Continuous measurement of plant and soil water status for irrigation scheduling in plum

The usefulness of continuous measurement of soil and plant water status for automated irrigation scheduling was studied in a drip-irrigation experiment on plum (Prunus salicina Black Gold). Two levels of water restriction were imposed at different phenological periods (from pit-hardening to harvest, post-harvest) and compared with a well irrigated control treatment. Soil matrix water potential (_soil) was measured with granular matrix sensors (Watermark); and short-period trunk diameter variation (TDV) was measured with linear variable displacement transformers. The Watermark sensor readings were in reasonable agreement with the irrigation regime and showed a good indication of plant water status across the season (r²=0.62), although they were a better predictor of stem water potential (_stem) in the dry range of _soil. Nonetheless, the most important drawback in their use was the high variability of readings (typical CV of 35–50%). From TDV measurements, maximum daily shrinkage (MDS) and trunk growth rate (TGR) were calculated. Their performance was also compared with _stem, which had the lowest variability (CV of 7%). During most of the fruit growth period, when TGR was minimum, MDS was higher in the less-irrigated treatment than in the control and correlated well (r²=0.89) with _stem. However, after harvest, when TGR was higher, this correlation decreased as the season progressed (r²=0.73–0.52), as did the slope between MDS and _stem, suggesting tissue elasticity changes. Later in the season, TGR was better related to plant water status. These observations indicate some of the difficulties in obtaining reference values useful for irrigation scheduling based exclusively on plant water status measurements.     

Numerical investigation of irrigation scheduling based on soil water status

Improving the sustainability of irrigation systems requires the optimization of operational parameters such as irrigation threshold and irrigation amount. Numerical modeling is a fast and accurate means to optimize such operational parameters. However, little work has been carried out to investigate the relationship between irrigation scheduling, irrigation threshold, and irrigation amount. Herein, we compare the results of HYDRUS 2D/3D simulations with experimental data from triggered drip irrigation, and optimize operational parameters. Two field experiments were conducted, one on loamy sand soil and one on sandy loam soil, to evaluate the overall effects of different potential transpiration rates and irrigation management strategies, on the triggered irrigation system. In both experiments, irrigation was controlled by a closed loop irrigation system linked to tensiometers. Collected experimental data were analyzed and compared with HYDRUS 2D/3D simulations. A system-dependant boundary condition, which initiates irrigation whenever the matric head at a predetermined location drops below a certain threshold, was implemented into the code. The experimental model was used to evaluate collected experimental data, and then to optimize the operational parameters for two hypothetical soils. The results show that HYDRUS 2D/3D predictions of irrigation events and matric heads are in good agreement with experimental data, and that the code can be used to optimize irrigation thresholds and water amounts applied in an irrigation episode to increase the efficiency of water use.     

Trickle irrigation timing and its effect on plant and soil water status

Tomato plants on Sinai sand dunes were irrigated daily by drip irrigation. The irrigation was supplied during daytime hours for one field and a short time after sunset for the second. Results showed that daytime irrigation of soil with low water holding capacity increased the yield significantly and improved plant water potential as well as water use efficiency. The dominant component of water balance under these conditions was found to be deep percolation, which accounted for more than 70% of the water budget. Controlling this component rather than soil water status requires measurements of flux as input for managing the quantity of water to be applied. It is concluded that during hours of high net radiation flux, transpiration rate can best compete with deep percolation rate. Based on this conclusion, the use of net radiation flux as input is recommended for the best irrigation timing.     

A method for spatial prediction of daily soil water status for precise irrigation scheduling

Available water holding capacity (AWC) and field capacity (FC) maps have been produced using regression models of high resolution apparent electrical conductivity (EC_a) data against AWC (adj. R² = 0.76) and FC (adj. R² = 0.77). A daily time step has been added to field capacity maps to spatially predict soil water status on any day using data obtained from a wireless soil moisture sensing network which transmitted hourly logged data from embedded time domain transmission (TDT) sensors in EC_a-defined management zones. In addition, regular time domain reflectometry (TDR) monitoring of 50 positions in the study area was used to assess spatial variability within each zone and overall temporal stability of soil moisture patterns. Spatial variability of soil moisture within each zone at any one time was significant (coefficient of variation [% CV] of volumetric soil moisture content (θ) = 3-16%), while temporal stability of this pattern was moderate to strong (bivariate correlation, R = 0.52-0.95), suggesting an intrinsic soil and topographic control. Therefore, predictive ability of this method for spatial characterisation of soil water status, at this site, was limited by the ability of the sensor network to account for the spatial variability of the soil moisture pattern within each zone. Significant variability of soil moisture within each EC_a-defined zone is thought to be due to the variable nature of the young alluvial soils at this site, as well as micro-topographic effects on water movement, such as low-lying ponding areas. In summary, this paper develops a method for predicting daily soil water status in EC_a-defined zones; digital information available for uploading to a software-controlled automated variable rate irrigation system with the aim of improved water use efficiency. Accuracy of prediction is determined by the extent to which spatial variability is predicted within as well as between EC_a-defined zones.     

A historical perspective on measurement of soil and plant water status and plant water status

Summary Surrounded by many sophisticated instruments, data acquisition systems and connecting computers, contemporary soil and plant scientists may not fully appreciate what really was known in earlier times and how the science has developed. A short review and a few examples of the growth of measurements and ideas are presented here.     

Plant response to evapotranspiration and soil water sensor irrigation scheduling methods for papaya production in south Florida

An irrigation study was conducted to determine the effects of implementing different irrigation practices on growth and yields of papaya plants in south Florida. Treatments included using automated switching tensiometers based on soil water status, irrigation based on ET calculated from historic weather data and a set schedule irrigation regime. The study consisted of two trials (2006-2007 and 2008-2009). Water volumes applied, plant height and diameter, leaf gas exchange, leaf petiole nutrient levels, fruit yields and fruit total soluble solids were measured throughout the study. For both trials, significantly more water was applied in the set schedule irrigation treatment than in all other treatments; historic ET and soil water based treatments received only about 31-36% of the water applied in the set schedule irrigation. Trunk diameter and plant height per unit water volume applied values for the set schedule treatment were significantly lower than those from all other treatments during both trials. The set schedule treatment in both trials also had the lowest crop production water use efficiency (CP-WUE); CP-WUE values among all other treatments were generally not significantly different from each other. Soil water and historic ET-based irrigation methods were identified as more sustainable practices compared to set schedule irrigation due to the lower water volumes applied while maintaining plant nutrient content, growth, photosynthetic rates, and fruit yields for this production system.     

Integrating and applying soil and plant water status measurements

Summary Application of soil and plant water status measurements requires some model of the soil-plant-atmosphere system because the measurements made refer to only part of the complex whole. Measurements need to be made to check on the validity of the model and to facilitate adjustment. Since models are only a small imitation of reality they need to be continually checked if application of the results are to be useful. The temptation to use models without checking should be discouraged — modelers should keep one foot in the field.     

节水灌溉发展规划中有必要澄清的几个问题

国家计划在新世纪开始的若干年内 ,加大投入力度 ,支持大型灌区进行技术改造 ,以推动全国节水灌溉的发展 ,这是中央在世纪交替之际 ,针对具有全球性质的水资源紧缺问题 ,所做的一项重大战略布置。为了妥善用好这笔资金 ,使之发挥应有的节水增收效益 ,根据水利部的安排 ,目前许多大型灌区续建配套与节水改造规划都在紧张进行。根据笔者所接触到的一些情况 ,深感有几个与规划成果质量有密切关系的问题 ,有必要提出来加以讨论澄清。由于气候、环境、水土资源、种植结构、产量水平、城镇生活工业用水等情况较之三、四十年前有了比较大的变化 ,规…     

Effects of deficit irrigation scheduling on yields and soil water balance of irrigated maize

This paper presents the findings of the effect of some selected deficit irrigation scheduling practices on irrigated maize crop in a sub-catchment in south western part of Tanzania. Field experiments, in which maize (TMV1-ST) variety was planted under total irrigation, were conducted during the dry seasons of 2004 and 2005. Surface irrigation method was used and the crop was planted in basins. The seasonal water applied ranged from 400 to 750 mm. Soil moisture content from both cropped and bare soils, leaf area index, dry matter, and grain yields were measured. The dry matter yield ranged between 6,966 and 12,672 kg/ha, and grain yields obtained were between 1,625 and 4,349 kg/ha. The results showed that deficit irrigation at any crop growth stage of the maize crop led to decrease in dry matter and grain yields, seasonal evapotranspiration and deep percolation. Deficit irrigation in any one growth stage of the maize crop only seems to affect grain production and no significant effect on biomass production, but deficit irrigation that spanned across two or more growth stages affect both biomass and grain production drastically. Crop water use efficiency (WUE) and Irrigation water use efficiency (IWUE) were strongly influenced by the number of growth stages in which deficit irrigations were applied and how critical the growth stages were to moisture stress rather than the amount of irrigation water applied. While maximum WUE was obtained under full irrigation, maximum IWUE was obtained in the deficit irrigation treatment at vegetative growth stage, which suggest that IWUE may be improved upon by practicing deficit irrigation at the vegetative growth stage of the maize crop.     

Root growth, available soil water, and water-use efficiency of winter wheat under different irrigation regimes applied at different growth stages in North China

Field experiments were conducted at the Luancheng Agro-Ecosystem Experimental Station of the Chinese Academy of Sciences during the winter wheat growing seasons in 2006-2007 and 2007-2008. Experiments involving winter wheat with 1, 2, and 3 irrigation applications at jointing, heading, or milking were conducted, and the total irrigation water supplied was maintained at 120 mm. The results indicated that irrigation during the later part of the winter wheat growing season and increase in irrigation frequency decreased the available soil water; this result was mainly due to the changes in the vertical distribution of root length density. In ≤30-cm-deep soil profiles, 3 times irrigation at jointing, heading, and milking increased the root length density, while in >30-cm-deep soil profiles, 1 time irrigation at jointing resulted in the highest root length density. With regard to evapotranspiration (ET), there was no significant (LSD, P < 0.05) difference between the regimes wherein irrigation was applied only once at jointing; 2 times at jointing and heading; and 3 times at jointing, heading, and milking. Compared with 1 and 3 times irrigation during the winter wheat growing season, 2 times irrigation increased grain yield and 2 times irrigation at jointing and heading produced the highest water-use efficiency (WUE). Combining the results obtained regarding grain yield and WUE, it can be concluded that irrigation at the jointing and heading stages results in high grain yield and WUE, which will offer a sound measurement for developing deficit irrigation regimes in North China.     

Responses of grain sorghum to variable water supply under two irrigation frequencies

Summary Two experiments were conducted with Sorghum bicolor (L.) Moench Cv. Pioneer 846 in a deep loam soil in the 1977 season. Experiment I consisted of two line-source sprinkler plots, one irrigated at two-day intervals (HF) and the other every 10 to 14 days (NF). In comparable treatments of both regimes, the seasonal amount of water applied was the same. Since the subsoil was very dry at planting, the different amounts of water applied led to the development of a range of water stresses from none to severe. Experiment II was conducted in an adjacent area with a fully wetted soil profile. The treatments were irrigated every week (I) and nonirrigated (NI). Applied water was measured after each irrigation. Soil water content, leaf water potential, leaf-area index, ground cover and dry-matter accumulation were measured at frequent intervals, and yields were taken at the end of the growing season. Linear relations were found between both total dry-matter production and grain yield and seasonal evapotranspiration (ET) in both HF and NF regimes. With seasonal values of ET near the potential, grain yield and dry-matter production did not differ between irrigation frequencies. At low seasonal ET values, however, the NF regime gave greater yields of dry-matter and grain than did the HF regime.     

Grapefruit response to variable salinity in irrigation water and soil

Summary Results are reported from a long-term field experiment designed to determine the effect of irrigation water salinity on the yield and water uptake of mature grapefruit trees. Treatments were started in 1970 and consisted of chloride concentrations in the irrigation water of 7.1, 11.4 and 17.1 meq/1 added as NaCl+CaCl₂ at a 1 : 1 weight ratio.For the last four years of the experiment, 1973 to 1976, yield was linearly related to the mean chloride concentration in the soil saturation extract weighted according to the distribution of water uptake with depth and time (Fig. 2, Table 1). There was a 1.45% (1.68 Mg/ha) yield reduction for each 1 meq/1 increase in chloride concentration above a threshold value of 4.5 meq/1. This corresponded to a 13.5% (14.7 Mg/ha) decrease per 1 mmho/cm increase in the electrical conductivity of the soil saturation extract above a threshold value of 1.2 mmho/cm.Total water uptake was reduced as salt concentration in the soil increased (Fig. 3, Table 2). In the high salinity treatment, root concentration in, and water uptake from, the lower portion of the root zone were decreased. The maximum electrical conductivity (ECe) measured at the bottom of the root zone was 7.90 mmho/cm similar to the values of EC, obtained by linear extrapolation to zero yield and also to zero water uptake.Salt accumulation in the soil depended on the quantity and salt concentration of the irrigation water, rainfall, and on the amount of leaching. SAR and the Na⁺ concentration of the soil remained low throughout the experiment (Table 3). No leaf symptoms of either Cl^– or Na⁺ injury were observed. The results indicate an osmotic — rather than a specific ion effect — of salinity on grapefruit yield.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. 1977 Series No. 197-E     

Behavior of temperature-based water stress indicators in BIOTIC-controlled irrigation

A subsurface drip irrigation study with cotton used canopy temperature to determine signals for irrigation control during 2002–2004. Timing of irrigation applications was controlled by the biologically identified optimal temperature interactive console (BIOTIC) protocol, which used stress time (ST) and a crop-specific optimum temperature to indicate water stress. ST was the cumulative daily time quantity when cotton canopy temperature exceeded 28°C. STs between 5.5 and 8.5 h in 1 h increments were irrigation signal criteria, which produced different irrigation regimes. This investigation examined the association among ST, daily average canopy temperature (T _c), canopy and air temperature difference (T _c−T _a), and the relative crop water stress index (RCWSI) including their relationship with lint yield. Number of irrigation signals decreased linearly with ST at the rate of −10.2 and −8.7 irrigations per 1 h increase of ST in 2003 and 2004. There were significant curvilinear relationships between ST and the average daily stress on days with irrigation signals and for days without irrigation signals across years. The percentage of positive daily (T _c−T _a) values increased with ST level. ST and T _c were positively related in all irrigation signal treatments with 5.5 and 6.5 h being significant in 2003 and 2004. Yield declined at the rate of 343 kg lint/ha for each 1 h increase of ST for days with irrigation signals. ST, mathematically the most simple of the canopy temperature-based parameters, provided the most consistent estimate of crop water stress and correlation with lint yield. The power of ST to characterize water stress effects on crop productivity evolves from being an integrated value of time while canopy temperature exceeds a physiologically based threshold value.

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Effect of saline water irrigation and manure application on the available water content, soil salinity, and growth of wheat

Good water management combined with appropriate soil management is necessary for sustainable crop production in drylands. A pot culture experiment was conducted using sand dune soil under greenhouse conditions to evaluate the response of wheat (Triticum aestivum L.) to the application of farmyard manure (FYM) or poultry manure (PM), and irrigation with water at two salinity levels (0.11 and 2.0 dS m⁻¹) and two irrigation intervals (daily and every second day). The manure was applied at a rate of 20 Mg ha⁻¹. The soil water content, measured 1 h before every irrigation, showed that soil treated with PM retained more water than that treated with FYM, while the control (no manure) contained the least water. FYM treatment resulted in 78 and 21% higher dry matter yield compared to the control and PM treatments, respectively, under daily irrigation using good-quality water. The increase was 29 and 55%, respectively, when saline water was used for daily irrigation. A similar trend was observed with the alternate day irrigation treatment; FYM gave the highest dry matter yield. The number of tillers and plant height showed that FYM was better than PM, which in turn was better than the control under irrigation with good-quality water regardless of the irrigation interval. When water of the highest salinity was used for irrigation, FYM was still always the best, but the control was now better than the PM treatment. The electrical conductivity of the soil measured at the end of the experiment was slightly higher with PM, as compared to the FYM and control treatments. A significant interaction between irrigation water quality and manure application was observed, affecting plant growth. PM aggravated the adverse affect of saline water on plant growth by increasing soil salinity.     

Response of cotton water stress indicators to soil salinity

Summary A field study was conducted on cotton (Gossypium hirsutum L. c.v. Acala SJ-2) to investigate the effects of soil salinity on the responses of stress indices derived from canopy temperature, leaf diffusion resistance and leaf water potential. The four salinity treatments used in this study were obtained by mixtures of aqueduct and well water to provide mean soil water electrical conductivities of 17, 27, 32 and 38 dS/m in the upper 0.6 m of soil profile. The study was conducted on a sandy loam saline-alkali soil in the lower San Joaquin Valley of California on 30 July 1981, when the soil profile was adequately irrigated to remove any interference of soil matric potential on the stress measurements. Measurements of canopy temperature, leaf water potential and leaf diffusion resistance were made hourly throughout the day.Crop water stress index (CWSI) estimates derived from canopy temperature measurements in the least saline treatment had values similar to those found for cotton grown under minimum salinity profiles. Throughout the course of the day the treatments affected CWSI values with the maximum differences occurring in mid-afternoon. Salinity induced differences were also evident in the leaf diffusion resistance and leaf water potential measurements. Vapor pressure deficit was found to indicate the evaporative demand at which cotton could maintain potential water use for the various soil salinity levels studied. At vapor pressure deficits greater than 5 kPa, cotton would appear stressed at in situ soil water electrical conductivities exceeding 15 dS/m. The CWSI was as sensitive to osmotic stress as other, more traditional plant measures, provided a broader spatial resolution and appeared to be a practical tool for assessing osmotic stress occurring within irrigated cotton fields.     

Relations between available and extractable soil water and evapotranspiration from a bean crop

The accuracy of ‘available’ and ‘extractable’ soil water estimates was investigated using irrigated and unirrigated beans (Vicia faba) grown in an alluvial silt loam in Canterbury, New Zealand. Available water capacity was defined as the difference between soil water contents in the root zone at the drained upper limit (DUL) and at the lower limit (LL) as estimated by laboratory procedures. Extractable water capacity was specified as the difference between field estimates of DUL and LL for the whole profile affected by roots. DUL was estimated in the laboratory by equilibrating soil cores at matric potentials at ?10, ?20 or ?30 kPa, and in the field by neutron moderation. Laboratory estimates of LL were made from soil samples equilibrated at ?1.5 MPa matric potential. In the field LL was measured by neutron moderation on plots where evaporation had apparently ceased due to drought stress.When compared at intervals down the profile laboratory estimates of DUL and LL showed poor agreement with field observations. However, the final estimates of available and extractable water capacities were similar because of compensatory inaccuracies in the laboratory estimates. Furthermore, field measurements of evapotranspiration, using neutron moderation and tensiometry, indicated that the accuracy of the available water estimates was much reduced by upward fluxes of water into the rooting zone. These fluxes resulted in water extraction to at least 1.0 m although the apparent maximum rooting depth (measured by counting roots washed from soil cores) was only 0.7 m.Particular attention was paid to the influence of subsoil textural variability, which is pronounced in such soils. Laboratory and field estimates of the LL had to be carefully matched texturally before relevant comparisons could be made. Problems associated with subsoil textural variability affected laboratory methods of DUL estimation more than field methods.     

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. Camelina 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 (K _cb ) methods for predicting camelina crop evapotranspiration (ET) and irrigation scheduling. Besides K _cb methods, additional treatment factors included two different irrigation scheduling 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 K _cb 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 K _cb 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 K _cb method and significantly greater for the 45 than 65 % SWD level. However, 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. Differences in camelina seed yields due to seasonal N applications (69–144 kg N ha^?1 over the 2 years) were not significant. Further investigations are needed to characterize camelina yield response over a wider range of irrigation and N inputs.