Butajira Irrigation Project Summary

1. Project Overview

• Location: 25 km west of Ziway on the Ziway-Butajira road
• Initial identified irrigable area: ~500 ha (later revised to ~200 ha gravity-fed)
• Total potential irrigable area with pumping: ~2500 ha
• Primary water source: Woja River (mean annual runoff ~75×10⁶ m³)

2. Key Components

Component	Findings
Water Resources	Proposed dam site on Woja River identified Initial storage capacity estimate: >500×10⁶ m³ at 1820 contour Revised estimate: 75×10⁶ m³ mean annual runoff Water quality suitable for irrigation
Soil Resources	Soils generally suitable for irrigation Formed in volcanic ashes covering ignimbrites Some areas of less suitable soils need exclusion Detailed soil profile analysis completed
Crop Water Requirements	Current crops: maize, sorghum, red peppers, wheat Estimated requirement: ~1450 mm for wheat/cotton rotation Assumed irrigation efficiency: 40%
Project Outline	200 ha irrigable by gravity below 1812 contour Remaining area would require pumping Two options considered: electric power or mini-hydro station

3. Geological Findings (Annex A)

• Dam site geology dominated by volcanic rocks (tuff and ashes)
• Rocks are highly weathered, fractured, and permeable
• Weathered crust thickness >15m complicates construction
• Key recommendations:
  • Remove weathered rock from cutoff trench
  • Conduct additional drilling/geophysical surveys
  • Locate suitable construction materials

4. Soil Investigation (Annex B)

• 700 ha study area on west bank of Woja River
• Flat terrain (0-2% slope) with uniform soils
• Soil profile shows:
  • Loamy topsoil (10-60cm)
  • Bleached eluvial horizon (evidence of waterlogging)
  • Dark silty clay layer at 30-50cm depth
  • Brownish loamy layers below
• pH increases with depth from 6.5 to 8.3

5. Next Steps

• Further geological investigations needed
• Detailed design for gravity-fed and pumped irrigation areas
• Final decision on power source for pumping
• Additional soil surveys for excluded areas