KrishiMela.bd
Scientific ReferenceStandard: UN FAO-56

Precision Irrigation Methodology & Agronomic Guide

A comprehensive, step-by-step breakdown of KrishiMela.bd’s mathematical modeling, soil physics, plant population dynamics, atmospheric evapotranspiration, and pump runtime scheduling.

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Jump to:1. Six-Step Pipeline2. Worked Step-by-Step Example3. Complete Terminology Glossary4. Data Fusion & Error Margins5. Academic Citations

1. The 6-Step Irrigation Calculation Pipeline

KrishiMela.bd executes an integrated physical-agronomic algorithm to convert atmospheric meteorological data, crop physiological parameters, soil hydrology, and pump flow rates into an actionable irrigation schedule.

1

Atmospheric Evaporative Demand (Reference ET0)

Unit: mm / day

What it represents: The atmospheric "thirst" or evaporative suction power of the ambient weather. It models how much water an extensive surface of green grass (0.12 m tall, actively growing, well-watered) would evaporate and transpire under current solar radiation, ambient temperature, relative humidity, and wind speed.

UN FAO-56 Penman-Monteith Equation:

ET₀ = [ 0.408 × Δ × (Rₙ - G) + γ × (900 / (T + 273)) × u₂ × (eₛ - eₐ) ] ÷ [ Δ + γ × (1 + 0.34 × u₂) ]

Solar Radiation (Rₙ):Thermal energy driving vapor phase change.
Temperature (T):Mean daily air temp at 2m height (°C).
Vapor Deficit (eₛ - eₐ):Drying capacity between wet leaves and air.
Wind Speed (u₂):Removal of saturated boundary layer vapor.
2

Crop Specific Evapotranspiration (ETc)

Formula: ETc = ET0 × Kc

What it represents: While ET0 is weather-dependent, different crops consume water differently depending on leaf area, stomatal resistance, crop height, and growth phase. The Crop Coefficient (Kc) acts as an empirical multiplier.

1. Initial StageKc,ini: ~0.30 - 0.45

Germination / small seedling. Water loss is mostly soil evaporation.

2. DevelopmentKc,dev: ~0.70 - 0.85

Rapid vegetative growth, canopy expanding to 70-80% ground cover.

3. Mid-Season (Peak)Kc,mid: ~1.05 - 1.20

Flowering, pollination, and fruit set. Maximum water consumption.

4. Late-SeasonKc,end: ~0.60 - 0.80

Ripening, fruit maturation, leaf senescence, and dry-down.

3

Plant Spacing, Stand Density & Individual Allocation

A_plant = Sr × Sp

What it represents: Farm fields are organized in geometric rows and beds. The spatial arrangement dictates root competition, solar canopy interception, and emitter distribution.

Ground Area Per PlantArea_plant = Row Spacing × Plant SpacingE.g., 0.60 m row × 0.40 m plant = 0.24 m² / plant.
Total Plant PopulationTotal Plants = Plot Area (m²) ÷ Area_plantE.g., 4,046.86 m² (1 acre) ÷ 0.24 m² = 16,861 plants.
Per-Plant Daily Water DemandLiters / Plant / Day = ETc (mm) × Area_plant (m²)Converts field-wide millimeter depths directly to drip emitter volumes.
4

Soil Hydrology & The Root Zone "Bucket" Model

TAW = 1000(θ_FC - θ_WP)Z_r = ((w_FC - w_WP)/100)·ρ_b·Z_r

What it represents: Soil is not a solid block; it is a porous sponge. The plant root zone acts as a storage reservoir that fills during rainfall/irrigation and steadily empties through evapotranspiration. Per FAO-56 Equation 82, when Field Capacity (θ_FC) and Wilting Point (θ_WP) are expressed as volumetric percentages (% v/v), TAW = ((θ_FC - θ_WP)/100) × Z_r(mm). When measured as gravimetric dry-weight percentages (% w/w), multiply by Bulk Density (ρ_b) since θ_v = w_g × ρ_b.

Field Capacity (FC)

The maximum capillary moisture held against downward gravity drainage (usually measured 24-48 hours after heavy wetting). Water above FC is gravitationally lost as deep percolation.

Management Depletion (MAD)

The allowable depletion fraction (p), typically 40% to 55%. This defines the Readily Available Water (RAW) buffer:RAW = MAD × AWC

Wilting Point (PWP)

The suction threshold (-15 bars / -1500 kPa) where soil holds remaining water so tightly that roots cannot extract it. If reached, plants permanently desiccate.

Irrigation Interval Formula:Interval (Days) = Readily Available Water (RAW in mm) ÷ Daily Crop Water Need (ETc in mm/day)
5

Application Efficiency (η)

No irrigation system delivers 100% of pumped water to the crop root zone due to wind drift, evaporation, canal seepage, and deep percolation.

  • Drip / Micro-Irrigation:85% - 92% Efficiency
  • Overhead Sprinkler:70% - 80% Efficiency
  • Furrow / Border Basin:55% - 65% Efficiency
  • Traditional Flood / Canal:40% - 50% Efficiency
Gross Water Needed = Net Root Need (mm) ÷ Efficiency (η)
6

Pump Discharge & Run Time

Translating field water volume into pump run minutes using pump capacity (m³/hr or liters/minute).

Run Time (Hours) = Total Volume (m³) ÷ Flow Rate (m³/hr)
Run Time (Minutes) = Run Time (Hours) × 60

Prevents over-pumping, reducing diesel/electricity waste, leaching of costly nitrogen fertilizers, and waterlogging.

2. Fully Worked End-to-End Agronomic Example

Let’s walk through a realistic, actual field scenario in Bangladesh to see how KrishiMela.bd connects every equation together.

Field Scenario Parameters
Crop:Tomato (Mid-Season, Fruit Set)
Plot Size:1 Acre (4,046.86 m² / 100 decimals)
Soil Texture:Loam (Doash Soil)
Irrigation System:Drip System (η = 90%) with 25 m³/hr Pump
Step 1Calculate Reference Evapotranspiration (ET₀) from Weather Data

NASA POWER / Local Meteorological Station records: T_max = 32°C, T_min = 21°C, Relative Humidity = 65%, Wind = 2.1 m/s, Solar Rad = 21 MJ/m²/day.

ET₀ = 4.80 mm / day
Step 2Apply Crop Coefficient (K_c) for Mid-Season Tomato

According to FAO-56 Table 12, tomato at full mid-season fruit development has a crop coefficient of K_c = 1.15.

ETc = ET₀ × Kc = 4.80 mm/day × 1.15 = 5.52 mm / day

The crop transpires exactly 5.52 mm of water per day across the field.

Step 3Compute Spacing, Total Plant Population, and Daily Need per Plant

Bed arrangement: Row Spacing = 0.60 m (60 cm), Plant Spacing = 0.40 m (40 cm).

Ground Footprint / Plant:0.60m × 0.40m = 0.24 m²
Total Plant Population (1 Acre):4,046.86 ÷ 0.24 = 16,861 plants
Daily Net Need / Plant:5.52 mm × 0.24 m² = 1.32 Liters / day
Step 4Determine Soil Storage Capacity & Days Between Irrigation Events

Medium Loam soil properties (CROPWAT 8.0 / FAO-56 Eq. 82): Volumetric Field Capacity (θ_FC) = 28.0% v/v (equivalent to gravimetric w_FC = 20.0% w/w at Bulk Density ρ_b = 1.40 g/cm³), Volumetric Wilting Point (θ_WP) = 14.0% v/v (w_WP = 10.0% w/w), Effective Root Depth (Z_r) = 700 mm (0.70 m), Management Depletion (MAD) = 45% (0.45).

Total Available Water (TAW) = ((28.0 - 14.0)/100) × 700 mm = ((20.0 - 10.0)/100) × 1.40 × 700 mm = 98.0 mm
Readily Available Water (RAW) = 98.0 mm × 0.45 (MAD) = 44.1 mm
Safe Irrigation Interval = 44.1 mm (RAW) ÷ 5.52 mm/day (ETc) = 7.98 ≈ 8 Days
Steps 5 & 6Calculate Gross Water Volume and Pump Run Duration

To refill the depleted 44.1 mm of moisture over the 8-day interval through a Drip System (90% efficiency) with a 25 m³/hr irrigation pump:

Gross Depth Needed = 44.1 mm ÷ 0.90 (Efficiency) = 49.0 mm
Total Plot Volume = 49.0 mm × 4,046.86 m² = 198,296 Liters (198.3 m³)
Total Volume per Plant over 8 Days = 198,296 Liters ÷ 16,861 plants = 11.76 Liters / plant
Pump Runtime = 198.3 m³ ÷ 25 m³/hr = 7.93 Hours ≈ 7 Hours and 56 Minutes

3. Precision Irrigation Terminology & Glossary

Clear, jargon-free explanations for every scientific and agronomic parameter used in KrishiMela.bd.

ET₀ (Reference Evapotranspiration)

The evaporative water loss from a standardized well-watered green grass surface. It represents the atmospheric demand independently of crop type.

Typical Range: 2.0 - 3.5 mm/day (Winter/Rabi) up to 5.0 - 7.5 mm/day (Pre-Monsoon Kharif-1).

K_c (Crop Coefficient)

A physiological multiplier representing how much more or less water a crop transpires compared to reference grass at a specific stage of growth.

Typical Range: 0.30 (seedling emergence) to 1.25 (peak flowering / panicle initiation).

Field Capacity (FC)

The upper moisture limit of the soil reservoir. The percentage of water remaining in soil after saturation has drained by gravity (usually 1-2 days).

Typical Values: Sand: 10-15%, Loam: 22-28%, Clay: 32-38% (volumetric %).

Permanent Wilting Point (PWP)

The lower limit of available water. Soil particles bind remaining water with such intense capillary suction (-1.5 MPa) that roots cannot extract it.

Typical Values: Sand: 4-6%, Loam: 10-12%, Clay: 16-20% (volumetric %).

AWC / TAW (Available Water Capacity)

The total water storage capacity between Field Capacity and Wilting Point across the active root depth: AWC = (FC - PWP) × BD × Zr.

Rule of Thumb: Loams offer the highest usable AWC (~140-180 mm/meter of root depth).

MAD / p (Management Allowed Depletion)

The percentage of available soil water that can be depleted before the plant experiences moisture stress and yield loss.

Typical Values: Sensitive vegetables: 30-40%; Cereals/Maize: 50-55%; Drought-tolerant crops: 60-65%.

Bulk Density (BD)

The dry mass of soil per unit volume (g/cm³). High bulk density indicates compaction, which restricts aeration, water infiltration, and root penetration.

Ideal Agricultural Soil: 1.10 - 1.45 g/cm³. Values > 1.60 g/cm³ severely stunt root development.

Infiltration Rate (IR)

The velocity at which ponded surface water penetrates downward into the soil profile (mm/hour). Irrigation application rate must not exceed this to prevent runoff.

Typical Rates: Heavy Clay: 1 - 5 mm/hr, Silt Loam: 10 - 20 mm/hr, Coarse Sand: 30 - 70 mm/hr.

AWD (Alternate Wetting and Drying)

A water-saving management practice for lowland puddle rice (Boro). Rather than keeping fields continuously flooded, water is allowed to recede 15 cm below soil surface (observed via perforated "pani pipe") before re-flooding.

Benefits: Saves 25-30% irrigation water, cuts diesel pumping costs by 30%, and reduces methane emissions by up to 50%.

Effective Root Depth (Z_r)

The depth of the soil profile containing over 80-90% of the active water-absorbing feeder root mass. Increases dynamically as the plant matures.

Examples: Onion/Leafy greens: 300 mm; Rice: 400 mm; Tomato/Potato: 600 mm; Maize: 800-1000 mm; Fruit Trees: 1200-1500 mm.

4. Data Sources, Fusion Model & Error Margins

To provide accurate forecasts even in remote rural areas without internet or ground hardware, KrishiMela.bd leverages automated satellite assimilation, machine learning pedotransfer, regional station networks, and IoT telemetry.

Data StreamPrimary Provider / SensorResolution / ModelError MarginUsage Role
NASA POWERNASA Langley MERRA-2 Assimilation0.5° × 0.5° Gridded (~50 km)±10% to 15%10-year climatological baseline & solar irradiance.
Regional Weather StationsBMD / DAE Automated StationsPoint sensors with IDW spatial interpolation±5% to 8%Near-real-time temperature, humidity, and rainfall.
FAO SoilGrids 2.0ISRIC Global Soil Information250m Machine Learning Pedotransfer±12% to 18%Baseline Sand/Silt/Clay ratios and organic carbon.
KrishiMela.bd IoT ProbesRS-485 Modbus FDR Soil Moisture ProbesDirect In-Situ Root Profile< ±2%Ground Truth calibration and automated pump switching.
Farmer Manual EntryKrishiMela.bd Profile & Field Input WizardUser-verified field dataExactOverrides generic models with true plant dates, spacing, & varieties.
Agriculturist Recommendation for Low-Resource Fields:Always verify model recommendations with tactile physical soil checks. If the model predicts available moisture but the surface soil exhibits crusting and deep fissures, irrigate immediately and switch your profile soil setting to "Clay / Etel" in the Data Sources & Variables tab.

5. Scientific Literature & Authoritative References

KrishiMela.bd’s computational algorithms adhere strictly to peer-reviewed agronomic literature and international agricultural research bodies.

1. FAO Irrigation and Drainage Paper No. 56 (FAO-56)

Allen, R.G., Pereira, L.S., Raes, D., & Smith, M. (1998). Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. Food and Agriculture Organization of the United Nations (FAO), Rome, Italy. (The definitive global standard for the Penman-Monteith equation, single and dual K_c coefficients).

2. Bangladesh Agricultural Research Institute (BARI) Agro-Technology Guidelines

BARI (2019). Krishi Projukti Hatboi (Handbook on Agro-Technology), 8th Edition. Gazipur, Bangladesh. (Source for local planting distances, row-to-row spacing, seed tuber depths, and crop yields).

3. Bangladesh Rice Research Institute (BRRI) & IRRI AWD Manual

BRRI & International Rice Research Institute (2013). Alternate Wetting and Drying (AWD) Technology for Irrigated Boro Rice Production in Bangladesh. (Standard protocol for the -15 cm water table threshold).

4. ISRIC SoilGrids 2.0 Global Pedotransfer Model

Poggio, L., de Sousa, L.M., Batjes, N.H., et al. (2021). SoilGrids 2.0: Producing soil information for the globe with quantified spatial uncertainty. SOIL, 7(1), 217–240. (Algorithmic derivation of volumetric Field Capacity and Wilting Points from soil texture).

5. USDA Natural Resources Conservation Service (NRCS) National Engineering Handbook

USDA-NRCS (1997). Part 623: Irrigation, Chapter 2: Irrigation Water Requirements. Washington, D.C. (Standards for application efficiencies, drip emitter distribution uniformity, and irrigation intervals).

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