Air Vapor Pressure Deficit Research Articles

Usefulness of thermography for plant water stress detection in citrus and persimmon trees

The feasibility of using canopy temperature (Tc) measured with a hand-operated infrared thermographic camera as a water stress indicator was evaluated in the field during two seasons on citrus and persimmon trees subjected to different levels of deficit irrigation. In both species, which differ in leaf anatomy and stomatal response to environmental conditions, Tc was compared with midday stem water potential (Ψs) measurements. In persimmon trees, leaf stomatal conductance (gs) was also measured. In 2009, images were taken from the sunlit and shady sides of the canopies. Based on the results obtained, during the second experimental season images were taken from the sunlit side of the trees and also from above the canopy. In persimmon, trees under deficit irrigation had lower Ψs and gs what resulted in a clear increase in Tc regardless of the position from where the pictures were taken. The maximum Tc difference between deficit-irrigated and control trees observed was of 4.4°C, which occurred when the stressed trees had Ψs values 1.1MPa lower than the control ones. In persimmon trees, Tc was the most sensitive indicator of plant water status particularly due to the lower tree-to-tree variability as compared to Ψs and gs. On the other hand, in citrus trees Tc was not always affected by plant water stress. Only in the second experimental season, when air vapour pressure deficit values were below 2.7kPa and images were also taken from above the canopies, deficit-irrigated trees had higher Tc than the control ones, this difference being at most 1.7°C. Overall, the results show that hand-operated thermographic cameras can be used to detect plant water stress in both fruit tree species. Nevertheless, the use of Tc measurements to detect plant water stress appears to be more precise in persimmon than in orange citrus. This might be because persimmon trees have larger leaf size which determines higher canopy resistance allowing for higher increases in canopy temperature in response to water stress via stomatal closure.

Leaf phenological traits and leaf longevity of three mangrove species (Rhizophoraceae) on Okinawa Island, Japan

Data on leaf phenological traits and longevity of mangroves are important for improved understanding of the adaptation strategies, growth, and productivity of these trees. We studied leaf phenological traits and longevity of Rhizophora stylosa, Bruguiera gymnorrhiza, and Kandelia obovata monthly from April 2008 to March 2009. Both tree sampling (direct observation) and litterfall (indirect observation) were used. All leaves and branches of each sample tree were numbered and marked with tags at the beginning of the experiment. Peaks of leaf recruitment and death occurred in July and June, respectively, for B. gymnorrhiza and K. obovata but both occurred in July for R. stylosa. Leaf recruitment for all the species was lowest in January; leaf death was minimum in December for R. stylosa and B. gymnorrhiza, and in January for K. obovata. Leaf recruitment for the three species was significantly correlated with monthly mean air temperature and monthly hours of sunshine. K. obovata and R. stylosa leaf death was correlated with monthly mean air temperature, monthly hours of sunshine, monthly mean air vapor pressure deficit, and monthly rainfall; B. gymnorrhiza leaf death was not correlated with any environmental factors. Specific leaf area for R. stylosa (45.4 ± 1.0 cm2 g−1) and K. obovata (48.6 ± 0.8 cm2 g−1) was not significantly different, but means for these two species were significantly different from that for B. gymnorrhiza (71.0 ± 2.8 cm2 g−1). Mean leaf longevity was 13.9 months for R. stylosa, 17.2 months for B. gymnorrhiza, and 12.1 months for K. obovata. Different measured variables between B. gymnorrhiza and the other two species may have been because of the growth strategy and shade tolerance of B. gymnorrhiza. Mean leaf longevity increased with increasing mean annual air temperature.

Comparing the impact of cloudiness on carbon dioxide exchange in a grassland and a maize cropland in northwestern China

AbstractLight quantity and quality strongly influence plant growth. However, different ecosystems have different capabilities to assimilate solar radiation. In this study, the effects of cloudiness intensity on the net ecosystem exchange of carbon dioxide (NEE) were compared between an alpine grassland (with lower leaf area index) at A'Rou and an oasis maize cropland (with higher leaf area index) at Yingke, using flux data obtained during the middle of the growing season (July–August) in 2008 and 2009. The results showed that the response of NEE to photosynthetically active radiation (PAR) was more negative (carbon uptake) under cloudy than under clear skies at both sites. The maximum NEE occurred when the clearness index (CI) ranged from 0.4 to 0.7 under cloudy skies. The maximum enhancements were 11.9% for solar elevation angles of 60–65° in the grassland, and 34.9% for solar elevation angles of 60–65° and 10.3% for angles of 35–40° in the maize cropland before the irrigation period. The response of NEE to CI changed slightly with solar elevation angle in the grassland compared to the maize cropland. The results indicate that enhanced NEE under cloudy skies can be attributed to increasingly diffuse PAR and interactions with environmental factors (air temperature and vapor pressure deficit).

EFFECT OF TWO NEW COVER MATERIALS ON GREENHOUSE ENERGY CONSUMPTION AND COOLING LOAD

The aim of this work was to study the effects of a new polyethylene (PE) film with high reflectance and absorption in near infrared radiation (NIR-PE film) and of a PE film with low transmittance in infrared radiation IR (IR-PE film), on the greenhouse microclimate, energy consumption and on growth and yield of a hydroponic tomato crop. Compared to a common PE film (C-PE), the IR-PE film, had 75% lower transmittance in IR, while the NIR-PE had 78% lower transmittance in IR and 42% higher reflectance in NIR. Films were evaluated experimentally in three similar arched roof greenhouses, two covered with the new covering materials and the third one by a C-PE film. The greenhouse and outside microclimate parameters along with crop growth and production were recorded. During the winter period, no significant differences of air temperature and relative air humidity between the tested greenhouses were found. The air vapour pressure deficit (VPD) values observed were relatively low (i.e below 0.5 kPa) during the night period, leading to condensation over the cover. The total energy consumption was 10% lower in the IR-PE covered greenhouse than in the C-PE covered greenhouse. During the summer period, the air temperature in the greenhouse covered by the NIR-PE was 2°C lower than the air temperature of the C-PE covered greenhouse.

Effects on microclimate, crop production and quality of a tomato crop grown under shade nets

SummaryThe effects of four shade nets with different colour transmission properties and shading intensities (two black nets with shading intensities of 40% and 49%, a green net with a shading intensity of 34%, and a mixed green and black shade net with a shading intensity of 40%) on the screenhouse microclimate, and on the growth and development of a tomato crop, were investigated. Experiments were carried out during Spring and Summer 2003 in New Anchialos, a coastal area of Eastern Central Greece. Tomato seedlings were planted in soil and the following environmental variables were recorded at regular intervals: solar radiation, air temperature and relative humidity, leaf temperature, and several variables related to crop growth and development. Air temperature and vapour pressure deficit (VPD) values were similar under all four shade nets and without shading. However, canopy temperature and, accordingly, the canopy-to-air VPD were significantly lower under shading than without shading. Shading did not reduce the total plant dry matter content, but increased the leaf area index, the number of fruit per plant, and the total fresh tomato yield. Shading reduced losses caused by tomato cracking by 50%, and thus increased the marketable tomato fruit yield by approx. 50% compared to growth under non-shaded conditions.

Shading Effects on Greenhouse Microclimate and Crop Transpiration in a Cucumber Crop Grown Under Mediterranean Conditions

The aim of this study was to investigate the effects of greenhouse shading on greenhouse microclimate and energy balance, and on crop production. Experiments were carried out in the experimental farm of the University of Thessaly at Velestino, in three similar, plastic-covered greenhouses using hydroponically-grown cucumbers as a test crop. One of the greenhouses was used as a control (without shading); the other two were shaded using two different shade nets (shading intensity of approximately 35% and 50%, respectively). Climatic parameters were measured during two growing seasons from April to June and from September to November 2008 and seven selected days of the above periods are presented. The results showed that shading could not keep greenhouse air temperature and vapor pressure deficit below 30C and 1.5 kPa, respectively, values that are considered acceptable for cucumber crop growth (Growers Books, 1980; Bakker et al., 1987; Olympios and Hanan, 1992). From the crop production data it was found that shading intensity should not exceed 35%. The analysis of greenhouse microclimate and energy balance showed that shading is necessary from the middle of spring, while even shading of approximately 50% was not sufficient to cool the greenhouse during noon time of summer days in Central Greece and that an additional cooling system was required.

Modelling leaf photosynthetic and transpiration temperature-dependent responses in Vitis vinifera cv. Semillon grapevines growing in hot, irrigated vineyard conditions.

Grapevines growing in Australia are often exposed to very high temperatures and the question of how the gas exchange processes adjust to these conditions is not well understood. The aim was to develop a model of photosynthesis and transpiration in relation to temperature to quantify the impact of the growing conditions on vine performance. Leaf gas exchange was measured along the grapevine shoots in accordance with their growth and development over several growing seasons. Using a general linear statistical modelling approach, photosynthesis and transpiration were modelled against leaf temperature separated into bands and the model parameters and coefficients applied to independent datasets to validate the model. Photosynthesis, transpiration and stomatal conductance varied along the shoot, with early emerging leaves having the highest rates, but these declined as later emerging leaves increased their gas exchange capacities in accordance with development. The general linear modelling approach applied to these data revealed that photosynthesis at each temperature was additively dependent on stomatal conductance, internal CO(2) concentration and photon flux density. The temperature-dependent coefficients for these parameters applied to other datasets gave a predicted rate of photosynthesis that was linearly related to the measured rates, with a 1 : 1 slope. Temperature-dependent transpiration was multiplicatively related to stomatal conductance and the leaf to air vapour pressure deficit and applying the coefficients also showed a highly linear relationship, with a 1 : 1 slope between measured and modelled rates, when applied to independent datasets. The models developed for the grapevines were relatively simple but accounted for much of the seasonal variation in photosynthesis and transpiration. The goodness of fit in each case demonstrated that explicitly selecting leaf temperature as a model parameter, rather than including temperature intrinsically as is usually done in more complex models, was warranted.

Stomatal response of an anisohydric grapevine cultivar to evaporative demand, available soil moisture and abscisic acid

Stomatal responsiveness to evaporative demand (air vapour pressure deficit (VPD)) ranges widely between species and cultivars, and mechanisms for stomatal control in response to VPD remain obscure. The interaction of irrigation and soil moisture with VPD on stomatal conductance is particularly difficult to predict, but nevertheless is critical to instantaneous transpiration and vulnerability to desiccation. Stomatal sensitivity to VPD and soil moisture was investigated in Semillon, an anisohydric Vitis vinifera L. variety whose leaf water potential (Ψ(l)) is frequently lower than that of other grapevine varieties grown under similar conditions in the warm grape-growing regions of Australia. A survey of Semillon vines across seven vineyards revealed that, regardless of irrigation treatment, midday Ψ(l) was dependent on not only soil moisture but VPD at the time of measurement. Predawn Ψ(l) was more closely correlated to not only soil moisture in dry vineyards but to night-time VPD in drip-irrigated vineyards, with incomplete rehydration during high night-time VPD. Daytime stomatal conductance was low only under severe plant water deficits, induced by extremes in dry soil. Stomatal response to VPD was inconsistent across irrigation regime; however, in an unirrigated vineyard, stomatal sensitivity to VPD-the magnitude of stomatal response to VPD-was heightened under dry soils. It was also found that stomatal sensitivity was proportional to the magnitude of stomatal conductance at a reference VPD of 1kPa. Exogenous abscisic acid (ABA) applied to roots of Semillon vines growing in a hydroponic system induced stomatal closure and, in field vines, petiole xylem sap ABA concentrations rose throughout the morning and were higher in vines with low Ψ(l). These data indicate that despite high stomatal conductance of this anisohydric variety when grown in medium to high soil moisture, increased concentrations of ABA as a result of very limited soil moisture may augment stomatal responsiveness to low VPD.

Response of ‘Hass’ avocado trees to irrigation management and root constraint

Abstract The performance of ‘Hass’ avocado trees grown in lysimeters under different irrigation regimes obtained by manipulating drip irrigation frequency and root volume was examined. The experimental design comprised six treatments (3 × 2) with three irrigation frequencies and two container volumes (100- and 200-L). The three irrigation frequencies were: pulsed irrigation (10–20 min every 30 min) throughout the day (Irg1), one daily irrigation event beginning at night and terminated in the morning every day (Irg2) and one irrigation event every two days (Irg3). Irrigation management induced significant differences in water availability in the root zone and subsequently, the diurnal and periodic water uptake. Water uptake of trees in the Irg1 treatment closely followed changes in the meteorological conditions (air temperature, pan evaporation and vapour pressure deficit) while that of trees in the Irg2 and Irg3 treatments was depressed by intermediate and severe water stress, respectively. The experimental treatments had little effect on the vegetative growth, flowering or fruit-set processes. However there were significant treatment differences on fruitlet abscission and accordingly, on fruit yield. Black spots initiated from the seed became apparent on some of the fruits at the beginning of June, and about two weeks later an intensive abscission of fruitlet begun that ended at the beginning of July. The abscission was more intense in the 100-L than the 200-L containers, in the following order: Irg3 > Irg2 ≫ Irg1 regardless of the container volume. Net CO2 assimilation during periods of fruit growth decreased in trees exposed to moderate or severe water stress (Irg2 and Irg3, respectively) and therefore, it is plausible that fruitlet abscission resulted from carbohydrate stress. Improvement of water and nutrient availability, especially in periods where the activity of the root system was weak as a result of low carbohydrate supply presumably played a dominant role in seeds or fruit function.

POTENCIAL HÍDRICO XILEMÁTICO EN CUATRO ESPECIES ARBUSTIVAS NATIVAS DEL NORESTE DE MÉXICO

Since water stress is the most limiting factor in this region, the aim of the current study was to establish how the xylem water potential (Ψ) in native shrubs such as Celtis pallida (Ulmaceae), Acacia amentacea (Leguminosae) Forestiera angustifolia (Oleaceae), and Parkinsonia texana (Leguminosae) is influenced by soil water content, air temperature, relative humidity, and rainfall. The study was carried out in an area located the municipality of China, state of Nuevo Leon. Using a Scholander pressure bomb the Ψ were estimated at 15 days intervals between January 15 and September 28, 2009 in five different plants per species at 06:00 h (predawn) and 14:00 h (midday). At the wettest period, Ψ oscillated from -0.30 (C. pallida) to -0.90 MPa (P. texana) at predawn, in contrast, at the driest period, Ψ oscillated from -2.18 (P. texana) to -3.94 MPa (F. angustifolia) at predawn. At midday, P. texana and F. angustifolia achieved the highest (-1.14 MPa) and lowest (-3.38 MPa) Ψ values, respectively. Average soil water content at depth profile 0-50cm showed between 82 and 33 % of the variation of Ψ at predawn. Soil water content at different soil depths, and relative humidity were significantly and positively correlated with Ψ at predawn; whereas air temperature and vapor pressure deficit was negatively correlated. A. amentacea and P. texana maintainedhigh values in Ψ at predawn and midday under water stress conditions, so these species may be considered as drought tolerant species.

Biophysical characteristics of maize pollen: Variability during emission and consequences on cross-pollination risks

Modelling of cross-pollination risks using physical-based model of pollen dispersal requires estimations of pollen viability rate as well as settling velocity distribution of viable pollen grains at emission. These two biophysical characteristics are mainly determined by pollen water content. Three years of experiments were carried out to measure biophysical characteristics of pollen at different times of emission within the day. In a first part, we proposed a simple model to predict pollen water content at emission as a function of air vapour pressure deficit. We found that by setting a constant duration of pollen dehydration in the anthers the model predicts accurately pollen water content variations at emission. In a second part, we related pollen water content and settling velocity distribution of viable and non-viable pollen grains. We used with success settling velocity distribution as a proxy to estimate fraction of spheroid pollen, water content distribution of spheroid pollen and pollen viability rate. In addition, we validated with success the estimations of viability rate. In a third part, based on above models, we estimated the effects of air humidity on cross-pollination risks.

Modelling light use efficiency using vegetation index and land surface temperature from MODIS in Harvard Forest

Light use efficiency (LUE) is of great importance for carbon cycle and climate change research. This study presents a new LUE model incorporation of vegetation indices (VIs) and land surface temperature (LST) derived from the Moderate-Resolution Imaging Spectroradiometer (MODIS) in Harvard Forest. Three indices, including the normalized difference vegetation index (NDVI), the two-band enhanced vegetation index (EVI2) and the soil-adjusted vegetation index (SAVI), were selected as indicators of forest canopy greenness. A single VI provided moderate estimates of LUE with coefficients of determination (R 2) 0.6219, 0.7094 and 0.7502 for NDVI, EVI2 and SAVI, respectively. Our results demonstrated that canopy LUE was related both to the canopy photosynthesis efficiency and air temperature (R 2 = 0.5634). Therefore, the MODIS LST product was incorporated as a surrogate for monitoring of environmental stresses as the observed relationship between LST and both air temperature (R 2 = 0.8828) and vapour pressure deficit (VPD) (R 2 = 0.6887). The new model in terms of (VI) × (Scaled (LST)) provided improved estimates of LUE estimation with R 2 of 0.7349, 0.7561 and 0.7879 for NDVI, EVI2 and SAVI, respectively. The results will be useful for the development of future LUE models based entirely on remote-sensing observations.

Ecosystem services and urban heat riskscape moderation: water, green spaces, and social inequality in Phoenix, USA

Urban ecosystems are subjected to high temperatures--extreme heat events, chronically hot weather, or both-through interactions between local and global climate processes. Urban vegetation may provide a cooling ecosystem service, although many knowledge gaps exist in the biophysical and social dynamics of using this service to reduce climate extremes. To better understand patterns of urban vegetated cooling, the potential water requirements to supply these services, and differential access to these services between residential neighborhoods, we evaluated three decades (1970-2000) of land surface characteristics and residential segregation by income in the Phoenix, Arizona, USA metropolitan region. We developed an ecosystem service trade-offs approach to assess the urban heat riskscape, defined as the spatial variation in risk exposure and potential human vulnerability to extreme heat. In this region, vegetation provided nearly a 25 degrees C surface cooling compared to bare soil on low-humidity summer days; the magnitude of this service was strongly coupled to air temperature and vapor pressure deficits. To estimate the water loss associated with land-surface cooling, we applied a surface energy balance model. Our initial estimates suggest 2.7 mm/d of water may be used in supplying cooling ecosystem services in the Phoenix region on a summer day. The availability and corresponding resource use requirements of these ecosystem services had a strongly positive relationship with neighborhood income in the year 2000. However, economic stratification in access to services is a recent development: no vegetation-income relationship was observed in 1970, and a clear trend of increasing correlation was evident through 2000. To alleviate neighborhood inequality in risks from extreme heat through increased vegetation and evaporative cooling, large increases in regional water use would be required. Together, these results suggest the need for a systems evaluation of the benefits, costs, spatial structure, and temporal trajectory for the use of ecosystem services to moderate climate extremes. Increasing vegetation is one strategy for moderating regional climate changes in urban areas and simultaneously providing multiple ecosystem services. However, vegetation has economic, water, and social equity implications that vary dramatically across neighborhoods and need to be managed through informed environmental policies.

Seasonal photosynthetic gas exchange and water-use efficiency in a constitutive CAM plant, the giant saguaro cactus (Carnegiea gigantea)

Crassulacean acid metabolism (CAM) and the capacity to store large quantities of water are thought to confer high water use efficiency (WUE) and survival of succulent plants in warm desert environments. Yet the highly variable precipitation, temperature and humidity conditions in these environments likely have unique impacts on underlying processes regulating photosynthetic gas exchange and WUE, limiting our ability to predict growth and survival responses of desert CAM plants to climate change. We monitored net CO(2) assimilation (A(net)), stomatal conductance (g(s)), and transpiration (E) rates periodically over 2 years in a natural population of the giant columnar cactus Carnegiea gigantea (saguaro) near Tucson, Arizona USA to investigate environmental and physiological controls over carbon gain and water loss in this ecologically important plant. We hypothesized that seasonal changes in daily integrated water use efficiency (WUE(day)) in this constitutive CAM species would be driven largely by stomatal regulation of nighttime transpiration and CO(2) uptake responding to shifts in nighttime air temperature and humidity. The lowest WUE(day) occurred during time periods with extreme high and low air vapor pressure deficit (D(a)). The diurnal with the highest D(a) had low WUE(day) due to minimal net carbon gain across the 24 h period. Low WUE(day) was also observed under conditions of low D(a); however, it was due to significant transpiration losses. Gas exchange measurements on potted saguaro plants exposed to experimental changes in D(a) confirmed the relationship between D(a) and g(s). Our results suggest that climatic changes involving shifts in air temperature and humidity will have large impacts on the water and carbon economy of the giant saguaro and potentially other succulent CAM plants of warm desert environments.

Spatiotemporal analysis of particulate matter, sulfur dioxide and carbon monoxide concentrations over the city of Rio de Janeiro, Brazil

Time series of pollutants and weather variables measured at four sites in the city of Rio de Janeiro, Brazil, between 2002 and 2004, were used to characterize temporal and spatial relationships of air pollution. Concentrations of particulate matter (PM10), sulfur dioxide (SO2) and carbon monoxide (CO) were compared to national and international standards. The annual median concentration of PM10 was higher than the standard set by the World Health Organization (WHO) on all sites and the 24 h means exceeded the standards on several occasions on two sites. SO2 and CO did not exceed the limits, but the daily maximum of CO in one of the stations was 27% higher on weekends compared to weekdays, due to increased activity in a nearby Convention Center. Air temperature and vapor pressure deficit have both presented the highest correlations with pollutant’s concentrations. The concentrations of SO2 and CO were not correlated between sites, suggesting that local sources are more important to those pollutants compared to PM10. The time series of pollutants and air temperature were decomposed in time and frequency by wavelet analysis. The results revealed that the common variability of air temperature and PM10 is dominated by temporal scales of 1–8 days, time scales that are associated with the passage of weather events, such as cold fronts.

Drought-associated changes in climate and their relevance for ecosystem experiments and models

Abstract. Drought periods can have important impacts on plant productivity and ecosystem functioning, but climatic conditions other than the lack of precipitation during droughts have never been quantified and have therefore not been considered explicitly in both experimental and modeling studies. Here, we identify which climatic characteristics deviate from normal during droughts and how these deviations could affect plant responses. Analysis of 609 years of daily data from nine Western European meteorological stations reveals that droughts in the studied region are consistently associated with more sunshine (+45 %), increased mean (+1.6 °C) and maximum (+2.8 °C) air temperatures and vapour pressure deficits that were 51 % higher than under normal conditions. These deviations from normal increase significantly as droughts progress. Using the process-model ORCHIDEE, we simulated droughts consistent with the results of the dataset analysis and compared water and carbon exchange of three different vegetation types during such natural droughts and droughts in which only the precipitation was affected. The comparison revealed contrasting responses: carbon loss was higher under natural drought in grasslands, while increased carbon uptake was found especially in decidious forests. This difference was attributed to better access to water reserves in forest ecosystems which prevented drought stress. This demonstrates that the warmer and sunnier conditions naturally associated with droughts can either improve growth or aggravate drought-related stress, depending on water reserves. As the impacts of including or excluding climatic parameters that correlate with drought are substantial, we propose that both experimental and modeling efforts should take into account other environmental factors than merely precipitation.

Xylem water potentials of native shrubs from northeastern Mexico

Xylem water potentials (Ψ) were evaluated in browse plants such as Forestiera angustifolia (Oleaceae), Celtis pallida (Ulmaceae), Zanthoxylum fagara (Rutaceae), and Eysenhardtia texana (Fabaceae). Collections were carried out at Linares County, a semi-arid region of northeastern Mexico. Ψ (MPa) were estimated at 10-day intervals between July 10 and September 30, 2008 by using a Scholander pressure bomb, and were monitored in five different plants per species at 06:00 h (predawn) and 14:00 h (midday). Air temperature, relative humidity, precipitation, and soil water content were registered throughout. Data were subjected to one-way ANOVA and linear correlation analysis. At the wettest period, Ψ at predawn varied from − 0.29 (C. pallida and E. texana) to − 0.37 (F. angustifolia and Z. fagara); in contrast, at the driest period, predawn Ψ ranged from − 3.28 (F. angustifolia) to − 4.50 (Z. fagara). At midday E. texana achieved the highest (−1.14) and lowest (−4.20) values at wettest and driest sampling dates, respectively. It seems that air temperature and vapor pressure deficit negatively influenced Ψ values in all species; conversely, Ψ values augmented as relative humidity increased. Since F. angustifolia and C. pallida maintained higher predawn and midday Ψ values under water stress, these species may be considered as drought-adapted species while, Z. fagara and E. texana that acquired lower values, may be less adapted to drought and in physiological disadvantage under limited water conditions.

The crop water stress index (CWSI) for drip irrigated cotton in a semi-arid region of Turkey

This study was carried out to determine the crop water stress index (CWSI) for drip irrigated cotton grown on a heavy clay texture soil ( Palexerollic Chromoxerert ) under semi-arid climatic condition of East Mediterranean region for three years (2005 to 2007) in Adana, Turkey. Four irrigation treatments designated as full (I 100 ) with no water stress and slight (DI 70 ), moderate (DI 50 ) and strong water stress (continuous stress, dry land) (DI 00 ) were tested. The treatments of DI 70 and DI 50 received water amount of 70 and 50% of the control treatment and the DI 00 was not irrigated except for germination water given at the beginning of the growing season. Irrigation was initiated when leaf water potential (LWP) reached to -15 bar for full (I 100 ), -17 bar for DI 70 and -20 bar for DI50 irrigation treatments. After first irrigation, all the treatments were irrigated at one week interval. The deficit irrigation affected, the irrigation water use, seed cotton yield, dry matter and some yield components such as plant height and number of boll per plant of cotton. Average values of water use, seed cotton yield, dry matter and water use efficiency of full irrigated cotton were 578 mm, 3.28 tha -1 , 13.44 tha -1 and 0.59 kgm -3 , respectively. CWSI values were calculated from the measurements of canopy temperatures by infrared thermometer (IRT), ambient air temperatures and vapor pressure deficit values for all the irrigated treatments. A non-water stressed baseline (lower baseline) equation for cotton was developed using canopy temperature measured from full irrigated plots as, T c −T a = −1.7543VPD +1.56; R2=0.5327 and the non-transpiring baseline (upper baseline) equation was built using canopy temperature data taken from continuous stress plots as, T c −T a = −0.0217VPD + 3.2191. The trends in CWSI values were consistent with the varying soil water content due to the deficit irrigation programs. The relationships between mean CWSI and plant parameters considered in this study were linear except for irrigation water amount. Both dry matter and seed cotton yield decreased with increased soil water deficit. Seed cotton yield (SY) and seasonal mean CWSI values relationship were obtained as, SY = −2.3552CWSI + 3.5657 ; R 2 =0.499. This relationship can be used to predict the seed cotton yield. The results suggest that the cotton crop for this particular climate and soil conditions, should be irrigated when CWSI approaches 0.36. The CWSI approach, according to results of this study, can be accepted as a useful tool to schedule irrigations for cotton. Key words : Crop water stress index (CWSI), evapotranspiration, cotton, drip irrigation.

Climate, canopy conductance and leaf area development controls on evapotranspiration in a boreal coniferous forest over a 10-year period: A united model assessment

The aim of this work was to test a process-based model (hydrological model combined with forest growth model) on the simulation of seasonal variability of evapotranspiration (ET) in an even-aged boreal Scots pine ( Pinus sylvestris L.) stand over a 10 year period (1999–2008). The water flux components (including canopy transpiration ( E t) and evaporation from canopy ( E c) and ground surface ( E g) were estimated in order to output the long-term stand water budget considering the interaction between climate variations and stand development. For validation, half-hourly data on eddy water vapor fluxes were measured during the 10 growing seasons (May–September). The model predicted well the seasonal course of ET compared to the measured values, but slightly underestimated the water fluxes both in non-drought and drought (2000, 2003 and 2006) years. The prediction accuracy was, on average, higher in drought years. The simulated ET over the 10 years explained, on average, 58% of the daily variations and 84% of the monthly amount of ET. Water amount from E t contributed most to the ET, with the fractions of E t, E c and E g being, on average, 67, 11 and 23% over the 10-year period, respectively. Regardless of weather conditions, the daily ET was strongly dependent on air temperature ( T a) and vapor pressure deficit ( D a), but less dependent on soil moisture ( W s). On cloudy and rainy days, there was a non-linear relationship between the ET and solar radiation ( R o). During drought years, the model predicted lower daily canopy stomatal conductance ( g cs) compared with non-drought years, leading to a lower level of E t. The modeled daily g cs responded well to D a and W s. In the model simulation, the annual LAI increased by 35% between 1999 and 2008. The ratio of E c: ET correlated strongly with LAI. Furthermore, LAI reduced the proportion of E g as a result of the increased share of E c and E t and radiation interception. Although the increase of LAI affected positively E t, the contribution of E t in ET was not significantly correlated with LAI. To conclude, although the model predicted reasonably well the seasonal course of ET, the calculation time steps of different processes in the model should be homogenized in the future to increase the prediction accuracy.

Generalizing plant-water relations to landscapes

Changing climate and land use patterns make it increasingly important that the hydrology of catchments and ecosystems can be reliably characterized. The aim of this paper is to identify the biophysical factors that determine the rates of water vapor loss from different types of vegetation, and to seek, from an array of currently available satellite-borne sensors, those that might be used to initialize and drive landscape-level hydrologic models. Spatial variation in the mean heights, crowd widths, and leaf area indices (LAI) of plant communities are important structural variables that affect the hydrology of landscapes. Canopy stomatal conductance (G) imposes physiological limitation on transpiration by vegetation. The maximum value of G (Gmax) is closely linked to canopy photosynthetic capacity, which can be estimated via remote sensing of foliar chlorophyll or nitrogen contents. G can be modeled as a nonlinear multipliable function of: (i) leaf–air vapor pressure deficit, (ii) water potential gradient between soil and leaves, (iii) photosynthetically active radiation absorbed by the canopy, (iv) plant nutrition, (v) temperature and (vi) the CO2 concentration of the air. Periodic surveys with Light Detection and Ranging (LiDAR) and interferometric RADAR, along with high-resolution spectral coverage in the visible, near-infrared, and thermal infrared bands, provide, along with meteorological data gathered from weather satellites, the kind of information required to model seasonal and interannual variation in transpiration and evaporation from landscapes with diverse and dynamic vegetation.