Simple and Two-Level Hierarchical Bayesian Approaches for Parameter Estimation with One- and Two-Layer Evapotranspiration Models of Crop Fields

: The two-layer Shuttleworth–Wallace (SW) evapotranspiration ( ET ) model has been widely used for predicting ET with good results. Since the SW model has a large number of speciﬁc parameters, these parameters have been estimated using a simple non-hierarchical Bayesian (SB) approach. To further improve the performance of the SW model, we aimed to assess parameter estimation using a two-level hierarchical Bayesian (HB) approach that takes into account the variation in observed conditions through the comparison with a traditional one-layer Penman–Monteith (PM) model. The difference between the SB and HB approaches were evaluated using a ﬁeld-based ET dataset collected from ﬁve agricultural ﬁelds over three seasons in Myanmar. For a calibration period with large variation in environmental factors, the models with parameters calibrated by the HB approach showed better ﬁtting to observed ET than that with parameters estimated using the SB approach, indicating the potential importance of accounting for seasonal ﬂuctuations and variation in crop growth stages. The validation of parameter estimation showed that the ET estimation of the SW model with calibrated parameters was superior to that of the PM model, and the SW model provided acceptable estimations of ET , with little difference between the SB and HB approaches.


Introduction
Evapotranspiration (ET) is the process of moving water from vegetation to the atmosphere by a combination of soil water evaporation (E) and crop transpiration (T). The measurement and prediction of ET are essential for many applications in agriculture, such as irrigation scheduling, crop yield forecasting, and general hydrologic studies [1] (Flumignan et al., 2011). ET and its components can be directly measured using lysimeter, sap flow, eddy covariance, and stable isotope techniques [2,3]. However, these methods are expensive and labor intensive. Several models of ET have been developed since the 1950s. Over the last decade, the two-layer Shuttleworth-Wallace (SW) evapotranspiration model, which is physically sound and rigorous, has been widely used, and has shown performance superior to that of one-layer models such as the Penman-Monteith model (PM) [4] for different types of sparse crops, and different climatic conditions [5][6][7][8][9][10]. The SW model calculates ET as the sum of the PM equation for E and T, weighted by a set of coefficients that represent the combination of soil and canopy resistance [11]. However, the practical application of this approach is somewhat limited, since it is a highly complex

Study Sites and Observation
The study sites were experimental fields of the Department of Agricultural Research in the Zayarthiri Township of Naypyidaw (latitude 19 • 49.5 N, longitude 96 • 16.44 E, 100 m above sea level) (Figure 1a). This area has a moderate monsoon climate, with an average annual rainfall of 1100 mm, of which about 75% falls during the monsoon season-between June and September. The observation of ET using the BREB method was conducted using a tripod micrometeorological station installed at five agricultural experimental fields growing four crops-rice, sunflower, sesame, and groundnut-over three seasons-dry, monsoon, and post-monsoon-from 2017 to 2019 (Figure 1b). The abbreviations of each observation representing seasons and crops (i.e., sesame in the dry (SsD), paddy rice in the dry (PrD), paddy rice in the monsoon (PrM), groundnut in the post-monsoon (GnPm), and sunflower in the post-monsoon (SfPm) seasons) are defined in Table 1. The soil at a depth of 0-70 cm in the study fields was primarily sandy loam and clay loam with poor organic matter content, and partially contained a layer of loam and loamy sand. Due to the limited dataset in Myanmar, only PrD2 was used to validate the model parameters. The other observations were used for the calibration. The calibration period for the parameters of the ET model was 87 days for all observations. In the HB approach, the parameters were calibrated using a dataset of 87 days which was divided equally into three calibration periods of 29 days, each considering three growth stages of crops (i.e., vegetative, reproductive, and ripening stages). The irrigated fields included both paddy rice and SsD fields; the soil condition of the paddy field was almost saturated duration the observation; thus, the amount of water provided by basin irrigation was only observed for SsD.
Water 2021, 13, x FOR PEER REVIEW 3 of 23 using a tripod micrometeorological station installed at five agricultural experimental fields growing four crops-rice, sunflower, sesame, and groundnut-over three seasons-dry, monsoon, and post-monsoon-from 2017 to 2019 (Figure 1b). The abbreviations of each observation representing seasons and crops (i.e., sesame in the dry (SsD), paddy rice in the dry (PrD), paddy rice in the monsoon (PrM), groundnut in the postmonsoon (GnPm), and sunflower in the post-monsoon (SfPm) seasons) are defined in Table 1. The soil at a depth of 0-70 cm in the study fields was primarily sandy loam and clay loam with poor organic matter content, and partially contained a layer of loam and loamy sand. Due to the limited dataset in Myanmar, only PrD2 was used to validate the model parameters. The other observations were used for the calibration. The calibration period for the parameters of the ET model was 87 days for all observations. In the HB approach, the parameters were calibrated using a dataset of 87 days which was divided equally into three calibration periods of 29 days, each considering three growth stages of crops (i.e., vegetative, reproductive, and ripening stages). The irrigated fields included both paddy rice and SsD fields; the soil condition of the paddy field was almost saturated duration the observation; thus, the amount of water provided by basin irrigation was only observed for SsD.  The BREB ET observation system was composed of thermo-hygrometers (air temperature and capacitive relative humidity sensor instruments) (HMP110; Vantaa, Vaisala, Finland) with multi-plate radiation shields (FP1806; Field Pro, Tokyo, Japan) located at 1.0 m and 2.5 m above ground level; solar radiometers (PCM−01NB−L3CS; PREDE Co.,  The BREB ET observation system was composed of thermo-hygrometers (air temperature and capacitive relative humidity sensor instruments) (HMP110; Vantaa, Vaisala, Finland) with multi-plate radiation shields (FP1806; Field Pro, Tokyo, Japan) located at 1.0 m and 2.5 m above ground level; solar radiometers (PCM−01NB−L3CS; PREDE Co., Tokyo, Japan) for shortwave radiation and infrared radiometers (PRI−01B−L3CS; PREDE Co,.) for long-wave radiation; a heat flux sensor (PHF02−L5CS; PREDE Co.); and an anemometer (03101Y−L3CS; RM Young, Traverse City, MI, USA). These data were automatically logged every 10 min using a data logger (CR1000; Campbell Scientific Inc., Logan, UT, USA). The soil moisture content was observed using capacitance sensors (GS-1; METER Group Inc., Pullman, WA, USA) installed in each field, which recorded into a data logger at 30 min intervals (Em50; METER Group Inc.). The daily mean datasets were used for the ET calculation of the BREB and the simulation of the SW model. We did not measure the leaf area index (LAI) and no research into LAI has been conducted in Myanmar. The LAI plays important roles in determining the coefficients of soil surface and canopy resistances in ET models for weighting ET partitioning into E and T. However, since in this study we did not aim to rigorously evaluate ET by the models, we estimated the values of LAI from plant height using formulas from the literature, as described below.

Two-Layer Model Based on the SW Model
The SW model is composed of the sum of two terms, E and T, as follows: where the latent heat flux (λET) is the sum of the latent heat of water evaporation of the soil surface (λE) and crops (λT), respectively (W m −2 ). C s and C c represent the soil surface and canopy resistance coefficients, respectively. λ is the latent heat of water vaporization (MJ kg −1 ). ET s and ET c represent the soil evaporation and the canopy transpiration (W m −2 ), respectively, and are calculated as: where ∆ is slope of the saturation vapor pressure curve at the mean air temperature (kPa • C −1 ), ρ is the mean air density (kg m −3 ), C p is the specific heat capacity of dry air at constant pressure (approximately 1.01 kJ kg −1 • C −1 ), γ is the psychrometric constant (approximately 0.066 kPa • C −1 ), D is the air water vapor pressure deficit at the reference height (kPa), r a and r s are the aerodynamic resistance and stomatal resistance (s m −1 ), respectively, and A and A s represent the available energy input above the canopy and above the soil surface (W m −2 ), respectively. These are expressed as: where R n and R ns are the net radiation fluxes (W m −2 ) into the canopy and the soil surface and G is the soil heat flux (W m −2 ). R ns can be calculated using Beer's law: where K a is the extinction coefficient of light attenuation depending on the crop development stage. For example, the K a of rice increases from 0.35 to 0.62 from the vegetative period to the post-heading stage [26]. In this study, K a was set to 0.4 for all crop and growth stages. The LAI is defined as the effective value of the leaf area index that accounts for illumination-induced stomatal closure deeper in the canopy (m 2 m −2 ). The value of estimated LAI (LAI est ) was estimated from the measured plant height (h c ) using the following formula for typical values of field crops [27,28]: Water 2021, 13, 3607

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The maximum LAI (LAI max ) for normal conditions of plant density were obtained from the literature, and were 4.0 for rice [29,30], 5.0 for groundnut [31,32], 3.0 for sunflower [33], and 2.2 for sesame [34]. The coefficients C s and C c are given by the expressions: in which R a , R s , and R c are calculated as: The three aerodynamic resistance values (r c a , r a a , and r s a ) (s m −1 ) were calculated using the methodology of [35], as detailed in Appendix A. The bulk stomatal resistance of the canopy (r c s ) (s m −1 ), which is the equivalent resistance of all the individual stoma in a canopy and depends on other environmental variables, can be calculated using the Jarvis-type model [36,37] and following the protocol of [17] as; where r STmin indicates the minimal stomatal resistance (s m −1 ) and is estimated using a Bayesian approach. Following previous studies [17,38], the stress functions F 1 to F 4 , representing the effect of the variables related to soil surface and aerodynamic resistance R s , air temperature T a ( • C), D (kPa), and volumetric moisture content wc (m 3 m −3 ), are defined as follows: where k 1 (W m −2 ), k 2 ( • C), and k 3 (kPa −1 ) are constants optimized using a Bayesian approach. T a,low and T a,high are the low and high temperature limits and were assumed to be 0 and 40 • C, respectively [39]; wc root is the moisture content in the root zone, estimated from observed data at a depth of 0-50 cm (m 3 m −3 ); wc wp is the moisture content at the wilting point (m 3 m −3 ) measured in the laboratory, and wc cr is the critical moisture content at which plant stress starts, set to 60% of saturated moisture content [16]. When the value of the stress function is zero, the canopy resistance is not calculated. The soil surface resistance (r s s ) (s m −1 ) is expressed as: where b 1 and b 2 are empirical constants (s m −1 ) representing the estimation parameters using the Bayesian approaches, wc sur is the observed volumetric soil moisture content 5 cm below the soil surface, and wc SAT is the saturated moisture content.

Traditional One-Layer PM Model
The PM model can be formulated as [4]: where the canopy resistance r c s was computed following Equation (13). r a is the aerodynamic resistance (s m −1 ) and is usually calculated using the following equation, assuming neutral stability conditions [40]: where k is von Karman's constant (0.41), u is the wind speed at 2 m height (m s −1 ); z is the reference height above the crop at which meteorological measurements are available (2 m); d is zero plane displacement (m), estimated as d = 0.63·h c ; and z 0 is the roughness length for momentum transfer (m), estimated as z 0 = 0.13·h c.

Simple and HB Approaches
The probabilistic models for the estimation of parameters of the ET model are described as follows: For the SB approach, and for the HB approach, where the SB approach does not include the constant c, representing different groups; in this study, the three calibration periods. O ET [c] [t] indicates the ET data observed at day t (1, 2, . . . , 87 in this study) at each observation period, divided evenly into three calibration periods c (1, 2, 3), S ET [c] [t] are the ET data estimated by the SW and PM models at day t for each period c, and σ is the standard deviation, representing the measurement error variance of ET estimates (mm day −1 ). Normal indicates a normal distribution from which the ET estimates are generated. θ s [c] represents a prior uncertainty in the parameters of the calibration period and can be described as being stochastically generated from a normal distribution of θ s, mean and σ θ s . θ s, mean is the overall mean distribution of each parameter in a dataset, while σ θ s is a random variable distribution that represents the difference in the calibration period of the parameter. We assumed that the specific unknown parameters θ s are distributed uniformly within a specified prior distribution range, as shown in Table 2.
In this study, all the simulations and calculations were performed in R version 4.0.2. For estimating the posterior distribution for the SB and HB approaches, we used RStan version 2.19.3 developed by [41], which employs a Markov Chain Monte Carlo (MCMC) technique to sample from the posterior distribution of a given model. We ran four MCMC chains with 50,000 iterations and monitored them to confirm that the MCMC chains converged to the target distributions. When the Rhat of RStan was less than 1.05, we judged the chains to have converged.

Evaluation of the Performance of the SW Model Using Bayesian Approaches
The ET values were calculated from the posterior mean values of the model parameters were evaluated using four statistical criteria: the mean absolute error (MAE), the root mean square error (RMSE), the mean absolute percentage error (MAPE), and the root mean square percentage error (RMSPE). The MAE can potentially be used to identify bias. RMSE provides an overall measure of the degree to which the data differ from the model estimation. MAE and RMSE values of zero indicate a perfect fit. MAPE and RMSPE, which can express the estimation accuracy as a ratio, were included because the model was estimated under different observation conditions: the dry and the monsoon. These statistical criteria are described as follows: where E i and O i represent the estimated and observed values, respectively. N (or n) is the number of data points in the dataset.

Differences in Observation Conditions in a Dataset
In the HB approach, the model parameters were estimated taking into account the uncertainty in group levels within a dataset. The growth period of crops is generally separated into three stages: the vegetative, reproductive, and ripening stages. Therefore, daily observed data for the 87 days were divided into three periods to calibrate the parameters using the HB approach, as described in Figure 2.
The difference between R n and T a in the first and third periods of the dry season and the post-monsoon was greater than that for the monsoon season. The T a of PrM was almost constant and SsD, PrD1, and PrD2 gradually increased, but GnPm and SfPm gradually decreased over the three periods. The difference between the minimum and maximum T a in a dataset was 3.6 • C in the monsoon, 6.1 • C and 8.1 • C in the dry, and 7.2 • C and 8.0 • C in the post-monsoon seasons, respectively. Changes in wc, except in the paddy field, indicated that SsD, GnPm, and SfPm had decreased residual wc, and PrM, PrD1, and PrD2 with ponded water maintained high. As calculated using Equation (7), the estimated LAI values increased rapidly in the first and second periods and gradually reached a constant

Comparison of Posterior Distributions of Model Parameters
A violin plot, depicting the posterior distribution of the SW and PM models parameters using the SB and HB approaches, is presented in Figure 3. The values of the posterior parameter distribution for the two models corresponding to the means and the 95% confidence intervals are given in Appendix B (Table A1).
These results showed that parameterizations using SB and HB were, in most cases, successful in reducing the uncertainties in the parameters. Several posterior distributions, such as the k2 of SsD and the b1 of SfPm for the SW model were strongly constrained, indicating their importance as a parameter for controlling ET estimation and partition (Figure 3a), as discussed below. In the estimation of the k2 of SsD, we assumed that the strong correlation between Ta and the observed ET with the small LAI was a factor that constrained the posterior distributions, in order to reduce the value of T. SsD had the

Comparison of Posterior Distributions of Model Parameters
A violin plot, depicting the posterior distribution of the SW and PM models parameters using the SB and HB approaches, is presented in Figure 3. The values of the posterior parameter distribution for the two models corresponding to the means and the 95% confidence intervals are given in Appendix B (Table A1).

Effect of Difference in Observation Conditions on Parameter Estimation
The differences between the posterior distributions of the model parameters, estimated using the HB approach based on observed ET data over the three calibration periods, were clear in several cases (Figure 3), including the of PrM and GnPm, the k1 of GnPm, the k2 of SfPm, and the b1 of SsD in the SW model, and the k2 of SsD and SfPm, These results showed that parameterizations using SB and HB were, in most cases, successful in reducing the uncertainties in the parameters. Several posterior distributions, such as the k 2 of SsD and the b 1 of SfPm for the SW model were strongly constrained, indicating their importance as a parameter for controlling ET estimation and partition (Figure 3a), as discussed below. In the estimation of the k 2 of SsD, we assumed that the strong correlation between T a and the observed ET with the small LAI was a factor that constrained the posterior distributions, in order to reduce the value of T. SsD had the highest correlation with T a and the observed ET of all crops. The correlation coefficients between daily T a and the observed ET in descending order were SsD 0.66 > PrD1 0.57 > PrM 0.47 > GnPm 0.36 > SfPm 0.21. This strongly constrained k 2 of SsD at the low side of the prior bound indicated that the stomatal resistance of the canopy largely limited the value of T. In the estimation of the b 1 of SfPm in the SW model, the soil moisture of SfPm near the soil surface was the lowest of all observations (Figure 2). The posterior distribution range of b 1 associated with the soil surface resistance was, therefore, constrained to the low side of the prior range to reduce the value of E. The parameter k 3 , which is related to D, was poorly constrained and displayed large variability in all observations (Figure 3). Although the D value differed considerably between the dry and monsoon seasons (Figure 2), there were no differences in the posterior distribution of k 3 between the two seasons for the SW model. As previous studies [17,18] showed, k 3 is an insensitive parameter of the models. Some parameters showed large differences between the SB and the HB on the posterior distribution. For example, the b 2 of SsD for the SB in the SW model was constrained at the low side on the prior bound, but that for the HB was restricted at the upper side, with no differences in group levels ( Figure 3a). This observation indicated that the parameter b 2 , calibrated by the SB and the HB, had different abilities under the same environmental factors, and thus might have made a difference in the estimated and partitioned ET between the SB and HB approaches (Figure 3a).
The range of posterior distributions for several parameters differed substantially between the SW and PM models. For example, the r STmin of PrM of the SW model was well constrained compared to that of the PM model. For the PM model, the b 2 of SsD had lower uncertainty than that for the SW model. The differences in model structure between the SW and PM models affected the reduction in uncertainty in the parameter estimation. However, for most of the parameters and conditions, there were no large differences in the range of posterior distributions between the models.

Effect of Difference in Observation Conditions on Parameter Estimation
The differences between the posterior distributions of the model parameters, estimated using the HB approach based on observed ET data over the three calibration periods, were clear in several cases (Figure 3), including the r STmin of PrM and GnPm, the k 1 of GnPm, the k 2 of SfPm, and the b 1 of SsD in the SW model, and the k 2 of SsD and SfPm, the b 1 of GnPm, and the b 2 of SfPm in the PM model. However, in the k 3 of all conditions and all parameters for PrD1, these differences were unclear.
The degrees of variation in the posterior mean value of the three calibration periods in a dataset-the coefficient of variation (CV) for each parameter and observation-are shown in Figure 4. The CV was calculated from the posterior mean value of σ θ s divided by θ s,mean in Equation (23). The CV values of r STmin , which are associated with stomatal resistance, varied greatly depending on the observations; the difference between the SW and PM models was also larger than those of other parameters. Since the two-layer SW model is sensitive to error in the values of canopy and soil resistance [10,43], measurement errors or improperly used constants and variables, such as K a and LAI, may have increased the variation in the estimation of this parameter. In paddy fields, which have continuously saturated surfaces in which the fluctuations in the surface resistance are small, the variations in r STmin may be strongly affected by the growth of the plant.
In contrast, the variability of k 3 was nearly constant, regardless of differences in the models and observations. The posterior distributions of k 3 were poorly constrained and widespread along prior ranges ( Figure 3). As has been shown in previous studies [17,18,20], this observation suggests that k 3 is not very susceptible to seasonal fluctuations with large uncertainties. Therefore, k 3 may not be necessary for strict parameter estimation, and empirical values may suffice.
In contrast, the variability of k3 was nearly constant, regardless of differences in the models and observations. The posterior distributions of k3 were poorly constrained and widespread along prior ranges ( Figure 3). As has been shown in previous studies [17,18,20], this observation suggests that k3 is not very susceptible to seasonal fluctuations with large uncertainties. Therefore, k3 may not be necessary for strict parameter estimation, and empirical values may suffice.

Comparison of ET Estimation by SW and PM Models Using SB and HB Approaches
Figures 5a and 6a show the daily changes in observed and estimated ET using the SW and the PM models with parameters calibrated using the SB and HB approaches with irrigation + rainfall amount for SsD, while rainfall amount only was used for the others. Figures 5b and 6b show the values corresponding to the difference in the SB and HB approaches on the estimated ET, calculated as the estimated ET of the SB approach divided by that of the HB approach. The ET observed and estimated by the two models with parameters calibrated by the SB and HB approaches with portioned E and T only for the SW model; the results of statistical evaluation for the model performances are shown in Table  3, and the regression analysis between the observed ET and that estimated by the models are presented in Figure 7.
For the SW model, generally, the posterior mean value of the parameters calibrated using the SB and HB approaches provided acceptable estimations of ET over 87 days (Figure 5a), and the differences in the estimated total values of ET between the SB and HB approaches were small: 0 to 1 mm for PrD1 and SfPm, and 3 to 10 mm for SsD, PrM, and GnPm (Table 3). However, for the PM model, there was a large difference between the

Comparison of ET Estimation by SW and PM Models Using SB and HB Approaches
Figures 5a and 6a show the daily changes in observed and estimated ET using the SW and the PM models with parameters calibrated using the SB and HB approaches with irrigation + rainfall amount for SsD, while rainfall amount only was used for the others. Figures 5b and 6b show the values corresponding to the difference in the SB and HB approaches on the estimated ET, calculated as the estimated ET of the SB approach divided by that of the HB approach. The ET observed and estimated by the two models with parameters calibrated by the SB and HB approaches with portioned E and T only for the SW model; the results of statistical evaluation for the model performances are shown in Table 3, and the regression analysis between the observed ET and that estimated by the models are presented in Figure 7.
For the SW model, generally, the posterior mean value of the parameters calibrated using the SB and HB approaches provided acceptable estimations of ET over 87 days (Figure 5a), and the differences in the estimated total values of ET between the SB and HB approaches were small: 0 to 1 mm for PrD1 and SfPm, and 3 to 10 mm for SsD, PrM, and GnPm (Table 3). However, for the PM model, there was a large difference between the estimated and observed ET, except for PrD1 and PrM (Figure 6a and Table 3). The confidence intervals of the posterior distributions on the estimated ET for the PM model were generally wider than those for the SW model, indicating that large uncertainty in the parameter values led to poor model performance. For the PrD1 of both models, the estimated total value of ET produced by the SB and HB approaches were very similar ( Table 3). The values of ET sw.SB /ET sw.HB and ET pm.SB /ET pm.HB were close to 1.0 over the entire period (Figures 5b and 6b), indicating a lack of variation in the posterior mean distributions at three calibration periods using the HB approach (Figure 3). The wc and LAI values changed constantly or linearly over all calibration periods ( Figure 2). Therefore, even when the variables that affected the estimation of the parameters varied considerably over the calibration period, the variables would not have affected the parameter estimation for each period if the change ratio was a constant. For PrM with a small fluctuation in environmental factors, a slight difference in ET estimates between SB and HB was apparent in the third calibration period (Figures 5b and 6b), possibly because the estimated LAI value varied in a different change rate at the third period ( Figure 2). The averaged change rate of LAI for the three stages in PrM were 3.2%, 1.0%, and 0.1%. For SsD and SfPm, large fluctuations in environmental factors produced differences between the ET estimates obtained using the SB and HB approaches for almost the entire period, especially in the PM model (Figures 5 and 6). This observation suggests that the SW model could adapt well to changes in seasons and surface conditions with respect to parameter estimation, but the PM model was subject to environmental fluctuation in the calibration period.  The ET estimates obtained using the HB approach in both models were better than those produced using the SB approach in both models, indicating that the performance of the SW model was superior to that of the PM model ( Table 3). The slope of the estimates obtained using the HB approach in the regression was closer to 1 than that produced by the SB for most observation conditions in both models (Figure 7). Considering the magnitude of errors for the BREB method, which are reported to be in order from several to 10 percent (e.g., [44]), it is not appropriate to evaluate the accuracy of estimated ET with parameters calibrated using the SB and HB approaches. However, the HB approach improved the fit of the model to the observed data, indicating the potential importance of accounting for seasonal fluctuations and variations in crop growth stages in calibrating models which can reliably predict ET. Water 2021, 13, x FOR PEER REVIEW 14 of 23  For ET partitioning in PrM for the SB and HB approaches in the SW model (Table 3), the E est and E est /ET est values were very small, at 43 mm and 0.08, respectively, indicating the improper use of estimated LAI values, or the uncertainty in ET partitioning by the SW model calibrated using only observed ET data. However, due to the lack of direct measurements of the different components of ET, it is not possible to validate the partitioned ET values using the SB and HB approaches in the SW model, although the estimated total ET was acceptable. Thus, multiple observations obtained using lysimeters and measures of sap flow should be used to validate ET partitioning into E and T by the SW model using the Bayesian approach in future studies. Water 2021, 13, x FOR PEER REVIEW 16 of 23

Validation of Model ET Estimation
Applying the posterior mean values of the parameters generated by the SB and the HB approaches using the dataset of PrD1 (Table A1), the model performance of the SW and the PM models were validated using the dataset of PrD2, as shown in Figure 8.
The ET estimates of the SW model were approximately equal to the observed ET, except during the initial stage, and there were few differences in the estimated ET using parameters calibrated by the SB and the HB approaches. The PM model overestimated ET in the last half period; the estimated ET using parameters obtained using the HB approach was slightly larger than those obtained using the SB approach.
Numerous studies have shown that the two-layer SW model has higher performance for ET estimation than other ET models, a finding consistent with those of our study. These studies have confirmed that the differences in the model structure by which ET is partitioned into E and T is related to the model performance (e.g., [16,17,[45][46][47][48]). The ET estimation of the SW model with parameters calibrated using the Bayesian approaches was acceptable for the paddy field condition, suggesting the usefulness of the two-layer SW model. The low level of difference between the SB and HB approaches in the ET estimation of the SW model may be related to the small environmental fluctuations in the calibration period (PrD1).
The overestimation of ET by the PM model in this study agrees with the results of several previous studies (e.g., [9,48]), but some studies have found that the difference between the SW and PM models is not significant [8,49]. Generally, the performance of ET model is strongly related to the surface resistance, which is calculated as the integration of the canopy and soil surface resistances and reflects the effects of soil moisture and variation in LAI [50]. Some studies (e.g., [46]) reported that when the surface resistance in the PM model is lower than the observed resistance, the PM model overestimates ET. Thus, this overestimation in this study indicates an inadequate parameter estimation of the surface resistance, affected by the model structure of the PM model. The estimated mean value of r c s in the PM model using the parameters from the HB approach was slightly lower (54 m s −1 ) than that from the SB approach (69 m s −1 ) during the last half period. The posterior means of r STmin for PrD1 generated by the SB and HB approaches were 52 m s −1 and 47 m s −1 , respectively, and those of other parameters also differed slightly between the SB and HB approaches (Table A1). Since the validation was performed using only the dataset of the paddy field conditions, the validity of the HB approach must be further verified using calibration data including large seasonal fluctuations and variations in crop growth.

Conclusions
In this study, we assessed the estimation of ET and the parameters of the SW and PM models calibrated using SB and HB approaches, based on a field-based ET dataset collected from five agricultural fields over three seasons in Myanmar. The main conclusions were as follows: 1. Parameterization using the SB and HB approaches was, in most cases, successful in reducing the uncertainties in the parameters. Using the HB approach to parameter estimation, we identified the parameters which are sensitive to seasonal fluctuations and differences in crop growth stages. The parameter k3, which is related to the water vapor pressure deficit, was not very susceptible to seasonal fluctuations, while rSTmin, which is related to stomatal resistance, was sensitive to variations in observation conditions. 2. In the calibration of model parameters, the statistical criteria for all conditions observed indicated that the models with parameters calibrated using the HB approach had a better fit to the observed ET data than those with parameters calculated using the SB approach, indicating the potential importance of seasonal fluctuations and variability in crop growth stages for the calibration of model parameters. The performance of the SW models was superior to that of the PM model for most of the obser-

Conclusions
In this study, we assessed the estimation of ET and the parameters of the SW and PM models calibrated using SB and HB approaches, based on a field-based ET dataset collected from five agricultural fields over three seasons in Myanmar. The main conclusions were as follows: 1.
Parameterization using the SB and HB approaches was, in most cases, successful in reducing the uncertainties in the parameters. Using the HB approach to parameter estimation, we identified the parameters which are sensitive to seasonal fluctuations and differences in crop growth stages. The parameter k 3 , which is related to the water vapor pressure deficit, was not very susceptible to seasonal fluctuations, while r STmin , which is related to stomatal resistance, was sensitive to variations in observation conditions.

2.
In the calibration of model parameters, the statistical criteria for all conditions observed indicated that the models with parameters calibrated using the HB approach had a better fit to the observed ET data than those with parameters calculated using the SB approach, indicating the potential importance of seasonal fluctuations and variability in crop growth stages for the calibration of model parameters. The performance of the SW models was superior to that of the PM model for most of the observation conditions, using both the SB and HB approaches. 3.
The SW model, with parameters calculated using the SB and HB approaches with only observed ET data, could provide an acceptable estimation of the ET. The PM model with parameters calculated using both SB and HB approaches overestimated the ET in the last half period; the ET estimates for the HB approach were slightly overestimated compared with the SB approach.
where r a a (α) and r s a (α) are values of r a a and r s a for a crop with complete canopy cover (LAI = 4) (s m −1 ), respectively; r a a (0) and r s a (0) are values of r a a and r s a for bare soil (s m −1 ), respectively; z is the reference height above the crop where meteorological measurements are available (2 m); h c is plant height (m); d is zero plane displacement, usually estimated as d = 0.63·h c ; z 0 is roughness length of the crop, which can be calculated as z 0 = 0.13·h c ; z 0 is effective roughness length of bare soil (0.01 m); n is extinction coefficient of the eddy diffusion (2.5 [54]); and u * is the friction velocity (m s −1 ), which is given by u * = ku/ln{(z − d)/z 0 } where k is von Karman's constant (0.41) and u is wind speed (m s −1 ).

Appendix B. Posterior Probability Distributions of the Parameters
The values of the posterior parameter distribution using the SB and HB approaches for the SW and PM models corresponding to the means and the 95% confidence intervals are given in Appendix B (Table A1). Table A1. Posterior probability distributions of the parameters in the Shuttleworth-Wallace (SW) and the Penman-Monteith (PM) models using simple Bayesian (SB) and hierarchical Bayesian (HB) approaches for five observations, characterized by the posterior means and 95% Bayesian confidence intervals.