The influence of crop canopy on evapotranspiration and crop coefficient of beans (Phaseolus vulgaris L.)

Experiments were conducted to investigate the effect of crop development on evapotranspiration and yield of beans (Phaseolus vulgaris L.) at the Instituto Agronômico (IAC), Campinas, State of São Paulo, Brazil, during the dry season of 1994. A completely randomized design was carried out with three population density treatments and four replications. The treatments were: (a) crop sown in evapotranspirometers at a density of 50 plants m⁻², and thereafter thinned to 25 plants m⁻², when the canopy achieved full ground cover; (b) crop sown with population densities of 14 and 28 plants m⁻² in an irrigated field. Crop growth was evaluated considering dry matter (DM), vegetative ground cover (GC%) and leaf area index (LAI). These parameters were successfully related to basal crop coefficient (k_cb) and crop coefficient (k_c), demonstrating the strong dependence of both coefficients on canopy development. A simulation study was carried out and showed that k_cb based on LAI would allow good estimates of water use for different plant density populations in the field.     

Phenology based irrigation scheduling and determination of crop coefficient of winter maize in rice fallow of eastern India

Vast rainfed rice area (12 million ha) of eastern India remains fallow after rainy season rice due to lack of appropriate water and crop management strategies inspite of having favourable natural resources, human labourers and good market prospects. In this study, a short duration crop, maize, was tried as test crop with different levels of irrigation during winter season after rainy season rice to increase productivity and cropping intensity of rainfed rice area of the region. Maize hybrid of 120 days duration was grown with phenology based irrigation scheduling viz., one irrigation at early vegetative stage, one irrigation at tassel initiation, two irrigation at tassel initiation + grain filling, three irrigation at early vegetative + tassel initiation + grain filling and four irrigation at early vegetative + tassel initiation + silking + grain-filling stages. Study revealed that one irrigation at tassel initiation stage was more beneficial than that of at early vegetative stage. Upto three irrigation, water use efficiency (WUE) was increased linearly with increased number of irrigation. With four irrigations, the yield was higher, but WUE was lower than that of three irrigations, which might be due to increased water application resulted in increase crop water use without a corresponding increase of yield for the crop with four irrigations. The crop coefficients (K_c) at different stages of the crop were derived after computing actual water use using field water balance approach. The crop coefficients of 0.42–0.47, 0.90–0.97, 1.25–1.33, and 0.58–0.61 were derived at initial, development, mid and late season, respectively with three to four irrigation. Study showed that leaf area index (LAI) was significantly correlated with K_c values with the R² values of 0.93. When LAI exceeded 3.0, the K_c value was 1. 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.     

Estimation of evapotranspiration,transpiration ratio and water-use efficiency from a sparse canopy using a compartment model

Using the Shuttleworth and Wallace (S–W) model, evapotranspiration (ET); transpiration ratio (T/ET), which is the ratio of transpiration (T) to ET; and water-use efficiency (WUE) were estimated for a sparsely planted sorghum canopy that was well irrigated. That model is designed to estimate separately the evaporation from soil and transpiration from crops.The evapotranspiration estimates for both short- and long-term measurement periods coincided closely with the Bowen ratio energy balance (BREB) measurements. The transpiration ratios were affected by the canopy resistances and the soil surface resistances during the day. The regression curve between leaf area index (LAI) and transpiration ratio suggests that LAI, less than 1.6, determined the transpiration ratio in the absence of water stresses by soil water drought and extreme weather condition. The WUEs for transpiration (WUEt) and evapotranspiration (WUEet), which are the total dry matter (TDM) production for 1 kg T and ET, reached the peaks of 9.0 and 4.5 g kg⁻¹ H₂O, respectively, in the end of July when the total dry matter increasing rate was greatest. These two WUEs degraded to less than zero in the end of August when the plant biomass decreased due to drying and death. The WUEs are largely affected by the TDM seasonal increment rate.Thus, in a sparse crop, the crop growth properties (i.e. LAI and TDM increment) mainly determine the crop water uses (i.e. the transpiration ratio and water-use efficiency) in the absence of water stresses.     

Water use and lint yield response of drip irrigated cotton to the length of irrigation season

A 2-year experiment was conducted at Tal Amara Research Station in the Bekaa Valley of Lebanon to determine water use and lint yield response to the length of irrigation season of drip irrigated cotton (Gossypium hirsutum L.). Crop evapotranspiration (ET_crop) and reference evapotranspiration (ET_rye-grass) were directly measured at weekly basis during the 2001 growing period using crop and rye-grass drainage lysimeters. Crop coefficients (K_c) in the different growth stages were calculated as ET_crop/ET_rye-grass. Then, the calculated K_c values were used in the 2002 growing period to estimate evapotranspiration of cotton using the FAO method by multiplying the calculated K_c values by ET_rye-grass measured in 2002. The length of irrigation season was determined by terminating irrigation permanently at first open boll (S1), at early boll loading (S2), and at mid boll loading (S3). The three treatments were compared to a well-watered control (C) throughout the growing period. Lint yield was defined as a function of components including plant height at harvest, number of bolls per plant, and percentage of opened bolls per plant.Lysimeter-measured crop evapotranspiration (ET_crop) totaled 642 mm in 2001 for a total growing period of 134 days, while when estimated with the FAO method in 2002 it averaged 669 mm for a total growing period of 141 days from sowing to mature bolls. Average K_c values varied from 0.58 at initial growth stages (sowing to squaring), to 1.10 at mid growth stages (first bloom to first open boll), and 0.83 at late growth stages (early boll loading to mature bolls).Results showed that cotton lint yields were reduced as irrigation amounts increased. Average across years, the S1 treatment produced the highest yield of 639 kg ha⁻¹ from total irrigations of 549 mm, compared to the S2 and S3 treatments, which yielded 577 and 547 kg ha⁻¹ from total irrigations of 633 and 692 mm, respectively, while the control resulted in 457 kg ha⁻¹ of lint yield from 738 mm of irrigation water. Water use efficiency (WUE) was found to be higher in S1 treatment and averaged 1.3 kg ha⁻¹ mm⁻¹, followed by S2 (1.1 kg ha⁻¹ mm⁻¹), and S3 (1.0 kg ha⁻¹ mm⁻¹), while in the control WUE was 0.80 kg ha⁻¹ mm⁻¹. Lint yield was negatively correlated with plant height and the number of bolls per plant and positively correlated with the percentage of opened bolls. This study suggests that terminating irrigation at first open boll stage has been found to provide the highest cotton yield with maximum WUE under the semi-arid conditions of the Bekaa Valley of Lebanon.     

The effects of different irrigation regimes on cucumber (Cucumbis sativus L.) yield and yield characteristics under open field conditions

A study was conducted to determine the effects of different drip irrigation regimes on yield and yield components of cucumber (Cucumbis sativus L.) and to determine a threshold value for crop water stress index (CWSI) based on irrigation programming. Four different irrigation treatments as 50 (T-50), 75 (T-75), 100 (T-100) and 125% (T-125) of irrigation water applied/cumulative pan evaporation (IW/CPE) ratio with 3-day-period were studied.Seasonal crop evapotranspiration (ET_c) values were 633, 740, 815 and 903 mm in the 1st year and were 679, 777, 875 and 990 mm in the 2nd year for T-50, T-75, T-100 and T-125, respectively. Seasonal irrigation water amounts were 542, 677, 813 and 949 mm in 2002 and 576, 725, 875 and 1025 mm in 2003, respectively. Maximum marketable fruit yield was from T-100 treatment with 76.65 t ha⁻¹ in 2002 and 68.13 t ha⁻¹ in 2003. Fruit yield was reduced significantly, as irrigation rate was decreased. The water use efficiency (WUE) ranged from 7.37 to 9.40 kg m⁻³ and 6.32 to 7.79 kg m⁻³ in 2002 and 2003, respectively, while irrigation water use efficiencies (IWUE) were between 7.02 and 9.93 kg m⁻³ in 2002 and between 6.11 and 8.82 kg m⁻³ in 2003.When the irrigation rate was decreased, crop transpiration rate decreased as well resulting in increased crop canopy temperatures and CWSI values and resulted in reduced yield. The results indicated that a seasonal mean CWSI value of 0.20 would result in decreased yield. Therefore, a CWSI = 0.20 could be taken as a threshold value to start irrigation for cucumber grown in open field under semi-arid conditions.Results of this study demonstrate that 1.00 IW/CPE water applications by a drip system in a 3-day irrigation frequency would be optimal for growth in semiarid regions.     

Development and evaluation of integrated water and nitrogen model for maize

Maize (Zea mays L.) is an important food crop for irrigated regions in the world. Its growth and production may be estimated by different crop models in which various relationships between growth and environmental parameters are used. For simulation of maize growth and grain yield, a simulation model was developed (Maize Simulation Model, MSM). Dynamic flow of water, nitrogen (N) movement, and heat flow through the soil were simulated in unsteady state conditions by numerical analysis in soil depth of 0–1.8 m. Hourly potential evapotranspiration [ET_p(t)] for maize field was estimated directly by Penman–Monteith method. Hourly potential evaporation [E_p(t)] was estimated based on ET_p(t) and canopy shadow projection. Actual evaporation of soil surface was estimated based on its potential value, relative humidity of air, water pressure head and temperature at soil surface layer. Actual transpiration (T_a(t)) was estimated based on soil water content and root distribution at each soil layer. Hourly N uptake by plant was simulated by N mass flow and diffusion processes. Hourly top dry matter production (HDMA^j + 1, where j is number of hours after planting) was estimated by hourly corrected intercepted radiation (RSLT^j + 1) by plant leaves [determined from leaf area index (LAI^j + 1)] with air temperature, the maximum and minimum plant top N concentration and the amounts of nitrogen uptake. The value of LAI^j + 1 at each hour was estimated by the accumulated top dry matter production at previous hour using an empirical equation. Maize grain yield was estimated by a relationship between harvest index and seasonal plant top dry matter production. The model was calibrated using data obtained under field conditions by a line source sprinkler irrigation. When the values of water and nitrogen application were optimum, grain yield (moisture content of 15.5%) was 16.2 Mg ha⁻¹. Model was validated using two independent experimental data obtained from other experiments in the Badjgah (Fars province). The experimental results validated the proposed simulation model fairly well.     

Estimation of crop evapotranspiration of irrigation command area using remote sensing and GIS

This paper describes the use of satellite-based remote sensing (RS) data and geographic information system (GIS) tools for estimating seasonal crop evapotranspiration in Mahi Right Bank Canal (MRBC) command area of Gujarat, India. Crop coefficients (K_c) for various major crops grown in MRBC were estimated, empirically, from the RS derived soil adjusted vegetation index (SAVI) values. A reference crop evapotranspiration (ET₀) map was generated from point meteorological observations. The K_c and ET₀ maps were combined to generate seasonal crop evapotranspiration (ET_crop) map which highlighted spatial variation in ET_crop ranging from more than 600 mm for healthy tobacco crops to less than 150 mm for very poor wheat crops.     

Water requirement of drip irrigated tomatoes grown in greenhouse in tropical environment

Four different levels of drip fertigated irrigation equivalent to 100, 75, 50 and 25% of crop evapotranspiration (ET_c), based on Penman–Monteith (PM) method, were tested for their effect on crop growth, crop yield, and water productivity. Tomato (Lycopersicon esculentum, Troy 489 variety) plants were grown in a poly-net greenhouse. Results were compared with the open cultivation system as a control. Two modes of irrigation application namely continuous and intermittent were used. The distribution uniformity, emitter flow rate and pressure head were used to evaluate the performance of drip irrigation system with emitters of 2, 4, 6, and 8 l/h discharge. The results revealed that the optimum water requirement for the Troy 489 variety of tomato is around 75% of the ET_c. Based on this, the actual irrigation water for tomato crop in tropical greenhouse could be recommended between 4.1 and 5.6 mm day⁻¹ or equivalent to 0.3–0.4 l plant⁻¹ day⁻¹. Statistically, the effect of depth of water application on the crop growth, yield and irrigation water productivity was significant, while the irrigation mode did not show any effect on the crop performance. Drip irrigation at 75% of ET_c provided the maximum crop yields and irrigation water productivity. Based on the observed climatic data inside the greenhouse, the calculated ET_c matched the 75–80% of the ET_c computed with the climatic parameters observed in the open environment. The distribution uniformity dropped from 93.4 to 90.6%. The emitter flow rate was also dropped by about 5–10% over the experimental period. This is due to clogging caused by minerals of fertilizer and algae in the emitters. It was recommended that the cleaning of irrigation equipments (pipe and emitter) should be done at least once during the entire cultivation period.     

Water productivity analysis of irrigated crops in Sirsa district,India

Water productivity (WP) expresses the value or benefit derived from the use of water, and includes essential aspects of water management such as production for arid and semi-arid regions. A profound WP analysis was carried out at five selected farmer fields (two for wheat–rice and three for wheat–cotton) in Sirsa district, India during the agricultural year 2001–02. The ecohydrological soil–water–atmosphere–plant (SWAP) model, including detailed crop simulations in combination with field observations, was used to determine the required hydrological variables such as transpiration, evapotranspiration and percolation, and biophysical variables such as dry matter or grain yields. The use of observed soil moisture and salinity profiles was found successful to determine indirectly the soil hydraulic parameters through inverse modelling.Considerable spatial variation in WP values was observed not only for different crops but also for the same crop. For instance, the WP_ET, expressed in terms of crop grain (or seed) yield per unit amount of evapotranspiration, varied from 1.22 to 1.56 kg m⁻³ for wheat among different farmer fields. The corresponding value for cotton varied from 0.09 to 0.31 kg m⁻³. This indicates a considerable variation and scope for improvements in water productivity. The average WP_ET (kg m⁻³) was 1.39 for wheat, 0.94 for rice and 0.23 for cotton, and corresponds to average values for the climatic and growing conditions in Northwest India. Including percolation in the analysis, i.e. crop grain (or seed) yield per unit amount of evapotranspiration plus percolation, resulted in average WP_ETQ (kg m⁻³) values of 1.04 for wheat, 0.84 for rice and 0.21 for cotton. Factors responsible for low WP include the relative high amount of evaporation into evapotranspiration especially for rice, and percolation from field irrigations. Improving agronomic practices such as aerobic rice cultivation and soil mulching will reduce this non-beneficial loss of water through evaporation, and subsequently will improve the WP_ET at field scale. For wheat, the simulated water and salt limited yields were 20–60% higher than measured yields, and suggest substantial nutrition, pest, disease and/or weed stresses. Improved crop management in terms of timely sowing, optimum nutrient supply, and better pest, disease and weed control for wheat will multiply its WP_ET by a factor of 1.5! Moreover, severe water stress was observed on cotton (relative transpiration < 0.65) during the kharif (summer) season, which resulted in 1.4–3.3 times lower water and salt limited yields compared with simulated potential yields. Benefits in terms of increased cotton yields and improved water productivity will be gained by ensuring irrigation supply at cotton fields, especially during the dry years.     

Developing crop coefficients for field-grown tomato (Lycopersicon esculentum Mill.) under drip irrigation with black plastic mulch

A 2 years field study was conducted to develop crop coefficients for field-grown tomato (Lycopersicon esculentum Mill.), a major irrigated crop in the Jordan Valley, under drip irrigation system with black plastic mulch. The area of the study field was 1.5 ha surrounded by many similar tomato fields. Actual crop evapotranspiration (ET_C) was measured using eddy covariance technique which distinguishes this study from other previous studies conducted in the Jordan Valley that relied on the old indirect approach for ET_C estimation based on the soil water balance.Grass reference evapotranspiration (ET_O) was determined by using the FAO Penman–Monteith method utilizing the agrometeorological parameters measured at the study site. The crop coefficient (K_C) was determined as the ratio of ET_C to ET_O. The tomato crop coefficients were determined following the FAO crop coefficient model. The average crop coefficient during the midseason growth stage (K_C mid) was 0.82 which is far below the adjusted FAO crop coefficient of 1.19 by about 31%. Also, the late season crop coefficient (K_C end) was much lower than the adjusted FAO crop coefficient of 0.76 by about 40%. Moreover, the weighted average crop coefficient over the entire growing season (K_C GS) was 0.69, which is about 36% lower than the FAO corresponding value. In fact, the low K_C values obtained reflect the effect of practicing both localized drip irrigation and plastic mulch covering. This study showed that there is a big difference between the reported FAO crop coefficients and the one measured in the filed using a precise approach. These exact updated values of crop coefficients will enhance future estimation of crop water requirements and hence irrigation management of tomato crop which is the major irrigated crop in the Jordan Valley.     

Evapotranspiration and seed yield of field grown soybean under deficit irrigation conditions

Evapotranspiration was measured for a reference crop, rye grass (Lolium prerenne) and soybean (Glycine max L. Merril) grown over two seasons in 2000 and 2001 at Tal Amara Research Station, Lebanon, using drainage and weighing lysimeters. Climatic data from the field weather station were recorded daily. Within the experimental plots, irrigation was withheld at full bloom, R2 stage (S1 treatment), at seed enlargement, R5 stage (S2 treatment) and at mature seeds, R7 stage (S3 treatment). Further, a control (C) was fully-irrigated throughout the growing period.Average crop evapotranspiration (ETc) as measured by the drainage lysimeters in 2000 totaled 800 mm for a total growing period of 140 days. However, when ETc was measured by the weighing lysimeter in 2001, it was 725 mm during a growing period of 138 days. Average crop coefficients (K_c) were computed for different growth stages for the two growing periods by dividing the measured crop evapotranspiration (ETc) by the corresponding measured reference evapotranspiration (ETo-rye grass). K_c values ranged from 0.62 at V10 stage (10th node on the main stem beginning with the unifoliolate node) to 1.0 at pod initiation, then to 0.81 at mature pods.Growth parameters, leaf area index (LAI) and dry matter accumulation, have been shown to be sensitive to water stress caused by the deficit irrigations. However, growth parameters were found to compensate for water stress at early stages, while at seed maturity the compensation ability was decreased.Plants of the lysimeters produced average aboveground biomass and seed yield of 8.1 and 3.5 t ha⁻¹, respectively. However, in the well-irrigated field treatment, aboveground biomass and seed yield averaged 7.3 and 3.2 t ha⁻¹, respectively. Deficit irrigation at R2 stage reduced aboveground biomass and seed yield by 16 and 4%, respectively, while deficit irrigation at R5 stage reduced these two parameters by 6 and 28%, respectively, with comparison to the control. The significant decrease in biomass at R2 stage due to water deficit may be attributed to a pronounced reduction in the number of vegetative nodes. However, limited irrigation at this stage did not reduce significantly (P < 0.01) neither seed number nor seed weight, while at R5 stage these two parameters were reduced by 20 and 10%, respectively, with comparison to the control. Results showed also that deficit irrigation at R7 stage (S3) was more profitable than irrigation deficit at any other crop phenology and did not cause significant reductions either in seed number or seed weight.     

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     

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.     

Crop coefficient of sesame in a semi-arid region of I.R. Iran

Crop coefficient of sesame is necessary for the water requirement estimation in irrigation water planning and management. This study has been initiated to determine the crop coefficient (K_c) of sesame in a semi-arid climate. The relationships between K_c and ET_p/E_p (pan evaporation) and leaf area index (LAI), growing degree-day (GDD) and days after sowing (DAS), were also investigated. The seasonal ET_p for sesame in the study area with a 5 month growth period was 910 m. The mid-season and late-season K_c values for sesame were 1.08 and 0.64, respectively. These values are somewhat lower and higher than those for other oil seed crops. The K_c value for the initial stage was close to that obtained by the procedure proposed by Allen et al. [Allen, R.G., Smith, M., Pereira, L.S., Pruitt, W.O., 1997. Proposed revision to the FAO procedure for estimating evapotranspiration. In: The Second Iranian Congress on Soil and Water Issues, 15–17 February 1997. Tehran, I.R. of Iran, pp. 1–18]. The ratio of ET_p/E_p varied between 0.49–1.0 from the beginning to the middle of the growing season which is a sign of mild local advection in the region. The maximum ratios of ET_p/ET₀ and ET_p/E_p occurred at a LAI of 3.0. Furthermore, third-order polynomials were presented to predict the K_c values from days after sowing (DAS), percent days after sowing (%DAS) and growing degree-day (GDD).     

Regional cotton lint yield,ETc and water value in Arizona and California

Water value as agriculture production may be overlooked, though it is an important factor to rational water allocations within a region. An analysis of cotton (Upland and Pima) lint yield, lint yield-consumptive use ratio (LY:ET_c), water-use efficiency (WUE) and lint price for Arizona (AZ) and California (CA) during 1988–1999 is considered as part of an attempt to determine lint water value, or benefit. It included determination of means and variability of cotton lint production, LY:ET_c ratios and associated irrigation water values (IWVs) and compared these numbers with published estimates of WUE, forage hay water values and municipal water costs. Available rainfall, reference evapotranspiration (ET_o), lint yields and price data for counties in both states were used. Consumptive use was estimated using a four-stage crop coefficient function verified by literature values or County Advisor experience. As with dry matter production, cotton lint yields in interior valley regions of CA were weakly correlated with ET_c and averaged 1.33 Mg/ha (Upland) and 1.08 Mg/ha (Pima). Cotton lint yields in desert regions of AZ and CA were not correlated with ET_c. The greatest LY:ET_c ratios (1.9–2.1 kg/ha-mm) were in the San Joaquin valley of CA, were similar to that from WUE type studies and resulted in gross IWVs (∼3400–3800 US$/ha-m), with relatively moderate variability at a net irrigation water requirement (IWR) of approximately 720 mm. While this IWV is 2.5 times greater than water delivery prices below the California Delta, it is less than average municipal water costs of ∼4200 US$/ha-m for Los Angeles, San Francisco and Pheonix while the overall AZ/CA average cotton lint IWV is considerably less. However, cotton lint IWV is two to three times greater than that obtained for alfalfa and sudangrass hay crops in all regions.     

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.     

Latent heat flux over a furrow-irrigated tomato crop using Penman–Monteith equation with a variable surface canopy resistance

The Penman–Monteith (P–M) model with a variable surface canopy resistance (r_c) was evaluated to estimate latent heat flux (LE) or crop evapotranspiration (ET) over a furrow-irrigated tomato crop under different soil water status and atmospheric conditions. The hourly values of r_c were computed as a function of environmental variables (air temperature, vapor pressure deficit, net radiation, and soil heat flux) and a normalized soil water factor (F), which varies between 0 (wilting point, θ_WP) and 1 (field capacity, θ_FC). The Food and Agricultural Organization (FAO-56) method was also evaluated to calculate daily ET based on the reference evapotranspiration, crop coefficient and water stress coefficient. The performance of the P–M model and FAO-56 method were evaluated using LE values obtained from the Bowen ratio system. On a 20 min time interval, the P–M model estimated daytime variation of LE with a standard error of the estimate (SEE) of 46 Wm⁻² and an absolute relative error (ARE) of 3.6%. Thus, daily performance of the P–M model was good under soil water content ranging from 118 to 83 mm (θ_FC and θ_WP being 125 and 69 mm, respectively) and LAI ranging from 1.3 to 3.0. For this validation period, the calculated values of r_c and F ranged between 20 and 114 s m⁻¹ and between 0.87 and 0.25, respectively. In this case, the P–M model was able to predict daily ET with a SEE of 0.44 mm h⁻¹ (1.1 MJ m⁻² d⁻¹) and an ARE of 3.9%. Furthermore, the FAO-PM model computed daily ET with SEE and ARE values of 1.1 mm h⁻¹ (2.8 MJ m⁻² d⁻¹) and 5.2%, respectively.     

Performance of sugarcane genotypes grown under sodic soil and water conditions

An experiment was conducted to evaluate the effect of residual sodium carbonates (RSC) of irrigation water on the growth and yield of sugarcane grown on sierozem light textured alkaline soil with sodic ground water and to study the performance of some promising sugarcane genotypes under these conditions. Treatments consisted of five levels of irrigations water viz RSC 2.8, 6.5, 12 me l⁻¹ and RSC 6.5 and 12.0 me l⁻¹ fully amended with gypsum. Plant and ratoon crops of eight genotypes of sugarcane were harvested. Cane yield and yield attributing characters like cane height, number of internodes per cane and number of millable canes were recorded. Juice quality viz percent juice extraction, percent sucrose, and commercial cane sugar (CCS%) in juice were determined at the harvest of crop. For both plant and ratoon crops, the average cane yield of all the genotypes of sugarcane and cane yield attributing characters decreased significantly with the increase in RSC of irrigation water to 6.5 and 12.0 me l⁻¹ (35% and 51% decline in the average cane yield for plant crop). For ratoon crop, the corresponding decrease in the average cane yield was less than the plant crop (only 14% and 21%). Amending RSC with gypsum increased the yield in all genotypes. The cane yield of various genotypes obtained under amended RSC with gypsum treatments were almost equal to the yield obtained under RSC 2.8 me l⁻¹ treatment (89% to 92% average cane yield for plant crop and 93% to 96% for ratoon crop). The effect of RSC of irrigation was variable for different genotypes (for example, for the plant crop of CoH 97, 65% and 76% and for CoH 108, 9% and 20% decline in the cane yield was observed with the application of high RSC irrigation water). As compared to plant crop, the ratoon crop of all genotypes recorded higher average cane yield and lesser decline in the cane yield with the application of high RSC irrigation water. Average juice extraction % decreased from 40.5% to 35.8%, and sugar yield decreased significantly (5.61 to 2.91 t ha⁻¹ for plant crop and 6.18 to 5.38 t ha⁻¹ for ratoon crop) with the increase in RSC of irrigation water, and amending RSC with gypsum increased the juice extraction % and sugar yield per unit area.     

Simulations on direct and cyclic use of saline waters for sustaining cotton–wheat in a semi-arid area of north-west India

A validated agro-hydrological model soil water atmosphere plant (SWAP) was applied to formulate guidelines for irrigation planning in cotton–wheat crop rotation using saline ground water as such and in alternation with canal water for sustainable crop production. Six ground water qualities (4, 6, 8, 10, 12 and 14 dS/m), four irrigation schedules with different irrigation depths (4, 6, 8 and 10  cm) and two soil types (sandy loam and loamy sand) were considered for each simulation. The impact of the each irrigation scenario on crop performance, and salinization/desalinisation processes occurring in the soil profile (0–2 m) was evaluated through Water Management Response Indicators (WMRIs). The criterion adopted for sustainable crop production was a minimum of pre-specified values of ET_rel (≥0.75 and ≥0.65 for wheat and cotton, respectively) at the end of the 5th year of simulation corresponding to minimum deep percolation loss of applied water. The extended simulation study revealed that it was possible to use the saline water upto 14 dS/m alternatively with canal water for cotton–wheat rotation in both sandy loam and loamy sand soils. In all situations pre-sown irrigation must be accomplished with canal water (0.3–0.4 dS/m). Also when the quality of ground water deteriorates beyond 10 dS/m, it was suggested to use groundwater for post-sown irrigations alternately with canal water. Generally, percolation losses increased with the increase in level of salinity of ground water to account for leaching and thus maintain a favourable salt balance in the root zone to achieve pre-specified values of ET_rel.     

Maximum and Actual Evapotranspiration for Barley (Hordeum vulgare L.) through NOAA Satellite Images in Castilla-La Mancha,Spain

An easy-to-follow methodology is developed for the assessment of regional evapotranspiration in Castilla-La Mancha, a semi-arid region of Spain. The methodology is applied to barley crops to monitor the irrigation scheduling over the region, by using remote sensing techniques supplemented by ground measurements. The methodology can be based on either of two models. In the first one, established by Caselles and Delegido,¹the reference evapotranspiration,ET_o, derives from the expressionET_o=AR_g(T_a)_max+BR_g+CwhereA, BandCare empirical coefficients, depending on climatic parameters and determined for each region;R_gis the daily global radiation; and (T_a)_maxis the maximum air temperature. The second model, proposed by Jacksonet al.,²considers the actual evapotranspirationER=R_n+D(T_a−T_s) whereR_n, is the net radiation,T_aandT_sare the air and crop surface temperatures, respectively, andDis a semi-empirical coefficient. Both methods were compared with the method of Penman (considered standard), and resulted in differences of ±1 mm  d⁻¹. The developed methodology has been applied to map the reference and the actual evapotranspiration over a 10×10 km area, using the thermal-infrared information provided by the AVHRR (advanced very high resolution radiometer) sensor on board the NOAA (national oceanic atmosphere administration) satellite on a selected date.