Evaluation of grapevine water status from trunk diameter variations

We evaluated the usefulness of short-term trunk diameter variations (TDV) as water stress indicator in field-grown grapevines cv. Tempranillo. Two indices were calculated from TDV, maximum daily trunk shrinkage (MDS), and trunk growth rate (TGR). The seasonal evolution of both indicators was compared with occasional determinations of pre-dawn leaf water potential and stem water potential, measured at early morning (Ψ_s^em) and at midday (Ψ_s^md) in irrigated and non-irrigated vines. In the second season, the effect of crop load on the vine water status indicators was also studied. Crop load did not affect either the vine water relations or the TDV. All water potential determinations had much lower variability and were more sensitive than both MDS and TGR to water restrictions. The ability of both indices to detect plant water stress varied largely depending upon the phenological period. In fact, MDS and TGR were only able to detect vine water stress during a short period of time before veraison. During this period, TGR was linearly related to both Ψ_s^em and Ψ_s^md, while for MDS a curvilinear, quadratic equation, better described the relationship with plant water status. After veraison no apparent relationship existed between plant water status and MDS or TGR. Hence, our results question the practical use of both MDS and TGR as variables to automate irrigation scheduling for grapevine.     

Assessment of trunk diameter variation derived indices as water stress indicators in mature olive trees

Plant age and size, seasonal growth patters and crop load, among other factors, have been reported to decrease the usefulness of trunk diameter variation (TDV) derived indices as water stress indicators in olive trees. Our hypothesis, however, is that indices derived from TDV records in old, big olive trees are sensitive enough to detect levels of water stress in trees of orchards under deficit irrigation that, although severe, are below the threshold for fruit shrivelling. This is of importance for the production of good quality oils, since fruit shrivelling may affect oil quality. The aim of this work was to assess different TDV-derived indices as water stress indicators in 40-year-old ‘Manzanilla’ olive trees with heavy crop load. We derived the maximum daily shrinkage (MDS), daily growth (DG) and daily recovery (DR) from TDV records taken during the 2008 dry season both in well-irrigated FAO trees and in deficit-irrigated RI trees. Measurements of volumetric soil water content (θ_v), leaf water potential (Ψ_l), stomatal conductance (g_s), net CO₂ assimilation rate (A), water and oil accumulation in the fruits and yield parameters were made for both treatments. The trunks did not grow during the experimental season, either in the FAO or RI trees, likely because of the heavy crop load. Therefore, DG was useless as water stress indicator. For MDS and DR, which were responsive to the increase of the trees’ water stress, we calculated the variability, quantified by the coefficient of variation (CV), the signal intensity (S_I) and the sensitivity (S_I/CV) values. In addition, we derived reference equations for irrigation scheduling from the relationships between MDS values in the FAO trees and main meteorological variables. Values both of S_I-MDS and S_I-DR were steady until September 9, despite of increasing differences in θ_v between treatments from early in the dry season. The Ψ_l vs θ_v values showed an outstanding capacity of the RI trees to take up water from the drying soil, and the Ψ_l vs g_s values showed a near-isohydric behaviour of those deficit-irrigated trees. These results explain, at least in part, the lack of response of MDS and DR on that period. Both S_I-MDS and S_I-DR peaked for the first time on September 9, 16 days before the appearance of fruit shrivelling. Our results suggest that using TDV-derived indices as water stress indicators for irrigation scheduling in old olive orchards with medium to low plant densities, i.e. with large root zones, may be useless in case the irrigation strategy is aimed at keeping the soil close to field capacity. Nevertheless, the MDS and DR indices may be useful indicators for the avoidance of fruit shrivelling in deficit irrigated olive orchards for the production of good quality oil. Reliable reference equations for scheduling irrigation with the signal intensity approach were obtained from the regression of MDS values vs the daily maximum values of both the air temperature and the vapour pressure deficit of the air.     

Evaluating water stress in irrigated olives: correlation of soil water status,tree water status,and thermal imagery

Irrigation of olive orchards is challenged to optimize both yields and oil quality. Best management practices for olive irrigation will likely depend on the ability to maintain mild to moderate levels of water stress during at least some parts of the growing season. We examined a number of soil, plant and remote sensing parameters for evaluating water stress in bearing olive (var. Barnea) trees in Israel. The trees were irrigated with five water application treatments (30, 50, 75, 100 and 125% of potential evapotranspiration) and the measurements of soil water content and potential, mid-day stem water potential, and stomatal resistance were taken. Remote thermal images of individual trees were used to alternatively measure average canopy temperature and to calculate the tree’s crop water stress index (CWSI), testing empirical and analytical approaches. A strong non-linear response showing similar trends and behavior was evident in soil and plant water status measurements as well as in the CWSI, with decreasing rates of change at the higher irrigation application levels. No statistically significant difference was found between the analytical and the empirical CWSI, suggesting that the relative simplicity of the analytical method would make it preferable in practical applications.     

Seasonal sensitivity of stem water potential to vapour pressure deficit in grapevine

Boundary lines of stem water potential (Ψ_stem) responses to vapour pressure deficit (VPD) have been reported for several species and are generally referred to as VPD reference lines (RL). In order to study the response of Ψ_stem to VPD, RLs were determined in plants that received full (Control) and deficit (SSDI) irrigation during three consecutive years. The Control plants received irrigation equivalent to full crop water evapotranspiration minus effective rainfall and the SSDI plants were irrigated at 50% of the Control level. Ψ_stem values for the Control treatment during crop development tended to decrease, and those corresponding to SSDI plants were always lower than those for Control plants. Considering the 3-year data set, no relationship was found between Ψ_stem and VPD. However, there was a differential seasonal response between Ψ_stem and VPD, which depended on the stage of fruit development. A separate phenological analysis enabled the detection of RL for stage II (from fruit-set to veraison) and for stage III (post-veraison), whereas during stage I (from bud-break to fruit set) RL was not apparent. RL slopes increased as the season progressed and were significantly correlated to average values of Ψ_stem. The seasonal decrease in midday Ψ_stem for Control plants was interpreted as being a result of a progressive increase in canopy size and water consumption, which led to increased water depletion before each afternoon’s daily irrigation event. The apparent lack of RL during stage I was related to lower levels of water demand and high Ψ_stem.     

Interaction of applied water amounts and leaf removal in the fruiting zone on grapevine water relations and productivity of Merlot

A study was conducted in the San Joaquin Valley of California on Merlot to determine the interaction of applied water amounts [at 0.4, 0.8, and 1.2 of estimated vineyard evapotranspiration (ET_c)] and leaf removal (at berry set or veraison) in the fruiting zone on productivity. Shaded area was measured beneath the canopy of the 1.2 irrigation treatment at solar noon throughout the study to provide an estimate of seasonal crop coefficients (K _c). Vine water status was assessed across treatments and years by measuring midday leaf water potential (Ψ_l). The maximum K _c determined from the percent shaded area was 0.7 at the row spacing of 3.66?m and canopy type that developed a “California Sprawl.” Irrigation treatment had a significant effect on midday Ψ_l and no such effect for leaf removal. Clusters exposed to direct solar radiation had significantly higher temperatures and lower cluster Ψ than clusters in the shade. Irrigation treatment had a significant effect on berry weight, soluble solids, and titratable acidity. Yields of vines significantly increased as applied water amounts increased. In this wine grape production area, profitability is dependent upon yield. This study provided a reliable estimate of ET_c and applied water amounts to maximize yield.     

无人机热红外反演土壤含水率的方法

以不同生育期夏玉米为对象,讨论无人机热红外反演夏玉米田土壤含水率的精度及反演方法.利用无人机获取试验区的可见光和热红外图像.通过可见光图像提取冠层掩膜并叠加在热红外图像上提取玉米冠层温度,分析冠层温度的变化趋势及与叶面积指数(LAI)的相关性.最后,利用冠气温差的相反数与叶面积指数构建了一个新指标(D_TL),讨论了冠气温差或D_TL指标反演土壤含水率的准确性.结果表明：冠层温度随着土壤含水量的增加而降低,夏玉米LAI在一定程度上可以表征冠层温度;对比4个时期的数据,发现冠气温差反演效果在灌溉后较好(如2次灌后R²分别为0.614 6和0.463 7);与冠气温差相比,D_TL指标可以提高土壤含水量反演的精度,如0～20 cm深度的R²从0.614 6和0.463 7提高到0.661 6和0.485 0.该研究对热红外反演夏玉米田间土壤含水率方法进行了新的尝试.     

基于纹理特征与植被指数融合的水稻含水量无人机遥感监测

含水量是表征水稻生理和健康状况的关键参数,精确预测水稻含水量对于水稻育种和大田精准管理具有重要意义。目前,利用无人机搭载光谱图像传感器监测作物生长的研究主要集中在利用植被指数评估作物在单一或者几个生育期的生长参数,针对作物含水量监测的研究非常有限。本研究主要利用多旋翼无人机低空遥感平台获取不同生育期水稻冠层的RGB图像和多光谱图像,通过提取植被指数和纹理特征,分析水稻的动态生长变化,并构建了基于随机森林回归方法的含水量预测模型。试验结果表明：（1）从无人机图像提取的植被指数、纹理特征以及地面测量的含水量都能用于监测水稻生长,并且这些参数随水稻生长呈现出了相似的动态变化趋势;（2）与RGB图像相比,多光谱图像评估水稻含水量具有更高的潜力,其中归一化光谱指数NDSI_771,611实现了更好的预测精度（R²=0.68,RMSEP=0.039,rRMSE =5.24%）;（3）融合植被指数和纹理特征能够进一步改善含水量的预测结果（R²=0.86,RMSEP=0.026,rRMSE=3.51%）,预测误差RMSEP分别减小了16.13%和18.75%。上述结果表明,基于无人机遥感技术监测水稻含水量是可行的,可为农田精准灌溉和田间管理决策提供新思路。     

Influence of water stress on kiwifruit growth

Summary An irrigation experiment was conducted on young kiwifruit vines over two seasons to examine effects of water stress on fruit development. Vines were grown outdoors in a sandy, rooting medium enclosed within a polythene-lined trench with removable surface covers to enable strict control of the water supply. Measurements of fruit growth, leaf water potential, and stomatal conductance were made throughout the season in conjunction with periods of water stress imposed at different times, and for varying durations. Fruit development was very responsive to water stress with mean fruit size per vine at harvest varying from 60 to 130 cm³ as a result of various stress treatments. Fruit expansion ceased when predawn leaf water potentials fell below –0.1 MPa. Upon rewatering, leaf turgor was regained within 24 h even after severe, prolonged stress. Any turgor loss associated with fruit softening was quickly made up, and thereafter fruit growth continued at the same rate concurrently exhibited on continuously well-watered vines. Suggesting that stomatal conductance did not follow the rapid recovery of leaf water potentials and fruit expansion may be more closely linked to water supply than to the concurrent rate of photosynthesis. Despite the large range in mean fruit size, the shape of the fruit size distribution at harvest was not affected by water stress and it is concluded that harvest yields can be adequately modelled by assuming a normal distribution with a fixed standard deviation.     

An analysis of the effects of water stress on leaf growth and yield of three barley cultivars

The main objective of the study was to evaluate the sensitivity of Iraqi local barley cultivar (Black) to soil water deficit as compared to other barley cultivars, namely: CM-72 and Arivat. The local cultivar proved to be susceptible during germination and emergence, but it developed resistance during the vegetative growth and yield formation stages. Growth analysis of individual leaves clearly showed that Black barley produced leaves of long growth duration which could affect the growth processes especially cell division. Thus, the reduction in the number of cells composing those leaves was small in the cultivar Black. Leaf growth rates as soil matric potential decreased, were reduced considerably, although, no clear trends were observed between cultivars. On the other hand, the smaller cell volume of local cultivar may have a role in its ability to tolerate water deficit. Some evidence that the local cultivar is post-flowering resistant is discussed.     

Spectral vegetation indices for benchmarking water productivity of irrigated cotton and sugarbeet crops

Irrigation performance and water productivity can be benchmarked if estimates of spatially distributed yield and crop water use are available. A commonly used method to estimate crop evapotranspiration in irrigated areas is to multiply reference evapotranspiration values by appropriate crop coefficients. This study evaluated convenient ways to derive such coefficients using multispectral vegetation indices obtained by remote sensing. Detailed ground radiometric measurements were taken in small plots perpendicular to the crop rows to obtain canopy reflectance values. Ancillary measurements of green ground cover, plant height, leaf area index and biomass were taken in the cropped strip covered by the radiometer field-of-view. The results were up-scaled using 10 Landsat-5 and 1 Landsat-7 images. Crop measurements and ground radiometry were made at the time of Landsat overpass on two commercial fields, one grown with sugarbeet and the other with cotton. Crop height and ground cover were determined weekly in these two fields, three additional sugarbeet fields and one additional cotton field. The ground and satellite observations of canopy reflectance yielded similar results. Two vegetation indices, the normalized difference vegetation index (NDVI) and the soil adjusted vegetation index (SAVI) were evaluated. Both indices described the crop growth well, but SAVI was used in further evaluations because it could be conveniently related to both ground cover and the basal crop coefficient using a simple model. Based on these findings, crop water use variability was analyzed in a large sample of sugarbeet and cotton fields, within a homogeneous irrigation scheme in Southern Spain. The yield versus evapotranspiration data points were highly scattered for both cotton and sugarbeet. The yield values obtained from the sugarbeet fields and cotton fields were substantially lower than values predicted by a linear yield function, and close to a curvilinear yield function, respectively. Evapotranspired water productivity varied in the cotton fields from 0.3 to 0.78 kg m⁻³, and in the sugarbeet fields from 7.15 to 14.8 kg m⁻³.     

Interrelations of yield,evapotranspiration, and water use efficiency from marginal analysis of water production functions

The study uses the concepts of marginal water use efficiency (MWUE), and elasticity of water production (EWP) to reveal the dynamic interrelations of crop yield (Y), seasonal evapotranspiration (ET), and water use efficiency (WUE) based on the functional relation of an ET production function (ETPF). When the ETPF is linear, the changing trend of WUE with ET is directly affected by the intercept of the function, and the EWP will be numerically equivalent to a yield response factor (K_Y) when ET reaches maximum ET (ET_m). When the ETPF is quadratic, the ET needed to maximise WUE is less than the ET for maximum yield (Y_m), and the ET value that occurs at maximum WUE equals the arithmetic square root of the ratio of the intercept of the function to the coefficient of function quadratic term. The interrelationships of Y, ET, and WUE are demonstrated using a quadratic ETPF developed for maize from data obtained in a field experiment.     

A reexamination of the crop water stress index

Summary Hand-held infrared radiometers, developed during the past decade, have extended the measurement of plant canopy temperatures from individual leaves to entire plant canopies. Canopy temperatures are determined by the water status of the plants and by ambient meteorological conditions. The crop water stress index (CWSI) combines these factors and yields a measure of plant water stress. Two forms of the index have been proposed, an empirical approach as reported by Idso et al. (1981), and a theoretical approach reported by Jackson et al. (1981). Because it is simple and requires only three variables to be measured, the empirical approach has received much attention in the literature. It has, however received some criticism concerning its inability to account for temperature changes due to radiation and windspeed. The theoretical method is more complicated in that it requires these two additional variables to be measured, and the evaluation of an aerodynamic resistance, but it will account for differences in radiation and windspeed. This report reexamines the theoretical approach and proposes a method for estimating an aerodynamic resistance applicable to a plant canopy. A brief history of plant temperature measurements is given and the theoretical basis for the CWSI reviewed.     

Efficiency gains from water markets: Empirical analysis of Watermove in Australia

We analyse supply, demand, and welfare measures in markets where the temporary trading of water rights is reasonably active and liquid on a weekly basis. We identify four important characteristics of water trading within the Watermove program: (1) demand is highly responsive to price changes, supporting evidence that farmers’ participation is driven by the advantages water markets provide in reducing seasonal risk; (2) some efficiency enhancing trades are prevented from taking place due to physical limitations of the delivery system, thereby reducing the ability of the market to move toward equilibrium. In addition, these limitations also created price premiums in some zones; (3) variations in the types of products traded are explained by differences in the administrative charges for trading and current drought conditions; and (4) relatively large gains have been made thus far, suggesting that water markets will expand and generate additional increases in consumer and producer surplus in the future.     

Assessing water stress in a hedgerow olive orchard from sap flow and trunk diameter measurements

We used sap flow and trunk diameter measurements for assessing water stress in a high-density ‘Arbequina’ olive orchard with control trees irrigated to replace 100 % of the crop water needs, and 60RDI and 30RDI trees, in which irrigation replaced ca. 60 and 30 % of the control, respectively. We calculated the daily difference for both tree water consumption ( $ D_{{E_{\text{p}} }} $ ) and maximum trunk diameter (D _MXTD) between RDI trees and control trees. The seasonal dynamics of $ D_{{E_{\text{p}} }} $ agreed reasonably well with that of the stem water potential. We identified peculiarities on the response $ D_{{E_{\text{p}} }} $ to changes in water stressing conditions, which must be taken into account when using the index. An analysis of the water stress variability in the orchard is required for choosing the instrumented trees. The reliability of the D _MXTD index was poorer than that of $ D_{{E_{\text{p}} }} $ . The maximum daily shrinkage (MDS) was not a reliable water stress indicator.     

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.     

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.

---

Plant indicators of wheat and soybean crop water stress

Summary The onset of water stress within a crop is defined as the time at which the rate of water loss declines below that of a well watered crop in the same locality. The relation to the onset of water stress and soil water status of several readily measured plant parameters was investigated in crops of wheat and soybeans over three years. Evapotranspiration ET was monitored with weighing lysimeters. A noticeable decline in the rate of ET for both wheat and soybeans was detected once 20% to 30% of the total plant available water PAW remained in the 1 m deep lysimeter soil profile. Extension growth of wheat declined when PAW was 33% and 34% in two years of measurement. In soybeans, the decline in the rate of leaf extension coincided with the decline in the rate of ET. Midmorning measurement of exposed leaf water potential _L, covered leaf water potential _CL and covered plant leaf water potential _CP yielded similar results for both wheat and soybeans. Day-to-day variability was least in _CP and most in _L. Values of _CP, _L and _CL decreased rapidly with PAW < 30%. Daily values of leaf diffusive conductance were variable but there was a general decline in conductance with PAW < 30%. It is suggested that _CL may be the easiest and most reliable parameter to monitor as a means of detecting the onset of stress. The results indicated that PAW levels in the root zone of 50% for wheat and 30% for soybean probably do not affect extension growth or plant water status parameters and can thus be used as criteria for irrigation scheduling.Seconded from the Water Research Commission, Pretoria; present address: CSIRO, Division of Irrigation Research, Griffith, N SW 2680, Australia     

Daylight crop water stress index for continuous monitoring of water status in apple trees

We studied the suitability of empirical crop water stress index (CWSI) averaged over daylight hours (CWSI_d) for continuous monitoring of water status in apple trees. The relationships between a midday CWSI (CWSI_m) and the CWSI_d and stem water potential (ψ _stem), and soil water deficit (SWD) were investigated. The treatments were: (1) non-stressed where the soil water was close to field capacity and (2) mildly stressed where SWD fluctuated between 0 and a maximum allowable depletion (MAD of 50 %). The linear relationship between canopy and air temperature difference (ΔT) and air vapor pressure deficit (VPD) averaged over daylight hours resulted in a non-water-stressed baseline (NWSBL) with higher correlation (?T = ?0.97 VPD – 0.46, R ² = 0.78, p < 0.001) compared with the conventional midday approach (?T = ?0.59 VPD – 0.67, R ² = 0.51, p < 0.001). Wind speed and solar radiation showed no significant effect on the daylight NWSBL. There was no statistically meaningful relationship between midday ψ _stem and CWSI_m. The CWSI_d agreed well with SWD (R ² = 0.70, p < 0.001), while the correlation between SWD and CWSI_m was substantially weaker (R ² = 0.38, p = 0.033). The CWSI_d exhibited high sensitivity to mild variations in the soil water content, suggesting it as a promising indicator of water availability in the root zone. The CWSI_d is stable under transitional weather conditions as it reflects the daily activity of an apple crop.     

Simulation of root water uptake: III. Non-uniform transient combined salinity and water stress

Six different reduction functions for combined water and salinity stress are used in the macroscopic root water extraction term. The reduction functions are classified as linear additive, non-linear multiplicative, and that which is neither additive nor multiplicative. All these reduction functions are incorporated in the numerical simulation model HYSWASOR. The relation between the experimental relative transpiration and the joint soil water osmotic and pressure heads appears to be linear (with an exception for the salinity near the threshold value). As the mean soil solution salinity increases, the trend becomes more linear. The simulations indicated that for most treatments the newly proposed reduction term provides the closest agreement with the experimental transpiration. Soil water content, and particularly soil solution salinity simulated with this equation agree reasonably with the experimental data: in spite of the observed differences, the trend of the simulated data is good. A reason for the disagreement between the simulated and experimental water contents can be attributed to the influence of roots and the soil solution concentration on the soil hydraulic conductivity. The input soil hydraulic parameters were obtained from soil samples without roots and salinity and assumed constant during the simulations.