Developing a reliable strategy to infer the effective soil hydraulic properties from field evaporation experiments for agro-hydrological models

The Richards equation has been widely used for simulating soil water movement. However, the take-up of agro-hydrological models using the basic theory of soil water flow for optimizing irrigation, fertilizer and pesticide practices is still low. This is partly due to the difficulties in obtaining accurate values for soil hydraulic properties at a field scale. Here, we use an inverse technique to deduce the effective soil hydraulic properties, based on measuring the changes in the distribution of soil water with depth in a fallow field over a long period, subject to natural rainfall and evaporation using a robust micro Genetic Algorithm. A new optimized function was constructed from the soil water contents at different depths, and the soil water at field capacity. The deduced soil water retention curve was approximately parallel but higher than that derived from published pedo-tranfer functions for a given soil pressure head. The water contents calculated from the deduced soil hydraulic properties were in good agreement with the measured values. The reliability of the deduced soil hydraulic properties was tested in reproducing data measured from an independent experiment on the same soil cropped with leek. The calculation of root water uptake took account for both soil water potential and root density distribution. Results show that the predictions of soil water contents at various depths agree fairly well with the measurements, indicating that the inverse analysis is an effective and reliable approach to estimate soil hydraulic properties, and thus permits the simulation of soil water dynamics in both cropped and fallow soils in the field accurately.     

Analysis of soil-water uptake from a drying loess soil by an oat crop using a simulation model

Summary A simulation model of water uptake by a crop was developed to facilitate synthesis of field and laboratory observations with existing knowledge, and to analyze and predict affects of management practices, such as tillage, on water uptake from a drying soil. Radial water flow resistance in soil Rs was estimated by the single root flow model. Leaf stomata closure was represented by an observed minimal leaf water potential. Flow resistances, per unit root length Rr and in the plant Rp, were assumed to be constant and were evaluated together with an effective root length factor Frl, in the course of simulating a ten week period of observed soil water depletion by a crop of oats. Rr, Rp, and Frl were found to have similar values to those reported in the literature. Potential transpiration and evaporation and their ratio were estimated by the methods of Van Bavel (1966) and Denmead (1973). Evaporation reduction due to soil drying was estimated empirically.Cessation of soil water depletion (attainment of a permanent wilting soil water content) in the 0 – 20 cm soil layer, during the last ten-day period, was explained to be the net result of soil water extraction by the roots and backflow of water from the roots into the soil. Simulated onset of crop stress (closure of stomata) was found to be characterized by: (a) a steady decrease in average soil water potential, at a rate of about 500 cm-water per cm-soil water depletion; (b) a tenfold increase in the average soil resistance to radial flow, to about the same magnitude as average radial flow resistance in the roots; and (c) soil water diffusivities in the 0 – 50 cm layer being about 6 cm²/day. Sensitivity analyses showed that the ratio of actual to potential cumulative transpiration RCT depended primarily on potential evapotranspiration, rainfall, the unsaturated-to-saturated hydraulic conductivity exponent and plant cover. RCT was affected similarly by changes in Rr and in Rs. Under the conditions tested, zero tillage may increase RCT significantly only if it increases deep rooting beyond the 50 cm depth.Joint contribution from the Georg-August University, Göttingen, FRG, and the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, A.R.O. No. 207-E, 1980 Series     

滴灌湿润比对成龄库尔勒香梨树根系分布的影响

在充分灌溉条件下采用3种滴灌湿润比(20%、40%、60%).以漫灌为对照.研究库尔勒成龄香梨吸水根(根直径≤1 mm)在0～70 cm土层内分布的变化,探讨滴灌湿润比对根系分布的影响.结果表明,漫灌成龄库尔勒香梨树吸水根水平方向上在距树体1～2 m内从树行由内向外呈递增趋势;垂直方向上根系随深度呈递增趋势.滴灌对成龄...     

塔里木灌区膜下滴灌棉田土壤水分动态与耗水特性

土壤水分条件是棉花生长和发育的重要因素。为了研究塔里木灌区膜下滴灌棉田土壤水分特征,于2014年4月18日至10月31日采用中子仪对膜下滴灌棉田0~120cm土壤水分进行观测,分析了不同生育期土壤含水率的时空变化,采用水量平衡原理计算了膜下滴灌棉田耗水量。结果表明:4月中旬到7月中旬为土壤水分稳定期,7月中旬到8月底为土壤水分剧烈变化期,8月底到10月底为缓慢消耗期;0~20cm为土壤水分活跃层,20~60cm为土壤水分次活跃层,60~120cm为土壤水分稳定层;灌溉入渗水主要分布在0~40cm;膜下滴灌棉田苗期、蕾期、花铃期、吐絮期的耗水强度分别为0.63、2.62、7.01、0.71mm/d。     

Long term use of saline water for irrigation

Use of saline drainage water in irrigated agriculture, as a means of its disposal, was evaluated on a 60 ha site on the west side of the San Joaquin Valley. In the drip irrigation treatments, 50 to 59% of the irrigation water applied during the six-year rotation was saline with an EC_w ranging from 7 to 8 dS/m, and containing 5 to 7 mg/L boron and 220 to 310 g/L total selenium. Low salinity water with an EC_w of 0.4 to 0.5 dS/m and B 0.4 mg/1 was used to irrigate the furrow plots from 1982 to 1985 after which a blend of good quality water and saline drainage water was used. A six-year rotation of cotton, cotton, cotton, wheat, sugar beet and cotton was used. While the cotton and sugar beet yields were not affected during the initial six years, the levels of boron (B) in the soil became quite high and were accumulated in plant tissue to near toxic levels. During the six year period, for treatments surface irrigated with saline drainage water or a blend of saline and low salinity water, the B concentration in the soil increased throughout the 1.5 m soil profile while the electrical conductivity (EC_e) increased primarily in the upper l m of the profile. Increaszs in soil EC_e during the entire rotation occurred on plots where minimal leaching was practiced. Potential problems with germination and seedling establishment associated with increased surface soil salinity were avoided by leaching with rainfall and low-salinity pre-plant irrigations of 150 mm or more. Accumulation of boron and selenium poses a major threat to the sustainability of agriculture if drainage volumes are to be reduced by using drainage water for irrigation. This is particularly true in areas where toxic materials (salt, boron, other toxic minor elements) cannot be removed from the irrigated area. Continual storage within the root zone of the cropped soil is not sustainable.     

Irrigation based on a nomogram using soil suction measurements

Pivot-irrigation was managed with tensiometers on a field cropped with sugarcane at Analaiva (west coast of Madagascar). The volumes of water delivered by the pivot were 20 mm or less (expressed in terms of equivalent rain), depending on the stage of the crop. These applications were made when suctions at certain depths attained fixed values: a single value of 500  hPa in the top layer of the soil in the installation and growth stages of the crop, double values (600 hPa at 50 cm, 400 hPa at 150 cm) at the ripening stage.To characterize water movement in the soil, lines of isoflux were calculated from the hydrodynamic properties of the soil, and plotted so that the flux of water determined by the Darcy–Buckingham law would be read on the profile of hydraulic head. Maps of two-dimensional water movement were drawn independently from the data collected by an array of 30 tensiometers crossing a cane row. Before the rainy season, the water consumed by the crop corresponds to the irrigation water. At the ripening stage, an upward flux from the deep layer of the soil (wetted during the previous months of rains) is obtained by the strict management of irrigation.     

黄河三角洲盐渍土蒸发对土壤盐分变化的响应特征

为了探明黄河三角洲盐渍土蒸发对土壤盐分变化的响应特征,采用矿化度分别为5,10,30,50,70,90 g/L的咸水灌溉黄河三角洲0~40 cm土壤,获得不同盐分梯度的盐渍土处理,依次标记为处理T1—T6,并测定各处理的土壤含水率和电导率、蒸发强度和累积蒸发量等指标.结果表明,蒸发过程中表层土壤含水率和电导率均随土壤含盐量增加呈逐渐增加趋势;蒸发结束时,处理T1—T6的土壤表层平均含水率比试验初期降低了80.0%~95.8%,表层含水率的降低幅度随着含盐量增加而逐渐降低;土壤表层电导率分别增加135%~330%,且蒸发前期表层电导率增加幅度明显高于蒸发后期.土壤含盐量对土壤剖面含水率及电导率分布影响差异具有统计学意义,蒸发结束时,处理T1—T6表层0~2 cm比3~6 cm土壤含水率低了8.3%~30.5%,土壤电导率则高了82%~196%,且随着土壤含盐量增加,盐分对土壤剖面盐分分布的影响逐渐增强,表层与深层土壤含盐量差异逐渐增大.蒸发过程中,土壤平均蒸发强度和累积蒸发量随土壤含盐量增加呈降低趋势,处理T1—T6的平均蒸发强度为3.5×10^-4,3.5×10^-4,3.4×10^-4,3.2×10^-4,3.0×10^-4和2.7×10^-4 mm/d,土壤累积蒸发量分别为26.13,26.20,25.50,24.26,22.50和20.58 mm,且蒸发前期各处理的土壤平均蒸发强度及累积蒸发量均高于蒸发后期,土壤含盐量对土壤蒸发的抑制作用主要在蒸发前期.研究表明土壤含盐量可影响土壤剖面含水率与电导率分布以及土壤蒸发强度和累积蒸发量.     

滨海吹填土灌排工程改良效果研究

通过田间试验,对灌排工程改良措施改良滨海吹填土的效果进行了研究。结果表明,改良过程中排水工程从小区排出了大量的盐分,排出水矿化度从55.109g/L下降到5g/L左右。灌排措施下,300mm左右的淋洗水量能使0～40cm土层土壤脱盐率达到约65%,但不能使0～100cm土体整体脱盐;约700mm淋洗用水量能使0～100...     

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

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

Comparison between mechanistic and functional models for estimating soil water balance: deterministic and stochastic approaches

In many models used to simulate soil-water relationships, representations of the transport mechanisms in the soil-plant-atmosphere continuum, range from mechanistic to functional. The objective of this paper is to compare two functional models, FAO (Doorenbos and Pruitt, 1977) and Ritchie (1985)models, with a mechanistic model (Maraux and Lafolie, 1998) to simulate the soil water balance of maize and sorghum grown in sequence in Nicaragua. In the FAO model, the soil is described as a single reservoir which is characterized by its amount of water varying on a daily time scale, depending on the rain, drainage, and actual evapotranspiration. In the Ritchie model, the soil is regarded as a multilayered soil profile. The maximum evapotranspiration is divided between soil evaporation and plant transpiration, and drainage occurs if the amount of water arriving in the last layer corresponds to a water content greater than the field capacity. The mechanistic model is based on the Richards' equation. Comparison of the three models was first made according to a deterministic approach with parameters coming from the same database. We then considered a stochastic approach for which 800 hydraulic characteristics of the soil were generated, according to the spatial variability observed at the field scale and to the scaling theory applied to similar porous media. A distribution of the stochastic parameters used in the three models was thus derived. Results showed that the order of magnitude of the evapotranspiration was similar for the three models (902, 874, 842 mm cumulative evapotranspiration for a 203 day period for the MM, Ritchie, and FAO models, respectively). Adding a capillary rise mechanism in the functional models improved moderately the soil-water balance. Evapotranspiration and drainage showed moderate sensitivity to spatial variability in soil hydraulic properties (coefficients of variation less than 1.6%), whereas final water storage (after 203 days) showed a greater sensitivity (coefficients of variation from 7.9–15.7%, depending on the model).     

A simulation model of water dynamics in winter wheat field and its application in a semiarid region

Many models for water flow in cropped soil contain parameters such as rooting density, root permeability, and root water potential. Usually these parameters are chosen by trial-and-error method and direct measurements are difficult and impractical in some cases. This study presents a simulation model capable of analyzing water transport dynamics in a soil–plant–atmosphere continuum (SPAC). This model is developed by combining an existing mathematical model for soil water flow, a modified transpiration model taking into account of the air pressure and diurnal changes of the extinction coefficient of crop canopies, and a new simple model for root water uptake. Using data from lysimeters in a field experiment carried out on a wheat crop, we also developed two new empirical equations for the estimation of total canopy resistance and soil evaporation.We then applied the model for 2 years (1990–1991, 1991–1992) on winter wheat in a semiarid area of northwest China. Required parameters, particularly soil hydraulic and crop parameters, were determined by field and laboratory tests. Outputs from the simulation were in good agreement with the independent field measurements of seasonal changes in soil water content, canopy transpiration, surface evaporation, and root water uptake along the soil profile. In addition, this simulation agreed well with the actual measurements of seasonal crop water consumption and soil water balance among the treatments for different irrigation amounts.     

分散流控制壁对无压渗漏计收集效率影响的定量评价

无压渗漏计(Zero-tension lysimeter,ZTL)多用于非饱和带土壤溶质通量的监测,但由于ZTL安装时与原状土壤相接触会存在毛管障碍界面,易形成分散流使其土壤溶液收集效率降低。为准确描述田间水分渗漏量或土壤溶质的运移过程与规律,基于HYDRUS模型模拟结果,对ZTL不同设计(加装不同高度分散流控制壁)和不同适用环境条件(土壤质地、灌水量、土壤蒸发量和初始土壤含水率)的土壤渗漏水收集效率及影响因素进行数值模拟和定量评价。结果表明,无分散流控制壁的ZTL(ZTL0),在0.35 cm3/cm3土壤初始含水率、0.2 cm/d蒸发量和1 000 mm灌水量条件下的砂壤土、壤土和粉土处理,收集效率分别仅为11%、13%和26%,而在相同环境条件下安装分散流控制壁的ZTL(ZTLd),当控制壁高度为20 cm时可使收集效率提升到50%以上。安装的分散流控制壁高度随灌水量的降低、土壤持水能力的提高和土壤蒸发量的增大而升高,初始土壤含水率降低会使偏砂性土壤中安装的ZTLd收集效率降低,但在壤土和粉土中安装时可使其收集效率增大。增加ZTLd安装深度可能会导致其收集效率降低,在某一特定安装深度对ZTL收集效率计算的结果并不适用于其他深度。     

A study of root water uptake of crops indicated by hydrogen and oxygen stable isotopes: A case in Shanxi Province, China

Mechanisms of crop root water uptake play an important role in agricultural water management. In this study, stable isotopes were used to understand root water uptake patterns for the main crops (summer corn and cotton) in Shanxi Province, China. Precipitation, irrigation water, soil water, groundwater and stem water were sampled for stable isotopes analyses, and supported by hydrological observations. Both direct inference of hydrogen and oxygen isotopes between stem water and the soil water profile, and multiple-source mass balance assessment were applied to estimate the main depths of root water uptake of crops in different growing seasons. The results show that summer corn and cotton have different root water uptake patterns: summer corn mainly uses the shallow soil water from 0 to 20 cm layer (96-99%) in jointing stage and extending to 20-50 cm (58-85%) in flowering stage, then 0-20 cm (69-76%) again in full ripe stage. In contrast, the main depth of root water uptake of cotton gradually increases during the whole growth stage: from 0 to 20 cm (27-49%) in seedling stage, 20-50 cm (79-84%) in bud stage, 50-90 cm (30-92%) in blooming stage and >90 cm (69-92%) in boll open stage.     

Effect of soil moisture regimes on the yield and water use of lentil (Lens culinaris Medic)

Summary The effect of soil moisture regimes on the grain and straw yield, consumptive water use (Cu) and its relation with evaporation from free water surface (Eo), water use efficiency and soil moisture extraction pattern of lentil was studied in a field experiment conducted at the Indian Agricultural Research Institute, New Delhi during the fall-spring season of the crop years 1979–1980 and 1980–1981. The grain and straw yield, consumptive water use rate, Cu/Eo ratio and water use efficiency increased with an increase in irrigation frequency. Consumptive water use rate increased as the crop season advanced and reached its peak value during flowering and grain filling stage. The Cu/Eo ratio attained its minimum values 35 and 105 days after sowing at branching and grain filling stages. Depletion of soil moisture was most from the top 0–30 cm soil layer followed by 30–60 cm soil layer and was least from 90–120 cm soil layer. The pattern of soil moisture depletion was also influenced by soil moisture regime. During the vegetative and flowering stage the percent contribution from the top 0–30 cm soil layer decreased and that from the lower soil layers (30–60, 60–90, and 90–120 cm) increased with an increase in the soil moisture tension, however, the actual amount of moisture depleted from all the soil layers was always higher under low soil moisture tension regime than under high soil moisture tension regime. During the grain development stage the soil moisture treatment had no significant effect on the relative contribution from different soil layers under low and high soil moisture tension as the crop was irrigated at the same time under both these treatments. However, with no irrigation, the percent contribution from top soil layer continued to decrease, and from lower soil layers continued to increase, as the crop advanced from flowering stage to grain development stage.     

裸地蒸发过程土壤盐分运移的实验及数值模拟研究

本文利用农田灌溉研究所洪门试验场地中渗透仪,对不同地下水埋深条件下裸地蒸发过程粉砂壤土的盐分运移进行了实验及数值模拟研究。结果表明,对于三种地下水埋深,即150cm、200cm和250cm,在无降雨裸地蒸发情况下,表层积盐均不明显;但在有小水量降雨和裸地蒸发情况下,地下水埋深为150cm的表层积盐强烈。地下水埋深为200cm及250cm的表层土壤虽有积盐,但积盐量较小,这种积盐主要是上层土壤中盐分的重新分布。     

Modeling irrigated cotton with shallow groundwater in the Aral Sea Basin of Uzbekistan: I. Water dynamics

In Khorezm, a region located in the Aral Sea basin of Uzbekistan, water use for irrigation of predominantly cotton is high whereas water use efficiency is low. To quantify the seasonal water and salt balance, water application, crop growth, soil water, and groundwater dynamics were studied on a sandy, sandy loam and loamy cotton field in the years 2003 and 2005. To simulate and quantify improved management strategies and update irrigation standards, the soil water model Hydrus-1D was applied. Results showed that shallow groundwater contributed a substantial share (up to 399 mm) to actual evapotranspiration of cotton (estimated at 488–727 mm), which alleviated water stress in response to suboptimal quantities of water applied for irrigation, but enhanced concurrently secondary soil salinization. Thus, pre-season salt leaching becomes a necessity. Nevertheless, as long as farmers face high uncertainty in irrigation water supply, maintaining shallow groundwater tables can be considered as a safety-net against unreliable water delivery. Simulations showed that in 2003 around 200 mm would have been sufficient during pre-season leaching, whereas up to 300 mm of water was applied in reality amounting to an overuse of almost 33%. Using some of this water during the irrigation season would have alleviated season crop-water stress such as in June 2003. Management strategy analyses revealed that crop water uptake would only marginally benefit from a permanent crop residue layer, often recommended as part of conservation agriculture. Such a mulch layer, however, would substantially reduce soil evaporation, capillary rise of groundwater, and consequently secondary soil salinization. The simulations furthermore demonstrated that not relying on the contribution of shallow groundwater to satisfy crop water demand is possible by implementing timely and soil-specific irrigation scheduling. Water use would then not be higher than the current Uzbek irrigation standards. It is argued that if furrow irrigation is to be continued, pure sandy soils, which constitute <5% of the agricultural soils in Khorezm, are best to be taken out of annual cotton production.     

Growth,yield and soil water extraction of irrigated and dryland peanuts in South Sulawesi,Indonesia

Summary Irrigation at 35 and 70 mm of pan evaporation applied during the pre and/or post early podfilling stages increased pod yield of Spanish peanuts (100 day maturity) three fold compared to a dryland crop. There was no difference in pod yield in crops receiving 12 compared to 6 irrigations. Soil water sampling immediately after irrigations on selected treatments revealed that infiltration of irrigation water was probably restricted to less than ca. 20 cm, a response which resulted in poor soil water replenishment and low irrigation efficiency (Fig. 3). Even though roots extracted soil water below the compaction layer which was at 20 cm severe crop water deficits had developed by the end of irrigation cycles during later but not early stages of growth. The dryland crop, which received no rainfall during the season, presumably extracted significant amounts of soil water at depths to and below 1.2 m (Fig. 3). Despite producing ca. 2.9 t ha^-1 of total dry matter yield, pod yield was extremely low (0.5 t ha^-1) arising from low pod numbers and high percentage of empty pods.This research was funded by the Australian Centre for International Agricultural Research (ACIAR-Project 8419) in collaboration with the Agency for Agricultural Research and Development (AARD).     

In situ water transmission characteristics of a tropical soil under rice-based cropping systems

In soils under rice-based cropping systems in Asia water movement and distribution in the root zone of rice and dryland crops are important for efficient water management. Saturated hydraulic conductivities in the wetland soil profile were evaluated from measurements of hydraulic gradients and percolation rates in the field. The subsoil layer (15–60 cm) restricted percolation rate to a greater degree than the puddled top soil.Unsaturated hydraulic conductivities and soil water diffusivities in the soil profile under dryland conditions were obtained from simultaneous measurements of soil water content using the neutron moderation technique and the soil matric potential by tensiometers over time and soil depth. Soil matric potential versus hydraulic conductivity and soil water content versus soil water diffusivity relations of various soil depths were established. At equivalent soil matric potentials, the hydraulic conductivity of surface soil was greater than that of the subsoil layers. Soil water diffusivity at different depths responded similarly. The study describes a simple in situ technique to measure percolation rates in wetland rice fields and evaluation of water transmission properties of field soil profiles.     

沙棘微扦插的生根研究

本实验将沙棘枝条从顶端依次向下剪取5段,每段3cm。选用5种生根素（ABT1、ABT2、GGR6、GGR7、硝普钠＋ABT1）对3cm的沙棘嫩枝处理,并采用3种育苗方式（土培、水培、沙培）进行微扦插实验。结果表明：土培、水培、沙培的生根率依次为49%、65%、80%。在土培、水培、沙培三种基质中,对比5种生根素处理的沙棘枝条生根率,生根素 GGR6处理的枝条生根率最高,依次为：55%、83.8%、96.8%。在土培、水培、沙培三种基质中,比较不同段的生根率,第3段生根率最高,依次为：67.36%、73.75%、84.33%。在土培、水培、沙培三种基质中,同一时期根的长度土培、水培、沙培依次为：1.90cm、3.60cm、6.45cm;根的数量依次为：2.10条、2.75条、4.45条;根的直径依次为：2.35mm、2.60mm、3.25mm。本试验为沙棘的大规模产业化种植提供理论性依据。     

Evapotranspiration of orange trees in greenhouse lysimeters

Eight-year-old Murcott orange trees (Citrus sinensis (L.) Murcott) grown in greenhouse lysimeters filled with sandy soil were used to investigate seasonal variations in daily and hourly evapotranspiration. The study was conducted in Japan during the summer of 2000 and the winter of 2001. Weighing lysimeters of 1.5 m diameter and 1.6 m depth (three replications) planted with a tree were irrigated when average soil moisture in 0-120 cm of soil depth was depleted to below 70% of the field capacity (FC). Evapotranspiration (ET) showed significant seasonal variations. Average ET rate exceeded 4.4 mm/day in the summer period, and dropped to 0.6 mm/day in the winter months. The average seasonal crop coefficient (K_C) was 0.91 and 0.75 during the summer and winter periods, respectively. Hourly variations in ET exhibited a time difference with season. The time of maximum ET was 0900 hours for winter and 1200 hours for summer. Moreover, some evaporative losses of soil water occurred even during the night in both summer and winter seasons. Soil evaporation (E) was 33% of ET during the winter period, while E was only 11% of ET during summer. Maximum water uptake by the trees was found at a depth of 30-60 cm, and soil water depletion was observed in the 0-120 cm depth of the profile during the summer period. However, during the winter season, water depletion occurred only from 0-30 cm depth of the soil profile.