Spate Irrigation Potential Assessment for Ethiopian Watershed

In the low lands of Logia sub basin, Ethiopia, because of shortage of rain to fully grow crops, irrigation is an obligation in general and the presence of seasonal rivers flowing in the region in particular makes flood utilization ideal for spate irrigation. The subjects of the present study were to assess the spate irrigation potential of Logiya watershed that has been brought under irrigation on the basis of flood water availability and land suitability. A GIS based technique combined with analytical hierarchy process (AHP) was applied to access the potential of the watershed for spate irrigation development. Potentially suitable sites for spate irrigation development were assessed for Maize, Sorghum and Tomato crops. Spate irrigation area was evaluated based on land use/cover, slope and soil suitability. CROPWAT software was used to estimate the reference crop evapotranspiration, effective rainfall, net irrigation and gross irrigation water requirement. The suitability model developed shows that only 26.15% of the total area falls under marginally to highly suitable categories for spate irrigation development. The Logiya seasonal river flow from July to October was 301.64 Mm3. However, the annual flood water available from the river was less than the total GIWR by 8.77 Mm3 during growing period. The surplus water available from the river before July might be stored and used for irrigation during water deficit period during growing seasons.


Introduction
Ethiopia is the second most populous country in Africa (Awulachew et al., 2005). About 85 percent of which lives in the rural areas depending on subsistence agriculture. Though agriculture is the dominant sector, most of Ethiopia's cultivated land is under rainfed agriculture. Due to lack of water storage and large spatial and temporal variations in rainfall, there is not enough water for most farmers to produce more than one crop per year and hence there are frequent crop failures due to dry spells and droughts which has resulted in a chronic food shortage currently facing the country (Awulachew et al., 2007).
The total potential irrigable land in Ethiopia is estimated to be around 3.7 million hectares. The estimated irrigation potential for Awash River basin is about 134,121 hectares. Out of these, a potential, 30,556 hectares are for small-scale, 24,500 hectares for medium-scale and 79,065 hectares for large-scale development. Even if the country has large potential of land for irrigation, only about 4 to 5% is under irrigation.
To feed an ever increasing population, assessment of all unutilized resource potential of land and water resource in the country is prime importance for planning of sustainable food production in the national as well as regional level. The existing irrigation development in Ethiopia, as compared to the resources potential, is not significant and the contribution of irrigation is not satisfactory.
Substantial amount of studies have already been performed on the subject of irrigation potential based on surface water over the last decade since formal irrigation started in 1960 in Awash basin (Megersa, 2017). When analysis of those studies done on the subject of irrigation potential almost all concerned on surface source of water and this perceptions ignore the fact that one of the world's oldest forms of irrigation practice arose in arid zones and remains a crucial element in the agricultural economies of the country generally known as spate irrigation has hardly received any serious attention from policy makers, technical experts or donors and detail potential is not documented yet (van Steenbergen et al., 2010).
Due to geological setting of Ethiopia no drainage is coming in but flows out radiating in all directions and mean annual specific runoff which is 54.4 BCM (Alamayehu, 2008) has been flow as flash flood along all its basin through arid and semiarid regions which constitute 70% of the total landmass of the country's (Bogale & Shimeles, 2009). Historical record states that in Ethiopia, flash flood occurred in almost all basin which Awash basin is one. Based on hydrological analysis, Awash basin divided in to 21 sub-basins which contribute annual runoff of 900 Mm 3 out of 4.6 Bm 3 annual potential of surface and runoff water of basin (Nigatu, 2006). Destructive flood is occurring at any rainy season in the basin in general and further downstream of Awash basin in particular. To overcome this challenge changing this flood which is unpredictable and destructive in to something that is useful is the last option to alleviate the moisture stress for spate irrigation as well as flood control in low land area where rapid flood putting huge pressure on the natural resource (Daniel et al., 2014).
Limited research has been done on the assessment of irrigable land for spate irrigation to enable irrigated agriculture practice in Ethiopia. This study contributed by incorporating the following points: (1) Assessment of land suitability for spate irrigation; (2) Assessment of Spate irrigation water requirement for Logia sub basin; (3) produce a spate irrigation suitability map using GIS. The results from this study would help local decision makers and stakeholders on the expansion of small-scale irrigated agriculture in the region. Policy makers need to consider spate irrigation options in their agriculture development programs.

Problem Statement
The understanding of many people is that floods can be one of the most destructive force of nature will continue to cause serious economic and environmental losses. In Awash basin, When an example looks back in to history and analysis from recent memory is the flood that had been caused in 2006 in the eastern catchment of Awash basin, Dire Dawa administrative council which caused deaths of 256 people, 244 missing and more than 9956 people displaced (Billi et al., 2015). The other is Tributaries of lower Awash basin from wollo high lands such as Mille and Logia seasonal rivers flows have also caused severe impacts on human lives and property especially in the area between Dubti and Assaita woreda of Afar regional state lower Awash basin. But flood is not always hazard as no one could imagine that this barren land would ever show signs of life if there is no flood. In addition to these, if this destructive flood is managed it is possible to change in to something which is useful and productive especially for spate irrigation since irrigation has had direct benefits in terms of production and incomes, and indirect benefits in terms of reduced incidence of downstream flood damage (FAO, 2010). On the other hand Tesfay (2002) state that flood and famine is the twin and alternative curses, flood is a blessing in disguise; thus, famines may be effectively prevented by floodwater. In order to plan the development of this water resource carefully, especially for agriculture which is by far the largest user of water, an assessment of irrigation potential becomes a very timely and crucial issue (FAO, 1995). As Daniel et al. (2014) state the Afar region has one of the highest spate irrigation potential and practices in Ethiopia as the runoff generated from the highlands of Amhara and Tigray, however, detail assessment of the potential of spate irrigation in Afar as general and lower Awash basin of Afar regional state in particular is not carried out to date. Rational assessment is crucial to know the potential of spate irrigation in the Logiya sub basin of Awash, Afar national regional state using Logia seasonal flood. The principal objective of this study is both to identify potential areas suitable for spate irrigation and to quantify the available Logia flood water potential for spate irrigation by using GIS techniques.

Study Area
The logia sub basin of Awash basin considered for this study are geographically setup in the arid lowlands in the north-eastern parts of Ethiopia in great rift valley between 11°35′ to 12°5′ N latitude and 39°35′ to 41°5′ E longitude ( Figure 1). The study area is dominated by flat lowland areas with mountainous boundaries with an elevation varying from 362 m to 3453 m. The annual rainfall varies from 176.2 mm to 344.1 mm. The mean annual minimum and maximum temperatures ranges between 16 °C and 38.0 °C and month of June had the highest temperature while December extremely lowest temperature. In Ethiopia there are five agro-ecological zones classification; very cold (Wurch), cold (Dega), sub-moist cool (Weina-dega), sub-moist warm (Kolla) and dry-hot (Bereha). The study area is classified as the dry-hot (Bereha)-ecological Zone. The long term monthly average rainfall for the study area has bimodal rainfall pattern with peaks in August and July. The soil in the study area is dominated by Fluvisols, Leptosols and Regosols (Halcrow, 1989 Where, NIWR and GIWR are net and gross irrigation water requirement (mm), respectively; ETc = crop evapotranspiration (mm day -1 ); Pef = effective precipitation (mm day -1 ); P = daily rainfall (mm day -1 ); C = constant equal to 0.20; E = irrigation efficiency in fraction.

Land Suitability for Spate Irrigation
Slope: The slope analysis result reveled that about 88.2% of the total study area was classified as highly to marginally suitable for spate irrigation (Figure 2g). The remaining 11.8% of the sub-basin area having slope of greater than 8%, which is marginally to permanently not suitable for spate irrigation. According to FAO (1999) suitability classification, most of the area of the Logiya sub-basin was falls below 8%, which is in suitable range of slop classification for spate irrigation.
Soil type and depth: The major soil groups recognized in the study area were Calcaric fluvisol (45.8%), Orthic solonchaks (16%) and Orthic solonchaks (6.5%) (Figure 2(c)). Soil depth was among the significant physical soil parameters used to assess soil suitability for surface irrigation development. Soil depth was inferred from geomorphology and the soil map of FAO, 1991. The soil depth in the sub-basin varied from < 30 to > 120 cm. Further, the soil depth was reclassified into four classes; 30-50, 50-80, 80-100 and > 120 and soil depth of the study area was analysis in Arc GIS 10.41 version (Figure 2(d)). From Figure 2(c) and Table 1, it is manifested that most of the eastern and central part of the Logiya sub-basin soil is categorized as highly to moderately suitable (97%) and the areas with soil type Eutric cambisols, Leptosols and Orthic acrisols are classified as not a suitable class for the use of spate irrigation development.
As shown in Figure 2(d), based on soil depth suitability 138905.9 ha (41.6%) is highly suitable, the area 166171.9 ha (49.8%) is marginally suitable and the area 28541 ha (8.5%) is not suitable class. Note. S1 = highly suitable, S2 = moderately suitable and N = conditionally not suitable.
Well-drained soils are characterized under the high suitability rating class and imperfectly drained soils are considered as moderately suitable for surface irrigation development (FAO, 1984) (Figure 3(f)).
Drainage: Soil drainage is one of the most important parameters for spate irrigation potential assessment. In the study area, three soil drainage classes, i.e., well-drained, imperfectly drained and not drained classes were identified based on the FAO (1985) guidelines. The drainage suitability map (Figure 2(f)) and Table 1

Assessment of Flood Water Availability and Irrigation Requirement
Average annual monthly stream flow, maximum and minimum flow was evaluated. Available flood water was estimated after some percentage water released for ecological purpose (Table 3 and Table 4). The mean annual flow of Logiya seasonal flow is 48.5 Mm 3 before 25% of ecological purpose is not released and 36.4 Mm 3 after ecological purpose is released. The flow of the river is highly seasonal and 85% of the flow occurs during the months of June up to September. According to the discharge analysis, mean flow in each month from July to September were estimated to be 7.7 m 3 /s, 12.8 m 3 /s, 12.3 m 3 /s and 6.6 m 3 /s respectively. The study area receives total average annual rainfall of 344 mm. The effective rainfall was calculated by CROPWAT model using the average recorded rainfall data at four climate gauging stations (Figure 4). Table 3. Mean, max, and min monthly flow of Logia River at gauged station before 25% released for ecological purpose  Table 4. Mean, max, and min annual monthly flow of Logiya flood River at gauged station after 25% released for ecological purpose  Vol. 12, No. 7;2020 142 The highest effective rainfall during irrigation time was at Mersa station which varied from 4.8 mm during October and 98 mm during August at Mersa Climatic Stations and the lowest is found in Dubti climatic station which varied from 0.7 mm during October and 74 mm during August. Crop evapotranspiration (ETc) was calculated from CROPWAT8.0 model using monthly average value of ETo and crop coefficient for each of the crop Maize, Sorghum and Tomato at Dubti, Logia, Chifra and Mersa climatic stations. Crop water requirements of selected crops grown in the area were calculated assuming mono cropping pattern in the area. The ETc was calculated using the monthly average value of ET0 and the crop coefficient for maize, sorghum and tomato crops.  Figure 5. Crop evapotranspiration for both the crops at both the climatic stations was more than the effective rainfall.
The effective rainfall was not sufficient to meet the crop water requirements. Therefore, irrigation was needed in the study area.
The NIR and GIWR from the weekly irrigation schedule for each of the crops for each of the stations were estimated using CROPWAT software ( Figure 5(c)-(d)).The monthly values of net and gross irrigation requirement in mm is given in (Figure 5(c)-(d)). The mean monthly values of irrigation water demand and water flow available are clearly identified (Table 4). Results revealed that high irrigation demand observed on the month of September (130.08 Mm 3 ) and lesser demand on the month of July (39.8 Mm 3 ). Total irrigation water demand for the catchment 310.41 Mm 3 of water was maintained to irrigate the whole irrigable area of 69529.9 ha of land. The Logiya seasonal river flow from July to October was 301.64 Mm 3 . However, the annual flood water available from the river was less than the total GIWR by 8.77 Mm 3 during growing period. The surplus water available from the river before July might be stored and used for irrigation during water deficit period during growing seasons.

Conclusions
A consistent irrigation water supply will upsurge agricultural productivity, and increase food security level and the rural economy, thus developing irrigation infrastructure is important. However, this can be accomplished by assessing available land and water resources for irrigation. Therefore, the spate irrigation potential of the Logiya watershed was evaluated in this study. The study results showed that about 91.4% and 88.2% of the study area is in the range of highly suitable to marginally suitable for spate irrigation development based on the soil and slope criteria, respectively. The overall suitability using a weighted overlay of the above factors in ArcGIS shows that potentially irrigable land suitable for spate irrigation is about 26.15% of the catchment area. This is expected to reduce further if more factors are considered in the weighted evaluation process and may provide a better estimate of the land potential for spate irrigation. The gross irrigation water requirement (GIWR) of the maize, sorghum and tomato crops was different for the Logiya, Chifra, Dubti and Mersa climatic stations.  Vol. 12, No. 7;2020 seasonal river flow from July to October was 301.64 Mm 3 . However, the annual flood water available from the river was less than the total GIWR by 8.77 Mm 3 during growing period. The surplus water available from the river before July might be stored and used for irrigation during water deficit period during growing seasons.