ROBUST EVAPORATION ESTIMATES: RESULTS FROM A COLLABORATIVE PROJECT

ROBUST EVAPORATION ESTIMATES: RESULTS FROM A COLLABORATIVE PROJECT MG MCGLINCHEY 1 AND NG INMAN-BAMBER 1 Swaziland Sugar Association Technical Services, Simunye, Swaziland CSIRO, Davies Lab, Townsville, Australia Abstract Water Use Efficiency (WUE) can be defined in a number of ways. In Southern Africa one of the most commonly used definitions is that of yield per unit Evapotranspiration (Et) the widely used 9 tons cane/1 mm Et rule of thumb was developed using this definition of WUE. Central to this calculation is a reliable estimate of Et. Historically class-a pan evaporation was used as an estimate of Et, however with improvements in computer technology and simulation techniques this estimate is now frequently obtained using simulation models like CANEGRO and APSIM or FAO based Penman-Monteith (PM) type estimates and crop factors (FAO ). A question often raised is how robust are these simulation models and can they be used with confidence to estimate Et. This paper discusses the results of a collaborative project between the Swaziland Sugar Association Technical Services and the CSIRO (Australia) to test these simulation techniques under different environmental conditions and confirm or otherwise the crop factors published in FAO for sugarcane. Et was measured above mature sugarcane crops using two similar Bowen Ratio systems in Australia and Swaziland. Measured Et was compared with the standard FAO reference evaporation estimate at both sites. Agreement between the two sites was good. Mean mid-season crop coefficients of 1. and 1.9 were obtained in Australia and Swaziland respectively, confirming the value recommended in FAO. Both CANEGRO and APSIM were able to predict measured Et values with an acceptable degree of accuracy. At both sites net radiation was underestimated using the algorithms in FAO and those currently employed in CANEGRO. This is of concern bearing in mind that radiation is the most important component of the Penman- Monteith equation. A possible solution to this is site-specific calibration. Introduction Water Use Efficiency can be defined in a number of ways, depending on the definition of the water component in the relationship between water and yield. Yang (1997) reviewed current literature on sugarcane CWU and reported published values ranging from 3. to 19.9 TC/ha/ML. This range was due largely to the varying definitions of water-use employed by the authors. In Southern Africa the most commonly used definition of WUE is that of yield per unit ET. Historically ET was estimated using class-a pan evaporation and a simple table of canopy factor to cope with conditions of partial canopy. More recently computer crop models like CANEGRO and APSIM and energy balance methods (United Nations Food and Agriculture Organisation (FAO) type approach) have become increasingly popular as tools for estimating sugarcane crop water use and benchmarking WUE. Both the CANEGRO and the FAO techniques rely on a Penman-Monteith type equation, the former uses a modified two step approach developed specifically for sugarcane (ETcane) (McGlinchey and Inman-Bamber 199), whereas the latter uses a reference evaporation estimate (ET) adjusted using sugarcane-specific crop factors (Kc) (Allen et. al. 199). In contrast the APSIM model uses a transpiration-use-efficiency (TUE) to calculate ET from an estimate of crop biomass corrected for vapour pressure deficit. The question often posed is how reliable are theses estimates and can they be used with confidence to estimate ET across contrasting environments and different sugarcane cultivars. A collaborative project between SSA Technical Services and CSIRO (Australia) was initiated to test theses various techniques for estimating Et at Simunye ( o 1'S and 31 o 'E) in Swaziland and Kalamia, Ayr (19.7 S, 17. E) in Australia using Bowen Ratio Energy Balance (BREB). In addition this work was designed to

confirm or otherwise the Kc values for sugarcane published in FAO. Instrumentation Methods In Swaziland a BREB system was installed above a mature 3.m high crop for a period of 7 days. In Australia a similar system was erected above a young crop (.3-. m) for the remaining duration of the crop cycle. A brief description of the BREB system installed at Kalamia, Australia follows. Differences between this system and the BREB system used in Swaziland are highlighted in italics The BREB system (Campbell Scientific Inc, Logan, UT, USA) consisted of a Q7.1 REBS net radiometer, five HFT3 (REBS) soil heat flux plates (four in Swaziland) and two identical sensor arms each supporting an air intake through a mm diameter, 1. µm pore filter and an aspirated fine wire chromel-constantan thermocouple (in Swaziland the thermocouples were un-aspirated and exposed). Air was sampled alternately from each arm every 1 s. This air was passed through a chamber, housing a dew point hygrometer and dew point was measured at 1 s intervals. The mean dew point temperature for the final s of each sampling period was stored in a 3X data logger (CSI). Air temperature at the arms was also measured and logged every 1 s. The net radiometer was installed about 1. m above the canopy on a separate mast. The arms and net radiometer were raised each week as the canopy height increased. The soil heat flux (SHF) plates were installed at a depth of mm across the 1. m distance between two crop rows. Thermocouples were installed at depths of and mm in two positions either side of the central SHF plates so that SHF plates and thermocouples were evenly spaced. Two frequency domain reflectometers (model CS1, Campbell Scientific, Inc.) were inserted horizontally in the soil at a depth of 7 mm and were scanned and logged every minutes. Omhura s (19) criteria for instrument resolution were used to reject arm measurements when necessary and Bowen ratio (BR) values were interpolated to replace missing -minute values. Daily ETc calculations were rejected when more than 3% of the -minute intervals between and 1 hours readings required interpolation. At each site an automatic weather station (AWS) was erected -1 m from the BREB system above a well-watered grass surface. Short-wave radiation, temperature, relative humidity, wind speed and rainfall were logged hourly. Daily records required by the models were constructed from these hourly values. Et estimates AWS data were used to determine (ET) from equation 1 (Allen et. al., 199), where R n = net radiation at the crop surface (MJ.m -.day -1 ), G = soil heat flux density (MJ.m -.day -1 ), T = air temperature at m height ( o C), u = wind speed at m height (m.s -1 ), VPD = vapour pressure deficit (kpa), = slope vapour pressure curve (kpa. o C -1 ) and γ = psychrometric constant (kpa. o C -1 ). ET 9. ( Rn - G) γ u VPD = T 73 γ (1.3u ) (1) An estimate of ETcane was also obtained from the AWS, using a modified two-step approach that was fully described by McGlinchey and Inman-Bamber (199). A brief description follows. During the first step ET was calculated using Eq. 1. The latent and sensible heat components and appropriate profile equations (Eqs. -) were used to estimate temperature, vapour pressure and wind speed at a new reference height of 1 metres. u e t ln(( z d ) / z r r = u1 () ln(( z1 d ) / z r r γet (ln(( z d ) /( z d ))) r 1 r = e1 (3) cp ( u1 u) k H(ln(( z d )/( z d ))) r 1 r = t1 () c p( u1 u) k where: u 1 and u = wind speed at and 1 m (m.s -1 ) e 1 and e = vapour pressure at and 1 m (kpa) t 1 and t = temperature at and 1 m ( o C) z 1 and z = heights and 1 m above the ground d r = zero plane displacement of reference surface =.7 m z or = roughness length of reference surface =.13 m H = sensible heat flux (MJ.m -.day -1 )

c p = specific heat of air at constant pressure (MJ.kg -1. o C -1 ) k = von Karmans s constant =.1 During step two ETcane was calculated using Eq. and the estimated vapour pressure deficit at the new 1 m reference height (VPD ) where ρ = air density (kg.m -3 ). To comply with the definition of a sugarcane reference, crop height was fixed at 3. m during the calculation of the aerodynamic resistance (r a ) component and surface resistance (r s ) was set at s.m -1 ET ( cane) ( Rn - G) ρcpvpd = γ (1 r / r ) s a / r Results and Discussion a () Net radiation Net radiation (Rn) is the most important variable in both FAO reference ET and in the sugarcane reference ETcane. Rn used to calculate ETcane in Swaziland was estimated using an equation developed by Wright (19). Empirical constants in the equation were adjusted during initial model development (McGlinchey and Inman-Bamber 19). Rn estimated with this method was similar to Rn estimated using the FAO approach, however both underestimated measured Rn at Simunye. The bias was similar to that obtained at the Kalamia site in Australia (Fig 1). It should be emphasized that ET is a reference for comparison with crop ET (ETc) and errors in estimating Rn will affect the comparison between ETc and ET but not necessarily ETc which is the main reason for calculating ET in the fist place (Allen et al, 199). The similarity in the bias in Rn estimate at both sites provides common ground for comparisons between measured ETc and ET in Australia and Swaziland. FAO crop factor determination Determination of Kc described in FAO is in relation to crop development and is essentially obtained from measured ETc divided by ET. The aim of this initial work was to confirm the Kc(mid) value for sugarcane. In Australia the period when the fraction of intercepted radiation (FIR) <. was excluded from the analysis. Mean ETc for the period when FIR >. was. ±.13 mm and mean ET was. ±.7 mm (n=11). Weighted mean Kc was thus 1.3. It is suggested that canopy closure is essentially complete when FIR >. and that Kc be rounded to 1. for sugarcane crops in this condition. Over the 7 days duration of the Swaziland experiment a total of 3 days were considered useable. This excluded days during which a large number of data points were rejected for either instrument failure or when atmospheric conditions were unsuitable. Mean ETc for this period was.19 ±. mm and mean ET was 3.9 ±.1 mm. The Kc (mid) varied between a low of.91 and a high of 1.. These lower values occurred during the first half of the measurement period when evaporation was generally low due to overcast conditions. The average Kc(mid) for this period was 1.9. The relationship between daily ET and daily ETc measured when canopy was closed (FIR>. at Kalamia) was similar to the relationship between daily ET and daily ETc measured in Swaziland (Fig ). Differences between intercept and slope coefficients were not statistically significant. This constitutes a remarkable agreement between two sets of data for determining Kc across different countries. The similarity between Kc(mid) determined in Australia and Swaziland indicates that crop coefficients derived from these experiments are sufficiently robust to be used across contrasting environments and cultivars. Rn(ETcane) (MJ/m /day) 1 1 n= 3, R =. RMSE= 1.37 MJ/m /d Y=.7.X (a) Rn(FAO) (MJ/m /day) 1 1 n= 3, R =.9 RMSE=.9 MJ/m /d Y= 3.1.7X (b) FAO Rn (MJ m d -1 ) 1 1 n= 1, R =. RMSE= 1. MJ m d -1 Y= 3..7X (c) 1 1 Meas. Rn (MJ/m /day) 1 1 Meas. Rn (MJ/m /day) 1 1 Measured Rn (MJ m d -1 ) Figure 1. Measured and modeled Rn at Simunye, Swaziland, estimated using CANEGRO (a) and FAO (b) and Kalamia, Australia, estimated using FAO (c).

Bowen ratio ET C (mm d -1 ) 1 1 FAO daily reference ET mm d -1 ) Figure. Daily ETc measured with Bowen ratio in Kalamia (open symbols) and Swaziland (solid symbols) versus FAO daily reference ET. Kalamia ETc with FIR >.. Kalamia regression function (broken line), Y = -.99 (±.) 1. (±.) X, n = 97; Swaziland regression function (solid line), Y = -.9 (±.1) 1. (±.1) X, n = 3. APSIM estimate of ET The BREB work in Australia was used to calibrate the TUE estimate in the model. The default value of. g KPa Kg -1 was increased to. g KPa Kg -1 to adequately explain the ETc values measured with the BREB (Fig 3). As an independent validation APSIM was used to estimate cumulative ETc for three weighing lysimeters at Pongola, South Africa. APSIM simulated cumulative ETc of lysimeter almost exactly during the plant crop (Fig ). ETc of the other two lysimeters was slight above and below that of lysimeter for the plant crop. In the ratoon crop the agreement between lysimeters was good. Simulated ETc was similar to measured ETc until the summer of 19/9 when the simulated values were increasingly greater than measured. Simulated and measured mean cumulative ETc differed by 11 mm in mid 199. At the end of the first ratoon crop the difference between simulated and measured ETc was negligible (Fig ). CANEGRO estimate of ET The ETcane model underestimated ETc measured in Swaziland particularly on days of high evaporative demand (Fig ). The bias in the comparison was similar to that obtained for the Rn estimate which could account to some extent for the poor performance of the ETcane model APSIM ETc (mm d -1 ) 1 n= 1 R =. RMSE = 1.mm Y= -. 1.11X 1 Measured ETc (mm d -1 ) during peak demand periods. ETcane totalled 1 mm for the 3 valid days compared with 1 mm measured with BREB during the same period. This % error was only reduced to % (1 mm vs 19 mm) when measured Rn was substituted for simulated Rn in the calculation of ETcane (eqs. 1 and ). This suggests the remaining bias was inherent elsewhere in the PM. Figure 3. APSIM ET calibration data-set. TUE was increased from to. g KPa Kg -1 to adequately explain the Kalamia BREB data. Cumulative ET (mm) 1 1 Lys. 1 Lys. Lys. 3 APSIM Plant crop 19/1/7 7// /11/ 13//9 31/1/9 Figure. Cumulative evapotranspiration (ET) from three weighing lysimiters (Thompson, 19) and simulated ET using the APSIM-Sugarcane model with revised transpiration use efficiency. Conclusions 1 st ratoon Both simulation models were able to simulate ETc with an acceptable degree of accuracy. Given the difficulties in estimating net radiation, the predominant factor in ETc, it is perhaps fortuitous that bias was similar at the two experimental sites. It is possible that bias would be different at another site with different atmospheric properties. If this was the case then Kc may have been different to some extent. This is a problem for methods using ET or ETcane

ETOcane (mm/day) n= 3, R =.7 RMSE=.3 mm/d Y= 1..9X ETc (mm/day) Figure. Measured and simulated ETc for the Swaziland BREB data. ETc was estimated using the ETcane model. which estimate Rn, but not for APSIM which uses measured solar radiation and vapour pressure deficit for climatic effects on ETc. It would not be difficult to measure net radiation at new locations to make appropriate adjustments to Kc for a new site. In addition the results from this collaborative project support the Kc(mid) values for sugarcane (= 1.) published in FAO. The similarity between Kc(mid) determined in Australia and Swaziland indicates that crop coefficients derived from these experiments are sufficiently robust to be used across contrasting environments and cultivars. References Allen RG, Pereira LS, Raes D. and Smith M (199). Crop evapotranspiration - Guidelines for computing crop water requirements. FAO Irrigation and drainage paper. FAO, Rome. McGlinchey MG and Inman-Bamber NG (199). Predicting sugarcane water use with the Penman- Monteith equation. In: Evapotranspiration and irrigation scheduling: proceedings of the international conference, 3- November 199, San Antonio. CR Camp, EJ Sadler and RE Yoder (Eds); ASAE, St Joseph, Michigan. pp 9-9. Ohmura A (19). Objective criteria for rejecting data for Bowen ratio flux calculations. J Applied Meteorol 1: 9-9. Wright JL (19). New evaporation crop coefficients. J Irrig Drain Div, Am Soc Civ Eng 1(): 7-7. Yang SJ (1997). The water use efficiency of a sugarcane crop a review. Proc of the ISSCT Irrigation Workshop, Townsville, Australia.