Salt sensitivity of cowpea at various growth stages

Summary The relative salt tolerance of cowpea (Vigna unguiculata (L.) Walp. cv. California Buckeye No. 5) at different stages of growth was determined in a greenhouse. Plants were grown in sand cultures that were irrigated four times daily with modified half-strength Hoagland's solution. Salination with NaCl and CaCl₂ (2:1 molar ratio) provided seven treatment solutions with osmotic potentials (_s) ranging from –0.05 to –1.05 MPa (electrical conductivities of 1.4 to 28 dS/m). Salt stress was imposed for 20 days beginning at either 7, 27, or 52 days after planting. The three 20-day stages are referred to here as vegetative, flowering, and pod filling stages. Pod and seed yields from plants stressed during either the vegetative, flowering, or pod-filling stages indicated that cowpea was the most sensitive to salinity during the vegetative stage and became less sensitive the later plants were stressed. Seed yield was reduced 50% at _s =–0.45, –0.76, and –0.88 MPa for plants salinized during the vegetative, flowering, and pod-filling stages, respectively. Salinity reduced seed yield by reducing seed number; it had little, if any, effect on the weight of individual seeds. Vegetative growth was significantly reduced by salt stress during all three stages but the effect was much less when stress was imposed during the last two stages than during the first stage.     

Salt sensitivity of wheat at various growth stages

Summary The relative salt tolerance of two wheat species (Triticum aestivum L., cv. Probred and Triticum turgidum L., Durum Group, cv. Aldura) at different stages of growth was determined in a greenhouse experiment. Plants were grown in sand cultures that were irrigated four times daily with modified Hoagland's solution. Salinization with NaCl and CaCl₂ (2:1 molar ratio) provided seven treatment solutions with osmotic potentials ( _s ) ranging from –0.05 to –1.25 MPa (electrical conductivities of 1.4 to 28 dS/m). Salt stress was imposed for 45 days beginning at either 10, 56, or 101 days after planting. The three 45-day stages are referred to here as the vegetative, reproductive, and maturation stages although the first stage included spikelet differentiation. In a separate experiment, seedling growth was measured after 21 days of salt stress ( _s = –0.05 to –0.85 MPa) initiated at 0, 7, 11, and 16 days after planting. Salt stress ( _s = –0.65 MPa) delayed germination by 4 days for both wheats but full emergence occurred. Relative growth response curves of the seedlings were alike regardless of whether salt stress was imposed at planting or at the 1st, 2nd, or 3rd-leaf stage of growth. Salt stress also retarded leaf development and tillering but hastened plant maturity. Grain yields from plants stressed during either the vegetative, reproductive, or maturation stages indicated that both species became less sensitive to salinity the later plants were stressed. Grain yield was reduced 50% at _s = –0.76, –1.53, and –1.58 MPa for Probred and –0.65, –1.08, and –1.34 MPa for Aldura when salinized during stages 1, 2, and 3, respectively. Salinity reduced grain yield by reducing seed number more than seed weight indicating that salt stress during stage 1 affected spikelet differentiation. Straw yield was significantly reduced by salt stress only during stage 1. Leaf mineral analyses revealed that Aldura readily accumulated Na whereas Probred did not. Both species accumulated Cl but the concentrations were much higher in Aldura. K uptake was severely inhibited by salt stress imposed during the first stage but not when imposed the second stage.     

The effect of sowing date,irrigation and cultivar on the growth and yield of wheat in the Namoi River Valley,New South Wales

Summary A factorial experiment which examined the effects of sowing date, cultivar and irrigation frequency on the growth and grain yield of irrigated wheat was conducted at Narrabri, New South Wales. Irrigation scheduling was based on morning values of leaf water potentials (_l): plots were watered when _l, had fallen to either –0.8 MPa or –0.4 MPa or were not irrigated during the season.Maximum leaf areas, tiller numbers and total dry matter production were increased by more frequent irrigation, but subsequent tiller death and leaf senescence were generally not reduced by increasing watering. A delay in sowing from 23 June to 23 July reduced yields by 20%, on average. More frequent irrigation increased yields at both sowing dates, but a high protein, locally bred wheat (Songlen) responded less than a cultivar derived from the CIMMYT program (WW 15). The highest yield for Songlen was 570 g m^–2 which was lower than the highest yield for WW 15 (730 g m^–2); both were obtained from the –0.4 MPa treatment sown on 23 June. Compared with irrigated wheat grown in Mexico or southern New South Wales, dry matter production after anthesis at Narrabri was low. It was suggested that high temperatures after anthesis may limit post-anthesis productivity and subsequently, grain yields. The results of this experiment suggested that yields of irrigated wheat in the lower Namoi Valley can be improved through better irrigation management and varietal improvement, but the magnitude of this response may be limited by high spring temperatures.     

Evaluation of the Penman-Monteith model for estimating soybean evapotranspiration

The Penman-Monteith model with a variable surface canopy resistance (r_cv) was evaluated to estimate hourly and daily crop evapotranspiration (ET_c) over a soybean canopy for different soil water status and atmospheric conditions. The hourly values of r_cv were computed as a function of environmental variables (air temperature, vapor pressure deficit, net radiation) and a normalized soil water factor (F), which varies between 0 (wilting point, _WP) and 1 (field capacity, _FC). The performance of the Penman-Monteith model (ET_PM) was evaluated using hourly and daily values of ET_c obtained from the combined aerodynamic method (ET_R). On an hourly basis, the overall standard error of estimate (SEE) and the absolute relative error (ARE) were 0.06 mm h^–1 (41 W m^–2) and 4.2%, respectively. On a daily basis, the SEE was 0.47 mm day^–1 and the ARE was 2.5%. The largest disagreements between ET_PM and ET_R were observed, on the hourly scale, under the combined influence of windy and dry atmospheric conditions. However, this did not affect daily estimates, since nighttime underestimations cancelled out daytime overestimations. Thus, daily performances of the Penman-Monteith model were good under soil water contents ranging from 0.31 to 0.2 (_FC and _WP being 0.33 and 0.17, respectively) and LAI ranging from 0.3 to 4.0. For this validation period, calculated values of r_cv and F ranged between 44 s m^–1 and 551 s m^–1 and between 0.19 and 0.88, respectively.Communicated by R. Evans     

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

A study was conducted to determine the relationship between midday measurements of vine water status and daily water use of grapevines measured with a weighing lysimeter. Water applications to the vines were terminated on August 24th for 9 days and again on September 14th for 22 days. Daily water use of the vines in the lysimeter (ET_LYS) was approximately 40 L vine⁻¹ (5.3 mm) prior to turning the pump off, and it decreased to 22.3 L vine⁻¹ by September 2nd. Pre-dawn leaf water potential (Ψ_PD) and midday Ψ_l on August 24th were −0.075 and −0.76 MPa, respectively, with midday Ψ_l decreasing to −1.28 MPa on September 2nd. Leaf g _s decreased from ~500 to ~200 mmol m⁻² s⁻¹ during the two dry-down periods. Midday measurements of g _s and Ψ_l were significantly correlated with one another (r = 0.96) and both with ET_LYS/ET_o (r = ~0.9). The decreases in Ψ_l, g _s, and ET_LYS/ET_o in this study were also a linear function of the decrease in volumetric soil water content. The results indicate that even modest water stress can greatly reduce grapevine water use and that short-term measures of vine water status taken at midday are a reflection of daily grapevine water use.     

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)     

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

A field experiment was carried out to evaluate the METRIC (mapping evapotranspiration at high resolution with internalized calibration) model to estimate the actual evapotranspiration (ET_a) and crop coefficient (K _c) of a drip-irrigated Merlot vineyard during the 2007/2008 and 2008/2009 growing seasons. The Merlot vineyard located in the Talca Valley (Chile) was trained on a vertical shoot positioned system. The performance of METRIC was evaluated using measurements of ET_a and K _c from an eddy covariance (EC) system. METRIC overestimated ET_a by about 9?% with a root mean square error (RMSE) and mean absolute error (MAE) of 0.62 and 0.50?mm?d^?1, respectively. For the main phenological stages of the Merlot vineyard, METRIC overestimated the K _c by about 10?% with RMSE?=?0.10 and MAE?=?0.08. Furthermore, the indexes of agreement were 0.70 for K _c and 0.85 for ET_a. Mean values of K _c measured from EC were 0.41, 0.53, 0.56, and 0.46, while those estimated by METRIC were 0.46, 0.54, 0.59, and 0.62 for the bud break to flowering, flowering to fruit set, fruit set to veraison, and veraison to harvest stages, respectively.     

Evaluation of plant-based water status indicators in mature apple trees under field conditions

The performance of different indicators of plant water status as a tool for irrigation management was evaluated in mature field grown ‘Golden Delicious’ apple trees during the late summer of 1998. Control (C) and stress (S) treatments were studied. In the C treatment trees were irrigated daily at 100% ET_c whereas in the S treatment water was withheld during 31 days (DOY’s 236–266). Predawn water potential (Ψ_pd) and midday stem water potential (Ψ_stem) were measured several times a week during the experimental period. Three daily measurements of stomatal conductance (g_s) and stem water potential were made during five consecutive days in mid-September. Trunk diameter changes (TDC) were recorded by LVDT sensors, and from these measurements, maximum daily shrinkage (MDS), daily growth (DG), and cumulative growth (CG) were calculated. Midday Ψ_stem showed the best ratio between the response to moderate water stress and tree variability (“signal/noise” ratio) among the indicators studied here, followed closely by Ψ_pd. On the other hand, the poorest water status indicator was g_s. Due to the low trunk growth rate of the trees, and its high variability, DG and CG were not adequate indicators. MDS showed a lower sensitivity to water stress and a higher variability (CV = 0.19) than midday Ψ_stem (CV = 0.08) and Ψ_pd (CV = 0.10). However, MDS correlated well with ET₀ and with midday Ψ_stem (R ² = 0.79) thus, making this parameter an interesting and promising tool for irrigation management in apple orchards. More research needs to be done in order to define reference values for MDS and plant water potential indicators, in relation to evaporative conditions and in different phenological periods, and to quantify the relationship between water status indicators values and apple tree yield and fruit quality.     

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.     

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.     

Yield sensitivities of short season soybeans to irrigation management

Summary Field experiments were conducted for three seasons on a loam textured soil in East-central North Dakota. Soybean seed yield responses were measured for three cultivars (maturity groups 00 and 0) as a function of water management treatments. Water stress development per treatment was generally limited to one of three growth periods, planting to first bloom (R1), R1 to full seed (R6), or R6 to physiologic maturity (R8). Stress levels were defined as low (L), moderate (M) or severe (S) and were based on the degree of root zone water depletion and/or an allowable depression of leaf water potential in midafternoon before stress relief by irrigation. Yield vs seasonal ET relationships were linear. Mean slope of the regression based functions (per three cultivars) was 1.01 kg/m³. Maximum seasonal seed yields were generally produced with an irrigation regime that maintained low stress levels in each growth period. Results when normalized as relative yield (Y/Y_m) vs relative seasonal evapotranspiration (ET/ET_m) indicated an average slope of 1.26% yield loss per 1% decline in seasonal ET from the ET_m level. Average observed Y/Y_m ratios were 0.57 for dryland/non-irrigated treatments, 0.88 for MLL (i.e., a moderate stress in planting to R1 growth period followed by the maintenance of low stress levels in the R1 to R6 and R6 to physiologic maturity growth periods), 0.87 for LML, 0.80 for LSL, 0.91 for LLM and 0.97 for LLL stress sequences. Stress effects were most detrimental to yield if imposed in the R4 (full pod) to R6 (full seed) period. The average Y_m yields were 3,266, 2,937 and 3,248 kg/ha for McCall, Maple Amber and Ozzie cultivars, respectively. Yield reductions of less than 5% from Y_m levels appear likely in this climatic setting if remaining available water levels are maintained above 40–50%. Depressions of midafternoon leaf water potentials to about-1.20 MP_a were estimated as the threshold at which seed yield declines from potential levels. Relative yields vs relative irrigation (IR/IR_m) relationships suggest that irrigation amounts could be reduced by 20% from IR_m levels with MLL and LLM regimes for less than a 5% yield reduction in average rainfall seasons. In very dry seasons this yield reduction could result with about a 10% reduction in seasonal irrigation.This work was supported by ND Agr. Exp. Sta. Proj. 1442 and by funds provided by the U.S. Dept. of Interior, USBR     

Cotton root and shoot responses to subsurface drip irrigation and partial wetting of the upper soil profile

The ability of cotton roots to grow downwards through a partially-wetted soil (Calcic Haploxeralf) profile toward a water source located beneath them was investigated. Plants were grown in 60-cm-high soil columms (diameter 10 cm), the bottom 15 cm of which was kept wet by frequent drip irrigation, while the upper 45 cm was wetted three times per week up to 20, 40, 60, 80 or 100% of pot capacity. Pot capacity was defined as the water content which gave uniform distribution within the pot and was at a soil matric potential ( _m ) of –0.01 MPa. Plants were harvested 42 and 70 days after emergence (DAE). Root length density was reduced by decreased soil moisture content. At 42 DAE, density was reduced in the soil profile down to 36 cm. The density in the middle segment of the cylinder (24–36 cm) increased at the second harvest, from 0.1 to 0.35 cm · cm^–3 at 40% and from 0.2 to 0.5 cm · cm^–1 at 60% of pot capacity, respectively. A significant rise in root length density was found at all moisture contents above 20% in the two deepest soil segments. It was most marked at 40% where the rise was from 0.2 to 0.8 cm · cm^–3, due to the development of secondary roots at the wetted bottom of the column. When only 20% of pot capacity was maintained in the top 45 cm of the profile, almost no roots reached the wetted soil volume, and root length density was very low. Hydrotropism, namely root growth through dry soil layers toward a wet soil layer was thus not apparent. Root dry weight per unit length decreased with increasing depth in the column at all moisture levels. However, the only significant decrease was, found between the top and the second soil segments and was due to thicker primary roots in the top segment. There was no clear relationship between length and dry weight of roots. Total plant dry weight and transpiration were reduced significantly only at 20% of pot capacity. Dry matter production by roots was less severely inhibited than that by shoots, under decreased moisture content in the soil profile. Leaf water potential decreased when the soil moisture content of the top 45 cm of the profile was reduced below 60% of pot capacity. It was concluded that even at soil moisture content equivalent to a _m of 0.1 MPa, the rate of root growth was sufficient to reach a wetted soil layer at the bottom of the soil column, where the plant roots then proliferated. This implies that as long as the soil above the subsurface dripper is not very dry there is no real need for early surface irrigation.     

Components of the water balance in soil with sugarcane crops

The objective of this study was to analyze the components of the water balance in an Ultisol, located in the municipality of Jaboticabal, SP, Brazil (21°20′20″S, 48°18′35″W), that was cultivated with sugarcane. The monitoring was performed during the agricultural cycle of the first ratoon between 11/16/2006 and 7/9/2007. Three treatments were established in four blocks with doses of ammonium sulfate, as follows: Treatment 1 (T₁), without fertilizer; Treatment 2 (T₂), 100 kg ha⁻¹ of nitrogen (N) and 114 kg ha⁻¹ of sulfur (S); and Treatment 3 (T₃), 150 kg ha⁻¹ of N and 172 kg ha⁻¹ of S. Rainy precipitation (P) in the area was measured with a rain gauge. The soil water storage (H) and the soil water storage variations (ΔH) were determined by the gravimetric method, and the internal drainage (D)/capillary rise (CR) at a depth of 0.9 m was quantified by the water flux density using the Darcy–Buckingham equation. The actual evapotranspiration (ET_a) was calculated as follows: ET_a = P − D + CR ± ΔH. During the study period, the amount of rainfall was 1406 mm, 121 mm greater than the historic average for the region (1285 mm), with a notable peak in the month of January of 402 mm (historic average: 251 mm). The internal drainage was 300 mm under T₁, 352 mm under T₂, and 199 mm under T₃, and this was relevant during times with elevated P, when the actual H was greater than the field capacity H. The actual evapotranspiration (T₁: −897.7 mm, T₂: −847.5 mm, and T₃: −970.8 mm) and the water use efficiency (T₁: −131.3 kg mm⁻¹, T₂: −146.6 kg mm⁻¹, and T₃: −127.5 kg mm⁻¹) did not differ among the treatments. The dispersion of D was greater than the other components of the water balance, especially during the period of elevated P, with the errors of this process propagated in the estimation of ET_a. Despite of this propagated standard deviation of ET_a, it accounted less than 15% of the total ET_a, showing that the method may be conveniently used in field studies with sugarcane crops.     

Estimation of actual evapotranspiration for a drip-irrigated Merlot vineyard using a three-source model

A study was performed in order to evaluate the three-source model (Clumped model) for direct estimation of actual evapotranspiration (ET_a) and latent heat flux (LE) over a drip-irrigated Merlot vineyard trained on a vertical shoot positioned system (VSP) under semi-arid conditions. The vineyard, with an average fractional cover of 30%, is located in the Talca Valley, Region del Maule, Chile. The performance of the Clumped model was evaluated using an eddy covariance system during the 2006/2007 and 2007/2008 growing seasons. Results indicate that the Clumped model was able to predict ET_a with a root mean square error (RMSE), mean bias error (MBE), and model efficiency (EF) of 0.33, −0.15 mm day⁻¹ and 74%, respectively. Also, the Clumped model simulated the daytime variation of LE with a RMSE of 36 W m⁻², MBE of −8 W m⁻², and EF of 83%. Major disagreement (underestimated values) between observed and estimated values of ET_a was found for clear days after rainfall or foggy days, but underestimated values were less than 10% of the data analysis. The results obtained in this study indicate that the Clumped model could be used to directly estimate vine water requirements for a drip-irrigated vineyard trained on a VSP. However, application of the Clumped model requires a good characterization of the drip-irrigated vineyard architecture.     

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

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

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     

The effect of soil surface temperature on the crop water stress index

Summary A coupled soil-vegetation energy balance model which treats the canopy foliage as one layer and the soil surface as another layer was validated againt a set of field data and compared with a single-layer model of a vegetation canopy. The two-layer model was used to predict the effect of increases in soil surface temperature (T _s ) due to the drying of the soil surface, on the vegetation temperature (T _v ). In the absence of any change in stomatal resistance the impact of soil surface drying on the Crop Water Stress Index (CSWI) calculated from T _v was predicted. Field data came from a wheat crop growing on a frequently irrigated plot (W) and a plot left un watered (D) until the soil water depletion reached 100 mm. Vegetation and soil surface temperatures were measured by infrared thermometers from tillering to physiological maturity, with meteorological variables recorded simultaneously. Stomatal resistances were measured with a diffusion porometer intensively over five days when the leaf area index was between 5 and 8. The T _v predicted by the single-layer and the two-layer models accounted for 87% and 88% of the variance of measured values respectively, and both regression lines were close to the 11 relationship. Study of the effect of T _s on the CWSI with the two-layer model indicated that the CWSI was sensitive to changes in T _s . The overestimation of crop water stress calculated from the CWSI was predicted to be greater at low leaf area indices and high levels of stomatal resistance. The implications for this bias when using the CWSI for irrigation scheduling are discussed.List of Symbols C Sensible heat flux from the soil-vegetation system (W m^–2) - c _{l shade} Mean stomatal conductance of the shaded leaf area (m s^–1) - c _{l sun} Mean stomatal conductance of the sunlit leaf area (m s^–1) - c _max Maximum stomatal conductance (m s^–1) - c ₀ Minimum stomatal conductance (m s^–1) - C _p Specific heat at constant pressure (J kg^–1 °C^–1) - C _s Sensible heat flux from the soil (W m^–2) - C _v Sensible heat flux from the vegetation (W m^–2) - c _v Bulk stomatal conductance of the vegetation (m s^–1) - CWSI Crop Water Stress Index (dimensionless) - e _a Vapor pressure at the reference height (kPa) - e _b Vapor pressure at the virtual source/sink height of heat exchange (kPa) - e ₀ ^* Saturated vapor pressure at T ₀ (kPa) - e _s Vapor pressure at the soil surface (kPa) - e _v ^* Saturated vapor pressure at T _v (kPa) - G Soil heat flux (Wm^–2) - GLAI Green leaf area index (dimensionless) - GLAI_shade Green shaded leaf area index (dimensionless) - GLAI_sun Green sunlit leaf area index (dimensionless) - k Extinction coefficient for photosynthetically active radiation (dimensionless) - k ₁ Damping exponent for Eq. A 5 (m² W^–1) - LAI Leaf area index (dimensionless) - LE Latent heat flux from the soil-vegetation system (W m^–2) - LE _s Latent heat flux from the soil (W m^–2) - LE _v Latent heat flux from the vegetation (W m^–2) - p _a Density of air (kg m^–3) - PAR_a Photosynthetically active radiation above the canopy (W m^–2) - PAR_u Photosynthetically active radiation under the canopy (W m^–2) - r _a Aerodynamic resistance (s m^–1) - r _b Heat exchange resistance between the vegetation and the adjacent air boundary layer (s m^–1) - r _c Bulk stomatal resistance of the vegetation (s m^–1) - R _n Net radiation above the canopy (W m^–2) - R _s Net radiation flux at the soil surface (W m^–2) - r _st Mean stomatal resistance of leaves in the canopy (s m^–1) - R _v Net radiation absorbed by the vegetation (W m^–2) - r _w Heat exchange resistance between the soil surface and the boundary layer (s m^–1) - S Photosynthetically active radiation on the shaded leaves (W m^–2) - S _d Diffuse photosynthetically active radiation (W m ^–2) - S ₀ Photosynthetically active radiation on a surface perpendicular to the beams (W m^–2) - T _a Air temperature at the reference height (°C) - T _b Temperature at the virtual source/sink height of heat exchange (°C) - T ₀ Aerodynamic temperature (°C) - T _s Soil surface temperature (°C) - T _v Vegetation temperature (°C) - w ₀ Single scattering albedo (dimensionless) - Psychrometric constant (kPa °C) - ₀ Cosine of solar zenith angle (dimensionless)     

Growth and yield responses of almond (Prunus amygdalus) to trickle irrigation

Growth and yield responses of developing almond trees (Prunus amygdalus, Ruby cultivar) to a range of trickle irrigation amounts were determined in 1985 through 1987 (the fifth through seventh year after planting) at the University of California's West Side Field Station in the semi-arid San Joaquin Valley. The treatments consisted of six levels of irrigation, ranging from 50 through 175% of the estimated crop evapotranspiration (ET_c), applied to a clean-cultivated orchard using a line source trickle irrigation system with 6 emitters per tree. ET_c was estimated as grass reference evapotranspiration (ET₀) times a crop coefficient with adjustments based upon shaded area of trees and period during the growing season. Differential irrigation experiments prior to 1984 on the trees used in this study significantly influenced the initial trunk cross-section area and canopy size in the 50% ET_c treatment and 125% ET_c treatment. In these cases, treatment effects must be identified as relative effects rather than absolute. The soil of the experimental field was a Panoche clay loam (nonacid, thermic, Typic Torriorthents). The mean increase in trunk cross-sectional area for the 3-year period was a positive linear function (r ² = 0.98) of total amounts of applied water. With increases in water application above the 50% ET_c treatment, nut retention with respect to flower and fertile nut counts after flowering, was increased approximately 10%. In 1985 and 1987, the nut meat yields and mean kernel weights increased significantly with increasing water application from 50% to 150% ET_c. Particularly in the higher water application treatments, crop consumptive use was difficult to quantify due to uncertainty in estimates of deep percolation and soil water uptake. Maintenance of leaf water potentials higher than –2.3 MPa during early nut development (March through May) and greater than –2.5 MPa the remainder of the irrigation season (through August) were positively correlated with sustained higher vegetative growth rates and higher nut yields.     

Salinity and drought

Summary Water deficit (water stress — WS) and excess salt (salt stress — SS) evoke similar plant responses, yet clear differences have been observed. The effect of the two forms of stress applied consecutively to cotton (Gossypium hirsutum) and pepper (Capsicum annuum) was studied in a growth chamber (29/20°C day/night temperature, 50% RH, 12-h photoperiod) in 2.5-liter containers packed with a silt loam soil.Leaf water potential () under increasing WS [soil water potential decrease from –0.16 to –1.10 MPa] of transpiring cotton and pepper plants declined to lower levels than under equivalent SS. The decline of leaf solute potential ₀ on the other hand, was less under WS than under SS, resulting in reduced turgor potential ( _p ), in contrast with turgor maintenance under SS. Predawn turgor potential of WS plants was maintained at all levels of soil water potential. Transpiration, CO₂ assimilation and light period leaf extension rate were higher under low soil water potential produced by salinity than an equivalent value produced by water deficit.The more severe effect of WS was attributed to incomplete osmotic adjustment — the reduction in solute potential did not keep pace with the reduction in leaf water potential, and to increased root interface resistance in the dry soil.The leaf sap of cotton under WS had a higher proportion of sugars (65%) than electrolytes, compared to SS. When WS was converted to SS and plant solute potential decreased, electrolytes were taken up at the expense of a reduction in the sugar concentration. Water stress and salt stress may have an additive effect in depressing growth. But at equivalent levels, the relative magnitude of the effect of low soil matric potential (WS) on plant growth was twice as great as that of low soil solute potential (SS).     

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.