Rainfall Analysis and Length of Growing Season (LGS)

1. Rainfall Analysis

Definition: Rainfall analysis is the study of the amount, distribution, and variability of rainfall over time to understand its impact on crop production and water availability in drylands.

Purpose:

• To determine the suitability of crops for an area.
• To plan sowing dates, cropping patterns, and moisture management strategies.
• To assess drought risk and crop failure probability.

Key Parameters of Rainfall Analysis

Facts:

• Rainfall less than 500 mm/year: Arid areas (high risk, dry farming).
• Rainfall 500–1000 mm/year: Semi-arid areas (dryland farming).
• Rainfall 1000–1150 mm/year: Dry sub-humid areas (rainfed farming).
• Southwest monsoon contributes ~70% of rainfall in drylands.
• CV of rainfall: 20–60% in semi-arid/arid zones → high drought probability.

Methods of Rainfall Analysis:

• Statistical methods: Mean, standard deviation, coefficient of variation, probability of exceedance.
• Water balance approach: Compare rainfall with potential evapotranspiration (PET) to assess moisture availability.
• Rainfall threshold analysis: Minimum rainfall required at critical crop growth stages.

2. Length of Growing Season (LGS) / Length of Growing Period (LGP)

Definition: LGS is the period (in days) during which soil moisture is adequate for the growth of crops without supplemental irrigation.

Significance:

• Determines crop selection and cropping system in drylands.
• Short LGS → need for short-duration, drought-tolerant crops.
• Long LGS → possible multiple cropping or intercropping.

Classification of LGS in Drylands

Factors Affecting LGS:

• Rainfall onset and distribution – irregular rains shorten LGS.
• Soil water-holding capacity – sandy soils → shorter effective LGS.
• Evapotranspiration (ET) – high ET reduces effective moisture.
• Temperature – very high or very low temperatures affect crop growth.

Facts:

• Drylands of India have LGS ranging from <90 days (arid) to 180 days (dry sub-humid).
• LGS is shorter than total calendar season due to mid-season droughts and uneven rainfall.
• Short LGS crops are mostly millets, pulses, and drought-tolerant oilseeds.

Relationship Between Rainfall Analysis & LGS

• Rainfall amount & distribution → Determines LGS.
• Erratic rainfall → Shortens effective LGS, increasing risk of crop failure.
• LGS is used in agro-climatic zoning, crop planning, and risk assessment.

Quick Facts for ASRB NET:

• Arid (<500 mm rainfall): LGS <90 days → pearl millet, moth bean.
• Semi-arid (500–1000 mm rainfall): LGS 90–150 days → sorghum, pigeon pea, sunflower.
• Dry sub-humid (1000–1150 mm rainfall): LGS 150–180 days → rice, maize, soybean.
• LGS helps in delineating dryland tracts for research and crop management.

Distribution of Rainfall in India

Facts:

• India’s rainfall is highly spatially and temporally variable.
• Coefficient of variation (CV):
  • Arid/semi-arid → 20–60% (high risk of drought)
  • Humid regions → <20% (more reliable rainfall)

Temporal Distribution

• Southwest Monsoon (June–Sept): Major rainfall contributor (~70% of annual rainfall).
• Northeast Monsoon (Oct–Dec): Important for Tamil Nadu, Kerala, Andhra Pradesh.
• Pre-Monsoon & Post-Monsoon Showers: Minor contribution; help in sowing summer crops.

Facts:

• Late onset or early withdrawal of monsoon → shortens effective growing season.
• Mid-season dry spells → reduce yield by 20–50% in rainfed areas.

Effective Rainfall

• Definition: Portion of total rainfall available for crop use after losses due to runoff, deep percolation, evaporation.
• Fact: Effective rainfall in arid regions <300 mm, even if total rainfall = 500 mm.