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1.
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. 相似文献
2.
Xiaodong Gao Pute Wu Xining Zhao Yinguang Shi Jiawen Wang 《Agricultural Water Management》2011,102(1):66-73
Estimating spatial mean soil water contents from point-scale measurements is important to improve soil water management in sloping land of semiarid areas. Temporal stability analysis, as a statistical technique to estimate soil water content, is an effective tool in terms of facilitating the upscaling estimation of mean values. The objective of this study was to examine temporal stability of soil water profiles (0–20, 20–40, 40–60 and 0–60 cm) in sloping jujube (Zizyphus jujuba) orchards and to estimate field mean root-zone soil water based on temporal stability analysis in the Yuanzegou catchment of the Chinese Loess Plateau, using soil water observations under both dry and wet soil conditions. The results showed that different time-stable locations were identified for different depths and the temporal stability of soil water content in 20–40 cm was significantly (P < 0.05) weaker than that in other depths. Moreover, these time-stable locations had relatively high clay contents, relatively mild slopes and relatively planar surfaces compared to the corresponding field means. Statistical analysis revealed that the temporal stability of root zone soil water (0–60 cm) was higher in either dry or wet season than that including both, and soil water exhibited very low temporal stability during the transition period from dry to wet. Based on the temporal stability analysis, field mean soil water contents were estimated reasonably (R2 from 0.9560 to 0.9873) from the point measurements of these time-stable locations. Since the terrains in this study are typical in the hilly regions of the Loess Plateau, the results presented here should improve soil water management in sloping orchards in the Loess Plateau. 相似文献
3.
黄淮海平原冬小麦对盐分胁迫的响应研究 总被引：1，自引：0，他引：1
针对黄淮海平原地区淡水资源匮乏的问题,以当地浅层地下水配置不同矿化度的微咸水进行盐分胁迫实验,对冬小麦主根区土壤含盐量、冬小麦产量以及叶面积指数对盐分胁迫的响应进行了分析。结果表明,在不同的生育期降水量条件下,冬小麦对盐分胁迫有着不同的响应。为获得满意产量,在当地的土壤条件下,生育期一般年和湿润年可以采用的最高矿化度为3g/L,而在生育期偏旱年,如果不采取其他措施的条件下,可以采用的最高矿化度为2g/L,该结果为合理开发利用当地的地下咸水资源提供了一定的依据。 相似文献
4.
利用Pro/E软件对某柴油机气缸盖进行三维实体建模,并对该模型进行了热负荷分析,其中水腔的热边界条件用CFD软件Fluent进行求解,将CFD计算出的对流边界条件影射到缸盖水腔表面,最后利用ANSYS-workbench软件计算出缸盖的温度场,并进行热-机耦合应力分析。结果表明,气门周围的应力较大,应作为设计重点考虑的部位。 相似文献
5.
《Journal of Agricultural Engineering Research》1998,69(4):331-341
This study concerns the evaluation of the root zone water quality model (RZWQM) to simulate the seasonal water and nitrate movement in a level basin irrigated corn field under three different nitrogen (N) fertilizer treatments. The three N treatments, superimposed over a split basal dose applied before and at planting, were: a single broadcast application of 150 kg N/ha as urea (100% amidic form), a single fertigation application of the same N as UAN (50% amidic, 25% ammonium and 25% nitrate) with the first irrigation, and multiple UAN fertigations with three irrigations. Certain variety-specific maize crop parameters in the model were obtained by fitting these parameters to field data from the single fertigation treatment. The model was then evaluated on water and N results for the treatments. The model adequately simulated the water and nitrate transport for the season, with the seasonal averages of measured and predicted values differing by less than 5%. The most significant differences between measured and simulated water and nitrate occurred near the soil surface (15 cm depth), mostly during the days when the soil was extremely wet following irrigations. With the soil hydraulic properties estimated by simple means, the model tends to overestimate downward water fluxes and related nitrate transport through a compacted layer; however, it is found to be a useful tool to study the relative impacts of alter- nate nitrogen fertilizer and irrigation practices on root zone water quality. 相似文献
6.
《Agricultural Water Management》2006,81(3):335-357
The relative yield decline that is expected under specific levels of water stress at different moments in the growing period is estimated by integrating the FAO Ky approach [Doorenbos, J., Kassam, A.H., 1979. Yield response to water. FAO Irrigation and Drainage Paper No. 33. Rome, Italy] in the soil water balance model BUDGET. The water stored in the root zone is determined in the soil water balance model on a daily basis by keeping track of incoming and outgoing water fluxes at its boundary. Given the simulated soil water content in the root zone, the corresponding crop water stress is determined. Subsequently, the yield decline is estimated with the Ky approach. In the Ky approach the relation between water stress in a particular growth stage and the corresponding expected yield is described by a linear function. To account for the effect of water stresses in the various growth stages, the multiplicative, seasonal and minimal approach are integrated in the model. To evaluate the model, the simulated yields for two crops under various levels of water stress in two different environments were compared with observed yields: winter wheat under three different water application levels in the North of Tunisia, and maize in three different farmers’ fields in different years in the South West of Burkina Faso. Simulated crop yields agreed well with observed yields for both locations using the multiplicative approach. The correlation value (R2) between observed and simulated yields ranged from 0.87 to 0.94 with very high modeling efficiencies. The root mean square error values are relatively small and ranged between 7 and 9%. The minimal and seasonal approaches performed significantly less accurately in both of the study areas. Estimation of yields on basis of relative transpiration performed significantly better than estimations on basis of relative evapotranspiration in Burkina Faso. A sensitivity analysis showed that the model is robust and that good estimates can be obtained in both regions even by using indicative values for the required crop and soil parameters. The minimal input requirement, the robustness of the model and its ability to describe the effect on seasonal yield of water stress occurring at particular moments in the growing period, make the model very useful for the design of deficit irrigation strategies. BUDGET is public domain software and hence freely available. An installation disk and manual can be downloaded from the web. 相似文献
7.
大面积水盐监测和调查的难度在于受土壤空间变异性的影响。为揭示水盐变异规律 ,经地质统计学初步分析 ,实验取样结构性较弱而随机较强 ,从而采用随机理论 ,将传统随机理论—时间序列分析法和最大熵谱分析用于空间序列 ,以空间代替时间 ,对分层土壤作空间序列的最大熵谱分析 ,系统揭示出田间土壤不同尺度下 ,水分盐分变化规律 ,对大面积土壤水盐监测具有一定的指导意义 相似文献
8.
山地履带拖拉机坡地等高线作业土壤压实应力研究 总被引：1，自引：0，他引：1
山地履带拖拉机（配备姿态调整机构）具有良好的稳定性和越障性能,特别适宜在丘陵山区坡地作业,然而由于坡地角的存在导致拖拉机两侧履带下的应力分布极不均匀,使得拖拉机附着性和通过性均降低。本文针对山地履带拖拉机坡地等高线行驶/作业时,坡地土壤内部应力分布规律不明确以及如何提高应力均匀性缓解土壤压实等问题,在深入分析坡地工况下履带最大接地比压与应力传递基本规律的基础上,采用EDEM-RecurDyn耦合方法进行了仿真试验,并采取土压力盒埋设法分别开展了基于小型坡地土槽的静态试验和坡地试验田的动态试验;其中,静态试验探究了不同深度土壤在含水率、初始紧实度、加载质量及坡地角等影响下的垂直应力分布规律;动态试验探究了山地履带拖拉机坡地等高线行驶/旋耕作业时履带下方土壤应力随作业速度、车身状态（调平/未调平）及牵引负载的变化规律;并分析了履带张紧力对土壤垂直、水平应力分布的影响。试验结果表明：履带下垂直应力在各支重轮的轴线处呈现一个应力峰值;水平应力在各支重轮轴线的前、后方分别出现一个应力峰值;适当增大作业速度,可减小土壤内部垂直和水平应力峰值,拖拉机速度由0.5 km/h增加到1.5 km/h,垂直... 相似文献
9.
《Agricultural Water Management》1997,33(1):63-78
The oft-touted reason for the efficiency of drip irrigation is that roots can preferentially take up water from localised zones of water availability. Here we provide definitive evidence of this phenomenon. The heat-pulse technique was used to monitor rates of sap flow in the stem and in two large surface roots of a 14 year old apple tree (Malus domestica Borkh. cv. Braeburn). The aim was to determine the ability of an apple tree to modify its pattern of root water uptake in response to local changes in soil water content. We monitored the water status of the soil close to the instrumented roots by using time domain reflectometry (TDR) to measure the soil's volumetric water content, θ, and by using ceramic-tipped tensiometers to measure the soil's matric pressure head, h. A variation in soil water content surrounding the two roots was achieved by supplying a single localised irrigation to just one root, while the other root remained unwatered. Sap flow in the wetted root increased straight away by 50% following this drip irrigation which wetted the soil over a zone of approximately 0.6 m in diameter and 0.25 m in depth. Sap flow in the wetted root remained elevated for a period of about 10 days, that is until most of the irrigation water had been consumed. A comparative study of localised and uniform irrigation was then made. Following irrigation over the full root zone no further change in sap flow in the previously wetted root was observed when referenced to the corresponding sap flow measured in the stem of the apple tree. However sap flow in the previously dry root responded to subsequent irrigations by increasing its flow rate by almost 50%. These results show that apple roots have the capacity to transfer water from local wet areas at much higher rates than normally occurs when the entire root zone is supplied with water. They are also able to shift rapidly their pattern of uptake and begin to extract water preferentially from those regions where it is more freely available. Such an ability supports the use of drip irrigation for the efficient use of scarce water resources. We conclude that the soil-to-root pathway represents a major resistance to water uptake by apple, even at the relatively high soil water pressure heads developed during parts of this experiment, during which the tree was not even under any stress. 相似文献
10.
通过测定高温处理后水培和土培樱桃番茄的几项生理指标,研究了樱桃番茄在高温胁迫条件下的叶水势、光合速率、SOD酶活性等生理指标的变化特征,结果表明,高温胁迫条件下土培番茄叶水势、光合速率和SOD活性大幅度下降,而水培番茄上述生理指标虽有下降,但幅度远较土培番茄为少,因此,水培樱桃番茄对高温的抵抗能力要强于土培。 相似文献
11.
Integrating satellite-based evapotranspiration with simulation models for irrigation management at the scheme level 总被引：2，自引：2，他引：0
Improvements in irrigation management are urgently needed in regions where water resources for irrigation are being depleted.
This paper combines a water balance model with satellite-based remote-sensing estimates of evapotranspiration (ET) to provide
accurate irrigation scheduling guidelines for individual fields. The satellite-derived ET was used in the daily soil water
balance model to improve accuracy of field-by-field ET demands and subsequent field-scale irrigation schedules. The combination
of satellite-based ET with daily soil water balance incorporates the advantages of satellite remote-sensing and daily calculation
time steps, namely, high spatial resolution and high temporal resolution. The procedure was applied to Genil–Cabra Irrigation
Scheme of Spain, where irrigation water supply is often limited by regional drought. Compared with traditional applications
of water balance models (i.e. without the satellite-based ET), the combined procedure provided significant improvements in
irrigation schedules for both the average condition and when considering field-to-field variability. A 24% reduction in application
of water was estimated for cotton if the improved irrigation schedules were followed. Irrigation efficiency calculated using
satellite-based ET and actual applied irrigation water helped to identify specific agricultural fields experiencing problems
in water management, as well as to estimate general irrigation efficiencies of the scheme by irrigation and crop type. Estimation
of field irrigation efficiency ranged from 0.72 for cotton to 0.90 for sugar beet. 相似文献
12.
Selection of the time period when liquid N fertilizer is introduced during an irrigation (timing), can potentially lead to more accurate placement of N in the root zone of perennial crops. The effect of four timing treatments, T1, T2, T3 and T4 the four quarters of a water application on in situ redistribution of ammonium nitrate at two water application rates (23 and 58 mm) was investigated. Irrespective of applied water quantity, soil ammonium-N content decreased with depth from the soil surface. Retarding the timing from T1 to T4 resulted in a significant increase in ammonium-N content in the uppermost 50 mm depth increment at both water application rates. In all timing treatments, the negatively charged nitrate ion moved to greater depths than its positively charged counterpart. These differences were greater where 58 vs. 23 mm of water was applied. Except for the T3 and T4 treatments at the 23 mm level, peak soil nitrate concentrations appeared below the soil surface. Retarding the timing at the 58 mm level gave rise to consecutive nitrate peaks between the surface and the 500 mm depth. A less distinct trend was also apparent where 23 mm was applied. Evidence for preferential movement of N in a uniform course sandy profile is also presented. 相似文献
13.
为了实现黄土高原地区滴灌条件下枣树园土壤含水率的小范围快捷监测,根据FAO-56水分胁迫系数的定义和相关计算公式,得到了基于土壤水分胁迫系数的黄土高原地区滴灌条件下枣树根层土壤平均含水率估算公式．2009年4-9月将该公式应用于西北农林科技大学陕西米脂孟岔试验站的枣树试验园,配置了3种不同的土壤含水率控制下限,对枣树2个重要生育期的土壤含水率进行了估算,模拟了水分动态变化过程,并对估算值和实测值进行对比和误差分析．结果表明:采用基于FAO-56水分胁迫系数的计算公式对土壤含水率的动态模拟达到了较高的精度,估算值与实测值之间误差较小:其中开花坐果期各处理(灌水下限为60％,50％,40％的田间持水率)的估算值与实测值之间的相关系数分别为0828 0,0907 3,0935 1;标准误差分别为0055,0093,0068．果实膨大期各处理的相关系数分别为0777 2,0766 7,0905 5;标准误差分别为0057,0092,0079．估算值与实测值之间的相关系数随土壤含水率的增大而减小,随土壤水分胁迫程度的增大而增大,即土壤含水率较高时对公式精度有一定的影响．该方法较适用于黄土高原半干旱地区,对农业用水管理具有一定参考价值． 相似文献
14.
Soil evaporation from drip-irrigated olive orchards 总被引：1，自引：3，他引：1
Santiago Bonachela Francisco Orgaz Francisco J. Villalobos Elías Fereres 《Irrigation Science》2001,20(2):65-71
Evaporation from the soil (Es) in the areas wetted by emitters under drip irrigation was characterised in the semi-arid, Mediterranean climate of Córdoba (Spain). A sharp discontinuity in Es was observed at the boundary of the wet zone, with values decreasing sharply in the surrounding dry area. A single mean value of evaporation from the wet zone (Esw) was determined using microlysimeters. Evaporation from the wet zones of two drip-irrigated olive orchards was clearly higher than the corresponding values of Es calculated assuming complete and uniform soil wetting (Eso), demonstrating the occurrence of micro-scale advection in olive orchards under drip irrigation. Measurements over several days showed that the increase in evaporation due to microadvection was roughly constant regardless of location and of the fraction of incident radiation reaching the soil. Thus, daily evaporation from wet drip-irrigated soil areas (Esw) could be estimated as the sum of Eso and an additive microadvective term (TMA). To quantify the microadvective effects, we developed variable local advective conditions by locating a single emitter in the centre of a 1.5 ha bare plot which was subjected to drying cycles. Esw increased relative to Eso as the soil dried and advective heat transfer increased evaporation from the area wetted by the emitter. The microadvective effects on Es were quantified using a microadvective coefficient (Ksw), defined as the ratio between Esw and Eso. A model was then developed to calculate TMA for different environmental and orchard conditions. The model was validated by comparing measured Esw against simulated evaporation (Eso+TMA) for different soil positions and environmental conditions in two drip-irrigated olive orchards. The mean absolute error of the prediction was 0.53 mm day-1, which represents about a 7% error in evaporation. The model was used to evaluate the relative importance of seasonal Es losses during an irrigation season under Córdoba conditions. Evaporation from the emitter zones (Esw) represented a fraction of seasonal orchard evapotranspiration (ET), which ranged from 4% to 12% for a mature (36% ground cover) and from 18% to 43% of ET for a young orchard (5% ground cover), depending on the fraction of soil surface wetted by the emitters. Estimated potential water savings by shifting from surface to subsurface drip ranged from 18 to 58 mm in a mature orchard and from 28 to 93 mm in a young orchard, assuming daily drip applications and absence of rainfall during the irrigation season. 相似文献
15.
Canopy temperature as a measure of salinity stress on sorghum 总被引：1，自引：0，他引：1
Summary A complete understanding of plant response to combined water and salinity stress is desirable. Previous growth chamber and greenhouse experiments with sorghum and maize indicate that soil salinity, by negatively affecting growth processes, may reduce consumptive water use, thus prolonging the supply of available soil moisture. In the present field experiment, canopy temperature measurements were used to examine the effect of soil salinity on the plant-soil water relations of sorghum (Sorghum bicolor L. cv. Northrup King 1580). An infrared thermometer was used to measure canopy temperature during a 9-day period including two irrigations in plots of various salinities. The salinity treatments were created by a dual line-source sprinkler irrigation system, which applied waters of different quality. Excess irrigation allowed soil moisture to be uniform across the salinity treatments at the beginning of the measurement period. Consumptive water use and soil salinity were measured to quantify the salinity and water treatments. Grain and dry matter yields provided measures of plant response. Canopy temperature measurements were sensitive enough to detect differences across the salinity treatments when soil moisture was uniform for several days following irrigation. However, over the 9-day measurement period, plants in the low-salt plots used more water than plants in the high-salt plots. This differential water use eventually offset the salinity-induced stress, with the result that temperature differences were eliminated. Differences in temperature were observed again following irrigation. The results demonstrate that canopy temperature can be used as a tool to detect salinity stress on sorghum. Timing of measurements with regard to irrigation is identified as a key factor in detecting temperature differences that can be attributed to the presence of soil salinity. 相似文献
16.
为了阐明不同沟向与沟灌方式下作物根区土壤温度的变化规律,通过垄作玉米田不同点位处的根区地温观测,研究了南北沟向和东西沟向下交替隔沟灌溉(AFI)和常规沟灌(CFI)田的地温变化。结果表明,东西沟向时AFI在湿沟、垄位和干沟处0～20cm土温分别高于南北沟向0～4.60、0～5.37、0～6.57℃;同一沟向条件下,AFI的平均土温比CFI提高了0.02～7.00℃,且AFI在非灌水区域的土温比灌水区域高0.23～6.20℃。东西沟向AFI的作物根区土温最高,提高了根区土壤热能,以其通过栽培方式或灌溉方式的改变调节田间光热环境以适应不同作物的生长需求。 相似文献
17.
Plant response to the soil water reserve: Consequences of the root system environment 总被引：4，自引：0，他引：4
Summary The ability of water balance models based on the concept of Transpirable Soil Water to predict the occurrence of water stress and the need for irrigation was tested for several environmental conditions of the root system, to determine in which conditions errors are likely to be appreciable. The response of evapotranspiration, stomatal conductance and leaf water potential to soil water reserve was studied under three conditions: (i) in pots with maize plants, (ii) in the field with deep soil and the root system placed in favourable conditions, with wheat during a dry year and with maize during four years with contrasting climate, (iii) in the field, with soil compaction which disturbed the maize root system, decreasing its efficiency for water uptake, during four year. (i) In the pot experiment, where the volume of the Transpirable Soil Water (TSW) is well defined, the responses followed the hypothesis of water balance models. (ii) The soil depletion was higher than the calculated TSW during two dry years in the field, because of an appreciable contribution of the non-rooted soil layers to the water balance. As a consequence, evapotranspiration, stomatal conductance and predawn water potential did not decrease over the whole range of soil water reserve. Grain yield was no lower in those years than in the wet years, in spite of the fact that the soil water reserve was depleted. Thus, a water balance based on the TSW would have underestimated in these conditions the ability of plants to withdraw soil water, overestimating the necessity of irrigation. Predawn water potential gave, on the contrary, indications consistent with the responses of the stomatal conductance and the net CO2 assimilation. (iii) The water uptake by plants would have been overestimated in the case of compacted soil. Stomatal conductance was low even for high levels of the soil water reserve, except if the densely rooted top 0.1 m layer of soil was rewatered by irrigation. Water stress could not have been diagnosed in this case from indications of soil water potential or of pre-dawn water potential. These data confirm that some knowledge of the environmental conditions of the root system is necessary to determine if errors made using water balance models are likely to be appreciable, and to know if they lead to an underestimation or overestimation of the risk of water stress. 相似文献
18.
《Agricultural Water Management》2006,85(3):305-313
The evapotranspiration from a 3 to 4 years old drip irrigated peach orchard, located in central Portugal, was measured using the eddy covariance technique during two irrigation seasons, allowing the determination of crop coefficients. These crop coefficient values differed from those tabled in FAO Irrigation and Drainage Paper 56. In order to improve evapotranspiration estimates obtained from FAO tabled crop coefficients, a dual crop coefficient methodology was adopted, following the same guidelines. This approach includes a separation between the plant and soil components of the crop coefficient as well as an adjustment for the sparse nature of the vegetation. Soil evaporation was measured with microlysimeters and compared with soil evaporation estimates obtained by the FAO 56 approach. The FAO 56 method, using the dual crop coefficient methodology, was also found to overestimate crop evapotranspiration. During 2 consecutive years, measured and estimated crop coefficients were around 0.5 and 0.7, respectively. The estimated and measured soil evaporation components of the crop coefficient were similar. Therefore, the overestimation in evapotranspiration seems to result from an incorrect estimate of the plant transpiration component of the crop coefficient. A modified parameter to estimate plant transpiration for young, yet attaining full production, drip irrigated orchards is proposed based on field measurements. The method decreases the value of basal crop coefficient for fully developed vegetation. As a result, estimates of evapotranspiration were greatly improved. Therefore, the new approach seems adequate to estimate basal crop coefficients for orchards attaining maturity established on sandy soils and possibly for other sparse crops under drip irrigation conditions. 相似文献
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When the soil water balance method is applied at a field scale, estimation of the spatial variability and confidence interval of actual evapotranspiration is rare, although this method is sensitive to the spatial variability of the soil, and thus to the sampling strategy. This work evaluated the effect of soil sampling strategies for soil water content and water flux at the bottom of the soil profile on the estimation of the daily and cumulative evapotranspirations. To do that, according to the statistical properties of daily measurements in a field experiment with a soybean crop, the water content and flux through the base to the soil profile in space (field scale) and time (daily scale) were simulated. Four different sampling strategies were then compared, and their effects on daily and seasonal cumulative evapotranspirations quantified. Strategy 1 used ten theoretical sites randomly located in the field. The daily water content estimates were assumed to be available each day from these same ten locations, which were located from 0.15 m to 1.55 m in depth, with space steps of 0.10 m. Strategy 2 assumed that daily water content estimates combined two sources: in the 0.00–0.20 m soil layer, ten theoretical sites were selected but changed every day, with thin soil layers for soil moisture sampling, from 1 to 5 cm in thickness. In the 0.20–1.60 m soil layer, the daily water content estimates were assumed to come from the same ten locations (the first soil moisture estimate was located at 0.25 m, and the others were located every 0.10 m until 1.55 m). Strategy 3 used ten theoretical sites located in the field, as in strategy 1, however the water content estimates in the 0.00–0.20-m soil layer were assumed to come from accurate water content measurements (soil layers from 1 to 5 cm in thickness), while for the 0.20–1.60 m soil layer, the strategy was similar to strategies 1 and 2. Strategy 4 used 10 new theoretical locations of measurement every day. Precise water content estimates for thin layers were assumed to be available in the 0.00–0.20 m soil layer as in strategy 2. The layers for water content estimates in the 0.20–1.60 m were similar to those of strategies 1, 2, and 3. Results showed that the spatial variability of the daily actual evapotranspiration may not be negligible, and differences from approximately ±1.0 mm d –1 to ±3.0 mm d –1 were calculated between the four sampling strategies. Strategy 1 gave the worst results, because variations in the water content of the top soil layers were neglected, and thus the daily evapotranspiration was underestimated. Strategy 2 led to a considerable variability for estimating daily evapotranspiration which was explained by the effect of the spatial variability due to the daily site sampling for the top soil layers (0 to 0.2 m). Strategy 3 appeared to be the best practical compromise between practical field considerations and the necessity to obtain accurate evapotranspiration measurements. The accuracy of daily evapotranspiration could reach ± 0.5 mm d–1, and could be further improved by increasing the number of measurement sites. The best results were obtained with strategy 4, although such a destructive and time-consuming strategy is not likely to be practical. 相似文献