Limited Soil Water Research Articles

Irrigation water saving strategies in Citrus orchards: Analysis of the combined effects of timing and severity of soil water deficit

The knowledge of the crop response to soil water deficit is essential to predict the actual crop water requirements under limited soil water conditions. The mathematical schematization of the crop response under RDI can allow identifying the exact irrigation timing. The threshold of soil water status below which crop transpiration decreases represents a key parameter for the water stress functions. The main objective of this paper was to investigate the effects of several RDI treatments, applied during the three stages of fruit growth, on soil-plant-water relations of drip-irrigated mandarin trees. Experiments were carried out in seven irrigation treatments: a control, irrigated at 125% of potential evapotranspiration measured in weighting lysimeters, ETlys, during the whole year, and six RDI treatments in which 25% and 50% of ETlys were applied during each of the three stages of fruit growth.The critical threshold of soil water status expressed in terms of soil water depletion below which predawn (PLWP) and midday (MSWP) leaf water potential is affected by decreasing soil water content, were identified. The dependency of crop water status from the climate forcing, occurring under soil water contents higher than the critical threshold was also demonstrated. Moreover, a strong relationship between water stress integral (SMSWP) evaluated during the three stages of fruit growth, and the corresponding amount of water applied (irrigation and precipitation) was observed after normalizing the variables. The robust relationship between these two variables (R2 = 0.83) confirmed that the water stress integral represents a good indicator of the plant-water relationship and allows the prediction of the total irrigation depth to be applied to achieve a desired stress level during the fruit growth. Moreover, this relationship depends on the irrigation schedule adopted to regulate the deficit, which includes the irrigation variables, such as irrigation dose and its variability.

Leaf gas exchange and bean quality fluctuations over the whole canopy vertical profile of Arabic coffee cultivated under elevated CO2

Leaves in different positions respond differently to dynamic fluctuations in light availability, temperature and to multiple environmental stresses. The current hypothesis states that elevated atmospheric CO2 (e[CO2]) can compensate for the negative effects of water scarcity regarding leaf gas exchanges and coffee bean quality traits over the canopy vertical profile, in interactions with light and temperature microclimate during the two final stages of berry development. Responses of Coffea arabica L. were observed in the 5th year of a free air CO2 enrichment experiment (FACE) under water-limited rainfed conditions. The light dependent leaf photosynthesis curves (A/PAR) were modelled for leaves sampled from vertical profile divided into four 50-cm thick layers. e[CO2] significantly increased gross photosynthesis (AmaxGross), the apparent quantum yield efficiency, light compensation point, light saturation point (LSP) and dark respiration rate (Rd). As a specific stage response, considering berry ripening, all parameters calculated from A/PAR were insensitive to leaf position over the vertical profile. Lack of a progressive increase in AmaxGross and LSP was observed over the whole canopy profile in both stages, especially in the two lowest layers, indicating leaf plasticity to light. Negative correlation of Rd to leaf temperature (TL) was observed under e[CO2] in both stages. Under e[CO2], stomatal conductance was also negatively correlated with TL, reducing leaf transpiration and Rd even with increasing TL. This indicated coffee leaf acclimation to elevated temperatures under e[CO2] and water restriction. The e[CO2] attenuation occurred under water restriction, especially in A and water use efficiency, in both stages, with the exception of the lowest two layers. Under e[CO2], coffee produced berries in moderate- and high light level layers, with homogeneous distribution among them, contrasted to the heterogeneous distribution under actual CO2. e[CO2] led to increased caffeine content in the highest layer, with reduction of chlorogenic acid and lipids under moderate light and to raised levels of sugar in the shaded low layer. The ability of coffee to respond to e[CO2] under limited soil water was expressed through the integrated individual leaf capacities to use the available light and water, resulting in final plant investments in new reproductive structures in moderate and high light level layers.

Root water uptake under heterogeneous soil moisture conditions: an experimental study for unraveling compensatory root water uptake and hydraulic redistribution

Plant roots often encounter heterogeneity in soil water content and respond by compensating water uptake from wet zones to cope with the transpiration demand. Simultaneously, plants may also exhibit root-mediated hydraulic redistribution from wet to dry zones. Experiments were conducted to simultaneously monitor compensated root water uptake and hydraulic redistribution in the vadose zone. Vertical and horizontal split-root lysimeters were used to hydraulically isolate maize roots under altering soil water conditions. Compensated root water uptake and root-mediated hydraulic redistribution were monitored by continuous measurement of water content in the lysimeter compartments. Soil water heterogeneity and limited soil water availability were found to accelerate the root water uptake from moist region to compensate the reduced water availability in dry zone. However, no measurable root mediated hydraulic redistribution was observed in short term basis, despite high water potential gradient in the root zone of both lysimeters. The night-time transpiration and xylem refilling processes seem to override the hydraulic redistribution on a diurnal basis in our experiment. Our study shows that compensated root water uptakeplays a major role in meeting the transpiration demand. In contrast, the role of root mediated hydraulic redistribution is found to benegligible in maize plant.

A single tree model to consistently simulate cooling, shading, and pollution uptake of urban trees

Extremely high temperatures, which negatively affect the human health and plant performances, are becoming more frequent in cities. Urban green infrastructure, particularly trees, can mitigate this issue through cooling due to transpiration, and shading. Temperature regulation by trees depends on feedbacks among the climate, water supply, and plant physiology. However, in contrast to forest or general ecosystem models, most current urban tree models still lack basic processes, such as the consideration of soil water limitation, or have not been evaluated sufficiently. In this study, we present a new model that couples the soil water balance with energy calculations to assess the physiological responses and microclimate effects of a common urban street-tree species (Tilia cordata Mill.) on temperature regulation. We contrast two urban sites in Munich, Germany, with different degree of surface sealing at which microclimate and transpiration had been measured. Simulations indicate that differences in wind speed and soil water supply can be made responsible for the differences in transpiration. Nevertheless, the calculation of the overall energy balance showed that the shading effect, which depends on the leaf area index and canopy cover, contributes the most to the temperature reduction at midday. Finally, we demonstrate that the consideration of soil water availability for stomatal conductance has realistic impacts on the calculation of gaseous pollutant uptake (e.g., ozone). In conclusion, the presented model has demonstrated its ability to quantify two major ecosystem services (temperature mitigation and air pollution removal) consistently in dependence on meteorological and site conditions.

Superabsorbent polymer and rabbit manure improve soil moisture, growth and yield of eggplant (Solanum melongena L.)

Production of eggplant (Solanum melongena L.) is influenced by limited soil water and fertility conditions that affect its growth and yield. The use of superabsorbent polymer (SAP), also known as slush powder and rabbit manure are among the strategies that can improve soil conditions, hence growth and yield of crops. The objective of the study was to evaluate the effects of SAP and rabbit manure on soil moisture, growth and yield of eggplant. The study was conducted at Pwani University, Kilifi, Kenya. A randomized complete block design with three replications was used. The treatments were SAP, rabbit manure, SAP plus rabbit manure and control (without SAP or rabbit manure). Soil moisture, growth and yield parameters were determined. Superabsorbent polymer and/or rabbit manure improved soil moisture, growth and yield of eggplant compared with the control. Use of SAP had a better soil moisture retention, growth and yield comparable to SAP combined with rabbit manure. The finding demonstrates that use of SAP and/or rabbit manure may help in better soil water and nutrient management particularly in arid and semi-arid areas to improve growth and yield of eggplant.

Effects of Drought and Heat on Photosynthetic Performance, Water Use and Yield of Two Selected Fiber Hemp Cultivars at a Poor-Soil Site in Brandenburg (Germany)

Hemp currently regains certain importance as fiber, oil and medical crop not least because of its modest requirements of biocides, fertilizer and water. During recent years, crops were exposed to a combination of drought and heat, even in northern Central-Europe. Dynamic responses of photosynthesis and stomatal conductance to these stresses and their persistent effects had been studied, if at all, in controlled environment experiments. Comprehensive field studies on diurnal and long-term net photosynthesis and gas exchange, and yield properties of hemp during a drought prone, high-temperature season in northern Central-Europe are obviously missing. Thus, in whole season field trails, the essential actual physiological (rates of net photosynthesis and transpiration, stomatal conductance, water use efficiencies, ambient and internal CO2 concentrations) and the yield performance of modern high-yielding multi-purpose hemp cultivars, ‘Ivory’ and ‘Santhica 27’, were evaluated under extreme environmental conditions and highly limited soil water supply. This provides comprehensive information on the usability of these cultivars under potential future harsh production conditions. Plants of both cultivars differentially cope with the prevailing climatic and soil water conditions. While ‘Ivory’ plants developed high rates of CO2 gain and established large leaf area per plant in the mid-season, those of ‘Santhica 27’ utilized lower CO2 uptake rates at lower leaf area per plant most time. This and the higher germination success of ‘Santhica 27’ resulted in nearly twice the yield compared to ‘Ivory’. Although stomatal control of CO2 gain was pronounced in both cultivars, higher stomatal limitations in ‘Ivory’ plants resulted in higher overall intrinsic water use efficiency. Cultivation of both hemp cultivars with only basic irrigation during seed germination was successful and without large effects on yield and quality. This was valid even under extremely hot and dry climatic conditions in northern Central Europe.

Assessing HC27 Soil Database for Modeling Plant Production

Soil information is a vital input for crop models applications in various large area studies including climate change impact and food security. One of the global soil databases that provide full information for crop models is HC27 of IFPRI. The quality of the database has not been assessed for crop modeling so far. A tested crop simulation model (SSM-iCrop2) was used for this purpose that needs soil water related properties (i.e., depth, albedo, curve number for runoff, drainage coefficient, and soil water limits at saturation, drained upper limit and lower limit) for the simulation of crop properties. Actual data of two soil profiles from three different climate zones (locations) were used as model inputs to simulate potential yield, evapotranspiration (under rainfed conditions) or net irrigation water requirement (under irrigated conditions) of some important plant species (alfalfa, sugar beet, sugar cane, wheat, olive, soybean, apricot and chickpea) under rainfed and irrigated conditions of Iran. Results showed that the application of HC27 soil information in the SSM-iCrop2 model resulted in model output that was not different from the model output with actual soil information with respect to mean, variance, and distribution. No statistically significant difference was found in the simulation of various combinations of soil profiles-plant species-locations. It was concluded that HC27 information can be used in simulation studies with SSM-iCrop2 or other similar simple models for the simulation of potential yield, net irrigation water, or evapotranspiration that are commonly required for food security and climate change studies.

Comparison of evapotranspiration methods in the DSSAT Cropping System Model: I. Global sensitivity analysis

Global sensitivity analysis (GSA) is useful for evaluating the responsiveness of agroecosystem models to input parameter adjustments. Evaluations of model sensitivity for diverse water status conditions and evapotranspiration (ET) algorithms will facilitate better use of models to provide water management recommendations. The objective of this study was to conduct a GSA to identify influential parameters in the Decision Support System for Agrotechnology Transfer (DSSAT) Cropping System Model (CSM), specifically using the CROPGRO-Cotton module with data from cotton field studies conducted in 2000, 2001, and 2008 at Bushland, Texas. The field studies tested fully-irrigated, deficit-irrigated, and dryland cotton production in a semi-arid environment. Using high performance computing resources, a GSA was conducted to evaluate the sensitivity of 24 model outputs with respect to 37 model input parameters. The GSA was conducted for six different ET methodologies available in the model. With first-order sensitivity indices <0.05, nearly half of the tested input parameters did not influence any model output for any of the tested simulation scenarios. Among the parameters with first-order sensitivity indices >0.05, eleven were cultivar parameters that controlled crop development and growth, and five were soil parameters that specified initial soil water conditions, soil water limits, drainage rate, and root growth characteristics. The influences of another soil parameter and one ET parameter were relevant only for the ET methods that required them. Large differences in sensitivity indices were found based on the choice between two soil water evaporation methods. In addition to providing insights for other applications of this model, the results specifically informed further efforts to evaluate the model using measured data from the Bushland cotton studies to compare performance among the six ET methods, as reported in a companion paper.

Mesophyll CO2 conductance and leakiness are not responsive to short‐ and long‐term soil water limitations in the C4 plant Sorghum bicolor

Breeding economically important C4 crops for enhanced whole-plant water-use efficiency (WUEplant ) is needed for sustainable agriculture. WUEplant is a complex trait and an efficient phenotyping method that reports on components of WUEplant , such as intrinsic water-use efficiency (WUEi , the rate of leaf CO2 assimilation relative to water loss via stomatal conductance), is needed. In C4 plants, theoretical models suggest that leaf carbon isotope composition (δ13 C), when the efficiency of the CO2 -concentrating mechanism (leakiness, ϕ) remains constant, can be used to screen for WUEi . The limited information about how ϕ responds to water limitations confines the application of δ13 C for WUEi screening of C4 crops. The current research aimed to test the response of ϕ to short- or long-term moderate water limitations, and the relationship of δ13 C with WUEi and WUEplant , by addressing potential mesophyll CO2 conductance (gm ) and biochemical limitations in the C4 plant Sorghum bicolor. We demonstrate that gm and ϕ are not responsive to short- or long-term water limitations. Additionally, δ13 C was not correlated with gas-exchange estimates of WUEi under short- and long-term water limitations, but showed a significant negative relationship with WUEplant . The observed association between the δ13 C and WUEplant suggests an intrinsic link of δ13 C with WUEi in this C4 plant, and can potentially be used as a screening tool for WUEplant in sorghum.

Leaf senescence did not account for variations in grain and oil yield observed in sunflower under radiation limiting conditions

The objective of this work was to study the effect of the decrease in incident solar radiation on leaf senescence, its consequence on the fraction of radiation intercepted by the leaves and on the grain and oil yield in sunflower. Five field experiments were conducted in Balcarce, with three hybrids, maintaining non limiting soil water conditions and nutritional status, and controlling weeds and insects. Treatments of different shading levels (50 or 80% of radiation interception) were applied during different periods of crop development (around flowering, during a window within the grain filling period or during the complete grain filling period). Shading levels were more intense than those expected by cloudiness in field conditions. The number, weight and oil concentration of grains, leaf area, incident radiation (I0) and the transmitted to incident radiation ratio (the fraction of radiation intercepted by leaves, I/I0) by different leaves strata were measured. Intercepted radiation (iPAR) was calculated as the product between I0 and I/I0. Shading treatments reduced grain and oil yield between 23 and 51%. No differences among periods or levels of shading were observed on grain and oil yield. Both the number and weight of grains decreased by shadings. Oil concentration decreased (between 5 and 21%) only when shading was applied during the window within the grain filling period or during the complete grain filling period. Shading accelerated leaf senescence but it did not affect the ratio I/I0. Intercepted PAR accumulated during the grain filling period was not related to the ratio I/I0. Incident radiation (I0) accounted for differences in grain and oil yield observed among treatments and experiments. It is concluded that the higher leaf senescence by decrease in I0 did not affect grain and oil yield through a decrease in the ratio I/I0.

Assessments of Humic Substances Application and Deficit Irrigation in Triploid Watermelon

Soil organic matter degradation and water limitation caused by intense farming activities are some of the major threats affecting agricultural production. Accordingly, the concepts of sustainable agricultural systems with optimized irrigation and improved soil quality can be adapted to address these issues. During this 2-year field study, two management factors—humic substances (HS) as organic inputs (HS vs. control) and deficit irrigation as the irrigation method (50% vs. 100% based on evapotranspiration)—were evaluated based on triploid watermelon (Citrullus lanatus cv. Fascination) yield and soil property changes. HS application increased watermelon early yield by 38.6% and total yield by 11.8% compared with the control; the early yield mainly increased under deficit irrigation. Compared with full irrigation, deficit irrigation increased water use efficiency (WUE) without significantly affecting total yield. In addition, HS application significantly increased the soil organic carbon (SOC) content, which was found to be positively correlated with crop WUE. These results indicate that soil organic inputs with HS and deficit irrigation are valuable strategies to establish sustainable systems for watermelon production, which will not only increase yield and WUE but also significantly improve soil quality and save irrigation water.

Simulating the effects of water limitation on plant biomass using a 3D functional-structural plant model of shoot and root driven by soil hydraulics.

Improved modelling of carbon assimilation and plant growth to low soil moisture requires evaluation of underlying mechanisms in the soil, roots, and shoots. The feedback between plants and their local environment throughout the whole spectrum soil-root-shoot-environment is crucial to accurately describe and evaluate the impact of environmental changes on plant development. This study presents a 3D functional structural plant model, in which shoot and root growth are driven by radiative transfer, photosynthesis, and soil hydrodynamics through different parameterisation schemes relating soil water deficit and carbon assimilation. The new coupled model is used to evaluate the impact of soil moisture availability on plant productivity for two different groups of flowering plants under different spatial configurations. In order to address different aspects of plant development due to limited soil water availability, a 3D FSP model including root, shoot, and soil was constructed by linking three different well-stablished models of airborne plant, root architecture, and reactive transport in the soil. Different parameterisation schemes were used in order to integrate photosynthetic rate with root water uptake within the coupled model. The behaviour of the model was assessed on how the growth of two different types of plants, i.e. monocot and dicot, is impacted by soil water deficit under different competitive conditions: isolated (no competition), intra, and interspecific competition. The model proved to be capable of simulating carbon assimilation and plant development under different growing settings including isolated monocots and dicots, intra, and interspecific competition. The model predicted that (1) soil water availability has a larger impact on photosynthesis than on carbon allocation; (2) soil water deficit has an impact on root and shoot biomass production by up to 90 % for monocots and 50 % for dicots; and (3) the improved dicot biomass production in interspecific competition was highly related to root depth and plant transpiration. An integrated model of 3D shoot architecture and biomass development with a 3D root system representation, including light limitation and water uptake considering soil hydraulics, was presented. Plant-plant competition and regulation on stomatal conductance to drought were able to be predicted by the model. In the cases evaluated here, water limitation impacted plant growth almost 10 times more than the light environment.

PENGARUH PUPUK ORGANIK PADAT TERHADAP HASIL BEBERAPA VARIETAS KEDELAI ( Glycine max (L)Merr) PADA TANAH VERTISOL

Soybean (Glycine max (L.) Merr.) was one of the food commodities whose the demand has not been fulfilled. Local soybean production is predicted to stagnate in the range of 520,000 tons, while consumption is estimated to reach 3.07 million tons. The causal factor is suspected to be due to limited soil water supplies and unsuitable physical properties of the soil, especially in Vertisol . The purpose of this study was to determine the effect of organic fertilizer doses on several soybean varieties on Vertisol. The study design uses the Split Plot Design. As Main Plot are Varieties (Grobogan, Burangrang, and Baluran Varieties), while as Sub Plots are Doses of Organic Fertilizers (0, 10, 20, and 30 tons / ha). The data obtained were analyzed by Analysis of Varians. Then carried out further tests with the DMRT test (Duncan Multiple Range Test) at a significant level of 5%. The results showed that the administration of organic fertilizer at a dose of 30 tons / ha increased the yield of all soybean varieties tested, while the administration of organic fertilizer below that dose only increased in vegetative growth. Baluran varieties provide the best response to the provision of organic fertilizer, following Burangrang, and Grobogan.

The production and tolerance level of 22 accessions of Sukabumi robusta coffee to water stress

Extensification of the Robusta coffee to enhance national productivity can be expanded to marginal areas with limited soil water. Robusta coffee clones that have a good tolerance for these conditions are needed. This study aimed to evaluate production and tolerance level of 22 Sukabumi Robusta coffee accessions to water stress. The study was conducted in a greenhouse at Pakuwon Experimental Station, altitude of 450 m asl with Latosol type of soil and B type of climate according to Schmidt and Fergusson classification, start from January 2015 until July 2016. Randomized complete block design with 22 treatments of Robusta coffee accessions and two replications was performed. Variables observed were the height of seedlings, number of leaves, and diameter of stems at 15 and 45 days after water stress treatment (DAS), as well as the percentage of viable seedlings at 60 and 75 DAS. The results showed that seven Robusta coffee accessions were classified as high production accessions (ROS 212, 335, 337, 508, 514, 519, and ROS 520), while two accessions were classified as tolerant to water stress (ROS 010 and ROS 507). However, there is no accession that classified as high production and tolerant to water stress.

Vegetables cultivation under protected conditions

In addition, the biotic stresses also do not allow successful production of vegetables like capsicum, cucumber, tomato, chilli, okra/bhindi, cauliflower, cabbage, leafy vegetables etc. in the fields mainly during rainy and winter season. In spite of the great importance of vegetable crops, it faces a lot of restraints like temperature stress, photostress, moisture stress and weeds growth, nutrients deficiency in the soil, excessive wind velocity and atmospheric CO2. Some possible ways to reduce green house gas emissions are suggested and some sustainable adaptation strategies are indicated. We expand our survey of the existing research to the entire production system, including greenhouse structure, energy, electricity, growing media, and fertilizers. Different protected cultivation structures having different temperature, humidity, UV radiation ranges and also having different cost involvement in construction of structure which may results the production of vegetables with distinct advantage of good quality fruits, productivity and favorable market price to the growers/farmers. These limitations can be alleviated by adopting a unique, specialized hi-technology is called as protected cultivation. Intense heat and high incidence of UV rays in dry months reduce the quality of leafy vegetables in open which otherwise very good in protected structures. Protected cultivation has great scope to achieve self-sufficiency in vegetable sector even with less area. The shade net house is ideal for cultivation of vegetable during dry period in the tropical islands when extreme sunshine and heat are limiting factor to vegetable production. These structures are constructed using a UV stabilized agro-shade net (35%-70% shade) materials for creating partial shade condition inside the structure to favour the growth and development of partial shade requiring crops or to protect crop from scorching heat during hot/summer months. The partial shade effect helps in fast and better growth of leafy vegetables besides reducing the evapo-transpiration loss of water. Low cost shade net for high value leafy vegetables like coriander, palak, fenugreek and insect free brinjal crop. Consequently more pesticides are being used for protecting the crops which deteriorate the vegetable quality by the presence of residues. This also creates environmental concern as the limited soil and fresh water bodies are get contaminated. The net house can be used for year round cultivation of crops such as Capsicum, tomato, okra, cauliflower, cabbage, brinjal, beans, parthenocarpic cucumber and other cucurbits with reduced incidence of insect-pest and diseases and consequent low chemical residues on the produces. The cost can be reduced drastically by the use of local wood materials like bamboo/wooden poles. Vegetable growers can substantially increase their income by protected cultivation of vegetables in off-season as the vegetables produced during their normal season generally do not fetch good returns due to large availability of these vegetable in the markets. Off-season cultivation of cucurbits under low plastic tunnels is one of the most profitable technologies under northern plains of India. Protected cultivation enables us to grow vegetables in the off-season and also to extend the vegetable growing seasons for a much longer period than is possible under open field conditions. Though certain breakthroughs in vegetable cultivation in adverse climates using protected devices have been registered, more extensive research and generation of awareness are required to tap potential of protected cultivation of vegetable crops and also increase the area under protected devices.

Biochar made from low density wood has greater plant available water than biochar made from high density wood

Soil water limitations often restrict plant growth in unirrigated agricultural, forestry and urban systems. Biochar amendment to soils can increase water retention, but not all of this additional water is necessarily available to plants. Differences in the effectiveness of biochar in ameliorating soil water limitations may be a result of differences in feedstock cell structure. Previous research has shown that feedstock cell structure influences the pore structure of biochar and consequently the volume available for water storage. The availability of this water for plant uptake will be determined by biochar pore diameters, given its role in determining capillary forces which plants must overcome to access pore water. Therefore, we hypothesized that differences in hardwood feedstock cell structure would result in differences in the plant available water holding capacity of biochar. Before pyrolysis, we measured the wood morphology of 18 Eucalyptus species on three replicates of equal age on a gradient of wood density (572–960 kg m−3). Wood samples were then pyrolysed (550 °C) and the resulting biochars were sieved and their particle size distribution was standardised before their physical properties, including water holding capacity, plant available water and bulk density were measured. Our results show that biochar made from lower density eucalypt wood had up to 35% greater water holding capacity and up to 45% greater plant available water than biochar made from higher density eucalypt wood. Further, feedstock wood density related well to fibre cell wall thickness and fibre lumen diameter. Therefore, wood density could be used as a proxy for wood cell structure, which can in turn be used to predict plant available water in biochar. The simple measure of feedstock wood density can inform feedstock choices for producing biochars with greater plant available water, optimal for the use as soil amendment in water limited environments.

Large regional differences of soil water limitation effect on ozone induced yield loss for wheat and potato in Switzerland

The accumulated stomatal ozone (O3) uptake over a threshold (Phytotoxic Ozone Dose POD6), calculated by an ozone deposition model, has been shown to be the most appropriate metric to quantify negative effects of O3 on food crops. In this study we used data of 13 sites in different regions of Switzerland with multiple years of O3 measurements to quantify the stomatal O3 uptake and the related yield loss of wheat and potato. Flux patterns for different years were calculated with the DO3SE model to disentangle the influence of contrasting seasonal environmental conditions. Regional and inter-annual differences in meteorological conditions led to considerable variations in soil water conditions and the POD6 values for wheat. Potato stomatal uptake was much less influenced by soil water and showed a more even distribution of POD6 values across sites and years. The estimated nationally and temporally average yield loss was 3.2 ± 1.2% for wheat and 2.4 ± 0.8% for potato, calculated based on an area weighting.It was found that soil water deficit, observed frequently in the western part of Switzerland, had a large attenuation effect on stomatal O3 uptake by wheat and on corresponding yield losses. This highlights the importance of including soil moisture limitation in O3 uptake modelling even in moist climatic regions. The comparison of modelled evapotranspiration with water flux measurements over a wheat field showed a reasonable agreement concerning the temporal pattern and the magnitude. But it is also concluded that the DO3SE soil moisture module will need further testing and adaptation to improve accuracy of the model in dryer conditions.

Time-Lag Effect Between Sap Flow and Environmental Factors of Larix principis-rupprechtii Mayr

A time lag between sap flux density (Js) and meteorological factors has been widely reported, but the controlling factors of the time lag are poorly understood. To interpret the time lag phenomenon systematically, thermal dissipation probes were placed into each of eight trees to measure the Js of Larix principis-rupprechtii Mayr. in the Liupan Mountains in Northwest China. Meteorological factors, including vapor pressure deficit (VPD), solar radiation (Rs) and air temperature (Ta), were synchronously measured with Js, and the dislocation contrast method was used to analyze the time lag between Js and the meteorological factors. The analysis indicated the following for the whole experimental period. (1) The time lag between Js and VPD (TLV) and the time lag between Js and Rs (TLR) both exhibited different patterns under different weather conditions, and Js could precede Rs on dry days. (2) Both TLV and TLR varied with the day of the year (DOY) throughout the experimental period; namely, both exhibited a decreasing tendency in September. (3) Reference crop evapotranspiration (ETref) had a greater influence on the time lag than the other meteorological factors and directly controlled the length and direction of TLV and TLR; relative extractable water (REW) modified the relationship between ETref and time lag. (4) The regression analysis results showed differences between the time lags and the environmental factors (ETref and REW) within different ranges of REW. Namely, TLR was better determined by ETref and REW when REW &lt; 0.38, while TLV was better correlated with ETref and REW in the absence of soil water limitations (REW &gt; 0.38). This project provided an important opportunity to advance the understanding of the interaction between plant transpiration and meteorological factors in a changing climate.

Spatio-temporal transpiration patterns reflect vegetation structure in complex upland terrain

Topography exerts control on eco-hydrologic processes via alteration of energy inputs due to slope angle and orientation. Further, water availability varies with drainage position in response to topographic water redistribution and the catena effect on soil depth and thus soil water storage capacity. Our understanding of the spatio-temporal dynamics and drivers of transpiration patterns in complex terrain is still limited by lacking knowledge of how systematic interactions of energy and moisture patterns shape ecosystem state and water fluxes and adaptation of the vegetation to these patterns. To untangle the effects of slope orientation and hillslope position on forest structure and transpiration patterns, we measured forest structure, sap flux, soil moisture, throughfall and incoming shortwave radiation along two downslope transects in a forested head water catchment in south-east Australia. Our plot locations controlled for three systematically varying drainage position levels (topographic wetness index: 5.0, 6.5 and 8.0) and two levels of energy input (aridity index: 1.2 and 1.8). Vegetation patterns were generally stronger related to drainage position than slope orientation, whereas sap velocity variations were less pronounced. However, in combination with stand sapwood area, consistent spatio-temporal transpiration patterns emerged in relation to landscape position, where slope orientation was the primary and drainage position the secondary controlling factor. On short temporal scales, radiation and vapor pressure deficit were most important in regulating transpiration rates, whereas soil water limitation only occurred on shallow soils during summer. The importance of stand structural parameters increased on longer time scales, indicating optimization of vegetation in response to the long-term hydro-climatic conditions at a given landscape position. Thus, vegetation patterns can be conceptualized as a ‘time-integrated’ predictor variable that captures large fractions of other factors contributing to transpiration patterns.

Transpiration dynamics and water sources for selected indigenous trees under varying soil water content

The major route through which water from the earth’s surface re-enters the hydrologic cycle in forested ecosystems is via tree transpiration (T). It is therefore important to have detailed understanding of the quantity and source of water transpired by different tree species. The aims of this study were to i) assess the trends of T for selected tree species (camel thorn, sweet thorn, shepherd’s tree and buffalo thorn) across a range of soil water content conditions and ii) partition the total T of the selected tree species growing in arid environments dominated by open cast mining activities into soil water and groundwater. Tree T was measured using the compensation heat pulse velocity (CHPV) method, while soil water content was monitored using DFM capacitance probes. The soil water content within the upper 50 cm soil profile ranged from 11 mm during the dry season to 20 mm during the wet season. The deeper soil layer (50–120 cm) was generally wetter compared to the top layer with water content was up to >30 mm during the wet season. The measured tree T ranged from 0.2 mm day−1 on buffalo thorn during the dry season to 1.9 mm day−1 on sheperd’s tree in summer. It was also revealed that T of large (diameter at breast height =46 cm) camel thorn trees is not responsive to seasonal variations of soil water availability and remained constant at approximately 1.2 mm day−1. Diurnal patterns of T did not effect changes on the soil water depletions within the top 120 cm soil profile, which indicated that the trees sourced water beyond this zone. Signs of daytime redistribution were observed within the canopy areas of the investigated trees during very limited soil water conditions of the dry season. It was concluded that the water use of trees is inclined to the seasonal variations, which however is not the case in old trees. Close to 100% of the water transpired by trees in the study area is sourced below 1.2 m (vadose zone and water table). We recommended investigation of daytime redistribution among the indigenous tree species of the study area. We also recommended extension of tree water use studies to other species for comprehensive catchment tree water use calculations to inform water budgets.