Spate Irrigation

Title Modelling exercise HEC-RAS Prepared by: Dr Adane Abebe and Abebe Demissie

Outline of presentation Introduction Hydrology and Sediment Transport Water Diversion and Control Structure Soil and Field Water Management Agricultural Practices Management Arrangements Case study on spate irrigated field

Introduction Flood is known for its destructive image Question: Can flood be put to a beneficial use ?

Introduction Flood which is known for its destructive image, can be beneficial Spate/flood/ irrigation Flood recession farming Other benefits, such as Groundwater recharging Ecosystem Conservation Fish farming Leaching of saline soils Fig. Flood from micro catchment flowing to Boru Dodota Spate Irrigation Scheme (Yohannes G.) Fig. Flood recession farming on the riverbeds in Gambela. (Dr Frank)

Introduction … Fig. Farmers working on flood for farm to farm water distribution

Introduction… Flood (Spate) irrigation makes use of occasional, unpredictable and often destructive floods in ephemeral rivers to irrigate farmland, rangeland and forestry, recharge groundwater, fill drinking water reservoirs and mitigate climate change and variability (Dr. Abraham M., 2011)

Introduction … Spate irrigation is a resource system, whereby flood water is emitted through normally dry wadis and conveyed to irrigable fields”. Definition by Mehari et al. (2007) Spate irrigation development requires high levels of cooperation between farmers to divert and distribute flood flows.

Introduction … Spate irrigation is the main source of livelihoods for large numbers of economically marginal people and it is found: - Global, Africa and Ethiopia perspective Spate irrigation types: Spate irrigation can be categorized based on: Size of the scheme type of infrastructure types of operation and maintenance Source of water

Hydrology and Sediment Transport in Spate Spate hydrology is characterized by a great variation in the size and frequency of floods Flood hydrographs are characterized by an extremely rapid rise in time, followed by a short recession (Dr Abraham M.)

Hydrology and Sediment Transport … The combination of poor cover, steep slopes and high-intensity rainfall in wadi catchments results in high rates of soil erosion and a large supply of sediments to the wadi systems Total sediment concentrations rising to and exceeding 100,000 ppm, or 10 percent by weight can occur in floods in some wadis. Sediment concentrations up to 5 percent by weight in floods are common.

Hydrology and Sediment Transport … The following information should ideally be available to designers of intakes and canals: the annual volumes of water available at the diversion point(s) in terms of seasonal incidence and reliability; the distribution of flows during runoff events, particularly the shape of the recession limb of the hydrograph, which provides the bulk of the water that can be diverted to irrigation command areas;

Hydrology and Sediment Transport … wadi bed seepage rates; the magnitude and return periods of extreme discharges for the design and protection of the permanent works; the concentrations and size range of the sediments transported by spate events and their relationship with wadi discharges; and the sediment-transporting capacity of existing canals.

Flood Diversions and Control Structures Intakes in spate systems have to divert large and varying levels of flood flows and prevent large uncontrolled flows from entering canals, deliver at high level to ensure command over the irrigated field

Flood Diversions and Control Structures… Intakes in spate systems limit the entry of the very high concentrations of coarse sediments function over the longer term with rising irrigation command levels, degradation of wadi bed levels canal capacities have to cope with a wide range of discharge

Flood Diversions and Control Structures… Advantages of traditional intakes: Flexible (the intake can easily be adjusted to suit the changing bed conditions Low cost; based on local technology (material) High overall diversion efficiency (cascades of intakes) limit diversion of high flood and high sediment load (Breaching bunds)

Flood Diversions and Control Structures… Disadvantages of traditional intakes: Enormous input of labour and resources to maintain and reconstruct intakes After the large flood destroys upstream intake, a following flood cannot be diverted until repairs are made!

What considerations should be taken for the Improvement/modernization?

Flood Diversions and Control Structures… the Improvement should: make it easier and less labour-intensive to operate and maintain; minimize the capacity of large and uncontrolled flood flows to damage canals and field systems; help maintain the distribution of water within the system in line with established rules and rights;

Flood Diversions and Control Structures… …the Improvement should: avoid unintentional alteration of water distribution (including drinking water and water for animals) within the watershed between upstream and downstream water users; avoid excessive sediment load in spate systems and ensure that suspended sediments are deposited on the land and not in the canals; cope with frequent and sometimes large changes in wadi bed conditions.

What improvement options do you envisage?

Flood Diversions and Control Structures… Options for improvement include: more durable diversion spurs; improved diversion bunds; controlling flows admitted to canals; - head regulator that permits flows up to the maximum capacity of a main canal but that rejects higher flows

Flood Diversions and Control Structures… …Options for improvement include: provision of basic gated intakes; - wider intake considering the requirements and the response time for the operation of the gates should be less than the time to flood peak (10 – 30 min) provision of rejection spillways - The spillway needs to be designed as a lateral-flow weir capable of passing all the flow in excess of the downstream canal capacity.

Flood Diversions and Control Structures… The engineering structures involved when spate schemes are improved can be described under three headings: diversion structures (intakes); spate canals and water control/dividing structures; and Bank protection and wadi-training structures.

Flood Diversions and Control Structures… New permanent diversion structures A typical diversion structure includes a raised weir, with or without a fuse plug, a scour or under-sluice, a canal head regulator and a guide or divide wall; Single versus multiple intake; Location of intake At the outside of relatively mild wadi bend, just d/s from the point of maximum curvature

Flood Diversions and Control Structures Discuss Traditional Vs improved structures on spate irrigated scheme for the following lists of hydraulic structures Canal, water control/ division structures Canal, check and drop structures, flow splitting structures, field offtakes, infield structures. Bank Protection

Soil and Field Water Management Spate soils are largely built up sedimentation and generally have good water-holding capacities with relatively moderate infiltration rates that vary with soil texture, density and soil management practices

Soil and Field Water Management… Field water management in spate irrigation systems is as important as effective water diversion. Water distribution is regulated by prevailing water rights and rules and generally follows a number of principles that includes: A: rapidly spreading the available flows to prevent spate water rapidly disappearing in low-lying areas; B: dividing the floods into manageable quantities’ C: ensuring large enough amounts of water are conveyed in the short time to irrigate the downstream areas

Soil and Field Water Management…. Soil fertility management in spate systems Soil organic matter and fertility can be improved by incorporating crop residues into the soil (but crop residues are often used as fodder), by growing leguminous crops and by practising crop rotation

Soil and Field Water Management… Field water distribution methods Water distribution is regulated by water rights and rules, and at field level water distribution methods are grouped into two practices: practices in command area water distribution: field-to-field distribution or individual field distribution; sizing of command area: extensive distribution or intensive distribution.

Soil and Field Water Management… Moisture conservation Several techniques to conserve soil moisture are applied in spate systems: ploughing prior to and after irrigation; conservation tillage and soil mulching; breaking soil crusts.

Agricultural Practices Farmers in spate systems have developed various cropping strategies to cope with the risks inherent in spate irrigation. These include: growing local varieties that are adapted to the local agroclimatic conditions and have a high tolerance to drought; growing crops that produce some fodder even if the floods fail and grains cannot be grown;

Agricultural Practices… ….Farmers in spate systems have developed various cropping strategies to cope with the risks inherent in spate irrigation. These include: practising intercropping, so that, in bad years, one of the planted crops can be harvested; selecting crops in relation to of the timing and volume of the first irrigation and, where possible, of subsequent irrigations; selecting crops in relation to the soil moisture available after irrigation.

Agricultural Practices… There is wide ranges in yields observed in spate schemes, this can be attributed to: the unreliability of irrigation; degree of control that farmers can exercise over spate flows; the farming skills in soil moisture conservation practices; and the less priority that farmers give to spate irrigation, because of the low return to labour in spate.

Agricultural Practices… Areas for improvement include: introducing an integrated farming systems approach, including livestock and agroforestry; use of improved seed varieties and a better understanding of the balance of nutrients; cultivating more minor crops and wild plants/truffle mushrooms or vegetables; and a better control of post-harvest losses.

Management Arrangements… The viability of spate systems is mostly determined by the strength of the organizations involved in their operation and maintenance. While farmer management exists at some level in all spate systems, there are essentially three types of management arrangement: predominantly farmer-managed; farmer-managed with involvement from local government or other external support; and managed by a specialized irrigation agency, in which case farmers may become passive recipients of water delivered.

Case study: Boru Dodota Spate Irrigation Scheme Boru Dodota Spate Irrigation Scheme which is located in Arsi zone, Oromia Regional State at 135 km from the capital Addis Ababa of Ethiopia, is established to increase agricultural production through supplementing the existing rainfed agriculture over 5,000 ha with spate irrigation (Aman, 2007). The diverted flood that is used for spate irrigation purposes at Boru Dodota Scheme are originated from Boru sub-basins and the adjacent micro sub-basins.

Case study: Boru Dodota Spate Irrigation Scheme Diversion headwork (Ogee weir ) is built to divert the flood from Boru Wadi. Fig. (Abebe D.)

Case study: Boru Dodota Spate Irrigation Scheme(BDSIS) There is enormous room to increase the efficiency of Sediment management, moisture conservation, scheme operation, flood diversion, flood management and scheme operation at BDSIS. Though the scheme was operated at overall poor efficiency level, in the year 2009 substantial improvement in yield on the spate irrigated field compared with the adjacent rainfed agriculture was observed.

Case study: Boru Dodota Spate Irrigation Scheme(BDSIS) Crops *Rainfed (Kg ha-1) *At Boru Dodota Spate Irrigation (Kg ha-1) High producing variety (kg ha-1) Wheat 80 250 4000-6000 Barley 100 270 - Teff 109 160 2200-2800 Maize 85 60 6000-8000 Haricot Bean 52 1500-2500 Fig. Actual yield for 2009; on rainfed field versus Boru Dodota Spate Irrigated field (source, Abebe D.)