基于MARS模拟鄱阳湖地区参考作物蒸散量

为了寻找最适宜的鄱阳湖作物蒸散量替代计算方法,文中以FAO Penman-Monteith模型参考作物蒸散量计算结果(ET₀)为标准值,使用江西省南昌站1966—2015年逐日最高温度、最低温度、日照时数、风速和相对湿度数据(其中1966—1990年数据用于建立模型,1991—2015年数据用于验证模型),建立12种不同气象要素组合条件下的多元自适应回归样条(MARS)ET₀计算模型,并将计算结果与广义回归神经网络(GRNN)、支持向量机(SVM)和经验模型(Hargreaves法、Irmak-Allen法、Makkink 法、Pristley-Taylor法)的计算结果相比较.结果表明:3种人工智能算法的ET₀计算结果精度均优于相同输入数据下的经验模型.3种人工智能算法中MARS的精度最高,在全参数组合下RMSE为0.227 mm/d,R²为0.982,NRMSE为0.086,其次是支持向量机,其在全参数组合下RMSE为0.266 mm/d,R²为0.978,NRMSE为0.101,GRNN排第三,其在全参数组合下RMSE为0.323 mm/d,R²为0.962,NRMSE为0.123.缺少温度参数时,模型精度总体较差,3种人工智能算法下R²仅为0.8左右.MARS法不但精度更高,而且具有明确的数学表达式,是鄱阳湖地区适宜的ET₀计算方法.     

Evaluation of pan coefficient for reference crop evapotranspiration for semi-arid region

Pan coefficient (K _pan) is the important factor for computation of reference evapotranspiration (ET _o ) from pan evaporation (E _pan). In this paper, the approaches proposed by Cuenca (Irrigation system design: an engineering approach. Prentice-Hall, Englewood Cliffs, 1989), Snyder (J Irrig Drain Eng 118(6):977–980, 1992), Orang (Potential accuracy of the popular non-linear regression equations for estimating pan coefficient values in the original and FAO-24 tables. Unpublished Report, Calif. Dept. of Water Resources, Sacramento, 1998), Raghuwanshi and Wallender (J Irrig Drain Eng 118 (6):977–980, 1998) and Pereira et al. (Agric Water Manage 76:75–82, 1995) were evaluated for a semi-arid region. By comparing with the FAO 56 Penman-Monteith (F-PM) method the Snyder (J Irrig Drain Eng 118(6):977–980, 1992, 1992) approach was best suited for the semi-arid region.     

Applicability of support vector machines and adaptive neurofuzzy inference system for modeling potato crop evapotranspiration

Estimation of crop evapotranspiration (ET_C) for certain crops such as potato is very important for irrigation planning, irrigation scheduling and irrigation systems management. The primary focus of this study was to investigate the accuracy of the adaptive neurofuzzy inference system (ANFIS) and support vector machines (SVM) for potato ET_C estimation when lysimeter measurements or the complete weather data for applying the FAO method are not available. The estimates of the ANFIS and SVM models were compared with the empirical equations of Blaney–Criddle, Makkink, Turc, Priestley–Taylor, Hargreaves and Ritchie. The performances of the different SVM and ANFIS models were evaluated by comparing the corresponding values of root mean square error (RMSE), mean absolute error (MAE) and correlation coefficient (r). The drawn conclusions confirmed that the SVM and ANFIS models could provide more accurate ET_C estimates than the empirical equations. Overall, the minimum RMSE (0.042 mm/day) and MAE (0.031 mm/day) values and the maximum r value (0.98) were obtained using the SVM model with mean air temperature, relative humidity, solar radiation, sunshine hours and wind speed as inputs.     

Evaluation of estimated weather data for calculating Penman-Monteith reference crop evapotranspiration

Utilizing the weather generator ClimGen, daily solar radiation (R_s) and vapor pressure deficit (VPD) were estimated from temperature data and used to calculate evapotranspiration at five locations, representing tropical, temperate, semi-arid, and arid climates. ClimGen was calibrated for each location using the most recent 2 or 5 years of complete daily weather records. Actual and estimated values were compared on a daily and weekly (7-day running average) basis. Error indices were defined to indicate excellent to poor performance of the estimation methods. Overall in all locations, the ClimGen estimates for both daily R_s and VPD were poor to acceptable. The weekly analyses showed significant improvement in performance for both R_s and VPD estimations in arid and semi-arid locations. Daily reference crop evapotranspiration values using the FAO Penman-Monteith equation (PM ET_o) were calculated using complete daily weather records. These values were compared with (1) ET_o calculated with the PM model, actual temperature data, and ClimGen estimates of daily R_s, VPD, and generated wind speed (PM_Est ET_o), and (2) ET_o calculated solely from actual daily temperature data using a calibrated version of the Hargreaves method (HG_Adj ET_o). The daily PM_Est ET_o results were poor to acceptable in all locations, but analyses for weekly periods showed improved performance to acceptable and good levels for arid and semi-arid locations. The performance of the HG_Adj ET_o method was also poor to acceptable for daily ET estimates in all locations, while weekly analyses showed improvement. A non-calibrated version of the Hargreaves method did not work for either daily or weekly periods. The PM_Est ET_o and HG_adj ET_o methods appeared suitable for weekly periods in arid and semi-arid locations provided that at least 2 years of complete weather records were available to calibrate the parameters required. There was no advantage in using 5 years of weather records for calibration.Communicated by E. Fereres     

Modelling weekly rainfall data for crop planning in a sub-humid climate of India

In this study, two and three-parameter probability distributions have been compared to identify the most appropriate distribution to describe the weekly rainfall data in a sub-humid climate of India. The “best” distribution among different data sets has been identified using probability plot and Anderson–Darling (AD) test for goodness-of-fit, along with the appropriateness of estimated percentiles. One single probability distribution has not been found appropriate to represent all the data sets though Weibull distribution has been found promising for most of the data sets. Gamma probability distribution, which is generally employed for describing weekly rainfall data, was found to grossly approximate the underlying process. Likelihood ratio (LR) test revealed that three-parameter distributions did not significantly improve the fit over two-parameter distributions within the same family. The developed models for different data sets have been employed for computing minimum assured amount of rainfall at different probability levels. Minimum assured rainfall at 40% probability level was found to be in close agreement with the long-term average weekly rainfall data as depicted by L2 and Chebycheff norms. The effect of departure from minimum assured rainfall at 70% probability level on yield of rainfed maize in the region has also been studied using the available experimental data collected from three different field experiments. The effect of intervening drought was found to be most serious in reducing the crop yield by 37–58% as compared to other scenarios representing rainfall amount and its distribution under a sub-humid climate.     

参考作物蒸发蒸腾量计算方法在海河流域的适用性

参考作物蒸发蒸腾量（ET0）的计算公式很多,各公式所需参数各异,为寻找一种所需资料少而又精度较高的替代方法,选用1998年FAO-56分册推荐的Penman－Monteith（PM）、Hargreaves、Irmark-Allen等6种方法分别计算海河流域10个典型气象站30 a的参考作物蒸发蒸腾量,并以PM公式为标准,对其他方法进行评价。结果表明,10个站点中除了五台山地区,Hargreaves与FAO-24 Radiation 这2种方法更接近于PM方法的计算结果,其误差较小,在海河流域缺少辐射和风速     

A comparison of the Priestley-Taylor and Penman methods for estimating reference crop evapotranspiration in tropical countries

An equation for Potential Evaporation (PE) proposed by Priestley and Taylor in 1972 has fewer data requirements than the well established Penman Potential Transpiration (Et) equation. From their definitions, PE and Et values should both provide acceptable estimates of Reference Crop Evapotranspiration (ETo), as defined by Doorenbos and Pruitt. Analysis of mean monthly climatic data from 30 tropical stations, widely spread within the latitude zone 25°N to 25°S, showed that PE and Et estimates agreed closely when monthly rainfall exceeded monthly Et. The minimum data requirements for the Priestley-Taylor equation are daily net radiation and mean air temperature. The Penman equation additionally requires daily data for humidity and run of wind. As reliable field net radiometers become more widely available, the Priestley-Taylor PE equation offers a satisfactory alternative to the Penman Et equation for estimating ETo in humid tropical climates.     

Using energy balance data for assessing evapotranspiration and crop coefficients in a Mediterranean vineyard

Improving water use efficiency is a key element of water management in irrigated viticulture, especially in arid or semi-arid areas. In this study, the micrometeorological technique “Eddy Covariance” was used to directly quantify the crop evapotranspiration (ET) and to analyze the complex relationships between evapotranspiration, energy fluxes, and meteorological conditions. Both observed Direct measurements (DIR) of latent heat flux (LE) and observed from the residual of the energy balance (REB) equation were used for crop evapotranspiration calculations. Observed crop coefficients (K _cms) were then determined using the standardized reference evapotranspiration (ET_o) equation for short canopies. In addition, linear approximations from observations were used to model the seasonal trend lines for crop coefficients and K _cs values were parameterized by first identifying the beginning and end of each growth stage. The modeled K _cs values were used to predict daily ET from ET_o measurements and compared with values from literature. The daily observed DIR ET values (ET_do) were lower than REB ET (ET_ro) during periods with precipitation, but they were similar during dry periods, which implies that energy balance closure is better when the surface is drier. Comparisons between modeled ET and crop ET estimated using K _c values from best agreement was observed between the modeled REB and FAO 56 and the local K _c values provided by the Regional Agency ARPAS showed good agreement with observed ET (from DIR and REB data) than the FAO 56 ones. The study confirmed that the availability of locally driven K _c could be relevant to quantify the crop water requirement and represents the starting point for a sustainable management of water resources.     

西北地区冬小麦腾发量估算模型适用性评价

为实现对西北地区冬小麦腾发量(ET)的准确估算,在对不同生育期ET的影响因子进行分析后分别采用双作物系数模型、单作物系数模型和Priestley-Taylor(PT)模型模拟ET,并以大型蒸渗仪实测ET为标准值对比其精度.结果表明:气象因子是播种-返青(Ⅰ期)和抽穗-乳熟(Ⅲ期)ET的主导因子,作物因子是乳熟-收获(Ⅳ期)ET的主导因子,2种因子对返青-抽穗(Ⅱ期)和全生育期ET的驱动作用相近;Ⅰ期双作物系数模型、单作物系数模型和PT模型的R²分别为0.511 8,0.239 3,0.374 2,RMSE变化范围为0.284 6~0.366 3 mm/d,总体评价指标GPI排名分别为1,3,2;Ⅱ期3个模型的R²均在0.700 0 以上,RMSE为0.540 9~0.844 0 mm/d,双作物系数模型模拟效果最好;Ⅲ期各模型的R²均高于0.600 0,RMSE为0.828 8~1.258 7 mm/d,双作物系数模型GPI排名第1;Ⅳ期3个模型的R²分别为0.799 1,0.671 6,0.270 8,RMSE为0.968 1~1.946 2 mm/d,作物系数模型模拟精度明显高于PT模型;全生育期各模型RMSE为0.551 5~0.893 6 mm/d,双作物系数模型的R²达到0.902 2.     

Irrigation for crops in a sub-humid environment VII. Evaluation of irrigation strategies for cotton

Summary Empirical functions to predict the nitrogen uptake, increase in LAI and minimum leaf water potential (LWP) of cotton were incorporated into a water balance model for the Namoi Valley, N.S.W. A function was then developed to describe the lint yield of irrigated cotton as a function of water stress days at 4 stages of development, total nitrogen uptake and days of waterlogging. A water stress day was defined as predicted minimum leaf water potential less than -1.8 MPa up to 90 days after sowing and -2.4 MPa there-after; stress reduced yield by up to 40 kg lint ha^–1 d^–1 with greatest sensitivity at 81–140 days after sowing and when N uptake was highest. Nitrogen uptake was reduced by 0.98 kg per ha and yield reduced by 33.2 kg lint ha^–1 for each day of waterlogging. The model was used to evaluate various irrigation strategies by simulating production of cotton from historical rainfall data. With a water supply from off farm storage, net returns ($ M1^–1) were maximized by allocating 7 Ml ha^–1 of crop. The optimum practice was not to irrigate until 60 days from sowing and until the deficit in the root zone reached 50%. When the supply of water was less than 7 Ml ha^–1 there was no advantage in either delaying the start of irrigation or irrigating at a greater deficit; it was economically more rational to reduce the area shown or, if already sown, to irrigate part with 6 Ml ha^–1 and leave the rest as a raingrown crop. Irrigation decisions are compromises between reducing the risk of water stress and increasing the risk of waterlogging. The simulation showed that there is no single set of practices that is always best in every season; in a number of seasons practices other than those which on average are best, give better results.     

Irrigation for crops in a sub-humid environment

Summary A four year study examined the effect of irrigating at various water deficits at different times in the growing season, in combination with a range of nitrogen fertilizer rates, on the growth, yield and quality of cotton. The major effect of irrigation treatment on growth was to increase leaf area and plant size; net assimilation rate in the vegetative phase was not affected by irrigation treatment. The initial rate of boll setting was slightly faster in low nitrogen and less frequent irrigation treatments, but by day 180 (immediately prior to defoliation), all treatments had 60% of total dry weight as bolls and 7% as leaf. The best irrigation strategy varied from year to year due to the variable rainfall pattern. Irrigation when 80% of the available soil moisture had been depleted in the first half of the season only decreased total lint yield by up to 12% in two of the four seasons. During the second half of the season the 80% level of depletion decreased yield by an average of 15% but gave an earlier crop. Yield was reduced by up to 17% if irrigation at 40–60% of available moisture depletion in the first half of the season was followed by irrigation at 80% of available moisture depletion in the second half of the season. A rainfed treatment yielded from 16 to 43% less than the heaviest yielding irrigation treatment. After irrigation there was evidence of poor aeration in the soil which was most severe and lasted the longest at 30 cm depth. Heaviest yields were obtained with 100–150 kgN ha^–1, except in rainfed treatments where 0–50 kgN ha^–1 was sufficient. Irrigation at only 40% of available moisture depletion decreased nitrogen uptake in all seasons. Treatment effects on fibre quality in these experiments were small and variable. Nitrogen fertilizer generally increased length and strength but decreased micronaire. Stress during boll filling decreased micronaire and length in two of the four seasons.     

Contrasting methods for estimating evapotranspiration in soybean

Crop scientists are often interested in canopy rather than leaf water estimates. Comparing canopy fluxes for multiple treatments using micrometeorological approaches presents limitations because of the large fetch required. The goal of this study was to compare leaf-scale to field-scale data by summing soil water evaporation (E) and leaf transpiration (T) versus ET using tower eddy covariance (EC) and scaling leaf transpiration to the canopy level using a two-step scaling approach in soybean [Glycine max (L.) Merr.]. Soybean transpiration represented 89-96% of E + T when combining the soil water evaporation with leaf transpiration on the five measurement days during reproductive growth. Comparing E + T versus ET from the EC system, the E + T method overestimated ET from 0.68 to 1.58 mm. In terms of percent difference, the best agreement between the two methods was 15% on DOY 235 and the worst agreement occurred on DOY 234 (41%). A two-step scaling method predicted average ET within 0.01 mm of the EC ET between 10:00 and 14:15 on an hourly time-step on DOY 227 under uniform sky conditions and average ET within 0.03 mm of the EC ET on DOY 235 under intermittent sky conditions between 10:00 and 15:15. Pooling the scaled-leaf data and comparing them with the measured EC ET data exhibited a strong linear relationship (r = 0.835) after accounting for bias (6%). Findings from this study indicate satisfactory results comparing absolute differences are likely not obtainable by summing leaf transpiration with soil water evaporation to calculate canopy water fluxes. However, scaling leaf transpiration provided a robust measure of canopy transpiration during reproductive growth in soybean under these conditions and merits additional study under different climatic and crop conditions.     

Irrigation for crops in a sub-humid environment

Summary The water use of two soybean cultivars (Bragg and Ruse) was measured for three seasons for a range of irrigation treatments. The seasonal totals of plant and soil evaporation ranged from 450 to 750 mm or from 36 to 64% of class A pan evaporation for the same period. Both cultivars extracted approximately 60% of the total extractable soil water in the top 1.2 m of soil before actual evaporation (Ea) dropped below potential evaporation (Eo). Up to this point the ratio between Ea and class A pan evaporation averaged 0.8. Ruse used water at a faster rate than Bragg but Ruse was not as effective in extracting the deep (below 1.0 m) soil water as Bragg. Water use efficiency (kg seed ha^–1 mm^–1 water) showed a small but general increase with decreasing irrigation water application. Runoff losses varied from zero for non-irrigated Ruse in 1977/78 to 352 mm for frequently-irrigated Bragg in 1976/77, generally increasing with the number of irrigations.     

Irrigation for crops in a sub-humid environment

Summary In this paper the soil water balance model developed and tested in Part III (Mason and Smith, 1980) for soybeans grown in the variable rainfall environment of the Namoi Valley of New South Wales was used to investigate the potential advantages of a computer-based system of irrigation scheduling. The advantages were evaluated using historical rainfall data for the 25 seasons from 1953/54 to 1977/78. The effects on irrigation efficiency of soil water holding capacity, the allowable soil water deficit prior to irrigation, and ordering irrigation water in advance were evaluated with the model. Reducing the allowed deficit prior to irrigation by 20% compared to the recommended level increased the number of irrigations by an average of 2.8 per year and irrigation requirements by 0.73 X 10³ m³ ha^–1. The need to order water 6 days in advance because of delays in delivery also increased requirements by 1.46 X 10³ m³ ha^–1 due to a reduced ability to utilize natural rainfall. Average farm irrigation efficiencies calculated from actual pumping records were found to be low by world standards for the 3-year period 1975/76 to 1977/78. It was concluded that if increased production per unit of water became a high priority in the Namoi Valley, then irrigation efficiency for the three year period discussed could have been increased from 35 to 47%, a saving of 1.3 X 10³ m³ ha^–1 year^–1.     

Fluctuation of crop evapotranspiration coefficients with weather: a sensitivity analysis

In transferring crop coefficients (K_c) from one location to another, it has been assumed that basal K_c (minimal soil evaporation) and/or K_c during full-canopy cover will be universally valid if the variation in weather is accounted for by reference ET variations. The sensitivity of full-canopy-cover crop coefficients (K_c) to variations in solar radiation, air temperature, air vapor density and wind speed was investigated using an energy balance model. Interpretation of the sensitivity involved analyzing the components of the energy balance, which varied as a result of differences in aerodynamic and canopy resistance between the reference crop and the crop to be irrigated. Instability of crop coefficients was shown to increase with decreasing crop canopy resistance and with increasing crop height, indicating that the expectation of universal validity for basal or full-canopy-cover coefficients is not fulfilled. For crops taller and/or with lower canopy resistance than the short clipped grass (0.1 m) used as reference, the magnitude of K_c fluctuations with changes in weather elements suggests caution when using K_c values under environmental conditions different from those prevailing at the site where they were experimentally derived. Values of K_c derived for crops whose height and canopy resistance are not too different from the reference are more stable across environments. Thus, full-cover alfalfa (0.5-m height) would be a better reference crop choice than a short clipped grass (0.1 m) because its canopy resistance and roughness better approximate those of most crops. Research to develop operational methods for directly estimating crop ET, as an alternative to the two-step approach of calculating reference ET and determining site specific empirical crop coefficients, seems desirable.Contribution from the College of Agriculture and Home Economics Research Center, Washington State University, Pullman, WA 99164, USA     

Measurement and estimation of plastic greenhouse reference evapotranspiration in a Mediterranean climate

The standard FAO methodology for the determination of crop water requirements uses the product of reference evapotranspiration (ET_o) and crop coefficient values. This methodology can be also applied to soil-grown plastic greenhouse crops, which occupy extended areas in the Mediterranean basin, but there are few data assessing methodologies for estimating ET_o in plastic greenhouses. Free-drainage lysimeters were used between 1993 and 2004 to measure ET_o inside a plastic greenhouse with a perennial grass in Almería, south-eastern Spain. Mean daily measured greenhouse ET_o ranged from values slightly less than 1 mm day⁻¹ during winter to values of approximately 4 mm day⁻¹ during summer in July. When the greenhouse surface was whitened from March to September (a common practice to control temperature), measured ET_o was reduced by an average of 21.4%. Different methodologies to calculate ET_o were checked against the measurements in the greenhouse without and with whitening. The methods that performed best in terms of accuracy and statistics were: FAO56 Penman–Monteith with a fixed aerodynamic resistance of 150 s m⁻¹, FAO24 Pan Evaporation with a constant Kp of 0.79, a locally-calibrated radiation method and Hargreaves. Given the data requirements of the different methods, the Hargreaves and the radiation methods are recommended for the calculation of greenhouse ET_o because of their simplicity.     

Evaluation of the VegSyst model with muskmelon to simulate crop growth, nitrogen uptake and evapotranspiration

Like many intensive vegetable production systems, the greenhouse-based system on the south-eastern (SE) Mediterranean coast of Spain is associated with considerable NO₃⁻ contamination of groundwater. Drip irrigation and sophisticated fertigation systems provide the technical capacity for precise nutrient and irrigation management of soil-grown crops which would reduce NO₃⁻ leaching loss. The VegSyst crop simulation model was developed to simulate daily crop biomass production, N uptake and crop evapotranspiration (ET_c). VegSyst is driven by thermal time and consequently is adaptable to different planting dates, different greenhouse cooling practices and differences in greenhouse design. It will be subsequently incorporated into a practical on-farm decision support system to enable growers to more effectively use the advanced technical capacity of this horticultural system for optimal N and irrigation management.VegSyst was calibrated and validated for muskmelon grown in Mediterranean plastic greenhouse in SE Spain using data of four melon crops, two grown in 2005 and two in 2006 using two management strategies of water and N management in each year. VegSyst very accurately simulated crop biomass production and accurately simulated crop N uptake over time. Model performance in simulating dry matter production (DMP) over time was better using a double radiation use efficiency (RUE) approach (5.0 and 3.2 g MJ⁻¹ PAR for vegetative and reproductive growth phases) compared to a single RUE approach (4.3 g MJ⁻¹ PAR). The simulation of ET_c over time, was very accurate in the two 2006 muskmelon crops and somewhat less so in the two 2005 crops. The error in the simulated final values, expressed as a percentage of final measured values was −1 to 6% for DMP, 2-11% for crop N uptake, and −11 to 6% for ET_c. VegSyst provided effective simulation of DMP, N uptake and ET_c for crops with different planting dates. This model can be readily adapted to other crops.     

Determination of evapotranspiration and basal crop coefficient of alfalfa with a weighing lysimeter

The aim of this study was to determine the evapotranspiration and basal crop coefficient of alfalfa with a weighing lysimeter. Lysimeter experiments were conducted at Ankara Research Institute of Rural Services, Turkey during 1995–1997. Penman–Monteith, Makkink (FAO-radiation (FAO-Rad)) and Penman-FAO methods performed satisfactorily for estimating the reference evapotranspiration of alfalfa in semi-arid climate conditions. Measured evapotranspiration rates were 1470, 1557 and 1161 mm in 1995, 1996 and 1997, respectively. Basal crop coefficients were computed from evapotranspiration measurements and weather data. The estimated values of the basal crop coefficients for alfalfa at the four growth stages of an individual cutting period were 0.71, 1.78 and 1.51 for initial, mid and late season, respectively.     

基于ELM的西北旱区参考作物蒸散量预报模型

为实现气象资料缺失情况下ET₀的精确预报,选取中国西北旱区4个代表性站点的气象资料,建立15种基于极限学习机(ELM)的ET₀预报模型,并通过与其他ET₀计算模型对比和可移植性分析探究ELM在西北旱区的适用性.结果表明:基于温度和风速的ELM₇预报精度较高(整体评价指标GPI排名第4);基于温度和辐射的ELM₅预报精度(GPI排名第6)明显高于Iramk模型和Jensen-Haise模型;仅基于温度的ELM₉预报精度(GPI排名第8)高于Hargreaves-Samani模型.通过模型可移植性分析发现,ELM₇在西北旱区内各训练站点和预测站点组合下预报精度良好.因此,可将ELM₅(输入温度和辐射)、ELM₇(输入温度和风速)和ELM₉(输入温度)作为西北旱区较少气象参数输入情况下精确预报ET₀的推荐模型.     

Relations between available and extractable soil water and evapotranspiration from a bean crop

The accuracy of ‘available’ and ‘extractable’ soil water estimates was investigated using irrigated and unirrigated beans (Vicia faba) grown in an alluvial silt loam in Canterbury, New Zealand. Available water capacity was defined as the difference between soil water contents in the root zone at the drained upper limit (DUL) and at the lower limit (LL) as estimated by laboratory procedures. Extractable water capacity was specified as the difference between field estimates of DUL and LL for the whole profile affected by roots. DUL was estimated in the laboratory by equilibrating soil cores at matric potentials at ?10, ?20 or ?30 kPa, and in the field by neutron moderation. Laboratory estimates of LL were made from soil samples equilibrated at ?1.5 MPa matric potential. In the field LL was measured by neutron moderation on plots where evaporation had apparently ceased due to drought stress.When compared at intervals down the profile laboratory estimates of DUL and LL showed poor agreement with field observations. However, the final estimates of available and extractable water capacities were similar because of compensatory inaccuracies in the laboratory estimates. Furthermore, field measurements of evapotranspiration, using neutron moderation and tensiometry, indicated that the accuracy of the available water estimates was much reduced by upward fluxes of water into the rooting zone. These fluxes resulted in water extraction to at least 1.0 m although the apparent maximum rooting depth (measured by counting roots washed from soil cores) was only 0.7 m.Particular attention was paid to the influence of subsoil textural variability, which is pronounced in such soils. Laboratory and field estimates of the LL had to be carefully matched texturally before relevant comparisons could be made. Problems associated with subsoil textural variability affected laboratory methods of DUL estimation more than field methods.