Water Content In Root Zone Research Articles

Stress Induced Water Content Variations in Mango Stem by Time Domain Reflectometry

Close, direct, and accurate monitoring of the plant water status may serve as a practical (irrigation scheduling) and a research (climate–environmental induced physiologic changes) tool. Methods for high‐frequency capacitance measurement (e.g., time domain reflectometry [TDR]) possess the potential for high resolution dielectric measurements with minimal dependence on properties of the measured matrix. The objective of this study is to test the accuracy, response time, and sensitivity of the TDR methodology in measuring changes in water status in a mango (Mangifera indica L., Cultivar ‘Kent’) tree stem exposed to several perturbations concerning water, salinity, fruit load, and radiation. Under several induced stress conditions, stem and root zone water content (θ) and electrical conductivity (σ) were simultaneously measured. Our study is distinct in its detailed and frequent measurements of stem water content (θstem) using short (29–70 mm) TDR probes in trees growing in a detached medium. We have found that θstem response to root zone applied salinity and water stress were negative and positive, respectively. Stem electrical conductivity (σstem) was primarily dependent on θstem and only negligibly on stem cells salinity. The θstem response time to water application was ∼4 h. Two practical outcomes of our study were: (1) Because the salt content of the tree cells only slightly affected σstem, stem resistivity measurements could be used to represent dielectric changes, and (2) quite short probes could be used to include young trees of slim tree branches.

Above-ground biomass accumulation and nitrogen fixation of broom ( Cytisus scoparius L.) growing with juvenile Pinus radiata on a dryland site

Annual above-ground biomass growth and nitrogen accumulation was investigated in a mixed stand of juvenile Pinus radiata and broom ( Cytisus scoparius L.), growing on a dryland site in Canterbury, New Zealand. Nitrogen fixation by broom, and the extent of fixed nitrogen transfer to the P. radiata was assessed using the 15 N natural abundance method. Broom biomass increased five-fold over the course of the year to 4250 kg ha −1. Values of δ 15 N were highest in P. radiata growing without broom, intermediate in P. radiata growing with broom, and lowest in field-grown broom. Broom was an effective fixer, deriving 81% of nitrogen in above-ground tissues from the atmosphere, which was equivalent to 111 kg N ha −1 per year. The intermediate values of δ 15 N found for trees growing with broom suggest that there was some transfer of fixed nitrogen from the broom to the P. radiata. Relatively small differences in δ 15 N between nitrogen derived from the non-fixer and fixation, combined with a lack of information on mycorrhizal associations and sources of nitrogen suggested that estimates of broom nitrogen fixation could be subject to some error. Despite these uncertainties there was close agreement between estimates of nitrogen fixation obtained from sequential harvests with that obtained using the 15 N natural abundance technique. The comparatively low values of above-ground biomass growth exhibited by broom at this drought prone site highlight the importance of root-zone water content in regulating growth in this species. Competition by broom for water and soil nitrogen completely outweighed any beneficial influence to the tree resulting from fixed nitrogen transfer. However, the high rates of nitrogen fixation documented in this study suggest that the presence of broom may enhance long-term growth of P. radiata on wet, nitrogen-deficient sites.

Multi-scale assimilation of surface soil moisture data for robust root zone moisture predictions

In the presence of uncertain initial conditions and soil hydraulic properties land surface model performance can be significantly improved by the assimilation of periodic observations of certain state variables, such as the near surface soil moisture as observed from a remote platform. Recently, Montaldo et al. [Water Resour Res 37 (2001) 2889] derived a framework that uses biases between observed and modeled time rates of change of surface soil moisture to quantify biases between modeled and actual root-zone-average soil moisture contents. For very large errors in the saturated conductivity the soil moisture assimilation procedure is continuously working against the drainage errors, resulting in a persistent bias in its predictions. In this paper, we adopt this persistent (directional) bias in soil moisture as evidence of an error in the saturated conductivity. From manipulations of soil water balance equations we derived an expression that quantitatively relates the persistent bias in soil moisture to the estimated error in the saturated hydraulic conductivity. We combined this result with the approach of Montaldo et al. [Water Resour Res 37 (2001) 2889] to form a multi-scale assimilation approach. The multi-scale assimilation system is shown to provide marked improvements in the prediction of root zone soil moisture for a case study using data taken from an experimental catchment near Cork, Ireland. In effect, the root zone moisture is updated to provide a temporal trajectory of the near surface moisture that follows the trajectory of the observed surface moisture, and the hydraulic conductivity is adjusted on the basis of the time averaged corrections applied to the root zone water content. It is anticipated that this approach would be useful in operational forecasting models over large domains, where system parameters would be uncertain and occasional distributed observations would be limited to the near surface zone.

Effective Rainfall in Poorly Drained Microirrigated Citrus Orchards

Effective rainfall (ER) is the portion of total rainfall that plants use to help meet their consumptive water requirements, and is an important component of water resource budgeting for irrigation. The USDA's Technical Release no. 21 (TR-21) is used to predict ER and irrigation requirements for south Florida citrus, but its accuracy is in question due to high-intensity rainfall, poorly drained soils, and partial irrigation coverage in microirrigated orchards. We evaluated the calculation of ER by TR-21 under these conditions by monitoring rainfall, irrigation, water table depth, evapotranspiration (ET), and soil water content inside and outside of the microirrigation-wetted pattern in four orchards for a total of 83 site-months. We developed a soil water budget to calculate daily water table upflux, root zone water content, water used, and ER separately for the irrigated and nonirrigated root zones. Water budget ER calculated by site ranged between −3.3% and +18.2% of TR-21 ER, with a mean of +10%. A linear correlation between water budget ER and TR-21 ER using pooled data from all four sites yielded the equation: Water Budget ER (mm) = 0.79 [TR-21 ER (mm)] + 17.7, r = 0.84. A hypothetical ER comparison using 30-yr mean rainfall and ET data showed that annual ER calculated by TR-21 amounted to 673 mm, while water budget ER totaled 744 mm, or +10.5%. We suggest that the TR-21 method has the level of accuracy needed to allocate water for microirrigated citrus on poorly drained south Florida soils.

Effective Rainfall in Poorly Drained Microirrigated Citrus Orchards

Effective rainfall (ER) is the portion of total rainfall that plants use to help meet their consumptive water requirements, and is an important component of water resource budgeting for irrigation. The USDA's Technical Release no. 21 (TR‐21) is used to predict ER and irrigation requirements for south Florida citrus, but its accuracy is in question due to high‐intensity rainfall, poorly drained soils, and partial irrigation coverage in microirrigated orchards. We evaluated the calculation of ER by TR‐21 under these conditions by monitoring rainfall, irrigation, water table depth, evapotranspiration (ET), and soil water content inside and outside of the microirrigation‐wetted pattern in four orchards for a total of 83 site‐months. We developed a soil water budget to calculate daily water table upflux, root zone water content, water used, and ER separately for the irrigated and nonirrigated root zones. Water budget ER calculated by site ranged between −3.3% and +18.2% of TR‐21 ER, with a mean of +10%. A linear correlation between water budget ER and TR‐21 ER using pooled data from all four sites yielded the equation: Water Budget ER (mm) = 0.79 [TR‐21 ER (mm)] + 17.7, r = 0.84. A hypothetical ER comparison using 30‐yr mean rainfall and ET data showed that annual ER calculated by TR‐21 amounted to 673 mm, while water budget ER totaled 744 mm, or +10.5%. We suggest that the TR‐21 method has the level of accuracy needed to allocate water for microirrigated citrus on poorly drained south Florida soils.

Impact of non-drained irrigated rice cropping on soil salinization in the Senegal River Delta

Soils in the Senegal River Delta are naturally saline due to the presence of a shallow saline water table (electrical conductivity>20 dS m −1; water table depth between 1 m and >2 m throughout the year). Private irrigation schemes for rice in the Delta entail low investments and generally lack a surface drainage infrastructure. Such schemes are often abandoned after a few years, which is commonly blamed on soil salinization because of lack of drainage. However, no surveys have been conducted so far to sustain this assumption. The objective of this study was to obtain quantitative information on the impact of non-drained irrigated rice cropping on soil salinization in the Delta. We selected a 2000 ha study area with predominantly private non-drained irrigation in the Gorom region in the Delta. Soil salinity was assessed for 118 sample fields at the onset of the wet rice growing season using an electromagnetic conductivity meter (EM38) and conventional laboratory techniques (0 to 0.3 m depth). The number of years under rice cropping for individual fields was obtained using satellite imagery from previous years (1992–1995) and a geographical information system. EM38 measurements were poorly correlated with root zone EC lab data, which was attributed to a low root zone water content at the time of measurement (i.e., end of the dry season). EM38 measurements reflected, therefore, mainly subsoil salinity and could not be used to predict root zone salinity. Contrary to common thinking, 1:5 soil extract conductivity (EC lab) at 0–0.30 m depth declined with increased cropping intensity. Measurements conducted shortly before the start of the growing season resulted in an average EC lab of 2.2 dS m −1 for four years of rice cropping without drainage, and in an average EC lab of 8.9 dS m −1 for zero years of rice cropping. Rice cropping therefore reduced soil salinity, even when conducted without drainage. Results are explained by the leaching effect of the ponded water layer in an irrigated rice field, temporarily transporting salts downwards and blocking capillary rise of salt from the water table to the soil surface. EC lab determined for rice fields at harvest ranged from 0.4–1.7 dS m −1 for 0–20 cm depth. Comparison with previous data showed that a yield decline due to salinity can be expected if EC lab>0.8 dS m −1. Installation of surface drainage facilities may, therefore, in some cases be worth the investment in the Delta.

A dynamic model of crop growth and partitioning of biomass

A model is presented of growth and partitioning to leaves, stems and roots in herbaceous, vegetative crops in response to atmospheric conditions and water supply. It comprises 12 state variables and 33 parameters (including four functional relationships), all of direct physiological significance. The important characteristic of the model is the simultaneous consideration of crop assimilate and water balances achieved by calculations made at short time steps (1 h or less) in order to capture the physiological responses of crop growth and water use as they respond to diurnal environmental patterns. In the model, root-zone water content decreases with transpiration and soil evaporation, and increases with rainfall, irrigation and deepening of the root zone as the crop develops. Photosynthesis depends upon intercepted radiation and temperature and also on canopy conductance determined from crop water status. Respiration of organs is calculated as separate requirements for maintenance and growth. Transpiration proceeds with photosynthesis but in response to evaporative demand, reducing crop water content, which is in turn replenished from the root zone based on its water content and the root length that explores it. Partitioning of assimilate to leaves, stems, and roots depends upon diurnal oscillations in assimilate supply, temperature, and crop water content within limits set by phenological development. Phenological development, here the initiation and expansion of leaves and the maturity and senescence of canopy and root systems, is determined by temperature. Examples, and trends, of model performance are compared with measured physiological and agronomic responses of sunflower to strategies of irrigation.

Variation among Red and Freeman Maples in Response to Drought and Flooding

Freeman maples (Ace×freemanii E. Murray) are marketed as stress-resistant alternatives to red maples (Acer rubrum L.). Our objective was to compare two cultivars of Freeman maple [`Jeffersred' (Autumn Blaze®) and `Indian Summer'] and five red maples [`Franksred' (Red Sunset®), `Autumn Flame', `PNI 0268' (October Glory®), `Fairview Flame', and unnamed selection 59904] for effects of flooding and water deficit on plant growth, biomass partitioning, stomatal conductance, and leaf osmotic potential. Plants grown from rooted cuttings in containers were subjected to three consecutive cycles during which root-zone water content decreased to 0.12, 0.08, and 0.02 m3·m–3, respectively. Additional plants were flooded for 75 days, while plants in a control treatment were irrigated frequently. Stomatal conductance immediately before imposing drought and after three drought cycles did not differ among genotypes and averaged 220 and 26 mmol·s–1·m–2, respectively. Differences in stomatal conductance after recovery from the first drought cycle and at the end of the second drought cycle did not vary with species. Drought reduced estimated leaf osmotic potential similarly for all genotypes; means for drought-stressed and control plants were –1.92 and –1.16 MPa, respectively. Freeman maples had a higher mean root: shoot weight ratio and a lower leaf surface area: root dryweight ratio than did red maples. Across genotypes, stomatal conductance of flooded plants initially increased by ≈20% and then fell to and remained below 50 mmol·s–1·m–2. Stomatal conductance of `Indian Summer' decreased to ≈20 mmol·s–1·m–2 after 8 days of flooding, indicating that this cultivar may be particularly sensitive to root-zone saturation.

The SAWAH Riceland Hydrology Model

The paper presents a robust Darcy‐based one‐dimensional soil water balance model (SAWAH) for simulating riceland hydrology. Experimental data from an upland rice toposequence showing highly variable groundwater and volumetric soil water contents θ are used to calibrate the model and to validate it by a modified cross‐validation method. Analysis of variance is applied to estimate the mean squared prediction error and to split it into errors in predicting horizontal, vertical, and temporal gradients of θ. Root mean squared error of prediction (RMSEP) of soil water content at strip‐depth‐time points was about equal to the standard deviation of unreplicated measurements. Measured strip and field means were more accurate than predicted means, but RMSEP remained below 0.03 m3 m−3. Ranking of crop and soil parameters for their effects on root zone water content and seasonal flux totals depended strongly on the imposed hydrology regime. Capillary rise contributed 5–55% of seasonal evapotranspiration, depending on season and topographic position.

077 Modeling Nitrogen Status of Chrysanthemum in a Growing Container

A model was constructed to dynamically simulate how the nitrogen concentration changes in the root zone of a pot grown chrysanthemum. The root zone concentration of nitrogen is predicted at any time during the crop by predicting the root zone contents of nitrogen and water. The root zone content of nitrogen is predicted by integrating the rates of nitrogen applied, taken up by the plant and entering the top layer of the pot. The root zone water content is predicted by integrating the rates of water applied, evaporated from the media surface and transpired by the plant. Simple models were constructed to predict the various rate processes. For example the rate of nitrogen uptake was modeled as a function of the dry mass accumulation and was broken down into demands of nitrogen by the plant for maintenance of the current dry mass and for support of new growth. Dry mass accumulation was modeled as a function of the amount of PPF which could be intercepted by the plant. The model was validated using plants grown in growth chambers and greenhouses at various PPF levels and fertilizer concentrations. The model will be used to test the risks involved in using various fertilizer strategies and to develop more efficient fertilization strategies.

Ratio of stomatal resistance on two sides of wheat leaves as affected by soil water content

Observations of stomatal diffusion resistance on the two surfaces of wheat flag leaves were made concurrently in two plots of different soil water content in the main part of the growing season in 1984 and 1985. The results indicate that adaxial and abaxial stomatal apertures have different sensitivities to soil water stress. The resistance of abaxial stomata increases substantially with decreasing soil water content. The increase of stomatal resistance on the abaxial surface is larger than that on the adaxial surface under the stress conditions. The daytime ratio of abaxial to adaxial stomatal resistance (rb/rd) remained constant at about 1.5 during the main part of the growing season in a well-watered plot (relative soil water content in root zone above 75%). When soil water was in short supply the ratio rb/rd was larger and changed during the day. There was a linear relationship between the daily mean ratio of rb/rd and soil water potential in root zone when the relative soil water content was below 75%.

Evapotranspiration over an Agricultural Region Using a Surface Flux/Temperature Model Based on NOAA-AVHRR Data

The possibility of using infrared surface temperatures from satellites (NOAA, GOES) for inferring daily evaporation and soil moisture distribution over large areas (102 to 105 km2) has been extensively studied during the past few years. The methods are based upon analysis of the surface energy budget, but treating surface transfers as over bare soils. In this context, we have developed a methodology using infrared surface data (from NOAA-7) as input data, in a one-dimensional boundary layer/vegetation/soil model, including a parameterization of transfers within the canopy, based on the formalism of Deardorff which allows the use of a small number of mesoscale surface vegetation parameters. As shown from the model sensitivity tests, a single surface temperature measured near midday (provided by NOAA-7) is sufficient for obtaining the surface energy fluxes over dense vegetation and for deriving the only governing parameter that remains, the bulk canopy resistance to evaporation, a different concept from moisture availability used for bare soils. The objective of the model in predicting the area-averaged surface fluxes and canopy resistances over dense vegetation is analyzed in conjunction with experimental surface flux measurements for three cases with cloudless NOAA images over a flat monocultural region (the Beauce in France). In the absence of a current capability for routine daily soil moisture observation over an agricultural region, an area-averaged evaluation of the soil moisture can be derived from the canopy resistance obtained by this methodology, using an empirical expression relating this resistance to the root zone water content. Spatial gradient of water content between two areas of Beauce with different soil drainage properties is thus evaluated.

TEST AND ANALYSIS OF A MODEL OF WATER USE BY SORGHUM1

We carried out a field experiment with a well-developed sorghum crop to validate a water use and uptake model proposed by Van Bavel and Ahmed (1976). Actual transpiration is determined from standard daily weather data and the leaf area index and root penetration development, as inputs, and from a canopy resistance, which is a function of the effective water potential of the leaf canopy. This canopy potential is, in turn, a function of the transpiration rate, a constant hydraulic root/canopy resistance, and an effective soil water potential. The latter is a root density weighted average value of the water potential over a number of soil layers. Water uptake from each layer is determined from the relative root density and the difference in water potential between the canopy and that layer. The algorithm allows water uptake as well as water excretion by the roots. The calculated evapotranspiration over a 50-d period was not significantly different from the experimental values. Soil water content profiles calculated with the model showed some systematic deviation from the measured values, possibly because the physical nature of the profile was assumed to be homogeneous. The spread of the simulated results reflects the field variation of hydraulic soil properties. A sensitivity analysis showed minor response to rooting depth and distribution and moderate response to the excretion of water by the roots into dry soil layers. Finally, we examined the ability of the model to predict root zone water content and canopy temperatures, the latter with a view toward potential application of remote sensing in the determination of canopy water stress.

A growing season water balance model applied to two Douglas fir stands

The forest water balance model presented requires only daily solar radiation, maximum and minimum air temperature, and rainfall as the input weather data. Site parameters are root zone depth, soil water retention and drainage characteristics, estimated canopy leaf area index, and the coefficients of the evapotranspiration and rainfall interception submodels. The evapotranspiration submodel calculates the forest evapotranspiration rate as the lesser of energy‐limited and soil‐limited rates. The former is calculated from the 24‐hour net radiation and the latter from the fraction of extractable water in the root zone. Solar radiation and air temperature are used to calculate net radiation. Interception is calculated from the daily rainfall. The root zone is treated as a single layer with drainage calculated as a function of the root zone water content. Water deficits and the matric potential of the root zone are used to indicate tree Water stress. The model was tested on two Douglas fir stands of different stand density and leaf area index. The coefficients used in the evapotranspiration submodel were found to be the same for both stands. It was also found that over 20% of the growing season rainfall was lost through interception.

Measuring and modelling forest evapotranspiration

AbstractMicrometeorological methods of measuring and modelling forest evapotranspiration are reviewed and evaluated. Measurement methods include eddy correlation, aerodynamic, Bowen ratio/energy balance and stomatal diffusion resistance techniques. Modelling approaches discussed include a model relating evapotranspiration rate to the root zone water content and the net radiation flux, the Penman equation, and vapour diffusion models requiring stomatal resistance characteristics of the stand. Measured and modelled estimates of evapotranspiration from a Douglas‐fir stand are compared.