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.     

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%.     

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.     

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.     

Determination of the crop coefficient for grafted ‘Tahiti’ lime trees and soil evaporation coefficient of Rhodic Kandiudalf clay soil in Sao Paulo,Brazil

The expansion of permanent trickle irrigation systems in Sao Paulo (Brazil) citrus has changed the focus of irrigation scheduling from determining irrigation timing to quantifying irrigation amounts. The water requirements of citrus orchards are difficult to estimate, since they are influenced by heterogeneous factors such as age, planting density and irrigation system. In this study, we estimated the water requirements of young ‘Tahiti’ lime orchards, considering the independent contributions from soil evaporation and crop transpiration by splitting the crop coefficient (Kc = ETc/ETo) into two separate coefficients; Ke, a soil evaporation coefficient and Kcb, a crop transpiration coefficient. Hence, the water requirement in young ‘Tahiti’ lime (ET_y) is ET_y = (Ke + Kcb) · ETo, where ETo is the reference crop evapotranspiration. Mature tree water requirement (ET_m) is ET_m = Kcb · ETo, assuming no soil water evaporation. Two lysimeters were used; one was 1.6 m in diameter and 0.7 m deep, and the other was 2.7 m in diameter and 0.8-m deep. The first one was used to calculate evaporation and the second one was used for transpiration. ETo was estimated by the Penman–Monteith method (FAO-56). The measurements were conducted during a period between August 2002 and April 2005 in Piracicaba, Sao Paulo state, Brazil. The lysimeters were installed at the center of a 1.0-ha plot planted with ‘Tahiti’ lime trees grafted on ‘Swingle’ citrumelo rootstock. The trees were 1-year old at planting, spaced 7 × 4 m, and were irrigated by a drip irrigation system. During the study period, Kc varied between 0.6 and 1.22, and Kcb varied between 0.4 and 1.0. The results suggested that for young lime trees, the volume of water per tree calculated by Ke + Kcb is about 80% higher than the volume calculated using Kc. For mature trees, the volume of water per tree calculated using just Kcb can be 10% less than using Kc. The independent influence of soil evaporation and transpiration is important to better understand the water consumption of young lime trees during growth compared to mature lime trees.     

Sprinkler irrigation scheduling of winter wheat in the North China Plain using a 20 cm standard pan

Based on evaporation from a 20 cm diameter pan placed above the crop canopy, sprinkler irrigation scheduling of winter wheat was studied in the North China Plain (NCP) in the 2001–2004 winter wheat seasons. Results showed that pan evaporation (E _pan,C) was closely related to actual evapotranspiration (ET) measured using weighing lysimeters. The combined pan–crop coefficient (K _c,pan), the ratio of ET to E _pan,C, was closely related to leaf area index (LAI ) and plant height. Data from the 2002–2003 season were used to establish the relationships between K _c,pan and LAI (method A) or plant height (method B), and used to determine the crop coefficient (method C). ET computed by the three methods was compared with measured ET using lysimeters in the 2001–2002 and 2003–2004 seasons. Mean relative error of estimated daily ET by the three methods ranged from 20 to 30%, and the relative error in cumulative ET in the experimental periods ranged from 1 to 19%. Among the three methods, results from methods A and B were not significantly different from each other (P > 0.01), and were closer to the lysimeter data than results from method C (P < 0.001). Method B, being easier to measure, was recommended for ET estimation in NCP.     

Methods for estimating irrigation needs of spring wheat in the middle Heihe basin,China

Understanding reference crop evapotranspiration (ET₀) is essential in planning the most effective use of water resources in the arid northwest China. The objective of the present work in the middle Heihe River basin were: (1) to determine the best model for calculating the areal distribution of reference crop evapotranspiration in this region, and (2) to estimate the spatial distribution of the irrigation requirements of spring wheat. Note that eight commonly used formulates were tested and that FAO-Penman was the best.The irrigation amount of spring wheat in 2000 was estimated by three steps. First, DEM-based and GIS-assisted methods were employed to estimate the spatial distribution of reference crop evapotranspiration (ET₀) according to FAO-Penman model. Then, spring wheat evapotranspiration (ET) was calculated by ET₀ and crop-coefficient (K_c). Finally, the maximum irrigation amount of spring wheat was estimated with the spring wheat evapotranspiration and precipitation in the different growing stage. The maximum irrigation has temporal–spatial variation. Temporally the irrigation amount appears the largest in June when it is the peak period of spring wheat development. The irrigation amount is the smallest in July because spring wheat was in late-season stage. In April, spring wheat was in seedling stage during which the water demand is also small. Spatially the irrigation amount increases from southeast to northwest.     

An evaluation of common pan coefficient equations to estimate reference evapotranspiration in a subtropical climate (north of Iran)

Accurate reference evapotranspiration (ET₀) data are essential to water resources project planning and farm irrigation scheduling. Evaporation pans are widely used to estimate reference ET₀. Via the pan coefficient (K _p), ET₀ is estimated from evaporation pan data. Four common K _p equations (Orang in Potential accuracy of the popular non-linear regression equations for estimating pan coefficient values in the original and FAO-24 tables, unpublished report, 1998; Allen and Pruitt in J Irrig Drain Eng 117(5):758–773, 1991; Cuenca in Irrigation system design: an engineering approach, p 133, 1989; Snyder in J Irrig Drain Eng 118(6):977–980, 1992) to calculate daily K _p coefficients to estimate ET₀ were evaluated using a 10-year mean climate dataset for a subtropical climate (north of Iran). Overall results showed that ET₀ calculated using the daily K _p values from Orang (Potential accuracy of the popular non-linear regression equations for estimating pan coefficient values in the original and FAO-24 tables, unpublished report, 1998) provided more accurate daily, monthly, and annual total estimates compared to the others equations.     

A model of crop yield response to irrigation water salinity: theory,testing and application

A relationship between crop yield and irrigation water salinity is developed. The relationship can be used as a production function to quantify the economic ramifications of practices which increase irrigation water salinity, such as disposal of surface and sub-surface saline drainage waters into the irrigation water supply system. Guidelines for the acceptable level of irrigation water salinity in a region can then be established. The model can also be used to determine crop suitability for an irrigation region, if irrigation water salinity is high. Where experimental work is required to determine crop yield response to irrigation water salinity, the model can be used as a first estimate of the response function. The most appropriate experimental treatments can then be allocated. The model adequately predicted crop response to water salinity, when compared with experimental data.Abbreviations A Crop threshold rootzone salinity in Equation of Maas and Hoffman (dS/m) - B Fractional yield reduction per unit rootzone salinity increase (dS/m)^–1 - C_i Average salinity of applied water (dS/m) - C_r Average salinity of rainfall (dS/m) - C_s Linearly averaged soil solution salinity in the rootzone (dS/m) - C_se Linearly averaged soil saturation extract salinity in the rootzone (dS/m) - C_w Average salinity of irrigation supply water (dS/m) - C_z Soil solution salinity at the base of the crop rootzone (dS/m) - C Mean root water uptake weighted soil salinity in equation of Bernstein and François (1973) (dS/m) - E_p Depth of class A pan evaporation during the growing season (m) - ET_a Actual crop evapotranspiration during the growing season (m) - ET_m Maximum crop evapotranspiration during the growing season (m) - I The total depth of water applied during the growing season (including irrigation water and rainfall) (m) - K Empirical coefficient in leaching equation of Rhoades (1974) - K_c Crop coefficient for equation of Doorenbos and Pruit (1977) to estimate crop water use - K_y Yield response factor in equation of Doorenbos and Kassam (1974) - LF The leaching fraction - Ro Depth of rainfall runoff during the growing season (m) - R Depth of rainfall during the growing season (m) - W Depth of irrigation water applied during the growing season (m) - Y Relative crop yield - Y_a Actual crop yield (kg) - Y_m Maximum crop yield (kg) - /z Dimensionless depth for equation of Raats (1974), and empirical coefficient for the leaching equation of Hoffman and van Genutchen (1983)     

The dual crop coefficient approach to estimate and partitioning evapotranspiration of the winter wheat–summer maize crop sequence in North China Plain

An accurate estimation of crop evapotranspiration (ET _c) is very useful for appropriate water management; hence, an accurate and user-friendly model is needed to support related irrigation decisions. In this view, a study was developed aimed at estimating the ET _c of winter wheat–summer maize crop sequence in the North China through eddy covariance measurements, to calibrate and validate the SIMDualKc model, to estimate the basal crop coefficients (K _cb) for both crops and to partition ET _c into soil evaporation and crop transpiration. Two years of field experimentation of that crop sequence were used to calibrate and validate the SIMDualKc model and to derive K _cb using eddy covariance measurements. Various indicators have shown the goodness of fit of the model, with estimated values very close to the observed ones and estimate errors close to 0.5 mm d^?1. The initial, mid-season and end basal crop coefficients for wheat were 0.25, 1.15 and 0.30, respectively, and those for maize were 0.15, 1.15 and 0.45, thus close to those proposed in FAO56 guidelines. The soil evaporation represented near 80 % of ET _c for the initial stages of winter wheat and summer maize and decreased to only 5–6 % during the mid-season period. Evaporation during the full crop season averaged 28 % for winter wheat and 40 % for summer maize. The importance of wetting frequency and crop ground coverage in controlling soil evaporation was evidenced.     

Crop coefficient and evaptranspiration of onion at Awash Melkassa, Central Rift Valley of Ethiopia

Improved water management through precise crop water requirement determination is needed to improve the efficiency of water use in agricultural production. As a result, appropriate irrigation scheduling which can lead to water saving, improvements in the yield and income can be designed. In this study, three non-weighing lysimeters having dimensions of 2 m × 1 m × 2 m were used to determine water requirement (ET_c) and crop coefficient (Kc) of onion (Bombay Red cultivar). Reference crop evapotranspiration (ET_o) was determined using weather data recorded at the site. The measured ET_c values were 51.3 mm, 140.5 mm, 144.8 mm, and 53.9 mm during the initial, development, mid-season and late season growth stages respectively. Crop coefficient (K_c) values, calculated as ratio of ET_c to ET_o, were 0.47, 0.99, and 0.46 during the initial and mid-season stages and end of late season. Furthermore, third-order polynomials were fitted well to predict the crop coefficient values as functions of growing degree-days (GDD).     

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.     

Energy balance and crop water stress in winter maize under phenology-based irrigation scheduling

In eastern India, cultivation of winter maize is getting popular after rainy season rice and farmers practice irrigation scheduling of this crop based on critical phenological stages. In this study, crop water stress index of winter maize at different critical stages wase determined to investigate if phenology-based irrigation scheduling could be optimized further. The components of the energy budget of the crop stand were computed. The stressed and non-stressed base lines were also developed (between canopy temperature and vapor pressure deficit) and with the help of base line equation, [(T _c − T _a) = −1.102 VPD − 3.772], crop water stress index (CWSI) was determined from the canopy-air temperature data collected frequently throughout the growing season. The values of CWSI (varied between 0.42 and 0.67) were noted just before the irrigations were applied at critical phenological stages. The soil moisture depletion was also measured throughout the crop growing period and plotted with CWSI at different stages. Study revealed that at one stage (silking), CWSI was much lower (0.42–0.48) than that of recommended CWSI (0.60) for irrigation scheduling. Therefore, more research is required to further optimize the phenology-based irrigation scheduling of winter maize in the region. This method is being used now by local producers. The intercepted photosynthetically active radiation and normalized difference vegetation index over the canopy of the crop were also measured and were found to correlate better with leaf area index.     

Varietal differences in the response of durum wheat (Triticum turgidum L. var. durum) to irrigation strategies in a semi-arid region of Algeria

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

Bahiagrass crop coefficients from eddy correlation measurements in central Florida

Bahiagrass (Paspalum notatum) is a warm-season grass used primarily in pastures and along highways and other low maintenance public areas in Florida. It is also used in landscapes to some extent because of its drought tolerance. Bahiagrass can survive under a range of moisture conditions from no irrigation to very wet conditions. Its well-watered consumptive use has not been reported previously. In this study, bahiagrass crop coefficients (K _c) for an irrigated pasture were determined for July 2003 through December 2006 in central Florida. The eddy correlation method was used to estimate crop evapotranspiration (ET_c) rates. The standardized reference evapotranspiration (ET_o) equation (ASCE-EWRI standardization of reference evapotranspiration task committee report, 2005) was applied to calculate ET_o values using on site weather data. Daily K _c values were estimated from the ratio of the measured ET_c and the calculated ET_o. The recommended K _c values for bahiagrass are 0.35 for January–February, 0.55 for March, 0.80 for April, 0.90 for May, 0.75 for June, 0.70 for July–August, 0.75 for September, 0.70 for October, 0.60 for November, and 0.45 for December in central Florida. The highest K _c value of 0.9 in May corresponded with maximum vapor pressure deficit conditions as well as cloud free conditions and the highest incoming solar radiation as compared to the rest of the year. During the summer (June to August), frequent precipitation events increased the cloud cover and reduced grass water use. The K _c annual trend was similar to estimated K _c values from another well-watered warm-season grass study in Florida.     

Variation in vineyard evapotranspiration in an arid region of northwest China

Grapevines are extensively grown in the arid region of China, but little information is available on the diurnal, seasonal and interannual variability of vineyard evapotranspiration (ET). To address this question, two vineyards in the arid region of northwest China were taken as an example to study the variation of ET using Bowen ratio-energy balance method in 2005-2008. Results indicate that the Bowen ratio method provided accurate estimate of vineyard ET as the instrument was correctly installed. Irrigation and rainfall increased daily ET by 38 and 175%, respectively, but frost decreased it by 32%. Daily ET had a maximum value of 1.6-3.5 mm/d at the berry development stage, and a minimum value of 0.8-1.7 mm/d at the early and later stages. The total ET was 226-399 mm over the growing season. The ratio of transpiration to evapotranspiration was 0.52 and the modified crop coefficient (K_cm) was 0.71-0.88 (except 2005) over the whole growing stage. Larger interannual difference of ET and K_cm mainly resulted from the difference of irrigation and rainfall between different years.     

Effect of irrigation method and quantity on squash yield and quality

Squash yield and quality under furrow and trickle irrigation methods and their responses to different irrigation quantities were evaluated in 2010 spring and fall growing seasons. A field experiment was conducted using squash (Cucurbita pepo L.) grown in northern Egypt at Shibin El Kom, Menofia. A randomized split-plot design was used with irrigation methods as main plots and different irrigation quantities randomly distributed within either furrow or trickle irrigation methods. Irrigation quantity was a fraction of crop evapotranspiration (ET_c) as: 0.5, 0.75, 1.0, 1.25, and 1.5 ET_c. Each treatment was repeated three times, two of five rows from each replicate were left for squash seed production. In well-watered conditions (1.0 ET_c), seasonal water use by squash was 304 and 344 mm over 93 days in spring and 238 and 272 mm over 101 days in fall under trickle and furrow irrigation methods, respectively. Squash fruit yield and quality were significantly affected by season and both irrigation method and quantity. Fruit number and length were not affected by irrigation method and growing season, respectively. Interaction between season and irrigation quantity significantly affected leaf area index, total soluble solid (TSS), and fruit weight. Moreover, seed yield and quality were significantly affected by growing season and both irrigation method and quantity except harvest index, which was not affected by irrigation method. Significant differences for the interaction between season and irrigation method were only found for seed yield and 100 seeds weight. Except for harvest index, no significant difference was observed by interaction between season and irrigation quantity. Both fruit and seed yields were significantly affected in a linear relationship (r² ≥ 0.91) by either deficit or surplus irrigation quantities under both irrigation methods. Adequate irrigation quantity under trickle irrigation, relative to that of furrow, enhanced squash yield and improved its quality in both growing seasons. Fall growing season was not appropriate for seed production due to obtaining many of empty seeds caused by low weather variables at the end of the season. The results from small experiment were extrapolated to large field to find out optimal irrigation scheduling under non-uniform of irrigation application.     

Sensitivity analysis of FAO 33 crop water production function

This paper presents the results of sensitivity analysis of FAO 33 method and its modified forms that were developed by Rao et al. (Agric Water Manage 13:25–32, 1988). Results show that positive error of PET and K_y and negative error of AET result in over predicting of relative yield (Y_r). Sensitivity of Doorenbos and Kassam (FAO irrigation and drainage paper No. 33. FAO, Rome, Italy, 193 pp, 1979) equation (FAO 33 method) and additive form of Rao et al. (Agric Water Manage 13:25–32, 1988) equation is equal for positive or negative error of K_y and AET, but their sensitivities are greater for negative error of PET than positive error. However, sensitivity of multiplicative form of Rao et al. (Agric Water Manage 13:25–32, 1988) equation is greater for negative error of K_y and PET and positive error of AET. Error percentage on estimation of Y_r by multiplicative form of Rao et al. (Agric Water Manage 13:25–32, 1988) equation arising from error of PET, AET, or K_y is less than additive form. In addition, calculated Y_r by multiplicative equation is higher than additive form and the difference between two forms of this equation increases severely when water shortage increases. According to the results, it is recommended that multiplicative form of Rao et al. (Agric Water Manage 13:25–32, 1988) equation instead of additive form be used in optimization models and deficit irrigation planning.     

The effects of applied water at various fractions of measured evapotranspiration on reproductive growth and water productivity of Thompson Seedless grapevines

The reproductive growth and water productivity (WP_b) of Thompson Seedless grapevines were measured as a function of applied water amounts at various fractions of measured grapevine ET_c for a total of eight irrigation treatments. Shoots were harvested numerous times during the growing season to calculate water productivity. Berry weight was maximized at the 0.6–0.8 applied water treatments across years. As applied water amounts increased soluble solids decreased. Berry weight measured at veraison and harvest was a linear function of the mean midday leaf water potential measured between anthesis and veraison and anthesis and harvest, respectively. As applied water amounts increased up to the 0.6–0.8 irrigation treatments there was a significant linear increase in yield. Yields at greater applied water amounts either leveled off or decreased. The reduction in yield on either side of the yearly maximum was due to fewer numbers of clusters per vine. Maximum yield occurred at an ET_c ranging from 550 to 700 mm. Yield per unit applied water and WP_b increased as applied water decreased. The results from this study demonstrated that Thompson Seedless grapevines can be deficit irrigated, increasing water use efficiency while maximizing yields.     

Use of a water deficit sensitivity index for partial irrigation scheduling of wheat and barley

Shortage of water is the most important limiting factor for crop production in the arid and semi-arid regions in Iran. More land can become productive by using partial irrigation at strategic times during the growing season. This may be accomplished if a proper index of crop sensitivity to water deficit at various growth stages is used. A theoretical procedure was applied to determine the savings in water and the economic benefit derived from partial irrigation, using a water stress sensitivity index for winter wheat (Triticum aestivum L.) and spring barley (Hordeum vulgare L.) in a dry region of Fars province in Iran. The results indicated that some water reduction is possible. In general, the suggested maximum allowable water reduction was unreasonably high for the sensitivity index (λ _i ) proposed by Nairizi and Rydzewski. Their index is not applicable in arid and semi-arid areas. However, the field-derived λ _i of Aryan resulted in a reasonable water reduction close to the field-applied water reduction to achieve a corresponding relative yield. Therefore, appropriate values of λ _i for different climatic conditions should be used to calculate rational water reductions. The maximum allowable water reduction for spring barley was higher than that for winter wheat. The reduction increased as the benefit to cost ratio (B/C) was increased. Water reductions of 7 and 26% were allowed for winter wheat and spring barley, respectively, at a B/C ratio of 1.5. This corresponded to an 8 and 35% increase in cultivated area, respectively. These results need to be validated in more extensive field experimentation. Received: 9 December 1994