Crop coefficients and actual evapotranspiration of a drip-irrigated Merlot vineyard using multispectral satellite images

A field experiment was carried out to evaluate the METRIC (mapping evapotranspiration at high resolution with internalized calibration) model to estimate the actual evapotranspiration (ET_a) 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 ET_a and K _c from an eddy covariance (EC) system. METRIC overestimated ET_a 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 ET_a. 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.     

Latent heat flux over Cabernet Sauvignon vineyard using the Shuttleworth and Wallace model

The Shuttleworth and Wallace model (SW) was evaluated to estimate latent heat flux above a drip-irrigated Cabernet Sauvignon vineyard, located in the Pencahue Valley, Region del Maule, Chile (35°22′ LS; 71°47′ LW; 150 m above sea level). The performance of the WS model (LE_ws) was evaluated against the eddy-covariance method (LE_ed) on a 30 min time interval. Results indicate that the root mean square error (RMSE) and mean absolute error (MAE) were 29 W m⁻² and 22 W m⁻², respectively. For the vine evapotranspiration (ETv), RMSE was 0.42 mm day⁻¹ and MAE was 0.36 mm day⁻¹. The largest disagreements between LE_ed and LE_ws were observed under dry atmospheric conditions. Also, the sensitivity analysis indicates that predicted ETv by the SW model was sensitive to errors of ±30% in leaf area index and mean stomatal resistance, but it was not affected by errors in the estimation of aerodynamic resistances.     

Water use and the development of seasonal crop coefficients for Superior Seedless grapevines trained to an open-gable trellis system

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 (ET_c) 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 ET_c of the lysimeter-grown vines, were applied in the vineyard. Reference evapotranspiration (ET_o) 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 = ET_c/ET_o. Maximum ET_c values in different seasons ranged from 7.26 to 8.59 mm day⁻¹ and seasonal ET_c (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 ET_c and K _c values that were observed are explained by the wide canopy layout that characterize the large open-gable trellis system.     

Use of thermal units to estimate corn crop coefficients under semiarid climatic conditions

Two crop coefficient equations were derived as a function of fraction of thermal units from lysimeter measured corn evapotranspiration (ET_c-lys) during 1997 and 1998, and reference evapotranspiration obtained from: (a) lysimeter measurements (Kc_mes) or FAO Penman-Monteith (ET_o-PM) estimates (Kc_est-PM). For validation, corn evapotranspiration (ET_c-est) was estimated in 2005 and 2006 from ET_o-PM and: (a) the equation for Kc_mes with (ET_c-est-lyslc) or without (ET_c-est-lys) locally calibrated ET_o-PM; (b) the equation for Kc_est-PM; and (c) the FAO approach (ET_c-est-FAO). The ET_{c-est_lys} estimates showed the lowest bias (0.09 mm day⁻¹); the ET_c-est-PM and ET_c-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 ET_c-est-lys, ET_c-est-lyslc and ET_c-est-PM were similar. Therefore, ET_c-est-lys is recommended although the ET_c-est-lyslc was similarly accurate. The ET_c-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 ET_c-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.     

Midday measurements of leaf water potential and stomatal conductance are highly correlated with daily water use of Thompson Seedless grapevines

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 (ET_LYS) 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 (Ψ_PD) and midday Ψ_l on August 24th were −0.075 and −0.76 MPa, respectively, with midday Ψ_l 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 Ψ_l were significantly correlated with one another (r = 0.96) and both with ET_LYS/ET_o (r = ~0.9). The decreases in Ψ_l, g _s, and ET_LYS/ET_o 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.     

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

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

Vineyard evaporative fraction based on eddy covariance in an arid desert region of Northwest China

Studying farmland evaporative fraction (EF) plays an important role in interpreting the components of energy budget and evapotranspiration (ET). The present study examines the pattern of vineyard EF after monitoring energy components by eddy covariance for 2 years, and estimates the crop ET by EF in the arid desert region of Northwest China. Main results indicate that EF during daytime is nearly constant on sunny days when the available energy exceeds 200 W m⁻², but EF becomes relatively unsteady when the available energy is lower than 200 W m⁻². Furthermore, daytime average EF is relatively low in the early growth stage, nearly constant in the mid-later stage, and significantly reduced in the later stage; Moreover, mean EF in different periods of daytime is in good agreement with daytime average EF, mean EF during 10:00–15:00 h is relatively close to daytime average EF and mean EF during 14:00–15:00 h is approximately equal to daytime average EF. The estimated daytime ET from mean EF during 14:00–15:00 h is highly correlated to the measured ET by Bowen ratio-energy balance though the value is partially underestimated. This study demonstrated that daytime ET can be estimated from midday EF and the relationship can be used to guide irrigation practice in the arid region.     

Daily evapotranspiration estimates from extrapolating instantaneous airborne remote sensing ET values

In this study, six extrapolation methods have been compared for their ability to estimate daily crop evapotranspiration (ET_d) 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. ET_d estimation errors for all six methods and both crops varied from −5.7 ± 4.8% (MBE ± RMSE) to 26.0 ± 15.8%. Extrapolated ET_d values based on the evaporative fraction (EF) method better compared to eddy covariance measured ET values. This method reported an average corn ET_d 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 ET_d values derived from eddy covariance energy balance systems. A solar radiation-based ET extrapolation method performed relatively well with ET_d estimation error of 2.2 ± 10.1% for both crops. An alfalfa reference ET-based extrapolation fraction method (ET_rF) yielded an overall ET_d 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 ET_d 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 ET_d. In addition, all methods need to be further tested under a variety of vegetation surface homogeneity, crop growth stage, environmental and climatological conditions.

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Evaluation of FAO-56 methodology for estimating reference evapotranspiration using limited climatic data: Application to Tunisia

The Food and Agriculture Organization of the United Nations had improved the version of the Penman–Monteith method (FAO-56 PM) which has recently been proposed as the standard for estimating reference evapotranspiration (ET_o). Unfortunately, some weather variables, especially solar radiation, relative humidity and wind speed, are often missing which could impede the estimation of ET_o with the FAO-56 PM method. To overcome the problem of the availability of climatic parameters, procedures to estimate ET_o with missing climate data are proposed as part of the FAO methodology. Therefore, assessing the accuracy of these procedures for different Tunisian locations is important. The comparison of ET_o estimates using limited data to those computed with full data set revealed that the difference between ET_o obtained from full and limited data set is small considering the 8 locations studied. Both the Mean Bias Error (MBE) and the Root Mean Square Error (RMSE) of the comparison were less than 0.6 and 0.8 with a minimum of −0.4 and 0.2 mm day⁻¹, respectively, leading to small errors in the ET_o estimates. The higher deviations occur when the only available information is minimum and maximum air temperature. These deviations were significantly higher when using the Hargreaves equation to calculate ET_o.     

Regional calibration of Hargreaves equation for estimating reference ET in a semiarid environment

The Hargreaves equation provides reference evapotranspiration (ET_o) estimates when only air temperature data are available, although it requires previous local calibration for acceptable performance. This equation has been evaluated under semiarid conditions in Southern Spain using data from 86 meteorological stations, comparing daily estimates against those from the FAO-56 Penman–Monteith equation, which was used as standard. Variability of results among location was clearly apparent, with MBE ranging from 0.74 to −1.13 mm d⁻¹ and RMSE from 0.46 to 1.65 mm d⁻¹. Maxima under- and overestimation amounted to 24.5 and 22.5%, respectively. In general, larger under- and overestimations occurred in stations located close to the coast and at inland areas, respectively. Yearly means of windspeed (V) and daily temperature range (ΔT) fairly influenced the accuracy of the equation. It was more accurate for windy locations with large ΔT, and for locations with light wind conditions combined with low to moderate values of ΔT. According to the values taken by V and ΔT, the stations were represented by points on the ΔT–V coordinate plane, in which four regions were delimited. A regional calibration was carried out considering only temperature and wind conditions. Correction was not necessary for stations located within two of them; for the other two regions, new values for the empirical coefficient of the equation are suggested (0.0027 and 0.0021). After correction, average RMSE and maximum and minimum MBE decreased substantially (12, 24 and 41%, respectively), and 74 out of the 86 locations gave quite accurate results, with relative values of MBE lower than 10% in most cases. Alternatively, another method based on kriging interpolation was proposed to obtain, for each individual station, locally adjusted values for the empirical coefficient as a function of the same variables. This second correction procedure behaved even better than the first one. There was a 15% improvement in the average RMSE, and maximum and minimum MBE values decreased 50 and 70%, respectively. At all locations, relative values of MBE were less than 10% and in 70% of them were lower than 5%. Validation was done by using data from 14 meteorological stations for other Spanish regions, and the consequences from the application of the corrections proposed for an irrigation district are discussed.     

Tomato root distribution, yield and fruit quality under different subsurface drip irrigation regimes and depths

Tomato rooting patterns, yield and fruit quality were evaluated in a field trial where three irrigation regimes [0.6 (DI), 0.9 (DII) and 1.2 ET_c (DIII)] and three drip irrigation depths [surface (R0), subsurface at 20 cm depth (RI) and subsurface at 40 cm depth (RII)] were imposed following a split-plot experimental design, with four replications. The behaviour of the root system in response to the irrigation treatments was evaluated using minirhizotrons installed between two plants, near the plant row. Root-length intensity (L _a)—length of the root per unit of minirhizotron surface area (cm cm⁻²)—was measured at four crop stages. For all sampling dates, none of the factors studied were found to influence L _a or rooting depth significantly or the interaction between treatments. For all treatments most of the root system was concentrated in the top 40 cm of the soil profile, where the root-length density ranged from 0.5 cm cm⁻³ to 1.4 cm cm⁻³ . The response of tomato fruits to an increase in the water applied was similar in quantitative and qualitative terms for the different drip irrigation depths. Water applied by drip irrigation had the opposite effect on commercial yield (t ha⁻¹) and soluble solids (°Brix) (r=−0.82, P<0.001), however, yield in terms of total soluble solids (t ha⁻¹) was the same for the 0.9 and 1.2 ET_c. The increase in commercial yield can be described by the equation      

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.     

Accuracy of canopy temperature energy balance for determining daily evapotranspiration

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%.     

Estimation of actual evapotranspiration in winegrape vineyards located on hillside terrain using surface renewal analysis

Sensible and latent heat flux densities were estimated in a level vineyard, a northeast aspect vineyard and a southwest aspect vineyard in the Napa Valley of California using the eddy covariance and surface renewal methods. Surface renewal is theoretically not limited to level or extensively homogeneous terrain because it examines a more localized process of scalar exchange as compared with eddy covariance. Surface renewal estimates must be calibrated against eddy covariance data to account for unequal heating of the air parcels under a fixed measurement height. We calibrated surface renewal data against eddy covariance data in a level vineyard, and the calibration factor (α) was applied to the surface renewal measurements on the hillside vineyards. Latent heat flux density was estimated from the residual of the energy balance. In the level vineyard, the average daily actual evapotranspiration (ET _a ) for the period of June through September was 2.4?mm per day. In the northeast aspect vineyard, the average daily ET _a was 2.2?mm per day, while in the southwest aspect vineyard it was 2.7?mm per day. The net radiation values for the level vineyard, the northeast aspect vineyard, and the southwest aspect vineyard were compared against the Ecosystem Water Program with good agreement.     

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.     

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.     

Wheat basal crop coefficients determined by normalized difference vegetation index

Crop coefficient methodologies are widely used to estimate actual crop evapotranspiration (ET_c) for determining irrigation scheduling. Generalized crop coefficient curves presented in the literature are limited to providing estimates of ET_c for “optimum” crop condition within a field, which often need to be modified for local conditions and cultural practices, as well as adjusted for the variations from normal crop and weather conditions that might occur during a given growing season. Consequently, the uncertainties associated with generalized crop coefficients can result in ET_c estimates that are significantly different from actual ET_c, which could ultimately contribute to poor irrigation water management. Some important crop properties such as percent cover and leaf area index have been modeled with various vegetation indices (VIs), providing a means to quantify real-time crop variations from remotely-sensed VI observations. Limited research has also shown that VIs can be used to estimate the basal crop coefficient (K _cb) for several crops, including corn and cotton. The objective of this research was to develop a model for estimating K _cb values from observations of the normalized difference vegetation index (NDVI) for spring wheat. The K _cb data were derived from back-calculations of the FAO-56 dual crop coefficient procedures using field data obtained during two wheat experiments conducted during 1993–1994 and 1995–1996 in Maricopa, Arizona. The performance of the K _cb model for estimating ET_c was evaluated using data from a third wheat experiment in 1996–1997, also in Maricopa, Arizona. The K _cb was modeled as a function of a normalized quantity for NDVI, using a third-order polynomial regression relationship (r ²=0.90, n=232). The estimated seasonal ET_c for the 1996–1997 season agreed to within −33 mm (−5%) to 18 mm (3%) of measured ET_c. However, the mean absolute percent difference between the estimated and measured daily ET_c varied from 9% to 10%, which was similar to the 10% variation for K _cb that was unexplained by NDVI. The preliminary evaluation suggests that remotely-sensed NDVI observations could provide real-time K _cb estimates for determining the actual wheat ET_c during the growing season.     

Model-based evaluation of low-cost drip-irrigation systems and management strategies using saline water

A drip-irrigation module was developed and included in an ecosystem model and tested on two independent datasets, spring and autumn, on field-grown tomato. Simulated soil evaporation correlated well with measurements for spring (2.62 mm d⁻¹ compared to 2.60 mm d⁻¹). Changes in soil water content were less well portrayed by the model (spring r ² = 0.27; autumn r ² = 0.45). More independent data is needed for further model testing in combination with developments of the spatial representation of below-ground variables. In a fresh-water drip-irrigated system, about 30% of the incoming water was transpired, 40% was lost as non-productive evaporative flows, and the remainder left the system as surface runoff or drainage. Simulations showed that saline water irrigation (6 dS m⁻¹) caused reduced transpiration, which led to higher drainage and soil evaporation, compared with fresh water. Covering the soil with plastic mulch resulted in an increase in yield and transpiration. Finally, two different drip-irrigation discharge rates (0.2 and 2.5 l h⁻¹) were compared; however the simulations indicated that the discharge rate did not have any impact on the partitioning of the incoming water to the system. The model proved to be a useful tool for evaluating the importance of specific management options.

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Estimating reference evapotranspiration with atmometers in a semiarid environment

Reference evapotranspiration (ET₀) estimations require accurate measurements of meteorological variables (solar radiation, air temperature, wind speed, and relative humidity) which are not available in many countries of the world. Alternative approaches are the use of Class A pan evaporimeters and atmometers, which have several advantages compared to meteorological stations: they are simple, inexpensive and provide a visual interpretation of ET₀. The objectives of the study were to compare the evaporation from atmometers (ET_gage) with the evapotranspiration estimated by the FAO-56 Penman-Monteith equation (ET_0PM) and to evaluate the variability between three modified atmometers of a commercial model. Comparison between daily ET_gage measured by the atmometer and ET_0PM showed a good correlation. However, ET_gage underestimated ET_0PM by approximately 9%. Differences between ET_gage and ET_0PM ranged from −2.4 to 2.2 mm d⁻¹ while the mean bias error was −0.41 mm d⁻¹. Underestimations occurred more frequently on days with low maximum temperatures and high wind speeds. On the contrary, atmometer overestimations occurred on days with high maximum temperatures and low wind speeds. Estimates of ET₀ using the atmometer appeared to be more accurate under non-windy conditions and moderate temperatures as well as under windy conditions and high temperatures. Atmometers 2 and 3 overestimated the evaporated water by atmometer 1 with a maximum variability of cumulative water losses of 4.5%. A temperature-based calibration was performed to improve the atmometer accuracy, using maximum temperature as an independent variable, with good results.