Velocity profile modeling in rectangular and compound open-channel cross sections

A 3-D hydraulic model was developed for computing velocity profiles, surface velocity coefficients, and discharge under steady, uniform flow conditions for rectangular and compound open-channel cross sections. Reynolds-averaged Navier–Stokes equations, Reynolds stress equations, and kinetic energy and dissipation equations were applied in the model using the finite-volume method with the k–ε turbulence model. Many previously unpublished approaches to solving the numerical details of this type of hydraulic model are presented herein. Four different sets of Reynolds stress equations (one using the Boussinesq hypothesis and three algebraic stress models of varying complexity) were tested. Only one of the four stress models was successful in predicting the depression of the maximum stream-wise velocity below the water surface. The model was verified using data collected at the Utah Water Research Laboratory. A companion paper (Marjang and Merkley in Irrig Sci, 2009, in press) describes the application of this model to the calculation of surface velocity coefficients for the float method to estimate discharge in rectangular and compound irrigation canals.     

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of the popular non-linear regression equations for estimating pan coefficient values in the original and FAO-24 tables, unpublished report, 1998; Allen and Pruitt in J Irrig Drain Eng 117(5):758–773, 1991; Cuenca in Irrigation system design: an engineering approach, p 133, 1989; Snyder in J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of the popular non-linear regression equations for estimating pan coefficient values in the original and FAO-24 tables, unpublished report, 1998) provided more accurate daily, monthly, and annual total estimates compared to the others equations.     

Evaluation of pan coefficient for reference crop evapotranspiration for semi-arid region

Pan coefficient (K _pan) is the important factor for computation of reference evapotranspiration (ET _o ) from pan evaporation (E _pan). In this paper, the approaches proposed by Cuenca (Irrigation system design: an engineering approach. Prentice-Hall, Englewood Cliffs, 1989), Snyder (J Irrig Drain Eng 118(6):977–980, 1992), Orang (Potential accuracy of the popular non-linear regression equations for estimating pan coefficient values in the original and FAO-24 tables. Unpublished Report, Calif. Dept. of Water Resources, Sacramento, 1998), Raghuwanshi and Wallender (J Irrig Drain Eng 118 (6):977–980, 1998) and Pereira et al. (Agric Water Manage 76:75–82, 1995) were evaluated for a semi-arid region. By comparing with the FAO 56 Penman-Monteith (F-PM) method the Snyder (J Irrig Drain Eng 118(6):977–980, 1992, 1992) approach was best suited for the semi-arid region.     

Modeling the water transport and nitrogen dynamics in irrigated salad crops

The use of N fertilizers in agriculture is crucial, and agricultural techniques need to be implemented that improve significantly N fertilizer management by reducing downward movements of solutes through the soil. To achieve this, it is essential to develop and test models against experimental conditions in order to improve them and to make sure that they can be applied to a broad range of soil and climatic conditions. A field experiment was carried out in the French department of Gard. The soil was a clay loam (26.7% clay, 44.7% fine and coarse silt, and 28.6% fine and coarse sand). Salad vegetables (Cichorium endivia, Lactuca sativa) were cultivated during two consecutive periods (spring and summer crops). The crops were planted on punched and permeable plastic mulching bands. The field was irrigated with a sprinkler watering system. Local measurements were made combining a neutron probe, tensiometers, and ceramic porous cups to estimate NO₃-N concentrations. The model is one-dimensional and is based on Richards' equation for describing saturated-unsaturated water flow in soil. At the soil surface, the model is designed to handle flux-type or imposed-pressure boundary conditions. In addition, provision is made in the model, for example, to account for a mulch plastic sheet that limits evaporation. The model accounts for heat transport by diffusion and by convection, while the modeling of the displacement of nitrate and ammonium in the soil is based on the convection-dispersion equation. Nitrate uptake by the crop is modeled assuming Michaelis-Menten kinetics. Nitrogen cycle modeling accounts for the following major transformations: mineralization of organic matter, nitrification of ammonium, and denitrification. The results showed that the overall trend of the water potential in the soil profile was correctly described during the crop seasons. Mineralization was high for the spring crop (4.7 kg NO₃-N day^–1 ha^–1), whereas the other sink components, such as root uptake, drainage, and denitrification, were smaller (1.9, 1.4, and 0.2 kg NO₃-N day^–1 ha^–1, respectively). For the summer crop, intensive denitrification was found in the soil layer at 0.15–0.90 m (5.7 kg NO₃-N day^–1 ha^–1), while the mineralization was always an important component (9.2 kg NO₃-N day^–1 ha^–1) and the sink terms were 1.7 and 1.7 kg NO₃-N day^–1 ha^–1 for root uptake and drainage, respectively. Similar high denitrification rates were found in the literature under intensive irrigated field conditions. Received: 25 October 1995     

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.     

Estimating in situ soil–water retention and field water capacity in two contrasting soil textures

A priori knowledge of the in situ soil field water capacity (FWC) and the soil-water retention curve for soils is important for the effective irrigation management and scheduling of many crops. The primary objective of this study was to estimate the in situ FWC using the soil-water retention curve developed from volumetric water content (θ), and water potential (ψ) data collected in the field by means of soil moisture sensors in two contrasting-textured soils. The two study soils were Lihen sandy loam and Savage clay loam. Six metal frames 117 cm × 117 cm × 30 cm high were inserted into the soil to a depth of 5–10 cm at approximately 40 m intervals on a 200 m transect. Two Time Domain Reflectrometry (TDR) sensors were installed in the center of the frame and two Watermark (WM) sensors were installed in the SW corner at 15 and 30 cm depths to continuously monitor soil θ and ψ, respectively. A neutron probe (NP) access tube was installed in the NE corner of each frame to measure soil θ used for TDR calibration. The upper 50–60 cm of soil inside each frame was saturated with intermittent application of approximately 18–20 cm of water. Frames were then covered with plastic tarps. The Campbell and Gardner equations best fit the soil–water retention curves for sandy loam and clay loam soils, respectively. Based on the relationship between soil ψ and elapsed time following cessation of infiltration, we calculated that the field capacity time (t _FWC) were reached at approximately 50 and 450 h, respectively, for sandy loam and clay loam soils. Soil-water retention curves showed that θ values at FWC (θ _FWC) were approximately 0.228 and 0.344 m³ m⁻³, respectively, for sandy loam and clay loam soils. The estimated θ _FWC values were within the range of the measured θ _FWC values from the NP and gravimetric methods. The TDR and WM sensors provided accurate in situ soil–water retention data from simultaneous soil θ and ψ measurements that can be used in soil-water processes, irrigation scheduling, modeling and chemical transport.     

Characterisation of water use by Sultana grapevines (Vitis vinifera L.) on their own roots or on Ramsey rootstock drip-irrigated with water of different salinities

Seasonal evapotranspiration (ET) was determined for Sultana grapevines grown on their own roots (Own-rooted) or grafted onto Ramsey rootstock (Grafted), and irrigated with water of three salinity levels – low (0.4 dS m^–1), medium (1.8 dS m^–1) and high (3.6 dS m^–1) – during the 1994/1995 growing season in south-eastern Australia. Transpiration (T) was determined from sap flux, soil evaporation (E _s) with a model, and soil water (S) with a neutron probe. Total ET for the season was similar for both Own-rooted and Grafted, averaging 382 mm. However, Grafted partitioned a mean of 193.5 mm (50.8%) of the ET through T compared to 146.7 mm (38.4%) by Own-rooted. Daily rates of T were generally low and attained peaks of 1.2 mm (9.9 l per vine) for Grafted and 0.9 mm (7.5 l) for Own-rooted in late November, and changed very little until after harvest in February. In contrast to T, the E _s rate was consistently higher for Own-rooted than for Grafted from November onwards, and at the end of the season totalled 237 mm for Own-rooted compared to 187 mm for Grafted. Differences between Own-rooted and Grafted in their partitioning of ET into T and E _s were associated with their canopy development. Grafted had a higher rate of canopy development than Own-rooted, and in mid-season, the former intercepted about 50% more incident radiation than Own-rooted. The crop factors, i. e. ratio of water use to evaporative demand, based on ET were similar for both vine types with an average seasonal value of 0.25, but when based on T were 0.12 for Grafted and 0.10 for Own-rooted. The ratio of fresh fruit weight to total water used at harvest, i. e. crop water use efficiency (CWUE), based on ET, had a mean of 86 kg mm^–1 ha^–1 for Grafted and 43 kg mm^–1 ha^–1 for Own-rooted, and when based on T, was 165 and 115 kg mm^–1 ha^–1, respectively; however, supplementary data obtained during the 1993/1994 season, indicated a CWUE based on T of 294 and 266 kg mm^–1 ha^–1 for Grafted and Own-rooted, respectively. Salinity did not have significant effects on canopy development and water use for most of the 1994/1995 growing season. The study shows ET and crop factors for the drip-irrigated grapevines to be much lower than previously reported for this district. Received: 6 May 1996     

Net changes of main ions in the soil profile of irrigated field plots in central Spain

Implementation of improved irrigation schedules in some semiarid zones improve water efficiency and can be recommended where occasional periods of heavy rainfall may remove some of the accumulated salts. We hypothesized, however, that the leaching pattern of the main ions may differ regarding their potential contribution to the total salt discharge. The experiment was conducted near Madrid in Spain on a typical Xerofluvent soil with sandy-loam texture in the first 0.5 m. For 4 years, a traditional crop rotation of corn–wheat–corn–oat (Zea mays L.–Triticum aestivum L.–Zea mays L.–Avena sativa L.) was planted and two irrigation treatments (traditional and improved) were applied only to the corn. In an experimental set-up of 24 plots, samples of the soil solutions were extracted 61 times during the experiment at soil depths of 0.4, 0.9 and 1.4 m. During the experiment, drainage volume was estimated in plots under the two irrigation schedules. Main ions in the soil solution were SO₄²⁻, Cl⁻, Ca²⁺, Na⁺ and HCO₃⁻. These solutes accounted for 88% of total salt discharge under the two irrigation treatments. Two main patterns of salt leaching were observed. For most main ions, except HCO₃⁻, the input to discharge ratio was lower than one. Also for HCO₃⁻ , the irrigation treatment did not affect the leaching pattern (higher input than discharge under the two irrigation treatments). Improved irrigation schedules can be implemented without increasing the total salt load, but attention should be paid to specific leaching patterns of individual ions.     

Water-yield relations and water-use efficiency of winter wheat in the North China Plain

Limited precipitation restricts yield of winter wheat (Triticum aestivum L.) grown in the North China Plain. Water stress effects on yield can be avoided or minimized by application of irrigation. We examined the multiseasonal irrigation experiments in four locations of the piedmont and lowland in the region, and developed crop water-stress sensitivity index, relationship between seasonal evapotranspiration (ET) and yield, and crop water production functions. By relating relative yield to relative ET deficit, we found that the crop was more sensitive to water stress from stem elongation to heading and from heading to milking. For limited irrigation, irrigation is recommended during the stages sensitive to water stress. Grain yield was 258–322 g m⁻² in the piedmont and 260–280 g m⁻² in the lowland under rainfed conditions. The corresponding seasonal ET was 242–264 mm in the piedmont and 247–281 mm in the lowland. Irrigation significantly increased seasonal ET and therefore grain yield as a result of increased kernel numbers per m⁻² and kernels per ear. On average, one irrigation increased grain yield by 21–43% and two to four irrigations by 60–100%. Grain yield was linearly related to seasonal ET with a slope of 1.15 kg m⁻³ in the lowland and 1.73 kg m⁻³ in the piedmont. Water-use efficiency was 0.98–1.22 kg m⁻³ for rainfed wheat and 1.20–1.40 kg m⁻³ for the wheat irrigated 2–4 times. Grain yield response to the amount of irrigation (IRR) was developed using a quadratic function and used to analyze different irrigation scenarios. To achieve the maximum grain yield, IRR was 240 mm in the piedmont and 290 mm in the lowland. When the maximum net profit was achieved, IRR was 195 mm and 250 mm in the piedmont and lowland, respectively. The yield response curve to IRR showed a plateau over a large range of IRR, indicating a great potential in saving IRR while maintaining reasonable high levels of grain yield.     

Partitioning of seasonal evapotranspiration from a commercial furrow-irrigated Sultana vineyard

Seasonal partitioning of evapotranspiration (ET) between transpiration by grapevines (Vitis vinifera) (T _gp) and by cover crops of a ryegrass/clover mixture (T _cc), and soil evaporation (E _s) was performed for a furrow-irrigated vineyard during the 1994/1995 and 1995/1996 growing seasons in south-eastern Australia. ET, determined with a water balance approach, averaged 622 mm. The ET rate averaged over the two seasons increased from 2 mm day^–1 in spring (September to November), when it was dominated by T _cc, to peak rates of around 5 mm d^–1 in summer (December to February) when it was dominated by E _s. T _gp, determined with either heat-pulse sensors or the Penman-Monteith equation, attained peak rates of 0.75 and 0.98 mm d^–1, or 6.2 and 8.1 l vine^–1 day^–1 in the first and second seasons, respectively. Total seasonal T _gp of 109.1 mm (900 l vine^–1) in 1994/1995 and 118.8 mm (980 l vine^–1) in 1995/1996 constituted just 18 – 19% of total ET. T _cc totalled 214 mm (34% of ET) in the first season, when pasture cover was sparse and present for 5 months of the growing season (September to February), and 196 mm (30% of ET) in the second season when pasture cover was heavy but present for only 3 months (September to November). E _s averaged 49% of total ET over both seasons. At least 30% of water used for ET was contributed by antecedent soil water in both seasons. The crop factor (K _c) was largely constant throughout the season with an average value of 0.48. The depletion pattern of soil water indicated that the vine explored the soil profile well beyond 1.0 mm depth and almost evenly up to a distance of 1.5 m from the trunk. Water use efficiencies for fresh fruit yield (WUE), i. e., the ratio of fruit weight to total water use at harvest,were 13.3 and 40.5 kg ha^–1 mm^–1 when based on ET in 1994/1995 and 1995/1996, respectively, and 84.0 and 211.1 kg ha^–1 mm^–1, respectively, when based on T _gp. The T _gp data were used to verify three models of vine transpiration developed in an earlier study. Models based on the green area index or on fraction of incident radiation intercepted by the vine canopy produced good agreement with T _gp. The model based on canopy resistance performed poorly, indicating the difficulty of extrapolating the stomatal response to environmental variables from one set of experimental conditions to another. Received: 23 September 1996     

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.     

Spatially distributed monthly reference evapotranspiration derived from the calibration of Thornthwaite equation: a case study, South of Iran

The Penman-Monteith equation is the most common method for estimating reference crop evapotranspiration (ET_o). Using this method reqiures many different meteorological data, yet few stations with adequate meteorological data may exist in a region. Setting up a station that records the required data for Penman-Monteith equation is expensive. Alternatively, the Thornthwaite equation is a simpler method for estimating ET_o since it is a temperature-based method. In this study, the Thornthwaite equation was spatially calibrated based on the Penman-Monteith method (as the standard and reference method to compute ET_o) for every month of the year, using the meteorologica data of seven synoptic weather stations in Fars province, and seven synoptic stations outside the Fars province. The Thornthwaite equation using effective temperature that has been introduced recently in other studies was used (Camargo et al. in Revista Brasileira de Agrometeorologica 7:251–257, 1999). For this purpose a calibration coefficient k must be determined. The results of the spatial and temporal calibration of the new approach using the Thornthwaite equation showed that for each station different k values should be used monthly. Generally, the k values fluctuated between 0.55 and 1.12, and the mean RMSE for all stations was less than 1 mm day⁻¹, which showed good and reliable agreement between the ET_o estimations obtained from the Penman-Monteith and calibrated Thornthwaite equations. Depending on the geographical location of each station, spatial distribution maps of monthly k values were created for the study area using the inverse distance weighting (IDW) interpolation method. It is therefore possible to estimate monthly ET_o using the appropriate k map and the Thornthwaite equation for different regions of study area instead of using the Penman-Monteith method. This case study showed that the same analysis might be used for the other parts of the country or any part of the world and would result in efficient scheduling of water resources for agriculture.     

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.     

离心泵动—静子流动耦合数值算法研究

针对离心泵动-静子流动耦合问题,采用有限体积方法(FVM)离散不可压旋转流动方程组。研究对流、扩散及源项时、空计算格式。对动子采用非惯性参考系求解相对运动方程组,对静子采用惯性参考系求解绝对运动方程组,动-静界面通过Neuman条件保持通量守恒。将动子出口相对速度转为绝对速度,作为静子进口来流条件,实现动-静子流场的耦合求解。采用模式切换实现算法。为验证算法,对Ubaldi实验用的ERCOFTAC离心叶轮及扩压器耦合流场进行数值研究,获得定常及非定常数值计算结果,并与文献进行比较,结果表明数值预测与实验在数量和分布趋势上均较贴近,验证本算法具有准确预测动-静子流场能力,可用于离心泵流动预测与研究。     

Olive tree response to irrigation with wastewater from the table olive industry

A field experiment was conducted to examine the effect of drip irrigation using wastewater from a table olive industry on physiological, nutritional and yield parameters of olive trees (Olea europaea L.). Very limited information, if any, exists, on the potential of recycling this kind of wastewater in agriculture. Two types of wastewater were used in the experiment, the first with SAR and EC values of 12–56 and 3.5–4.2 dS m⁻¹, respectively, and the second 73–90 and 4.3–6.0 dS m⁻¹. In general, this kind of wastewater has a highly variable composition and SAR values that are too high for agricultural purposes. Olive trees rapidly responded to wastewater application. Compared to the control (fresh water), the more saline wastewater caused important decreases in leaf water potential, stomatal conductance to H₂O and the photosynthesis rate after only 15 days of irrigation, the reduction being more pronounced after 2 months of irrigation. This treatment also caused a rapid, significant reduction in leaf N concentration, as compared with the N level in the trees before irrigation. Both types of wastewater significantly reduced olive yield, compared to that obtained in the control. These results indicate that this kind of wastewater is unsuitable for application to olive orchards under irrigation. Received: 16 August 1999     

混流式水轮机尾水管旋涡流的LES

基于N－S方程和大涡模拟（LES）模型，采用贴体坐标和四面体网格系统，用SIMPLE算法求解，对混流式水轮机内部流动进行了三维非定常紊流计算，较准确地预测了一混流式水轮机在各工况下的内部流动，尤其是尾水管和转轮内的旋涡流动。     

Monitoring the pollution risk and water use in orchard terraces with mango and cherimoya trees by drainage lysimeters

Agricultural nonpoint-source pollution is the leading cause of water-quality degeneration of rivers and groundwater. In this context, the coast of Granada province (SE Spain) is economically an important area for the subtropical fruit cultivation. This intensively irrigated agriculture often uses excessive fertilizers, resulting to water pollution. Therefore, a 2-year experiment was conducted using drainage lysimeters to determine the potential risk of nutrient pollution in mango (Mangifera indica L. cv. Osteen) and cherimoya (Annona cherimola Mill. cv. Fino de Jete) orchards. These lysimeters were used to estimate the nutrient budgeting for each crop. NO₃-N, NH₄-N, PO₄-P and K losses according to lysimeters were, respectively, 55.1, 12.4, 3.7, and 0.6 for mango and 61.8, 17.8, 4.9, and 0.5 kg ha⁻¹ yr⁻¹, for cherimoya. NO₃, concentrations in the leachates ranged from 1.8 to 44.3 mg L⁻¹, and from 23.0 to 51.0 mg L⁻¹, for mango and cherimoya, respectively, in some cases exceeding the limits for safe drinking water. PO₄ also exceeded the permitted concentrations related to eutrophication of water, ranging from 0.07 to 0.5 mg L⁻¹ and from 0.12 to 0.68 mg L⁻¹ from mango and cherimoya lysimeters, respectively. With respect to the nutrient balance, N, P, and K removed by cherimoya fruits was 76.4, 5.5, and 22.6 kg ha⁻¹ yr⁻¹, and for mango fruits 30.2, 3.3 and 27.8 kg ha⁻¹ yr⁻¹, respectively. Nutrient losses in the leachates were surprisingly low, considering total N, P, and K applied during the year, in mango lysimeters 3.8, 0.11, and 12.6%, and in cherimoya lysimeters 7.7, 0.23 and 16.0%, respectively, indicating a potential soil accumulation and eventual loss risk, especially during torrential rains. Crop coefficient (Kc) values of mango trees varied within ranges of 0.35–0.67, 0.55–0.89, and 0.39–0.80 at flowering, fruit set, and fruit growth, respectively. Kc values for cherimoya trees had ranges of 0.58–0.67, 0.61–0.68, and 0.43–0.62 at flowering, fruit set and fruit growth, respectively. In this study, the Kc values of mango and cherimoya were significantly correlated to julian days. Therefore, the estimated WUE in the mango and cherimoya orchards reached 21.2 and 14.0 kg ha⁻¹ mm⁻¹, respectively. Thus, this study highlights the urgency to establish the optimal use of fertilizers and irrigation water with respect to crop requirements, to preserve surface-water and groundwater quality, thereby achieving more sustainable agriculture in orchard terraces.     

Neural networks with artificial bee colony algorithm for modeling daily reference evapotranspiration

The study investigates the ability of artificial neural networks (ANN) with artificial bee colony (ABC) algorithm in daily reference evapotranspiration (ET₀) modeling. The daily climatic data, solar radiation, air temperature, relative humidity, and wind speed from two stations, Pomona and Santa Monica, in Los Angeles, USA, are used as inputs to the ANN–ABC model so as to estimate ET₀ obtained using the FAO-56 Penman–Monteith (PM) equation. In the first part of the study, the accuracy of ANN–ABC models is compared with those of the ANN models trained with Levenberg–Marquardt (LM) and standard back-propagation (SBP) algorithms and those of the following empirical models: The California Irrigation Management System (CIMIS) Penman, Hargreaves, and Ritchie methods. The mean square error (MSE), mean absolute error (MAE) and determination coefficient (R²) statistics are used for evaluating the accuracy of the models. Based on the comparison results, the ANN–ABC and ANN–LM models are found to be superior alternative to the ANN–SBP models. In the second part of the study, the potential of the ANN–ABC, ANN–LM, and ANN–SBP models in estimation ET₀ using nearby station data is investigated.     

Evaluation of crop water stress index for LEPA irrigated corn

This study was designed to evaluate the crop water stress index (CWSI) for low-energy precision application (LEPA) irrigated corn (Zea mays L.) grown on slowly-permeable Pullman clay loam soil (fine, mixed, Torrertic Paleustoll) during the 1992 growing season at Bushland, Tex. The effects of six different irrigation levels (100%, 80%, 60%, 40%, 20%, and 0% replenishment of soil water depleted from the 1.5-m soil profile depth) on corn yields and the resulting CWSI were investigated. Irrigations were applied in 25 mm increments to maintain the soil water in the 100% treatment within 60–80% of the “plant extractable soil water” using LEPA technology, which wets alternate furrows only. The 1992 growing season was slightly wetter than normal. Thus, irrigation water use was less than normal, but the corn dry matter and grain yield were still significantly increased by irrigation. The yield, water use, and water use efficiency of fully irrigated corn were 1.246 kg/m², 786 mm, and 1.34 kg/m³, respectively. CWSI was calculated from measurements of infrared canopy temperatures, ambient air temperatures, and vapor pressure deficit values for the six irrigation levels. A “non-water-stressed baseline” equation for corn was developed using the diurnal infrared canopy temperature measurements as T _c–T _a = 1.06–2.56 VPD, where T _c was the canopy temperature (°C), Ta was the air temperature (°C) and VPD was the vapor pressure deficit (kPa). Trends in CWSI values were consistent with the soil water contents induced by the deficit irrigations. Both the dry matter and grain yields decreased with increased soil water deficit. Minimal yield reductions were observed at a threshold CWSI value of 0.33 or less for corn. The CWSI was useful for evaluating crop water stress in corn and should be a valuable tool to assist irrigation decision making together with soil water measurements and/or evapotranspiration models. Received: 19 May 1998     

Field study of water mass balance in a wastewater land application system

A water mass balance approach was proposed as a design approach for surface application of effluent for on-site wastewater systems, but little field study was available to verify the validity of the approach. The mass balance was investigated using plots at the wastewater treatment plant of the City of Littlefield, Texas, from October 2005 to September 2007. The highest monthly precipitation is 181 mm while the lowest is 1 mm; the individual Christiansen’s coefficient of uniformity (UCC) ranged from 31 to 75%, yet the annual UCC was 83%. The average daily leaching amount was measured in the range of 0–2 mm. The ratio of leaching to total water input in each sampling period is 0.49–37.05%, while the ratio of cumulative leaching to cumulative total water input is relatively constant and ranged from 12 to 14%. The regression model between cumulative leaching (y) and cumulative total input (x) was analyzed and resulted in an R ² of 0.98.