Effects of soil type on soybean crop water use in weighing lysimeters

Summary Different soils are known to affect the amount and distribution of both available water and roots. Optimising irrigation water use, especially when shallow water-tables are present requires accurate knowledge of the root zone dynamics. This study was conducted to determine the effect of two soil types on root growth, soil water extraction patterns, and contributions of a water-table to crop evaporation (E). Two weighing lysimeters (L1 and L2) with undisturbed blocks of soil were used. The soil in L1 had higher hydraulic conductivity and lower bulk density than that in L2. Well watered conditions were maintained by irrigation for the first 110 days from sowing (DFS). Root length density (RLD) was calculated from observations made in clear acrylic tubes installed into the sides of the lysimeters. Volumetric soil water contents were measured with a neutron probe. A water-table (EC = 0.01 S m^-1) was established 1 m below the soil surface 18 DFS. RLD values were greater in L1 than L2 at any depth. In L1, maximum RLD values (3 × 10⁴ m m^-3) were measured immediately above the water-table at physiological maturity (133 DFS). In L2, maximum RLD values (1.5 × 10⁴ m m^-3) were measured at 0.42 m on 120 DFS and few roots were present above the water-table. From 71 to 74 DFS, 55 and 64% of E was extracted from above 0.2 m for L1 and L2, respectively. In L2, extraction was essentially limited to the upper 0.4 m, while L1 extraction was to 0.8 m depth. Around 100 DFS the water-table contributed 29% (L1) and 7% (L2) of the water evaporated. This proportion increased rapidly as the upper soil layers dried following the last substantial irrigation 106 DFS. Over the whole season the water-table contributed 24% in L1 and 6.5% in L2 of total E.     

Response of wheat to saline irrigation and drainage

The response of wheat (Triticum aestiuum L.) to varying depths of irrigation, quantity of water applied and to the drainage conditions was studied in 2 m × 2 m × 2 m size lysimeters filled in with a sandy loam soil. Saline water with an electrical conductivity of 8.6 dS m⁻¹ was used for irrigation. The treatments included four irrigations of 5 cm depth, four irrigations of 7 cm, and three irrigations of 9 cm, scheduled on the basis of cumulative pan evaporation, while the drainage conditions were represented by the drained and undrained lysimeters. Another treatment, using good quality water for irrigation, represented the potential yield of the crop. The growth parameters, as well as the yield, showed an improvement with larger irrigation depth increments in the drained lysimeters. But, in contrast, in the undrained lysimeters, the yield was reduced with larger irrigation depth increments, mainly due to a sharp rise in water table depth during the irrigation cycles. The rise and fall in water table showed a high sensitivity and were also highly disproportionate to the irrigation and evapotranspiration events. The yield tended to be higher with a smaller depth of water applied more frequently in the undrained lysimeters. But, in view of the limitations of conventional surface irrigation to apply water in smaller depth increments, an improved drainage is imperative for cropping in shallow saline water table conditions.     

Sprinkler irrigation scheduling of winter wheat in the North China Plain using a 20 cm standard pan

Based on evaporation from a 20 cm diameter pan placed above the crop canopy, sprinkler irrigation scheduling of winter wheat was studied in the North China Plain (NCP) in the 2001–2004 winter wheat seasons. Results showed that pan evaporation (E _pan,C) was closely related to actual evapotranspiration (ET) measured using weighing lysimeters. The combined pan–crop coefficient (K _c,pan), the ratio of ET to E _pan,C, was closely related to leaf area index (LAI ) and plant height. Data from the 2002–2003 season were used to establish the relationships between K _c,pan and LAI (method A) or plant height (method B), and used to determine the crop coefficient (method C). ET computed by the three methods was compared with measured ET using lysimeters in the 2001–2002 and 2003–2004 seasons. Mean relative error of estimated daily ET by the three methods ranged from 20 to 30%, and the relative error in cumulative ET in the experimental periods ranged from 1 to 19%. Among the three methods, results from methods A and B were not significantly different from each other (P > 0.01), and were closer to the lysimeter data than results from method C (P < 0.001). Method B, being easier to measure, was recommended for ET estimation in NCP.     

灌水频率对绿洲菊芋耗水特征及水分利用效率的影响

灌水频率对作物耗水特征和水分利用效率有重要影响,但对以收获地下块茎为主要目的的经济作物研究并不多见。以菊芋为材料,分别在苗期(S)、枝繁叶茂期(L)、现蕾期(B)和开花期(F)施加不同灌水组合,包括苗期600 m3/hm2(S600,下同)处理J1、S600+L600为J2、S600+B600为J3、S600+L300+B900为J4、S600+L900+B300为J5、S600+L600+B300+F900为J6及全生育期不灌水的对照CK,研究菊芋农田水分利用状况。结果表明,枝繁叶茂期和苗期是菊芋水分消耗和需求最大的时期,其次为现蕾期。不同灌水频率下菊芋苗期耗水量、耗水强度及耗水模数差异不大(p0.05),但与不灌水相比差异显著(p0.05)。枝繁叶茂期和现蕾期缺水导致菊芋耗水量和耗水强度显著下降,而开花期灌水菊芋耗水量、耗水强度及耗水模数显著增大。菊芋全生育期耗水量随灌水频率和灌水量增大而增大,但灌水频率和总灌水量一定时,不同生育期灌水量分配对菊芋耗水量无显著影响。不同灌水频率下菊芋全生育期耗水量比不灌水显著增加55.3%~205.6%,灌水频率越高耗水量越大。菊芋水分利用效率随灌水频率和灌水量增大而降低,但相同灌水频率和灌水量下菊芋生育期内灌水量不同时水分利用效率差异不大。总体而言,以收获地下块茎为主要目的的经济作物在耗水规律和水分利用特征方面明显不同于以收获籽粒为目的的粮食作物,应保证营养生长期和并进生长阶段的水分供应以满足蒸腾需水和光合同化对水分的需求,而生殖生长期则应适当减少灌水量以抑制作物奢侈蒸腾,提高水分利用效率。     

基于无人机航拍图像的大田玉米冠层结构建模

针对目前采用三维数字化等方法获取大田作物冠层结构信息时需要手动干预、费时费力的问题,利用超微小型无人机分别获取了苗期大田玉米群体的航拍图像、去掉周边植株后成熟期单株及多株的玉米小群体航拍图像.基于伪极点-Crust方法构建了玉米苗期和成熟期的冠层结构模型,并基于大田原位手动测量的株高、叶长、最大叶宽、叶面积等参数对所构...     

Effect of root and water distribution in lysimeters and in the field on the onset of crop water stress

Summary Lysimeters have been frequently used to study crop response to the onset of water stress. To test the representativeness of lysimeter derived criteria for the onset of crop water stress, spring wheat (Triticum aestivum L.) was grown in two field plots with 1.0 m deep lysimeters in the center of each plot. One plot was well-watered while the second was subjected to a drying period with no irrigation. Crop water stress was assessed by monitoring leaf water potential ( _l ), stomatal diffusive resistance (r _s ), canopy temperature (CT), evapotranspiration (ET), and soil water content in both plots and lysimeters. The rate of change of all these measured parameters, when compared to the well-watered field control-plot revealed that the field-grown plants showed signs of water stress long before the lysimeter-grown plants. Water stress developed gradually for the field crop, but the transition from the well-watered to the stressed condition happened abruptly for the lysimeter-grown plants. Once this transition occurred, the lysimeter-grown plants were more drought stressed than the field-grown plant. Water profiles measured inside the lysimeter were different from those measured in the adjacent plots. An increase in root length density with depths below 0.6 m was observed in the lysimeters as opposed to a quasimonotonic decrease with depth in the field. The response of the lysimeter-grown plants was a result of the anomalous water content and root distribution. We conclude that threshold values of ET, _l , r _s , and CT for the onset of water stress obtained when deep-rooted crops grown in a shallow lysimeter are subjected to drought periods may not be directly applicable to field situations.     

分散流控制壁对无压渗漏计收集效率影响的定量评价

无压渗漏计(Zero-tension lysimeter,ZTL)多用于非饱和带土壤溶质通量的监测,但由于ZTL安装时与原状土壤相接触会存在毛管障碍界面,易形成分散流使其土壤溶液收集效率降低。为准确描述田间水分渗漏量或土壤溶质的运移过程与规律,基于HYDRUS模型模拟结果,对ZTL不同设计(加装不同高度分散流控制壁)和不同适用环境条件(土壤质地、灌水量、土壤蒸发量和初始土壤含水率)的土壤渗漏水收集效率及影响因素进行数值模拟和定量评价。结果表明,无分散流控制壁的ZTL(ZTL0),在0.35 cm3/cm3土壤初始含水率、0.2 cm/d蒸发量和1 000 mm灌水量条件下的砂壤土、壤土和粉土处理,收集效率分别仅为11%、13%和26%,而在相同环境条件下安装分散流控制壁的ZTL(ZTLd),当控制壁高度为20 cm时可使收集效率提升到50%以上。安装的分散流控制壁高度随灌水量的降低、土壤持水能力的提高和土壤蒸发量的增大而升高,初始土壤含水率降低会使偏砂性土壤中安装的ZTLd收集效率降低,但在壤土和粉土中安装时可使其收集效率增大。增加ZTLd安装深度可能会导致其收集效率降低,在某一特定安装深度对ZTL收集效率计算的结果并不适用于其他深度。     

Evapotranspiration components and dual crop coefficients of coffee trees during crop production

Quantifying crop water consumption is essential for many applications in agriculture, such as crop zoning, yield forecast and irrigation management. The objective of this study was to determine evaporation (E), transpiration (T) and dual crop coefficients (Ke and Kcb) of coffee trees during crop production (3rd and 4th year of cultivation), conducted under sprinkler and drip irrigation and no irrigation, in Londrina, Paraná State, Brazil. Crop evapotranspiration (ET) was measured by weighing lysimeters cultivated with plants of cultivar IAPAR 59, E was measured by microlysimeters installed on the lysimeters and T was obtained by the difference between ET and E. The crop coefficient (Kc) was determined for the irrigated treatments as the ratio between ET and the reference evapotranspiration (ETo). Similarly, evaporation coefficient (Ke) and basal crop coefficient (Kcb) were determined as the ratio of E and T, respectively, to the value of ETo, which was estimated by the ASCE Penman-Monteith method on an hourly basis. The values of E and Ke varied due to atmospheric demand and water application method. Those factors, in addition to crop phenology and leaf area evolution, also influenced T and Kcb. Regardless irrigation treatment, the measured values of E represented 35% of ET, while T was 65% of ET. The recommended values of Ke were 0.46 and 0.26 for sprinkler and drip irrigation, respectively. The recommended values of Kcb were 0.52 and 0.82 for sprinkler-irrigated, and 0.5 and 0.65 for drip-irrigated treatments, varying as a function of daily ETo (ETo ≥ or <3 mm day⁻¹, respectively).     

Skin layer evaporation to account for small precipitation events—An enhancement to the FAO-56 evaporation model

The FAO-56 soil water evaporation model is a simple ‘slab’ model that has been found to produce good estimates of evaporation from bare soil over a range of conditions due to its adherence to conservation of mass and energy. The simplicity of the model makes it straightforward to apply and to parameterize. An enhancement is made to the original formulation to accommodate light wetting events that wet the soil ‘skin’ near the surface and evaporate relatively quickly, even when the underlying soil is dry. In effect, the evaporation process, when the soil skin is wetted, reverts temporarily into stage 1 evaporation. The enhancement utilizes the ‘readily evaporable water’ (REW) term of the original model so that no new parameters are required. The extended model performs similar to the original model in the absence of small precipitation events, but increases the evaporation rate when small events occur. The FAO-56 method with the skin evaporation enhancement is shown to compare well against simulations made using the HYDRUS 1D model that bases evaporation on the Richards equation. The enhanced model also closely followed evaporation recorded by weighing lysimeter for a silt loam soil at Kimberly, Idaho, with root mean square difference of 0.39 and 0.69 mm d⁻¹ for two wetting/drying sequences. Total cumulative evaporation during the longest and wettest sequence was estimated at 92% of the measured value.     

Canopy temperature and water stress of cotton crops with complete and partial ground cover

Summary The use of canopy and air temperature differences to compute a crop water stress index (CWSI) for assessing plant water status was investigated using cotton crop canopies that either fully or partially covered the ground. The complete ground cover canopy condition was studied in a well watered moisture regime in a rainout shelter with measurements made on six Texas cotton race stocks. The partial ground cover canopy situation was investigated in a well watered moisture regime of a commercial cotton variety Paymaster 266 grown in the field. The slope of the nonstressed baseline of the CWSI for a cotton canopy with about 50% ground cover was approximately one-half that reported for full canopies. Values of CWSI calculated with theoretical and empirical procedures agreed more closely under a complete canopy condition than under a partial canopy situation. Values of aerodynamic resistance (r _a ) and canopy resistance for well watered soil moisture conditions (r _ep )were estimated in order to use the theoretical procedure of computing CWSI. Values of r _a ranged from 10 to 15 sm^–1 and r _cp from 50 to 60 sm^–1. Both the theoretical and empirical procedures showed much promise, but more information is needed to develop techniques for evaluating r _a and r _cp under differing canopy and environmental conditions.     

基于冠气温差的淮北地区水稻日需水量估算模型研究

【目的】构建水稻日需水量估算模型,为灌区动态用水管理提供科学指导。【方法】以淮北淮涟灌区水稻为研究对象,通过称质量法获取水稻日需水量,同步采集水稻冠层温度和相关气象要素,分析各要素与实测水稻日需水量关系,构建了以太阳净辐射和冠气温差为参数的水稻日需水量估算模型,并利用叶面积指数对其进行修正。【结果】太阳净辐射量和冠气温差是反映水稻日需水量的关键因子,水稻日需水量与太阳净辐射呈正线性相关,与冠气温差呈负线性相关,冠气温差随着生育期延续有增大的趋势。通过叶面积指数修正的模型相对误差为5.07%,均方根误差为0.183 mm/d。【结论】利用冠气温差估算水稻日需水量,方法较为简单,精度满足灌溉管理要求。     

Effect of level and timing of moisture stress on soybean plant development and yield components

Field studies were conducted in 1984 and 1985 using non-weighing lysimeters of 1 m² to determine the effect of level of soil moisture stress (L) and timing of moisture stress (T) on reproductive phenology, plant characteristics, and yield components of Maple Amber soybean [Glycine max (L.) Merr.]. The study incorporated the most probable combinations of moisture stress in order to provide a basis for planning and managing irrigation for optimum soybean production. The effect of L was consistent over T and Year. Neither L nor T affected days to full bloom stage (R2). Increases in L linearly (r² = 0.71) increased days to beginning seed stage (R5), but decreased (r² = 0.93) days to full maturity stage (R8). L was related quadratically to plant height (r² = 0.88) and number of pod-bearing nodes (r² = 0.98), and linearly to number of pods (r² = 0.75) and total dry matter (r² = 0.91). No consistent effect of T on the timing of R5 to R8 stages was found, mainly because of the presence of a Year X T interaction (P = 0.05). Withholding irrigation during the R5 stage from previously adequately watered crops caused significant reduction in plant height (7%), number of pod-bearing nodes on the main stem (13%), number of pods (18%), number of seeds (20%), total seed weight (25%), and total dry matter (23%), indicating the importance of irrigation during this stage. It was concluded that, if irrigation water is limiting, with-holding irrigation at R2 may be acceptable, but irrigation at R5 is essential for optimizing yield components and thus yield.     

Surface energy balance model of transpiration from variable canopy cover and evaporation from residue-covered or bare soil systems: model evaluation

A surface energy balance model (SEB) was extended by Lagos et al. Irrig Sci 28:51–64 (2009) to estimate evapotranspiration (ET) from variable canopy cover and evaporation from residue-covered or bare soil systems. The model estimates latent, sensible, and soil heat fluxes and provides a method to partition evapotranspiration into soil/residue evaporation and plant transpiration. The objective of this work was to perform a sensitivity analysis of model parameters and evaluate the performance of the proposed model to estimate ET during the growing and non-growing season of maize (Zea Mays L.) and soybeans (Glycine max) in eastern Nebraska. Results were compared with measured data from three eddy covariance systems under irrigated and rain-fed conditions. Sensitivity analysis of model parameters showed that simulated ET was most sensitive to changes in surface canopy resistance, soil surface resistance, and residue surface resistance. Comparison between hourly estimated ET and measurements made in soybean and maize fields provided support for the validity of the surface energy balance model. For growing season’s estimates, Nash–Sutcliffe coefficients ranged from 0.81 to 0.92 and the root mean square error (RMSE) varied from 33.0 to 48.3 W m^?2. After canopy closure (i.e., after leaf area index (LAI = 4) until harvest), Nash–Sutcliffe coefficients ranged from 0.86 to 0.95 and RMSE varied from 22.6 to 40.5 W m^?2. Performance prior to canopy closure was less accurate. Overall, the evaluation of the SEB model during this study was satisfactory.     

滴灌核桃不同灌水定额下综合效益优选

为了评估核桃综合效益,以8 a生核桃树为研究对象,通过田间试验,对比了灌水定额为150 m3/hm~2(C1处理)、300 m3/hm~2(C2处理)和450 m3/hm~2(C3处理)的冠层参数,并依托数学分析方法,筛选了果树的最佳灌水定额。结果表明,在核桃果实膨大期,C2处理提高了冠层叶面积指数,而C1、C3处理下透光率、直射光立地系数、散射光立地系数均大于C2处理。核桃生育中期及末期,C2、C3处理明显影响冠层截获的辐射能,但C1处理的作用较弱。随着果实生长,冠层叶面积指数、截获辐射能监测值均不断增大直至平稳,透光率、直射光立地系数、散射光立地系数则不断减小。叶面积指数、灌水定额可以显著影响核桃的最终产量。因此,在滴灌条件下,灌水定额300 m3/hm~2可为核桃的最优生产和最佳经济效益提供有力保障。     

Role of transpiration suppression by evaporation of intercepted water in improving irrigation efficiency

Sprinkler irrigation efficiency declines when applied water intercepted by the crop foliage, or gross interception (I_gross), as well as airborne droplets and ponded water at the soil surface evaporate before use by the crop. However, evaporation of applied water can also supply some of the atmospheric demands usually met by plant transpiration. Any suppression of crop transpiration from the irrigated area as compared to a non-irrigated area can be subtracted from I_gross irrigation application losses for a reduced, or net, interception (I_net) loss. This study was conducted to determine the extent in which transpiration suppression due to microclimatic modification resulting from evaporation of plant-intercepted water and/or of applied water can reduce total sprinkler irrigation application losses of impact sprinkler and low energy precision application (LEPA) irrigation systems. Fully irrigated corn (Zea Mays L.) was grown on 0.75 m wide east-west rows in 1990 at Bushland, TX in two contiguous 5-ha fields, each containing a weighing lysimeter and micrometeorological instrumentation. Transpiration (Tr) was measured using heat balance sap flow gauges. During and following an impact sprinkler irrigation, within-canopy vapor pressure deficit and canopy temperature declined sharply due to canopyintercepted water and microclimatic modification from evaporation. For an average day time impact irrigation application of 21 mm, estimated average I_gross loss was 10.7%, but the resulting suppression of measured Tr by 50% or more during the irrigation reduced I_gross loss by 3.9%. On days of high solar radiation, continued transpiration suppression following the irrigation reduced I_gross loss an additional 1.2%. Further 4–6% reductions in I_gross losses were predicted when aerodynamic and canopy resistances were considered. Irrigation water applied only at the soil surface by LEPA irrigation had little effect on the microclimate within the canopy and consequently on Tr or ET, or irrigation application efficiency.     

Irrigation effects on the growth, yield, and water use efficiency of alfalfa

A field study was conducted in the semiarid region of northern Sudan to investigate the effects of variable irrigation on the growth, yield, and water use efficiency (WUE) of alfalfa (Medicago sativa L.). Treatments were 65 mm of water applied every 7 days, 80 mm of water applied every 10 days, or 104 mm of water applied every 13 days. The heavy, infrequent irrigation reduced stem height, stem density, leaf area index (LAI), total biomass production, and the WUE of alfalfa plants. Maximum yields for six harvests were 15.3, 12.9, and 11.2 ton ha^–1 and the WUE values were 0.12, 0.10, and 0.08 ton ha^–1 cm^–1 for the frequent, less-frequent, and infrequent irrigation regimes, respectively. In all the treatments, alfalfa dry matter yield was positively correlated with stem height and LAI. The relationship between dry matter yield and total water use was a linear function (R ²=0.99), regardless of the irrigation treatment. Alfalfa growth, yield, and WUE remained high during the relatively cool months and declined during the hot period under the three water regimes. It was concluded that alfalfa grown under semiarid conditions should be watered lightly and frequently to attain high yields and high WUE. Received: 19 February 1996     

Influence of different water quantities and qualities on lemon trees and soil salt distribution at the Jordan Valley

The influences of water quantity and quality on young lemon trees (Eureka) were studied at the University of Jordan Research Station at the Jordan Valley for 5 years (1996–2000). Five water levels and three water qualities were imposed via trickle irrigation system on clay loam soil. The primary effect of excess salinity is that it renders less water available to plants although some is still present in the root zone. Lemon trees water requirements should be modified year by year since planting according to the percentage shaded area, and this will lead into substantial water saving. Both evaporation from class A pan and the percentage shaded area can be used to give a satisfactory estimate of the lemon trees water requirement at the different growth stages. The highest lemon fruit yield was at irrigation water depth equal to evaporation depth from class A pan when corrected for tree canopy percentage area. Increasing irrigation water salinity 3.7 times increased average crop root zone salinity by about 3.8–4.1 times.The high salt concentration at the soil surface is due to high evaporation rate from wetted areas and the nature of soil water distribution associated with drip irrigation system. Then, the salt concentration decreased until the second depth, thereafter, salt concentration followed the bulb shape of the wetted soil volume under trickle irrigation. Irrigation water salinity is very important factor that should be managed with limited (deficit) irrigation. But increasing amount of applied saline water could result in a negative effect on crop yield and environment such as increasing average crop root zone salinity, nutrient leaching, water logging, increasing the drainage water load of salinity which might pollute ground water and other water sources.     

农膜残留对砂壤土和砂土水分入渗和蒸发的影响

通过室内试验设置5个不同残膜量(0、50、100、200、400 kg/hm~2)处理,研究不同残膜量对砂壤土和砂土水分入渗湿润锋、入渗速率、累积入渗量、土壤累积蒸发量和蒸发速率的影响,并评价了主要土壤入渗、蒸发模型在农膜残留土壤的适用性。结果表明:随着土壤中残膜量增多,砂壤土和砂土入渗速率变慢,土壤湿润锋运移相同距离所需时间均显著增加,其中运移30 cm时,砂壤土残膜量400 kg/hm~2处理(SL5)比无残膜处理(SL1)运移时间增加了27.56%;相同入渗时间内累积入渗量随残膜量增加均显著减小(P0.05),入渗结束后SL5处理比SL1处理累积入渗量减小了52.01 m L(23.12%);残膜量增加导致蒸发速率、累积蒸发量都显著减小(P0.05),蒸发结束后SL5处理比SL1处理累积蒸发量减小了30.63%,且不同残膜量对砂壤土的影响大于砂土。对4个土壤水分入渗及蒸发模型进行拟合,结果显示Kostiakov和Philip入渗模型均能较好模拟残膜条件下土壤水分入渗,其中Philip入渗模型拟合精度高于Kostiakov入渗模型,且对砂土中农膜残留下的土壤水分入渗模拟效果更好;Black蒸发模型随着残膜量增加拟合精度下降,而Rose蒸发模型受残膜量的影响较小,更适合于农膜残留土壤累积蒸发量估算。     

面向植物抗旱性研究的多源表型信息采集和分析技术

利用表型信息采集系统获取不同生长环境下的植物形态结构和生理生化数据,研究植物体对不同胁迫的反应,从而进行抗性育种和筛选优质良种.本文构建了一套由双CCD相机、热成像仪、水分控制模块、称量模块、光源等组成的多源表型信息采集系统,采用YOLO v3目标检测算法和图像处理算法提取了植物投影叶面积、株高、叶片数量、冠层温度等表...     

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