Levels Of Soil Water Depletion Research Articles

Application of Some Drought Stress Indices to Study Response of Durum Wheat Triticum durum to Different Levels of Water Depletion

A pot experiment was conducted to investigate the effect of five levels of soil water depletion from field capacity (30, 40, 50, 60, and 70%) on some growth, yield and yield components of durum wheat (Triticum durum L.) VAR-ACSAD/65, with testing ten stress and drought indices to indicate the suitable index that could be simply used to interpret drought stress conditions. Results indicated significant effect of soil water depletion levels on wheat plant height, tillers per plant, spikes per plant, spike length, and total chlorophyll content, increasing water depletion levels decreased all recorded data. The effect on spike numbers per plant; spike length; number of seeds per spike; 1000 seeds weight (gm); seed yield per plant; straw yield per plant; biological yield per plant and harvest index were negatively dependent on drought stress to the level of no seed yield under the highest water depletion level 70%. Drought indices could classify to three categories; unity to zero, zero to higher value, high values to lower values. The relationship between drought indices were either positive or negatively correlated to plant seed yield. All studied crop drought indices were suitable for studying crop drought cases. The wheat variety acsad/65 was sensitive to drought stress.

Water management of mombaça grass (panicum maximum) in sandy and loamy soil

The objective of the present study was to assess the influence of irrigation management in the production of Mombaça grass in two distinct types of soil. The experimental material used was the Panicum maximum (Jacq.) cv. Mombaça, developed by Embrapa – Centro Nacional de Pesquisa de Gado de Corte (National Beef Cattle Research Center). Research took place in arched greenhouse located in the experimental area of the Escola Superior de Agricultura “Luiz de Queiroz”, in Piracicaba-SP - Brasil. Two experiments using different types of soil (sandy and loamy) were conducted using a randomized block design, in a factorial (4x3) arrangement, with four levels of soil water depletion (15%, 30%, 45% and 60%) in three harvests, with four repetitions. The experimental unit consisted of a 21 L pot, with 0.30 m of depth and a surface area of 0.07 m2. As results it was observed that the water use efficiency increased with the addition of water availability in the Nitosol. The grass showed sensitivity to water deficit, with higher production when irrigated with 30% depletion of available water capacity in both soils.

Irrigation Scheduling Based on Wireless Sensors Output and Soil-Water Characteristic Curve in Two Soils.

Data-driven irrigation planning can optimize crop yield and reduce adverse impacts on surface and ground water quality. We evaluated an irrigation scheduling strategy based on soil matric potentials recorded by wireless Watermark (WM) sensors installed in sandy loam and clay loam soils and soil-water characteristic curve data. Five wireless WM nodes (IRROmesh) were installed at each location, where each node consisted of three WM sensors that were installed at 15, 30, and 60 cm depths in the crop rows. Soil moisture contents, at field capacity and permanent wilting points, were determined from soil-water characteristic curves and were approximately 23% and 11% for a sandy loam, and 35% and 17% for a clay loam, respectively. The field capacity level which occurs shortly after an irrigation event was considered the upper point of soil moisture content, and the lower point was the maximum soil water depletion level at 50% of plant available water capacity in the root zone, depending on crop type, root depth, growth stage and soil type. The lower thresholds of soil moisture content to trigger an irrigation event were 17% and 26% in the sandy loam and clay loam soils, respectively. The corresponding soil water potential readings from the WM sensors to initiate irrigation events were approximately 60 kPa and 105 kPa for sandy loam, and clay loam soils, respectively. Watermark sensors can be successfully used for irrigation scheduling by simply setting two levels of moisture content using soil-water characteristic curve data. Further, the wireless system can help farmers and irrigators monitor real-time moisture content in the soil root zone of their crops and determine irrigation scheduling remotely without time consuming, manual data logging and frequent visits to the field.

Okra Crop Response Under Subsurface Drip and Conventional Furrow Irrigation with Varying N Fertilization

ABSTRACTThere is a growing concern about excessive use of nitrogen (N) and water in agricultural system with unscientific management in Indian and developing countries of the world. Field experiments were conducted on the lateritic sandy loam soils of Kharagpur, West Bengal, India, during spring–summer (February-June) seasons for three years (2015–2017) to evaluate okra crop response under subsurface drip and conventional furrow irrigation with varying amount of nitrogen treatments. Irrigation treatments had three levels of soil water depletion from field capacity (i.e., 20%, 35%, and 50%) under subsurface drip system. There was no soil water depletion under conventional furrow irrigation system. There were four levels of nitrogen fertilizer treatments (i.e., 0, 80, 100, and 120 kg ha−1). This was supplied using urea as a nitrogenous fertilizer. The yield response of okra crop under subsurface drip was found to be 56.4% higher than that of the furrow irrigation treatment. Best yield response and maximum water use efficiency and nitrogen use efficiency were recorded under 20% soil water depletion with 100 kg ha−1 of nitrogen fertigation. Among the various soil moisture depletions, subsurface drip at 20% soil water depletion treatment responded least quantity of water lost through deep drainage and nitrogen loss beyond the root zone as compared to other irrigation treatments. The water loss through subsurface drainage was observed as 33.11 mm lesser under subsurface drip as compared to that of the furrow irrigation, and this may due to low-volume and frequent irrigation water application with subsurface drip. Hence, irrigation through subsurface drip should be used for improving water and nitrogen fertilizer use efficiency of okra crop cultivation.

Responses of Vitis vinifera L. cv. Sultanina to Level of Soil Water Depletion Under Semi-Arid Conditions

Sultanina grapevines were irrigated at 90%, 60% and 30% plant-available water (PAW) depletion during the growing season in a field trial on a fine sandy soil in the Lower Orange River region of South Africa. Grapevines of all treatments were irrigated at 60% PAW depletion during winter. Measurement of diurnal leaf water potential (tJ'1) cycles at different stages during the season showed that plant water status was not only a function of soil water availability, but also of meteorological conditions. Increasing vapour pressure deficit (VPD) decreased pre-dawn leaf water potential, suggesting nocturnal water loss via transpiration. On the other hand, tpl measured at 14h00 increased with an increase in VPD, indicating partial stomatal closure during the warmest part of the day. Vegetative growth, berry mass and production increased with increased plant water status as imposed by the different PAW depletion levels. PAW depletion level had almost no effect on sugar accumulation or acid breakdown. Soil water depletion level had no effect on the quality of dipped raisins. Hence, the ideal balance between vegetative growth, yield and water use efficiency for the production of dipped Sultanina raisins was obtained with irrigation at 60% PAW depletion. Since quality of sun-dried raisins was considerably reduced by irrigation at 60% as well as 90% PAW depletion, relative to 30% depletion, the latter seems to be the optimum depletion level for sun-dried raisin production. This optimum depletion level might not be applicable for irrigation of Sultanina on heavier, more fertile alluvial soils.

Water Relations of Vitis vinifera L. cv. Sunred Seedless in Response to Soil Water Depletion Before Harvest

Effects of soil water depletion level and irrigation cut off during ripening on water stress in Sunred Seedless table grapes were studied in a field trial in the Hex River Valley of South Africa. Water status of grapevines subjected to four irrigation treatments was determined prior to harvest during the 2000/01 season. Grapevines of all treatments were irrigated at 40% plant available water (PAW) depletion before veraison. From veraison, i.e. the onset of ripening in December, until harvest at the end of January, two treatments were irrigated at 20% and 40% PAW depletion, respectively. Two more treatments were applied by cutting off irrigation at 20% PAW depletion when total soluble solids reached 12°B and 15°B, respectively. Pre-dawn water stress in grapevines irrigated at 20% PAW depletion until harvest was less than in those where irrigation was cut off at 12°B. Sap flow measurements indicated that partial stomatal closure seemed to have occurred over the warmest part of the day. Apparently, stomatal control limited Ψl differences between treatments although extensive soil water depletion occurred where irrigation was cut off during ripening. If accumulated leaf water potential (ΨT) is considered, irrigation cut off at 12°B and 15°B induced more water stress in grapevines from 04:00 to 24:00 compared to those vines where irrigation was continued until harvest. Where irrigation was continued throughout the ripening period, the level of PAW depletion did not affect ΨT. However, during the sunshine period (08:00 until 18:00), ΨT in grapevines subjected to 40% PAW depletion, as well as those where irrigation was cut off, was higher than in the ones that were irrigated at 20% depletion until harvest. During the first half of the night, i.e. from 18:00 until 24:00, water potential in grapevines where irrigation was cut off at 12°B was unable to recover to the same level as in those exposed to the other treatments.

Model-Based Deficit Irrigation of Maize in Kansas

Abstract. Maize is the dominant irrigated crop in Kansas. In recent years, as a result of declining groundwater levels in the Ogallala aquifer and diminished well capacities, farmers are turning to deficit irrigation strategies. This study demonstrates the potential of model-based optimization for determining adequate soil water depletion levels. CERES-Maize was used as surrogate crop, while the AquaCrop model was used in an optimization procedure that determined the optimal water depletion levels. A multi-objective optimization framework was used to determine several combinations of optimal water depletion levels based on ten years of historical weather, and these combinations were tested using an additional 50 years of historical weather. The results show that, although imperfect modeling and weather fluctuations caused the actual yield to be different from the target yield, the fluctuations around the multi-year averages were not significantly larger when testing the irrigation schedule with the CERES-Maize model than when testing it with the AquaCrop model that had been used to develop the irrigation schedule. Keywords: AquaCrop, Center-pivot irrigation, CERES-Maize, Multi-objective optimization.

Spatial variability of grape yield and its association with soil water depletion within a vineyard of arid northwest China

Spatially variable soil properties, combined with the complex interrelationship between crop yield with environmental factors often lead to spatio-temporal variability of crop yield across the field. The objectives of this study were to investigate spatial variability of grape yield and estimate the potential yield at various soil water depletion levels within a vineyard of arid northwest China. Grape yield and soil water contents at different times were measured at 135 georeferenced points in 2012, and 147 points in 2013 within a 7.6-ha vineyard. Geostatistical approach was used to describe the spatial variation in grape yield, while boundary line analysis was used to estimate the potential yield at various soil water depletion levels. Some selected soil properties between the low and high yield gap groups, defined as the difference between the actual yield and the estimated potential yield, were compared. The spatial structure of yield distribution in the field was similar between the two years. The upper boundary line between grape yield with soil water depletion was best fitted by a quadratic function for both years. When soil water depletion was relatively high, soil sand content and saturated hydraulic conductivity of 0–20cm soil were found to be significantly different between the high and low yield gap groups, while none of soil properties in the top 40cm soil were found to be important in determining the yield gap when soil water supply was limited. Boundary line analysis method could be a valuable tool in developing better management practices to improve overall yield according to available soil water conditions in the field.

Camelina water use and seed yield response to irrigation scheduling in an arid environment

Camelina sativa (L.) Crantz is a promising, biodiesel-producing oilseed that could potentially be implemented as a low-input alternative crop for production in the arid southwestern USA. However, little is known about camelina's water use, irrigation management, and agronomic characteristics in this arid environment. Came- lina experiments were conducted for 2 years (January to May in 2008 and 2010) in Maricopa, Arizona, to evaluate the effectiveness of previously developed heat unit and remote sensing basal crop coefficient (Kcb) methods for predicting camelina crop evapotranspiration (ET) and irrigation scheduling. Besides Kcb methods, additional treatment factors included two different irrigation sched- uling soil water depletion (SWD) levels (45 and 65 %) and two levels of seasonal N applications within a randomized complete block design with 4 blocks. Soil water content measurements taken in all treatment plots and applied in soil water balance calculations were used to evaluate the predicted ET. The heat-unit Kcb method was updated and validated during the second experiment to predict ET to within 12-13 % of the ET calculated by the soil water balance. The remote sensing Kcb method predicted ET within 7-10 % of the soil water balance. Seasonal ET from the soil water balance was significantly greater for the remote sensing than heat-unit Kcb method and signifi- cantly greater for the 45 than 65 % SWD level. How- ever, final seed yield means, which varied from 1,500 to 1,640 kg ha -1 for treatments, were not significantly different between treatments or years. Seed oil contents averaged 45 % in both years. Seed yield was found to be linearly related to seasonal ET with maximum yield occurring at about 470-490 mm of seasonal ET. Differ- ences in camelina seed yields due to seasonal N applica- tions (69-144 kg N ha -1 over the 2 years) were not

IRRIGATION MANAGMENTAND THE PRODUCTION OF SORGHUM UNDER CALCAREAUS SOIL

In general, irrigation by surface methods is the most common to supply crops with frequent application of water. Pressurized irrigation systems have been introduced in Egypt to develop new irrigation technology suited to limited water supply as well as to specific topographic and soil conditions. In this study, a field experiment was carried out in the Agricultural Experimental Station of the Desert Research Center, Maryut, Alex. Governorate during the two successive seasons of 2006 and 2007 .Five methods of irrigation were studied: , gated pipe(GP), surface drip irrigation, one line of drippers for one line of plants(D1:1) , surface drip irrigation ,one line of drippers for two lines of plants( D1:2 ), sub surface drip irrigation, one line drippers for one line of plants (SD1:1) and sub-surface drip irrigation, one line of drippers for two lines of plants (SD1:2) .The irrigation methods were conducted under three water quantities (100%,70% and50%) of reference ETO calculated by modified Penman- Montieth equation ,with two soil water depletion levels (30% and 50%) of available water .The statistical analysis revealed highly significant increases in all the studied parameters with increasing water quantities. The maximum fresh weights i.e. ,41.73,41.51and39.66 ton/fed./ were obtained with SD1:1,D1:1and GP, respectively under water quantity 100% ( Q1) and soil water depletion 30% ( D1),in the year 2006 while the maximum dry masses of 8.23,8.2and 7.9 ton/fed. Respectively were achieved under SD1:1, D1:1and GP with Q1and D1 .In 2007, the maximum fresh masses were, 42.55, 41.85and 40.53 under D1:1, SD1:1 and GP, while the maximum dry weights were 8.4, 7.98 and 7.27 ton/fed respectively .under D1:1, GPand SD1:1 with Q1and D1. The study showed that water use efficiency decreased by increasing quantity of the applied water. KW: WUE, Evapotranspiration, Sub-Surface Drip Irrigation (SDI), Drip Irrigation (DI), Gated Pipe (GP), Soil depletion.

Water use, crop coefficients, and irrigation management criteria for camelina production in arid regions

Camelina sativa (L.) Crantz is an oilseed crop touted as being suitable for production in the arid southwestern USA. However, because any significant development of the crop has been limited to cooler, rain-fed climate-areas, information and guidance for managing irrigated-camelina are lacking. This study measured the crop water use of a November-through-April camelina crop in Arizona using frequent measurements of soil water contents. The crop was grown under surface irrigation using five treatment levels of soil water depletion. The seed yields of treatments averaged 1,142 kg ha−1 (8.0% seed moisture) and were generally comparable with camelina yields reported in other parts of the USA. Varying total irrigation water amounts to treatments (295–330 mm) did not significantly affect yield, whereas total crop evapotranspiration (ETc) was increased for the most frequently irrigated treatment. However, total ETc for the camelina treatments (332–371 mm) was markedly less than that typically needed by grain and vegetable crops (600–655 mm), which are commonly grown during the same timeframe in Arizona. The camelina water-use data were used to develop crop coefficients based on days past planting, growing degree days, and canopy spectral reflectance. The crop coefficient curves, along with information presented on camelina soil water depletion and root zone water extraction characteristics will provide camelina growers in arid regions with practical tools for managing irrigations.

Parameterization and Evaluation of the AquaCrop Model for Full and Deficit Irrigated Cotton

Predicting yield is increasingly important to optimize irrigation under limited available water for enhanced sustainability and profitable production. Food and Agriculture Organization (FAO) of the United Nations addresses this need by providing a yield response to water simulation model (AquaCrop) with limited sophistication. In this study, AquaCrop was parameterized and tested for cotton (Gossypium hirsutum L.) under full (100%) and deficit (40, 60, and 80% of full) irrigation regimes in the hot, dry, and windy Mediterranean environment of northern Syria. Model parameterization used the 2006 data and was straightforward within the designed user‐interface, owing to the limited number of key parameters. Accurate simulation of canopy cover was central to sound prediction of evapotranspiration and biomass accumulation. Key user‐input parameters for this purpose were identified as the coefficients defining canopy development and the threshold soil water depletion levels for the water stress indices. The parameterized model was tested using data from the 2004 and 2005 seasons, resulting in accurate prediction of evapotranspiration (<13% error). The predicted yield values were within 10% of measurements, except in the 60 and 80% irrigation regimes in 2004, with errors up to 32%. The model closely predicted the trend in total soil water, but deviation existed for individual soil layers. This study provides first estimate values for cotton parameters useful for future model testing and use. Model parameterization is site‐specific, and thus the applicability of key calibrated parameters must to be tested under different climate, soil, variety, irrigation methods, and field management.

Rose-scented geranium ( Pelargonium capitatum × P. radens) growth and essential oil yield response to different soil water depletion regimes

Effective irrigation management in arid and semi-arid regions, like South Africa, could increase crop yield and thereby improve productivity of scarce fresh water resources. Experiments were conducted at the Hatfield Experimental Farm of the University of Pretoria, South Africa, from 2004 to 2006, to investigate the effect of soil water depletion regimes on rose-scented geranium ( Pelargonium capitatum × P. radens cv. Rose) essential oil yield, essential oil composition and water-use efficiency in an open field and a rain shelter. Four maximum allowable soil water depletion levels (MAD), 20, 40, 60 and 80% of the plant available soil water (ASW) in the top 0.8 m root zone, were applied as treatments. Plant roots extracted most soil water from the top 0.4 m soil layer. Increasing the soil water depletion level to 60% and higher resulted in a significant reduction in herbage mass and essential oil yield. Water stress apparently increased the essential oil concentration (percentage oil on fresh herbage mass basis), but its contribution to total essential oil yield (kg/ha oil) was limited. Irrigation treatments did not affect essential oil composition. An increase in maximum allowable depletion level generally resulted in a decrease in leaf area and an increase in leaf to stem fresh mass ratio. Up to 28% of irrigation water could be saved by increasing maximum allowable depletion level of ASW from 20 to 40%, without a significant reduction in essential oil yield.

Use of crop simulation models to evaluate limited irrigation management options for corn in a semiarid environment

Increasing competition for land and water resources due to increasing demands from rapid population growth calls for increasing water use efficiency of irrigated crops. It is important to develop location‐specific agronomic practices to maximize water use efficiency (WUE). Adequately calibrated and validated agricultural systems models provide a systems approach and a fast alternative method for developing and evaluating agronomic practices that can utilize technological advances in limited irrigation agriculture. The objectives of this study were to (1) calibrate and validate the CERES‐maize model under both dryland and irrigated corn (Zea mays L.) production in northeastern Colorado and (2) use the model with a long‐term weather record to determine (1) optimum allocation of limited irrigation between vegetative and reproductive growth stages and (2) optimum soil water depletion level for initiating limited irrigation. The soil series was a Rago silt loam, and the initial water content on 1 January of each year was equal to field capacity in the upper 300 mm and half of the field capacity below this depth. Optimum production and WUE with minimum nitrogen (N) losses were found when (1) a water allocation ratio of 40:60 or 50:50 (uniform) between vegetative and reproductive stages for irrigations up to 100 mm, and a ratio of 20:80 for irrigations above 100 mm was used; and (2) irrigation was initiated at 20% plant‐available water (PAW) (80% depletion). When available water for irrigation is limited to 100 mm, irrigating 50% of the area with 200 mm of water at 20:80 split irrigations between the vegetative and reproductive stages produced greater yield than irrigating 100% of the area with 100 mm water. Concepts developed in the study can potentially be adapted to other locations, climates, and crops. However, precise site‐specific recommendations need to be developed for each soil‐climate zone using the validated system model.

Evapotranspiration of Vitis vinifera L. cvs Sunred Seedless and Muscat Supreme in response to soil water depletion and irrigation cutoff during berry ripening

The effects of soil water depletion and irrigation cutoff during ripening on yield, quality and evapotranspiration (ET) of table grapes were studied to refine irrigation strategies needed during hot, dry summers. Six irrigation treatments were applied to two cultivars, viz. Sunred Seedless and Muscat Supreme, from the 1998/99 until 2000/01 seasons in a field trial in the Hex River valley of South Africa. Grapevines of all treatments were irrigated with micro-sprinklers at 40% plant available water (PAW) depletion before véraison and after harvest. Two treatments were irrigated at 20% and 40% PAW depletion from véraison to harvest. Four more treatments were applied by cutting off irrigation at these two depletion levels when total soluble solids reached 12°B and 15°B, respectively. According to earlier results, best overall performance for Sunred Seedless was obtained where irrigation at 40% PAW depletion was cut off at 15°B compared to the worst, i.e. where irrigation at 20% depletion was cut off at 15°B. For Muscat Supreme, irrigation at 20% PAW depletion, either continued until harvest, or cut off at 15°B, resulted in optimum overall performance. Although yield and quality responded differently, this study showed that the ET of the two cultivars responded in a similar way to the irrigation treatments under the given conditions. The ET of grapevines irrigated at 40% PAW increased from 1.6 mm day−1 in September to a maximum of 5.4 mm day−1 in January. Irrigation at 20% PAW depletion increased the maximum ET to 7.3 mm day−1. For both soil water depletion levels, irrigation cutoff at 12°B and 15°B resulted in, respectively, ca. 75% and 65% PAW depletion at harvest. Mean daily ET during January decreased to 4.1 mm day−1 and 5.0 mm day−1 after the irrigation had been cut off at 12°B and 15°B, respectively. The crop coefficients, which decreased accordingly, can be used to reduce irrigation to obtain optimum table grape yield and quality, particularly for Sunred Seedless.

Variable upper and lower crop water stress index baselines for corn and soybean

Upper and lower crop water stress index (CWSI) baselines adaptable to different environments and times of day are needed to facilitate irrigation scheduling with infrared thermometers. The objective of this study was to develop dynamic upper and lower CWSI baselines for corn and soybean. Ten-minute averages of canopy temperatures from corn and soybean plots at four levels of soil water depletion were measured at North Platte, Nebraska, during the 2004 growing season. Other variables such as solar radiation (R s), air temperature (T a), relative humidity (RH), wind speed (u), and plant canopy height (h) were also measured. Daily soil water depletions from the research plots were estimated using a soil water balance approach with a computer model that used soil, crop, weather, and irrigation data as input. Using this information, empirical equations to estimate the upper and lower CWSI baselines were developed for both crops. The lower baselines for both crops were functions of h, vapor pressure deficit (VPD), R s, and u. The upper baselines did not depend on VPD, but were a function of R s and u for soybean, and R s, h, and u for corn. By taking into account all the variables that significantly affected the baselines, it should be possible to apply them at different locations and times of day. The new baselines developed in this study should facilitate the application of the CWSI method as a practical tool for irrigation scheduling of corn and soybean.

Physiological Responses of Carambola Trees to Soil Water Depletion

Two-year-old `Arkin' carambola (Averrhoa carambola L.) trees were grown in containers in a greenhouse and the field in a very gravelly loam soil. Trees in the field were subjected to four soil water depletion (SWD) levels which averaged, 10.5%, 26.5%, 41.0%, and 55.5% and trees in the greenhouse were maintained at field capacity or dried continuously to produce a range of SWD levels. The relationships between SWD and leaf (ΨL) and stem (Ψs) water potential, net CO2 assimilation (ACO2), stomatal conductance of water (gs) and transpiration (E) were determined. Coefficients of determination values between physiological variables were higher for trees in the greenhouse than in the field, which may have been due to greater fluctuations in vapor pressure deficit (VPD) in the field. Soil water depletion levels above 50% caused a reduction in Ψs that subsequently decreased gs. This reduction in Ψs was correlated with a linear reduction in E and a considerable decline in ACO2 when gs fell below about 50 mmol·m–2 ·s–1. Leaf gas exchange parameters were better correlated with Ψs than with SWD level. Therefore, Ψs may be a better predictor of carambola tree water status than SWD in a well-drained, very gravelly loam soil.

Four Levels of Soil Water Depletion Minimally Affect Carambola Phenological Cycles

Phenological cycles were determined for carambola (Averrhoa carambola) trees in a gravelly loam soil at four different soil water depletion (SWD) levels in containers and in an orchard in southern Florida. The phenological cycles of young trees grown in containers were not as well defined as those of mature trees in an orchard. Shoot extension of trees in the orchard and containers was observed from the first week of March until the third week of December. Two peak flowering periods occurred during the first week of March, and from mid-September to mid-October. The major fruit harvest periods were August and December. Shoot flushing, extension shoot growth, flowering, and fruiting showed little response to irrigation at four SWD levels. This lack of response was likely caused by sufficient soil water due to precipitation and capillary rise from the high water table located about 1–2 m below the soil surface. Regardless of the lack of SWD effects on phenological cycles of carambola, the periodicity of shoot flushing, extension shoot growth, flowering, and fruiting and the intensity of these phenological events elucidated in this study should provide useful guidelines for carambola orchard management in southern Florida.

Physiological Response of Carambola Trees to Soil Water Depletion in Krome Soils

The effect of soil water depletion on plant water potential and leaf gas exchange of carambola (Averrhoa carambola L. cv. Arkin) in Krome very gravelly loam soil was studied in an orchard and in containers in the field and in a greenhouse. The rate of soil water depletion was determined by continuously monitoring soil water content with multi-sensor capacitance probes. Stem water potential and leaf gas exchange of carambola in containers were reduced when the soil water depletion level fell below 50% (where field capacity = 100%). Although there was a decrease in the rate of soil water depletion in the orchard as the soil dried, soil water depletion did not go below an average of 70%. This was presumably due to sufficient rainfall and capillary movement of water in the soil. Therefore, soil water content did not decline sufficiently to affect leaf gas exchange and leaf and stem water potential of orchard trees. A decline in soil water depletion below 40% resulted in a concomitant decline in stem water potential of the container trees in the field and greenhouse to below -1.0 MPa. Stomatal conductance, net CO2 assimilation, and transpiration declined significantly when stem water potential was below -1.0 MPa. The reduction of net CO2 assimilation and transpiration was proportional to the decline in stomatal conductance of container trees in the field and greenhouse. Thus, soil water depletion in Krome very gravelly loam soil must be less than 50% before water potential or leaf gas exchange of carambola is affected. Based on these results, irrigation scheduling should be based on physiological variables such as stem water potential and stomatal conductance or the amount rather than the rate of soil water depletion.

An Analysis of Simulated Long-Term Soil Moisture Data for Three Land Uses under Contrasting Hydroclimatic Conditions in the Northern Great Plains

Soil moisture (SM) plays an important role in land surface and atmosphere interactions. It modifies energy balance near the surface and the rate of water cycling between land and atmosphere. The lack of observed SM data prohibits understanding of SM variations at climate scales under varying land uses. However, with simulation models it is possible to develop a long-term SM dataset and study these issues. In this paper a water balance model is used to provide a quantitative assessment of SM climatologies for three land uses, namely, irrigated corn, rain-fed corn, and grass, grown under three hydroclimatic regimes in Nebraska. These regimes are stops along an east–west decreasing precipitation gradient of the Great Plains. The simulated SM climatologies are provided for the root zone as a whole and for the five layers of the soil profile to a depth of 1.2 m. As expected, the soil water content in the root zone of irrigated corn was higher than rain-fed corn or grass. The lowest levels of soil water depletion were found under rain-fed corn cultivation due to its complete reliance on naturally available SM. The annual total evapotranspiration (ET) was 34% and 36% higher for irrigated corn than for rain-fed corn and grass, respectively. The study suggests that due to interannual variability the SM variability is higher for deeper depths, as compared to near-surface depths. Growing season SM depletion and prevailing soil water content at various depths of the soil profile varies with crops, soils, and prevailing hydroclimatic conditions. The results show that land use affects the magnitude of SM variability at all time scales. At a daily temporal scale, SM variability is less under irrigated land use and sharply increases under rain-fed land uses. At the monthly scale, SM variability largely follows the trend of the daily time scale. Year-to-year SM variability is significant. Extremely dry or wet conditions enhance and reduce, respectively, the forcing of land use on SM variability at an annual time scale. Thus, large-scale interannual climate variations and land use jointly affect SM variability at this scale.