Research Article - (2020) Volume 9, Issue 2
Date of Submission: 20-Aug-2019
, DOI: 10.37421/idse.2020.9.245
Citation: Rediet Girma, Eshetu Gebre and Teshale Tadesse. Ã¢Â€ÂœLand Suitability Evaluation for Surface Irrigation Using Spatial Information Technology in Omo-Gibe River Basin, Southern Ethiopia.Ã¢Â€Â Irrigat Drainage Sys Eng 9(2020): 245 doi: 10.37421/IDSE.2020.9.245
Copyright: Â© 2020 Girma R, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Irrigation would provide farmers with sustained livelihoods and improve their general well-being. The aim of this study was to evaluate the suitability of the land for surface irrigation using GIS based weighted overlay analysis of individual parameters for better utilization of land resources. Factors considered included physical land features (land use/land cover, soil and slope), and proximity to water sources. Based on soil depth, 82.4% of the study area is potential suitable for the intended uses; the drainage class scores 70% suitability; 80% the soil texture was clay dominant hence it was moderately suitable for surface irrigation. Considering the terrain, 11.75% of the basin is suited for irrigation practice. The LULC classification revealed that, 54.42% was found to be highly suitable and 16.7% is found to be unsuitable. In reference to river proximity, around 81% of the area could be highly recommended for the intended use. Excluding the national parks, 71% (7% is S1 and 64% is S2) is suitable for the intended use. Hence, future surface irrigation development is feasible. Based on the findings, to increase the land area to be irrigated; an appropriate drainage provision and cost wise land leveling should be taken into consideration, further land suitability analysis for other types of irrigation and water source should be carried out. The study result could assist policy makers for better decisions during the development of irrigation projects in Omo-Gibe river basin.
Land suitability • Surface irrigation • Weighted overlay • ArcGIS • Pair-wise comparison
Agriculture is a mainstay of Ethiopian economy. The country is endowed with ample water resources with 12 river basins. Irrigation in Ethiopia is considered as a basic strategy to alleviate poverty and hence to achieve food security and improve the economy of the farming community through generating additional income during the dry season. It is useful to transform the rain-fed agricultural system which depends on rainfall into the combined rain-fed and irrigation agricultural system. This is believed to be the most prominent way of sustainable development in the country [1-4]. The process of land suitability classification is the appraisal and grouping of specific areas of land in terms of their suitability for defined uses . Proper use of land depends on the suitability or capability of land and water resources for the development of irrigation facilities could lead to substantial increase in food production .
For land suitability analysis of surface irrigation, particular attention is given to the physical properties of the soil, distance from available water sources and terrain conditions in relation to methods of irrigation considered . In addition, land cover/land use types are considered as limiting factors in evaluating suitability of land for irrigation . Irrigation suitability land classification investigations are an integral part of multi objective planning for the development and operation of water resource projects with an irrigation component. They support planning and management by identifying land resource potentials or problems through the collection, evaluation, and presentation of land resources data . Availability of irrigation leads to land use change as well as intensive cropping system. Improper use of irrigation water has resulted in environmental degradation of natural resources that leads to decline in the productivity of land resources and deterioration of land quality for its future use .
Irrigation would provide farmers with sustained livelihoods and improve their general well-being. Thus, to bring food security in the national as well as in house hold level, improvement and expansion of irrigated agriculture must be enhanced as noted by Negash . Soil, terrain feature (DEM and its derivatives), water potential and land use classification criteria are the basis used to define the suitability. With this respect, the Geographic Information System (GIS) facilities will be extensively used. To ensure adequate management and design of a particular irrigation system, a welldeveloped and suitable database is quite important. Thus, it should be able to deal with spatially and temporally varying factors affecting the system.
Being Ethiopia‘s second largest river system, the Omo-Gibe basin contributes 90% of the runoff to Lake Turkana (located downstream in Kenya) and accounts for 14% of the country‘s annual runoff, second only to the Blue Nile in runoff volume . According to Woodroofe et al. , it is considered as a very significant resource within Ethiopia. Appropriate management and selection of applicable irrigation method is a prerequisite for better utilization of land resources which help to optimize and sustain the productivity of these land resources . Therefore, the objective of this study is to spatially evaluate and map suitable parcel of land for surface irrigation in the Omo-Gibe river basin using weighted overlay analysis in ArcGIS environment though the characterization of biophysical suitability factors aimed to assist in land use policy decisions.
Description of the study area
Omo-Gibe River Basin is situated in the South-West part of Ethiopia, between 4°30’ and 9°30’ N latitude, and between 35° and 38° E longitude (Figure 1). It encompasses parts of two National Regional States; Oromia which occupies the north-eastern part of the basin and the rest of the basin, this study focused, is situated in the Southern People’s Regional States (SNNPRS). It is drained by two major rivers from the highlands, the Gibe River flowing southwards and Gojeb River flowing eastwards. The Gibe River is called the Omo River in its lower valley south and south westwards from its confluence with the Gojeb River. The northern part of the basin has a number of tributaries from the northeast of which the largest are the Walga and Wabe rivers. The Tuljo and Gilgel Gibe rivers are important rivers that drain to the Gibe .
Factors used to assess land suitability for irrigation
The process of land suitability classification is the appraisal and grouping of specific areas of land in terms of their suitability for defined uses . This kind of test “land suitability analysis” is an essential starting point for the development. Where, it provides the necessary information about the deferent limitations and the possible opportunities for the land use under investigation based on the land capabilities. The factors were identified from different relevant sources and data availability. For this study, factors considered included physical land features (land use/land cover, soil and slope), and proximity to water sources. To achieve this, characterization and geo-referencing the biophysical database is required, hence the following datasets were collected and reclassified in to different suitability classes.
Soil database: Soil is a key factor in determining the suitability of an area for agriculture in general and irrigation in particular. The soil data (physical property; texture, drainage class and soil depth) covering the study area was accessed from the FAO website Harmonized World Soil Database, in Environmental System Research Institute (ESRI) shape file format (version 1.21) .
Terrain feature: The slope gradient of the land has great influence on selection of the irrigation methods. According to FAO standard guidelines for the evaluation of slope gradient, slopes which are less than 2%, are very suitable for surface irrigation. But slopes, which are greater than 8%, are not generally recommended . The slope of the study area was reclassed in ArcGIS environment using 30m resolution Digital Elevation Model (DEM) data from freely available Shuttle Radar Topography Mission (SRTM) and the percentage slope was determined.
Distance to water sources: It is important to make sure that there will be no lack of irrigation water. If water is in short supply during some part of the irrigation season, crop production will suffer, returns will decline and part of the scheme's investment will lay idle . The suitability class of a land parcel with respect to river proximity is determined by its distance in relation to the perennial rivers. Distance to the existing river was calculated in ArcGIS tool by projecting the locations to a Mercator (UTM) Zone 37N. After categorizing the distance map of the river in to four different classes, the farthest distances were assigned as not suitable and closer distances was classified as highly suitable. In addition to river proximity, electrical conductivity (EC) of the river water was included as an information.
Land use/Land cover (LULC): The 2018 LULC of the Omo-Gibe river basin was assessed and mapped using Landsat-8 satellite images. Prior to image classification, detailed image pre-processing including radiometric and geometric correction were performed to correct the surface feature reflectance characteristics. Image classification was processed using supervised image classification technique according to the desired decision rule of maximum likelihood algorithm for the respective years by using ERDAS IMAGINE. To improve classification accuracy and reduction of misclassifications, post-classification refinement was therefore used for simplicity and effectiveness of the method [17-20]. Visual interpretation of satellite images was made by using ERDAS IMAGINE and ArcGIS software package for satellite image processing and LU/LC analysis. Agricultural land use is considered as highly suitable, grass land is moderately suited, shrub land and forest areas assigned as marginally and not suitable for irrigation respectively.
Framework of land suitability evaluation
The FAO approach (Table 1) defines land suitability as aptitude of a given type of land to support a defined use . After all the required data were collected from different data sources, further analysis was carried out for each biophysical factor for a better understanding of their contribution in assessment of land suitability potential for irrigation.
|Class S1||Highly suitable||Land without any significant limitations|
|Class S2||Moderately suitable||Moderately severe limitations which reduce productivity or benefits or increase required inputs.|
|Class S3||Marginally suitable||Overall severe limitations; given land use is only marginally justifiable|
|N||Not suitable||Limitations not currently overcome with existing knowledge within acceptable cost limits|
Classification of individual factors: According to FAO , each factor determining the suitability of the land for irrigation was classified into four classes such as highly suitable (Class S1), moderately suitable (Class S2), marginally suitable (Class S3), currently not suitable (Class N) (Table 2 and Figure 2) and data layers were prepared for further overlay [21,22].
|Soil suitability||Land use/land cover||Slope (%)||Suitability class|
|depth (cm)||texture class||drainage class|
|>100||Loam/loamy sand/Sandy loam||Well||Cultivated land||0-2||S1|
|75-100||clay loam/ sandy clay loam/clay (light)/silty clay loam||Moderately Well||Grass land||2-5||S2|
|50-75||Sand||Imperfect||Barren & shrub land||5-8||S3|
|<50||Silty clay||Poor/very poor||Constraints (Forest, built-up, water, wetland)||>8||N|
Overall land suitability/weighted overlay: Potential irrigable land was obtained by creating irrigation suitability model analysis which involves weighting of values of all individual data sets. The purpose of weighting in land suitability analysis is to determine the importance of each factor relative to other factors. Analytical Hierarchy Process (AHP) (Figure 3), a well-known Multi-Criteria Decision Analysis method developed by Saaty  was used to express the relative importance of each suitability factors.
The pair-wise comparison matrix is a rating of the relative importance of the two factors regarding the suitability of the land. A scale of importance is broken down from a value of 1 to 9 (Table 3). The intensity of importance and their explanation is given in table 3. Despite there are various scales to rate the related stakeholders’ judgments, the 1-9 point scale has strongly been recommended to be used as an acceptable scale in the AHP and it was implemented in this study too. The 1-9 point scale is simple, straightforward, and easy to use [20-21].
|Equally important||1||Equally important||1/1|
|Equally or slightly more important||2||Equally or slightly less important||1/2|
|Moderately/Slightly more important||3||Moderately/Slightly less important/ Experience and judgment slightly favor one over the other.||1/3|
|Slightly to much more important||4||Slightly to way less important||1/4|
|Strongly more important / Much more important||5||Way less important/ Experience and judgment strongly favor one over the other.||1/5|
|Much to far more important||6||Way to far less important||1/6|
|Very much more important/Far more important||7||Far less important/ Experience and judgment very strongly favor one over the other.||1/7|
|Far more important to extremely more important||8||Far less important to extremely far less important||1/8|
|Absolutely more important / Extremely more important||9||Extremely less important/ The evidence favoring one over the other is of the highest possible validity.||1/9|
If factor X is exactly as important as Y, this pair receives an index of 1. If X is much more important than Y, the index is 9. All gradations are possible in between. For a "less important" relationship, the fractions 1/1 to 1/9 are available: if X is much less important than Y, the rating is 1/9. The values are entered row by row into a cross-matrix. The diagonal of the matrix contains only values of 1. If X to Y was rated with the relative importance of n, Y to X has to be rated with 1/n. Then to calculate the weight, a normalized comparison matrix was created: each value in the matrix was divided by the sum of its column. To get the weights of the individual criteria, the mean of each row of this second matrix was determined. These weights are already normalized; their sum is equal to 1.
In the application of the AHP method it is important that the weights derived from a pairwise comparison matrix are consistent. It should be noted that for preventing bias thought criteria weighting the Consistency Ratio was used (CR). Consistency for a comparison matrix was measured by calculating the consistency index (CI) (eqn. 1).
The consistency ratio (eqn. 2) is defined as
Where, n, is the number of criteria or sub-criteria in each pairwise comparison matrix and λmax is the maximum eigenvalue of the comparison matrix (the average of consistency vector). This consistency index was then compared to a random index (RI). The RI is the average CI of randomly generated reciprocal matrices using the scale 1/9, 1/8. . . 8, 9. For different dimension of the matrix (n), Saaty  generated random matrices and calculated their mean CI value. The random consistency index for different dimensions n is given in Table 4 . Thus, for n=3 and n=4, conventionally it is required that CR≤0.05 and 0.08, respectively to be acceptable. For n≥5, a consistency ratio of 0.10 or less is acceptable. A consistency ratio (CR) of 0.10 or less indicates a reasonable level of consistency . If the CR is >0.1, the comparison matrix should be revised again.
Using the ArcGIS weighted overlay analysis tool (Figure 4), overall land suitability was assessed spatially and suitability map was generated by aggregating the output from AHP.
The suitability analysis indicated that, 11.75% and 82.4% of the study area based on slope and soil depth respectively is suited for surface irrigation. In reference to soil drainage class 70% of Omo-Gibe river basin is accounted for suitable class (S1 and S2) at present condition and 17.5% is classed under marginal suitability class. Based on the LULC, 54.42% is highly suited for surface irrigation. In regard to water resource proximity, majority of the study area (80.85%) is currently suitable (S1). In overall, most of the study area is suitable for irrigation (71% excluding the national parks; 7% is S1 and 64% is S2) whereas the remaining 29% is not recommended for surface irrigation. Hence, future surface irrigation development is feasible based the land features and river proximity.
Based on the finding of this study, for the area which have slope above 8% (76.16% of the total area), land leveling operation and/or soil conservation work have to be incorporated to break surface slope and to make it suitable for surface irrigation. The result obtained reveled that, 12.5% of the study area has poor drainage condition. These soils could be made more suitable by adopting improved drainage system, soil and crop management practices. Hence an appropriate drainage provision should be taken into consideration in further development. In consideration to river proximity, almost 4% of the river basin is not suited for surface irrigation therefore looking for other sources of water in the nearby is recommended. Groundwater is a viable option for supplementing surface water resources for irrigation in several basins in the country. Further land suitability analysis for sprinkler and drip irrigation should be carried out to increase the land area to be irrigated.
The authors confirm and declared that there is no conflict of interest in this paper.
We would like to acknowledge the unreserved support of Southern Nation Nationalities People Regional State Water and Irrigation Development Bureau Irrigation Construction and Scheme Administration Agency. Our deepest thanks go to the staff members of Hawassa University, Institute of Technology, Faculty of Biosystems and Water Resource Engineering for their valuable comments and expert’s suggestion.