Artificial Rainmaking and Weather Modification

Weather modification refers to deliberate efforts to manipulate the climate or local weather conditions. It dates back to the 1940s when the US military began experimenting with cloud seeding to induce rainfall. Today, both public and private entities are involved in weather modification to reduce the damage caused by extreme weather events.

Principles of Rainmaking

• Cloud Classification:
  Clouds are classified into warm and cold types based on the temperature of their tops.
  • Warm Clouds: These have a positive temperature at the top.
  • Cold Clouds: These have a negative temperature at the top.
• Nucleation: The nucleus required for precipitation differs based on the cloud type:
  • Warm clouds need hygroscopic nuclei (water-absorbing materials).
  • Cold clouds require ice nuclei for precipitation.

History of Cloud Seeding

• Cloud seeding began with Vincent Schaefer’s experiments in the 1940s, where he found that ice crystals could promote precipitation.
• Initially, ice crystals were difficult to transport, so silver iodide was adopted due to its similar properties to ice.
• However, by the 1970s, due to limited successful results, the program was shelved until it was revived later with improvements.

Cloud Seeding: The Process

Cloud seeding involves injecting artificial condensation nuclei into clouds to encourage precipitation, thereby increasing rainfall in drought-prone areas. The practice has been employed in places like Texas and Utah to enhance crop production, though its effectiveness remains debated.

Techniques for Cloud Seeding

Seeding Cold Clouds

• Dry Ice Seeding Dry ice (solid carbon dioxide) is used to produce ice crystals when it evaporates. Aircrafts release dry ice pellets (0.5–1.0 cm) into the clouds, causing the formation of ice crystals and subsequent rainfall. However, it requires significant amounts of dry ice (about 250 kg per cloud), making it an expensive process.
• Silver Iodide Seeding Silver iodide is used because its atomic structure mimics that of ice, making it an effective ice-forming nucleus in supercooled clouds. The smoke produced by ground generators or aircraft disperses the silver iodide particles into the clouds, inducing precipitation. This method is more efficient than dry ice as it requires much less material and can cover larger areas.

Seeding Warm Clouds

• Water Drop Technique Large water droplets are introduced into warm clouds to encourage the coalescence process, where smaller droplets combine to form larger ones. Aircraft release water droplets (around 25 mm in size) to stimulate rain formation.
• Common Salt Technique Common salt, either in solution or solid form, is used to seed warm clouds. Salt and soap mixtures are applied using power sprayers or from the ground via generators. The balloon burst technique involves using gunpowder and sodium chloride to explode near the cloud base, dispersing salt particles.

Modification of Field Microclimate

In addition to rainmaking, technologies can be developed to modify field microclimates to support crop production, especially in regions with challenging environmental conditions.

1. Control of Heat Load
   • Heat Evasion: In tropical and subtropical regions, shading plants can reduce evapotranspiration. Using wood or fiber for shading helps plants tolerate higher heat loads.
   • Heat Trapping: In temperate climates, heat trapping is beneficial for crops with short growing periods. This can be achieved by planting trees on sunny slopes or erecting low stone walls that reflect sunlight.
2. Protection Against Frost Damage
   • Direct Methods: Measures like frost protection heaters or wind machines can be used to raise the temperature around plants.
   • Indirect Methods: Mulching or ploughing the soil reduces radiation from the ground, helping prevent frost formation on plants.

Applications of Cloud Seeding

• Increasing Precipitation: The primary application of cloud seeding is to increase precipitation, which can be done in both warm and cold clouds. The two main techniques for seeding are:
  • Hygroscopic Seeding (for warm clouds)
  • Glaciogenic Seeding (for cold clouds)
• Water Availability in Arid Regions:
   Cloud seeding can help alleviate water shortages, especially in areas with frequent droughts, by enhancing rainfall to support agriculture.

Indirect Methods for Improving Plant Water Management

i) Choice of Sites:

• • Site selection is critical for optimizing water use efficiency. Choosing locations that align with the plant’s water requirements is essential for reducing stress.
  • Soil conditions (such as texture, permeability, and drainage) should be considered to ensure that the soil can retain adequate moisture without excessive waterlogging.
  • Climatic conditions, including rainfall patterns, temperature, and evaporation rates, play a crucial role in determining the feasibility of planting certain crops. Selecting sites that have consistent moisture availability or are less prone to extreme weather conditions can help improve water efficiency.

ii) Use of Resistant Cultivars:

• • Drought-resistant cultivars are developed through selective breeding or genetic modification to tolerate dry conditions better. These plants have traits such as deeper root systems, thicker cuticles, or efficient stomatal control, which reduce water loss. Examples: Some varieties of maize, rice, and wheat have been bred to withstand drought conditions and maintain better yield stability even under water deficit conditions.

iii) Use of Crop Growth Regulators and Chemicals:

• • Crop growth regulators (CGRs), such as abscisic acid (ABA) and gibberellins, can help modify plant growth patterns to improve drought resistance.
  • Chemicals like antitranspirants (e.g., plastics, waxes) are applied to plant leaves to reduce water loss through transpiration. These chemicals create a barrier on the leaf surface, limiting evaporation and improving water retention.

Direct Methods for Managing Water

i) Mulches: Mulching involves covering the soil with materials (organic or inorganic) to reduce water loss by evaporation. Organic mulches, like straw, grass clippings, and compost, also improve soil fertility and structure as they break down. Inorganic mulches, such as plastic sheeting, reflect sunlight, keeping the soil cooler and reducing evaporation.

ii) Overhead Sprinkler Irrigation: Sprinkler irrigation involves dispersing water over crops using spray heads, mimicking natural rainfall. This method ensures a consistent water supply, which is particularly useful during drought conditions. Sprinklers can cover large areas and be automated to avoid wastage.

iii) Brushing: This technique involves creating a temporary barrier or shade over crops to reduce soil moisture evaporation and protect plants from extreme temperature fluctuations. It may involve using local materials, such as brushwood or temporary shade cloths, to provide a buffer between plants and environmental stress.

iv) Sand Application: Sand application can help reduce frost damage by improving soil structure and enhancing water drainage. It can also prevent soil crusting, which limits water infiltration. This practice is particularly beneficial in areas where winter frost damage is a concern for crops.

v) Windbreaks: Windbreaks, or shelterbelts, are rows of trees or shrubs planted to block wind and reduce moisture loss from the soil. By slowing wind speed, windbreaks help reduce evaporation rates and protect crops from desiccating winds.

Improving Water Balance in Agricultural Systems

i) Reducing Runoff:

• • Strip Cropping: Alternating rows of different crops to prevent soil erosion and reduce runoff.
  • Contour Ploughing: Plowing along the contour of the land to prevent water from flowing downhill, reducing soil erosion and runoff.
  • Terracing: Creating flat steps on hilly terrain to slow down water movement, allowing it to infiltrate the soil.
  • Inter-Terrace Land Management Practices: Implementing practices that maintain moisture in the soil between terraces, such as using cover crops or mulching.

ii) Increasing Infiltration:

• • Mulching with Organic Materials: Applying organic mulch, like straw, to improve the soil’s ability to absorb water by preventing surface crusting.
  • Tillage: Using tillage methods to break up compacted soils, facilitating better water penetration.
  • Fallowing: Leaving land fallow, without crops, for a season to allow the soil to rest, recover, and restore its water retention capacity.

iii) Reducing Evaporation:

• • Windbreaks: Planting trees or shrubs around fields to block winds and reduce moisture loss from the soil.
  • Use of Antitranspirants: Chemicals applied to plant surfaces that form a coating, reducing water loss through transpiration.
  • Mulching: Using organic or inorganic mulch to cover the soil surface and prevent moisture evaporation.

iv) Water Harvesting: Rainwater harvesting involves collecting and storing rainwater in ponds, tanks, or other storage facilities for later use. This water can be used for irrigation during dry spells, reducing dependency on other water sources.

Control of Atmospheric Turbulence

• Shelter Belts: Shelter belts consist of rows of trees or tall plants like corn, sorghum, and sunflowers, which act as barriers against wind. These belts reduce the wind speed, preventing soil erosion and conserving moisture.
• Afforestation: Afforestation involves planting trees in areas where forests did not previously exist. Forests act as natural windbreaks and moisture regulators, and their root systems improve soil water retention.
• Discouraging Urbanization: Urban sprawl often leads to the destruction of natural water systems, such as wetlands and floodplains. By discouraging urbanization in sensitive agricultural areas, natural hydrological cycles can be maintained, ensuring adequate water availability for agriculture.

Mid-Season Correction/Crop Life-Saving Techniques

1. Thinning: Thinning involves removing excess plants to reduce competition for water, nutrients, and light. This allows the remaining plants to access more resources and grow more efficiently.
2. Removal of Weaklings: Removing poorly growing or diseased plants prevents them from using up water and nutrients needed by the healthier plants, thus helping conserve moisture for the remaining crop.
3. Repeated and Deep Inter-Cultivation: Regular tilling or inter-cultivation helps reduce evaporation and improve moisture retention by breaking up compacted soil and incorporating organic matter. Deep inter-cultivation also encourages root development.
4. Mulching: Using crop residues or organic material as mulch helps retain soil moisture, suppress weed growth, and improve the overall soil structure.
5. Stripping of Older Leaves: Removing older leaves helps reduce water demand and allows the plant to focus resources on newer, healthier growth. This technique conserves water by reducing transpiration.
6. Ratooning: Ratooning involves harvesting a crop and allowing it to regenerate or produce a second crop without needing to be replanted. This technique is common in crops like sugarcane and rice, helping plants survive drought and continue producing after a dry spell.
7. Application of Nutrients: After rainfall, fertilizer applications help crops recover quickly and efficiently, ensuring that nutrients are available for recovery and growth during the growing season.
8. Life-Saving Irrigation: Life-saving irrigation is a practice where stored water from ponds, wells, or seasonal streams is used to irrigate crops during critical drought periods, preventing crop loss.
9. Use of Antitranspirants: Applying antitranspirants can help reduce water loss from plant surfaces, particularly in times of severe drought. These chemicals create a barrier that slows down transpiration, helping crops conserve water during critical stages.

Plant Adaptations to Drought

1. Escaping Drought (Ephemerals): Ephemeral plants complete their life cycle quickly, often before the onset of dry conditions. These plants only live for a short period, usually during the rainy season, avoiding drought stress altogether.
2. Drought Avoidance: Drought-avoiding plants possess adaptations such as deep root systems, leaf modifications, and stomatal regulation to minimize water loss and access water from deeper soil layers, allowing them to survive periods of low water availability.
3. Drought Tolerance: Drought-tolerant plants can survive severe water stress without significant damage. They achieve this by conserving water, maintaining cell turgidity, and using metabolic pathways that minimize water loss.
4. Conserving Water: Plants employ various mechanisms such as reduced stomatal conductance, thickened leaf cuticles, and water storage in tissues to minimize water loss and enhance survival in dry conditions.
5. Improving Water Uptake: Plants may have deeper or more extensive root systems that help them access water from deeper soil layers. Some species develop root hairs or mycorrhizal associations to improve water uptake.
6. Mitigating Stress: Drought-tolerant plants use mechanisms like osmotic adjustment (accumulating solutes like proline) to maintain water balance and reduce the stress of dehydration.
7. High Tolerance: Some plants can withstand extreme drought conditions by adapting over generations, developing traits like high osmotic potential and resilient tissues that allow them to survive even when 50% or more of their cells dry out. Examples include many desert plants and some xerophytes.

Escaping Drought:

Plants that escape drought do so by completing their life cycle before the onset of drought stress. These plants are known as drought escapers. They are not necessarily drought-resistant, but they can thrive in conditions where drought occurs after their life cycle is completed. Ephemeral plants: Many desert plants, such as certain types of millets, cowpea, greengram, and blackgram, are examples of drought escapers. These plants germinate at the beginning of the rainy season and complete their growth before the dry season begins, hence avoiding drought conditions.

Drought Resistance:

Drought resistance refers to a plant’s ability to endure drought conditions without suffering irreversible injury, allowing it to grow successfully and mature properly. Resistance can be further subdivided into drought avoidance and drought tolerance.

i) Drought Avoidance:

Drought avoidance is the ability of plants to maintain favorable water balance and turgidity, even under drought conditions, thus preventing the stress that typically comes with a water deficit. Several mechanisms contribute to drought avoidance:

• Stomatal Regulation: Drought-resistant plants can regulate their stomatal openings. Under drought stress, these plants can close their stomata to reduce transpiration and water loss. Some plants, such as semi-dwarf wheat, open their stomata early in the morning (when the stress is at a minimum), allowing for photosynthesis without significant water loss. In contrast, tall wheat varieties may open their stomata for only a few hours in the morning and remain closed the rest of the day.
• Increased Photosynthetic Efficiency: C4 plants (e.g., sorghum, pearl millet, maize, sugarcane) and CAM plants (e.g., pineapple, agave) are more efficient at photosynthesis compared to C3 plants under drought conditions. The enzyme phosphoenolpyruvic acid carboxylase in C4 plants has a high affinity for CO2, which enables higher photosynthetic activity with less stomatal opening, thereby conserving water.
• Low Cuticular Transpiration: The presence of a cuticle (waxy coating) on the leaves reduces water loss through the leaf surface. Plants growing under dry conditions, such as wheat, barley, and cacti, often develop this waxy coating to limit water loss.
• Reduction in Transpiration: Some plants produce lipids on their surfaces, which form a protective layer that reduces transpiration. Soybean and sorghum are examples of plants that use this mechanism.
• Reduced Leaf Area: Some plants reduce their leaf area to minimize water loss. For example: Xeromorphic plants reduce their transpiring surface, as seen in grasses. Some plants may roll or curl their leaves to minimize exposure to the sun, reducing water loss via transpiration. Groundnut (peanut) plants fold their leaves during midday to reduce the load of solar radiation and, in turn, water loss.
• Leaf Surface: Leaves with thick cuticles, waxy coatings, or spines (such as in cacti) are more effective in reducing transpiration. Pubescence (hairy surfaces) on leaves can increase reflectance, thus reducing solar radiation and water loss.
• Stomatal Frequency and Location: Plants with fewer stomata or stomata located in depressions can reduce water loss, as this limits the effect of air currents on the plant.

ii) Mechanisms for Improving Water Uptake:

Plants can also adopt strategies to improve water uptake, especially in deep soil layers, which is crucial for surviving drought.

• Effective Root System: Deep roots with extensive branching and root hairs help plants access water from deeper soil layers. For example, sorghum has a greater root branching density than maize, which contributes to its superior drought resistance. Some plants, such as xerophytes, grow secondary roots quickly after rainfall, enabling them to exploit available water more effectively.
• Root-to-Shoot Ratio: Under dry conditions, the root growth rate often exceeds that of the shoots, which helps reduce transpiration and ensures the plant can access a larger volume of soil for water.
• Increase in Liquid Phase Conductance: Increasing the diameter or number of xylem vessels in roots can enhance water uptake, especially in dry soils, helping to maintain water flow to the plant’s tissues.

iii) Osmotic Adjustment: Osmotic potential: Plants in drought conditions may break down carbohydrates and proteins to accumulate solutes in the cell sap, lowering the osmotic potential and helping the plant to retain more water and maintain turgor pressure.

Drought Tolerance:

Drought tolerance refers to a plant’s ability to survive and continue growing under severe drought conditions, even when 50% of the cells have died. Drought tolerance is achieved through several mechanisms:

1. Mitigating Stress:

• Resistance to Dehydration: Plants can accumulate solutes like proline to maintain osmotic pressure, which helps prevent dehydration.
• Prevention of Leaf Collapse: Thick cell walls, sunken stomata, and the presence of waxy coatings help plants prevent dehydration and leaf collapse under drought stress.

2. High Degree of Tolerance:

• Reducing Metabolic Strain: In xerophytes, the dehydration compensation point refers to the water potential at which photosynthesis and respiration are balanced, thus preventing metabolic strain during drought. This allows the plant to conserve energy and resources.
• Plastic Strain: This refers to the irreversible loss of plant tissues due to severe stress. Some perennial plants may lose above-ground parts, such as leaves and stems, during drought, while their underground parts (like rhizomes, bulbs, and tubers) remain dormant but viable.

3. Heat Resistance: Temperature Compensation Point: This is the temperature at which respiration and photosynthesis are equally rapid. Higher temperatures can slow down enzymatic activities, causing stress to the plant. Some plants, such as wheat, can be made more heat-resistant by foliar sprays of zinc, which activate enzymes and enhance metabolic processes.

Desirable Plant Characteristics for Drought-Resistant Crops:

Plants with the following traits are more likely to survive and thrive under drought conditions:

• Rapid germination and early root establishment.
• Rapid phenological development (early flowering and fruiting).
• Developmental plasticity (ability to adjust growth stages to avoid stress).
• Efficient osmotic adjustment.
• High photosynthetic rate despite limited water availability.
• Thick, small leaves to minimize water loss.
• Ability to adjust root growth in dry soils to access water.
• Dehydration tolerance, especially during the seedling and grain-filling stages.