CROP IRRIGATION WATER REQUIREMENTS

CRIWAR 3.0 for Windows

• Powerpoint presentation

• Demonstration of the program

• Exercises with Criwar

Outline of the topic

effective root zone

evaporation transpiration+ = evapotranspiration (ET )

Evapotranspiration

the plants extract water from the soil;this water leaves the plant during the daythrough the stem and the leaves

• an open water surface

• the soil• the leaves and

the stem of the plant

During the day waterEcapes as vapour toThe atmosphere from:

After M.G.bos

Potential evapotranspiration ETp is the evapotranspirationfrom cropped soils that have an optimum supply of water.

Definitions in Criwar

In CRIWAR, ETp is the volume of irrigation water required to meet the crops’ potential evapotranspiration during a specific time period, under a given cropping pattern and in a specific climate

Effective precipitation is that part of the total precipitation on the cropped area, during a specific time period, which is available to meet evapotranspiration in the cropped area.

What does CRIWAR calculate?

CWR = ETp - Pe

where,ETp = potential evapotanspirationPe = effective precipitation

Criwar calculates the crop irrigation water requirements (CWR) of a cropping pattern in an irrigated area on a daily base:

Irrigation

Precipitation ET

Drainage

GroundwaterGroundwater

Seepage

Capillary rise

Rootzone

Depth to ground-water

After M.G.bos

Field waterbalance

Influencing factors in Criwar

The following factors have effect on the water requirement of a crop:

• The climate

• The type of crop

• The growth stage of the crop

Influence of the climate on crop water needs

Climatic factor Crop water need

High Low

Sunny cloudyHot coolLow (dry) high (humid)Windy little wind

SunshineTemperatureHumidityWind speed

Highest crop water needs in hot, dry, windy and sunny areas

Crop water needs as compared to standard grass

Not all crops have the same water needs. The crop water requirements vary

- 30%- 10%

same as standard grass

+ 10% + 20%

CitrusOlivesGrapes

CucumberRadishesSquash

CarrotsCrucifersLettuceMelonsOnionsPeanutsPeppersSpinachTeaGrassCacaoCoffeeClean cultivated nuts & fruit treese.g. apples 

BarleyBeansMaizeCottonTomatoPotatoesOatsPeasPotatoesSorghumSoybeansSugarbeetSunflowerTobaccoWheat

Paddy riceSugarcaneBananaNuts & fruit trees with cover crop

The influence of growth stage on crop water needs

Small plants evaporation more important than transpiration

During initial stage, crop water need about 50% of mid-season

1. Initial stageGermination and early growth of the crop. Soil surface hardly covered by the crop canopy (ground cover <10%)

2. Crop developmentFrom the end of stage 1 to effective full ground cover of 70% to 80%. Note that the crop has not reached its mature height yet 3. Mid-seasonFrom the attainment of effective full ground cover to the start of maturing of the crop. Maturing may be indicated by discolouring of leaves or falling of leaves.

4. Late seasonFrom the end of the mid-season stage until full maturity or harvest of the crop

CALCULATING EVAPOTRANSPIRATION

In the past:Empirical correlation methods to estimate the potential evapotranspiration. These were often only valid for the local conditions and hardly transferable to other areas.

Examples:

Blaney and Criddle 1950; based on air temperature + day length Turc 1954; based on air temperature + radiation Jensen and Haise; based on air temperature + radiation

Presently most of the calculation methods for evapotranspitation are based on3 physical requirements in soil – plant - atmosphere:

• continous supply of water• energy to change liquid water into vapour• a vapour gradient to maintain a flux from the evaporating surface to the atmosphere

Penman was the first to apply this so-called ‘Combination method’

CALCULATING EVAPOTRANSPIRATION

Penman’s formula

  Rn - GE0 = -------- ------------ + ---------- Ea

+

< radiation term > < aerodynamic term >Eo = open water evaporation rate (kg/m2 s)

= proportionality constant dez/dTz (kPa/C)

Rn = net radiation (W/m2)

G = heat flux density into the water body (W/m2) = latent heat of vaporisation (J/kg) = psychometric constant (kPa/C)Ea = isothermal evaporation rate (kg/m2 s)

The Penman method

Penman method:

• Estimate the evaporation from an open water surface and use that as a reference evaporation

• Reference evaporation * crop factor = potential evapotranspiration

Etcrop = Eo * Kc

Data required are:- air temperature- air humidity- solar radiation- wind speed

Two other methods to calculate potential evapotranspiration

• The FAO Modified Penman Method

• The Penman-Monteith Approach

The FAO Modified Penman Method

Instead of open water they used the evapotranspiration from a reference crop Etg defined as:

“An extended surface of an 8 to 15 cm tall green grass cover of uniform height, actively growing, completely shading the ground and not short of water”

Again there are a radiation term and an aerodynamic term

Main differences are:

• Different short wave reflection coefficient (0.05 water, 0.25 grass)• More sensitive wind function• Adjustment factors for local condition compared to assumed standards

The formula reads:

ETp = Kc * ETref = Kc * ETg

The Penman-Monteith Approach

There was evidence that the Modified Penman method over-predicted the crop water requirements 

Monteith developed an equation that describes transpiration from a dry, extensive horizontal and uniformly vegetated surface that fully covered the ground, optimally supplied with water.

Canopy and air resistances to water vapour diffusion were introduced.

The characteristics of hypothetical reference crop

The main differences with the modified Penman method are:

Different reflection coefficients

Different aerodynamic resistance, resulting in a different wind function\

Modification of the psychromatic constant

ETp = Kc * ETref = Kc * ETh

The Penman-Monteith Approach

Eth = 0.85 ETg

Crop coefficients introduced for Modified penman method could still be used with Penman-Monteith

Relationship modified Penman vs Penman-Monteith

The modified Hargreaves method

ET0,mh = 0.0013 x 0.408RA x(Tav +17.0) x (TD – 0.0123P)0.76

RA = extraterrestrial radiation (MJ/m2 per day)Tav = average daily temperature (Celsius)TD = Tmax – Tmin

P = average monthly precipitation

Criwar uses 4 crop stages

The crop coefficient for the initial growth stage in CRIWAR

During the initial growth stage, the value of the crop coefficient, Kc1, depends largely on the level of ETref and on the frequency with which the soil is wetted by rain or irrigation. The figure shows the relationship between Kc, ETref, and the average interval between irrigation turns or significant rain.

Other crop stages

Values for the mid-season and late leason crop stagesare derived from tables based om field research.During the crop development stage, a straight line interpolation isAssumed to find the Kc2 value

EFFECTIVE PRECIPITATION

Effective precipitation is that part of total precipitation on the cropped area, during a specific time period, which is available to meet the evapotranspiration in the cropped area. 

CWR = ETp - Pe

Not all precipitation is effective:• part evaporates• part become surface runoff• part will recharge the groundwater

only that part that will be stored in the rootzone and that becomes readily available soil moisture will be taken up by the roots to meet the crop’s evapotranspiration needs

Some Factors Influencing Effective Rainfall

Amount and frequency of precipitation Time of occurrence of precipitation Rainfall intensity Type of crop Infiltration rate Water-holding capacity Field slope Land surface condition Depth to groundwater

CRIWAR combines a semi emperical method developed by the USDA combined with the net irrigation application depth on effective precipitation 3 factors are considered: mean cumulative monthly precipitation

effectiveness in areas with light precipitation relatively high

mean cumulative monthly evapotranspiration effectiveness is higher when the evaporation rate is higher

irrigation application depth

depth of water application per term equal to readily available water that can be stored in the root zone. This depends on the soil type and the rooting depth high storage capacity within the rootzone indicates a relatively high effectiveness

Method to calculate for effective rainfall in Criwar

The average monthly effective precipitation can not exceed the total monthly rainfall, nor thetotal evapotranspiration

USDA method to calculete Pe

Effective precipitation formula as used in CRIWAR

Criwar uses the following semi empirical formula to calculate Pe

Pe = (1.253P0.824 –2.935) X 100.001ETp

Pe = effective precipitation (mm/month)P = total precipitation per month (mm/month)ETp = total crop evapotranspiration per month (mm/month) = correction factor depending on depth of application term

When the irrigation water application Da = 75 mm/turn then = 1.0

If da < 75 mm/turn then = 0.133 + 0.201 ln Da orIf Da >= 75 mm/turn then = 0.946 + 7.3 x 10-4 x Da

• Estimate CWR by variation in planting dates

• Estimate CWR for different cropping patterns

• Estimate CWR with different varieties

A Criwar cropping pattern can exist of 40 different crops in one calculation. The same crop can be used more than once in one cropping pattern (staggering crops)

CRIWAR can be used to:

CWR module of criwar

Data requirement

• General data• Meteo data• Crop data• Cropping pattern

Main Screen

General data

Meteo data

Cropping pattern data

Crop factor file

Report screen

Report Screen

Description parameter Range DimensionLatitude 0 ≤ Latitude ≤ 66 Degrees N or SAltitude -500 ≤ Altitude ≤ 4500 MetresHeight wind speed measured

0 ≤ Height ≤15 Metres

Temperature 0 ≤ Temperature

≤ 45 Degrees oC

Precipitation 0 ≤ Precipitation

≤ 1000 mm per period

Sunshine hour 0 ≤ Sunshine r ≤ 24 Hours per dayRelative humidity 0 ≤ Rhum ≤ 100 PercentWind speed 0 ≤ Wind ≤ 15 Metre per secondMaximum rel. humidity rhum ≤ Rhmax ≤ 100 PercentWind speed ratio day/night

0 ≤ Ratio ≤ 5 Dimensionless

Range of input values to be used in Meteorological file

Range of Crop Input Parameters