Total Soil Water Research Articles

Effects of environmental factors on growth and mortality of Parashorea macrophylla (Dipterocarpaceae) planted on slopes and valleys in a degraded tropical secondary forest in Sarawak, Malaysia

Enrichment planting with climax trees such as dipterocarps is required to rehabilitate degraded tropical forests in Southeast Asia, because these forests have been damaged by various anthropogenic activities. Severely degraded tropical secondary forests are the most important targets for enrichment planting in the region, because secondary forest trees potentially provide moderate conditions for planted seedlings by preventing strong sunlight and high temperatures. In an enrichment planting site located in Sarawak, Malaysia, we explored how variation in secondary forest structure (biomass and stand density) and soil (nutrient and water) conditions influenced the mortality and growth rates of planted dipterocarp seedlings (Parashorea macrophylla, Wyatt-Sm. ex P.S. Ashton) over the period from 2000–2007. Experimental plots were set up on different topographical features (slopes and valleys). Using the data collected, we demonstrated the ways in which environmental factors, such as soil nitrogen content, relate to climax tree seedling growth and mortality in degraded tropical secondary forests dominated by the genera Macaranga, Ficus, and Glochidion. Stand density and total aboveground biomass of secondary forest trees were significantly lower in the valley plots than on the slopes, whereas soil total nitrogen and water contents were significantly higher in valley plots. Over seven years, the total biomass of trees increased, whereas the density of secondary forest trees decreased in all plots. Nutrient stocks (nitrogen, phosphorus, potassium, calcium, and magnesium) in the soil (0–5 cm) also decreased over seven years in all plots. Height, diameter, and mortality of planted seedlings were higher in the valley plots than on slopes. Multiple stepwise regression demonstrated significant positive effects of total soil nitrogen and water content on the relative growth rate of seedlings over seven years. There were no significant relationships between seedling mortality and any of the environmental factors measured, including soil conditions. We conclude that (1) seedling growth was enhanced by soil total nitrogen rather than by available phosphorus or potassium, and (2) planted seedlings grew faster and died more often in the valley plots than on the slopes.

Modelling Soil Water Dynamics under Rainfed Agriculture to Mitigate Climate Change

The model performance to simulate soil water dynamics was evaluated by comparing the predicted soil water content values with calculated soil water at different phenological stages of wheat and total soil available water using neutron probe. The pre-sowing soil water (mm) in this study varied from 40 to 50 mm in loam and sandy clay soil of Islamabad and Chakwal, respectively. When soil water is >50 mm, its effect on crop establishment is dependent on amount and temporal distribution of rainfall. Plant available soil water seems to be the most important factor if rainfall occurs between sowing and floral initiation period as happened during 2008-09 in the present study. The dynamics of soil water from emergence to maturity represented here as total soil water that remained maximum at earlier plant stages especially when crop roots were established and utilized soil water effectively that resulted in the lowest level of water at maturity which can be due to the evapotranspiration. The results depicted that the soil water distribution pattern mainly depends on soil properties and if sowing methodologies are resilient with available soil water then crop stand will be good and crop uses water much effectively. The results also depicted that when soil water is at drain upper limit (DUL) and the crop sown at proper time along with least soil evaporation, the soil water can be more easily taken up by the plant roots. Furthermore, the simulated soil water by the model was in close agreement with actual data. The validation skill scores like R2 confirmed the actuality of the model, therefore, dynamic model like Agricultural Production System Simulator (APSIM) could be used to describe the distribution of rainwater into different components like infiltration, runoff and drainage, and it can be used as a decision support tool for accurate management of different cultural operations for sustainable atmosphere-soil-plant (ASP) system.

In situ measuring soil ice content with a combined use of dielectric tube sensor and neutron moisture meter in a common access tube

For monitoring soil water dynamics in cold regions, the major concerns are not only liquid soil water content (LSWC) and total soil water content (TSWC), but soil ice content (SIC) as well. The field determination of SIC is more challenging than that of LSWC or TSWC and no practical method has been found. This study presents a method that combined a neutron moisture meter (NMM) with a dielectric tube sensor (DTS) lowered in a common access PVC-tube, in which NMM was for determining TSWC and DTS was for LSWC. Consequently, the absolute difference was of SIC. The proposed method was tested in an agricultural field over the winter of 2011–2012. Field calibration and a cross-check showed that DTS and NMM measurements in the common access PVC-tube were very similar (R2=0.9503, RMSE=0.0197cm3cm−3) to the volumetric soil water contents (VSWCs) ranging from 0.05 to 0.40cm3cm−3. More importantly, the results of overall experimental period confirmed that both sensors had the same responses in the unfrozen layers but different responses in the frozen layers. Based on this, we transformed the measurements of both sensors into SIC. Furthermore, for characterizing the SIC dynamics during the overall experimental period, we presented the curve-based results and the map-based results, respectively. These results verified that the proposed method is feasible to determine the SIC profiles in situ with a major advantage that allows both probes lowering multiple access PVC-tubes at the field scale.

Assessment of yield and water use efficiency of durum wheat as affected by irrigation practices and soil properties in the irrigated area of oued rmel in Tunisia

This work aims to study the impact of total available soil water in the root zone (TAW) on the relation between irrigation regime and water use efficiency of a durum wheat (Triticum durum Desf. Var. Karim.) cultivar in the pedoclimatic conditions of the irrigated area of Oued Rmel in Tunisia. After calibration, STICS model was used to simulate effects of two irrigation scenarios on water use efficiency of durum wheat over a 20-year period with regard to farmers irrigation practices in the study area. To carry out the study, three types of soil in the Oued rmel irrigation scheme were chosen on the basis of their TAW. Two scenarios of irrigation scheduling were designed through the STICS model: (i) irrigation was scheduled when 50% of the TAW was depleted (I50); (ii) irrigation was scheduled when 25% of the TAW was depleted (I25). Results showed that lowest values of grain yield were obtained in soil having low TAW as a consequence of a frequent water stress occurring during the reproductive stage of wheat. The highest values of WUE, as determined at grain and biomass basis (WUEg and WUEbio indexes), were obtained when irrigation was scheduled at 25% of the TAW depletion. In I50 and I25 scenarios, highest WUEg, averaging respectively 2.25 and 2.23 kg m -3 was

Estimating field-scale soil water dynamics at a heterogeneous site using multi-channel GPR

Abstract. We explore the feasibility to quantify the field-scale soil water dynamics through time series of GPR (ground-penetrating radar) measurements, which bridge the gap between point measurements and field measurements. Working on a 40 m × 50 m area in a heterogeneous agricultural field, we obtain a time series of radargrams after a heavy rainfall event. The data are analysed to simultaneously yield (i) a three-dimensional representation of the subsurface architecture and (ii) the total soil water volume between the surface and a reflection boundary associated with the presence of paleo sand dunes or clay inclusions in a rather uniform sand matrix. We assess the precision and the accuracy of these quantities and conclude that the method is sensitive enough to capture the spatial structure of the changing soil water content in a three-dimensional heterogeneous soil during a short-duration infiltration event. While the sensitivity of the method needs to be improved, it already produced useful information to understand the observed patterns in crop height and it yielded insight into the dynamics of soil water content at this site including the effect of evaporation.

PEDOLOGICAL ASPECTS OF LAND EVALUATION FOR IN‐FIELD RAINWATER HARVESTING IN SOUTH AFRICA

ABSTRACTThe in‐field rainwater harvesting (IRWH) technique was designed for improving crop yields of subsistence farmers in semi‐arid areas in South Africa. An appropriate soil survey to identify suitable land was developed by means of a field study near Thaba Nchu, South Africa. Results of the land Type Survey of South Africa (scale 1 : 250 000) were used to advantage. Areas suitable for IRWH on two soilscapes were delineated at a scale of 1 : 10 000. Although the procedure was satisfactory, to provide accuracy appropriate for small plots a scale of 1 : 5000 is recommended.The most important pedological characteristic of an IRWH‐suitable soil is its capacity to store plant‐available water, termed total extractable soil water (TESW). Satisfactory TESW values for maize and sunflower are > 150 mm, values which occur on fine‐textured soils at least 900 mm in depth. Other important characteristics are the absence of hydromorphy shallower than 700 mm, and a low final infiltration rate that promotes runoff into the basins.The reliability of land evaluation for IRWH can be enhanced by using appropriate crop models together with long‐term climate data to compute cumulative probability functions of yield to provide evidence regarding viability for a particular ecotope.It is concluded that the relevant technology and knowledge available in South Africa are adequate for the task ahead. Copyright © 2012 John Wiley & Sons, Ltd.

Effects of Different Tillage Systems on Soil Properties, Root Growth, Grain Yield, and Water Use Efficiency of Winter Wheat (Triticum aestivum L.) in Arid Northwest China

AbstractStudies on root development, soil physical properties, grain yield, and water-use efficiency are important for identifying suitable soil management practices for sustainable crop production. A field experiment was conducted from 2006 through 2008 in arid northwestern China to determine the effects of four tillage systems on soil properties, root development, water-use efficiency, and grain yield of winter wheat (Triticum aestivum L.). The cultivar Fan 13 was grown under four tillage systems: conventional tillage (CT) without wheat stubble, no-tillage without wheat stubble mulching (NT), no-tillage with wheat stubble standing (NTSS), and no-tillage with wheat stubble mulching (NTS). The soil bulk density (BD) under CT system increased gradually from sowing to harvest, but that in NT, NTSS, and NTS systems had little change. Compared to the CT system, the NTSS and NTS systems improved total soil water storage (0-150 cm) by 6.1-9.6 and 10.5-15.3% before sowing, and by 2.2-8.9 and 13.0-15.1% after harvest, respectively. The NTSS and NTS systems also increased mean dry root weight density (DRWD) as compared to CT system. The NTS system significantly improved water-use efficiency by 17.2-17.5% and crop yield by 15.6-16.8%, and the NTSS system improved that by 7.8-9.6 and 7.0-12.8%, respectively, compared with the CT system. Our results suggested that Chinese farmers should consider adopting conservation tillage practices in arid northwestern China because of benefits to soil bulk density, water storage, root system, and winter wheat yield.

The effects of partial root-zone irrigation and regulated deficit irrigation on the vegetative and reproductive development of field-grown Monastrell grapevines

The effects of regulated deficit irrigation (RDI) and partial root-zone irrigation (PRI) strategies that apply the same irrigation volumes on vegetative and reproductive development were analyzed during a 3-year-period in field-grown Monastrell grapevines under semiarid conditions. Five treatments were applied: control irrigated at 60 % ETc (crop evapotranspiration) for the whole season (308 mm year−1); RDI-1 and PRI-1 that received the same irrigation as the control before fruit set, 30 % ETc from fruit set to harvest and 45 % ETc post-harvest (192 mm year−1); and RDI-2 and PRI-2 that were the same, except with 15 % ETc from fruit set to harvest (142 mm year−1). Distinctive PRI effects on vegetative and reproductive development were observed depending on the total soil water content and the vine water stress level. PRI-1 vines showed less restriction of vegetative growth, lower leaf abscission, and higher leaf area during post-veraison than RDI-1 vines. Higher supply of water (close to field capacity) via half of the root system in PRI-1 vines maintained better water supply and more favorable phloem sap flow (water and carbohydrates) into the fruit during post-veraison and showed a positive differential effect on fruit growth compared with RDI-1 vines. This was reflected in a higher solute content per berry (12 % higher) and higher fresh berry weight (8 % higher) at harvest in PRI-1 compared to RDI-1 berries. However, this positive effect in fruit growth was not reflected in either an improved final yield or the water use efficiency of PRI-1 vines. In PRI-2, the soil water in the wet half was insufficient to maintain more favorable shoot water supply and phloem sap flow into the berry, and no substantial changes were observed in vine vigor, leaf and fruit growth between PRI-2 and RDI-2. Higher irrigation amount in the wet root zone and higher depth of irrigation under PRI seem to be more effective for 1103P–Mourvedre combination to produce a favorable effect in berry growth and development.

Model-based evaluation of irrigation needs in Mediterranean vineyards

The irregular rainfall distribution causes interannual variation of water status in Mediterranean vineyards. A frequential analysis of irrigation needs was carried out from continuous simulation of the soil water balance during 39 years in south France. The off-season soil water refilling was often incomplete, with a higher frequency in soils with a high total transpirable soil water (TTSW) and/or susceptible to runoff and high evaporation. On soils with high TTSW (over 250 mm), the irrigation need was nil or small (except in situations of high runoff) and focused on the beginning of the crop cycle. On soils with lower TTSW, the irrigation need increased on average and was spread all along the grapevine cycle due to the limited buffering effect of the soil water reservoir. For 100 mm TTSW, the irrigation need was 40–60 mm for half of the years. The calculated irrigation needs were sensitive to the soil (TTSW, susceptibility to runoff and evaporation) and canopy (crop coefficient) properties. Therefore, soil and canopy management should be considered together with irrigation for an integrated approach of water management.

Transpiration response of upland rice to water deficit changed by different levels of eucalyptus biochar

The objective of this work was to evaluate the effect of eucalyptus biochar on the transpiration rate of upland rice 'BRSMG Curinga' as an alternative means to decrease the effect of water stress on plant growth and development. Two-pot experiments were carried out using a completely randomized block design, in a split-plot arrangement, with six replicates. Main plots were water stress (WS) and no-water stress (NWS), and the subplots were biochar doses at 0, 6, 12 and 24% in growing medium (sand). Total transpirable soil water (TTSW), the p factor - defined as the average fraction of TTSW which can be depleted from the root zone before water stress limits growth -, and the normalized transpiration rate (NTR) were determined. Biochar addition increased TTSW and the p factor, and reduced NTR. Consequently, biochar addition was able to change the moisture threshold (p factor) of the growing medium, up to 12% maximum concentration, delaying the point where transpiration declines and affects yield.

Performance of the FAO AquaCrop model for wheat grain yield and soil moisture simulation in Western Canada

Wheat (Triticum aestivum L.) is the major grain crop grown in western Canada (Canadian Prairies) and soil water stress is considered the main limiting factor for crop growth. The objective of this study was to adapt and test the ability of the FAO developed AquaCrop model (v3.0) to simulate spring wheat yield and total soil water content (0–120cm layer) on the Canadian Prairies. Crop yield and soil water content data collected from five experimental sites across the Canadian Prairies from 2003 through 2006 were used in the study. Results showed that the AquaCrop model can be used to model both wheat grain yield and soil water content on the Canadian Prairies with acceptable accuracy. Overall, the relationship between observed and modelled wheat grain yield for all sites combined produced a R2 of 0.66, slope of 0.96, index of agreement (d) of 0.99, root mean square error (RMSE) of 743kgha−1 and mean absolute error (MAE) of 611kgha−1. Similarly, the comparison between observed and modelled soil water content yielded a R2 of 0.90, slope of 0.73, d of 0.99, RMSE of 49mm and MAE of 40mm. The difference between observed and modelled grain yield was only 3%, while that between observed and modelled total soil water was 2%. Consequently, AquaCrop can be a valuable tool for simulating both wheat grain yield and soil water content on the Canadian Prairies, particularly considering the fact that the model requires a relatively small number of explicit and mostly intuitive input data which can be readily available or easily collected. However, the performance of the model has to be evaluated and fine-tuned under a wider range of conditions, which we hope will be the next step.

Contrasting physiological effects of partial root zone drying in field-grown grapevine (Vitis vinifera L. cv. Monastrell) according to total soil water availability.

Different spatial distributions of soil moisture were imposed on field-grown grapevines by applying the same irrigation volumes to the entire (DI; deficit irrigation) or part of the (PRD; partial root zone drying) root zone. Five treatments were applied: controls irrigated at 60% ETc (crop evapotranspiration) for the whole season (308 mm year−1); DI-1 and PRD-1 that received the same irrigation as controls before fruit set, 30% ETc from fruit set to harvest and 45% ETc post-harvest (192 mm year−1); and DI-2 and PRD-2 that were the same, except that 15% ETc was applied from fruit set to harvest (142 mm year−1). Compared with DI-1, PRD-1 maintained higher leaf area post-veraison and increased root water uptake, whole-plant hydraulic conductance, leaf transpiration, stomatal conductance, and photosynthesis, but decreased intrinsic gas exchange efficiency without causing differences in leaf xylem abscisic acid (ABA) concentration. Compared with DI-2, PRD-2 increased leaf xylem ABA concentration and decreased root water uptake, whole-plant hydraulic conductance, leaf transpiration, stomatal conductance, and photosynthesis, mainly at the beginning of PRD cycles. Distinctive PRD effects (e.g. greater stomatal closure) depended on the volumetric soil water content of the wet root zone, as predicted from a model of root-to-shoot ABA signalling.

Determining Water Retention in Seasonally Frozen Soils Using Hydra Impedance Sensors

The soil freezing characteristic, defined as the relationship between freezing soil temperature and unfrozen water content, can be used to determine the soil water retention curve in situ. The objective of this study was to investigate whether freezing characteristics measured with Hydra impedance sensors (Stevens Water Monitoring Systems, Portland, OR) result in accurate depthwise soil water retention curves. Hydra sensors measuring the complex permittivity and soil temperature were installed at five depths at five sites in southeastern Wyoming and monitored for approximately 2 yr. A dielectric mixing model was calibrated using unfrozen soil data to predict the liquid water content and ice content in frozen soils. The total water potential in the frozen soils was calculated from the Hydra sensor soil temperature measurements using the Clapeyron equation. Soil water pressure heads were calculated from the total soil water head by subtracting the osmotic head. Comparison of the resulting Hydra sensor water retention data with depthwise laboratory retention data using a dew point potentiometer and a pressure plate extractor showed mixed results (coefficient of determination 0 ≤ R2 ≤ 0.94). The best results were obtained for the shallowest sensors at the five sites (0.74 ≤ R2 ≤ 0.93) because of the more significant and more prolonged soil freezing at these depths, resulting in relatively wide ranges for the calculated soil water pressure heads. Fitted curves for the Hydra sensor water retention data yielded unreliable parameters because of insufficient information on the wet end of the water retention curves.

Role of Soil Sorption and Microbial Degradation on Dissipation of Mesotrione in Plant-Available Soil Water

Mesotrione is a carotenoid biosynthesis-inhibiting herbicide labeled for pre-emergence and postemergence weed control in corn production. Understanding the factors that influence the dissipation of mesotrione in soil and in the plant-available water (PAW) is important for the environmental fate assessment and optimal weed management practices. The present research investigated the role of soil properties and microbial activities on the interrelated sorption and degradation processes of mesotrione in four soils by direct measurements of PAW. We found that mesotrione bound to the soils time dependently, with approximately 14 d to reach equilibrium. The 24-h batch-slurry equilibrium experiments provided the sorption partition coefficient ranging from 0.26 to 3.53 L kg(-1), depending on soil organic carbon and pH. The dissipation of mesotrione in the soil-bound phase was primarily attributed to desorption to the PAW. Degradation in the PAW was rapid and primarily dependent on microbial actions, with half-degradation time (DT(50)) <3 d in all four soils tested. The rapid degradation in the PAW became rate limited by sorption as more available molecules were depleted in the soil pore water, resulting in a more slowed overall process for the total soil-water system (DT(50) <26 d). The dissipation of mesotrione in the PAW was due to microbial metabolism and time-dependent sorption to the soils. A coupled kinetics model calibrated with the data from the laboratory centrifugation technique provided an effective approach to investigate the interrelated processes of sorption and degradation in realistic soil moisture conditions.

Influence of crust formation under natural rain on physical attributes of soils with different textures

One of the main negative anthropic effects on soil is the formation of crusts, resulting in soil degradation. This process of physical origin reduces soil water infiltration, causing increased runoff and consequently soil losses, water erosion and/or soil degradation. The study and monitoring of soil crusts is important for soil management and conservation, mainly in tropical regions where research is insufficient to explain how soil crusts are formed and how they evolve. The purpose of this study was to monitor these processes on soils with different particle size distributions. Soil crusts on a sandy/sandy loam Argissolo Vermelho-Amarelo (Typic Hapludult), sandy loam Latossolo Vermelho-Amarelo (Typic Hapludox) and a clayey Nitossolo Vermelho eutroférrico (Rhodic Kandiudalf) were monitored. The soil was sampled and data collected after 0, 3, 5 and 10 rain storms with intensities above 25 mm h-1, from December 2008 to May 2009. Soil chemical and particle size distribution analysis were performed. The changes caused by rainfall were monitored by determining the soil roughness, hydraulic conductivity and soil water retention curves and by micromorphological analysis. Reduced soil roughness and crust formation were observed for all soils during the monitored rainfall events. However, contrary to what was expected according to the literature, crust formation was not always accompanied by reductions in total porosity, hydraulic conductivity and soil water retention.

Photosynthetic, chlorophyll fluorescence and growth changes in hot pepper under deficit irrigation and partial root zone drying

There is increasing evidence that the spatial distribution of water within the root zone, as well as total soil water status, determines plant physiological and agronomic responses. To examine the response of photosynthesis, chlorophyll fluorescence and growth of hot pepper (Capsicum annuum L.) to deficit irrigation (DI50, 50% of the control) and partial root-zone drying (PRD, with half of the root system exposed to soil drying and the other half watered with 50% irrigation water of the control), two water deficit treatments were imposed on greenhouse grown hot pepper during the growing period in 2006. Control plants received irrigation to both halves of the root system when soil water content was < 80% of field capacity. Both PRD and DI50 treatments decreased total dry mass by 33 to 44%, shoot biomass by 31 to 44% compared to the control. These treatments increased root-shoot ratio by 35 to 44% in relation to the control, with significant differences between PRD, DI50 and the control. Deficit irrigation led to a relative leaf water content of about 77.91 to 92.71%. Two water deficit treatments reduced photosynthetic rate (Pn) slightly and transpiration rate (Tr) significantly, thus improving leaf water use efficiency (WUEL, defined as the ratio of Pn to Tr) by 24 to 26%. During water stress, a down-regulation of PSII activity was observed along with some impairment of photochemical activity, as revealed by decreases in the maximum quantum yield of PSII (Fv/Fm). Although Fv/Fm did not significantly differ between the deficit treatments and the control, Fv/Fm of PRD plants was higher than that of DI50 plants. The mean values of Fv, Fm, qP at four stages decreased and Fo and qN increased in the soil drought environment. Development of non-radiative energy dissipation mechanisms was evidenced during stress by increases in non-photochemical quenching and decreases in efficiency of excitation capture by open centers. Key words: Photochemical parameters, relative leaf water content, DI50, partial root-zone drying (PRD).

Forage quality, water use and nitrogen utilization efficiencies of pearl millet (Pennisetum americanum L.) grown under different soil moisture and nitrogen levels

Abstract The increasing scarcity of water for irrigation is becoming the most important problem for producing forage in all arid and semi-arid regions. Pearl millet is a key crop in these regions which needs relatively less water than other crops. In this research, a field study was conducted to identify the best combination of irrigation and nitrogen (N) management to achieve acceptable pearl millet forage both in quantity and quality aspects. Pearl millet was subjected to four irrigation treatments with interaction of N fertilizer (0, 75, 150 and 225 kg ha−1). The irrigation treatments were 40%, 60%, 80% and 100% of total available soil water (I40, I60, I80 and I100, respectively). The results showed that increasing moisture stress (from I40 to I100) resulted in progressively less total dry matter (TDM), leaf area index (LAI), and nitrogen utilization efficiency (NUzE), while water use efficiency (WUE) and the percentage of crude protein (CP%) increased. The highest TDM and LAI were found to be 21.45 t ha−1 and 8.65, in I40 treatment, respectively. TDM, WUE, CP% and profit responses to N rates were positive. The maximum WUE of 4.19 kg DM/m3 was achieved at I100 with 150 kg N ha−1. The results of this research indicate that the maximum profit of forage production was obtained in plots which were fully irrigated (I40) and received 225 kg N ha−1. However, in the situation which water is often limited and not available, application of 150 kg N ha−1 can produce high forage quality and guaranty acceptable benefits for farmers.

Interacting Vegetative and Thermal Contributions to Water Movement in Desert Soil

Thermally driven water‐vapor flow can be an important component of total water movement in bare soil and in deep unsaturated zones, but this process is often neglected when considering the effects of soil–plant–atmosphere interactions on shallow water movement. The objectives of this study were to evaluate the coupled and separate effects of vegetative and thermal‐gradient contributions to soil water movement in desert environments. The evaluation was done by comparing a series of simulations with and without vegetation and thermal forcing during a 4.7‐yr period (May 2001–December 2005). For vegetated soil, evapotranspiration alone reduced root‐zone (upper 1 m) moisture to a minimum value (25 mm) each year under both isothermal and nonisothermal conditions. Variations in the leaf area index altered the minimum storage values by up to 10 mm. For unvegetated isothermal and nonisothermal simulations, root‐zone water storage nearly doubled during the simulation period and created a persistent driving force for downward liquid fluxes below the root zone (total net flux ∼1 mm). Total soil water movement during the study period was dominated by thermally driven vapor fluxes. Thermally driven vapor flow and condensation supplemented moisture supplies to plant roots during the driest times of each year. The results show how nonisothermal flow is coupled with plant water uptake, potentially influencing ecohydrologic relations in desert environments.

Water Relations and Transpiration of Quinoa (Chenopodium quinoa Willd.) Under Salinity and Soil Drying

Drought and salinity are the two major factors limiting crop growth and production in arid and semi-arid regions. The separate and combined effects of salinity and progressive drought in quinoa (Chenopodium quinoa Willd.) were studied in a greenhouse experiment. Stomatal conductance (gs), leaf water potential (Wl), shoot and root abscisic acid concentration ([ABA]) and transpiration rate were measured in full irrigation (FI; around 95 % of water holding capacity (WHC)) and progressive drought (PD) treatments using the irrigation water with five salinity levels (0, 10, 20, 30 and 40 dS m )1 ); the treatments are referred to as FI0 ,F I10 ,F I20 ,F I30 ,F I40 ;P D0 ,P D10 ,P D20 ,P D30 ,P D40, respectively. The measurements were carried out over 9 days of continuous drought. The results showed that increasing salinity levels decreased the total soil water potential (WT) and consequently decreased gs and Wl values in both FI and PD. During the drought period, the xylem [ABA] extracted from the shoots increased faster than that extracted from the roots. A reduction in WT, caused by salinity and soil drying, reduced transpiration and increased apparent root resistance (R) to water uptake, especially in PD0 and PD40 during the last days of the drought period. The reasons for the increase in apparent root resistance are discussed. At the end of the drought period, the minimum value of relative available soil water (RAW) was reached in PD0. Under non-saline conditions, Wl decreased sharply when RAW reached 0.42 or lower, but under the saline conditions of PD10 and PD20, the threshold values of RAW were 0.67 and 0.96, respectively. In conclusion, due to the additive effect of osmotic and matric potential during soil drying on soil water availability, quinoa should be re-irrigated at higher RAW in salt-affected soils, i.e. before the soil water content reaches the critical threshold level causing the drop in Wl resulting in stomatal closure.

The effects of the degree of soil cover with an impervious sheet on the establishment of tree seedlings in an arid environment

Low precipitation and high evaporation rates hinder the establishment of tree seedlings in arid environments. The objective of this study was to assess the effect of covering different fractions (0, 30, 70 and 100%) of the wetted area surrounding seedlings of Acacia saligna (Labill.) H.L. Wendl. with a black polyethylene sheet on the seedlings development and water use. The cover treatments were applied to plots with and without Acacia seedlings. Within the mulched areas at all depths, soil temperatures close to the edges were lower than at the corresponding depth close to the center, but still higher than in the non-mulched plots. In plots with Acacia seedlings, the total soil water losses were similar among treatments. However, the cumulative evaporative losses near the soil surface were lower with larger mulched areas. During early growth stages, roots grew deeper in the 30% mulch treatment and more laterally in the other mulched treatments. The non-mulched treatment produced the smallest root system. Larger canopies developed in the mulched treatments and the gross water use efficiency increased with the increasing fraction of the mulched area. Optimum utilization of stored soil water and seedling development were attained with the 70% mulch treatment.