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.     

Evapotranspiration and water use of full and deficit irrigated cotton in the Mediterranean environment in northern Syria

Cotton (Gossypium hirsutum L.) is the most important industrial and summer cash crop in Syria and many other countries in the arid areas but there are concerns about future production levels, given the high water requirements and the decline in water availability. Most farmers in Syria aim to maximize yield per unit of land regardless of the quantity of water applied. Water losses can be reduced and water productivity (yield per unit of water consumed) improved by applying deficit irrigation, but this requires a better understanding of crop response to various levels of water stress. This paper presents results from a 3-year study (2004-2006) conducted in northern Syria to quantify cotton yield response to different levels of water and fertilizer. The experiment included four irrigation levels and three levels of nitrogen (N) fertilizer under drip irrigation. The overall mean cotton (lint plus seed, or lintseed) yield was 2502 kg ha⁻¹, ranging from 1520 kg ha⁻¹ under 40% irrigation to 3460 kg ha⁻¹ under 100% irrigation. Mean water productivity (WP_ET) was 0.36 kg lintseed per m³ of crop actual evapotranspiration (ET_c), ranging from 0.32 kg m⁻³ under 40% irrigation to 0.39 kg m⁻³ under the 100% treatment. Results suggest that deficit irrigation does not improve biological water productivity of drip-irrigated cotton. Water and fertilizer levels (especially the former) have significant effects on yield, crop growth and WP_ET. Water, but not N level, has a highly significant effect on crop ET_c. The study provides production functions relating cotton yield to ET_c as well as soil water content at planting. These functions are useful for irrigation optimization and for forecasting the impact of water rationing and drought on regional water budgets and agricultural economies. The WP_ET values obtained in this study compare well with those reported from the southwestern USA, Argentina and other developed cotton producing regions. Most importantly, these WP_ET values are double the current values in Syria, suggesting that improved irrigation water and system management can improve WP_ET, and thus enhance conservation and sustainability in this water-scarce region.     

Soil salinity related to physical soil characteristics and irrigation management in four Mediterranean irrigation districts

Irrigated agriculture is threatened by soil salinity in numerous arid and semiarid areas of the Mediterranean basin. The objective of this work was to quantify soil salinity through electromagnetic induction (EMI) techniques and relate it to the physical characteristics and irrigation management of four Mediterranean irrigation districts located in Morocco, Spain, Tunisia and Turkey. The volume and salinity of the main water inputs (irrigation and precipitation) and outputs (crop evapotranspiration and drainage) were measured or estimated in each district. Soil salinity (EC_e) maps were obtained through electromagnetic induction surveys (EC_a readings) and district-specific EC_a-EC_e calibrations. Gravimetric soil water content (WC) and soil saturation percentage (SP) were also measured in the soil calibration samples. The EC_a-EC_e calibration equations were highly significant (P < 0.001) in all districts. EC_a was not significantly correlated (P > 0.1) with WC, and was only significantly correlated (P < 0.1) with soil texture (estimated by SP) in Spain. Hence, EC_a mainly depended upon EC_e, so that the maps developed could be used effectively to assess soil salinity and its spatial variability. The surface-weighted average EC_e values were low to moderate, and ranked the districts in the order: Tunisia (3.4 dS m⁻¹) > Morocco (2.2 dS m⁻¹) > Spain (1.4 dS m⁻¹) > Turkey (0.45 dS m⁻¹). Soil salinity was mainly affected by irrigation water salinity and irrigation efficiency. Drainage water salinity at the exit of each district was mostly affected by soil salinity and irrigation efficiency, with values very high in Tunisia (9.0 dS m⁻¹), high in Spain (4.6 dS m⁻¹), moderate in Morocco (estimated at 2.6 dS m⁻¹), and low in Turkey (1.4 dS m⁻¹). Salt loads in drainage waters, calculated from their salinity (EC_dw) and volume (Q), were highest in Tunisia (very high Q and very high EC_dw), intermediate in Turkey (extremely high Q and low EC_dw) and lowest in Spain (very low Q and high EC_dw) (there were no Q data for Morocco). Reduction of these high drainage volumes through sound irrigation management would be the most efficient way to control the off-site salt-pollution caused by these Mediterranean irrigation districts.     

Assessing satellite-based basal crop coefficients for irrigated grapes (Vitis vinifera L.)

A combined methodology of basal crop coefficient (K_cb) derived from vegetation indices (VI) obtained from satellite images and a daily soil water balance in the root zone of the crop was proposed to accurately estimate the daily grape crop coefficient and actual evapotranspiration. The modeled values were compared with field measurements of crop evapotranspiration (ET) using an energy balance eddy-covariance flux tower and adjusted for closure using the measured Bowen ratio. A linear relation between K_cb and VI for vineyard was obtained, K_cb = 1.44 × NDVI-0.10 and K_cb = 1.79 × SAVI-0.08. The correlation of the measured crop coefficient (K_c) and modeled (K_crf) exhibits a linear tendency, K_c = 0.96K_crf, r² = 0.67. Other derived parameters such as weekly K_c and daily and weekly ET show good consistency with measurements and higher coefficients of determination. The study of the soil water balance suggests the importance of soil water storage in grapes within the La Mancha region. These results validate the use of remote sensing as a tool for the estimation of evapotranspiration of irrigated wine grapes planted on trellis systems.     

Evapotranspiration components determined by sap flow and microlysimetry techniques of a vineyard in northwest China: Dynamics and influential factors

Large areas of vineyards have been established in recent years in arid region of northwest China, despite limited water resources. Water to support these vineyards is mainly supplied by irrigation. Accurate estimation of vineyard evapotranspiration (ET) can provide a scientific basis for developing irrigation management. Transpiration and soil evaporation, as two main components of ET, were measured separately in a vineyard in this region by heat balance sap flow system and micro-lysimeters during the growing season of 2009. Diurnal and seasonal dynamics of sap flow and its environmental controls were analyzed. Daily sap flow rate (SR_l) increased linearly with solar radiation (R_s), but showed an exponential increase to its maximum curve as a function of vapor pressure deficit (VPD). Residuals of the two regressions both depended on volumetric soil water content to a depth of 1.0 m (VWC). VWC also significantly influenced SR_l. The relationship of them could be expressed by a piecewise regression with the turnover point of VWC = 0.188 cm³ cm⁻³, which was ∼60% of the field capacity. Conversely, soil evaporation (E_s) increased exponentially with VWC. Thus, we recommended keeping VWC in such vineyards slightly above ∼60% of the field capacity to maintain transpiration while reducing soil evaporation. Vineyard transpiration (T_s) was scaled from sap flow by using leaf area (A_l) as it explained 60% of the spatial variability of sap flow. Vine transpiration was 202.0 mm during the period from April 28 to October 5; while that of E_s was 181.0 mm. The sum of these two components was very close to ET estimated by the Bowen ratio energy balance method (386.9 mm), demonstrating the applicability of sap flow for measuring grape water use in this region.     

An alternative to define canopy surface temperature bounds

Canopy temperature, which may be estimated by infrared thermometry (IRT), can serve as an indicator of plant water status. [Idso et al., 1981a] and [Idso et al., 1986] proposed the nowadays much used concept of the crop water stress index, which relates observed canopy surface temperature (T_s) to maxima and minima temperature bounds. Jackson et al. (1981) defined those bounds on the basis of the energy balance. Those bounds vary with the meteorological situation. In this paper a chart is offered for general use with a fixed frame for the upper and lower bound. It relates canopy surface temperatures with r₁(=1 + r_c/r_a)-values (r_c the canopy resistance and r_a the aerodynamic resistance) as a function of a specifically defined temperature sum (S). It links the curved lower bound with the straight upper bound by a bundle of r₁-curves (the T_s-S-r₁-chart). The lower bound can be expressed by an equation, which approximates the energy balance solution with high accuracy. The sensitivity of the upper bound is also discussed. A comparison was made between bounds following Jackson et al. (1981) and the proposed alternative method, which, however, is limited by the short data-set available for this paper.     

Performance evaluation and calibration of soil water content and potential sensors for agricultural soils in eastern Colorado

This study evaluated the performance of three soil water content sensors (CS616/625, Campbell Scientific, Inc., Logan, UT; TDT, Acclima, Inc., Meridian, ID; 5TE, Decagon Devices, Inc., Pullman, WA) and a soil water potential sensor (Watermark 200SS, Irrometer Company, Inc., Riverside, CA) in laboratory and field conditions. Soil water content/potential values measured by the sensors were compared with corresponding volumetric water content (θ_v, m³ m⁻³) values derived from gravimetric samples, ranging approximately from the permanent wilting point (PWP) to field capacity (FC) volumetric water contents. Under laboratory and field conditions, the factory-based calibrations of θ_v did not consistently achieve the required accuracy for any sensor in the sandy clay loam, loamy sand, and clay loam soils of eastern Colorado. Salt (calcium chloride dihydrate) added to the soils in the laboratory caused the CS616, TDT, and 5TE sensors to experience errors in their volumetric water content readings with increased bulk soil electrical conductivity (EC; dS m⁻¹). Results from field tests in sandy clay loam and loamy sand soils indicated that a linear calibration (equations provided) for the TDT, CS616 and 5TE sensors (and a logarithmic calibration for the Watermark sensors) could reduce the errors of the factory calibration of θ_v to less than 0.02 ± 0.035 m³ m⁻³. Furthermore, the performance evaluation tests confirmed that each individual sensor needed a unique calibration equation for every soil type and location in the field. In addition, the calibrated van Genuchten (1980) equation was as accurate as the calibrated logarithmic equation and can be used to convert soil water potential (kPa) to volumetric soil water content (m³ m⁻³). Finally, analysis of the θ_v field data indicated that the CS616, 5TE and Watermark sensor readings were influenced by diurnal fluctuations in soil temperature, while the TDT was not influenced. Therefore, it is recommended that the soil temperature be considered in the calibration process of the CS616, 5TE, and Watermark sensors. Further research will be aimed towards determining the need of sensor calibration for every agricultural season.     

Evapotranspiration and crop coefficient of Populus euphratica Oliv forest during the growing season in the extreme arid region northwest China

Based on successive observation, fifteen-day evapotranspiration (ET_c) of Populus euphratica Oliv forest, in the extreme arid region northwest China, was estimated by application of Bowen ratio-energy balance method (BREB) during the growing season in 2005. During the growing season in 2005, total ET_c was 446.96 mm. From the beginning of growing season, the ET_c increased gradually, and reached its maximum value of 6.724 mm d⁻¹ in the last fifteen days of June. Hereafter the ET_c dropped rapidly, and reached its minimum value of 1.215 mm d⁻¹ at the end of growing season. The variation pattern of crop coefficient (K_c) was similar to that of ET_c. From the beginning of growing season, the K_c value increased rapidly, and reached its maximum value of 0.623 in the last fifteen days of June. Afterward, with slowing growth of P. euphratica, the value dropped rapidly to the end of growing season. According to this study, the ET_c of P. euphratica forest is affected not only by meteorological factors, but by water content in soil.     

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.     

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.     

Water use efficiency and crop coefficients of dry season oilseed crops

Eastern India receives higher average annual rainfall (1000–2000 mm) but 80% of it occurs within the June–September (rainy season), whereas the winter season (November–March) is dry. Due to a shortage of soil moisture, most rainfed areas of the region remain fallow during the winter season and cultivation (mainly rice) is confined to the rainy season only (June–September). To explore the possibility of double cropping in the rainfed rice areas, three oilseed crops, viz., linseed (Linum usitatissimum L.), safflower (Carthamous tinctorious L.), mustard (Brassica juncea L.), were grown in a representative rainfed area of eastern India, i.e. Dhenkanal, Orissa, during the dry/winter season by applying irrigation water at phonological stages. Study revealed that with three supplemental irrigations, the highest WUE was achieved by safflower followed by linseed with the mean values being 3.04 and 2.59 kg ha⁻¹ mm⁻¹, respectively. Whereas, with one irrigation, the highest water use efficiency (WUE) was achieved for safflower (1.23 kg ha⁻¹ mm⁻¹) followed by linseed (0.93 kg ha⁻¹ mm⁻¹). Of the three crops studied, safflower withdrew maximum water followed by mustard and crops were shown to use 90–105 mm more water than linseed. With three irrigations, average maximum rooting depths were 1.66, 1.17 and 0.67 m for safflower, mustard and linseed, respectively, which were 13.5, 10.6 and 11.4% higher than for single irrigated crops because of more wet sub soils and decrease of soil strength. The crop growth parameters like leaf area, dry biomass were also recorded with different levels of irrigation. The research work amply revealed the potential of growing these low water requiring oilseed crops in rice fallow during dry/winter season utilizing limited irrigation from harvested rainwater of rainy season. Crop coefficients (K_c) of three winter season oilseed crops were derived using field water balance approach. Study showed that LAI was significantly correlated with K_c values with the R² values of 0.91, 0.89 and 0.94 in linseed, safflower and mustard, respectively. When LAI exceeded 3.0, the K_c value was 1 in safflower and mustard whereas in linseed corresponding LAI was 2.5. Study revealed that the K_c values for the development and mid season stage were slightly higher to that obtained by the procedure proposed by FAO, which might be due to local advection.     

Increasing water productivity of irrigated crops under limited water supply at field scale

Borkhar district is located in an arid to semi-arid region in Iran and regularly faces widespread drought. Given current water scarcity, the limited available water should be used as efficient and productive as possible. To explore on-farm strategies which result in higher economic gains and water productivity (WP), a physically based agrohydrological model, Soil Water Atmosphere Plant (SWAP), was calibrated and validated using intensive measured data at eight selected farmer fields (wheat, fodder maize, sunflower and sugar beet) in the Borkhar district, Iran during the agricultural year 2004-2005. The WP values for the main crops were computed using the SWAP simulated water balance components, i.e. transpiration T, evapotranspiration ET, irrigation I, and the marketable yield Y_M in terms in terms of Y_MT⁻¹, Y_M ET⁻¹ and Y_M I⁻¹.The average WP, expressed as $ T⁻¹ (US $ m⁻³) was 0.19 for wheat, 0.5 for fodder maize, 0.06 for sunflower and 0.38 for sugar beet. This indicated that fodder maize provides the highest economic benefit in the Borkhar irrigation district. Soil evaporation caused the average WP values, expressed as Y_M ET⁻¹ (kg m⁻³), to be significantly lower than the average WP, expressed as Y_MT⁻¹, i.e. about 27% for wheat, 11% for fodder maize, 12% for sunflower and 0.18 for sugar beet. Furthermore, due to percolation from root zone and stored moisture content in the root zone, the average WP values, expressed as Y_MI⁻¹ (kg m⁻³), had a 24-42% reduction as compared with WP, expressed as Y_M ET⁻¹.The results indicated that during the limited water supply period, on-farm strategies like deficit irrigation scheduling and reduction of the cultivated area can result in higher economic gains. Improved irrigation practices in terms of irrigation timing and amount, increased WP in terms of Y_MI⁻¹ (kg m⁻³) by a factor of 1.5 for wheat and maize, 1.3 for sunflower and 1.1 for sugar beet. Under water shortage conditions, reduction of the cultivated area yielded higher water productivity values as compared to deficit irrigation.     

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.     

Yield and quality response of drip irrigated broccoli (Brassica oleracea L. var. italica) under different irrigation regimes, nitrogen applications and cultivation periods

The experiment aimed at evaluating the yield and quality response of broccoli (Brassica oleracea L. var. italica) to applied irrigation water and nitrogen by drip irrigation method during the spring and autumn cultivation periods of 2007. Irrigation water was applied based on a ratio of Class A pan evaporation (k_cp = 0.50, 0.75, 1.00 and 1.25) with 7 days interval. Also, the effect of four nitrogen levels (0 kg ha⁻¹, 150 kg ha⁻¹, 200 kg ha⁻¹ and 250 kg ha⁻¹) was compared with each treatment. The seasonal evapotranspiration in the treatments varied from 233 mm to 328 mm during the spring period and from 276 mm to 344 mm during the autumn period. The highest broccoli yield was obtained in the spring period as 11.02 t ha⁻¹ and in the autumn period as 4.55 t ha⁻¹. In general, there were statistical differences along nitrogen does with respect to yield and yield components while there were no statistically significant differences in the yield and yield components among irrigation regimes. Both yield and yield parameters in the spring period were found to be higher than that of the autumn period due to the low temperature and high rainy days in autumn. Irrigation water use efficiency (IWUE) ranged from 3.78 kg m⁻³ to 14.61 kg m⁻³ during the spring period and from 1.89 kg m⁻³ to 5.93 kg m⁻³ during the autumn period. On the other hand, nitrogen use efficiency (NUE) changed as 37.32-73.13% and 13.08-22.46% for spring and autumn season, respectively.     

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.     

Corn crop response under managing different irrigation and salinity levels

Non-uniformity of water distribution under irrigation system creates both deficit and surplus irrigation areas. Water salinity can be hazard on crop production; however, there is little information on the interaction of irrigation and salinity conditions on corn (Zea Mays) growth and production. This study evaluated the effect of salinity and irrigation levels on growth and yield of corn grown in the arid area of Egypt. A field experiment was conducted using corn grown in northern Egypt at Quesina, Menofia in 2009 summer season to evaluate amount of water applied, salinity hazard and their interactions. Three salinity levels and five irrigation treatments were arranged in a randomized split-plot design with salinity treatments as main plots and irrigation rates within salinity treatments. Salinity treatments were to apply fresh water (0.89 dS m⁻¹), saline water (4.73 dS m⁻¹), or mixing fresh plus saline water (2.81 dS m⁻¹). Irrigation treatments were a ratio of crop evapotranspiration (ET) as: 0.6ET, 0.8ET, 1.0ET, 1.2ET, and 1.4ET. In well-watered conditions (1.0ET), seasonal water usable by corn was 453, 423, and 380 mm for 0.89EC, 2.81EC and 4.73EC over the 122-day growing season, respectively. Soil salt accumulation was significantly increased by either irrigation salinity increase or amount decrease. But, soil infiltration was significantly decreased by either salinity level or its interaction with irrigation amount. Leaf temperature, transpiration rate, and stomata resistance were significantly affected by both irrigation and salinity levels with interaction. Leaf area index, harvest index, and yield were the greatest when fresh and adequate irrigation was applied. Grain yield was significantly affected in a linear relationship (r² ≥ 0.95) by either irrigation or salinity conditions with no interaction. An optimal irrigation scheduling was statistically developed based on crop response for a given salinity level to extrapolate data from the small experiment (uniform condition) to big field (non-uniformity condition) under the experiment constraints.     

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.     

Different drip irrigation regimes affect cotton yield, water use efficiency and fiber quality in western Turkey

Decreasing in water availability for cotton production has forced researchers to focus on increasing water use efficiency by improving either new drought-tolerant cotton varieties or water management. A field trial was conducted to observe the effects of different drip irrigation regimes on water use efficiencies (WUE) and fiber quality parameters produced from N-84 cotton variety in the Aegean region of Turkey during 2004 and 2005. Treatments were designated as full irrigation (T₁₀₀, which received 100% of the soil water depletion) and those that received 75, 50 and 25% of the amount received by treatment T₁₀₀ on the same day (treatments T₇₅; T₅₀ and T₂₅, respectively). The average seasonal water use values ranged from 265 to 753 mm and the average seed cotton yield varied from 2550 to 5760 kg ha⁻¹. Largest average cotton yield was obtained from the full irrigation treatment (T₁₀₀). WUE ranged from 0.77 kg m⁻³ in the T₁₀₀ to 0.98 kg m⁻³ in the T₂₅ in 2004 growing season and ranged from 0.76 kg m⁻³ in the T₁₀₀ to 0.94 kg m⁻³ in the T₂₅ in 2005 growing season. The largest irrigation water use efficiency (IWUE) was observed in the T₂₅ (1.46 kg m⁻³), and the smallest IWUE was in the T₁₀₀ treatment (0.81 kg m⁻³) in the experimental years. A yield response factor (k_y) value of 0.78 was determined based on averages of two years. Leaf area index (LAI) and dry matter yields (DM) increased with increasing water use for treatments. Fiber qualities were influenced by drip irrigation levels in both years. The results revealed that well-irrigated treatments (T₁₀₀) could be used for the semi-arid climatic conditions under no water shortage. Moreover, the results also demonstrated that irrigation of cotton with drip irrigation method at 75% level (T₇₅) had significant benefits in terms of saved irrigation water and large WUE indicating a definitive advantage of deficit irrigation under limited water supply conditions. In an economic viewpoint, 25.0% saving in irrigation water (T₇₅) resulted in 34.0% reduction in the net income. However, the net income of the T₁₀₀ treatment is found to be reasonable in areas with no water shortage.