Approaches for Crop ET estimation at field scale

Pravash Chandra Moharana, Roll No. 9905SOIL SCIENCE AND AGRICULTURAL CHEMISTRY

INDIAN AGRICULTURAL RESEARCH INSTITUTE, NEW DELHI

IntroductionET measurementET estimationCase studiesConclusion

CONTENTS

• ET = evaporation from soils, plant surfaces, and water bodies combined with water losses through plant leaves at particular time period

• Evaporation: net loss of water from a surface resulting from a change in state from liquid to vapor and the net transfer of vapor to the atmosphere

• Transpiration: net loss of water from plant leaves by evaporation through plant stomata

The Basics

Irrigation scheduling

Watershed planning

Regulating water supply of canal

Calculating water use efficiency of crop

Why to estimate the Crop ET

Parameters determining ET estimation

RadiationAir temperatureAir humidityWind speed

Crop type Crop growth stages Low soil fertility

Salt toxicity Soil waterloggingpests, diseases hard pan in the root zone

How to estimate Reference Crop ET (ET0)

How to measure evapotranspiration?

Hydrological approaches

(1) Soil water balance(2) lysimeter

ET measurement

Micrometeorological approaches

(3) Energy balance and Bowen ratio(4) Aerodynamic method(5) Eddy covariance

Plant physiology approaches

(6) Sap flow method

ET = I + P - RO - DP + CR ± Δ SF ± Δ SW

Soil water balance

I= Irrigation P= Rainfall add water to the root zone.RO= Surface runoffDP= Deep percolationCR= Capillary riseΔ SF =Subsurface flow in (SFin) or out of (SFout) the root zoneΔ SW =Change in soil water content

Daily evapotranspiration calculated by soil water balance budget

Gravimetric (dashed lines) or TDR (solid line) methods. Vertical bars represent amount of water provided to the soil by

irrigation (outline bar) or rainfall (solid Bar)Mastrorilli et al., 1998

Lysimeter

PE = R + A - P

http://www.llansadwrn-wx.co.uk/evap/lysim.html

Lysimeter studies involve the growing of crops in large containers (lysimeters) and measuring their water loss and gains.

Limitations of Lysimeter

Heavy and Fixed into soil, Difficult maintenance, Expensive.

Under arid and semi arid climates, atmospheric evaporation demand can worsen the performances.

Represent a closed environment

Lysimeter rim can influence ET measurement

Energy balance

Ra= Extraterrestrial radiationRs=Solar radiationRns= Net solar or net shortwave radiationRnl=Net longwave radiation

Rn-G=H+λE

Rn= Net radiation (W m-2)G =Soil heat flux, measure directly by net-radiometers and soil heat flux platesH = Sensible heat flux densityλE =Major used part of available energy due to the radiation balance.

Energy balance measurements

Comparison of ET estimated Bowen ratio and soil water balance

Mastrorilli et al., 1998

Aerodynamic method

Used when solar energy supply (net radiation Rn) is not limiting– unlimited supply of sun light

Ea= Evaporation rate due to air flowa=overlying air (ambient)s= saturatedas= saturated and overlying air (ambient)ea = Ambient vapor pressure in aireas = Vapor pressure at the surfaceB = Vapor transfer coefficient

Combination method

Aerodynamic method• Energy supply is not limitingEnergy method• Vapor transport is not limiting

• Normally, both are limiting, so use a combination method

• Used when both air flow and solar energy supply are limited

Eddy covariance

The eddy covariance technique is a key atmospheric flux measurement technique to measure and calculate vertical turbulent fluxes within atmospheric boundary layers. The transport of scalar (vapour, heat, CO2) and vectorial amounts (i.e. momentum) in the low atmosphere in contact with the canopies is mostly governed by air turbulence

Air flow can be imagined as a horizontal flow of numerous rotating eddies, that is, turbulent vortices of various sizes, with each eddy having horizontal and vertical components. The situation looks chaotic, but vertical movement of the components can be measured from the tower.

Comparison between ET measured by eddy covariance, Bowen ratio and water balance

Rana and Katerji, 1996 Li et al., 2008

Sap flow method

Reference crop approach

Reference crop as a hypothetical crop with an assumed height of 0.12 m having a surface resistance of 70 s m-1 and an albedo of 0.23, closely resembling the evaporation of an extension surface of green grass of uniform height, actively growing and adequately watered

Suitability of ET methods to measure or model actual evapotranspiration

Stewart , 1984

Analytical approach

FAO 56 PM

Modified Penman method

Hargreaves method

Radiation method

Blaney Criddle method

Pan Evaporation method

The reference evapotranspiration was computed as per the standard procedure described in FAO 56 and FAO 24.

Allen et al., 1998FAO 56

Analytical approach

Modified Penman method was considered to offer the best results with minimum possible error in relation to a living grass reference crop.

Pan method would give acceptable estimates, depending on the location of the pan.

Radiation method was suggested for areas where available climatic data include measured air temperature and sunshine, cloudiness or radiation, but not measured wind speed and air humidity.

Blaney-Criddle method for areas where available climatic data cover air temperature data only.

FAO 56 PM

A: available energy = Rn-G (W/m-2)T: air temperature (°C)u2: Wind speed (m/s)Δ: is the slope of saturated vapour pressure curve (Pa K-1)γ: is the psychometric constant (Pa K-1)

Allen et al. 1998

Modified Penman method

where: ea = saturation vapour pressure;e d = actual vapour pressure in the air;ed = ea. RH/1QQf(U) = 0.27(1 + U/100) U= wind speed (U in km day-1 at 2 m height)Rn = total net radiation in mm day-1

w = temperature- and altitude-dependent weighting factorC = adjustment factor for ratio Uday/Unight,RHmax and Rs

Hargreaves method

Ra is extra-terrestrial radiation in equivalent mm of water evaporation for the time period,

Tmean is the mean monthly temperature in °C, and TD is the difference between maximum and minimum temperatures

Hargreaves & Samani 1985

Radiation method

Rs equivalent evaporation in mm day-1

w = temperature- and altitude-dependent weighting factorC depends on RHmean and Uday

Doorenbos & Pruitt 1977

http://www.ametsoc.org

ET0=Kp·Epan

ETo: reference crop evapotranspirationKp: pan coefficient dependent on the type of pan involved, the pan environment in relation to nearby surfaces and the climate. Ranges between 0.4 and 0.85.Epan: pan evaporation (mm/day)

Pan Evaporation

Doorenbos & Pruitt, 1977

Class A pan Colorado sunken pan

Evaporation pan siting and their environment

Crop coefficient (Kc) Approach

Kc= Ks Kcb + Ke

Basal Crop Coefficient (Kcb): ratio of ET and ET0 under conditions when the soil surface is dry, but where the soil water content of the root zone is adequate to sustain full plant transpiration

Ke: Soil water evaporation coefficient

Ks: stress reduction coefficient

Crop coefficient used to compute ET for a period of time where average conditions are used to account for the effect of water stress and evaporation from wet soil surfaces.

Kc Values (FAO 56)

Crop Kc initial Kc midseason Kc end seasonSmall Vegetables 0.7 1.00 - 1.05 0.75 - 0.95

Tomatoes & Peppers 0.6 1.05 - 1.15 0.70 - 0.90

Cucumber Family 0.4-0.6 0.85 - 1.00 0.60 - 0.90

Roots & Tubers 0.3-0.5 1.05 - 1.20 0.70 - 0.95

Legumes 0.4-0.5 1.00 - 1.15 0.35 - 1.10

Oil Crops 0.3 1.00 - 1.15 0.25 - 0.55

Cereals 0.3-1.05 1.00 - 1.20 0.25 - 1.05

Allen et al. 1998

How to estimate Crop ET (ETc)

To evaluate the most reliable ETo estimate model and crop coefficient in wheat crop for estimating the water requirement of wheat

Objectives

Case study-1

Study area- Rahuri, MaharashtraClimate- semi-arid regionSoil types- clayeyComputation of reference Eto by- Radiation Balance,(RB),Blaney Criddle (BC),Modified Penman(MP)56 Penman Monteith (PM)Pan Evaporation(PE)

Material and methods

Weekly reference evapotranspiration and lysimeter ETc (mm week-1)

Radiation Balance,(RB),Blaney Criddle (BC), Modified Penman(MP),56 Penman Monteith (PM), Pan Evaporation(PE)

RMSE test for comparing different ETo models

Radiation Balance,(RB),Blaney Criddle (BC), Modified Penman(MP),56 Penman Monteith (PM), Pan Evaporation(PE)

Lysimeter ET, reference ETo and crop coefficient in wheat.

Inference of case study

The Hargreaves method was found highly acceptable for computing reference ETo due to minimum root mean square error

This method also resulted in minimum variation of ETo (overestimated by 6.54%) as compared to the remaining models.

The FAO 56 Penman Monteith is considered as one of the soundest methods in well-established weather stations, but many of the climatic data are required when micro level study is taken up

Objectives

Case study-2

Determination of ET0 through lysimeterPerformance evaluation of commonly used ET0 estimation methods

Material and methods

Study area- Kharagpur, IndiaClimate- Sub-humid regionCrop-Potato

Estimation methods used

Comparison of seasonal ET0 estimation by different methods

Ranking of ET0 estimation methods on basis of root mean square error

(+) Shows over estimation and (+) Shows under estimation with respect to the measured value

Comparison of measured and estimated values of ET0

Inference of case study

Comparison with lysimeter data revealed that Penman-Monteith gives the best estimate of ET0 In a sub-humid region.

Penman, Hargreaves, FAO-radiation and FAO- corrected penman are not to be recommended in a sub-humid region.

Conclusions

Most direct methods have limited practical application

ET is commonly computed from weather data. A large number of empirical or semi-empirical equations have been developed for assessing crop or reference crop evapotranspiration from meteorological data.

The FAO Penman-Monteith equation provides a standard to which evapotranspiration in different periods of the year can be compared and to which the evapotranspiration from other crops can be estimated

Penman-Monteith is the best method to estimate daily and stage wise crop coefficient (Kc), using the lysimeter measured crop ET

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