Water and nitrogen distribution as affected by fertigation of ammonium nitrate from a point source

The simultaneous distribution of water, nitrate, and ammonium from a point source discharging an ammonium nitrate (NH₄NO₃) solution was measured using the gravimetric method. A 15° wedge-shaped plexiglass container was used to represent one twenty-fourth of the complete cylinder. The variables affecting water and solute distributions, including application rate, input concentration, and applied volume were investigated and their effects are presented on a basis of a completely cylindrical system. In the experiments, the apparent cylindrical application rate was varied from 0.6 to 7.8 l/h, the apparent cylindrical applied volume from 6 to 15 l, and the input concentration from 100 to 700 mg/l. Monitoring of the water movement revealed that the ultimate saturated entry radius on the surface increased with the application rate. Surface wetted radius and vertical wetted depth were proportional to the applied water volume with power values of about 0.3 and 0.5, respectively. The shape of the wetted soil zone was clearly related to the application rate and the applied volume. An increase in rate resulted in an increase in the wetted horizontal area and a decrease in the wetted soil depth, while an increase in volume resulted in an increase in wetted horizontal and vertical areas. A uniform distribution of nitrate concentration in the soil was found 15 cm around the point source for a given input concentration. For any input concentration, the accumulation of nitrate at the boundary of the wetted volume was observed. The nitrate concentration in the soil was primarily affected by the input concentration; there was an increased nitrate concentration with a higher input concentration. The results of ammonium distribution measurements indicated that there existed an extremely high ammonium concentration in the proximity of the point source (about 2.5-7.5 cm from the source). An increased input concentration produced a higher ammonium concentration around the point source. Results also demonstrated that the influence of fertigation on the ammonium distribution was restricted in a small volume, about 10 cm around the point source. Beyond this range, input concentration, application rate, and total applied volume had insignificant effects on ammonium distribution. The information obtained from this research is useful in the design, operation, and management of a fertigation system with drip irrigation.     

Bell pepper response to surface and subsurface drip irrigation under different fertigation levels

A two-year field experiment was conducted in 2007 and 2008 to investigate different bell pepper responses to subsurface drip irrigation (SDI) and surface drip irrigation (DI) under four nitrogen levels: 0, 75, 150, and 300 kg/ha N (N₀, N₇₅, N₁₅₀, and N₃₀₀, respectively). Irrigation interval was set at 4 days. Bell pepper yield under SDI was significantly higher than that under DI by 4% in 2007 (13% in 2008). Water consumption under SDI was lower than that under DI by 6.7% in 2007 (7.3% in 2008). Meanwhile, root length density under SDI was obviously higher than that under DI by 11.8% in 2007 (12.5% in 2008). The percentage of root length below 10-cm soil depth under SDI was higher than that under DI by 7%, proving that SDI promotes crop root growth and enhances downward root development. Soil N residue under SDI was lesser than that under DI. Lastly, SDI with N application of 150 kg/ha is recommended as an optimal fertigation practice in improving bell pepper yield and water-use efficiency, as well as in NO₃ ⁻-N leaching.     

Evaluation of humic substances fertigation through surface and subsurface drip irrigation systems on potato grown under Egyptian sandy soil conditions

The aim of this study was to evaluate the effect of humic substances application in sandy soil under surface and subsurface drip irrigation systems on potato tubers yield quantity, quality, nutrients concentration in tubers and soil fertility after harvesting. For this purpose, field experiment was carried out at the experimental farm of the Agricultural Research Station, National Research Center, El-Nubaria district, Egypt during the winter season of 2007/2008. The used experimental design was split plot design with three replicates, main treatments were presented irrigation systems, i.e. surface and subsurface drip irrigation, while subtreatments were presented rates of humic substances additives which were 0, 60 and 120 kg ha⁻¹. Results showed that increasing humic substances application rates up to 120 kg ha⁻¹ enhanced tubers yield quantity, starch content and total soluble solids. The increase of humic substances application rates was associated with the decrease of nutrients leaching, which was reflected on increasing macro- and micronutrients concentration in potato tubers, as well as increasing concentration of these nutrients in soil after tubers harvesting. Subsurface drip irrigation system was found to be more efficient than surface drip irrigation system on improving tubers yield quantity, quality parameters and nutrients concentration content, in addition to soil fertility after harvesting.     

滴灌均匀系数对土壤水分和氮素分布的影响

为了确定滴灌均匀系数的设计与评价标准,在日光温室内研究了滴灌施肥灌溉均匀性和施氮量对土壤水氮分布特性的影响。试验中滴灌均匀系数（Cu）设置0.62、0.80和0.96 3个水平,施氮量设置150和300 kg/hm2 2个水平。土壤含水率和电导率采用沿毛管均匀布置的TDR探头（Hydra Probe）连续监测,并定期取土样测试土壤硝态氮和铵态氮含量。结果表明,在作物生育期内3种滴灌均匀系数处理的土壤含水率一直保持很高的均匀系数,滴灌均匀系数和施氮量对土壤含水率均值及其均匀系数的影响均不显著（α=0.05）。土壤电导率及硝态氮含量的均匀性在很大程度上取决于土壤初始氮素含量的均匀性,其均匀系数低于土壤含水率的均匀系数,滴灌均匀系数的影响也不显著。从获得均匀的土壤水氮分布的角度出发,现行滴灌均匀系数标准尚有降低的空间。     

Spatial and temporal distributions of nitrogen and crop yield as affected by nonuniformity of sprinkler fertigation

Deep percolation and nitrate leaching are important considerations in the design of sprinkler systems. Field experiments were therefore conducted to investigate the influence of nonuniformity of sprinkler irrigation on deep percolation and spatial distributions of nitrogen and crop yield during the growing season of winter wheat at an experiment station in Beijing, China. Three experimental plots of a sandy clay loam soil in the 0–40 cm depth interval and a loamy clay soil below 40 cm were irrigated with a sprinkler irrigation system that had a seasonal averaged Christiansen irrigation uniformity coefficient (CU) varying from 72 to 84%. Except for the fertilizer applied before planting, fertilizer was applied with the sprinkler irrigation system. The corresponding seasonal averaged CU for fertigation varied from 71 to 85%. Daily observation of matrix water potentials in the root zone showed that little deep percolation occurred. Consequently, the effect of sprinkler uniformity on deep percolation was minor during the irrigation season for the soil tested. Intensive gravimetric soil core samplings were conducted several times during the irrigation season in a grid of 5 m × 5 m for each plot to determine the spatial and temporal variation of NH₄-N and NO₃-N contents. Soil NH₄-N and NO₃-N exhibited high spatial variability in depth and time during the irrigation season with CU values ranging from 23 to 97% and the coefficient of variation ranging from 0.04 to 1.06. A higher uniformity of sprinkler fertigation produced a more uniform distribution of NH₄-N, but the distribution of NO₃-N was not related to fertigation. Rather it was related to the spatial variability of NO₃-N before fertigation began. At harvest, the distribution of dry matter above ground, nitrogen uptake, and yield were measured and the results indicated that sprinkler fertigation uniformity had insignificant effects on the parameters mentioned above. Field experimental results obtained from this study suggest that sprinkler irrigation if properly managed can be used as an efficient and environment-friendly method of applying water and fertilizers.     

Dynamics and modeling of soil water under subsurface drip irrigated onion

Subsurface drip irrigation provides water to the plants around the root zone while maintaining a dry soil surface. A problem associated with the subsurface drip irrigation is the formation of cavity at the soil surface above the water emission points. This can be resolved through matching dripper flow rates to the soil hydraulic properties. Such a matching can be obtained either by the field experiments supplemented by modeling. Simulation model (Hydrus-2D) was used and tested in onion crop (Allium cepa L.) irrigated through subsurface drip system during 2002-2003, 2003-2004 and 2004-2005. Onion was transplanted at a plant to plant and row to row spacing of 10 cm × 15 cm with 3 irrigation levels and 6 depths of placement of drip lateral. The specific objective of this study was to assess the effect of depth of placement of drip laterals on crop yield and application of Hydrus-2D model for the simulation of soil water. In sandy loam soils, it was observed that operating pressures of up to 1.0 kg cm⁻² did not lead to the formation of cavity above the subsurface dripper having drippers of 2.0 l h⁻¹ discharge at depths up to 30 cm. Wetted soil area of 60 cm wide and up to a depth of 30 cm had more than 18% soil water content, which was conducive for good growth of crop resulting in higher onion yields when drip laterals were placed either on soil surface or placed up to depths of 15 cm. In deeper placement of drip lateral (20 and 30 cm below surface), adequate soil water was found at 30, 45 and 60 cm soil depth. Maximum drainage occurred when drip lateral was placed at 30 cm depth. Maximum onion yield was recorded at 10 cm depth of drip lateral (25.7 t ha⁻¹). The application of Hydrus-2D confirmed the movement of soil water at 20 and 30 cm depth of placement of drip laterals. The model performance in simulating soil water was evaluated by comparing the measured and predicted values using three parameters namely, AE, RMSE and model efficiency. Distribution of soil water under field experiment and by model simulation at different growth stages agreed closely and the differences were statistically insignificant. The use of Hydrus-2D enabled corroborating the conclusions derived from the field experimentation made on soil water distribution at different depths of placement of drip laterals. This model helped in designing the subsurface drip system for efficient use of water with minimum drainage.     

Water and nitrate movement in drip-irrigated onion under fertigation and irrigation treatments

Frequent fertigation of crops is often advocated in the technical and popular literature, but there is limited evidence of the benefits of high-frequency fertigation. Field experiments were conducted on an Indo-American Hybrid var., Creole Red, of onion crop during three winter seasons of 1999–2000 through 2001–2002 in coarse-textured soil of Delhi under the semi-arid region of India. Three irrigation levels of 60, 80 and 100% of the crop evapotranspiration (ET) and four fertigation frequencies of daily, alternate day, weekly and monthly comprised the fertigation treatment. Analysis of soil samples indicated considerable influence of fertigation frequency on NO₃-N distribution in soil profile. NO₃-N in lower soil profiles (30.0–60.0 cm soil depth) was marginally affected in daily, alternate day and weekly fertigation. However, fluctuations of NO₃-N content in 0.0–15.0, 15.0–30.0, 30.0–45.0 and 45.0–60.0 cm soil depth was more in monthly fertigation frequency. The level of soil NO₃-N after the crop season shows that more NO₃-N leached through the soil profile in monthly fertigation frequency. Amounts of irrigation water applied in three irrigation treatments proved to be too small to cause significant differences in the content of NO₃-N leached beyond rooting depth of onion. Yield of onion was not significantly affected in daily, alternate day and weekly fertigation, though there was a trend of lower yields with monthly fertigation. The highest yield was recorded in daily fertigation (28.74 t ha⁻¹) followed by alternate day fertigation (28.4 t ha⁻¹). Lowest yield was recorded in monthly fertigation frequency (21.4 t ha⁻¹). Application of 56.4 cm irrigation water and 3.4 kg ha⁻¹ urea per fertigation (daily) resulted in highest yield of onion with less leaching of NO₃-N.     

Simulation of soil water dynamics under subsurface drip irrigation from line sources

A mathematical model which describes water flow under subsurface drip lines taking into account root water uptake, evaporation of soil water from the soil surface and hysteresis in the soil water characteristic curve θ(H) is presented. The model performance in simulating soil water dynamics was evaluated by comparing the predicted soil water content values with both those of Hydrus 2D model and those of an analytical solution for a buried single strip source. Soil water distribution patterns for three soils (loamy sand, silt, silty clay loam) and two discharge rates (2 and 4 l m⁻¹ h⁻¹) at four different times are presented. The numerical results showed that the soil wetting pattern mainly depends on soil hydraulic properties; that at a time equal to irrigation duration decreasing the discharge rate of the line sources but maintaining the applied irrigation depth, the vertical and horizontal components of the wetting front were increased; that at a time equal to the total simulation time the discharge rate has no effect on the actual transpiration and actual soil evaporation and a small effect on deep percolation. Also the numerical results showed that when the soil evaporation is neglected the soil water is more easily taken up by the plant roots.     

滴灌条件下沙地土壤水分分布与运移规律

通过对不同流量的滴头,不同灌水历时条件下沙地土壤滴灌湿润体的大小与形状的观测观察,以及对滴灌停止后不同间隔时间的土壤湿润锋运移,湿润体的发展,土壤水分的分布与再分布测量分析,使之对滴灌条件下土壤湿润体的大小及发展、土壤水分的分布与再分布规律等有了比较清楚的认识。本文不进行理论上的土壤水分分布模拟,仅从众多的实测资料分析着手,揭示土壤水分分布与运移规律,旨在为砂壤地玉米滴灌的参数确定（滴头间距、流量、灌水历时等）提供基本依据。通过土壤水分与湿润体实测资料分析认为,滴灌玉米的滴头流量在２．０ｌ／ｈ左右,灌水历时２～３ｈ,滴头间距为５０ｃｍ左右为宜。过长的灌水历时（大于３ｈ）即可能导致灌溉水的无为损失,造成先进的节水灌溉技术条件下新的水浪费问题。滴头间距过密,会加大不必要的工程投资。     

Soil water variability under subsurface drip and furrow irrigation

Non-uniformities in soil hydraulic properties and infiltration rates are considered to be major reasons for the inefficiencies of some surface irrigation systems. These non-uniformities may cause non-uniformities in soil water contents and could potentially affect plant growth. To investigate whether the non-uniformities in soil water contents can be overcome by well-managed irrigation systems, fields with clay loam soils and planted to cotton were irrigated with a continuous-flow, a surge flow, and a subsurface drip system. Measurements of water contents in each field were taken throughout the growing season at several depths. The water contents measured within the top 0–0.9 m in the three irrigations systems were evaluated in terms of their spatial and temporal variabilities. The analyses indicated that on this soil, use of the surge flow system did not lead to increased spatial uniformities of soil water contents compared with the continuous-flow system. Use of the subsurface drip system resulted in very non-uniform soil water contents above the depth of the emitters. Variability in water contents below the emitter depth was comparable to the surface irrigation systems. Received: 26 March 1996     

Nitrate leaching and soil nitrate content as affected by irrigation uniformity in a carrot field

High value crops such as carrot planted in coarse soils of the Southern San Joaquin Valley in California are prime candidates for nitrate leaching through irrigation nonuniformity. A 2-year study was carried out to explore the impact of irrigation uniformity on nitrate leaching. Irrigation uniformity was measured using catchcans. Soil nitrate (NO₃-N) and ammonium (NH₄-N) contents were measured from soil sampled at different depths and times during two growing seasons. Nitrate leaching was determined using ion-exchange resin bags at 1-m depth sampled three times during each season. Although, soil NO₃-N as well as seasonal irrigation was significantly higher along the lateral irrigation pipe than between the sprinklers, nitrate leaching was not significantly higher. As expected, soil nitrate content decreased as percolation increased for both years. Nitrate leaching, as estimated by anion-exchange resin bags, was positively correlated to soil NO₃-N content but was not correlated to irrigation depth, irrigation uniformity, or deep percolation. Field variation in saturated hydraulic conductivity (K_s), soil organic matter (OM), and soil water retention at field capacity had limited effect on NO₃-N and NH₄-N distributions in the profile and on nitrate leaching. The results of this experiment suggest that irrigation nonuniformity has less impact on nitrate movement than suggested by earlier studies.     

Effect of surface and subsurface drip fertigation on sweet corn rooting,uptake, dry matter production and yield

Summary Subsurface (SS) drip fertigation may increase sweet corn ear yield relative to surface (S) fertigation, because immobile nutrients are delivered at the center of the soil-root volume rather than on top of the soil. A container (1 × 1 × 1 m) experiment was conducted on a loessial soil (Haploxeralf) to test this hypothesis. Marketable and total ear yields were higher for tricklers placed 30 cm below the soil surface (3.22 and 4.90 kg m^–2, respectively) than on the surface (2.86 and 4.30 kg m^–2, respectively). Total fresh weight, dry matter production and plant height during the growing season were also greater for subsurface emitters. Deep trickler position significantly increased P and K content at the center of the root zone. The enhanced concentration apparently stimulated plant rooting which, together with the higher nutrient activity in the soil solution, increased P and K uptake rates, which in turn facilitated the higher dry matter production and commercial yield relative to surface trickler placement. The higher root activity in SS than in S fertigation was reconfirmed by soil air CO₂ concentration measurements which showed significant differences between the two treatments during the growth season.On studies in the ARO from The College of Postgraduates of Mexico, Montecillos, Mexico.     

Tape depth and germination method influence patterns of salt accumulation with subsurface drip irrigation

Subsurface drip irrigation (SDI) can result in accumulation of soluble salts at or near the soil surface. In the southwestern USA, rainfall is usually inadequate for stand establishment, thus supplemental irrigation is necessary. Use of sprinklers to minimize salt concentrations near the soil surface is an alternative to using SDI for stand establishment. Our objective was to evaluate the effects of germination method (irrigation with SDI or sprinklers), depth of SDI tape (0.18 and 0.25 m), and irrigation water salinity (1.5 and 2.6 dS m⁻¹) on salt and Br distribution after each of two consecutive growing seasons. Treatments consisted of factorial combinations of these three factors. Bromide was used to trace salt accumulation from the drip tape. After season 1, the highest salt concentrations (ECe up to 11 dS m⁻¹) were in the top 3 cm of soil. Below 3 cm, soil EC dropped significantly and remained constant to 1.05 m. Similarly, Br concentrations were highest in the top 3 cm of soil. The mass of salt and Br recovered in the top 3 cm were significantly affected by tape depth, and water EC significantly affected salt mass. Salt present in the soil after season 1 adversely affected crop emergence in season 2, where SDI was used for stand establishment. After season 2, the highest salt and Br concentrations were at about 25 cm depth, probably due to 210 mm of rainfall that occurred near the end of the growing season. There were no significant differences among treatments in the mass of either salt or Br in the top 3 cm or 16 cm of the soil profile after season 2. Timely rainfall, transplanting rather than direct seeding, and changing bed geometry can reduce dependence on sprinklers for stand establishment.     

Soil moisture distribution patterns under surface and subsurface drip irrigation systems in sandy soil using neutron scattering technique

This study was carried out at the experimental field station of the Atomic Energy Authority in Anshas, Egypt, by the aim of assessing the soil moisture status under surface and subsurface drip irrigation systems, as a function of the variation in the distance between drippers along and between laterals. Moisture measurements were carried out using neutron moisture meter technique, and water distribution uniformity was assessed by applying Surfer Model. The presented data indicated that the soil moisture distribution and its uniformity within the soil profile under surface drip was to great extent affected by the distance between drippers rather than that between laterals. Generally, the soil moisture distribution under using 30-cm dripper spacing was better than of that under 50 cm. Under subsurface drip irrigation, the allocation of the irrigation system was the factor that dominantly affected the moisture trend under the studied variables. Installing the system at 30 cm from the soil surface is the one to be recommended as it represents the active root zone for most vegetable crops, beside it leads to a better water saving in sandy soils than that allocated at 15 cm depth.     

Field evaluation of fertigation uniformity as affected by injector type and manufacturing variability of emitters

Fertigation with microirrigation systems is increasing in popularity. Uniformity of fertigation is important for many reasons. Field experiments were conducted to evaluate the effects of injector types and emitters on fertigation uniformity by simultaneously measuring the distributions of water application, solution concentration, and fertilizer applied within a subunit of microirrigation system. Three conventionally used injectors, a water-driven piston proportional pump, a venturi device, and a differential pressure tank, were evaluated with three different emitters. The results indicated that both manufacturing variability of emitters and injector types had a very significant effect on the uniformity of fertilizer applied, while the uniformity of water application was mainly dependent on emitter type. The uniformity of solution concentration was dependent on injection methods. Emitters having a higher manufacturer’s variation produced a more nonuniform distribution of water application and fertilizer applied. For a given emitter type, a differential pressure tank produced considerably higher coefficients of variation (Cv) for water application and fertilizer applied than a proportional pump or a venturi injector because a differential pressure tank released fertilizer in a decreasing rate with time. To obtain a uniform fertigation distribution, an injector that can inject fertilizers in a constant rate is recommended. The relationship between water application uniformity and fertigation uniformity for a microirrigation system was established for different injection methods. Cv for fertilizer applied was very close to water application Cv for a microirrigation system using a proportional pump or a venturi injector as an injection device. However, fertilizer Cv for a differential pressure tank was approximately double of the water application Cv. The injection method and injector performance should therefore be considered in the design of microirrigation systems.     

Sprinkler water distributions as affected by winter wheat canopy

Sprinkler uniformity is often used to evaluate irrigation system performance. The measurement of uniformity is generally made from one test when no crop is present. However, a developing crop canopy has significant potential to modify the distribution of water applied during irrigation. This study was conducted to evaluate the influence of a winter wheat canopy on sprinkler uniformity and on canopy-intercepted water by measuring water distributions above and below the canopy. The Christiansen uniformity coefficient (CU) was calculated on both a daily and a cumulative basis. The CU was higher below the canopy than above the canopy. Canopy-intercepted water, which is here defined as the sum of canopy storage and stemflow, increased with increasing water application depth. Sprinkler uniformity had no significant effect on the mean amount of water interception by the canopy. The ratio of water interception to total water application depth for the whole irrigation season was between 0.24 and 0.28. The CUs calculated from the cumulative depth caught above and below the canopy are larger than the averages of individual CU values during the irrigation season. Measurement of individual CUs during the irrigation season therefore underestimates the cumulative CU. Experimental results also demonstrated that sprinkler uniformity in this study had little effect on crop yield. Received: 1 February 2000     

循环曝气地下滴灌的温室番茄生长与品质

循环曝气滴灌可以大幅度提高灌溉水掺气比例,有效改善普通地下滴灌引起的黏质型土壤根区间歇性缺氧环境,提高作物生产力.以河南省中牟县黄河淤积黄黏土为供试土壤,以温室番茄为供试对象,研究循环曝气地下滴灌对番茄生理及品质的影响.结果表明,与普通地下滴灌(对照处理)相比,相同灌溉定额条件下曝气处理番茄果实前5次产量提高了29.15%;番茄的水分利用效率提高了20.72%.曝气处理气孔导度提高了30.51%,植物的生长活力得到增强.番茄果实维生素C含量提高了13.25%,可溶性固形物含量提高了8.62%,糖酸比提高了22.05%,而总酸含量和硬度分别下降了15.50%和11.19%.曝气处理最大根长增加了16.75%,根冠质量之比提高了25.81%.综合分析表明,曝气滴灌可显著促进黄黏土中番茄的生长,促进番茄果实成熟,有效提高作物产量,改善番茄品质.     

Simulation of 1D surface and 2D subsurface water flow and nitrate transport in alternate and conventional furrow fertigation

Increasing water and fertilizer productivity stands as a relevant challenge for sustainable agriculture. Alternate furrow irrigation and surface fertigation have long been identified as water and fertilizer conserving techniques in agricultural lands. The objective of this study was to simulate water flow and fertilizer transport in the soil surface and in the soil profile for variable and fixed alternate furrow fertigation and for conventional furrow fertigation. An experimental data set was used to calibrate and validate two simulation models: a 1D surface fertigation model and the 2D subsurface water and solute transfer model HYDRUS-2D. Both models were combined to simulate the fertigation process in furrow irrigation. The surface fertigation model could successfully simulate runoff discharge and nitrate concentration for all irrigation treatments. Six soil hydraulic and solute transport parameters were inversely estimated using the Levenberg–Marquardt optimization technique. The outcome of this process calibrated HYDRUS-2D to the observed field data. HYDRUS-2D was run in validation mode, simulating water content and nitrate concentration in the soil profiles of the wet furrows, ridges and dry furrows at the upstream, middle and downstream parts of the experimental field. This model produced adequate agreement between measured and predicted soil water content and nitrate concentration. The combined model stands as a valuable tool to better design and manage fertigation in alternate and conventional furrow irrigation.     

温室番茄对微小流量滴灌埋深与灌水量的响应

试验采用番茄品种为“中研998”,于2017年3-7月在中国农业大学通州实验站春秋大棚中进行.试验共6个处理,3个灌水梯度(以田持的85%,75%,65%计)与2个痕量带埋深梯度(15,30 cm).研究结果表明:埋深与灌水下限对番茄植株株高茎粗的影响规律性不强;深埋处理更有利于植株茎干物质积累,浅埋更有利于叶干物质积累,说明浅埋处理植株蒸腾量更大;埋深与灌水下限对根干重的影响规律性不强;深埋更有利于提高果实产量、氮素表观利用率与肥料偏生产力,浅埋更有利于提高水分利用效率;灌水下限越低,果实产量、水分利用效率、氮素表观利用率与肥料偏生产力越高.因此根据本试验研究的各项指标综合得出埋深为30 cm、灌水下限为65%田持时最适宜此种试验条件下温室番茄的生长.     

Distribution of rainfall and soil moisture content in the soil profile under citrus tree canopy and at the dripline

The plant canopy intercepts rain and thus can alter the distribution of water under the canopy as compared to that along the dripline. The effects of a citrus (Citrus sinensis L. Osbeck) tree (25-year-old, Valencia orange) canopy on the distribution of rainfall and soil moisture content within the soil profile either along the dripline (D) or under the canopy near the trunk (inner side; I), and midway between I and Dripline (M) were evaluated, on the east and west sides of trees planted along north-south rows. Results of eleven storm events in 1995 (mean of east and west sides) revealed that the amounts of precipitation at the D, M, and I positions were 97–140, 47–94, and 52–79% of the incident rainfall, respectively. Thus, canopy interception of incident rainfall was quite appreciable. The soil moisture content was greater along the dripline compared to that at the M and I positions, particularly in the deeper (≥60 cm) soil profile. The water flux was significantly greater at the dripline than under the canopy indicating a greater leaching potential of soil-applied fertilizers and other chemicals when placed along the dripline. A substantial reduction in the rainfall and water flux under the canopy as a result of canopy interception suggests that application of fertilizer and chemicals under the canopy could minimize leaching losses. Received: 10 November 1997