performance indicators were mentioned by Molden et al., (1998) to assess the performance of irrigation schemes. From the internal indicators storage, application, and conveyance efficiency were selected to evaluate the scheme performance (De Alwis and Wijesekera, 2011; Dessalew et al., 2016; Kartal et al., 2020). Similarly, agricultural, water supply, physical and financial performance indicators were selected from the external indicators based on Değirmenci et al., (2017), Agide Dejen (2012), Bumbudsanpharoke, and Prajamwong, (2015), and Al-Said et al., (2012).
Conveyance Efficiency (Ec) refers to the volume of water lost by the network of conveyance canals due to seepage and evaporation (Kassa and Ayana, 2019) and is determined as shown in equation 1.
\({E}_{c}={(Q}_{out}/{Q}_{in})\text{*}100\) (Eq. 1)
Where, Ec, Qin, and Qout are conveyance efficiency (%), inflow, and outflow discharge (m3/sec) in specified canal reach measured 3 times a month during the cropping season. The inflow and outflow discharge through the canal was determined as a product of the cross-sectional area with the average flow velocity. The current meter was used to determine the average flow velocity by counting the number of rotations of the propeller shaft as explained in Alebachew and Ing, (2018) using equation 2.
V = k*n + ⧍ (Eq. 2)
Where, n is the number of propeller rotations (i.e., revolution per second), k, and ∆ are constant values determined based on the number of propeller rotations (Table 2).
Table 2
Average velocity determination using a current meter (Alebachew and Ing, 2018)
S.no | Number of propeller rotation range (n) | Average Velocity (cm/sec) | Coefficients (constant values) |
k | ⧍ |
1 | [0,1.98] | 31.17n + 1.93 | 31.17 | 1.93 |
2 | [1.98,10.27] | 32.05n + 0.19 | 32.05 | 0.19 |
3 | [10.27,15] | 33.44n – 14.09 | 33.44 | -14.09 |
Note: n is the number of propeller rotations. k and ⧍ are constant values estimated based on the number of propeller rotation |
Application Efficiency (Ea) refers to the ratio of the average depth of irrigation water infiltrated and stored in the plant root zone to the average depth of irrigation water applied (Amer and Amer, 2010; van Halsema et al., 2011) and expressed as a percentage as shown in equation 3.
\({E}_{a}={(W}_{s}/{W}_{f})\text{*}100\) (Eq. 3)
Where, Ea, Ws, and Wf are application efficiency (%), the average depth of water stored in the plant root zone (mm), and average water depth delivered to the field (mm), respectively. The depth of water delivered to the field was recorded using a Parshall flume (Xiao et al., 2015) installed at the entrance of each field during irrigation. Soil moisture content before and after irrigation was determined by the gravimetric method (Fontenelli et al., 2017) to know the depth of water stored in the root zone by collecting soil samples from 0 to 60cm depth with 20cm depth increment.
Storage Efficiency (Es) relates the amount of water required by the plant with the amount of water stored in the root zone (Amer and Amer, 2010; van Halsema et al., 2011) and can be determined as shown in equation 4.
\({E}_{S}=({W}_{s}/{W}_{n})\text{*}100\) (Eq. 4)
Where, Es, Ws, and Wn are storage efficiency (%), water stored in the root zone during irrigation, and water needed in the root zone before irrigation (mm). The amount of water needed by the crop before irrigation depends on the type of soil and rooting depth (da Silva et al., 2017). It was estimated from the soil field capacity and permanent wilting point (Jabro et al., 2020) using equation 5.
\(TAW=100\text{*}\left({\text{ɵ}}_{Fc}-{\text{ɵ}}_{PWP}\right){\text{*}D}_{r}\) (Eq. 5)
Where, TAW, ɵFC, ɵPWP and Dr are volumetric total available water in the root zone (mm), volumetric moisture content at field capacity (m3/m3), volumetric moisture content at permanent wilting point (m3/m3), and plant root depth (m), respectively. According to Muluneh et al., (2017) the amount of water needed in the root zone before irrigation was taken as 75% of the total available water with a promising result of maximum crop productivity and used to determine the storage efficiency. The overall efficiency was computed as a product of conveyance and application efficiency to measure the performance of the irrigation schemes.
Agricultural performance indicators relate the value of production in dollar with the irrigated cropped area (ha), command area (ha), diverted and consumed irrigation water supply (m3) as an output of the scheme (Değirmenci et al., 2017; Sener and Albut, 2011) and is determined by Equation 6.
\(\left\{\begin{array}{c}Output=(Production Value/Irrigated area)\\ (Production Value/Command area)\\ (Production value/Irrigation supply)\\ (Production value/Water consumption)\end{array}\right\}\) (Eq. 6)
Where, the production value is the revenue in ($) collected from crop production, irrigated area (ha) is the sum of areas covered by crops, command area (ha) is the design area to be irrigated, irrigation supply (m3) is the volume of irrigation water diverted to the command area and water consumption is the amount of irrigation water consumed by the crops and evapotranspiration.
Water supply indicators. This indicator related the demand of crops with the amount of water supply to the field (Agide Dejen, 2012) according to equation 7. Two indicators were used to assess the performance of water supply namely; relative irrigation supply (RIS) and relative water supply (RWS).
\(\left\{\begin{array}{c}RWS=Total water supply/Crop water requirement\\ RIS=Irrigation supply/Irrigation requirement\end{array}\right\}\) (Eq. 7)
Where RWS and RIS are relative water supply and relative irrigation supply respectively. Total water supply is calculated as the volume of water diverted for irrigation plus rainfall (m3).
Physical performance indicators evaluate the status of the command area coverage by irrigation. The irrigation ratio was selected (Bumbudsanpharoke and Prajamwong, 2015; Degirmenci et al., 2006) and determined using equation 8.
\(IR=Irrigated area/Command area\) (Eq. 8)
Where IR is irrigation ratio, irrigated area is an area (ha) covered by irrigation, and command area is mean to say the estimated total irrigable area (ha) during the design stage of the schemes.
Financial performance indicators compare the value of production from the irrigation schemes with the total cost of infrastructure (Al-Said et al., 2012) and were determined using equation 9.
\(GRI=Production value/Infrastructure cost\) (Eq. 9)
Where GRI is the gross return on investment.