Harvesting water on a farm is a critical strategy for managing scarce water resources, particularly in Kenya where many regions face erratic rainfall and prolonged dry seasons. Efficient water harvesting structures enable farms to capture and store rainwater, reducing dependence on unreliable external water supply and enhancing resilience to drought. This chapter explores the various water harvesting structures commonly used in Kenyan farms, highlighting their functions, construction, and suitability for different farming systems. Understanding these structures equips students across diverse professional fields with practical knowledge to contribute to sustainable water use in agricultural and allied sectors.

2.1 Identify Water Harvesting Structures

Water harvesting structures are physical installations designed to collect, convey, store, and utilize runoff water for agricultural use. These structures vary widely depending on farm size, topography, soil type, and water demand. In Kenya, where smallholder farming predominates, simple yet effective structures are preferred, but commercial farms and institutions such as universities and county agricultural offices also implement more complex systems. Recognizing the types and purposes of water harvesting structures is essential for professionals involved in farm management, environmental conservation, and rural development.

2.1.1 Pits and Trenches for Water Harvesting

Pits and trenches are among the simplest water harvesting structures widely used in small-scale farming and community projects. They are designed to capture surface runoff and increase water infiltration into the soil, thereby enhancing soil moisture availability for crops.

Characteristics of Pits and Trenches

• Shallow Excavations: Typically, pits are shallow depressions dug into the soil to intercept rainfall runoff before it flows away. This increases the time water stays in the root zone.
• Runoff Collection: Trenches are linear excavations that channel runoff from sloping land into storage areas or directly to crops.
• Soil Moisture Improvement: By retaining runoff, pits and trenches help recharge soil moisture, which is crucial during dry spells.
• Simple Construction: These structures require minimal labor and tools, making them accessible to smallholder farmers and institutions such as rural schools.
• Erosion Control: Trenches particularly help reduce soil erosion by slowing water flow and trapping sediments.

In practice, a farm cooperative in Meru County enhanced maize yields by installing contour trenches that captured runoff during the rainy season, improving water availability during dry months. At a county referral hospital’s agricultural unit, pits were used to retain water for vegetable gardens, ensuring continuous food supply for patients.

2.1.2 Farm Dams and Reservoirs

Farm dams and reservoirs are larger-scale water harvesting structures that store substantial volumes of water for irrigation, livestock, and domestic use. They are commonly used on medium to large farms and institutional farms such as universities and county government agricultural offices.

Key Features of Farm Dams and Reservoirs

• Water Storage Capacity: These structures are designed to hold runoff water collected from catchment areas, providing a reliable water source during dry periods.
• Embankments or Earth Walls: Dams are formed by constructing embankments to impound water, often using local materials like soil and rocks.
• Multiple Uses: Water stored can be used for drip irrigation, livestock watering, and supporting agro-processing activities.
• Catchment Area Management: Proper management of the upstream catchment is necessary to reduce sedimentation and maintain water quality.
• Maintenance Requirements: Regular desilting and embankment repair are essential to sustain dam functionality.

For example, a tea cooperative in Kericho County constructed a farm dam that captured runoff from surrounding hills, enabling irrigation during dry spells and increasing tea leaf quality. Similarly, a county government agricultural office in Kitui uses a reservoir to supply water for demonstration plots and livestock, improving extension services.

2.1.3 Roof Water Harvesting Systems

Roof water harvesting involves collecting rainwater from building roofs through gutters and storing it in tanks or underground reservoirs. This method is increasingly adopted in urban and peri-urban farms, schools, hospitals, and commercial establishments such as hotels.

Advantages of Roof Water Harvesting Systems

• Clean Water Source: Roof runoff is generally cleaner than surface runoff, making it suitable for domestic use and irrigation of high-value crops.
• Space Efficient: This system requires minimal land area, suitable for institutions with limited space like universities and county offices.
• Modular Design: Storage tanks can be scaled according to water demand and roof size.
• Reduction of Flood Risk: By diverting roof water, these systems reduce local flooding and soil erosion around buildings.
• Cost-Effectiveness: Installation costs are moderate, and maintenance is straightforward, involving regular cleaning of gutters and tanks.

At a Nairobi-based hotel, roof water harvesting has been integrated with drip irrigation for the kitchen garden, reducing reliance on municipal water. A retail business with an adjacent nursery utilizes roof runoff stored in tanks for watering plants, lowering operational costs and conserving water.

2.1.4 Contour Bunds and Terraces

Contour bunds and terraces are earth structures built along the contour lines on sloping farms to slow runoff, increase infiltration, and reduce soil erosion. These are widely used in hilly areas and by institutions with demonstration farms such as TVET colleges and agricultural training centers.

Functional Aspects of Contour Bunds and Terraces

• Water Retention: By interrupting slope length, these structures allow water to infiltrate rather than run off rapidly.
• Soil Conservation: They reduce the velocity of surface water, preventing topsoil loss and maintaining soil fertility.
• Crop Yield Improvement: Enhanced moisture retention supports crop growth, especially in marginal rainfall zones.
• Labor Intensive Construction: Building terraces requires careful planning and significant labor but offers long-term benefits.
• Adaptability: Terraces can be combined with planting of cover crops or agroforestry to maximize conservation impact.

A TVET college in Nakuru County has integrated contour terraces on its demonstration farm, showing students how these structures improve maize and bean yields. Similarly, a county hospital’s agricultural unit uses bunds to protect medicinal herb gardens from erosion and optimize water use.

Chapter Summary

This chapter explored various types of water harvesting structures commonly used on farms, highlighting their purpose and features. It then detailed the principles of designing and drawing these structures to ensure they meet specific farm requirements. The construction process was covered with emphasis on following design specifications and work standards. Methods for testing the functionality of water harvesting structures were explained to confirm their effectiveness in capturing and storing water. Procedures for identifying and correcting faults in these structures were discussed to maintain optimal performance. Finally, the chapter addressed how to properly utilize water harvesting systems to maximize water conservation benefits on the farm. Together, these topics provide comprehensive guidance for managing farm water harvesting efficiently and sustainably.