Water harvesting and its techniques

Importance of Water Harvesting

Rainwater harvesting is an ancient practice that involves collecting and storing rainwater for various uses, such as domestic consumption, agriculture, or industrial purposes. This method has been widely practiced for over 4,000 years, as a response to the irregularity and variability of rainfall. Water harvesting plays an essential role in areas where conventional water supply systems are unavailable or where fresh water resources are limited or poor in quality. Below are some of the significant benefits of water harvesting:

• Improves Groundwater Quality: Water harvesting helps recharge underground aquifers, which may be depleted or of poor quality. This process can improve the quality of groundwater over time.
• Rises Water Levels in Wells/Borewells: Rainwater harvesting can raise water levels in wells and borewells that are drying up, helping to preserve local water supplies.
• Drought Mitigation: Harvested water provides a backup in case of droughts, improving water security during dry periods and reducing the impact of water scarcity.
• Reduction in Soil Erosion: By reducing the volume and velocity of surface runoff, water harvesting minimizes soil erosion, which can occur when heavy rainfall carries away fertile topsoil.
• Prevents Flooding: It prevents the choking of stormwater drains, which can otherwise lead to flooding in urban and rural areas.
• Energy Conservation: Water harvesting reduces the need for pumping groundwater, which can be energy-intensive, thereby conserving energy.

Types of Water Harvesting

Rainwater Harvesting:

Rainwater harvesting refers to collecting and storing runoff from roofs, courtyards, or treated surfaces for agricultural and domestic use. It includes:

1. Roof and Surface Water Collection: Water from rooftops and other compacted surfaces is collected for domestic or agricultural use.
2. Micro-catchment Water Harvesting: Involves the collection of surface runoff from small catchments and storing it in a basin or catchment area, typically for planting trees or annual crops.
3. Macro-catchment Water Harvesting: This technique collects runoff from larger catchments, such as hill slopes, and channels it to nearby cropping areas, usually on flat terrain.

Flood Water Harvesting:

Flood water harvesting is the collection and storage of water from creeks or streams for irrigation. It includes two methods:

1. Flood Water Harvesting within Stream Beds: Water is stored by damming the stream, inundating the floodplain, and allowing the water to infiltrate into the soil. This water can then be used for irrigation or pasture improvement.
2. Flood Water Diversion: This method involves diverting flood water from its natural course and channeling it to fields for irrigation purposes.

Groundwater Harvesting:

Groundwater harvesting involves extracting water from underground sources. Various techniques include:

• Qanat Systems: A traditional method where underground channels are used to transport water from a higher elevation to the surface.
• Subsurface Dams: These are constructed in riverbeds to store water below ground in sediments, which helps recharge the aquifers.
• Sand Storage Dams: These dams store water in porous layers of sand, where it can be gradually absorbed by the surrounding soil and aquifers.

Water Harvesting Techniques

Water harvesting techniques can be categorized into short-term and long-term methods, depending on the objectives and the storage capacities required.

Short-Term Runoff Harvesting Techniques

1. Contour Bunds:

• These are embankments constructed along the contour lines of the land. They collect surface runoff within the area between two bunds.
• The height of the bund varies from 0.3 to 1.0 meters, and the length ranges from tens to hundreds of meters.
• These bunds are effective in holding rainwater, which helps prevent soil erosion while allowing water to slowly infiltrate into the ground.

2. Semi-Circular Hoop:

• This structure consists of a semicircular earthen embankment placed along the contour.
• Water collects within the hoop and excess water is discharged from the tips to the next lower hoop, forming a staggered arrangement.
• The hoop’s height ranges from 0.1 to 0.5 meters, and the radius ranges from 5 to 30 meters.
• Ideal for irrigation of grasses, fodder crops, and small trees.

3. Trapezoidal Bunds:

• Similar to semicircular hoops but with a trapezoidal shape, these bunds are designed to manage large runoff volumes.
• The bunds are arranged in staggered rows to intercept overflow water.
• Suitable for areas with high rainfall intensity that would damage smaller contour bunds.
• Often used for irrigating crops, grasses, and shrubs.

4. Graded Bunds:

• These are embankments constructed at a slight grade (0.5 to 2% slope), different from contour bunds.
• They intercept surface runoff and direct it to the next area through channels.
• The height ranges from 0.3 to 0.6 meters, and the bunds are often arranged like modified trapezoidal bunds.
• Suitable for regions with intense rainfall and prone to soil erosion.

5. Rock Catchment:

• Exposed rock surfaces are used to collect runoff water, which is directed into a storage tank.
• These systems are common in areas with rocky terrain, where water runoff can be rapidly drained into a storage area.
• The catchment area can range from 100 m² to several thousand m².

6. Ground Catchment:

• A large, compacted area is used to collect surface runoff.
• The channels are designed to reduce seepage, often covered with gravel to minimize water loss.
• Ground catchments are historically significant, as seen in the Negev desert of Israel, where such techniques have been used for centuries.

Long-Term Runoff Harvesting Techniques

Long-term water harvesting involves the creation of larger storage systems, typically for irrigation, fish farming, or electricity generation.

1. Dugout Ponds:

• Dugout ponds are excavated areas where surface runoff or groundwater is stored.
• These ponds are ideal for areas with a gentle slope (less than 4%) and a water table within 1.5 to 2 meters from the surface.
• These ponds require significant investment but can store water efficiently for long-term use.

2. Embankment Type Reservoirs:

• These reservoirs are constructed by building embankments across valleys or depressions in the catchment area.
• The runoff is collected in these reservoirs and used as needed.
• There are different classifications of embankment reservoirs, including:
  • Irrigation Dams: For storing water for irrigation purposes, often equipped with gated spillways for controlled water release.
  • Silt Detention Dams: These reservoirs detain silt-laden runoff, allowing sediment to settle while storing cleaner water for irrigation.
  • High-Level Ponds: Located at the head of valleys, these ponds store water that can be used for irrigation downstream. They may be part of a series of ponds to collect runoff from higher catchment areas.

3. Farm Ponds: Multi-purpose ponds designed for various uses, including irrigation, livestock watering, fish farming, or even as water supplies for household needs. These ponds are strategically placed to ensure easy access and effective utilization of harvested water.

4. Water Harvesting Ponds: These ponds are specifically designed for water harvesting, similar to farm ponds, but with a primary focus on collecting rainwater for irrigation or other agricultural needs.

Flood Water Harvesting

Flood water harvesting, particularly water spreading and spate irrigation, is a critical method used to harness excess water during flood events, enabling it to be used productively in agricultural areas. This process involves managing floodwaters that may otherwise flow away, thereby mitigating the negative impacts of floods while improving water availability for crops. Below are the key aspects of floodwater harvesting:

Water Spreading (Spate Irrigation)

Water spreading, also known as spate irrigation, is a traditional practice where floodwater is directed onto agricultural fields or rangelands. The goal is to allow floodwaters to spread across broad, level terraces, thereby recharging the soil and supporting crop growth. This technique is highly beneficial in regions with sporadic rainfall and flood events.

Key Characteristics of Water Spreading:

• Turbulent Channel Flow Harvesting: Floodwater is harvested either through diversion (diverting floodwater into designated areas) or by allowing it to spread across the channel bed or valley floor.
• Runoff Storage: The water that is spread over the land is absorbed into the soil profile, helping recharge groundwater and providing moisture for crops.
• Large Catchment Areas: Water spreading often involves long catchments, which can span several kilometers, ensuring a large area is available for water collection.
• Catchment to Cultivated Area Ratio: In water spreading, the ratio of catchment area to cultivated area is typically greater than 10:1. This means the water collected from a large area is distributed over a smaller agricultural area.
• Overflow Provision: Excess water is allowed to overflow from the harvesting area to prevent flooding and ensure the system can handle large volumes of water.

Examples of Flood Water Harvesting Techniques

Several techniques are employed to harvest floodwater effectively. Below are some of the primary methods used in floodwater harvesting:

Permeable Rock Dams (for Crops)

• Description: Permeable rock dams are low dams constructed across valleys to slow down and spread floodwater. These dams also help in preventing gully erosion by stabilizing the land and promoting better water absorption.
• Usage: These are particularly beneficial in gently sloping valleys, which are prone to transformation into gullies during flood events. The dams help control water flow, prevent soil erosion, and provide water for agricultural use.

Water Spreading Bunds (for Crops and Rangeland)

• Description: Water spreading bunds are embankments constructed at a gradient along the land to direct floodwater into designated areas for irrigation. The embankments are typically earthen and feature a “dogleg” shape to help spread water evenly across the land.
• Usage: In arid areas, water is diverted from a watercourse and channeled onto crop or fodder fields through diversion drains. These bunds ensure that floodwater is utilized effectively for irrigation purposes.

Flood Control Reservoirs

• Description: Flood control reservoirs are built at strategically important sites to help manage floodwaters. These reservoirs feature self-operating mechanical outlets that allow controlled release of harvested water into streams or canals as needed.
• Usage: Flood control reservoirs play a significant role in flood management by preventing the spread of excess water to other areas while ensuring that the water can be used efficiently for agriculture and water supply during dry periods.

Groundwater Harvesting

Floodwater harvesting can also involve groundwater harvesting techniques, which focus on capturing runoff and replenishing underground water sources. One such system is the Qanat system.

Qanat System

• Description: The Qanat system is a traditional groundwater harvesting method that involves digging a well (known as the mother well) at a reliable source of groundwater, usually at the base of a hill or in the foothills. A long horizontal tunnel (qanat) is then constructed to transport water from the mother well to the communities in the valley below. The tunnel is sloped gently and often has intermittent access shafts for maintenance.
• Benefits:
  • The system works on gravity alone, meaning it does not require energy, making it sustainable and cost-effective.
  • The qanat system is effective in preventing leakage, evaporation, and contamination, as water is transported underground.
  • It ensures that the water table is not depleted, as the amount of water distributed is fixed by the natural supply from the spring or mountain.
  • It provides a steady, reliable water supply to communities, especially in arid or marginal landscapes where surface water may be scarce.

Runoff vs. Flood Water Harvesting While both runoff and floodwater harvesting aim to utilize excess water for productive purposes, there are key differences between the two:

Runoff Harvesting

• Definition: Runoff harvesting involves collecting water that runs off surfaces like rooftops or ground areas, usually after rain. This water is often stored for on-site use, particularly for irrigation or domestic consumption.
• Benefits:
  • Increases water availability for local vegetation or crops.
  • Reduces water flow velocity and erosion, preventing soil degradation and siltation issues.
  • Typically involves smaller catchment areas and is focused on managing water at the local level.

Flood Water Harvesting

• Definition: Floodwater harvesting, in contrast, focuses on managing water from natural watercourses or flood events. This water is collected and either stored or diverted into agricultural areas to recharge the soil and provide moisture for crops.
• Benefits:
  • Provides a valuable water source not only for local use but also for downstream users, helping replenish rivers, wetlands, and groundwater.
  • Plays a key role in maintaining the ecological balance of floodplains, enhancing biodiversity by supporting aquatic life (such as fish) and controlling the spread of harmful pests (e.g., mosquito larvae).
  • Helps reduce soil erosion by controlling the speed and flow of floodwater.