🗺️ Drought, Climate Classification & Agro-Climatic Zones of India

Why Drought and Climate Zones Matter for Agriculture

In the previous lesson, we explored clouds and cloud seeding — how clouds form, the 10 types, and artificial rainmaking techniques. Now we tackle the consequences when rain fails: drought — and the classification systems that help planners match crops to regions.

In 2018, a severe drought in Maharashtra forced thousands of farmers to abandon their soybean and cotton crops. Dryland farmers in Rajasthan routinely face crop failure when growing periods shrink below 5 weeks. Understanding drought classification helps design contingency plans, while knowing India's agro-climatic zones ensures that the right crop is grown in the right region — the foundation of agricultural planning.

This lesson covers:

1. Drought types — meteorological, agricultural, hydrological, socio-economic
2. Agricultural drought sub-types — early, mid, late, chronic, apparent
3. Drought indices — AAI, MAI, MDI, NDVI, SPI, Palmer Index
4. Weather forecasting — short, medium, and long range; synoptic vs statistical methods
5. IMD and agro-meteorological services — AMD, IAAS, DAAS, AMFUs
6. Growing Degree Days and crop-weather models — GDD formula, CERES, DSSAT
7. Length of Growing Period — LGP thresholds for cropping systems
8. Agro-climatic zones — Planning Commission's 15 zones, ICAR's 127 zones, NBSS&LUP's 20 AERs
9. Climate classification — Koppen, Thornthwaite, Troll systems
10. Meteorological instruments — heights, uses, and key devices

All topics are high-yield for IBPS AFO, NABARD Grade A, and FCI exams.

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What is Drought?

Drought can be defined in three ways:

1. The condition under which crops fail to mature because of insufficient water supply through rains.
2. A situation where water required for transpiration and evaporation by crops exceeds available soil moisture.
3. No precipitation in a rainy season for more than 15 days continuously — such stretches are called dry spells.

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Four Types of Drought

Type	Definition	Key Threshold	Agricultural Impact
Meteorological	Seasonal rainfall falls below 75% of the climatologically expected normal over a wide area	Moderate: 26–50% deficit; Severe: >50% deficit	Cropping pattern disrupted; sowing delayed
Agricultural	Inadequate rainfall followed by soil moisture deficit; 4 consecutive weeks (May–mid Oct) or 6 weeks (rest of year) without adequate soil moisture	Yield reduction due to soil moisture shortage	Growth reduction; yield loss; crop failure
Hydrological	Streams, rivers, reservoirs, lakes, and wells dry up due to marked depletion of surface water; ground water table falls	Lags meteorological drought by weeks/months	Irrigation sources fail; industry and power affected
Socio-economic	Drought impacts on livelihoods, food supply, and economy; cascading effect of the above three types	Human welfare indicators decline	Food prices rise; migration; employment loss; famine

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Five Types of Agricultural Drought

Sub-Type	When It Occurs	Cause	Agricultural Example
1. Early Season	Start of season	Delayed onset of monsoon or prolonged dry spell after onset	Seedling mortality in rainfed sorghum; need for resowing bajra
2. Mid-Season	During crop growth	Inadequate soil moisture between two successive rainfall events	Maize at tasseling stage suffers; grain filling reduced in rice
3. Late Season (Terminal)	End of season	Early cessation of monsoon; weak monsoon activity	Rainfed rice in dry sub-humid regions fails due to September rain failure (40–50% dry spells > 5 days)
4. Chronic	Most years	Rainfall and stored soil moisture inadequate most years	Assured growing period is hardly 6–7 weeks in arid Rajasthan; only short-duration bajra survives
5. Apparent	Any time	Mis-matching of cropping pattern with rainfall/moisture	Growing long-duration rice in a 90-day LGP zone

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Key Drought Terms

Term	Definition
Drought year	Annual rainfall ≤ 75% of normal
Drought-prone area	Severe or moderate drought in ≥ 20% of years
Chronic drought area	Severe or moderate drought in ≥ 40% of years
Palmer Drought Index	Given by Palmer (USA, 1965); uses temperature + rainfall to estimate relative soil moisture

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Drought and Moisture Indices

Index	Full Form / Formula	Use
AAI (Aridity Anomaly Index)	Departure from normal aridity	Used by IMD weekly to monitor agricultural drought in real-time
MAI (Moisture Availability Index)	P (at 50% probability) / PET × 100	Crop planning in dryland areas
MDI (Moisture Deficit Index)	(P − PET) / PET × 100	Estimate dryness of a region (used in ICAR classification)
TMI (Thornthwaite Moisture Index)	AET / PET	How well water supply meets atmospheric demand
NDVI (Normalised Difference Vegetation Index)	Satellite-derived; lower values = stressed vegetation	Drought severity assessment from vegetation greenness
SPI (Standardised Precipitation Index)	Based solely on monthly precipitation data	Versatile drought index; works across all time scales

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Drought Monitoring Systems

System	Full Form	Location	Function
NADAMS	National Agricultural Drought Assessment and Monitoring System	NRSA, Dehradun	Satellite-based vegetation monitoring using NDVI
NRSA	National Remote Sensing Agency	Hyderabad	Remote sensing data for crop and drought monitoring

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Weather Forecast Types

Drought monitoring is reactive — it tells us what has happened. Weather forecasting is proactive — it gives farmers advance warning to protect crops. Accurate weather forecasting is the first line of defence against drought and weather damage:

Forecast Type	Validity Period	Primary Users	What It Predicts
Short range	Up to 72 hours	Farmers, marine agencies, public	Rainfall, heavy rain, heat/cold waves, thunderstorms
— Now casting	0–2 hours	Same as above	Immediate weather conditions
— Very short range	0–12 hours	Same as above	Near-term weather
Medium range	3 to 10 days	Farmers	Rainfall occurrence, temperature trends
Long range	Beyond 10 days up to a month/season	Planners, policy makers	Monsoon rainfall; expressed as deviation from normal

Forecasting Methods

Method	Best For	Approach
Synoptic	Short-range forecasts	Weather maps, pressure systems, fronts
Statistical	Short and long range forecasts	Historical data, statistical relationships

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IMD and Agro-Meteorological Services

Agricultural Meteorology Division (AMD)

• Established at Pune in 1932 — one of the oldest agro-met services worldwide.
• Services: GKMS (Gramin Krishi Mausam Seva), Agromet advisories, Training for AMFUs.

Integrated Agro-meteorological Advisory Service (IAAS)

Milestone	Year	Detail
Weather broadcasts for farmers	1945	IMD radio broadcasts
State-level advisories	1976	Short-range forecast based
NCMRWF established	1988	Medium-range (3–10 day) numerical weather prediction
AMFUs created	—	130 units across all 127 agroclimatic zones in SAUs/ICAR/IITs
IAAS launched	1 April 2007	IMD took over Agromet Advisory Service
DAAS launched	2008	District-level Agromet Advisory Service

• IMD issues 5-day district-level forecasts twice a week using Multi-Model Ensemble technique.
• Seven parameters: rainfall, max/min temperature, wind speed/direction, RH, cloudiness.
• Advisories reach farmers through SMS, mobile apps, radio, TV, newspapers, Kisan Call Centres.

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Growing Degree Days (GDD)

Weather forecasts predict what is coming; Growing Degree Days help predict where a crop stands in its development cycle. GDD is one of the most important tools in agricultural meteorology for predicting crop development and planning farm operations.

• Formula: GDD = (T_max + T_min) / 2 − T_base
• Negative values are treated as zero (plants do not undergo negative growth).
• Base temperature = temperature below which plant growth is zero (e.g., 5°C for cereals).
• GDDs are accumulated daily from the beginning of the season.
• Used to compare growing season progress against long-term average and estimate maturity.

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Crop-Weather Modelling

Mathematical equations explaining crop interaction with environment, used to predict growth and yield. First used by Baier (1979).

Model	Full Form	Application
CERES	Crop Environment Resource Synthesis	Rice, Maize, Sorghum, Wheat simulation
DSSAT	Decision Support System for Agro-technology Transfer	Climate change impact on rice (TN)
PRECIS	Providing Regional Climates for Impacts Studies	Regional climate projections
InfoCrop-RICE	—	Impact of high temp + elevated CO₂ on rice yield
CROPGRO-Groundnut	—	Climate change impact on groundnut (Anantapur, Junagadh)

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Length of Growing Period (LGP)

LGP is the period when moisture availability from rainfall exceeds half the potential evapotranspiration (PET). It determines what cropping system is viable:

LGP and Cropping System

LGP (Days)	Viable Cropping System	Agricultural Example
< 75	Perennial vegetation only	Rangelands in western Rajasthan
75–140	Sole cropping	Single crop of bajra or short-duration pulses
140–180	Intercropping	Sorghum + pigeonpea intercrop in Maharashtra
> 180	Double cropping	Rice-wheat system in Punjab

LGP and Climate Region

LGP (Days)	Region
< 75	Arid
75–120	Semi-Arid
> 120	Humid

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15 Agro-Climatic Zones of India (Planning Commission, 1989)

The Planning Commission of India (1989) divided the country into 15 agro-climatic regions based on rainfall, temperature, topography, cropping systems, and water resources.

All 15 Agro-Climatic Zones — Detailed

1. Western Himalayan Zone

Feature	Detail
States	J&K, Himachal Pradesh, UP hills
Climate	Cool and humid
Rainfall	750–1500 mm
Temperature	July: 5–30°C; January: -5 to 5°C
Soils	Skeletal, podsolic, hilly brown; silty loams prone to erosion
Crops	Winter: barley, oats, wheat; Kharif: rice (valleys), maize (hills)
Horticulture	Apple, peach, apricot, pear, cherry, almond, litchi, walnut
Special	Saffron grown here; cropping intensity lowest in J&K, highest in HP

2. Eastern Himalayan Zone

Feature	Detail
States	Sikkim, Darjeeling, Arunachal Pradesh, NE states, parts of WB
Climate	Humid and sub-humid
Rainfall	1840–3530 mm
Crops	Rice, maize, potato, tea
Special	Shifting cultivation (Jhuming) in ~1/3 of cultivated area; major constraint: soil erosion

3. Lower Gangetic Plains Zone

Feature	Detail
States	West Bengal (lower Gangetic plain), Brahmaputra valley
Climate	Moist humid to dry humid
Rainfall	1300–1600 mm
Soils	Mostly alluvial; flood-prone
Crops	Rice (main), jute, rapeseed, wheat

4. Middle Gangetic Plains Zone

Feature	Detail
States	12 districts of eastern UP, 27 districts of Bihar
Area	16 million hectares
Irrigation	39% of gross cropped area
Cropping intensity	142%
Rainfall	1211–1470 mm
Crops	Sugarcane, paddy, maize, wheat

5. Upper Gangetic Plains Zone

Feature	Detail
States	32 districts of central and western UP
Irrigation intensity	131% (canals and tube wells)
Cropping intensity	145%
Rainfall	720–980 mm
Crops	Rice, wheat, sugarcane

6. Trans-Gangetic Plains Zone

Feature	Detail
States	Punjab, Haryana, Delhi, Chandigarh, Sriganganagar (Rajasthan)
Also called	Satluj Yamuna Plains
Climate	Semi-arid
Rainfall	650–1250 mm
Key distinction	Highest net sown area, highest irrigated area, high cropping intensity, highest ground water utilisation
Crops	Wheat, sugarcane, cotton, rice, gram, maize, pulses, oilseeds

7. Eastern Plateau and Hills Zone

Feature	Detail
States	Eastern MP, southern WB, most of inland Odisha
Climate	Moist sub-humid to dry sub-humid
Rainfall	1270–1435 mm
Soils	Red, yellow, laterites, alluvium; shallow to medium depth
Irrigation	Through tanks and tube wells
Crops	Rice, millets, maize, oilseeds, ragi, gram, potato
Special	Largest agro-climatic zone in India; deficient in water resources

8. Central Plateau and Hills Zone

Feature	Detail
States	46 districts of MP, parts of UP and Rajasthan
Climate	Semi-arid to dry humid
Rainfall	400–1550 mm
Rainfed area	75% with low-value cereal crops
Soils	Mixed red, yellow, black; ravines present
Crops	Wheat, gram, jowar, bajra, paddy, millets, oilseeds, cotton
Constraint	Scarcity of water

9. Western Plateau and Hills Zone

Feature	Detail
States	Major part of Maharashtra, parts of MP, one district of Rajasthan
Climate	Semi-arid
Rainfall	600–1040 mm (avg 904 mm)
Irrigated area	Only 12.4%
Soils	Regur (black) soil
Crops	Jowar, bajra, cotton, wheat
Special	Provides 50% of India's jowar; best quality orange, grape, and banana

10. Southern Plateau and Hills Zone

Feature	Detail
States	35 districts of AP, Karnataka, and TN
Climate	Semi-arid
Dryland farming	81% of area
Cropping intensity	111%
Rainfall	675–1000 mm
Crops	Coffee, cotton, tea, cardamom, spices

11. East Coast Plains and Hills Zone

Feature	Detail
States	East coast of TN, AP, and Odisha
Climate	Semi-arid and dry sub-humid
Rainfall	780–1290 mm
Soils	Alluvial and coastal sands
Rainfed area	75%
Crops	Rice, ragi, jowar, bajra
Special	Contributes 17.5% of India's groundnut production

12. West Coast Plains and Ghats Zone

Feature	Detail
States	West coast of TN, Kerala, Karnataka, Maharashtra, Goa
Rainfall	2230–3650 mm (highest rainfall zone)
Soils	Laterite and coastal alluvial
Crops	Rice, ragi, groundnut, tapioca
Special	Spices and plantation crops along Western Ghats hill slopes

13. Gujarat Plains and Hills Zone

Feature	Detail
States	19 districts of Gujarat
Climate	Arid and semi-arid
Irrigated area	Only 32.5%
Rainfed area	78%; about 60% drought-prone
Rainfall	340–1793 mm
Crops	Maize, wheat, groundnut, tobacco, cotton, jowar, bajra
Special	Known as "Oilseed Region"

14. Western Dry Zone

Feature	Detail
States	Nine districts of Rajasthan
Characteristics	Hot sandy desert; erratic rainfall; scanty vegetation; no perennial rivers
Rainfall	95 mm (lowest in India)
Ground water	Deep and often brackish
Forest area	1.2%; irrigated: 6.3%
Cropping intensity	105% (lowest)
Crops	Bajra, gram, wheat, rapeseed
Special	Famine and drought are common features

15. Islands Zone

Feature	Detail
States	Andaman & Nicobar Islands and Lakshadweep
Climate	Equatorial
Rainfall	3000 mm over 8–9 months
Crops	Rice, maize, millets, pulses, arecanut, turmeric, cassava
Special	Nearly half the cropped area under coconut

Key Zone Facts for Exams

Zone	Distinguishing Feature
Trans-Gangetic (Zone 6)	Highest net sown area, highest irrigated area, highest cropping intensity, highest groundwater use
Eastern Himalayan (Zone 2)	Shifting cultivation (Jhuming)
Eastern Plateau & Hills (Zone 7)	Largest agro-climatic zone
Gujarat Plains (Zone 13)	Oilseed zone; 60% drought-prone
Western Plateau (Zone 9)	50% of India's jowar; best orange, grape, banana
Western Dry (Zone 14)	Lowest rainfall (95 mm); famine and drought zone
West Coast (Zone 12)	Highest rainfall (2230–3650 mm)
Western Himalayan (Zone 1)	Saffron cultivation

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ICAR Classification — 127 Agro-Climatic Zones (NARP)

Under the National Agricultural Research Project (NARP), ICAR advised State Agricultural Universities to divide each state into sub-zones based on rainfall, cropping pattern, and administrative units. This yielded 127 agro-climatic zones. ICAR's climate classification uses Moisture Deficit Index (MDI) under the All India Coordinated Research Project on Dryland Agriculture, specifically designed for agricultural planning in dryland areas.

State-wise NARP Zones (Selected)

State	Zones	State	Zones
Madhya Pradesh	12 (most)	Uttar Pradesh	10
Karnataka	10	Rajasthan	9
Odisha	9	Maharashtra	9
Gujarat	8	Kerala	8
Andhra Pradesh	7	Tamil Nadu	7
West Bengal	6	Assam	6
Bihar	6	NE Hill region	6
Punjab	5	J&K	4
Himachal Pradesh	4	Haryana	2

Key Numbers for Exams

Classification	Number	Authority
Agro-climatic regions (Planning Commission)	15	Planning Commission, 1989
Agro-climatic zones (NARP)	127	ICAR/NARP
Total ACZs (ICAR reclassification)	131	ICAR
Agro-ecosystems under NARP	5	ICAR
Agro-ecological regions (NBSS&LUP)	20	NBSS&LUP/ICAR
State with most NARP zones	Madhya Pradesh (12)	ICAR

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20 Agro-Ecological Regions (NBSS & LUP)

The National Bureau of Soil Survey and Land Use Planning (NBSS & LUP) of ICAR delineated 20 agro-ecological regions (AERs) using the FAO 1978 concept — superimposing length of growing periods and bio-climate maps on soil physiographic maps.

All 20 Agro-Ecological Regions

Ecosystem	No.	Region	Key States	Eco-region	Soil Type
Arid	1	Western Himalaya	J&K, HP	Cold Arid	Shallow skeletal
Arid	2	Deccan Plateau	AP, Karnataka	Hot Arid	Red & Black
Arid	3	Western Plain, Kutch, part of Kathiawar	Gujarat, Rajasthan, Haryana, Punjab	Hot Arid	Desert & Saline
Semi-Arid	4	Northern Plain & Central Highlands (Aravallis)	Gujarat, Rajasthan, UP, MP, Haryana, Punjab	Hot Semi-Arid	Alluvium Derived
Semi-Arid	5	Central (Malwa) Highlands, Gujarat Plains	Gujarat, MP	Hot Semi-Arid	Medium & Deep Black
Semi-Arid	6	Deccan Plateau	Karnataka, AP, Maharashtra, MP	Hot Semi-Arid	Shallow & Medium Black
Semi-Arid	7	Deccan (Telangana) Plateau & Eastern Ghats	AP	Hot Semi-Arid	Red and Black
Semi-Arid	8	Eastern Ghats, TN Uplands, Deccan (Karnataka) Plateau	Karnataka, TN, Kerala	Hot Semi-Arid	Red Loamy
Sub-Humid	9	Northern Plain	Bihar, UP, Punjab	Hot Sub-humid (dry)	Alluvium-derived
Sub-Humid	10	Central Highlands (Malwa, Bundelkhand, Eastern Satpura)	MP, Maharashtra	Hot Sub-humid	Black & Red
Sub-Humid	11	Eastern Plateau (Chhattisgarh)	MP	Hot Sub-humid	Red & Yellow
Sub-Humid	12	Eastern Plateau (Chhota Nagpur) & Eastern Ghats	Odisha, WB, Bihar, MP, Maharashtra	Hot Sub-humid	Red & Laterite
Sub-Humid	13	Eastern Plain	UP, Bihar	Hot Sub-humid (moist)	Alluvium-derived
Sub-Humid	14	Western Himalayas	J&K, HP, UP	Warm Sub-humid to humid	Brown Forest & Podzolic
Humid/Per-humid	15	Bengal & Assam Plains	WB, Assam	Hot Sub-humid to Humid	Alluvium derived
Humid/Per-humid	16	Eastern Himalayas	Arunachal Pradesh, Sikkim, WB	Warm Per-humid	Brown & Red Hill
Humid/Per-humid	17	NE Hills (Purvanchal)	Tripura, Mizoram, Meghalaya	Warm Per-humid	Red & Lateritic
Coastal	18	Eastern Coastal Plain	TN, Puducherry, AP, Odisha, WB	Hot Sub-humid to Semi-arid	Coastal Alluvium
Coastal	19	Western Ghats & Coastal Plain	Kerala, Goa, Maharashtra, Gujarat	Hot Humid Per-humid	Red, Lateritic, Alluvium
Island	20	Andaman-Nicobar & Lakshadweep	A&N, Lakshadweep	Hot Humid Per-humid	Red Loamy & Sandy

AER Ecosystem Distribution

Ecosystem Type	Number of AERs
Arid	3
Semi-Arid	5
Sub-Humid	6
Humid/Per-humid	3
Coastal	2
Island	1
Total	20

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Climate Classification Systems

Agro-climatic zones are practical divisions for planning. Climate classification systems provide the scientific framework behind those divisions. Different scientists have classified climate using different criteria:

Classification By	Basis	Key Detail
De Candolle	Vegetation	Earliest; plant-based
Koppen (1884, modified 1918, 1936)	Mean monthly temperature and mean annual rainfall	Most widely used; first quantitative classification; 5 groups
Thornthwaite (1931)	P/E ratio and P/E index (sum of 12 monthly P/E ratios)	Recognised runoff and percolation losses; 6 temperature provinces (Tropical to Frost)
Thornthwaite and Mather (1948)	Moisture index (Im), PET	Im = 100 × [(P − PE) / PE]; modified by Mathur (1955)
Troll (1965)	Temperature and duration of humid months	Classification called "Seasonal Climates of the Earth"; humid month = month where P > PET; modified by ICRISAT for India

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Koppen Classification (A, B, C, D, E)

Koppen divided climate into five categories:

Group	Climate Type	Temperature/Precipitation Criteria	Indian Agricultural Region
A	Tropical rainy	Coolest month > 18°C; P > E	Kerala, coastal Karnataka — rice, coconut, spices
B	Tropical dry	Coolest month > 18°C; P < E	Rajasthan, Gujarat — bajra, guar, drought-tolerant crops
C	Humid temperate rainy	Coolest month 18°C to -3°C	Nilgiris, Western Ghats hills — tea, coffee
D	Cold forest	Coolest month < -3°C; precipitation as snow	Kashmir, HP — apple, walnut, saffron
E	Polar	Warmest month < 10°C	Ladakh — minimal agriculture

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Thornthwaite Classification — Moisture Index (Im)

Im Value	Climate	Agricultural Viability
100 and above	Per humid	Plantation crops; waterlogging risk
20 to 100	Humid	Rice, jute, tea
0 to 20	Moist sub-humid	Wheat, maize with some irrigation
-33.3 to 0	Dry sub-humid	Rainfed millets, pulses
-66.7 to -33.3	Semi-arid	Sorghum, groundnut, cotton
-100 to -66.7	Arid	Only drought-tolerant crops with irrigation

Moisture Deficit Index formula: MDI = 100 (P - PET) / PET

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Meteorological Institutions

The classification systems above were developed by international scientists, but India's own meteorological infrastructure implements them. These institutions and their founding details are frequently asked in exams.

Institution	Location & Year	Key Function
IMO (International Meteorological Organization)	Geneva, 1873	Predecessor of WMO
IMD (Indian Meteorological Department)	Calcutta, 1875 (later Shimla → Pune → New Delhi)	India's weather forecasting authority
WMO (World Meteorological Organization)	Switzerland, 1950	Global weather coordination
IITM (Indian Institute of Tropical Meteorology)	Pune, 1962	Monsoon and tropical weather research
IIRS (Indian Institute of Remote Sensing)	Dehradun, 1966	Remote sensing for agriculture
NCMRWF	New Delhi, 1988	Medium range weather forecasting
NRSA (National Remote Sensing Agency)	Hyderabad	Satellite-based crop monitoring

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Agro-Meteorological Observatory Standards

Standard	Specification
Distance from buildings/trees	10 times the height of the obstacle
Plot size	60 m (N-S) x 40 m (E-W)

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Meteorological Instruments — Heights and Uses

Every agro-meteorological observatory follows standardised placement rules to ensure consistent, comparable data. The heights below and the instrument-function pairings are heavily tested in competitive exams.

Standard Heights from Ground Level

Instrument	Height	What It Measures
Sunshine recorder, Anemometer, Wind vane	3.08 m	Sunshine hours, wind speed, wind direction
Stevenson screen	1.22 m	Houses thermometers (max, min, dry/wet bulb)
Automatic rain gauge	75 cm	Continuous rainfall recording
Ordinary rain gauge, Evaporation pan	50 cm	Daily rainfall, evaporation

Key Instruments and Their Uses

Instrument	Measures	Agricultural Use
IR Gun / Infrared Thermometer	Plant canopy surface temperature (8–14 μm range)	Estimates crop water stress without contact
Net Radiometer	Net radiation at earth's surface	Energy balance studies
Quantum Sensor	PAR above crop canopy	Light availability for photosynthesis
Tube Solarimeter	PAR reaching ground through crop canopy	Canopy light interception efficiency
Pyradiometer	Both long-wave and short-wave radiation	Total radiation balance studies
Assaman Psychrometer	RH in open and crop canopy	Humidity monitoring for pest management
Hygrograph	Continuous record of RH	Long-term humidity tracking
Rain Gauge	Rainfall	Basic rainfall measurement
Pluviograph	Continuous rainfall record	Rainfall intensity analysis
Drosometer / Dew Gauge	Dew measurement	Dew contribution to dryland crops
USWB Evaporimeter / Atmometer	Evaporation	Irrigation scheduling
Cone Penetrometer	Soil mechanical resistance	Tillage planning
Fortin Barometer (Mercurial)	Most accurate atmospheric pressure	Weather station standard
Barograph	Continuous atmospheric pressure	Pressure trend monitoring

Evapotranspiration, Soil, and Water Instruments

Instrument	Measures	Key Detail
Lysimeter	Evapotranspiration (ET)	Most accurate method for measuring ET; large soil container with vegetation
Tensiometer	Soil moisture tension	Range: 0–0.8 bar; widely used for irrigation scheduling
Infiltrometer	Infiltration rate	Double-ring type; measures vertical water entry into soil
Permeameter	Hydraulic conductivity	Measures rate of water flow through soil
Piezometer	Depth of water table / groundwater pressure	Monitoring well water levels

Temperature and Frost Instruments

Instrument	Measures	Key Detail
Cryometer	Temperature below 0°C	Uses alcohol instead of mercury (Hg freezes at −39°C)
Infra-Red Thermometer	Canopy temperature	Indicator of crop water stress (stressed crops = higher canopy temp)

Plant Growth and Physiology Instruments

Instrument	Measures	Key Detail
Auxanometer	Plant growth	Mechanical growth measurement device
Crescograph	Minute plant growth	Invented by Sir J.C. Bose; magnifies growth up to 10,000×
Potometer	Transpiration	Measures water uptake rate by cut shoot
Porometer	Stomatal opening area	Indicates plant water status and photosynthetic activity
Pressure Chamber / Thermocouple	Leaf water potential	Measures plant water stress directly
Pressure Membrane Apparatus	Moisture equivalents	Soil water retention curves

Density and Liquid Instruments

Instrument	Measures	Key Detail
Hydrometer	Relative density of liquids	Used in soil particle size analysis
Lactometer	Fat percentage in milk	Dairy quality testing
Pycnometer	Soil specific gravity	Soil physical property determination

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Exam-Important Rapid-Fire Facts

Quick-Reference Facts for Competitive Exams

Atmosphere and Radiation

• Average temperature of earth surface: 14–15°C
• 50% of atmospheric mass below 5.6 km
• 90% below 16 km; 99% below 40 km
• Air density at sea level: 1.2 kg/m³
• Density decreases with height
• Diurnal temperature variation: up to 16°C
• Seasonal variation: less near equator
• Incoming solar radiation: short wave (visible)
• Outgoing earth radiation: long wave (infrared)
• Temperatures at tropopause, stratopause, mesopause remain constant

Pressure and Wind

• Equatorial low-pressure belt: Doldrums (5°N and S)
• Atmospheric pressure = force per unit area
• Most accurate pressure instrument: Fortin barometer

Humidity

• Maximum RH: morning; Minimum RH: afternoon
• RH = 100% when temperature = dew point
• Pest incidence higher at higher RH

Agriculture-Specific

• Plant hardening causes: increased sugar concentration
• Highest solar energy utilisation: Sugarcane (10–12%)
• Acid rain caused by: SO₂ and NO₂
• Frost damage worst on: sandy soils
• Hailstorms: before onset and after withdrawal of monsoon
• Optimum weather data for climate: 31 years
• Weather data from sloppy land: Topo climate
• Weather data from standing crop: Microclimate
• Conversion efficiency: 3,700 to 4,100 cal/g of dry matter
• Oldest light intensity unit: Lux
• Biological amplification of DDT: Bio-magnification
• Atmospheric gases are in: mixed states

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Summary Table

Topic	Key Fact	Exam Value
Drought — dry spell	No rain for > 15 days continuously	Definition
Drought cascade	Met → Agri → Hydro → Socio-economic	Sequence
Meteorological drought	Rainfall < 75% of normal; Moderate: 26–50% deficit; Severe: >50%	Threshold
Agricultural drought sub-types	Early, Mid, Late, Chronic, Apparent (EML-CA)	Mnemonic
Drought year	Rainfall ≤ 75% of normal	Definition
Drought-prone area	Severe/moderate drought ≥ 20% of years	Definition
Chronic drought area	Severe/moderate drought ≥ 40% of years	Definition
Palmer Drought Index	Palmer, USA, 1965	Index
AAI	IMD weekly drought monitoring	Index
MAI	P at 50% probability / PET × 100	Index
NDVI	Satellite-based vegetation index for drought	Index
NADAMS	NRSA, Dehradun; satellite drought monitoring	Monitoring
Terminal drought	Early cessation of monsoon; rainfed rice fails	Crop-specific
Chronic drought	Growing period only 6–7 weeks	Duration
Short-range forecast	Up to 72 hours	Time period
Synoptic method	Best for short-range forecasts	Method
Statistical method	Best for short and long range forecasts	Method
Long-range forecast	Beyond 10 days to a season	Time period
AMD established	Pune, 1932	Institution
IAAS launched	1 April 2007	Institution
DAAS launched	2008	Institution
AMFUs	130 units	Institution
GDD formula	(Tmax + Tmin)/2 − Tbase; negative = zero	Formula
GDD base temp (cereals)	5°C	Threshold
Crop-weather modelling	Baier (1979); CERES, DSSAT, PRECIS	Models
Thornthwaite (1931)	P/E ratio, P/E index, 6 temperature provinces	Classification
Troll (1965)	"Seasonal Climates of the Earth"; humid month = P > PET; modified by ICRISAT	Classification
LGP < 75 days	Arid; perennial vegetation only	Classification
LGP 140–180 days	Intercropping viable	System choice
Planning Commission zones	15 agro-climatic regions (1989)	Number
NARP zones	127 (ICAR: 131)	Number
ICAR classification	Under All India Coordinated Research Project on Dryland Agriculture	Classification
NBSS&LUP regions	20 agro-ecological regions	Number
Trans-Gangetic Zone	Highest irrigation, sown area, cropping intensity	Zone identification
Eastern Plateau & Hills	Largest agro-climatic zone	Zone identification
Western Dry Zone	Lowest rainfall (95 mm)	Zone identification
Koppen — Group B	Only group classified by rainfall, not temperature	Classification basis
IMD established	Calcutta, 1875	Institution question
WMO established	Switzerland, 1950	Institution question
Lysimeter	Most accurate ET measurement	Instrument
Tensiometer	Soil moisture tension, 0–0.8 bar	Instrument
Crescograph	J.C. Bose; 10,000× magnification	Instrument
Stevenson screen height	1.22 m	Instrument height
Rain gauge height	50 cm	Instrument height

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Summary Cheat Sheet

Concept / Topic	Key Details
Drought dry spell	No rain for > 15 days continuously
Drought cascade	Met → Agri → Hydro → Socio-economic
Meteorological drought	Rainfall < 75% of normal; Moderate 26–50%; Severe >50%
Agricultural drought sub-types	Early, Mid, Late, Chronic, Apparent (EML-CA)
Drought year	Rainfall ≤ 75% of normal
Drought-prone area	Drought in ≥ 20% of years
Chronic drought area	Drought in ≥ 40% of years
Palmer Drought Index	Palmer, USA, 1965
AAI	IMD weekly drought monitoring
MAI	P at 50% probability / PET × 100
NDVI	Satellite-based vegetation drought index
SPI	Standardised Precipitation Index (monthly)
NADAMS	NRSA, Dehradun; satellite drought monitoring
Terminal drought	Early cessation of monsoon; rainfed rice fails
Chronic drought	Growing period only 6-7 weeks
Short-range forecast	Up to 72 hours
Synoptic method	Best for short-range
Statistical method	Best for short + long range
Long-range forecast	Beyond 10 days to a season
AMD established	Pune, 1932
IAAS launched	1 April 2007
DAAS launched	2008
AMFUs	130 units
GDD formula	(Tmax + Tmin)/2 − Tbase; negative = zero; cereals Tbase = 5°C
Crop-weather modelling	Baier (1979); CERES, DSSAT, PRECIS, InfoCrop-RICE, CROPGRO-Groundnut
Thornthwaite (1931)	P/E ratio, P/E index, 6 temperature provinces
Troll (1965)	"Seasonal Climates of the Earth"; humid month = P > PET; modified by ICRISAT
ICAR classification	Under All India Coordinated Research Project on Dryland Agriculture
LGP < 75 days	Arid; perennial vegetation only
LGP 140-180 days	Intercropping viable
Planning Commission zones	15 agro-climatic regions (1989)
NARP zones	127 (ICAR: 131)
NBSS&LUP regions	20 agro-ecological regions
Trans-Gangetic Zone	Highest irrigation, sown area, cropping intensity
Eastern Plateau & Hills	Largest agro-climatic zone
Western Dry Zone	Lowest rainfall (95 mm)
Koppen Group B	Only group classified by rainfall, not temperature
IMD established	Calcutta, 1875
WMO established	Switzerland, 1950
Lysimeter	Most accurate ET measurement
Tensiometer	Soil moisture tension 0–0.8 bar
Crescograph	J.C. Bose; 10,000× magnification
Stevenson screen height	1.22 m
Rain gauge height	50 cm
Optimum climate data	31 years