35 Agrometeorology

Sunayan Saha

1. Learning outcomes
2. Introduction
2.1. Definition and scope
3. Microclimate near the crop and its effect
3.1. Solar radiation
3.1.1. Energy balance near crop canopy
3.1.2. Radiation interception and distribution inside the canopy
3.2. Temperature
3.3. Frost
3.4. Wind
3.4.1. Wind profile near the crop canopy
3.4.2. Effect of wind on crop plants
3.4.3. Wind break and Shelter belts
4. Agrometeorological techniques
4.1. Weather based forewarning of crop pest and diseases
4.2. Crop growth simulation models
5. Summary

1. Learning outcomes

After studying this module, you shall be able to know:

• the definition and scope of agrometeorology
• the microclimate characteristics near the crop canopy and its effect the various applications of agrometeorological techniques

2. Introduction

2.1. Definition and scope

Agrometeorology is one of the branches of Biometeorology and abbreviated form of agricultural meteorology. It is an interdisciplinary science dealing with the interactions of physical environment i.e. meteorological, climatological and hydrological factors with agriculture (includes animal husbandry, horticulture and forestry apart from field crops) and the applications of such knowledge to improve agriculture production processes in terms of quantity or quality of produce, pest or disease control and sustainability of land and resources. It provides practical solutions for harnessing climate potential of an area and for protection against or avoidance of climate-related risks. Agrometeorology is construed as a four-pronged strategy. First stage deals with the accurate description of the physical environment and biological responses. In the second stage, interaction of biological systems with their physical environment is studied. Agrometeorological forecasting is done in third step. The fourth and last stage is to develop agrometeorological services and support systems for real time farming system management. The services include providing the farmer and policy makers with strategies for long-term sustainable use of natural resources, tailor made weather information, tactical solutions, and technological knowhow for short-term adjustments in day to day agricultural operations. Agrometeorological studies extend from a upper few centimeters of active roots zone in soil to few vertical meters of the atmosphere in the immediate vicinity of soil surface where crops and higher organisms grow and animals live. The scope of the subject includes

• characterization of agricultural climate for determining crop growing season and delineation of agroclimatic zones
• climatic crop planning based on water requirement of crop and its availability for sustainable production
• Weather based crop management practices like sowing, interculture operations, nutrient application, irrigation scheduling and harvesting.
• Developing crop weather relationships
• Large area crop monitoring to check crop health and growth, simulation modeling and pre-harvest yield forecasting
• Extreme climatic events and their management
• Agro advisory services

3. Microclimate near the crop and its effect

The state of the atmospheric in and around a crop surface is distinctly different from rest of the atmosphere. The crop architecture, surface characteristics and management among other things determine the ambience and have bearing on the crop production process.

3.1.  Solar radiation

Solar radiation intercepted by the crop canopy is used instantly and cannot be stored for future. It controls the principal physiological process of plants which is responsible for economic yield i.e. photosynthesis. It also regulates the evapotranspiration and consumptive water use by the plant canopy. The angle of incidence of solar radiation, spectral composition of the radiation and ratio of diffuse to direct radiation intensity are very important factors for crop growth and yield.

3.1.1. Energy balance of crop canopy

Energy balance of a crop canopy can be expressed as

Rn= A+LE+S+P+M

Where, Rn = net radiation

A = Sensible heat flux (10-15% of Rn)

LE = Latent heat flux (75-85% of Rn)

P = Energy used in photosynthesis (5-15% of Rn)

M = Metabolic heat and energy stored in crop (5% of Rn)

Different component of energy balance vary with crop stage, soil moisture condition, wind speed and direction. Maximum amount of net radiation is generally used as latent heat of evaporation. During day time net radiation is positive and in night, it is negative.

3.1.2. Radiation interception and distribution inside the crop canopy

About 75 % of the incident solar radiation on the plant canopy is absorbed, 15 % is reflected and 10 % is transmitted. Plant leaf strongly absorbs in blue and red wavelengths but much less so in green. Further, it absorbs near infrared (NIR) only weakly but strongly so in the IR region. The above optical features vary with many factors such as, to name a few, leaf type, crop species, biotic or abiotic stresses experienced by the plant canopy and moisture or organic matter content of the soil background underneath the canopy (Fig. 1). The difference in optical properties of earth objects at various spectral bands are explored through remote sensing technology for problem solving in agriculture such as land use land cover mapping, in season monitoring of sown area progress and crop health, agricultural drought, disease-pest occurrence and input applications in precision farming. Remote sensing data acquired through air borne or space borne platforms, spectral indices, canopy reflectance models, crop simulation models etc. are used for such purposes.

Intensity and spectral distribution of radiation within crop canopies is important as these control the plant photosynthesis and microclimate. Such information can be used to desirably  manipulate the crop environment for optimal use of sunlight and other resources as well as for pest and disease control. Breeders can utilize such information for breeding new ideotype i.e. an ideal plant model (Fig. 2). A planophyle (plant with perfectly horizontal foliage) is photosynthetically more efficient than an erectophyle (plant with perfectly vertical foliage) as the relative light interception of horizontal and erect foliage is in the ratio 1:0.44. In full sunlight, generally a vertically inclined leaf can harvest light more efficiently than a horizontal leaf and the optimum inclination is 81o. While considering the whole plant, for more efficient use of light, the upper leaves in a plant canopy should have a near-vertical orientation, whereas the lower foliage should be almost horizontal. An ideal arrangement of the plant canopy is for the lower 13 % of the leaves to be oriented at an angle of 0o to 30o, the middle 37 % should be at 30o to 60o, and the upper 50 % should be at 60o to 90o with the horizontal (Chang, 1968). For young plants, the percentage of light interception is smaller and also varies much throughout the day (minimum at noon and maximum during the morning and evening hours). Also, during sunrise and sunset, high proportion of diffuse light (favourable to photosynthesis) penetrates into the canopy. Crops sown in the north-south direction at all latitudes allows more intercepted radiation than that of east-west orientation.

As the radiation penetrates the canopy from top to bottom, its intensity gets reduced and such vertical light distribution profile is governed by Beer’s law as follows:

I = Io e-kf

(1)

Where, I is the intensity of light at a particular height within the canopy, Io is the intensity at the top, k is the light extinction coefficient of the leaf (the ratio between the light loss through the leaf to the light at the top), f is the leaf area index (LAI) and e is the base of natural log.

The rate of photosynthesis and radiation use efficiency of the plant is dependent on the PAR absorbed by the leaves and green tissues within the canopy. To compute absorbed PAR (APAR), four types of flux density measurement representing the incoming or outgoing fluxes are required. These are Io [Incident PAR at the canopy top (incoming)], Rc [PAR reflected from the canopy top (outgoing)], Tc [PAR transmitted through the crop canopy to soil surface (incoming)] and Rc [PAR reflected from the soil surface (outgoing)]. APAR is computed using the formula given below:

APAR = (Io + Rc) – (Tc+ Ro)

(2)

APAR is used to compute light/radiation use efficiency (LUE or RUE is the ratio of amount of dry matter produced to the amount of cumulative PAR absorbed) of plants and in simulation modelling for estimation of vegetation primary productivity (e.g., NPP i.e. Net primary productivity or GPP i.e. Gross Primary Productivity). For operational convenience of measurement, sometimes Intercepted PAR (IPAR) is used in lieu of APAR (IPAR is generally higher in magnitude than that of APAR i.e. all the intercepted PAR does not translate into absorption by the plant canopy).

IPAR = Io – Tc

(3)

RUE is higher for C4 than C3 crops. It is also higher in cereals than that of crops with oil- and protein-rich seeds. The average RUE for a wide range of crop species was reported to be between 0.85 and 3.0 g MJ-1 for C3 crops and up to 4.8 g MJ-1 for C4 crops (Kiniry et al., 1989). Atmospheric factors other than radiation that affect RUE include temperature, vapour pressure and drought.

3.2. Temperature

Environmental temperature has a primary role in plant growth and its geographical distribution over the earth. In comparison to air temperature, the amplitude of variation in soil surface temperature is much more pronounced. Soil temperature largely depends on the prevailing air temperature near the surface. However, a number of factors such as aspect and slope of the land, tillage, soil texture, organic matter and irrigation or moisture content determine the degree of difference between air and soil temperature. From crop germination, soil temperature is most important. Soil temperature also affects plant growth processes. Tables 1 & 2 contain the examples of effect of very high and very low temperature in plants.

3.3. Frost

Frost is a climatic hazard that causes serious damage to standing crops in temperate and subtropical climates. Frost is a weather hazard that occurs when the environmental temperature drops below the freezing point of water. It is formed through the physical processes of radiation (cooling due to radiation loss from the earth during calm winter nights) and advective cooling (due to incursion of cold air mass). The probability of frost damage is high if the cell size of the plants is large. Winter vegetables like potato grown in Northern part of India are very much susceptible to frost injury(blackening and browning of the leaves, leaf chlorosis, burning of leaf tips, floret sterility etc.) and eventual death. Chances of damage can be minimized by adopting some management practices as given below:

• Adjusting the sowing time in such a way that the growing season remains frost free
• Selection of resistant varieties
• Applying irrigation to the field when environmental temperature is becoming conducive to frost

3.4.  Wind

3.4.1.   Wind profile near the crop canopy

As wind blows over a surface, the effect of friction, which in turn depends on the characteristics of the surface, tends to slow it down to various degrees. The frictional resistance diminishes as one goes above the surface with the result that wind speed is slowest at the surface and increases with height. For this reason anemometers are placed at a chosen standard height, i.e., 10 m in meteorology and 2 or 3 m in agrometeorology. For the calculation of evapotranspiration, wind speed measured at 2 m above the surface is required. To adjust wind speed data obtained from instruments placed at heights other than the standard height of 2 m, a logarithmic wind speed profile above a short grassed surface may be used:

4.87
2 = ln⁡(67.8⁡z−5.42)	(4)

where, u2 is wind speed at 2 m above the ground surface (m s-1)

uz is the wind speed measured at z m above the ground surface (m s-1)

z  is the height of measurement above ground surface (m)

3.4.2.   Effect of wind on crop plants

Wind can bring physiological, morphological and anatomical changes in crop plants as given below:

• breaks petioles and twigs thus causing reduction in flower/fruit production
• causes discoloration and abrasion on leaves and fruit from windblown particles
• high wind causes increased CO2 supply to plant through turbulent mixing, thus enhances photosynthetic capacity of the plant higher wind velocity causes greater cuticular transpiration than stomatal transpiration
• hot, dry wind has desiccating effect on plants which may cause shriveled grains in cereal crops like rice and wheat
• wind induced damages are more prominent on sea coasts and hill slopes

3.4.3. Wind break and Shelter belts

Windbreaks are generally defined as any structure that reduces wind speed, whereas shelterbelts are plantation usually made up of one or more rows of trees or shrubs planted in such a manner as to provide primarily some wind stress protection and also other benefits to the crop plants grown in their leeward side (Fig.3). They are generally planted around the edges of crop fields, perpendicular to the dominant wind direction. Although the primary objective of wind breaks and shelter belt is to reduce the mechanical damage to farm crops or orchards, there are some additional benefits as well:

• Reduce evapotranspiration, improve moisture balance in the soil
• Reduce soil erosion due to wind and water and pesticide drift during spray
• Trapping and conserving a uniform layer of snow on the field (to protect the plants from winter frost)
• Promotes nesting by insect pollinators; creating low-turbulence zones for commercial insect pollinators
• Improve biodiversity
• Protect crops, pastures livestock from cold or hot winds
• Protect living and working areas from strong winds
• Provide firewood, timber, fodder, honey and other products
• Provide habitat for wildlife
• Act as firebreaks

4. Agrometeorological techniques

4.1.Weather based forewarning of crop pest and diseases

Indirect meteorological hazards like incidence of insects, pests and plant diseases cause significant loss to crop production. As per Stakman and Harrar (1957), 20% of world production was lost due to insect, pest and direct weather hazards. Hence, timely control of insect, pest and diseases is very much important.

Weather parameters decide the nature, number and activity of pest and virulence of diseases, rate and duration of spread, migration of insect pests, geographical damage and extend. Such weather factors like air temperature and humidity must have minimum, optimum and maximum values for different disease development. However, the relation between weather parameters and plant pest disease incidence is complex as several other factors like soil moisture, soil temperature, management practices, host susceptibility, availability of carriers, primary inoculant amount etc. also play crucial roles. It can be said that for a particular disease to occur, three conditions need to be fulfilled. These are: I) a susceptible host plant in a vulnerable stage of growth, II) a disease causing pathogen in infective stage and III) favorable environmental conditions. Temperature, humidity and wind are three vital environmental parameters, though solar radiation, rainfall and dew are also important in disease pest incidence. It can also be noted that extreme weather events like heavy rainfall, extreme cold and hot weather helps in controlling disease and pest.

By exploring the relationship between weather, crop disease pest occurrences and host plant relationship, agrometeorologist can forecast the disease pest risk of crops in advance. This advance forecasting may help in taking suitable plant protection measures such as spraying of chemicals on time.

All weather variables like air temperature, moisture, wind speed and radiation affect four stages viz., egg, larva, pupa and adult of life cycle of all insect pests. Increasing temperature tend to reduce the span of all stages of life cycle of cotton leaf worm whereas, incubation period of egg and longivity decrease with rise in temperature. Soil moisture plays a very important role in spread and distribution of soil diseases caused by soil organism or soil borne insect pests. Plant surface wetness duration (PSWD) is very much important in the development of insect pest and diseases. Wind speed and direction at ground surface and at the level of troposphere helps in short distance dispersal of insect and disease spores. High wind speed along with low RH and soil moisture are very conducive for some disease development. Mechanical damage of plants by high speed winds allows entry of causal organism to plants. Light intensity and duration determines the survival of inoculums and duration of incubation period.

Forecasting of disease pest using meteorological techniques was first started in 1941 in Japan.

There are two approaches for forecasting as described below:

I) Physiological or laboratory approach: In this method, effects of weather parameters on life cycles of different insect pest and disease organisms are studied. The results help in setting the minimum, optimum and maximum values of weather parameters required for particular insect pest or disease development.

II) Statistical approach: In this method, relationship between weather parameters and disease and insect pest development is quantified with some statistical equations based on long term multi location data. For example, Prasad and Chakravarti (2000) developed model equation for forecasting aphid population. The equation is as follows:

Pt = X1P(t-1) + X2 Tmax (t-1) + X3 Tmin (t-1) +X4 Tmean (t-1)

Where, Pt = percent aphid population

P(t-1)= Aphid population one week before observation

Tmax (t-1), Tmin (t-1) and Tmean (t-1) = maximum, minimum and mean temperatures one week before observation

Statistical relationships such as above lead to the development of disease and insect pest forecasting models. Some disease and insect pest forecasting model are EPIMAY (for maize leaf blight, castor et al., 1975), NEGFRY (for potato leaf blight) and BLASTAM (for rice blast).

4.2.Crop growth simulation models (CGSM)

A crop growth simulation model is a dynamic simulation model that helps estimate crop yield as a function of weather conditions, soil conditions and choice of crop management practices. Functioning of a simulation model is represented by the following block diagram (Fig. 4).

CSM Some of the applications of CGSMs are given below:

• Study on crop responses to variable sowing dates, seed rates, row spacing, irrigation schedules, fertilizer doses/schedules etc. to identify the best package of practices such as for obtaining highest benefit:cost ratio, maximum production or maximum resource use efficiency.
• Yield gap analysis i.e. to quantify the yield gap between actual and potential yields in different climatic regions.
• Yield forecasting can be done prior to harvest under expected weather.
• Models are used to evaluate consequences of climate change on quantity and quality of production, greenhouse gas emission etc. and to find out adaptation or mitigation options to counter any adverse effect.
• Dynamic CGSMs can be used as decision support systems as part of Agromet Advisory System (AAS) in the country.

5. Summary

• Agrometeorology is the study of crop physical environment (includes both atmosphere and soil) and its interaction with biological system so that technologies can be developed for improving agriculture.
• Radiation interception and distribution within the canopy, air and soil temperature in the canopy microclimate are among the most important factors in determining agricultural production.
• Agrometeorological techniques such as disease pest forewarning models and dynamic crop growth simulation models can be used to analyze and reduce risks, and improving profitability of the farming community.