CROP IRRIGATION WATER REQUIREMENTS CRIWAR 3.0 for Windows CROP IRRIGATION WATER REQUIREMENTS

Outline of the topic Powerpoint presentation Demonstration of the program Exercises with Criwar

= evapotranspiration (ET ) evaporation transpiration + = evapotranspiration (ET ) an open water surface the soil the leaves and the stem of the plant During the day water Ecapes as vapour to The atmosphere from: the plants extract water from the soil; this water leaves the plant during the day through the stem and the leaves effective root zone After M.G.bos

Definitions in Criwar Potential evapotranspiration ETp is the evapotranspiration from cropped soils that have an optimum supply of water. 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. 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

CWR = ETp - Pe What does CRIWAR calculate? where, ETp = potential evapotanspiration Pe = effective precipitation Criwar calculates the crop irrigation water requirements (CWR) of a cropping pattern in an irrigated area on a daily base:

Field water balance Precipitation ET Irrigation Drainage Depth to ground-water Drainage Capillary rise Rootzone Seepage Groundwater Groundwater After M.G.bos

The growth stage of the crop 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 cloudy Hot cool Low (dry) high (humid) Windy little wind Sunshine Temperature Humidity Wind 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% Citrus Olives Grapes Cucumber Radishes Squash Carrots Crucifers Lettuce Melons Onions Peanuts Peppers Spinach Tea Grass Cacao Coffee Clean cultivated nuts & fruit trees e.g. apples   Barley Beans Maize Cotton Tomato Potatoes Oats Peas Sorghum Soybeans Sugarbeet Sunflower Tobacco Wheat Paddy rice Sugarcane Banana Nuts & fruit trees with cover crop

The influence of growth stage on crop water needs 1. Initial stage Germination and early growth of the crop. Soil surface hardly covered by the crop canopy (ground cover <10%) 2. Crop development From 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-season From 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 season From the end of the mid-season stage until full maturity or harvest of the crop Small plants  evaporation more important than transpiration During initial stage, crop water need about 50% of mid-season

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

CALCULATING EVAPOTRANSPIRATION Presently most of the calculation methods for evapotranspitation are based on 3 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’

< radiation term > < aerodynamic term > Penman’s formula   D Rn - G g E0 = -------- ------------ + ---------- Ea D + g g D + g < radiation term > < aerodynamic term > Eo = open water evaporation rate (kg/m2 s) D = proportionality constant dez/dTz (kPa/C) Rn = net radiation (W/m2) G = heat flux density into the water body (W/m2) l = latent heat of vaporisation (J/kg) g = psychometric constant (kPa/C) Ea = isothermal evaporation rate (kg/m2 s)

Etcrop = Eo * Kc 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: ETp = Kc * ETref = Kc * ETg 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 Penman-Monteith Approach 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

Eth = 0.85 ETg Relationship modified Penman vs Penman-Monteith Crop coefficients introduced for Modified penman method could still be used with 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 stages are derived from tables based om field research. During the crop development stage, a straight line interpolation is Assumed to find the Kc2 value

EFFECTIVE PRECIPITATION CWR = ETp - Pe 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.   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

Method to calculate for effective rainfall in Criwar

USDA method to calculete Pe The average monthly effective precipitation can not exceed the total monthly rainfall, nor the total evapotranspiration

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 or If Da >= 75 mm/turn then  = 0.946 + 7.3 x 10-4 x Da

CWR module of criwar CRIWAR can be used to: 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)

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

Range of input values to be used in Meteorological file Description parameter Range Dimension Latitude 0 ≤ ≤ 66 Degrees N or S Altitude -500 ≤ ≤ 4500 Metres Height wind speed measured Height ≤15 Temperature ≤ 45 Degrees oC Precipitation ≤ 1000 mm per period Sunshine hour Sunshine r ≤ 24 Hours per day Relative humidity Rhum ≤ 100 Percent Wind speed Wind ≤ 15 Metre per second Maximum rel. humidity rhum ≤ Rhmax Wind speed ratio day/night Ratio ≤ 5 Dimensionless

Range of Crop Input Parameters