5. THE EFFECT OF IRRIGATION MANAGEMENT SYSTEMS AND TECHNOLOGIES ON
5.5. Analysis
5.5.1. Environmental impacts of irrigation
5.5.1.2. Normalized difference vegetation index (NDVI) Analysis
The maintenance of biodiversity and sustainability of natural resources are important components of poverty alleviation and sustainable development. In highly degraded areas of Ethiopia, improvement in vegetation is perceived to be significant. This section investigates the change in vegetation cover of irrigated plots by comparing the value of NDVI before farmers started to use irrigation water and afterwards. Table 5.2-5.8 and
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Figure 5.1 provide information on the normalized difference vegetation index (NDVI) before farmers started to irrigate and after, across various agro-ecological zones, irrigation water management systems, technologies, and years they have experienced irrigated agriculture.
As depicted in the Tables 5. 2, the overall trend observed in all irrigation sites included in the four regions of the country is that vegetation cover has been increasing since farmers started to use irrigation water. The NDVI analysis of the images of the irrigated farm sites included in the study reveals that there was less vegetation biomass on average, 0.05, in the study areas before irrigation started compared to the later years (0.22). This result is in line with the findings from FGD wtih irrigation beneficiaries which is discussed in section 5.5.1.1.
This result is not consistent with studies on large-scale irrigation projects in Ethiopia.
Studies by Amdihun (2008) and Ruffeis et al. (2008) show that the NDVI results around the Irrigation Project in Finchaa Valley Area show declining vegetation biomass, because of large scale deforestation caused by expansion of agricultural lands and growing settlements. As depicted from Table 5. 2, different scales of irrigation are included in the study. Large-scale irrigation schemes such as Kobo valley pressurized irrigation project, Koga irrigation and watershed management and Tibila irrigation-based integrated development project are considered in the analysis. Our evidence shows that there has been improvement in vegetation cover in those large-scale irrigation schemes, even if there was clearance of land due to construction of the irrigation infrastructures in the areas.
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Table 5. 2. The Normalized Vegetation index difference across salient feature of irrigation systems included in the study
Source: Author’s computation from own survey and images acquires form NASA/US geological survey earth observation satellites
Note: Standard errors in parenthesis
Figure 5. 1 presents the spatial and temporal change of NDVI of irrigation systems included in this study across different agro-ecological zones of the country. Irrigation plots in humid moisture-reliable lowlands have registered the highest improvement in vegetation biomass among irrigation systems included in the study that is 0.25, followed by moisture-reliable highland (Enset) areas (0.22). The improvement in the moisture reliable lowlands is mainly due to the establishment of Tibila Irrigation-based Integrated Development Project in the Arsi zone of Oromia region. The results from FGDs with irrigators who live around the irrigation project indicate that since the establishment of the scheme, which was diverted from Awash river, the micro-climate of the surrounding area has improved and the intensity and number of rainy days in Meher has been also increasing. According to KII with the project administrative staffs, out of the total gross
40NDVI before irrigation refers to the value of NDVI on the year before they started to irrigate which differs plot to plot.
Region Woreda
Scale of irriga-tion
No. of household
No. of irrigated plots
NDVI before irrigation40
NDVI in 2016
NDVI
difference
Tigray Atsebi Wemberta
Small,
micro 51 66 0.031
(0.009)
0.124 (0.003)
0.094***
(0.009)
Raya Alamata
Small,
micro 49 72 0.027
(0.013)
0.140 (0.0072)
0.113***
(0.014) Amhara Raya
Kobo
large, Small, micro
38 78
-0.036 (0.007)
0.139 (0.0054)
0.175***
(0.006)
Raya town
Large, small, micro
27 41
-0.057 (0.008)
0.120 (0.006)
0.178***
(0.010)
Mecha
Large, small, micro
66 170
-0.029 (0.01)
0.132 (0.011)
0.161***
(0.016) Oromia Illu Small,
micro 60 130 -0.045
(0.008)
0.116 (0.004)
0.162***
(0.008)
Wonchi
Medium, Small, micro
50 86
0.097 (0.016)
0.174 (0.005)
0.077***
(0.016)
Sire Large 12 37 0.046
(0.017)
0.298 (0.018)
0.252***
(0.020)
Jeju Large 8 17 -0.022
(0.012)
0.217 (0.017)
0.239***
(0.024) SNNPR Wondo
Genet
Small,
Micro 103 294 0.182
(0.008)
0.398 (0.003)
0.215***
(0.008)
4 10
464 1037 0.05
(0.005)
0.219 (0.005)
0.169***
(0.005)
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command area of 7,000 ha, 2,500 ha irrigated land has been developed so far, engaging around 5,000 households and total of 17,351 household members. The vegetation biomass improvement in the other agro-ecological zones has also been remarkable. For instance, average plots in cereal producing moisture-reliable highlands (0.0027), drought-prone lowlands (-0.0001), and drought-drought-prone highlands (-0.0338), had negative and close to zero NDVI values before they started using irrigation, which corresponds to barren areas of rock and sand. However, the NDVI result in 2015/16 shows a complete improvement of greenness representing some shrubs and grasslands, at 0.134. 0.14, 0.13, respectively.
Figure 5. 1. Mean spatial and temporal change NDVI analysis of irrigation systems included in the study across different agro-ecological zones of the country.
Source: Author’s computation from own survey and images acquires form NASA/US geological survey earth observation satellites
Table 5.3 provides information on changes in vegetation biomass of irrigation systems for various combinations of irrigation technologies and management systems since the year of their establishment. On average, the NDVI analysis results reveal that the general trend observed for plots in all types of irrigation water management systems has been increasing vegetation cover since irrigation commenced. Previously, it was hypothesized that, due to overexploitation (high discount rate) of the resource base, privately and open access irrigation schemes may be subject to environmental degradation due to lack of governance and collective management of the irrigation system. Surprisingly, the highest NDVI difference score is achieved in plots and surrounding areas that are in privately managed pump irrigated systems (0.237). Values are also positive for other systems.
Jointly managed gravity irrigation system indicates a slightly less NDVI value difference (0.144).
-0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
Moisture reliable, highland, cereal
Drought prone, lowlands
Drought Prone, highlands
Humid moisture relaible, lowlands
Moisture reliable, highlands, Enset
Total
NDVI at beginning NDVI in 2016 NDVI difference
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Table 5. 3. Temporal changes in vegetation biomass of irrigation systems at varying level of devolution and technologies
Irrigation water management system
No.
of plot
Before irrigation
NDVI in 2016
NDVI
difference Privately managed + pump 174 0.108 0.345 0.237***
(0.008) (0.009) (0.012) Collectively managed + pump 87 0.018 0.176 0.159***
(0.009) (0.012) (0.01) Collectively managed + Gravity 215 0.054 0.213 0.159***
(0.012) (0.008) (0.01) Jointly managed+ pump 72 -0.035 0.141 0.176***
(0.01) (0.008) (0.01) Jointly managed+ gravity 341 0.084 0.228 0.144***
(0.009) (0.009) (0.009)
Open access+ pump 136
-0.041 0.12 0.162***
(0.008) (0.00) (0.009) Source: Author’s computation from own survey and images acquires form NASA/US geological survey earth observation satellites
Note: Standard errors in parenthesis
Table 5.4 provides information on changes in vegetation biomass in irrigated plots across technologies applied. The result shows that plots that use irrigation water from groundwater sources reported major increases (0.207) in biomass. Similarly, among various irrigation water application mechanisms applied in the irrigated fields, the increase is larger for flood irrigation systems. This result is not surprising since flood irrigation (0.211) is a practice in which an entire field is covered with water. The excess water (overflow) is having a positive impact on the plots and the surrounding area. Plots under pressurized irrigation systems (0.163) perform a little better than those in furrow systems (0.136) in terms of changes in NDVI since irrigation began.
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Table 5. 4. Change in vegetation biomass in irrigated plots and surrounding areas by irrigation water source and appliance method.
Types of Irrigation Technologies
No. of plot
Before irrigation
NDVI in 2016
NDVI difference Irrigation water source structure
Surface 713 0.05
(0.006)
0.202 (0.005)
0.151***
(0.006)
Groundwater 324 0.05
(0.006)
0.258 (0.008)
0.208***
(0.008) Irrigation water application mechanism on the plot
Flooding 410 0.082
(0.008)
0.293 (0.007)
0.211***
(0.008) Pressurized system 106 0.006
(0.013)
0.169 (0.01)
0.164***
(0.009)
Furrow 521 0.037
(0.006)
0.173 (0.006)
0.136***
(0.006)
Total 1037 0.05
(0.0047)
0.219 (0.0045)
0.169***
(0.005)
Source: Author’s computation from own survey and images acquires form NASA/US geological survey earth observation satellites
Note: Standard errors in parenthesis
Table 5.5 presents the NDVI analysis that detects temporal change of vegetation biomass across the years they started to irrigate in the study area. The result indicates that the natural vegetation biomass has increased since the establishment of the irrigation systems. However, the results suggest that plots and surrounding areas that are irrigated for more years have a non-linear relationship with changes in NDVI. Differences in vegetation cover are higher for schemes with 10 to 20 years of establishment compared to those with less than 10 years of establishment, but those with 20 to 30 years show the smallest change in NDVI. Irrigation schemes that are more than 30 years old show higher differences in NDVI, similar to those aged between 10 to 20 years. A possible explanation for the low NDVI difference for those plots that started to be irrigated 20-30 years ago is that it was the time when promotion of small-scale agriculture came to the political agenda of the government. Immense effort was made an effort to benefit rural households by including their plots into nearby communally developed irrigation schemes which might not have considered environmental impacts of irrigation use.
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Table 5. 5. Temporal change of vegetation biomass across the year they started to irrigate in the study area
No. of years of irrigation experience
No.
of plot
Before irrigation
NDVI in 2016
NDVI
difference
<10 years 634
0.04 (0.004)
0.209 (0.006)
0.169***
(0.006) Between 10-20
years 311
0.037 (0.011)
0.219 (0.007)
0.181***
(0.009) Between 20-30
years 61
0.19 (0.018)
0.29 (0.017)
0.101***
(0.015) More than 30
years 31
0.11 (0.018)
0.294 (0.025)
0.184***
(0.032)
Source: Author’s computation from own survey and images acquires form NASA/US geological survey earth observation satellites
Note: Standard errors in parenthesis