Agriculture for Engineers 4 (3+1)

Lesson 21. Effect of Different Weather Parameters on Crop Growth and Development -II

21.1  RAINFALL

Precipitation is reaching of atmospheric humidity either as rain or snow to the ground. Precipitation occurs as rainfall when the temperature is high and as snowfall when the temperature is low. In India most of the precipitation, except in Jammu and Kashmir and some other hill station, occurs mainly as rainfall.

21.1.1 Rainfall climatology

 Study of rainfall over a long period is called rainfall climatology It reveals the general pattern of rainfall of a particular place. It helps in understanding the amount, intensity, distribution, and other rainfall characteristics. Rainfall analysis also helps in classification of climate. Suitable and efficient cropping systems can be developed by understanding the rainfall pattern. Rainfall analysis helps in taking decision on time of sowing, scheduling of irrigation, time of harvesting, etc. rainfall analysis is necessary for designing farm ponds, tanks, or irrigation projects. Amount, distribution and intensity of rainfall are the important aspects of rainfall that have considerable influence on crop production.

21.1.1.1 Amount of rainfall

Generally yield levels are determined by the amount of precipitation above the basic minimum required to enable the crop to achieve maturity. Though rainfall has major influence on yield of crops, yields are not always directly proportional to the amount of rainfall. When the rainfall is concentrated in 4-5 months of the year, there may be period when the rate of precipitation exceeds the intake rate of soil. As a result, considerable runoff occurs, plant nutrients are leached out of the root zone and crops are adversely affected by anaerobic conditions, especially if the excess precipitation occurs during the cool season.

21.1.1.2 Intensity of rainfall

Intensity of rainfall mainly influences erosion of soil. Study of rainfall intensity helps in probable period of floods, filling of irrigation tanks, etc. If the intensity of rainfall exceeds rate of infiltration of soil, runoff starts. High intensity rainfall causes soil erosion. The runoff from hills and mountain slopes is collected in tanks.

21.1.1.3 Distribution of rainfall

The amount of rainfall received at periodic intervals like weeks, months, season, etc. indicates distribution. In addition distribution of rainfall can be known by the length of dry spells, wet spells and rainy days. Distribution of rainfall is more important than total rainfall.  

 21.2 RELATIVE  HUMIDITY

Relative humidity (RH) directly influences the water relations of plant and indirectly affects leaf growth, photosynthesis, pollination, occurrence diseases and finally economic yield.

21.2.1 Water Relation

Relative humidity has considerable influence on evapotranspiration and hence on the water requirement of crops. At constant temperature, change in atmospheric humidity affects transpiration by modifying the vapour pressure gradient from leaf to air. In dry regions relative humidity tends to below which causes severe water deficits in plants and reduces leaf water potential.

21.2.2 Leaf growth

Leaf growth not only depends on synthetic activities resulting from biochemical process but also upon the physical process of cell enlargement. Cell enlargement occurs as a result of turgor pressure developed within the cells. Turgor pressure is high under RH due to less transpiration. Thus leaf enlargement is high in humid areas.

21.2.3 Photosynthesis

Photosynthesis is indirectly affected by RH. When RH is low, transpiration increases causing water deficits in the plant. Water deficit cause partial or full closure of stomata and increase mesophyll resistance blocking entry of carbon dioxide. There is large increase in both mesophyll and stomatal resistance causing decline in net uptake of carbon dioxide and thus photosynthesis is affected.

21.2.4 Pollination

Moderately low air humidity is favorable for seed set in many crops, provided soil moisture supply is adequate. For example, seed set in wheat was high at 60 per cent RH compared to 80 per cent when water availability in the soil was not limiting. When RH is high, pollen may not be dispersed from the anther.

21.2.5 Pests

The incidence of insect, pest and disease is high under high humidity conditions. High RH favors easy germination of fungal spores on plant leaves. The blight diseases of potato and tea spread more rapidly under humid condition. Several insects such as aphids and jassids thrive better under moist conditions.

21.2.6 Grain yield

Very high or very low RH is not conductive for high grain yield. Under high humidity, RH is negatively correlated with grain yield of maize. The yield reduction was 144 kg/ha with an increase in one per cent of mean monthly RH. Similarly, whet grain yield is reduced in high RH. It can be attributed to adverse effect of RH on pollination and high incidence of pests.

21.3 WIND

Wind direction and velocity have significant influence on crop growth. In general, wind increases transpiration. When wind is hot, it accelerates the desiccation of the crop by replacing the humid air by dry air.

Wind increases the turbulence in atmosphere, thus increasing the supply of carbon dioxide to the plants resulting in greater photosynthetic rates, Net Assimilation Rate (NAR) and Relative Growth Rate (RGR). the indices of growth, increase with increase in wind speed.

Wind also alters the balance of hormones. Wind increases ethylene production in barley and rice. Wind decreases Gibberallic acid content of roots and shoots in rice.

Hot dry winds cause reduction in plant height due to reduction in cell expansion as the cells cannot attain full turgidity. Lodging is another major injury or damage caused by high wind. The other adverse effect of winds are drying of leaf tips, Tearing of leaves, fruit drop, breaking branches, uprooting of plants, etc. Fruit drop due to heavy gales is a major problem in mango crop. Heavy winds accompanied by rainfall cause uprooting of crops.