The use of the scintillation technique for monitoring seasonal water consumption of olive orchards in a semi-arid region

To monitor seasonal water consumption of agricultural fields at large scale, spatially averaged surface fluxes of sensible heat (H) and latent heat (L_vE) are required. The scintillation method is shown to be a promising device for obtaining the area-averaged sensible heat fluxes, on a scale of up to 10 km. These fluxes, when combined with a simple available energy model, can be used to derive area-averaged latent heat fluxes. For this purpose, a Large Aperture Scintillometer (LAS) was operated continuously for more than one year over a tall and sparse irrigated oliveyard located in south-central Marrakesh (Morocco). Due to the flood irrigation method used in the site, which induces irregular pattern of soil moisture both in space and time, the comparison between scintillometer-based estimates of daily sensible heat flux (H_LAS) and those measured by the classical eddy covariance (EC) method (H_EC) showed a large scatter during the irrigation events, while a good correspondence was found during homogenous conditions (dry conditions and days following the rain events). We found, that combining a simple available energy model and the LAS measurements, the latent heat can be reliably predicted at large scale in spite of the large scatter (R² = 0.72 and RMSE = 18.25 W m⁻²) that is obtained when comparing the LAS against the EC. This scatter is explained by different factors: the difference in terms of the source areas of the LAS and EC, the closure failure of the energy balance of the EC, and the error in available energy estimates. Additionally, the irrigation efficiency was investigated by comparing measured seasonal evapotranspiration values to those recommended by the FAO. It was found that the visual observation of the physical conditions of the plant is not sufficient to efficiently manage the irrigation, a large quantity of water is lost (≈37% of total irrigation). Consequently, the LAS can be considered as a potentially useful tool to monitor the water consumption in complex conditions.     

Accuracy of the Bowen ratio-energy balance method for measuring latent heat flux in a semiarid advective environment

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.     

Evaluating eddy covariance cotton ET measurements in an advective environment with large weighing lysimeters

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%).     

Evaluating the Crop Water Stress Index and its correlation with latent heat and CO2 fluxes over winter wheat and maize in the North China plain

Plant water status is a key factor impacting crop growth and agricultural water management. Crop water stress may alter canopy temperature, the energy balance, transpiration, photosynthesis, canopy water use efficiency, and crop yield. The objective of this study was to calculate the Crop Water Stress Index (CWSI) from canopy temperature and energy balance measurements and evaluate the utility of CWSI to quantify water stress by comparing CWSI to latent heat and carbon dioxide (CO₂) flux measurements over canopies of winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.). The experiment was conducted at the Yucheng Integrated Agricultural Experimental Station of the Chinese Academy of Sciences from 2003 to 2005. Latent heat and CO₂ fluxes (by eddy covariance), canopy and air temperature, relative humidity, net radiation, wind speed, and soil heat flux were averaged at half-hour intervals. Leaf area index and crop height were measured every 7 days. CWSI was calculated from measured canopy-air temperature differences using the Jackson method. Under high net radiation conditions (greater than 500 W m⁻²), calculated values of minimum canopy-air temperature differences were similar to previously published empirically determined non-water-stressed baselines. Valid measures of CWSI were only obtained when canopy closure minimized the influence of viewed soil on infrared canopy temperature measurements (leaf area index was greater than 2.5 m² m⁻²). Wheat and maize latent heat flux and canopy CO₂ flux generally decreased linearly with increases in CWSI when net radiation levels were greater than 300 W m⁻². The responses of latent heat flux and CO₂ flux to CWSI did not demonstrate a consistent relationship in wheat that would recommend it as a reliable water stress quantification tool. The responses of latent heat flux and CO₂ flux to CWSI were more consistent in maize, suggesting that CWSI could be useful in identifying and quantifying water stress conditions when net radiation was greater than 300 W m⁻². The results suggest that CWSI calculated by the Jackson method under varying solar radiation and wind speed conditions may be used for irrigation scheduling and agricultural water management of maize in irrigated agricultural regions, such as the North China Plain.     

Evaluating eddy covariance method by large-scale weighing lysimeter in a maize field of northwest China

Accurate determination of evapotranspiration (ET) is useful to develop precise irrigation scheduling. Although eddy covariance (EC) is a direct method which is widely used to measure ET, its performance in arid region of northwest China is not clear. In this study, ET measured by EC (ET_EC) was compared with that by large-scale weighing lysimeter (ET_L) during the whole growing season of maize in 2009. Energy balance ratio was 0.84 for daytime fluxes, indicating that lack of energy balance closure occurred, so daytime ET_EC was adjusted by Bowen-ratio forced closure method. Compared to the corresponding ET_L, half-hourly daytime ET_EC was underestimated by 21.8% without the adjustment and 4.8% with the adjustment. Furthermore, nighttime ET_EC was adjusted using filtering/interpolation method. Mean error between half-hourly nighttime ET_EC and ET_L decreased from 30.2% without the adjustment to 10.3% with the adjustment. After such adjustment of day and night measurements, daily ET_EC was underestimated by 6.2% compared to ET_L. These results indicated that the adjusted ET_EC well matched with the ET_L. Moreover, the discrepancy of adjusted total ET_EC and ET_L was decreased to 3.2% after subtracting the overestimated ET by lysimeter resulting from irrigation and heavy rainfall events. Thus, after appropriate adjustments of observations, eddy covariance method is accurate in estimating maize ET in the arid region of northwest China.     

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

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.     

Corrected surface energy balance to measure and model the evapotranspiration of irrigated orange orchards in semi-arid Mediterranean conditions

In this paper, based on the analysis of a long-term energy balance monitoring programme, a Bowen ratio-based method (BR) was proposed to resolve the lack of closure of the eddy covariance technique to obtain reliable sensible (H) and latent heat fluxes (λE). Evapotranspiration (ET) values determined from the BR method (ET_c,corr) were compared with the upscaled transpiration data determined by the sap flow heat pulse (HP) technique, evidencing the degree of correspondence between instantaneous transpirational flux at tree level and the micrometeorological measurement of ET at orchard level. Using the BR-corrected λE fluxes, a crop ET model implementing the Penman–Monteith approach, where the canopy surface resistance was determined from standard microclimatic variables, was applied to determine the crop coefficient values. The performance of the model was evaluated by comparing it with the sap flow HP data. The results of the comparison were satisfactory, and therefore, the proposed methodology may be considered valid for characterizing the ET process for orange orchards grown in a Mediterranean climate. By contrast to reports in the FAO 56 paper, the crop growth coefficient of the orange orchard being studied was not constant throughout the growing season.     

Calibration and validation of a remote sensing algorithm to estimate energy balance components and daily actual evapotranspiration over a drip-irrigated Merlot vineyard

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 (Rn_day) 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 (Rn_ins), soil heat flux (G _ins), and Rn_day 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 (LE_ins), 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 Rn_ins, G _ins, H _ins, and LE_ins 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.     

Comparison and sensitivity of measurement techniques for spatial distribution of soil salinity

In Khorezm, a district of Uzbekistan situated in the Aral Sea Basin, soil salinization is an important driver of soil degradation in irrigated agriculture. The main objective of this study was to identify techniques that enable rapid estimation of soil salinity. Therefore, bulk electrical conductivity of the soil (EC_a-meas) was measured with three different devices (2P, 4P, and CM-138) and electrical conductivity of the soil paste (EC_p-meas) was measured with the so-called 2XP device. These measurements were compared with independent estimates of EC_a-calc and EC_p-calc based on laboratory measurements of the saturated extract, EC_e, of soil samples from the same sites. Soil salinity could be assessed satisfactorily with all four devices. EC_p-meas could be well reproduced by the 2XP device (R ² = 0.76), whereas EC_a-meas estimates using 2P, 4P, and CM-138 in the field were less accurate (R ² < 0.50). The sensitivity of all devices to the main ions Cl⁻ and Ca^{2 +} suggests that the measuring principles are similar for all instruments. The devices can therefore be used interchangeably. Field assessment of soil salinity was considerably enhanced by the use of CM-138, because large areas can be quickly assessed, which may be desirable in spite of the lower accuracy.     

Evaluation of the surface renewal method to estimate wheat evapotranspiration

The performance of the surface renewal method to estimate latent heat fluxes (LE) over a wheat crop was evaluated by comparison against values of LE measured independently using a weighing lysimeter. High-frequency temperature readings were taken at 1.5 m above ground from 29 April to 7 June 2000 over a 0.7–0.8 m high wheat crop. Surface renewal analysis was applied for two time lags r (0.75 and 0.25 s) to estimate half-hour sensible heat flux (H) and, subsequently, LE by solving the energy balance equation, using concurrent measurements of net radiation and soil heat flux. When H was estimated using sensor measurement height (z) in the computations, indices of agreement (IA) between lysimeter and surface renewal LE were above 0.94 and relative errors varied between 8.5 and 14.9% for time lag r=0.75 s for all analyzed days but 7 June. Results were slightly poorer for time lag r=0.25 s. When z−h_c or z−d (h_c being the crop height and d being the zero plane displacement) were used instead of z to compute H, surface renewal LE estimates slightly improved, particularly for the z−d case. The improvement was particularly noticeable for 7 June. The use of z−h_c or z−d was thus more appropriate for these measurements, with the result that it was not necessary to calibrate the weighing factor α, as required by the standard surface renewal method. Unfortunately, although of similar magnitude than those reported for other micrometeorological methods, surface renewal errors found in this paper were biased and LE was underestimated. Further research and testing of the surface renewal method is therefore required to remove biases from the estimates of LE.     

Delineating water management zones in a paddy rice field using a Floating Soil Sensing System

Paddy rice fields are kept inundated during most of the growing period. This requirement is challenging to achieve because of the lack of suitable technologies to detect rapidly percolation prone zones within these fields. The objective of this study was to evaluate a methodology to identify water leakage areas to support precision soil–water management at a within-field level. Therefore, a Floating Sensing System (FloSSy) was designed to record the soil apparent electrical conductivity (EC_a) of a paddy field both under dry and inundated conditions using the electromagnetic induction sensor EM38. Comparison of EC_a data sets obtained under inundated and dry conditions showed that the EC_a measurements under inundated condition (EC_a-i) were more strongly related to soil properties due to the absence of variability in soil moisture and the increased stability of the floating sensing platform. Therefore, we proceeded with the EC_a-i measurements and grouped them into two classes using a fuzzy k-means classification method. These classes showed significant differences in water infiltration: lower EC_a values represented a higher infiltration rate and vice versa. This effect was attributed to differences in soil texture, more specifically the sand content, and its effect on water retention. It was concluded that an EC_a-i survey with FloSSy allowed the detection of soil heterogeneity linked to downward water fluxes which has a potential to support precision soil–water management in inundated fields.     

Evapotranspiration of grapevine trained to a gable trellis system under netting and black plastic mulching

The evapotranspiration (ET _c) of a table grape vineyard (Vitis vinifera, cv. Red Globe) trained to a gable trellis under netting and black plastic mulching was determined under semiarid conditions in the central Ebro River Valley during 2007 and 2008. The netting was made of high-density polyethylene (pores of 12 mm²) and was placed just above the ground canopy about 2.2 m above soil surface. Black plastic mulching was used to minimize soil evaporation. The surface renewal method was used to obtain values of sensible heat flux (H) from high-frequency temperature readings. Later, latent heat flux (LE) values were obtained by solving the energy balance equation. For the May–October period, seasonal ET _c was about 843 mm in 2007 and 787 mm in 2008. The experimental weekly crop coefficients (K _cexp) fluctuated between 0.64 and 1.2. These values represent crop coefficients adjusted to take into account the reduction in ET _c caused by the netting and the black plastic mulching. Average K _cexp values during mid- and end-season stages were 0.79 and 0.98, respectively. End-season K _cexp was higher due to combination of factors related to the precipitation and low ET _o conditions that are typical in this region during fall. Estimated crop coefficients using the Allen et al. (1998) approach adjusting for the effects of the netting and black plastic mulching (K _cFAO) showed a good agreement with the experimental K _cexp values.     

Crop water stress index is a sensitive water stress indicator in pistachio trees

Regulated deficit irrigation (RDI) strategies, often applied in tree crops, require precise monitoring methods of water stress. Crop water stress index (CWSI), based on canopy temperature measurements, has shown to be a good indicator of water deficits in field crops but has seldom been used in trees. CWSI was measured on a continuous basis in a Central California mature pistachio orchard, under full and deficit irrigation. Two treatments—control, returning the full evapotranspiration (ET_c) and RDI—irrigated with 40% ET_c during stage 2 of fruit grow (shell hardening). During stage 2, the canopy temperature—measured continuously with infrared thermometers (IRT)—of the RDI treatment was consistently higher than the control during the hours of active transpiration; the difference decreasing after irrigation. The non-water-stressed baseline (NWSB), obtained from clear-sky days canopy–air temperature differential and vapour pressure deficit (VPD) in the control treatment, showed a marked diurnal variation in the intercept, mainly explained by the variation in solar radiation. In contrast, the NWSB slope remained practically constant along the day. Diurnal evolution of calculated CWSI was stable and near zero in the control, but showed a clear rising diurnal trend in the RDI treatment, increasing as water stress increased around midday. The seasonal evolution of the CWSI detected large treatment differences throughout the RDI stress period. While the CWSI in the well-irrigated treatment rarely exceeded 0.2 throughout the season, RDI reached values of 0.8–0.9 near the end of the stress period. The CWSI responded to irrigation events along the whole season, and clearly detected mild water stress, suggesting extreme sensitivity to variations in tree water status. It correlated well with midday leaf water potential (LWP), but was more sensitive than LWP at mild stress levels. We conclude that the CWSI, obtained from continuous nadir-view measurements with IRTs, is a good and very sensitive indicator of water stress in pistachio. We recommend the use of canopy temperature measurements taken from 1200 to 1500 h, together with the following equation for the NWSB: (T _c − T _a) = −1.33·VPD + 2.44. Measurements of canopy temperature with VPD < 2 kPa are likely to generate significant errors in the CWSI calculation and should be avoided.     

Response of alfalfa (<Emphasis Type="Italic">Medicago sativa</Emphasis> L.) to diurnal and nocturnal saline sprinkler irrigations. I: total dry matter and hay quality

Little information is available on the quantitative effects on crops of saline sprinkler irrigations and the presumable beneficial effects of nocturnal versus diurnal irrigations. We measured crude protein content, carbon isotope discrimination and total dry matter (TDM) of alfalfa (Medicago sativa L.) subject to diurnal and nocturnal saline sprinkler irrigations. The work was carried out in Zaragoza (Spain) during the 2004–2006 growing seasons with a triple line source sprinkler system using synthetic saline waters dominated by NaCl with an irrigation water EC ranging from 0.5 to 5.6 dS m⁻¹. The quality of alfalfa hay assessed through its crude protein concentration was not significantly affected by salinity. Carbon isotope discrimination, an indicator of the effect of osmotic stress on plant water status, tended to decrease with increases in salinity. Based on a piecewise linear response model, alfalfa grown under saline sprinkler irrigation was shown to be more tolerant (threshold soil salinity, EC_e = 3.5 dS m⁻¹) than in previous experiments under surface irrigation (threshold EC_e = 2.0 dS m⁻¹) at relatively low salinity values, but became more sensitive at higher salinity values as shown by the higher absolute slope (13.4%) for sprinkler as compared to surface irrigation (7.3%). No significant differences in TDM were found between diurnal and nocturnal saline sprinkler irrigations. The recommended practice of irrigating at night for sprinkler irrigation using saline water is therefore not supported by our results in alfalfa grown under semiarid conditions.     

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.     

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.     

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.     

Partitioning of seasonal evapotranspiration from a commercial furrow-irrigated Sultana vineyard

Seasonal partitioning of evapotranspiration (ET) between transpiration by grapevines (Vitis vinifera) (T _gp) and by cover crops of a ryegrass/clover mixture (T _cc), and soil evaporation (E _s) was performed for a furrow-irrigated vineyard during the 1994/1995 and 1995/1996 growing seasons in south-eastern Australia. ET, determined with a water balance approach, averaged 622 mm. The ET rate averaged over the two seasons increased from 2 mm day^–1 in spring (September to November), when it was dominated by T _cc, to peak rates of around 5 mm d^–1 in summer (December to February) when it was dominated by E _s. T _gp, determined with either heat-pulse sensors or the Penman-Monteith equation, attained peak rates of 0.75 and 0.98 mm d^–1, or 6.2 and 8.1 l vine^–1 day^–1 in the first and second seasons, respectively. Total seasonal T _gp of 109.1 mm (900 l vine^–1) in 1994/1995 and 118.8 mm (980 l vine^–1) in 1995/1996 constituted just 18 – 19% of total ET. T _cc totalled 214 mm (34% of ET) in the first season, when pasture cover was sparse and present for 5 months of the growing season (September to February), and 196 mm (30% of ET) in the second season when pasture cover was heavy but present for only 3 months (September to November). E _s averaged 49% of total ET over both seasons. At least 30% of water used for ET was contributed by antecedent soil water in both seasons. The crop factor (K _c) was largely constant throughout the season with an average value of 0.48. The depletion pattern of soil water indicated that the vine explored the soil profile well beyond 1.0 mm depth and almost evenly up to a distance of 1.5 m from the trunk. Water use efficiencies for fresh fruit yield (WUE), i. e., the ratio of fruit weight to total water use at harvest,were 13.3 and 40.5 kg ha^–1 mm^–1 when based on ET in 1994/1995 and 1995/1996, respectively, and 84.0 and 211.1 kg ha^–1 mm^–1, respectively, when based on T _gp. The T _gp data were used to verify three models of vine transpiration developed in an earlier study. Models based on the green area index or on fraction of incident radiation intercepted by the vine canopy produced good agreement with T _gp. The model based on canopy resistance performed poorly, indicating the difficulty of extrapolating the stomatal response to environmental variables from one set of experimental conditions to another. Received: 23 September 1996     

Water requirements of olive orchards: I simulation of daily evapotranspiration for scenario analysis

Water requirements of olive orchards are difficult to calculate, since they are influenced by heterogeneous factors such as age, planting density and irrigation systems. Here we propose a model of olive water requirements, capable of separately calculating transpiration (E _p), intercepted rainfall evaporation (E _pd) and soil evaporation (E _s) from the wet and dry fraction of the soil surface under localized irrigation. The model accounts for the effects of canopy dimension on E _p and of the wetted soil surface fraction on E _s. The model was tested against actual measurements of olive evapotranspiration (ET) obtained by the eddy covariance technique in a developing olive orchard during 3 years. The predicted ET and crop coefficients showed good agreement with the measured data. The model was then used to simulate the average water requirements of two mature orchards using 20-year meteorological datasets of Cordoba (Spain) and Fresno (CA, USA). Average annual ET of a 300 trees ha⁻¹ orchard at Cordoba was 1,025 mm, while the same orchard at Fresno had an average ET of 927 mm. Transpiration losses were 602 mm at Cordoba and 612 mm at Fresno. Evaporation from the soil can have a large effect on olive ET; thus, olive crop coefficients (K _c) are very sensitive to the rainfall regime.     

A Landsat-based energy balance and evapotranspiration model in Western US water rights regulation and planning

The quantification of evapotranspiration (ET) from irrigated projects is important for water rights management, water resources planning and water regulation. Traditionally, ET has been estimated by multiplying a weather-based reference ET by crop coefficients (K_c) determined according to the crop type and the crop growth stage. However, there is typically question regarding whether crops grown compare with the conditions represented by the K_c values, especially in water short areas. In addition, it is difficult to estimate the correct crop growth stage dates for large populations of crops and fields. METRIC (Mapping Evapotranspiration at high Resolution and with Internalized Calibration) is an image-processing model for calculating ET as a residual of the surface energy balance. METRIC is a variant of SEBAL, an energy balance process developed in the Netherlands by Bastiaanssen and was extended to provide tighter integration with ground-based reference ET. METRIC was applied with Landsat images in southern Idaho to predict monthly and seasonal ET for water rights accounting and for operation of ground water models. ET “maps” (i.e., images) provide the means to quantify, in terms of both the amount and spatial distribution, the ET on a field by field basis. The ET maps have been used in Idaho to quantify net ground-water pumpage in areas where water extraction from underground is not measured and to estimate recharge from surface-irrigated lands. Application and testing of METRIC indicates substantial promise as an efficient, accurate, and relatively inexpensive procedure to predict the actual evaporation fluxes from irrigated lands throughout a growing season.