Irrigation Engineering 4(3+1)

LESSON 25 Evapotranspiration

Consumptive use (CU) – CU is used to designate the losses due to ET and water that is used for its metabolic activities of plants thus CU exceed ET by the amount of water used for digestion, photosynthesis, transport of minerals and photosynthates, structural support and growth. Since this difference is usually less than 1%, ET and CU are normally assumed to be equal. But both (CU &ET) terms are used simultaneously.

 

Fig.25.1.Evapotranspiration process.

(Source:http://toolboxes.flexiblelearning.net.au/demosites/series6/605/html/resources/visitor_centre/fact_sheets/text_evap_trans.htm: accessed on June 3, 2013)

 

ET can be either directly measured for a given crop, soil and climatic conditions or computed using the reference crop ET, which is generally estimated by various methods depending upon availability of data for a particular case.

 

25.2 Concept of Reference Crop Evapotranspiration

Reference Crop ET – is the potential ET for a specific crop (usually either grass or alfalfa) and set of surrounding (advective) conditions. According to Doorenbos and Pruitt (1977) ”ET from an extensive surface of 8 10 15 cm tall, green grass covers of uniform height, actively growing, completely shading the ground and not short of water”. Allen et al. (1998) modified the above definition to represent hypothetical grass surface. Reference crop evapotranspiration from an extensive surface of green grass of uniform height(0.12m), actively growing, completely shading the ground with an albedo of 0.23 and having ample water supply is called reference crop evapotranspiration and is denoted by ET_o (Fig. 25.2).

Fig. 25.2.Estimation of reference crop evapotranspiration.

(Source: http://moreprofitperdrop.files.wordpress.com : accessed on June 3, 2013)

 

Climatic parameters are the only factors affecting ET_o and it can be computed from weather data. ET for specific crop can be estimated using reference crop ET and crop coefficients. Typical ranges for ET_o values for different agro-climatic regions are given in Table 25.1.

 

Table 25.1. Average ETo for different agro-climatic regions in mm/day

Regions	Mean daily temperature (°C)
	Cool (~10°C)		Moderate (20°C)	Warm (> 30°C)
Tropics and subtropics
Humid and Sub-humid		2 – 3	3 - 5	5 – 7
Arid and Semi-arid		2 – 4	4 - 6	6 – 8
Temperate region
Humid and Sub-humid		1 – 2	2 - 4	4 – 7
Arid and Semi-arid		1 – 3	4 - 7	6 – 9

(Source:http://www.angrau.ac.in/media/7380/agro201.pdf: accessed on June 3, 2013)

 

25.3 ET Estimation Methods

Due to wide application of evapotranspirtaion data, various indirect ET estimation methods have been developed over the years. These methods can be grouped into four major categories. Although there is large number of methods developed in each category, a few examples are given below:

1. Temperature based (Thornwaite, SCS BlaineyCriddle, FAO24 BlaneyCriddle, Hargreaves)
2. Radiation based (Turc, Preistly Taylor, Jensen-Haise, and FAO 24 Radiation)
3. Pan evaporation based ( Pan evaporation, FAO 24 Pan)
4. Combination (Penman, Penman Moneith, FAO 56 Penman Monteith)

 

Data requirement as well as performance of each of these methods varies. In order to help user with ETo estimation, A Decision Support System for ETo estimation is developed at IIT Kharagpur (Bandopadhyaya et al. 2012), which estimates ETo by 22 well established method and ranks them based on their ETo estimation performance.

 

25.3.1 Hargreaves Methods

Hargreaves method is a temperature based method and it was derived to overcome non availability of solar radiation data at many locations. Hargreaves and Samani (1985) recommended estimating solar radiation from extraterrestrial radiation and proposed the following equation for estimating ETo in mm/day

            (25.1)

 

Where,

TD = difference between mean monthly maximum and minimum temperatures in °C,

 R_A= extraterrestrial solar radiation in MJ m^-2 d^-1,

 T_mean = mean monthly air temperature in °C.

 

25.3.2 ThornthwaiteMethod

The relationship is expressed as:

             (25.2)

 

Where,

ETo = Monthly potential evapotranspiration (cm) or reference crop ET (i.e., ETo)

T= Mean monthly temperature (°C)

I = A heat index for a given area which is the sum of 12 monthly index values i

i is derived from mean monthly temperatures using the following formula:

           (25.3)

a = an empirically derived exponent which is a function of I

     25.4       

 25.3.3 Pan Method

The amount of water evaporating from a pan is determined by measuring change in water level in the pan and correcting from precipitation (assuming that water loss due to wind action, animals, birds etc., has been prevented or is negligible). USBR Class A Pan evporometer is most commonly used for estimation of pan evaporation and is shown in Fig. 25.3. It is relatively inexpensive and simple way of assessing the evaporative capabilities of the atmosphere.

 

Fig. 25.3. Pan evaporimeter. (Source: http://www.polylabindia.com/open-panevaporimeter-1622575.html, accessed on August 29, 2013)

 

Reference Crop ET (ET_o) is related to E_pas follows:

ET_o = E_p .k_p                                                (25.5)

 

Where,

ETo = Reference crop ETo in mm/day

E_p = pan evaporation (mm/day)

K_p = pan coefficient

 

K_p accounts for differences in pan type and conditions upwind of the pan, and for dissimilarities between plants and evaporation pans. Kp values for USBR class a Pan can be chosen from Table 25.2.

 

Table 25.2.PanCoefficient for USBR Class a pan at varying location of fetch, mean RH and wind run

Wind	Windward side distance of green crop	RH mean %
		low	medium 40-70	high > 70
km/day	m	< 40
Light	1	0.55	0.65	0.75
< 175	10	0.65	0.75	0.85
	100	0.7	0.8	0.85
	1000	0.75	0.85	0.85
Moderate	1	0.5	0.6	0.65
175-425	10	0.6	0.7	0.75
	100	0.65	0.75	0.8
	1000	0.7	0.8	0.8
Strong	1	0.45	0.5	0.6
425-700	10	0.55	0.6	0.65
	100	0.6	0.65	0.7
	1000	0.65	0.7	0.75
Very strong	1	0.4	0.45	0.5
> 700	10	0.45	0.55	0.6
	100	0.5	0.6	0.65
	1000	0.55	0.6	0.65

(Source: FAO 24)

 

25.3.4 FAO-56 Penman – MonteithMethod

Allen et al. (1998) modified the Penman–Monteith equation for estimation of grass reference crop evapotranspiration (ETo). The mathematicalrelationship is as follows:

 

                                (25.6)

 

Where,

ETo = Reference crop evapotranspiration (mm/day)

Rn = Net radiation at the crop surface (MJ/m²/day)

G = Soil heat flux density (MJ/m²/day)

T = Air temperature at 2 m height (°C)

U₂ = Wind speed at 2 m height (m/s)

e_s = Saturation vapour pressure (kPa)

e_a = Actual vapour pressure (kPa)

(e_s – e_a) = Saturation vapour pressure deficit (kPa)

Δ = Slope vapour pressure curve (kPa/°C)

γ = Psychrometric constant (kPa/°C)

Details of parameter estimation for the above ETo methods can be found in Allen et al. (1998) and Michael (2008). 

Application & Limitations

• Method requires all weather data, i.e., minimum and maximum temperature, minimum and maximum RH, wind speed, Solar radiation or sunshine hour
• Wide applicability i.e., in arid, semi-arid, humid, sub-humid conditions
• Gives a very satisfactory estimate of ETo
• Can provide basis for developing consistent crop coefficients

 

25.4 Crop Evapotranspiration

Estimation of evapotranpiration for a specific crop (ET_c) requires first calculating the reference evapotranspiration (ET₀) and then multiplying the crop coefficients (K_c) to estimate actual:

ET_c = ET_o x K_c                                                        (25.7)

Where, ET_c is the ET of a specific crop,ET_o is the grass-reference ET, and Kc is the crop coefficient for a given crop.

 

25.4.1 Crop Coefficient Concept

While ET_o accounts for variations in weather and is used as an indicator of atmospheric demand for water, K_c values account for the difference between ET_oand ET_c and link them. K_c is the crop coefficient for a given crop and growth stage, and is usually determined experimentally. Each agronomic crop has a set of specific crop coefficients used to predict water use rates at different growth stages.

 

There are four main crop growth stages: initial, crop development, mid-season, and late season:

a) Initial period – planting to 10% ground cover

b) Crop development – 10% ground cover to effective cover i.e., flowering

c) Mid-season – Effective cover to start of maturity i.e., senescence of leaves

d) Late season – Start of maturity to harvest.

 

These crop development stages along with crop coefficient variation for a typical crop are depicted in Fig. 25.3.Tables 25.3 presents length of growth stages of some of the representative crops whereas Table 25.4 shows values of crop coefficient during different stages. Crop coefficient values vary with the development stage of the crop. In the case of annual crops, Kc is typically low at seedling, emergence and establishment stage, increaseswith increase in ground cover and attains maximum value at mid-season stage andthereafter decreases towards ripening and maturity stage.

 

Fig. 25.3.Variation of crop coefficient with crop growth stages. (Source: Allen et al. 1998)

Table 7.7.Length of crop growth stages (days)

Crop	Initial (Lini)	Development (Ldev)	Mid   (Lmid)	Late   (Llat)	Panting date	Region
Bean, green	20	30	30	10	Feb/Mar	Calif., Mediterrian
Cabbage	40	60	50	15	Sept	Calif. Desert, USA
Cauliflower	35	50	40	15	Sept	Calif. Desert, USA
Tomato	35	40	50	30	Apr/May	Calif., USA
Cucumber	20	30	40	15	Jun/Aug	Arid region
Potato	30	35	50	30	Apr	Europe
Groundnut	35	35	35	35	May	High Latitudes
Lentil	25	35	70	40	Oct/Nov	Arid region
Soybeans	20	30/35	60	25	May	Central USA
Cotton	30	50	60	55	Mar-May	Egypt; Pakistan; Calif.
Sesame	20	30	40	20	Jun	China
Sunflower	25	35	45	25	Apr/May	Medit.; Calif.
Wheat	15	25	50	30	Nov	Central India
Maize (grain)	20	35	40	30	Oct	India (dry, cool)
Rice	30	30	60	30	Dec; May	Tropics; Medit
Sugarcane, ratoon	30	50	180	60		Tropics
Banana, Istyr	120	90	120	60	Mar	Mediterranean
Grapes	20	40	120	60	Apr	Low Latitudes
Citrus	60	90	120	95	Jan	Mediterranean

(Source: Allen et. al, 1998)

 

Table 7.8.Single crop coefficients for grass reference crop and mean maximum plant heights for well managed no stressed condition in sub-humid regions (RH_min = 45% and u₂ = 2 ms^-1)

Crop	Kc_ini	Kc_mid	Kc_end	Maximum crop height, h (m)
Bean, green	0.5	1.05	0.90	0.4
Cabbage	0.7	1.05	0.95	0.4
Cauliflower	0.7	1.05	0.95	0.4
Tomato	0.6	1.15	0.70 – 0.90	0.6
Cucumber	0.5	1.00	0.90	0.3
Potato	0.5	1.15	0.75	0.6
Groundnut	0.4	1.15	0.60	0.4
Lentil	0.4	1.10	0.30	0.5
Soybeans	0.4	1.15	0.50	0.5-1.0
Cotton	0.35	1.15-1.20	0.70-0.50	1.2-1.5
Sesame	0.35	1.10	0.25	1.0
Sunflower	0.35	1.0-1.15	0.35	2.0
Wheat	0.3	1.15	0.25 – 0.40	1.0
Maize (grain)	0.3	1.20	0.60, 0.35*	2.0
Rice	1.05	1.20	0.90-0.60	1
Sugarcane	0.40	1.25	0.75	3
Banana, Istyr	0.50	1.10	1.00	3
Grapes	0.30	0.85	0.45	2
Citrus**	0.65	0.60	0.65	3

(Source: Allen et. al., 1998)

 

* First and second values correspond to harvest at high grain moisture and at complete field dry conditions, respectively.

** No ground cover, 50% canopy

 

Example 25.1:

Determine ETo for March from pan evaporation data of Palakkad, Kerala. Daily mean pan evaporation for the month of March = 7.01mm, Average relative humidity = 63.45 %, Average wind speed at 2m height = 1.157 m/s. At Palakkad, evaporation pan is placed in an area surrounded by green crops. The windward side distance of green crops is about 100 m.

 

Solution:

For the given conditions, Kp from Table 25.2 is 0.8

          ETo = Kp x Epan

                  = 0.8 x 7.01

                  = 5.61 mm. Ans

 

Example 25.2:

Determine monthly water requirements and total water requirement of a groundnut crop grown in the rice fallows at Palakkad, Kerala.

Month	January	February	March	April
Kc Values	0.51	0.93	1.14	0.82
ETo mm/day	4.60	5.00	5.59	5.20

 

Solution:

For the month of January:

Kc =0.51

No of days =31

ETo = 4.60

 

Therefore, ETc = ETo x Kc x No of days

                    = 4.60 x 0.51 x 31

                    = 72.725 mm

 

For the month of February:

Kc =0.93

No of days =28

ETo = 5.00

 

Therefore, ETc = ETo x Kc x No of days

                    = 5.00 x 0.93 x 28

                    = 130.2 mm

 

For the month of March:

Kc =1.14

No of days =31

ETo = 5.59

 

Therefore, ETc = ETo x Kc x No of days

                    = 5.59 x 1.14 x 31

                    = 197.47 mm

 

For the month of April:

Kc =0.82

No of days =30

ETo = 5.20

 

Therefore, ETc = ETo x Kc x No of days

                    = 5.20 x 0.82 x 30

                    = 85.2 mm

 

Total water requirement = Jan +Feb+Mar+Apr = 485.72 mm Ans.

 

Irrigation requirement for the above months are calculated by subtracting monthly effective rainfall from ETc of respective months.

 

Key Words: Evapotranspiration, Crop coefficient

 

References

Allen R, G.,Pereira, L.S., Raes, D. and Smith, M. (1998). Crop evapotranspiration guidelines for computing crop water requirements. Irrigation and Drainage Paper 56, Food and Agriculture Organization of the United Nations (FAO), Rome, Italy.(www.kimberly.uidaho.edu/ref-et/fao56.pdf‎)

Doorenbos, J. and Pruitt, W. O.(1977). Guidelines for predicting crop water requirements.Irrig.and Drain. Paper No. 24, FAO, Rome, Italy.

Michael, A.M. (2008). Irrigation Theory and Practice. Vikas Publishing House PvtLtd. New Delhi.

Murty, V.V.N. (2002).Land and Water Management Engineering(Fourth Edition).Kalyani Publisher, New Delhi.

 

Internet References

http://www.angrau.ac.in/media/7380/agro201.pdf

http://gilley.tamu.edu/BAEN464/Handout%20Items/Cuenca%20Book%20Chapter%203%20Soil%20Physics.pdf

http://ilri.org/InfoServ/Webpub/fulldocs/IWMI_IPMSmodules/Module_3.pdf

ftp://ftp.wcc.nrcs.usda.gov/wntsc/waterMgt/irrigation/NEH15/ch1.pdf

 

Suggested Reading

Michael, A.M. (2008). Irrigation Theory and Practice. Vikas Publishing House Pvt.Ltd. New Delhi.

Murty, V.V.N. (2002).Land and Water Management Engineering(Fourth Edition).Kalyani Publisher, New Delhi.

http://www.angrau.ac.in/media/7380/agro201.pdf

http://gilley.tamu.edu/BAEN464/Handout%20Items/Cuenca%20Book%20Chapter%203%20Soil%20Physics.pdf

http://ilri.org/InfoServ/Webpub/fulldocs/IWMI_IPMSmodules/Module_3.pdf

ftp://ftp.wcc.nrcs.usda.gov/wntsc/waterMgt/irrigation/NEH15/ch1.pdf