Simulation of winter wheat evapotranspiration in Texas and Henan using three models of differing complexity

Crop evapotranspiration (ET) is an important component of simulation models with many practical applications related to the efficient management of crop water supply. The algorithms used by models to calculate ET are of various complexity and robustness, and often have to be modified for particular environments. We chose three crop models with different ET calculation strategies: CROPWAT with simple data inputs and no calibrations, MODWht for intensive inputs and limited calibrations, and CERES-Wheat with intensive inputs and more calibrations for parameters. The three crop models were used to calculate ET of winter wheat (Triticum aestivum L.) grown at two experimental sites of China and US during multiple growing seasons in which ET was measured using lysimeter or soil water balance techniques. None of the models calculated daily ET well at either Bushland or Zhengzhou as indicated by high mean absolute differences (MAD > 1.1 mm) and root mean squared errors (RMSE > 2.0 mm). The three models tended to overestimate daily ET when measured ET was small, and to underestimate daily ET when measured ET was large. The fitted values of daily crop coefficients (K_c), calculated from daily ET and reference ET (ET_o), were very similar to those of Allen et al. (1998) [Allen, R.G., Pereira, S.L., Raes, D., Smith, M., 1998. Crop evapotranspiration guidelines for computing crop water requirements. Irrigation and drainage paper 56, Rome] although some K_c were overestimated (≥1.0). Leaf area index (LAI) was poorly calculated by MODWht and CERES-Wheat, especially when using the Priestley-Taylor method to estimate potential ET (PET). Poor overall ET calculation of three models was associated with poorly estimated values of PET or ET_o, K_c and LAI as well as their interactions. Therefore, this suggested that considerable revisions and calibrations of ET algorithms of the three models are needed for the improvement of ET calculation.     

Evapotranspiration, crop coefficient and growth of two young pomegranate (Punica granatum L.) varieties under salt stress

Pomegranate (Punica granatum L.) is a drought-hardy crop, suited to arid and semi-arid regions, where the use of marginal water for agriculture is on the rise. The use of saline water in irrigation affects various biochemical processes. For a number of crops, yields have been shown to decrease linearly with evapotranspiration (ET) when grown in salt-stressed environments. In the case of pomegranate, little research has been conducted regarding the effect of salt stress. Our study focused on the responses of ET, crop coefficient (K_c) and growth in pomegranate irrigated with saline water. Experiments were conducted using lysimeters with two varieties of pomegranate, P. granatum L. vars. Wonderful and SP-2. The plants were grown with irrigation water having an electrical conductivity (EC_iw) of 0.8, 1.4, 3.3, 4.8 and 8 dS m⁻¹. Plants were irrigated with 120% of average lysimeter-measured ET. Seasonal variation in ET, crop coefficient (K_c) and growth were recorded. Variation in daily ET was observed 1 month after initiation of the treatments. While significant seasonal ET variation was observed for the EC-0.8 treatment, it remained more stable for the EC-8 treatment. Salinity treatment had a significant effect on both daily ET (F = 131, p < 0.01) and total ET (F = 112.68, p = 0.001). Furthermore, the electrical conductivity of the drainage water (EC_dw) in the EC-8 treatment was five times higher than that of the EC-0.8 treatment in the peak season. Fitting the relative ET (ET_r) to the Maas and Hoffman salinity yield response function showed a 10% decrease in ET per unit increase in electrical conductivity of the saturated paste extract (EC_e) with a threshold of 1 dS m⁻¹. If these parameters hold true in the case of mature pomegranate trees, the pomegranate should be listed as a moderately sensitive crop rather than a moderately tolerant one. Fitting 30-day interval ET_r data to the Maas and Hoffman salinity yield response function showed a reduction in the slope as the season progressed. Thus using a constant slope in various models is questionable when studying crop-salinity interactions. In addition, both of the varieties showed similar responses under salt stress. Moreover, the calculated value of K_c is applicable for irrigation scheduling in young pomegranate orchards using irrigation water with various salinities.     

Evaporation and canopy conductance of citrus orchards

Evaporation of citrus orchards has been widely studied, but differences in methodologies and management conditions have led to conflicting results, mainly due to differences in ground cover and soil evaporation. In this work the contribution of transpiration and soil evaporation has been studied in a drip-irrigated, clean cultivated mandarin (Citrus reticulata Blanco) orchard on a sandy soil in Southern Spain. Evapotranspiration (ET) was measured using eddy covariance while soil evaporation was determined with microlysimeters, during August 2000 and May 2001. Average ET was 2.6 mm day⁻¹ in August and 2.1 mm day⁻¹ in May. The crop coefficient (K_c) was 0.44 and 0.43 in 2000 and 2001, respectively. The coefficient of transpiration (K_p) was 0.30 in 2000 and 0.25 in 2001. The daily bulk canopy conductance (g_c) ranged from 1.2 to 2.2 (average 1.8) mm s⁻¹ in 2000 and from 1.2 to 2.7 (average 1.9) mm s⁻¹ in 2001. A model of daily canopy conductance as a function of intercepted radiation was derived and applied to calculate the transpiration of orchards with different values of ground cover (GC). The ratio of transpiration over reference ET of mandarin orchards is linearly related to ground cover (K_p = 0.7 GC). Calculated crop coefficients agree with values suggested by FAO for mature orchards (around 0.65) but are substantially lower than FAO values for young plantations.     

Combining FAO-56 model and ground-based remote sensing to estimate water consumptions of wheat crops in a semi-arid region

This study was performed to test three methods based on the FAO-56 “dual” crop coefficient approach to estimate actual evapotranspiration (AET) for winter wheat under different irrigation treatments in the semi-arid region of Tensift Al Haouz, Marrakech (center of Morocco). The three methods differ in the calculation of the basal crop coefficient (K_cb) and the fraction of soil surface covered by vegetation (f_c). The first approach strictly follows the FAO-56 procedure, with K_cb given in the FAO-56 tables and f_c calculated from K_cb (No-Calibration method). The second method uses local K_cb and f_c values estimated from field measurements (Local-Calibration method) and the last approach uses a remotely-sensed vegetation index to estimate K_cb and f_c (NDVI-Calibration method). The analysis was performed on three fields using actual (AET) measured by Eddy Correlation systems. It was shown that the Local-Calibration approach gave best results. Accurate estimates of K_cb and f_c were necessary for FAO-56 “dual” crop coefficient application. The locally derived K_cb for winter wheat taken at initial, mid-season, and maturity crop growth were 0.15, 0.90 and 0.23, respectively. The K_cb value at the mid-season stage was found to be considerably less than that suggested by the FAO-56.     

A simple irrigation scheduling approach for pecans

Pecans are a major crop in New Mexico's Lower Rio Grande Valley (LRGV). It is estimated that New Mexico is responsible for about 21% of the world's pecan production (Lillywhite et al., 2007). Currently, approximately 12,000 ha of pecan orchards at various stages of growth consume 45% of the area's irrigation water. Pecan evapotranspiration (ET) varies with age, canopy cover, soil type, crop density and method of water management. Intense competition for the LRGV's limited water supply has created a serious need for better water management through improved irrigation scheduling. Annual pecan ET ranges from as low as 500 mm to as high as 1400 mm. Diversity of the pecan crop coefficient (K_c) and ET makes the task of irrigation scheduling for this crop very complicated. Using remote sensing technology and field ET measurements, a simple relationship was developed to relate crop coefficient and ET to canopy cover. This relationship is then used in combination with climate data to calculate daily and weekly water requirements for each orchard. The difference between annual ET values estimated from canopy cover and values measured with an eddy covariance flux tower ranged from 2 to 5%. The average ratio of estimated monthly ET values over measured ET values was 1.03 with the standard error of the estimate ranging from 10 to 20 mm/month. This methodology provides a simple tool that farmers can use to schedule irrigation of pecan orchards. Even though the methodology was developed for irrigation scheduling in the LRGV, it can be used in other locations by transferring the reference crop coefficients using K_c-GDD relationships.     

Camelina water use and seed yield response to irrigation scheduling in an arid environment

Camelina sativa (L.) Crantz is a promising, biodiesel-producing oilseed that could potentially be implemented as a low-input alternative crop for production in the arid southwestern USA. However, little is known about camelina’s water use, irrigation management, and agronomic characteristics in this arid environment. Camelina experiments were conducted for 2 years (January to May in 2008 and 2010) in Maricopa, Arizona, to evaluate the effectiveness of previously developed heat unit and remote sensing basal crop coefficient (K _cb ) methods for predicting camelina crop evapotranspiration (ET) and irrigation scheduling. Besides K _cb methods, additional treatment factors included two different irrigation scheduling soil water depletion (SWD) levels (45 and 65 %) and two levels of seasonal N applications within a randomized complete block design with 4 blocks. Soil water content measurements taken in all treatment plots and applied in soil water balance calculations were used to evaluate the predicted ET. The heat-unit K _cb method was updated and validated during the second experiment to predict ET to within 12–13 % of the ET calculated by the soil water balance. The remote sensing K _cb method predicted ET within 7–10 % of the soil water balance. Seasonal ET from the soil water balance was significantly greater for the remote sensing than heat-unit K _cb method and significantly greater for the 45 than 65 % SWD level. However, final seed yield means, which varied from 1,500 to 1,640 kg ha^?1 for treatments, were not significantly different between treatments or years. Seed oil contents averaged 45 % in both years. Seed yield was found to be linearly related to seasonal ET with maximum yield occurring at about 470–490 mm of seasonal ET. Differences in camelina seed yields due to seasonal N applications (69–144 kg N ha^?1 over the 2 years) were not significant. Further investigations are needed to characterize camelina yield response over a wider range of irrigation and N inputs.     

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.     

Evapotranspiration and responses to irrigation of broccoli

In cold, semi-arid areas, the options for crop diversification are limited by climate and by the water supply available. Growing irrigated crops outside the main season is not easy, because of climatic and market constraints. We carried out an experiment in Albacete, Central Spain, to measure the water use (evapotranspiration, ET) of broccoli (Brassica oleracea L. var. italica Plenck) planted in late summer and harvested at the end of fall. A weighing lysimeter was used to measure the seasonal ET under sprinkler irrigation. Consumptive use reached 359 mm for a period of 109 days after transplanting. The crop coefficient (K_c) for broccoli was obtained and compared to the standard recommendations for normal planting dates. Dual crop coefficient computations of the lysimeter ET data indicated that evaporation represented 31% of seasonal ET. An analysis of the variation in daily K_c values at a time of full cover suggested that the use of a grass lysimeter as a reference ET (ET_o) was superior to using the ASCE Penman-Monteith (ASCE PM) equation at hourly time steps, which in turn caused less variability in K_c than when using the FAO-56 Penman-Monteith (FAO-56 PM) equation at daily time steps for the ET_o calculation. An additional experiment aimed at evaluating the yield response to applied irrigation water by the drip method (seven treatments, from 59 to 108% of ET_c) generated a production function that gave maximum yields of near 12 t ha⁻¹ at an irrigation level of 345 mm, and a water use efficiency of 3.37 kg m⁻³. It is concluded that growing broccoli in the fall season is a viable alternative for crop diversification, as the lower yields obtained here may be more than compensated for by the higher produce prices in autumn, at a time of the year where irrigation water demand for other crops is very low.     

Evapotranspiration and crop coefficient of spring maize with plastic mulch using eddy covariance in northwest China

Spring maize under plastic mulch is the staple food crop in northwest China. Studying its evapotranspiration (ET) and crop coefficient (K_c) is important for managing water-saving irrigation in the region. Eddy covariance (EC) was applied to measure spring maize ET in 2007 in northwest China, focusing on the characteristics of the maize ET and K_c processes under plastic mulch. An interesting result was that a higher K_c in this study relative to the value of FAO 56 was presented in the mid and late season, e.g. average K_c was 1.46, 1.39 and 1.22 during the heading, filling and maturity stage, respectively. This result was mainly due to that (1) the plastic mulch had an effect on anti-senescence of maize and great green leaf still existed before the harvest; (2) the FAO 56 PM model may underestimate the reference crop ET in the mid and late season of maize in the region; (3) the planting density was higher in the study, which was about 374,800 plants ha⁻¹. Though K_c during the mid and late season was high, a high water use efficiency of 25.2 kg ha⁻¹ mm⁻¹ was still obtained in the study. Our study confirmed that plastic mulch has beneficial effect on improving maize water use efficiency in this severe water shortage region of northwest China.     

Comparisons of energy balance and evapotranspiration between flooded and aerobic rice fields in the Philippines

The seasonal and annual variability of sensible heat flux (H), latent heat flux (LE), evapotranspiration (ET), crop coefficient (K_c) and crop water productivity (WP_ET) were investigated under two different rice environments, flooded and aerobic soil conditions, using the eddy covariance (EC) technique during 2008-2009 cropping periods. Since we had only one EC system for monitoring two rice environments, we had to move the system from one location to the other every week. In total, we had to gap-fill an average of 50-60% of the missing weekly data as well as those values rejected by the quality control tests in each rice field in all four cropping seasons. Although the EC method provides a direct measurement of LE, which is the energy used for ET, we needed to correct the values of H and LE to close the energy balance using the Bowen ratio closure method before we used LE to estimate ET. On average, the energy balance closure before correction was 0.72 ± 0.06 and it increased to 0.99 ± 0.01 after correction. The G in both flooded and aerobic fields was very low. Likewise, the energy involved in miscellaneous processes such as photosynthesis, respiration and heat storage in the rice canopy was not taken into consideration.Average for four cropping seasons, flooded rice fields had 19% more LE than aerobic fields whereas aerobic rice fields had 45% more H than flooded fields. This resulted in a lower Bowen ratio in flooded fields (0.14 ± 0.03) than in aerobic fields (0.24 ± 0.01). For our study sites, evapotranspiration was primarily controlled by net radiation. The aerobic rice fields had lower growing season ET rates (3.81 ± 0.21 mm d⁻¹) than the flooded rice fields (4.29 ± 0.23 mm d⁻¹), most probably due to the absence of ponded water and lower leaf area index of aerobic rice. Likewise, the crop coefficient, K_c, of aerobic rice was significantly lower than that of flooded rice. For aerobic rice, K_c values were 0.95 ± 0.01 for the vegetative stage, 1.00 ± 0.01 for the reproductive stage, 0.97 ± 0.04 for the ripening stage and 0.88 ± 0.03 for the fallow period, whereas, for flooded rice, K_c values were 1.04 ± 0.04 for the vegetative stage, 1.11 ± 0.05 for the reproductive stage, 1.04 ± 0.05 for the ripening stage and 0.93 ± 0.06 for the fallow period. The average annual ET was 1301 mm for aerobic rice and 1440 mm for flooded rice. This corresponds to about 11% lower total evapotranspiration in aerobic fields than in flooded fields. However, the crop water productivity (WP_ET) of aerobic rice (0.42 ± 0.03 g grain kg⁻¹ water) was significantly lower than that of flooded rice (1.26 ± 0.26 g grain kg⁻¹ water) because the grain yields of aerobic rice were very low since they were subjected to water stress.The results of this investigation showed significant differences in energy balance and evapotranspiration between flooded and aerobic rice ecosystems. Aerobic rice is one of the promising water-saving technologies being developed to lower the water requirements of the rice crop to address the issues of water scarcity. This information should be taken into consideration in evaluating alternative water-saving technologies for environmentally sustainable rice production systems.     

Evapotranspiration of an hedge-pruned olive orchard in a semiarid area of NE Spain

The evapotranspiration of hedge-pruned olive orchards (Olea europaea L. cv. Arbequina) was measured under the semiarid conditions of the middle Ebro River Valley in a commercial olive orchard (57 ha) during 2004 and 2005. No measured ET_c values for this type of olive orchards have previously been reported. An eddy covariance system (krypton hygrometer KH20 and 3D sonic anemometer CSAT3, Campbell Scientific) was used. The eddy covariance measurements showed a lack of the energy balance closure (average imbalance of 26%). Then sensible and latent heat (LE) flux values were corrected using the approach proposed by Twine et al. (2000) in order to get daily measured olive evapotranspiration (ET_c) and crop coefficient (K_c) values. The highest measured monthly ET_c averages were about 3.1-3.3 mm day⁻¹, while the total seasonal ET_c during the irrigation period (March-October) was about 585 mm (in 2004) and 597 mm (in 2005). Monthly K_c values varied from about 1.0 (Winter) to 0.4-0.5 (Spring and Summer). These K_c values were similar to K_c values reported for round-shape canopy olive orchards, adjusted for ground cover, particularly during late Spring and Summer months when differences among measured and published K_c values were about less than 0.1.     

Consumptive water use and crop coefficients of irrigated sunflower

In semi-arid environments, the use of irrigation is necessary for sunflower production to reach its maximum potential. The aim of this study was to quantify the consumptive water use and crop coefficients of irrigated sunflower (Helianthus annuus L.) without soil water limitations during two growing seasons. The experimental work was conducted in the lysimeter facilities located in Albacete (Central Spain). A weighing lysimeter with an overall resolution of 250 g was used to measure the daily sunflower evapotranspiration throughout the growing season under sprinkler irrigation. The lysimeter container was 2.3 m × 2.7 m × 1.7 m deep, with an approximate total weight of 14.5 Mg. Daily ET _c values were calculated as the difference between lysimeter mass losses and lysimeter mass gains divided by the lysimeter area. In the lysimeter, sprinkler irrigation was applied to replace cumulative ET _c, thus maintaining non-limiting soil water conditions. Seasonal lysimeter ET _c was 619 mm in 2009 and 576 mm in 2011. The higher ET _c value in 2009 was due to earlier planting and a longer growing season with the maximum cover coinciding with the maximum ET _o period. For the two study years, maximum average K _c values reached values of approximately 1.10 and 1.20, respectively, during mid-season stage and coincided with maximum ground cover values of 75 and 88 %, respectively. The dual crop coefficient approach was used to separate crop transpiration (K _cb) from soil evaporation (K _e). As the crop canopy expanded, K _cb values increased while the K _e values decreased. The seasonal evaporation component was estimated to be about 25 % of ET _c. Linear relationships were found between the lysimeter K _cb and the canopy ground cover (f _c) for the each season, and a single relationship that related K _cb to growing degree-days was established allowing extrapolation of our results to other environments.     

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.     

Estimating cotton evapotranspiration crop coefficients with a multispectral vegetation index

Crop coefficients are a widely used and universally accepted method for estimating the crop evapotranspiration (ET_c) component in irrigation scheduling programs. However, uncertainties of generalized basal crop coefficient (K_cb) curves can contribute to ET_c estimates that are substantially different from actual ET_c. Limited research with corn has shown improvements to irrigation scheduling due to better water-use estimation and more appropriate timing of irrigations when K_cb estimates derived from remotely sensed multispectral vegetation indices (VIs) were incorporated into irrigation-scheduling algorithms. The purpose of this article was to develop and evaluate a K_cb estimation model based on observations of the normalized difference vegetation index (NDVI) for a full-season cotton grown in the desert southwestern USA. The K_cb data used in developing the relationship with NDVI were derived from back-calculations of the FAO-56 dual crop coefficient procedures using field data obtained during two cotton experiments conducted during 1990 and 1991 at a site in central Arizona. The estimation model consisted of two regression relations: a linear function of K_cb versus NDVI (r²=0.97, n=68) used to estimate K_cb from early vegetative growth to effective full cover, and a multiple regression of K_cb as a function of NDVI and cumulative growing-degree-days (GDD) (r²=0.82, n=64) used to estimate K_cb after effective full cover was attained. The NDVI for cotton at effective full cover was ~0.80; this value was used to mark the point at which the model transferred from the linear to the multiple regression function. An initial evaluation of the performance of the model was made by incorporating K_cb estimates, based on NDVI measurements and the developed regression functions, within the FAO-56 dual procedures and comparing the estimated ET_c with field observations from two cotton plots collected during an experiment in central Arizona in 1998. Preliminary results indicate that the ET_c based on the NDVI-K_cb model provided close estimates of actual ET_c.Communicated by R. Evans     

Changes in evapotranspiration over irrigated winter wheat and maize in North China Plain over three decades

Evapotranspiration (ET) is an important component of the water cycle at field, regional and global scales. This study used measured data from a 30-year irrigation experiment (1979-2009) in the North China Plain (NCP) on winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.) to analyze the impacts of climatic factors and crop yield on ET. The results showed that grass reference evapotranspiration (ET_o, calculated by FAO Penmen-Monteith method) was relatively constant from 1979 to 2009. However, the actual seasonal ET of winter wheat and maize under well-watered condition gradually increased from the 1980s to the 2000s. The mean seasonal ET was 401.4 mm, 417.3 mm and 458.6 mm for winter wheat, and 375.7 mm, 381.1 mm and 396.2 mm for maize in 1980s, 1990s and 2000s, respectively. The crop coefficient (K_c) was not constant and changed with the yield of the crops. The seasonal average K_c of winter wheat was 0.75 in the 1980s, 0.81 in the 1990s and 0.85 in the 2000s, and the corresponding average grain yield (GY) was 4790 kg ha⁻¹, 5501 kg ha⁻¹ and 6685 kg ha⁻¹. The average K_c of maize was 0.88 in the 1980s, 0.88 in the 1990s and 0.94 in the 2000s, with a GY of 5054 kg ha⁻¹, 7041 kg ha⁻¹ and 7874 kg ha⁻¹, respectively, for the three decades. The increase in ET was not in proportion to the increase in GY, resulting improved water use efficiency (WUE). The increase in ET was possibly related to the increase in leaf stomatal conductance with renewing in cultivars. The less increase in water use with more increase in grain production could be partly attributed to the significant increase in harvest index. The results showed that with new cultivars and improved management practices it was possible to further increase grain production without much increase in water use.     

Temporal and spatial variations of evapotranspiration for spring wheat in the Shiyang river basin in northwest China

The methods for estimating temporal and spatial variation of crop evapotranspiration are useful tools for irrigation scheduling and regional water allocation. The purpose of this study was to develop a method for mapping spatial distribution of crop evapotranspiration and analyze the temporal and spatial variation of spring wheat evapotranspiration in the Shiyang river basin in Northwest China in the last 50 years. DEM-based methods were employed to estimate the spatial distribution of spring wheat evapotranspiration (ET_c). Reference crop evapotranspiration (ET₀) was calculated with the Penman–Monteith equation using meteorological data measured from eight stations in the basin. Crop coefficient (K_c) was determined from measured evapotranspiration in spring wheat season in the region. The results showed that ET_c gradually increased in the upper reaches of the basin in the last 50 years, while the middle reaches showed a significant decreasing trend, and in other regions, no significant trend was found. These changes can be attributed to expansion of irrigation areas and climate change. The multiple regression analysis between ET_c and altitude, latitude, and aspect were carried out for eight weather stations and the relationships were used to map ET_c for the basin. The spatial variations of ET_c were analyzed for three typical growing seasons based their precipitation. Results showed that long-term average ET_c over cultivated land was increasing from 270 mm in southwest mountainous area to 591 mm in northeast oasis of the basin, and the relative error between the estimated ET_c in spring wheat growing season by reference evapotranspiration (ET₀) and crop coefficient (K_c), and the interpolated ET_c was within 11.1%.     

Measured and estimated water use and crop coefficients of grapevines trained to overhead trellis systems in California’s San Joaquin Valley

The use of overhead trellis systems for the production of dry-on-vine (DOV) raisins and table grapes in California is expanding. Studies were conducted from 2006 to 2009 using Thompson Seedless grapevines grown in a weighing lysimeter trained to an overhead arbor trellis and farmed as DOV raisins for the first two years and for use as table grapes thereafter. Maximum canopy coverage for the two lysimeter vines across years was in excess of 80 %. Seasonal (15 March–31 October) evapotranspiration for the lysimeter vines (ET_Lys) was 952 mm in 2007 (farmed as DOV raisins) and 943 and 952 mm (when farmed as table grapes). The maximum crop coefficient (K _cLys) across all 4 years ranged from 1.3 to 1.4. These maximum values were similar to those estimated using the relationship where K _c is a function of the amount of shaded area measured beneath the canopy at solar noon (K _c = 0.017 × percent shaded area). Covering the lysimeter’s soil surface with plastic (and then removing it) numerous times during the 2009 growing season (1 June–14 September) reduced ET_Lys from an average of 6.4 to 5.6 mm day^?1 and the K _c from 1.07 to 0.93. A seasonal basal K _c (K _cb) was calculated for grapevines using an overhead trellis system with a 13 % reduction in the K _cLys across the growing season.     

A consolidated evaluation of the FAO-56 dual crop coefficient approach using the lysimeter data in the North China Plain

The main purpose of this paper was to evaluate whether or not the dual crop coefficient (DCC) method proposed in FAO-56 was suitable for calculating the actual daily evapotranspiration of the main crops (winter wheat and summer maize) in the North China Plain (NCP). The results were evaluated with the data measured by the large-scale weighing lysimeter at the Yucheng Comprehensive Experimental Station (YCES) of the Chinese Academy of Sciences (CAS) from 1998 to 2005 using the Nash-Sutcliffe efficiency (NSE), the root mean square error (RMSE) and the root mean square error to observations’ standard deviation ratio (RSR). The evaluation results showed that the DCC method performed effective in simulating the quantity of seasonal evapotranspiration for winter wheat but was inaccurate in calculating the peak values. The RMSE value of the winter wheat during the total growing season was less than 0.9 mm/d, the NSE and RSR values during the total growing stage were “Very Good”, but the results for summer maize were “Unsatisfactory”. The recommended basal crop coefficient values K_cbtab during the initial, mid-season and end stages for winter wheat and summer maize were modified and the variation scope of basal crop coefficient K_cb was analyzed. The K_c (compositive crop coefficient, K_c = ET_c/ET₀, ET_c here is the observed values by lysimeter, ET₀ is the reference evapotranspiration) values were estimated using observed weighing lysimeter data during the corresponding stages for winter wheat and summer maize were 0.80, 1.15, 1.25, 0.95; 0.90, 0.95, 1.25, 1.00, respectively. These can be a reference for irrigation planning.     

Soil water dynamics and water use efficiency in spring maize (Zea mays L.) fields subjected to different water management practices on the Loess Plateau, China

Soil water supply is the main limiting factor to crop production across the Loess Plateau, China. A 2-year field experiment was conducted at the Changwu agro-ecosystem research station to evaluate various water management practices for achieving favorable grain yield (GY) with high water use efficiency (WUE) of spring maize (Zea mays L.). Four practices were examined: a rain-fed (RF) system as the control; supplementary irrigation (SI); film mulching (FM); and straw mulching (SM) (in 2008 only). The soil profile water storage (W) and the crop evapotranspiration (ET) levels were studied during the maize growing season, and the GY as well as the WUE were also compared. The results showed that mean soil water storage in the top 200 cm of the profile was significantly (P < 0.05) increased in the SI (380 mm in 2007, 411 mm in 2008) and SM (414 mm in 2008) compared to the FM (361 mm in 2007, 381 mm in 2008) and RF (360 mm in 2007, 384 mm in 2008) treatments. The soil water content was lower at the end of growing season than before planting in the 60-140 cm part of the profile in both the RF and FM treatments. Cumulative ET and average crop coefficiency (K_c) throughout the whole maize growing season were significantly (P < 0.05) higher in the SI (ET, 501 mm in 2007, 431 mm in 2008; K_c, 1.0 in 2007, 0.9 in 2008) treatment than in the other treatments. Both FM and SI significantly improved the GY. The WUE were increased significantly (23-25%; P < 0.05) under the FM treatment. It was concluded that both SI and FM are beneficial for improving the yield of spring maize on the Loess Plateau. However, FM is preferable because of the shortage of available water in the area.     

Macromanagement of deficit-irrigated peanut with sprinkler irrigation

Precision irrigation management and scheduling, as well as developing site- and cultivar-specific crop coefficient (K_c), and yield response factor to water deficit (ky) are very important parameters for efficient use of limited water resources. This study investigated the effect of deficit irrigation, applied at different growth stages of peanut with sprinkler irrigation in sandy soil, on field peanut evapotranspiration (ETc), yield and yield components, and water use efficiencies (IWUE and WUE). Also, yield response factor to water deficit (ky), and site- and cultivar-specific K_c were developed. Four treatments were imposed to deficit irrigation during late vegetative and early flowering, late flowering and early pegging, pegging, and pod formation growth stages of peanut, and compared with full irrigation in the course of the season (control). A soil water balance equation was used to estimate crop evapotranspiration (ETc). The results revealed that maximum seasonal ETc was 488 mm recorded with full irrigation treatment. The maximum value of K_c (0.96) occurred at the fifth week after sowing, this value was less than the generic values listed in FAO-33 and -56 (1.03 and 1.15), respectively. Dry kernels yield among treatments differed by 41.4%. Deficit irrigation significantly affected yields, where kernels yield decreased by 28, 39, 36, and 41% in deficit-irrigated late vegetative and early flowering, late flowering and early pegging, pegging, and pod formation growth stages, respectively, compared with full irrigation treatment. Peanut yields increased linearly with seasonal ETc (R² = 0.94) and ETc/ETp (R² = 0.92) (ETp = ETc with no water stress). The yield response factor (ky), which indicates the relative reduction in yield to relative reduction in ETc, averaged 2.9, was higher than the 0.7 value reported by Doorenbos and Kassam [Doorenbos, J., Kassam, A.H., 1979. Yield response to water. FAO Irrigation and Drainage Paper 33, Rome, Italy, 193 pp.], the high ky value reflects the great sensitivity of peanut (cv. Giza 5) to water deficit. WUE values varied considerably with deficit irrigation treatments, averaging 6.1 and 4.5 kg ha⁻¹ mm⁻¹ (dry-mass basis) for pods and kernels, respectively. Differences in WUE between the driest and wettest treatment were 31.3 and 31.3% for pods and kernels, respectively. Deficit irrigation treatments, however, impacted IWUE much more than WUE. Differences in IWUE between the driest and wettest treatment were 33.9 and 33.9% for pods and kernels, respectively. The results revealed that better management of available soil water in the root zone in the course of the season, as well as daily and seasonal accurate estimation of ETc can be an effective way for best irrigation scheduling and water allocation, maximizing yield, and optimizing economic return.