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1.
A study was carried out to calibrate and validate a remote sensing algorithm (RSA) for estimating instantaneous surface energy balance components and daily actual evapotranspiration (ETa) over a drip-irrigated Merlot vineyard located in the Maule Region of Chile (35° 25′ LS; 71° 32′ LW; 125?m.a.s.l.). ETa was estimated as a function of instantaneous evaporative fraction and average daily net radiation (Rnday) using meteorological variables in combination with reflectance data measured by a hand-held multi-spectral radiometer. The sub-models used to estimate the instantaneous net radiation (Rnins), soil heat flux (G ins), and Rnday were calibrated and validated using measurements of the surface energy balance components, incoming longwave radiation $(L \downarrow_{\text{ins}})$ , outgoing longwave radiation $(L \uparrow_{\text{ins}})$ , and surface albedo (α). The validations of instantaneous sensible heat flux (H ins), latent heat flux (LEins), and ETa were carried out using turbulent energy fluxes obtained from an eddy correlation (EC) system. For reducing the moderate EC imbalance (about 11?%), turbulent energy fluxes were recalculated using the Bowen ratio method. The validation analysis indicated that the calibrated sub-models of the RSA were able to estimate Rnins, G ins, H ins, and LEins with a root-mean-square error (RMSE), mean absolute error (MAE), and index of agreement (IA) ranging between 16–54, 13–44?W?m?2, and 0.72–94, respectively. Also, the RSA was able to estimate ETa with RMSE?=?0.38?mm?day?1, MAE?=?0.32?mm?day?1 and IA?=?0.96. These results demonstrate the potential use of reflectance and meteorological data to estimate ETa of a drip-irrigated Merlot vineyard.  相似文献   

2.
Two crop coefficient equations were derived as a function of fraction of thermal units from lysimeter measured corn evapotranspiration (ETc-lys) during 1997 and 1998, and reference evapotranspiration obtained from: (a) lysimeter measurements (Kcmes) or FAO Penman-Monteith (ETo-PM) estimates (Kcest-PM). For validation, corn evapotranspiration (ETc-est) was estimated in 2005 and 2006 from ETo-PM and: (a) the equation for Kcmes with (ETc-est-lyslc) or without (ETc-est-lys) locally calibrated ETo-PM; (b) the equation for Kcest-PM; and (c) the FAO approach (ETc-est-FAO). The ETc-est_lys estimates showed the lowest bias (0.09 mm day−1); the ETc-est-PM and ETc-est-FAO, the highest (0.50-0.51 mm day−1). However, the root mean square error (RMSE, 1.23–1.27 mm day−1) and the index of agreement (IA, around 0.94) of the ETc-est-lys, ETc-est-lyslc and ETc-est-PM were similar. Therefore, ETc-est-lys is recommended although the ETc-est-lyslc was similarly accurate. The ETc-est-PM is less recommended due to poorer bias and systematic mean square error, and a general underestimation except for low corn ET values. For real time irrigation scheduling, the ETc-est-FAO should be avoided as RMSE (1.35 mm day−1), IA (0.93) and bias were slightly worse, corn ET was overestimated but for high values, and the length of the four phenological stages must be known in advance.  相似文献   

3.
A study was performed in order to evaluate the three-source model (Clumped model) for direct estimation of actual evapotranspiration (ETa) and latent heat flux (LE) over a drip-irrigated Merlot vineyard trained on a vertical shoot positioned system (VSP) under semi-arid conditions. The vineyard, with an average fractional cover of 30%, is located in the Talca Valley, Region del Maule, Chile. The performance of the Clumped model was evaluated using an eddy covariance system during the 2006/2007 and 2007/2008 growing seasons. Results indicate that the Clumped model was able to predict ETa with a root mean square error (RMSE), mean bias error (MBE), and model efficiency (EF) of 0.33, −0.15 mm day−1 and 74%, respectively. Also, the Clumped model simulated the daytime variation of LE with a RMSE of 36 W m−2, MBE of −8 W m−2, and EF of 83%. Major disagreement (underestimated values) between observed and estimated values of ETa was found for clear days after rainfall or foggy days, but underestimated values were less than 10% of the data analysis. The results obtained in this study indicate that the Clumped model could be used to directly estimate vine water requirements for a drip-irrigated vineyard trained on a VSP. However, application of the Clumped model requires a good characterization of the drip-irrigated vineyard architecture.  相似文献   

4.
The Bowen Ratio-Energy Balance (BREB) is an accurate method often used to measure the latent heat flux (λE) due to its simplicity and portability. However, its performance in advective areas is less clear and its accuracy may depend on the equality of eddy transfer coefficients for heat and water vapor. In this work, hourly measured λE of a reference crop (Festuca arundinacea Schreb.) using a BREB system was compared with lysimeter-measured λE under moderate to severe advective conditions. The lysimeter resolution for hourly records was 22.6 W m−2. The analysis of the eddy transfer coefficients was made using simultaneous measurements of fluxes and vertical gradients of temperature and humidity. To avoid computational problems when β→ −1, some hourly periods were discarded in the analysis. Rejected data amounted to 37% of the total, although the cumulative evapotranspiration (ET) during these hours did not exceed 13% of the total ET. The BREB method overestimated daily ET by an average of 5.5% and by 5.7% when only daylight hours were considered. Under stable atmospheric conditions the method was less accurate, with relative errors of 21% vs. 11% under unstable conditions. For daylight hours, accuracy was higher under unstable conditions (RMSE = 36.15 W m−2) than under stable conditions (RMSE = 50.20 W m−2), which had larger overestimations of ET (6.3 vs. 5.1%). The main source of error appears to come from insufficient fetch resulting in local advective conditions. Nevertheless, and from a purely practical perspective, under the advective conditions of these measurements the BREB technique provides accurate ET fluxes with limited errors.  相似文献   

5.
The application of a single-layer canopy temperature energy balance (CTEB) model for determining integrated daily ET rates was tested, with possible applications towards determining irrigation requirements (“how much to irrigate”) as a complement to crop water stress index (CWSI) measurements (“when to irrigate”), an irrigation scheduling tool which uses much of the same data. Evapotranspiration (ET) rates estimated using the CTEB model were compared to Bowen ratio energy balance (BREB) measurements made over substantial portions of the growing seasons of corn and potato crops. Canopy temperature, net radiation and soil heat flux data were collected and analyzed at 20-minute intervals, and ET for each interval was summed to obtain daily and multi-day estimations. Only full canopy conditions were examined. Two methods for atmospheric stability correction were applied to the aerodynamic resistance required by the CTEB model; an iterative procedure proposed by Campbell, and a second procedure proposed by Monteith which uses an adjustment coefficient. To reduce instrumentation requirements for combined CTEB/CWSI data collection, estimates of ET were also determined using net radiation and soil heat flux values estimated from solar radiation measurements. Results showed that uncorrected CTEB ET estimates agreed reasonably well with BREB measurements over corn and potato canopies (RMSE = 0.5 to 0.7 mm day− for observed average ET ranging from 4.8 to 5.5 mm day−, with a trend toward seasonal overprediction with corn. Stability corrections usually lowered the daily RMSE 0.1 to 0.2 mm day−, with seasonal ET more in agreement with BREB ET. The Monteith-based adjustment gave slightly better results. CTEB ET model with estimated net radiation and soil heat flux terms produced similar average and total ET, but somewhat larger daily errors (RMSE=0.5 to 0.9 mm day−). Seasonal total ET by the uncorrected CTEB model generally overestimated within 10% (ranging from 1% to 10%) of the observed BREB total ET, an acceptable error for most irrigation practices. Stability corrections generally caused seasonal ET to be underestimated within 1% to 9%.  相似文献   

6.
Irrigation with effluents can detrimentally affect soil physical and chemical properties and impact plant growth and development. Excessive irrigation can leach salts from the root zone; which can be accomplished by precipitation in some areas. This study was conducted to examine the effect of applications of Kraft pulp mill effluent (KPME) with and without distilled water (DW) to simulate precipitation would have on soil chemical properties and growth of hybrid poplar (Populus deltoides × P. petrowskyana L. cv. Walker). Distilled water (DW), KPME, and a 50% combination (v/v; COMB) of DW and KPME were applied at rates of 6 and 9 mm day−1. COMB resulted in heights, biomasses, and leaf areas that were greater than those for KPME and comparable to those for DW. Diluted KPME treatments (i.e., COMB) still significantly increased soil electrical conductivity and sodium adsorption ratio compared to DW. Leachate collected from KPME 9 mm day−1 had concentrations of HCO3 −, SO4 2−, Cl−, Ca2+, K+, and Mg2+ comparable to those collected from COMB 9 mm day−1, but Na+ concentrations were three times higher in KPME than COMB 9 mm day−1. Results indicate that precipitation or additional irrigation water could potentially provide the leaching necessary to prevent salt accumulation within the rooting zone; however, irrigating with saline or sodic effluents requires careful management.  相似文献   

7.
The standard FAO methodology for the determination of crop water requirements uses the product of reference evapotranspiration (ETo) 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 ETo in plastic greenhouses. Free-drainage lysimeters were used between 1993 and 2004 to measure ETo inside a plastic greenhouse with a perennial grass in Almería, south-eastern Spain. Mean daily measured greenhouse ETo 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 ETo was reduced by an average of 21.4%. Different methodologies to calculate ETo 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 ETo because of their simplicity.  相似文献   

8.
A field experiment was carried out to evaluate the METRIC (mapping evapotranspiration at high resolution with internalized calibration) model to estimate the actual evapotranspiration (ETa) and crop coefficient (K c) of a drip-irrigated Merlot vineyard during the 2007/2008 and 2008/2009 growing seasons. The Merlot vineyard located in the Talca Valley (Chile) was trained on a vertical shoot positioned system. The performance of METRIC was evaluated using measurements of ETa and K c from an eddy covariance (EC) system. METRIC overestimated ETa by about 9?% with a root mean square error (RMSE) and mean absolute error (MAE) of 0.62 and 0.50?mm?d?1, respectively. For the main phenological stages of the Merlot vineyard, METRIC overestimated the K c by about 10?% with RMSE?=?0.10 and MAE?=?0.08. Furthermore, the indexes of agreement were 0.70 for K c and 0.85 for ETa. Mean values of K c measured from EC were 0.41, 0.53, 0.56, and 0.46, while those estimated by METRIC were 0.46, 0.54, 0.59, and 0.62 for the bud break to flowering, flowering to fruit set, fruit set to veraison, and veraison to harvest stages, respectively.  相似文献   

9.
Eddy covariance (EC) systems are being used to assess the accuracy of remote sensing methods in mapping surface sensible and latent heat fluxes and evapotranspiration (ET) from local to regional scales, and in crop coefficient development. Therefore, the objective was to evaluate the accuracy of EC systems in measuring sensible heat (H) and latent heat (LE) fluxes. For this purpose, two EC systems were installed near large monolithic weighing lysimeters, on irrigated cotton fields in the Texas High Plains, during the months of June and July 2008. Sensible and latent heat fluxes were underestimated with an average error of about 30%. Most of the errors were from nocturnal measurements. Energy balance (EB) closure was 73.2–78.0% for daytime fluxes. Thus, daylight fluxes were adjusted for lack of EB closure using the Bowen ratio/preservation of energy principle, which improved the resulting EC heat flux agreement with lysimetric values. Further adjustments to EC-based ET included nighttime ET (composite) incorporation, and the use of ‘heat flux source area’ (footprint) functions to compensate ET when the footprint expanded beyond the crop field boundary. As a result, ET values remarkably matched lysimetric ET values, with a ‘mean bias error ± root mean square error’ of −0.03 ± 0.5 mm day−1 (or −0.6 ± 10.2%).  相似文献   

10.
A surface energy balance model based on the Shuttleworth and Wallace (Q J R Meteorol Soc 111:839–855, 1985) and Choudhury and Monteith (Q J R Meteorol Soc 114:373–398, 1988) methods was developed to estimate evaporation from soil and crop residue, and transpiration from crop canopies. The model describes the energy balance and flux resistances for vegetated and residue-covered surfaces. The model estimates latent, sensible and soil heat fluxes to provide a method to partition evapotranspiration (ET) into soil/residue evaporation and plant transpiration. This facilitates estimates of the effect of residue on ET and consequently on water balance studies, and allows for simulation of ET during periods of crop dormancy. ET estimated with the model agreed favorably with eddy covariance flux measurements from an irrigated maize field and accurately simulated diurnal variations and hourly amounts of ET during periods with a range of crop canopy covers. For hourly estimations, the root mean square error was 41.4 W m−2, the mean absolute error was 29.9 W m−2, the Nash–Sutcliffe coefficient was 0.92 and the index of agreement was 0.97.  相似文献   

11.
Daily pan evaporation modeling using linear genetic programming technique   总被引：3，自引：1，他引：2  
This paper investigates the ability of linear genetic programming (LGP), which is an extension to genetic programming (GP) technique, in daily pan evaporation modeling. The daily climatic data, air temperature, solar radiation, wind speed, pressure and humidity of three automated weather stations, Fresno, Los Angeles and San Diego in California, are used as inputs to the LGP to estimate pan evaporation. The LGP estimates are compared with those of the Gene-expression programming (GEP), which is another branch of GP, multilayer perceptrons (MLP), radial basis neural networks (RBNN), generalized regression neural networks (GRNN) and Stephens–Stewart (SS) models. The performances of the models are evaluated using root mean square errors (RMSE), mean absolute error (MAE) and determination coefficient (R 2) statistics. Based on the comparisons, it was found that the LGP technique could be employed successfully in modeling evaporation process from the available climatic data.  相似文献   

12.
Water consumption of table grapevines (Vitis vinifera cv. Superior Seedless) trained to a large open-canopy gable system was measured during six growing seasons (1999, 2001–2005) using 12 drainage lysimeters. The lysimeters (1.3 m3 each) were installed as part of a one-hectare vineyard in a semi-arid region in southern Israel. Water consumption of the lysimeter-grown vines (ETc) was used as the basis for the calculation of irrigation applications in the vineyard. Three irrigation treatments, 80% (high), 60% (medium) and 40% (low) of ETc of the lysimeter-grown vines, were applied in the vineyard. Reference evapotranspiration (ETo) was calculated from regional meteorological data according to the Penman–Monteith equation. Seasonal curves for the crop coefficient (K c) were calculated as K c = ETc/ETo. Maximum ETc values in different seasons ranged from 7.26 to 8.59 mm day−1 and seasonal ETc (from DOY 91 through DOY 304) ranged from 1,087 to 1,348 mm over the six growing seasons. Leaf area index (LAI) was measured monthly using the SunScan Canopy Analysis System. Maximum LAI ranged from 4.2 to 6.2 m2 m−2 for the 2002–2005 seasons. A second-order polynomial curve relating K c to LAI (R2 = 0.907, P < 0.0001) is proposed as the basis for efficient irrigation management. The effects of the irrigation treatments on canopy growth and yield are presented. The high ETc and K c values that were observed are explained by the wide canopy layout that characterize the large open-gable trellis system.  相似文献   

13.
The continuous flow furrow irrigation (COFFI), surge flow furrow irrigation (SUFFI), cutback flow furrow irrigation (CUFFI), variable alternate flow furrow irrigation (VAFFI), and tail water reuse system furrow (TWRSF) techniques with the same inflow rate of 0.072 m3 min−1 were compared in relation to the cotton yield and water use efficiency at a 3-year field study conducted on cotton (Gossypium spp.) in the Southeastern Anatolia Project (GAP) area of Turkey. Yields revealed significant statistical differences between the furrow management techniques (P < 0.05). The maximum yield was obtained from the COFFI treatment (2,630 and 2,920 kg ha−1) in the first 2 years, and from SUFFI and CUFFI treatments (3,690 and 3,780 kg ha−1, respectively) in the last year. There were significant yield reductions, which varied from 10 to 35% in TWRSF and from 11 to 19% in VAFFI treatments although 43 and 28% more water was applied to the TWRSF than to CUFFI and SUFFI treatments, respectively. The average total water use efficiencies (WUEET) varied from 4.14 (VAFFI) to 2.59 (COFFI). The corresponding values were 0.37 and 0.36 kg ha−1 m−3 for CUFFI and SUFFI, respectively. The average irrigation water use efficiency (WUEIR) for CUFFI and SUFFI treatments were 0.30 and 0.23 kg ha−1 m−3, respectively.  相似文献   

14.
The response of three durum wheat cultivars (C: Chen’s, V: Vitron, W: Waha) to irrigation was studied during 4 years in semi-arid Algeria (Chlef). The four treatments were NI (unirrigated), EI (early irrigation, up to heading), LI (late irrigation, from heading) and FI (full irrigation, over the entire season). FI increased rainfed grain yield (1,300 kg ha−1) by 270%, EI by 107%, and LI by 67%. The variety × irrigation interaction was significant each year. Under irrigation, cv. Vitron was generally the most productive cultivar while in rainfed conditions cv. Waha always resulted in the highest grain yield. Grain yield increased exponentially with seasonal evapotranspiration (r 2 = 0.741) and harvest index (r 2 = 0.873). Water use efficiency for grain ranged from 4.6–5.3 kg ha−1 mm−1 (NI) to 9.6–10.8 kg ha−1 mm−1 (FI) as a function of cultivar and irrigation, cv. Vitron and cv. Waha (full irrigation) and cv. Waha (rainfed) being the most efficient cultivars. According to the evaporation pan method, the seasonal crop coefficient (K c) values for the three cultivars were 0.64 (V), 0.62 (W) and 0.54 (C). The corresponding peak K c values were 1.0, 0.97 and 0.89, respectively. K c was closely related to leaf area index (LAI) and specific logarithmic relationships were calculated for each cultivar. Irrigation scheduling should be adapted to the type of cultivar in relation to its potential yield and LAI development pattern.  相似文献   

15.
Summary A coupled soil-vegetation energy balance model which treats the canopy foliage as one layer and the soil surface as another layer was validated againt a set of field data and compared with a single-layer model of a vegetation canopy. The two-layer model was used to predict the effect of increases in soil surface temperature (T s ) due to the drying of the soil surface, on the vegetation temperature (T v ). In the absence of any change in stomatal resistance the impact of soil surface drying on the Crop Water Stress Index (CSWI) calculated from T v was predicted. Field data came from a wheat crop growing on a frequently irrigated plot (W) and a plot left un watered (D) until the soil water depletion reached 100 mm. Vegetation and soil surface temperatures were measured by infrared thermometers from tillering to physiological maturity, with meteorological variables recorded simultaneously. Stomatal resistances were measured with a diffusion porometer intensively over five days when the leaf area index was between 5 and 8. The T v predicted by the single-layer and the two-layer models accounted for 87% and 88% of the variance of measured values respectively, and both regression lines were close to the 11 relationship. Study of the effect of T s on the CWSI with the two-layer model indicated that the CWSI was sensitive to changes in T s . The overestimation of crop water stress calculated from the CWSI was predicted to be greater at low leaf area indices and high levels of stomatal resistance. The implications for this bias when using the CWSI for irrigation scheduling are discussed.List of Symbols C Sensible heat flux from the soil-vegetation system (W m–2) - c l shade Mean stomatal conductance of the shaded leaf area (m s–1) - c l sun Mean stomatal conductance of the sunlit leaf area (m s–1) - c max Maximum stomatal conductance (m s–1) - c 0 Minimum stomatal conductance (m s–1) - C p Specific heat at constant pressure (J kg–1 °C–1) - C s Sensible heat flux from the soil (W m–2) - C v Sensible heat flux from the vegetation (W m–2) - c v Bulk stomatal conductance of the vegetation (m s–1) - CWSI Crop Water Stress Index (dimensionless) - e a Vapor pressure at the reference height (kPa) - e b Vapor pressure at the virtual source/sink height of heat exchange (kPa) - e 0 * Saturated vapor pressure at T 0 (kPa) - e s Vapor pressure at the soil surface (kPa) - e v * Saturated vapor pressure at T v (kPa) - G Soil heat flux (Wm–2) - GLAI Green leaf area index (dimensionless) - GLAIshade Green shaded leaf area index (dimensionless) - GLAIsun Green sunlit leaf area index (dimensionless) - k Extinction coefficient for photosynthetically active radiation (dimensionless) - k 1 Damping exponent for Eq. A 5 (m2 W–1) - LAI Leaf area index (dimensionless) - LE Latent heat flux from the soil-vegetation system (W m–2) - LE s Latent heat flux from the soil (W m–2) - LE v Latent heat flux from the vegetation (W m–2) - p a Density of air (kg m–3) - PARa Photosynthetically active radiation above the canopy (W m–2) - PARu Photosynthetically active radiation under the canopy (W m–2) - r a Aerodynamic resistance (s m–1) - r b Heat exchange resistance between the vegetation and the adjacent air boundary layer (s m–1) - r c Bulk stomatal resistance of the vegetation (s m–1) - R n Net radiation above the canopy (W m–2) - R s Net radiation flux at the soil surface (W m–2) - r st Mean stomatal resistance of leaves in the canopy (s m–1) - R v Net radiation absorbed by the vegetation (W m–2) - r w Heat exchange resistance between the soil surface and the boundary layer (s m–1) - S Photosynthetically active radiation on the shaded leaves (W m–2) - S d Diffuse photosynthetically active radiation (W m –2) - S 0 Photosynthetically active radiation on a surface perpendicular to the beams (W m–2) - T a Air temperature at the reference height (°C) - T b Temperature at the virtual source/sink height of heat exchange (°C) - T 0 Aerodynamic temperature (°C) - T s Soil surface temperature (°C) - T v Vegetation temperature (°C) - w 0 Single scattering albedo (dimensionless) - Psychrometric constant (kPa °C) - 0 Cosine of solar zenith angle (dimensionless)  相似文献   

16.
Coal bed natural gas (CBNG) extraction in the Powder River (PR) Basin of Wyoming and Montana produces modestly saline-sodic wastewater, which may have electrical conductivity (EC) and sodium adsorption ratios (SAR) exceeding accepted thresholds for irrigation (EC = 3 dS m−1, SAR = 12 (mmolc l−1)1/2. As an approach to managing large volumes of CBNG-produced water, treatment processes have been developed to adjust produced water salinity and sodicity to published irrigation guidelines and legislated in-stream standards. The objective of this laboratory study was to assess acute and chronic soil solution EC and SAR responses to various wetting regimes simulating repeated flood irrigation with treated CBNG product water, followed by single rainfall events. Fifty-four soil samples from irrigated fields in southeast Montana were subjected to simulated PR water or CBNG water treated to EC and SAR values accepted as thresholds for designation of saline × sodic water, in a single wetting event, five wetting–drying events, or five wetting–drying events, followed by leaching with distilled water. Resultant saturated paste extract EC (ECe) and SAR of soils having <33% clay did not differ from one another, but resulting ECe and SAR were all less than those for soil having >33% clay. Repeated wetting with PR water having EC of 1.56 dS m−1 and SAR of 4.54 led to SAR <12, but brought ECe near 3 dS m−1. Repeated wetting with water having salinity = 3.12 dS m−1 and SAR = 13.09 led to ECe >3 dS m−1 and SAR near 12. Subsequent inundation and drainage with distilled water, simulating rainfall-quality leaching, reduced ECe and SAR more often in coarse-textured, high salt content soils than in finer-textured, lower salt content soils. Decreases in ECe upon leaching with distilled water were of greater magnitude than corresponding decreases in SAR, reinforcing supposition of sodium-induced dispersion of fine-textured soils as a consequence of rainfall following irrigation with water having salinity and sodicity levels equal to previously published thresholds.  相似文献   

17.
In this study, six extrapolation methods have been compared for their ability to estimate daily crop evapotranspiration (ETd) from instantaneous latent heat flux estimates derived from digital airborne multispectral remote sensing imagery. Data used in this study were collected during an experiment on corn and soybean fields, covering an area of approximately 12 × 22 km, near Ames, Iowa. ETd estimation errors for all six methods and both crops varied from −5.7 ± 4.8% (MBE ± RMSE) to 26.0 ± 15.8%. Extrapolated ETd values based on the evaporative fraction (EF) method better compared to eddy covariance measured ET values. This method reported an average corn ETd estimate error of −0.3 mm day−1, with a corresponding error standard deviation of 0.2 mm day−1, i.e., about 5.7 ± 4.8% average under prediction when compared to average ETd values derived from eddy covariance energy balance systems. A solar radiation-based ET extrapolation method performed relatively well with ETd estimation error of 2.2 ± 10.1% for both crops. An alfalfa reference ET-based extrapolation fraction method (ETrF) yielded an overall ETd overestimation of about 4.0 ± 10.0% for both crops. It is recommended that the average daily soil heat flux not be neglected in the calculation of ETd when utilizing method EF. These results validate the use of the airborne multispectral RS-based ET methodology for the estimation of instantaneous ET and its extrapolation to ETd. In addition, all methods need to be further tested under a variety of vegetation surface homogeneity, crop growth stage, environmental and climatological conditions.
José L. Chávez (Corresponding author)Email:
Christopher M. U. NealeEmail:
John H. PruegerEmail:
William P. KustasEmail:
  相似文献   

18.
The use of N fertilizers in agriculture is crucial, and agricultural techniques need to be implemented that improve significantly N fertilizer management by reducing downward movements of solutes through the soil. To achieve this, it is essential to develop and test models against experimental conditions in order to improve them and to make sure that they can be applied to a broad range of soil and climatic conditions. A field experiment was carried out in the French department of Gard. The soil was a clay loam (26.7% clay, 44.7% fine and coarse silt, and 28.6% fine and coarse sand). Salad vegetables (Cichorium endivia, Lactuca sativa) were cultivated during two consecutive periods (spring and summer crops). The crops were planted on punched and permeable plastic mulching bands. The field was irrigated with a sprinkler watering system. Local measurements were made combining a neutron probe, tensiometers, and ceramic porous cups to estimate NO3-N concentrations. The model is one-dimensional and is based on Richards' equation for describing saturated-unsaturated water flow in soil. At the soil surface, the model is designed to handle flux-type or imposed-pressure boundary conditions. In addition, provision is made in the model, for example, to account for a mulch plastic sheet that limits evaporation. The model accounts for heat transport by diffusion and by convection, while the modeling of the displacement of nitrate and ammonium in the soil is based on the convection-dispersion equation. Nitrate uptake by the crop is modeled assuming Michaelis-Menten kinetics. Nitrogen cycle modeling accounts for the following major transformations: mineralization of organic matter, nitrification of ammonium, and denitrification. The results showed that the overall trend of the water potential in the soil profile was correctly described during the crop seasons. Mineralization was high for the spring crop (4.7 kg NO3-N day–1 ha–1), whereas the other sink components, such as root uptake, drainage, and denitrification, were smaller (1.9, 1.4, and 0.2 kg NO3-N day–1 ha–1, respectively). For the summer crop, intensive denitrification was found in the soil layer at 0.15–0.90 m (5.7 kg NO3-N day–1 ha–1), while the mineralization was always an important component (9.2 kg NO3-N day–1 ha–1) and the sink terms were 1.7 and 1.7 kg NO3-N day–1 ha–1 for root uptake and drainage, respectively. Similar high denitrification rates were found in the literature under intensive irrigated field conditions. Received: 25 October 1995  相似文献   

19.
A study was conducted to determine the relationship between midday measurements of vine water status and daily water use of grapevines measured with a weighing lysimeter. Water applications to the vines were terminated on August 24th for 9 days and again on September 14th for 22 days. Daily water use of the vines in the lysimeter (ETLYS) was approximately 40 L vine−1 (5.3 mm) prior to turning the pump off, and it decreased to 22.3 L vine−1 by September 2nd. Pre-dawn leaf water potential (ΨPD) and midday Ψl on August 24th were −0.075 and −0.76 MPa, respectively, with midday Ψl decreasing to −1.28 MPa on September 2nd. Leaf g s decreased from ~500 to ~200 mmol m−2 s−1 during the two dry-down periods. Midday measurements of g s and Ψl were significantly correlated with one another (r = 0.96) and both with ETLYS/ETo (r = ~0.9). The decreases in Ψl, g s, and ETLYS/ETo in this study were also a linear function of the decrease in volumetric soil water content. The results indicate that even modest water stress can greatly reduce grapevine water use and that short-term measures of vine water status taken at midday are a reflection of daily grapevine water use.  相似文献   

20.
This study focuses on CO2 and water vapor flux measurements, water use efficiency estimates and evapotranspiration modeling during the course of growth of a young banana crop in a screenhouse in northern Israel. An eddy covariance system was deployed at the center of the screenhouse during two growth periods of the banana crop: small and large plants. Results show that daily whole canopy evapotranspiration increased during the measurement period from 2.2 mm day?1 for the smaller plants to 3.4 mm day?1 for the larger plants. The increase in net daily CO2 consumption doubled during the same period, from about 11 to 21.5 g m?2 day?1 per unit ground area. Water vapor and CO2 fluxes per unit leaf area were independent of plant size and averaged with 51 and 0.29 g m?2 day?1, respectively. Consequently, water use efficiency, defined as the ratio between net vertical fluxes of CO2 and water vapor, was nearly constant during growth of the plants. Evapotranspiration models provided more accurate predictions for larger than for smaller plants. This was due to inadequate treatment of the partial cover of young plants, which could be overcome by the use of a crop coefficient. A modified Penman–Monteith evapotranspiration model adapted to the screenhouse environment, applied here for the first time to a banana screenhouse, was in better agreement with the measurements than an open canopy model.  相似文献   

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