Water Requirement for Different Crops and Related Problems

Md. Nurul KadirLecturerDepartment of Agricultural Engineering

Sher-e- Bangla Agricultural University, Dhaka

04/18/2023

Crop Water RequirementReference:Crop Evapotranspiration, FAO Irrigation and Drainage Paper 56, FAO,

RomeMajor functions of water are:1. Cell development2. Solvent for nutrients and for translocation within the plant

Water is adsorbed by the roots and only about 5% of the water adsorbed is actually used by the plant itself.

Transpiration: Plant leaves contain stomata (8,000 to 120,000 per cm2). In presence of light, the stomata opens and CO2 and HO2 react to form starch which is used in building the plant tissue. In the process, water escapes from the stomata which is known as transpiration.

Evaporation: Change of water from liquid to vapour state through transfer of heat.

Plant SoilPlant Soil

RootRoot

Stomata

In presence of sunlight:6H2O + 6CO2 ----------> C6H12O6+ 6O2

Crop Water Requirement (CWR): Rice

Crop water requirement (CWR): the quantity of water required by a crop for its normal growth under field condition. It includes ET and other unavoidable losses.

CWR of rice = Evapotranspiration (ET) + Seepage and Percolation (S&P)

The main reasons for keeping standing water in rice field are: (1) stress free high yield (2) fertilizer management (3)

reduction of weed growth and (4) easier water management.

PET is often estimated from evaporation of US Class A pan (125 cm in diameter and 25 cm high).

For lowland rice, which is grown with standing water the ET is about 1.2 times pan evaporation.

Evaporation Pan

CWR: RiceEstimation of ET:There are a number of methods to estimate Reference Crop

Evapotranspiration (ET0). The choice of a method depends upon the availability of data (solar radiation, temperature, wind speed, sunshine hours, relative humidity, crop reflectance etc.)

Blaney-Criddle MethodModified Penman MethodPenman-Monteith Method: Recommended by FAO and

NWMP. CROPWAT (FAO software)ET (crop) = ET0 x KC ; where ETo = Reference crop

evapotranspiration (mm/day) and KC is the crop coefficientThe crop coefficient KC is dependent on type of crop and crop

growth stage.

Evapotranspiration

The Penman-Monteith equation is:

Where, ET0 is the reference crop evapotranspiration, Rn is the net radiation, G is the soil heat flux, (es - ea) represents the vapour pressure deficit of the air, r a is the mean air density at constant pressure, cp is the specific heat of the air, D represents the slope of the saturation vapour pressure temperature relationship, g is the psychrometric constant, and rs and ra are the (bulk) surface, u2 is the wind velocity at 2 m height and aerodynamic resistances.

A software called CROPWAT has been developed by FAO to calculate the ET0 by Penman-Monteith method.

Penman-Monteith Formula:

Empirical Methods of Calculation of ETThe Blaney-Criddle formula: ETo = p (0.46 T + 8)ETo = Reference crop evapotranspiration (mm/day) T = mean daily temperature (°C)p = mean daily percentage of annual daytime hours

Determination of ET with CROPWAT (version 8.0)

Data RequirementLatitudeLongitudeAltitudeTemperature - Maximum - MinimumHumidityWind speedSunshine hours

Growth Stage

Crop

HYV Boro HYV Aus HYV Aman Wheat Potato Maize Winter Veg.

Kc Days Kc Days Kc Days Kc Days Kc Days Kc Days Kc Days

Nursery 1.20 30 1.20 30 1.10 30

Initial stage 1.10 30 1.10 30 1.10 20 0.59 30 0.58 25 0.45 25 0.55 15

Dev. Stage 1.10 30 1.10 30 1.10 30 1.05 30 1.12 30 0.45 35 0.55 25

Mid season 1.25 30 1.10 30 1.10 40 1.10 30 1.13 45 1.10 40 1.00 35

Late season

1.00 30 0.85 10 0.90 30 0.69 30 0.92 30 0.55 30 0.98 15

Total duration

150 130 150 120 130 130 90

Growth stages, durations and KC of major crops

Generally: Initial and development stages are considered as vegetative stage Mid-season as reproductive stage, and Late season as ripening stage

CWR: RiceAlthough ET is the true CWR, in applying it water is lost as

seepage (lateral movement) and percolation (vertical movement). As seepage (S) and percolation (P) loss occur simultaneously and it is difficult to measure them separately in the field, S&P loss is considered together.

S&P loss depends upon soil texture, existence of plow pan, depth of standing water, water table depth etc.

Methods of Measuring S&PSloping GagePVC GageLysimeters have been used for the measurement of P.Once the ET and S&P are known, the CWR can be determined

on daily, decade, monthly and seasonal basis.

Irrigation Water Requirement (IWR) of Rice

IWR = CWR - Effective Rainfall (RE ) = ET + S&P - RE

Effective Rainfall (RE )Useful or utilizable portion of rainfall that is stored and

supplied to meet the crop water requirement. It excludes deep percolation, soil storage, surface runoff and interception.

There are a number of methods for calculating RE:

For monthly total rainfall (RT ), RE (USDA) is :RE = RT (125 – 0.2 RT )/125 if RT is < 250 mmRE = 125 + 0.1 RT if RT is > 250 mm

IWR of RiceAs in Bangladesh, the rainfall is scarce during the dry season,

dependable rainfall (80% probable) is considered as effective rainfall.

Land Preparation (LP)LP of rice fields means plowing and puddling, which breaks

down the soil aggregates for ease of transplantation and reduces percolation. Puddling levels the land, incorporates weeds and stubbles, reduces weed growth and promotes good water management.

Water requirement for LP includes: evaporation from wetted fields water required to establish a water layer water absorbed in the plow layer (land soaking) S&P beyond the plow layer

IWR of RiceAbout 150-180 mm of water is required for LP provided the

operations are completed within two weeks.IWR = (CWR – RE ) + LP = (ET + S&P – RE) + LPNo irrigation is provided during last 15 days of crop season and

the fields are drained: Terminal Drainage

Gross IWR = IWR / EfficiencyEfficiency (%) is that part of water pumped/diverted which is

effectively used by the plants. It depends upon the conveyance and other losses and leakages.

For lined canals the efficiency is 95%.For unlined canals the efficiency varies from 40%-80%, depending

upon the soil type and level of maintenance and management.

Months

ETo (mm/day)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ave

ETo 1.9 2.7 3.8 4.8 5.2 3.9 4.1 3.9 4.0 3.5 2.6 2.0 3.53

Table-5: ETo of Rangpur Station.

Months

Rainfall (mm/day)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total

Rainfall (KIPNU) 5.58 11.84 37.40 153.11 338.52 548.04 589.21 392.72 384.54 210.69 9.53 7.49 2688

Rainfall (KIPSU) 6.33 14.28 45.76 157.23 331.03 459.90 505.30 302.83 313.70 190.28 8.58 8.98 2344

Table-6: Monthly Average Rainfall of the Project Area (KIPNU & KIPSU)

Growth Stage

Crop

HYV Boro HYV Aus HYV Aman Wheat Potato Maize Winter Veg.

Kc Days Kc Days Kc Days Kc Days Kc Days Kc Days Kc Days

Nursery 1.20 30 1.20 30 1.10 30

Initial stage 1.10 20 1.10 30 1.10 20 0.59 30 0.58 25 0.45 25 0.55 15

Dev. Stage 1.10 30 1.10 30 1.10 30 1.05 30 1.12 30 0.45 35 0.55 25

Mid season 1.25 40 1.10 30 1.10 40 1.10 40 1.13 45 1.10 40 1.00 35

Late season 1.00 30 0.85 10 0.90 30 0.69 30 0.92 30 0.55 30 0.98 15

Total duration 150 130 150 130 130 130 90

Table-8: Growth Stage, Duration and Kc

ExampleCalculate the HYV Boro command area of a STW at Rupnagar with a discharge of 15 l/s from the following data (120 day

growth period: 15 January -15 May):1. Conveyance efficiency is 70%.2. Maximum pumping hours is 12 hrs/day3. Seepage and percolation rate is 5 mm/day4. Zero effective rainfall during the season.ET (crop) = ET0 x KC = 5.5 x 1.25 = 6.9 mm/day (maximum calculated

ETcrop for the season)CWR = 6.9 + 5.0 = 11.9 mm/dayIWR = 11.9 – RE (0) mm/day = 11.9 mm/day (1.377 l/ha/sec)Gross IWR = 11.9 / 0.7 = 17 mm/day = 0.017 m/dayVolume pumped from STW = 15 x 3600 x 12 l/day = 0.648

million liters/day = 648 m3 /day Command area = 648 / 0.017 = 38117.6 m2 = 3.81 ha = 15 / (1.377/0.7) / 2 = 3.81 haEffect of rainfall:To be deducted from all the CWR to determine the peak requirement.

CWR of Non-Rice CropsSaturated Moisture Content: All voids are filled with waterField Capacity (FC): Moisture content after draining of gravitational water;

Moisture content 1-3 days after irrigation; Moisture content at 0.1- 0.3 barPermanent Wilting Point (PWP): Moisture content at 15 barAvailable Water = FC – PWPReadily Available Water = 50% of Available WaterMoisture content (by volume) cc/cc = (Dry density / Water density) x

Moisture content (by weight; gm/gm) = bulk (dry) density x Moisture content by weight

For Non-Rice crops:CWR = ETCROP

ETCROP = ET0 x KC where, KC is the crop coefficient which wouldvary from crop to crop and for a crop upon its different growth stages.Like rice, ET0 can be estimated by Penmen-Monteith method (Cropwat).IWR = ETCROP – RE ; where, RE is the effective rainfall.Irrigation is actually provided at the critical growth stages.

Water Holding Capacity of Soil

IWR of Non-Rice CropsIWRFIELD = IWR / Field Application Efficiency (EF )

GL

Plow pan

Field Application Efficiency (EF): 60% for surface irrigation 75% for sprinkler irrigation 90% for drip irrigationIWR= (ETCROP – RE ) / EF

Gross IWR = IWR / Efficiency (same as rice)

SWD sSWD

SWD

ExampleCalculate the command area of a STW (discharge of 15 l/s and

maximum 12 hours of pumping) for wheat grown at Rupnagar during 15 November-15 March in a soil with FC and PWP of 27% and 5% (by weight) respectively. If the crop root zone is 1 m, the dry density (bulk density) of the soil is 1.5, field and scheme irrigation efficiencies are 60% and 70%, then calculate the amount of each irrigation at field level.

CWR = ETCROP (peak) = 4.8 x 1.2 = 5.76 mm/day (in February)IWRFIELD = 5.76 / 0.6 = 9.6 mm/day = 1.11 l/s/haIWRSCHEME = 1.11 / 0.7 = 1.586 l/s/haCommand area = 15 / 1.586 / 2 = 4.73 haRoot Zone Storage = 0.5 x 1000 x (0.27 – 0.05) x 1.5 =165mm Irrigation amount = 165 / 0.6 = 275 mm

For the modern wheat varieties (BARI Gom 25) grown in Bangladesh, the crop growth period is 110 days: 20 + 30 + 30 + 30