Rainwater Harvesting & Recycling: Techniques and Practices

1. Definition and Importance in Agronomy

• Definition:Rainwater harvesting is the technique of collection and storage of rainwater from surfaces on which rain falls, or its diversion into structures that hold water for direct use or for recharging groundwater.

Agronomic Importance:

• Supplemental Irrigation:Provides critical moisture during dry spells, ensuring crop survival and yield stability.
• Water Security:Reduces dependence on unpredictable monsoon patterns and over-exploited groundwater.
• Erosion Control:Reduces surface runoff, which minimizes soil erosion and loss of valuable topsoil and nutrients.
• Groundwater Recharge:Helps in maintaining the water table, ensuring the sustainability of wells and tubewells.
• Micro-climate Improvement:Increases soil moisture and groundwater levels, creating a more favorable micro-climate for plant growth.

Key Components of a Rainwater Harvesting System; A systematic approach involves three core elements:

• Catchment:The surface that receives rainfall directly. This can be:
• • Natural (e.g., Hill slopes, rocky surfaces)
  • Agricultural (e.g., Contoured crop fields, catchment areas in inter-plot farming)
  • Artificial (e.g., Rooftops, plastic sheets, paved areas)
• Conveyance:The system (e.g., gutters, pipes, channels) that transports the harvested water from the catchment to the storage/recharge unit.
• Storage/Recharge Unit:The place where water is either stored for direct use (e.g., tanks, ponds) or directed to recharge the aquifer (e.g., pits, trenches, wells).

Techniques and Practices

Techniques are broadly classified into two categories:

• In-Situ Rainwater Harvesting (Within the Field)

The primary goal is to minimize runoff and maximize water infiltration into the soil profile right where the rain falls. This is a core agronomic practice for soil moisture conservation.

• Contour Bunding:Building earthen embankments along the contour line of a slope to intercept runoff, allow it to infiltrate, and reduce erosion.
• Broad Bed and Furrow (BBF) System:A conservation tillage practice where crops are sown on broad beds and excess water drains into furrows, allowing for both moisture conservation and drainage.
• Tied Ridging:Creating small ties or barriers within crop furrows to create small basins that hold rainwater.
• Deep Tillage:Breaking hard pans to increase infiltration and soil water storage capacity.
• Mulching:Using organic or synthetic materials on the soil surface to reduce evaporation, control runoff, and conserve soil moisture.
• Pitting and Zai System:Small pits are dug and filled with organic matter to concentrate water and nutrients for plant roots (common in arid regions).
• Negarim Microcatchments:Small, diamond-shaped basins surrounded by low earth bunds to trap rainwater for tree cultivation.

• Ex-Situ Rainwater Harvesting (Runoff Farming)

This involves collecting runoff from a larger “catchment area” and storing it for irrigation in a “command area.”

• Farm Ponds / Percolation Tanks:The most common structure. Excavated pits to store surface runoff for supplemental irrigation. Type: Dugout ponds, Embankment ponds.
• Check Dams / Gully Plugs:Small, temporary or permanent structures built across gullies or streams to slow water flow, impound water, and facilitate groundwater recharge.
• Subsurface Dykes/Barriers:Underground barriers constructed across streams to impede groundwater flow and raise the water table.
• Recharge Shafts/Pits:To recharge deeper aquifers. Runoff water is channeled into a pit filled with boulders, gravel, and sand, which filters the water and allows it to percolate.
• Rooftop RWH with Storage Tanks:Collecting water from farm building rooftops and storing it in tanks for high-efficiency irrigation (e.g., drip system) for nurseries or high-value crops.

Recycling of Harvested Water in Agriculture

“Recycling” in this context refers to the efficient use of stored water.

• Conjunctive Use:Using harvested surface water (from ponds) in combination with groundwater. For example, using pond water early in the season and conserving groundwater for critical stages.
• Coupling with Efficient Irrigation Systems:The true value of stored water is realized when applied via:
• Drip Irrigation: Delivers water directly to the root zone, minimizing losses.
• Sprinkler Irrigation: Efficient for light irrigation and for certain field crops.
• Water Lifting Devices:Stored water can be lifted using energy-efficient pumps (solar-powered pumps are ideal) for irrigation.

Factors Affecting RWH System Design

An agronomist must consider:

• Rainfall Pattern:Amount, intensity, and distribution.
• Topography (Slope):Determines runoff volume and suitable structures (e.g., contour bunding on slopes, farm ponds in flat areas).
• Soil Type:Infiltration rate (e.g., sandy soil has high infiltration, less runoff; clayey soil has low infiltration, more runoff).
• Land Use and Crop Type:The water requirement of the cropping system dictates the storage capacity needed.

Important Concepts for ASRB NET

• Runoff Coefficient (C):The fraction of rainfall that becomes surface runoff. It is critical for calculating potential harvest.
  • Formula for Potential Harvest:V = R * A * C
  • Where, V= Volume of harvestable runoff, R = Rainfall (in meters), A = Catchment Area (in m²), C = Runoff Coefficient.

• Crop Water Requirement (CWR):The volume of stored water must be planned based on the CWR of the target crops for supplemental irrigation.
• Watershed Management:RWH is a fundamental component of integrated watershed management programs, aiming for sustainable development of land and water resources.

Key Statistical Data

Overall Water Resource and Rainfall Statistics

• Annual Precipitation:India receives an average annual rainfall of about 1,170 mm (approx. 4,000 Billion Cubic Meters – BCM). This is highly variable, ranging from over 11,000 mm in Cherrapunji to less than 100 mm in parts of Rajasthan.
• Utilizable Water:Out of the total precipitation:
• Surface Water Utilizable:Only about 690 BCM is utilizable surface water.
• Groundwater Recharge:The annual replenishable groundwater resources are 433 BCM.
• Total Utilizable Water Resources:The total utilizable water resources in the country are thus estimated at 1,123 BCM (690 BCM surface + 433 BCM ground).
• The Critical Gap:Despite high rainfall, India has only 4% of the world’s freshwater resources to support 18% of the world’s population.

Rainwater Harvesting Potential and Current Usage

• Harvestable Rainwater:It is estimated that of the total rainfall, only about 8% is captured. A significant potential remains untapped.
• Rooftop Rainwater Harvesting Potential:A study by the National Institute of Urban Affairs (NIUA) indicates that: 60%of the urban water demand can be met through rooftop rainwater harvesting in major Indian cities. For example, Delhihas the potential to capture 76 billion litres of rainwater annually from rooftops, which is about 35% of its total water supply.
• Irrigation Dependency:Over 60% of India’s net sown area is still rainfed, supporting 40% of the population. RWH is critical for the sustainability of this agriculture.

• Farm Ponds:As per the 2019-20 Agricultural Census, the number of farm ponds in the country saw a substantial increase, with states like Andhra Pradesh and Maharashtra leading in numbers. A well-designed farm pond (e.g., 20m x 20m x 3m) can hold about 2 million litres, providing critical irrigation for a kharif crop failure or for a rabi crop.
• Micro-Irrigation Linkage:The potential efficiency gain is huge. Using harvested rainwater through drip irrigation can achieve 80-90% efficiency compared to 60-70% for sprinklers and <50% for flood irrigation.