Evapotranspiration Estimation Using Soil Water Balance, Weather and Crop Data

The rise in water demand for agriculture, industry, domestic, and environmental needs requires sagacious use of this limited resource. Since agriculture (mainly irrigation) is the major user of water, improving agricultural water management is essential. Efficient agricultural water management requires reliable estimation of crop water requirement (evapotranspiration). Evapotranspiration (ET) is the transfer of water from the soil surface (evaporation) and plants (transpiration) to the atmosphere. ET is a critical component of water balance at plot, field, farm, catchment, basin or global level. From an agricultural point of view, ET determines the amount of water to be applied through artificial means (irrigation). Reliable estimation of ET is important in that it determines the size of canals, pumps, and dams. The use of the terms ‘reference evapotranspiration’, ‘potential evapotranspiration’, ‘crop evapotranspiration’, ‘actual evapotranspiration’ in this chapter is based on FAO-56 (FAO Irrigation and Drainage publication No 56) (Allen et al., 1998). There are different methods of determining evapotranspiration: direct measurement, indirect methods from weather data and soil water balance. These methods can be generally classified as empirical methods (eg. Thornthwaite, 1948; Blaney and Criddle, 1950) and physical based methods (eg. Penman, 1948; Montheith, 1981 and FAO Penman Montheith (Allen et al., (1998)). They vary in terms of data requirement and accuracy. At present, the FAO Penman Montheith approach is considered as a standard method for ET estimation in agriculture (Allen et al., 1998). A case study from a semiarid region of Australia will be used to demonstrate ET estimation for a canola (Brassica napus L.) crop using soil water balance and crop coefficient approaches. Daily rainfall data, soil moisture measurement data using neutron probe, and AquaCrop (Steduto et al., 2009) -estimated deep percolation below the crop root zone will be used to determine actual evapotranspiration of the crop using soil water balance. Reference evapotranspiration ETo will be determined using FAO ETo calculator (Raes, 2009). Crop canopy cover measured using a handheld GreenSeekerTM and expressed as normalized difference vegetation index (NDVI) will be used to interpret evolution of evapotranspiration during the growing season (life cycle) of the canola crop.


Introduction
The rise in water demand for agriculture, industry, domestic, and environmental needs requires sagacious use of this limited resource.Since agriculture (mainly irrigation) is the major user of water, improving agricultural water management is essential.Efficient agricultural water management requires reliable estimation of crop water requirement (evapotranspiration). Evapotranspiration (ET) is the transfer of water from the soil surface (evaporation) and plants (transpiration) to the atmosphere.ET is a critical component of water balance at plot, field, farm, catchment, basin or global level.From an agricultural point of view, ET determines the amount of water to be applied through artificial means (irrigation).Reliable estimation of ET is important in that it determines the size of canals, pumps, and dams.The use of the terms 'reference evapotranspiration', 'potential evapotranspiration', 'crop evapotranspiration', 'actual evapotranspiration' in this chapter is based on FAO-56 (FAO Irrigation and Drainage publication No 56) (Allen et al., 1998).There are different methods of determining evapotranspiration: direct measurement, indirect methods from weather data and soil water balance.These methods can be generally classified as empirical methods (eg.Thornthwaite, 1948;Blaney and Criddle, 1950) and physical based methods (eg.Penman, 1948;Montheith, 1981 andFAO Penman Montheith (Allen et al., (1998)).They vary in terms of data requirement and accuracy.At present, the FAO Penman Montheith approach is considered as a standard method for ET estimation in agriculture (Allen et al., 1998).A case study from a semiarid region of Australia will be used to demonstrate ET estimation for a canola (Brassica napus L.) crop using soil water balance and crop coefficient approaches.Daily rainfall data, soil moisture measurement data using neutron probe, and AquaCrop (Steduto et al., 2009) -estimated deep percolation below the crop root zone will be used to determine actual evapotranspiration of the crop using soil water balance.Reference evapotranspiration ET o will be determined using FAO ET o calculator (Raes, 2009).Crop canopy cover measured using a handheld GreenSeeker TM and expressed as normalized difference vegetation index (NDVI) will be used to interpret evolution of evapotranspiration during the growing season (life cycle) of the canola crop.A field experiment was carried out during the growing season of 2010 at canola field experimental site of Wagga Wagga Agricultural Research Institute located at Wagga Wagga (35 o 03'N; 147 o 21'E; 235 m asl), NSW (Australia).There was enough rainfall (930 mm) in contrast to long term average of 522 mm in 2010 to provode ideal growing conditions.A popular variety of canola (Hyola50) was sown on 30 April 2010.The experiment was conducted on a 24 m x 24 m area.There were 24 plots, 12 experimental plots and 12 buffer plots.The plots were 6 m long with 1 meter buffer on either end.Plot width was 1.8 m with a 0.5 m walking strip between plots for data collection.About a month before the experimental season, neutron probe access tubes were installed to a depth of 1.5 m for soil moisture measurement.Two access tubes were installed at 2 m from either end of the plot and 2 m from each other.Soil moisture content was measured at 15, 30, 45, 60, 90, and 120 cm depths every two weeks.The probe was calibrated using gravimetric soil moisture measurements done when access tubes were installed on site.

Weather data
Daily weather data (rainfall, minimum and maximum temperature, solar radiation, relative humidity, and wind speed) were collected from the meteorological station of the Wagga Wagga Agricultural Institute located adjacent to the experimental site.Out of the total annual rainfall of 930 mm, the amount or proportion (in percentage) during the canola growing season (May to November) was 514 mm (53%) while the long term average was 333 mm (64% of the long term average of 522 mm).Monthly average maximum and minimum temperature was 26 o C and 3 o C respectively.Reference evapotranspiration ET o was calculated using the procedure described in the FAO Irrigation and Drainage Paper 56 (Allen et al., 1998) with the help of the program FAO ET o Calculator (Raes, 2009).

Crop parameters
The following crop phenological stages were recorded during the growing season: planting date, 90% emergence, beginning and end of flowering, senescence and maturity.The canopy cover was measured using GreenSeeker TM , an Optical Sensor Unit (NTech Industries, Inc., USA).GreenSeeker TM , is a handheld tool that determines Normalized Difference Vegetative Index (NDVI), is an integrated optical sensing and application system that measures green crop canopy cover.

Soil water balance method
Rain or irrigation reaching a unit area of soil surface, may infiltrate into the soil, or leave the area as surface runoff.The infiltrated water may (a) evaporate directly from the soil surface, (b) taken up by plants for growth or transpiration, (c) drain downward beyond the root zone as deep percolation, or (d) accumulate within the root zone.The water balance method is based on the conservation of mass which states that change in soil water content ∆S of a root zone of a crop is equal to the difference between the amount of water added to the root zone, Q i , and the amount of water withdrawn from it, Q o (Hillel, 1998) in a given time interval expressed as in Eq. ( 1).
Eq. ( 1) can be used to determine evapotranspiration of a given crop as follows where ∆S = change in root zone soil moisture storage, P = Precipitation, I = Irrigation, U = upward capillary rise into the root zone, R = Runoff, D = Deep percolation beyond the root zone, ET = evapotranspiration.All quantities are expressed as volume of water per unit land area (depth units).
In order to use Eq. ( 2) to determine evapotranspiration (ET), other parameters must be measured or estimated.It is relatively easy to measure the amount of water added to the field by rain and irrigation.In agricultural fields, the amount of runoff is generally small so is often considered negligible.When the groundwater table is deep, capillary rise U is negligible.The most difficult parameter to measure is deep percolation D. If soil water potential and moisture content are monitored, D can be estimated using Darcy's Principle.
In this study, deep percolation estimated using AquaCrop (Raes et al., 2009), was adopted.Runoff R was also estimated using AquaCrop following USDA curve number approach (Hawkins et al., 1985).The change in soil water storage ∆S is measured using specialized instruments such as neutron probe and time-domain reflectrometer.

Introduction
The crop coefficient approach relates evapotranspiration from a reference crop surface (ET o ) to evapotranspiration from a given crop (ET c ) through a coefficient.Estimation of crop water requirement from weather and crop data is a simpler and cost effective method compared to other methods such as soil water balance method.In this method, potential evapotranspiration of a crop is presumed to be determined by the evaporative demand of the atmosphere and crop characteristics.Evaporative demand of the air is determined as the evapotranspiration from a reference crop.The reference crop is a hypothetical crop (grass or alfalfa) with specific characteristics such as crop height of 0.12 m and albedo of 0.23 (Allen et al., 1998).Penman (1956) defined reference evapotranspiration as "the amount of water transpired in unit time by a shorter green crop, completely shading the ground, of uniform height and never short of water."It is a useful standard of reference for the comparison of different regions and of different measured evapotranspiration values within a given region.As such, ET o is a climatic parameter expressing the evaporation power of the atmosphere independent of crop type, crop development and management practices (Allen et al., 1998).FAO Penman Montheith approach is considered as the standard method.In this method, reference evapotranspiration ET o is estimated from weather data as given in Eq. (3).
Reference evapotranspiration ET o can be calculated using a spreadsheet or computer programs which are designed for various level of data availability eg.CROPWAT (Smith, 1992) and ET o Calculator (Raes, 2009).In this study, the latter program was used.It is important to make clear distinction between reference evapotranspiration ET o and potential crop evapotranspiration ET c .The latter is also called maximum crop evapotranspiration.
Evapotranspiration from a given crop grown and managed under standard conditions is called potential crop evapotranspiration ET c .Standard condition is a disease-free, wellfertilized crops, grown in large fields, under optimum soil water conditions, and achieving full production under the given climatic conditions.ET o depends evapotranspiration (ET c ) represents the climatic "demand" for water by a given crop.Potential crop depends primarily on the evaporative demand of the air.

Single crop coefficient method
The single crop coefficient (K c ) method is used to determine soil evaporation and transpiration lumped over a number of days or weeks.The single "time-averaged" K c curve incorporates averaged transpiration and soil wetting effects into a single K c factor.The FAO-56 publication divides the crop growth stages into four phenological stages.Initial stage is f r o m p l a n t i n g t o 1 0 % g r o u n d c o v e r .D e v e lopment stage is from 10% groundcover to maximum cover.Midseason stage is from the beginning of full cover to the start of senescence.The late season stage is from the start of senescence to full senescence or harvest.The evolution of crop coefficients during these stages is tabulated in FAO-56 for a number of crops including canola.Three coefficients are given for the initial, midseason, and end of season stages as K c ini , K c mid , and K c end respectively.K c ini is assumed to be constant and relatively small (<0.4).The K c begins to increase during the crop development stage and reaches a maximum value K c mid which is relatively constant for most growing and cultural conditions.During the late season period, as leaves begin to age and senesce, the K c begins to decrease until it reaches a lower value at the end of the growing period equal to K c end .
The K c during the development is estimated using linear interpolation between K c ini and K c mid .Similarly, K c during the late season stage is determined using linear interpolation between K c mid and K c end .The value of K c ini and K c end can vary considerably on a daily basis, depending on the frequency of wetting by irrigation and rainfall.The single crop coefficient method can be used for irrigation planning and design.It is accurate enough for systems with large interval such as surface and set sprinkler irrigation.It is also used for catchment level hydrologic water balance studies (Allen et al., 1998).
In the single crop coefficient method, potential crop evapotranspiration ET c is estimated from a single crop coefficient (K c ) and reference evapotranspirations ET o as in Eq. ( 4).
Eq. ( 4) gives the potential (maximum) evapotranspiration of the crop when the soil moisture is not limiting.Since localized K c values are not always available in many parts of the world, the values of K c as suggested by FAO (Allen et al., 1998) are being widely used to estimate evapotranspiration.
When rainfall amount and irrigation are not sufficient to keep the soil moisture high enough, the soil moisture content in the root zone is reduced to levels too low to sustain the potential crop evapotranspiration ET c .This results in an evapotranspiration less than the potential, and the plants are said to be under water stress.This evapotranspiration is called actual evapotranspiration (ET a ).In general, the actual evapotranspiration ET a from various crops will not be equal to the potential value ET c .Actual evapotranspiration ET a is generally a fraction of ET c depending on soil moisture availability.Actual evapotranspiration ET a from a well-watered crop might generally approach ET c during the active growing stage, but may fall below during the early growth stage, prior to full canopy coverage, and again toward the end of the growing season as the matured plant starts to dry out (Hillel, 1997).
The actual evapotranspiration ET a is calculated by combining the effects of K c and soil water stress coefficient (K s ) as shown in Eq. ( 5).
The stress reduction coefficient K s [0-1] reduces K c when the average soil water content of the root zone is not high enough to sustain full crop transpiration.The stress coefficient K s is determined by the amount of moisture the crop depleted from the rootzone of a crop.The amount of water depleted from the rootzone is expressed by root zone depletion D r , i.e. water storage relative to field capacity.Stress is presumed to initiate when D r exceeds the readily available water (RAW), Fig. 2. When more than RAW is extracted from the rootzone (D r >RAW), K s is expressed (Allen et al., 1998) as Where TAW = total plant available soil water in the root zone (mm), and p = fraction of TAW that a crop can extract from the root zone without suffering water stress.When D r ≤ RAW, K s =1 indicating no water stress.The total available water in the root zone (TAW, mm) is estimated as the difference between the water content at the field capacity and wilting point Where Z r = effective rooting depth (m); θ FC is soil moisture content at field capacity (m 3 m -3 ); θ WP is soil moisture content at permanent wilting point (m 3 m -3 ).
Readily available water (RAW) is the amount of water which the crop can extract without experiencing stress.It is expressed as Soil moisture depletion fraction (p) is the fraction of soil water in the root zone that can be depleted before stress occurred.It varies from crop to crop and also varies at different growth stages of a given crop.Shallow rooted and sensitive crops such as vegetables have low p value while deep rooted and stress tolerant crops have a higher p value.
Canola crop coefficient values given in FAO 56 (Allen et al., 1998) are K c ini = 0.35, K c mid = 1.0-1.15,K c end = 0.35.These values represent K c for a sub humid climate with RHmin = 45% and wind speed of 2 m/s.To take account for impacts of differences in aerodynamic roughness between crops and the grass reference with changing climate, the K c mid and K c end values larger than 0.45 must be adjusted using the following equation: Where K c (tab) is the value of K c taken from

Dual crop coefficient method
The single coefficient method does not separate evaporation and transpiration components of evapotranspiration.The dual crop coefficient approach calculates the actual increase in K c for each day as a function of plant development and the wetness of the soil surface.It is best for high frequency irrigation such as microirrigation, centre pivots, and linear move systems (Suleiman et al., 2007).The effects of crop transpiration and soil evaporation are determined separately using two coefficients: the basal crop coefficient (K cb ) to describe plant transpiration and the soil water evaporation coefficient (K e ) to describe evaporation from the soil surface, Eq (10).AquaCrop determines crop transpiration (T r ) and soil evaporation (E) by multiplying ETo with their specific coefficients K cb and K e (Eq.11) (Steduto et al., 2009).
The range of K cb and K e is [0-1.4].When soil moisture is limiting, K cb is multiplied by a coefficient K s which is equal to 1 when D r ≤RAW and declines linearly to zero when all the available water in the rooting zone has been used.Evapotranspiration under such a condition is calculated using Eq. ( 12).
Because the water stress coefficient impacts only crop transpiration, rather than evaporation from the soil, the application using Eq. ( 12) is generally more valid than is application using Eq. ( 5) in the single crop coefficient approach.Allen et al. (1998) reported that in situations where evaporation from soil is not a large component of ET c , use of Eq. ( 5) will provide reasonable results.The dual coefficient approach can be summarized into the following three steps: Calculate reference evapotranspiration (ET o ) from climatic data using Eq. ( 3), calculate individual crops potential evapotranspiration ET c using Eq. ( 11), and when the soil moisture content is limited, K cb coefficient is multiplied by stress factors K s to calculate actual evapotranspiration ET a using Eq. ( 12).

Basal crop coefficient
The basal crop coefficient K cb is defined as the ratio of ET c to ET o when the soil surface layer is dry but where the average soil water content of the rootzone is adequate to sustain full plant transpiration (Bonder et al., 2007).The dual crop coefficient approach uses daily time step and is readily adapted to spreadsheet program.Some models such as AquaCrop (Steduto et al., 2009) determine crop water productivity from the "productive" component of evapotranspiration i.e. transpiration.AquaCrop requires regression of daily values of biomass and crop transpiration to determine crop water productivity.Therefore, transpiration should be measured or estimated.FAO-56 has tabulated K cb values for a number of crops, including canola, at the initial, mid season, and end of season stages.Since localized K cb values were not available for the study area, the values of K cb suggested by FAO-56 (Allen et al., 1998) were used.For canola these value were K cb ini = 0.15, K cb mid = 0.95-1.10,and K cb end = 0.25.In this study, K cb of 0.15, 1, and 0.25, respectively, for the initial, mid-season, and end of season stages were selected.
The growing season of canola vary from 5 months to 7 months in Australia i.e. 150 -210 days depending on the planting date and the weather conditions (rainfall and temperature) during the season.Initial, development, mid-season, and late season stage lengths for canola grown during the 2010 winter season in Wagga Wagga (Australia) were 10, 64, 84, 48 days respectively.
The values for K cb in the FAO-56 table represent values for a sub humid climate with RH min = 45% and wind speed of 2 m/s.To take account for impacts of differences in aerodynamic roughness between crops and the grass reference, the K cb mid and K cb end values larger than 0.45 must be adjusted using the following equation: Where K cb (tab) is the value of K cb mid taken from Table 17 of Allen et al. (1998).The other parameters are as defined in Eq. ( 9).The K cb values for the mid-season stage was adjusted using Eq. ( 13) to 0.98 for for RH min = 48%, u 2 = 1.91 m/s, and plant height of 1.0 m.Once the K cb values for the initial stage, mid season stage, and end-of-season stage were determined, K cb values for development and late season stages were determined using linear interpolation.
The K cb coefficient for any period (day) of the growing season can be derived by considering that during the initial and mid-season stages K cb is constant and equal to the K cb value of the growth stage under consideration.During the crop development and late season stage, K cb varies linearly between the K cb at the end of the initial stage (K c ini ) and the K cb at the beginning of the midseason stage (K cb mid ).During the mid season stage K cb is constant as K cb mid .During late season stage, K cb varies linearly between K cb mid and K cb end .In the case of canola the end of season K cb does not need adjustment since it is 0.25 which is less than 0.45.

Soil evaporation coefficient
Similar to K cb , soil evaporation coefficient K e needs to be calculated on a daily basis.K e is a function of soil water characteristics, exposed and wetted soil fraction, and top layer soil water balance (Allen et al., 2005).In the initial stage of crop growth, the fraction of soil surface covered by the crop is small, and thus, soil evaporation losses are considerable.Following rain or irrigation, K e can be as high as 1.When the soil surface is dry, K e is small and even zero.K e is determined using Eq. ( 14).
Where K c max = maximum value of crop coefficient K c following rain or irrigation; K r = evaporation reduction coefficient which depends on the cumulative depth of water depleted; and f ew = fraction of the soil that is both wetted and exposed to solar radiation.K c max represents an upper limit on evaporation and transpiration from the cropped surface.K c max ranges [1.05-1.30](Allen et al., 2005).Its value is calculated for initial, development, mid-season, or late season using Eq. 15.
Evaporation occurs predominantly from the exposed soil fraction.Hence, evaporation is restricted at any moment by the energy available at the exposed soil fraction, i.e.K e cannot exceed f ew x K c max.The calculation of K e consists in determining K c max, K r , and f ew .K c max for initial, development, midseason, and late season stages were calculated to be 1.196, 1.181, 1.187, and 1.195 respectively.

Evaporation reduction coefficient
The estimation of evaporation reduction coefficient K r requires a daily water balance computation for the surface soil layer.Evaporation from exposed soil takes place in two stages: an energy limiting stage (Stage 1) and a falling rate stage (Stage 2) (Ritchie 1972) as indicated in Fig. 3.During stage 1, evaporation occurs at the maximum rate limited only by energy availability at the soil surface and therefore, K r = 1.As the soil surface dries, the evaporation rate decreases below the potential evaporation rate (K c max -K cb ).K r becomes zero when no water is left for evaporation in the evaporation layer.Stage 1 holds until the cumulative depth of evaporation D e is depleted which depends on the hydraulic properties of the upper soil.At the end of Stage 1 drying, D e is equal to readily evaporable water (REW).REW ranges from 5 to 12 mm and highest for medium and fine textured soils (Table 1 of Allen et al., 2005).The evolution of K r is presented in Fig. 3.The second stage begins when D e exceeds REW.Evaporation from the soil decreases in proportion to the amount of water remaining at the surface layer.Therefore reduction in evaporation during stage 2 is proportional to the cumulative evaporation from the surface soil layer as expressed in Eq. ( 16).where De, j-1 = cumulative depletion from the soil surface layer at the end of previous day (mm); The TEW and REW are in mm.The amount of water that can be removed by evaporation during a complete drying cycle is estimated as in Eq. ( 17).
  1000 0.5 Where TEW =maximum depth of water that can be evaporated from the surface soil layer when the layer has been initially completely wetted (mm).θ FC and θwp are in (m 3 m -3 ) and Ze (m) = depth of the surface soil subject to evaporation.FAO-56 recommended values for Ze of 0.10-0.15m,with 0.10 m for coarse soils and 0.15 m for fine textured soils.
Fig. 3. Soil evaporation reduction coefficient K r (adapted from Allen et al., 2005).REW stands for readily extractable water and TEW stands for total extractable water.
Calculation of K e requires a daily water balance for the wetted and exposed fraction of the surface soil layer (f ew ).Eq. ( 18) is used to determine cumulative evaporation from the top soil layer (Allen et al., 2005).

 
,, 1 , , jj e j e jj j ei j ei j we w where D e,j-1 and D e,j = cumulative depletion at the ends of days j-1 and j (mm); P j and R j = precipitation and runoff from the soil surface on day j (mm); I j = irrigation on day j (mm); E j = evaporation on day j (i.e., E j = K e x ET o ) (mm); T ei,j = depth of transpiration from exposed and wetted fraction of the soil surface layer (f ew ) on day j (mm); and D ei,j = deep percolation from the soil surface layer on day j (mm) if soil water content exceeds field capacity (mm).
Assuming that the surface layer is at field capacity following heavy rain or irrigation, the minimum value of D e,j is zero and limits imposed are 0≤D e,j ≤TEW.T ei can be ignored except for shallow rooted crops (0.5-0.6m).
Evaporation is greater between plants exposed to sunlight and with air ventilation.The fraction of the soil surface from which most evaporation occurs is f ew = 1-f c .
Where 1-f c = 1-CC; f w is fraction of soil surface wetted by irrigation or rainfall; f w is 1 for rainfall (Table 20 of Allen et al., 1998); f c is fraction of soil surface covered by vegetation.In this study f c is the canopy cover measured using GreenSeeker TM .Values of parameters used in the dual coefficient approach are presented in Table 1.

Parameter Value
Field capacity, θ FC (m 3 m -3 ) 30.1 Permanent wilting point, θ WP (m 3 m -3 ) 15.0 Effective rooting depth, Z r (m) 1.00 Depth of the surface soil layer, Z e (m) 0.15 Total evaporable water, TEW (mm) 33.7 Readily evaporable water, REW (mm) 9 Total available water, TAW (mm) 160 Readily available water, RAW (mm) 96 The ratio of RAW to TAW, p (fraction) 0.6 Wetting fraction, f w (fraction) 1 Table 1.The parameters of the soil used in the determination of K s , K e , and K r in the FAO dual coefficient method.
The top soil layer (0-0.15m) of the soil in this study is sandy clay loam.Readily extractable water (REW) is 9 mm for this soil texture (Table 1 of Allen et al., 2005).Field capacity and wilting point of this soil were determined as part of soil hydraulic properties characterization.Canola effective rooting depth was determined as part of National Brasicca Germaplasm Improvement Program (David Luckett, personal communication).Soil moisture content was monitored using on-site calibrated neutron probe.Soil moisture depletion fraction (p) of 0.6 m was taken from FAO-56 publication (Allen et al., 1998).Since the only source of water was rainfall, wetting fraction f w of 1 was used.

AquaCrop approach of determining dual evapotranspiration coefficients
Eq. ( 11) gives evapotranspiration when the soil water is not limiting.When the soil evaporation and transpiration drops below their respective maximum rates, AquaCrop simulates ET a by multiplying the crop transpiration coefficient with the water stress coefficient for stomatal closure (Ks sto ), and the soil water evaporation coefficient with a reduction K r [0-1] (Steduto et al., 2009) as AquaCrop calculates basal crop coefficient at any stage as a product of basal crop coefficient at mid-season stage K cb(x) and green canopy cover (CC).For canola K cb(x) = 0.95 was used.
Evaporation from a fully wet soil surface is inversely proportional to the effective canopy cover.The proportional factor is the soil evaporation coefficient for fully wet and unshaded soil surface (K e(x) ) which is a program parameter with a default value of K e(x) = 1.1 (Raes et al., 2009).
During the energy limiting (non-water limiting) stage of evaporation, maximum evaporation (E x ) is given by Where CC is green canopy cover; K ex is soil evaporation coefficient for fully wet and non shaded soil surface (Steduto et al., 2009).In AquaCrop, K ex is a program parameter with a default value of 1.10 ( Allen et al., 1998).When the soil water is limiting, actual evaporation rate is given by Maximum crop transpiration (T rx ) for a well-watered crop is calculated as K cbx is the basal crop coefficient for well-watered soil and complete canopy cover.

Soil water balance
The actual evapotranspiration determined using soil water balance method is presented in Table 2. Evapotranspiration was determined using Eq. ( 2) from measurement of 12 neutron probes several times during the season.Deep percolation and runoff were not measured.Therefore, values estimated by AquaCrop (Steduto et al., 2009;Raes et al., 2009) during the canola water productivity simulation were adopted.The runoff estimated using AquaCrop was low, supporting the consensus that runoff from agricultural land is low.However, deep percolation past the 1.2 m was significant.
The actual annual crop evapotranspiration estimated using this method was 313 mm.It can be observed that evapotranspiration was higher during the mid season and highly evaporative months.

Evapotranspiration coefficient
Single and dual evapotranspiration coefficients and crop canopy cover data are presented in Fig. 4. The K c and K cb values adopted from FAO-56 publication and adjusted for the local condition are shown in the Figure .The K c and K cb curves follow similar trend as the measured canopy cover curve.The canopy cover values were higher than the K c and K cb curves towards the end of the season.This is due to the fact that as an indeterminate crop, canola still had green canopy due to the ample rainfall during this late season stage of the crop.The soil evaporation coefficient K e was correctly simulated using the top-layer soil water balance model.It can be seen that K e is high during the initial and late season stages.
It remained low and steady during the midseason stage.The higher number of K e spikes are

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Evapotranspiration -Remote Sensing and Modeling 54 due to frequent rainfall during the season.The K e value estimated using AquaCrop followed similar trend to the manually calculated using Eq. ( 14).However, AquaCrop did not simulate response to individual rainfall events.In the development stage, the soil surface covered by the crop gradually increases and the K e value decreases.In the midseason stage, the soil surface covered by the crop reaches maximum and water loss is mainly by crop transpiration and K e is as low as 0.05.In the late season stage, the K e values are greater than that in the mid-season stage because of the senescence.Evaporation and transpiration estimated using the dual coefficient approach (Fig. 5) are correctly simulated, with high evaporation during the initial and late stages, and low during the developmental and mid season stages.The fluctuation in the evaporation component is high at these stages and low and steady during the mid season stage except minor spikes after rainfall events.Evaporation during the late stage (late spring months) was high compared with the initial stage which is a winter period.The transpiration component was steady increasing during the crop development stage before reaching a maximum in late mid season stage and declined during the late season stage due to senescence.The trends in evaporation and transpiration were in perfect phase with the weather and crop phenology.Evapotranspiration varies during the growing period of a crop due to variation in crop canopy and climatic conditions (Allen et al., 1998).Variation in crop canopy changes the proportion of evaporation and transpiration components of evapotranspiration.The spikes in basal crop coefficient were high during the initial and crop development phases and decreases as the soil dries (Fig. 4).The spikes decrease as the canopy closes and much of ET is by transpiration.During the late season stage, there were fewer spikes because soil evaporation was low and almost constant.The largest difference between K c and K cb is found in the initial growth stage where evapotranspiration is predominantly in the form of soil evaporation and crop transpiration.Because crop canopies are near or at full ground cover during the mid-season stage, soil evaporation beneath the canopy has less effect on crop transpiration and the value of K cb in the mid season stage is very close to K c .Depending on the ground cover, the basal crop coefficient during the mid season stage may be only 0.05-0.10lower than the K c value.In this study K cb mid is 0.10 lower than K c mid .Some studies, carried out in different regions of the world, have compared the results obtained using the approach described by Allen et al. (1998) with those resulting from other methodologies.From this comparison, some limitations should be expected in the application of the dual crop coefficient FAO-56 approach.Dragoni et al. (2004), which measured actual transpiration in an apple orchard in cool, humid climate (New York, USA), showed a significant overestimation (over 15%) of basal crop coefficients by the FAO 56 method compared to measurements (sap flow).This suggests that dual crop coefficient method is more appropriate if there is substantial evaporation during the season and for incomplete cover and drip irrigation.Fig. 6.Crop evapotranspiration determined using single and dual coefficient approaches of FAO 56 for a canola planted on 30 April 2010 at Wagga Wagga, NSW (Australia).ET c estimated using AquaCrop (dual coefficient) is also presented.

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Evapotranspiration -Remote Sensing and Modeling

56
Crop evapotranspiration estimated using single and double coefficients is presented in Fig. 6.ET c estimated using AquaCrop is also presented in the Figure .It can be observed that ET c estimated using the three approaches is similar except in the initial and late season stages.
During the initial stage, the ET c estimated using Eq. ( 14) and AquaCrop (Eqs.21 and 22) are very close.However, the single coefficient method underestimated ET c at this stage.During the initial stage when most of the soil is bare, evaporation is high especially if the soil is wet due to irrigation or rainfall.The single crop coefficient approach does not sufficiently take this into account.A similar pattern was observed during the late season stage.However, AquaCrop overestimated ET c during this stage compared to the other two methods.The annual evapotranspiration estimated using different approaches was as follows: soil water balance (ET a = 313 mm), single crop coefficient (ET c = 332 mm), dual coefficient approach (ET c = 366 mm with E of 79 mm and T of 288 mm), AquaCrop (ET c = 382 mm with E of 139 mm and T of 243 mm).The evapotranspiration determined using soil water balance method is the "actual" evapotranspiration while the other methods measure potential evapotranspiration ET c .Soil water depletion (Dr) in Eq. ( 6) was determined using soil moisture content measured during the season and it was found that Dr<RAW throughout the season indicating that there was no soil moisture stress (K s = 1).That might be why the ET c estimated using single coefficient method is close to the ET c determined using soil water balance method.Approaches using dual coefficient (Eq.14) and Eqs.(21 and 22) resulted in higher ET c values.This might be due to the fact that in these approaches, the evaporation during the initial and late season stages was well simulated.

Conclusion
Two approaches of estimating crop evapotranspiration were demonstrated using a field crop grown in a semiarid environment of Australia.These approaches were the rootzone soil water balance and the crop coefficient methods.The components of rootzone water balance, except evapotranspiration, were measured/estimated.Evapotranspiration was calculated as an independent parameter in the soil water balance equation.Single crop coefficient and dual coefficient approaches were based on adjustment of the FAO 56 coefficients for local condition.AquaCrop was also used to estimate crop evapotranspiration using the dual coefficient approach.It was found that the dual coefficients, basal or transpiration coefficient K cb and evaporation coefficient K e , correctly depict the actual process.The effects of weather (rainfall and radiation) and crop phenology were correctly simulated in this method.However, single coefficient does not show the high evaporation component during the initial and late season stages.Generally, there is a strong agreement among different estimation methods except that the dual coefficient approach had better estimate during the initial and late season stages.The evapotranspiration estimated using different approaches was as follows: soil water balance (ET a = 313 mm), single crop coefficient (ET c = 332 mm), dual coefficient approach (ET c = 366 mm with E of 79 mm and T of 288 mm), AquaCrop (ET c = 382 mm with E of 139 mm and T of 243 mm).Evapotranspiration estimated using soil water balance method is actual evapotranspiration ET a , while other methods estimate potential (maximum) evapotranspiration.Accordingly, ET estimated using rootzone water balance is lower than the ET estimated using the other methods.The single coefficient approach resulted in the lowest ET c as it is not taking into account the evaporation spikes after rainfall during the initial and late season stages.

Fig. 1 .
Fig. 1.(a) Rain and reference evapotranspiration ET o (long term average and in 2010) (b) Monthly average temperature (long term average and in 2010) at Wagga Wagga, NSW (Australia).

Fig. 4 .
Fig. 4. Single crop coefficient (K c ), basal coefficient (K cb ), soil evaporation coefficient (K e ), crop canopy cover (CC) curves for canola having growth stage lengths of 10, 64, 84, and 48 days during initial, development, midseason, and late season stages.Indicated on curve are also single and basal crop coefficient (K c and K cb ) at initial, midseason, and end of season stages.Day of planting is 30 April 2010.