Function Of Soil Water Content Research Articles

Depthwise Carbon Dioxide Production and Transport in a Rangeland Soil

The objective of this study was to determine depthwise CO2 production in a semiarid rangeland soil near Laramie, WY, and to quantify the controls exerted by soil temperature and moisture. Depthwise CO2 flux and production were calculated for the 1 Nov. 2010 to 31 Oct. 2011 study period using continuous measurements of soil CO2 concentration, temperature, and water content. Calculated soil efflux using two different tortuosity models was compared with ecosystem respiration (Reco) as measured by a surface flux chamber. The efflux was mostly lower than the Reco values, as would be expected. Calculated CO2 production for the growing season was dominated by the 7.5‐ to 15‐cm depth (43 g C m‐2), compared with the 0‐ to 7.5‐cm depth (17 g C m‐2) and the 15‐ to 25‐cm depth (9 g C m‐2). The relatively high production at the 7.5‐ to 15‐cm depth was attributed to the fine‐textured soil at this depth, resulting in higher surface area, higher soil water retention, and possibly more favorable conditions for biological activity. The soil CO2 production increased with soil temperature, as expected, with temperature quotient (Q10) values of 2.98 and 3.03 for the 0‐ to 7.5‐ and 7.5‐ to 15‐cm depths, respectively. Temperature‐normalized CO2 production was found to be a function of soil water content (?w) at all depths. Maximum production was observed at ?w = 0.20 to 0.21 m3 m‐3 (0‐7.5‐cm depth), ?w = 0.17 to 0.23 m3 m‐3 (7.5‐15‐cm depth), and ?w = 0.19 to 0.22 m3 m‐3 (15‐25‐cm depth). The range of water contents for which soil respiration is at its peak should be confirmed for other rangelands.

Evaluation of dielectric constant by clay mineral and soil physico-chemical properties

Time domain reflectometry (TDR) has capability of distinguishing the dielectric property of solid, air and liquid phases of the soils and has become increasingly popular for determining the soil water content (SWC). The purpose of this study was to find a relationship between soil bulk density (BD), clay content, clay mineralogy with dielectric constant. The latter is a function of SWC. For this purpose 10 soil samples were taken from five areas with different textures and depths (from topsoil, 0 to 0.3 m and subsoil, 0.3 to 0.6 m), The BDs were varied from 1.18 to 1.65 Mg m-3. Soil physical and chemical properties such as organic matter, clay, silt, and sand percentages as well as clay minerals were measured. Results showed that high clay contents underestimated the SWC in the low moisture range and overestimated the SWC in the high moisture range. Results also showed that soils with similar clay content but different minerals had different impact on dielectric constant. Soils with high BD, or low porosity, had higher dielectric constant value than soils with low BD and high porosity. Key words: Clay mineralogy, dielectric constant, bulk density, soil water content, time domain reflectometry.

Soil water dynamics at a midlatitude test site: Field measurements and box modeling approaches

► Simple box models can describe soil water dynamics at midlatitudes. ► The only meteorological forcing used is daily rainfall. ► Three models are validated both in “in-sample” and in forecast mode. ► Best model: leakage and evapotranspiration are nonlinear functions of soil water. We test the ability of three box models ( Milly, 1993 , Kim et al., 1996 , Laio et al., 2001b ) to describe soil moisture dynamics in a regularly monitored experimental site in northwestern Italy. The models include increasingly complex representations of leakage and evapotranspiration processes. We force the models with the local rainfall, and we determine model parameters with a least-square minimization procedure. We study separately summer and annual dynamics. When we use the models to reproduce annual dynamics, we include a simple representation of the annual cycle of evapotranspiration. We test the models both in simulation mode, estimating their parameters from the same data used to verify them, and in forecast mode, determining the model parameters from one period, and using them to forecast soil water dynamics in a different period. The results indicate that simple box models can estimate soil water dynamics at midlatitudes, using rainfall as the only meteorological driver. The models displaying the best performances include the representation of leakage and evapotranspiration as nonlinear functions of soil water content.

Comparação de métodos de determinação da condutividade hidráulica em um latossolo vermelho-amarelo

A determinação da condutividade hidráulica como variável do conteúdo de água volumétrico, K(θ), é essencial para resolver, por exemplo, problemas de drenagem. Todavia, diferentes métodos de determinação podem produzir resultados distintos, dependendo dos pressupostos assumidos. Objetivou-se com este estudo avaliar o desempenho de métodos diretos no campo (perfil instantâneo, métodos do fluxo e da umidade) e indireto no laboratório (a partir da condutividade hidráulica do solo saturado e da curva de retenção) na estimativa da relação K(θ). O experimento foi realizado em Piracicaba, Estado de São Paulo, Brasil, em um Latossolo Vermelho-Amarelo textura argiloarenosa, entre os meses de junho e agosto de 1999, por serem de baixa precipitação pluvial média mensal. A função K(θ) fornecida pelos diferentes métodos foi comparada utilizando-se o erro máximo absoluto e o índice da concordância. Todos os métodos apresentaram limitações na determinação da função K(θ) na tensão próxima à de saturação, sendo os maiores erros identificados nessa faixa de umidade. Os métodos do fluxo, da umidade e indireto no laboratório tiveram concordância de moderada a fraca quando comparados ao método do perfil instantâneo, mostrando que no solo Latossolo Vermelho-Amarelo textura argiloarenosa assumir que o gradiente é unitário resultou em estimativas errôneas dos valores de K(θ), causadas pela falta de homogeneidade no perfil do solo.

Different sensitivity of isoprene emission, respiration and photosynthesis to high growth temperature coupled with drought stress in black poplar (Populus nigra) saplings

The effects of the interaction between high growth temperatures and water stress on gas-exchange properties of Populus nigra saplings were investigated. Water stress was expressed as a function of soil water content (SWC) or fraction of transpirable soil water (FTSW). Isoprene emission and photosynthesis (A) did not acclimate in response to elevated temperature, whereas dark (R(n)) and light (R(d)) respiration underwent thermal acclimation. R(d) was ~30% lower than R(n) irrespective of growth temperature and water stress level. Water stress induced a sharp decline, but not a complete inhibition, of both R(n) and R(d). There was no significant effect of high growth temperature on the responses of A, stomatal conductance (g(s)), isoprene emission, R(n) or R(d) to FTSW. High growth temperature resulted in a significant increase in the SWC endpoint. Photosynthesis was limited mainly by CO(2) acquisition in water-stressed plants. Impaired carbon metabolism became apparent only at the FTSW endpoint. Photosynthesis was restored in about a week following rewatering, indicating transient biochemical limitations. The kinetics of isoprene emission in response to FTSW confirmed that water stress uncouples the emission of isoprene from A, isoprene emission being unaffected by decreasing g(s). The different kinetics of A, respiration and isoprene emission in response to the interaction between high temperature and water stress led to rising R(d)/A ratio and amount of carbon lost as isoprene. Since respiration and isoprene sensitivity are much lower than A sensitivity to water stress, temperature interactions with water stress may dominate poplar acclimatory capability and maintenance of carbon homeostasis under climate change scenarios. Furthermore, predicted temperature increases in arid environments may reduce the amount of soil water that can be extracted before plant gas exchange decreases, exacerbating the effects of water stress even if soil water availability is not directly affected.

Ozone deposition onto bare soil: A new parameterisation

The variables controlling ozone deposition onto bare soil are still unknown and it is necessary to understand this pathway well, as it represents a significant sink for ozone. Eddy-covariance measurements of ozone (O 3) fluxes were performed over bare soils in agricultural land. Three datasets with contrasted meteorological conditions and soil nitric oxide (NO) emissions were used to study the factors controlling soil deposition. It is considered that ozone deposition can be represented with an aerodynamic resistance ( R a), a quasi-laminar boundary layer resistance ( R b O 3 ), and an additional resistance, named soil resistance ( R soil). Although it is assumed in previous studies that soil resistance is a function of soil water content (SWC) and could be considered constant as variation of SWC at monthly scale are generally weak, the results of this study indicate that SWC is not the main factor controlling R soil which shows daily and hourly variations. The main factor controlling soil resistance is the surface relative humidity which is positively correlated with R soil, contrary to non stomatal resistance onto canopies which show a negative correlation with relative humidity. The relationship between R soil and the surface relative humidity is probably due to a decrease in the surface available for ozone deposition, due to an increasing adsorption of water molecules onto the ground with relative humidity. A new parameterisation of R soil was established, where R soil is a function of the surface relative humidity only ( R soil = R soil min × e ( k×RH surf) , and R soil min = 21 ± 1.01 s m −1 and k = 0.024 ± 0.001, mean ± SD). The measured and parameterised ozone deposition velocities agree well when soil NO emissions are negligible. However, when there are large soil NO emissions, the parameterised ozone deposition strongly underestimates the measured deposition velocity even if the chemical destruction of ozone by reaction with NO in the air column was evaluated to be negligible. This suggests that soil NO emissions enhance soil ozone deposition by chemical reaction at or near the soil surface. The new parameterisation allows a better estimation of soil deposition, especially during daytime when R soil is overestimated using previously published parameterisations. It is an important step towards a better parameterisation of the non-stomatal uptake of ozone.

Performance of Extended Shuttleworth-Wallace Model for Estimating and Partitioning of Evapotranspiration in a Partial Residue-Covered Subsurface Drip-Irrigated Soybean Field

Estimation of actual evapotranspiration (ET), especially its partitioning into plant transpiration (T) and soil evaporation (E), in agricultural fields is important for effective soil water management and conservation and for understanding the interactions between ET, T, and E with the management practices. Direct field measurements of ET, T, and E rates are difficult and costly; hence, mathematical models are used for estimating them. The objective of this study was to evaluate the practical applicability of the Shuttleworth-Wallace (S-W) model to estimate and partition ET in a subsurface drip-irrigated soybean (Glycine max L. Merr.) field with partial residue cover. While its performance has been studied for various surfaces, the performance evaluation of the S-W model for such surface has not been carried out. An integrated approach of calculating bulk stomatal resistance (r s c ) as a function of soil water content (θ i ) was incorporated into the model to allow simulation of T over a range of θ i , and a residue decomposition function was introduced to account for surface residue decay over time to more accurately account for the actual residue cover in field conditions. The model performance was evaluated for different plant growth stages during the 2007 and 2008 growing seasons at the University of Nebraska-Lincoln, South Central Agricultural Laboratory near Clay Center, Nebraska. The sum of estimated T and E was compared to the Bowen Ratio Energy Balance System (BREBS)-measured actual ET on a daily time-step. The model was able to capture the trends and magnitudes of measured ET, but its performance differed for various plant physiological growth stages. The root mean square difference (RMSD) values between the model-estimated and measured ET values for the growing season (day after emergence until physiological maturity) were 1.26 and 1.03 mm d -1 for 2007 and 2008, respectively. Best performance was observed during the mid-season during full canopy cover with a two-year average r 2 of 0.87, average RMSD of 0.94 mm d -1 , and average mean biased error (MBE) of 0.30 mm d -1 . Estimates for both initial and late season growth stages where E was dominant had the least agreement with BREBS measurements. The proportion of T and E in the estimated ET varied with growth stage. The S-W-estimated seasonal total ET and BREBS measurements were equal in 2007 (S-W model ET = 496 mm and BREBS ET = 498 mm), and in 2008 the model underestimated by only 8.2% (S-W model ET = 452 mm and BREBS ET = 489 mm). While, in general, the model was successful in tracking the trends and magnitude of the BREBS-measured ET, further re-parameterization of the T module of the model can improve its accuracy to estimate ET, especially T, during the initial and late season (before full canopy cover and after physiological maturity) for a subsurface drip-irrigated soybean canopy. Other enhancements needed in the model for improved estimation of the E component include accurate determination of soil surface resistance coefficients and accounting for direct evaporation of intercepted rainfall on the canopy.

Characteristics and modeling of evapotranspiration over a temperate desert steppe in Inner Mongolia, China

The magnitude and seasonal dynamics of evapotranspiration (ET) over a temperate desert steppe in Inner Mongolia, China, were investigated during a growing season (from 1st May to 15th October 2008) using the eddy covariance (EC) technique. Diurnal variations of latent heat flux (LE) at different phenological stages showed similar patterns with the peak value around noon. The sensible heat flux dominated the latent heat flux at midday during the whole growing season. Seasonal variation in ET followed closely the variation in soil water content (SWC) and leaf area index (LAI). The maximum daily ET rate was 3.8 mm day−1. Cumulative ET during the study period estimated directly by EC method was 150.9 mm, exceeding the precipitation (PPT) received at the site during the same period (133.0 mm) by 13.4%. The daily crop coefficient (Kc, calculated as the ratio of the measured ET (from the EC system) to the reference ET (from the Penman–Monteith model)) also showed a distinct seasonal pattern, however its value on average was considerable lower (0.17) during the measurement period than the results reported from other grasslands, suggesting that this steppe was strongly affected by the deficit of available water. SWC was the most important environmental factor controlling ET in this steppe. The correlation analysis between daily Kc and its major environmental controls indicated that SWC, relative humidity (RH), and net radiation (Rn) were the major explanatory variables for daily Kc. Daily Kc showed a linear increase with SWC and Rn, and an exponential increase with RH. Thus, a daily Kc model would be developed as a function of SWC, RH, and Rn. An empirical daily ET model derived from the FAO 56 crop coefficient approach was developed based on this daily Kc model and the Penman–Monteith model. This ET model was validated by the data from the growing season in 2009, and showed a good consistency between the simulated and the measured ET values. Based on the developed ET model, an alternative ET model with the variable of SWC replaced by PPT was established, which may have the advantage of ET estimation under PPT more easily available than SWC.

An evaluation of empirical regression models for predicting temporal variations in soil respiration in a cool-temperate deciduous broad-leaved forest

Soil respiration (<TEX>$R_S$</TEX>) is a critical component of the annual carbon balance of forests, but few studies thus far have attempted to evaluate empirical regression models in <TEX>$R_S$</TEX>. The principal objectives of this study were to evaluate the relationship between <TEX>$R_S$</TEX> rates and soil temperature (ST) and soil water content (SWC) in soil from a cool-temperate deciduous broad-leaved forest, and to evaluate empirical regression models for the prediction of <TEX>$R_S$</TEX> using ST and SWC. We have been measuring <TEX>$R_S$</TEX>, using an open-flow gas-exchange system with an infrared gas analyzer during the snowfree season from 1999 to 2001 at the Takayama Forest, Japan. To evaluate the empirical regression models used for the prediction of <TEX>$R_S$</TEX>, we compared a simple exponential regression (flux = <TEX>$ae^{bt}$</TEX>Eq. [1]) and two polynomial multiple-regression models (flux = <TEX>$ae^{bt}{\times}({\theta}{\nu}-c){\times}(d-{\theta}{\nu})^f:$</TEX> Eq. [2] and flux = <TEX>$ae^{bt}{\times}(1-(1-({\theta}{\nu}/c))^2)$</TEX>: Eq. [3]) that included two variables (ST: t and SWC: <TEX>${\theta}{\nu}$</TEX>) and that utilized hourly data for <TEX>$R_S$</TEX>. In general, daily mean <TEX>$R_S$</TEX> rates were positively well-correlated with ST, but no significant correlations were observed with any significant frequency between the ST and <TEX>$R_S$</TEX> rates on periods of a day based on the hourly <TEX>$R_S$</TEX> data. Eq. (2) has many more site-specific parameters than Eq. (3) and resulted in some significant underestimation. The empirical regression, Eq. (3) was best explained by temporal variations, as it provided a more unbiased fit to the data compared to Eq. (2). The Eq. (3) (ST <TEX>$\times$</TEX> SWC function) also increased the predictive ability as compared to Eq. (1) (only ST exponential function), increasing the <TEX>$R^2$</TEX> from 0.71 to 0.78.

Unsaturated Hydraulic Conductivity Estimation of a Forest Soil Assuming a Stochastic‐Convective Process

Measurement of soil hydraulic conductivity requires considerable time and effort, which makes it difficult to characterize this important parameter across larger areas, especially remote forest regions. Forest soils are frequently texturally coarser than those in agricultural areas, making them more probable candidates for applications building on the stochastic‐convective hypothesis. We developed a method for measuring unsaturated soil hydraulic conductivity based on the analysis of a dye tracer resident concentration profile. In the experiment, partially saturated, steady‐state water flow was established in a forest allophanic soil by a sprinkler operating at 50 mm h−1, after which the water application was switched to a 10 mg L−1 Brilliant Blue FCF solution. The tracer was applied continuously until its cumulative infiltration reached 125 mm, after which the stained soil profile was exposed and photographed. The picture was subjected to an image analysis procedure designed to obtain the resident concentration profile of the tracer. The concentration was fitted to the solution of the convective lognormal transfer function, whose parameters were used in further calculations using functional relationships derived from the stochastic‐convective framework. The resulting hydraulic conductivity as a function of soil water content agreed within an order of magnitude with the relationship obtained by the instantaneous profile method. While the Mualem–van Genuchten model better reproduced the shape of that relationship, it strongly underestimated the hydraulic conductivity across the soil water content interval of interest (0.1–0.4). Finally, ways to improve the predictive capacity of the stochastic‐convective approach in terms of the general trends of the functional relationship were proposed.

앙상블 칼만필터를 연계한 추계학적 연속형 저류함수모형 (I) : - 모형 개발 -

본 연구의 목적은 현재 국내 홍수예경보 시스템의 유출해석모형으로 이용되고 있으며 단일 호우사상에 대해 적용이 가능한 유역 및 하도 저류함수모형을 추계학적 연속형 저류함수모형으로 개발하고자 하는데 있다. 이를 위해 기존 저류함수모형에 토양수분 산정 컴포넌트를 추가하고 지표면유출, 중간유출, 지하수유출 및 실제증발산량을 토양수분의 함수로 나타내어 각 수문성분에 대한 연속적인 모의가 가능하도록 하였다. 또한 실시간 관측유량자료 동화를 위해 앙상블 칼만 필터 기법을 도입하여 확정론적 모형을 추계학적 모형으로 개선하였다. 따라서 본 연구에서 개발된 추계학적 연속형 저류함수모형은 장기간의 연속적인 유출해석이 가능할 뿐만 아니라 관측자료 동화를 통해 기존 저류함수 모형보다 신뢰성 있는 결과를 제시할 수 있을 것으로 판단된다. The objective of this study is to develop a stochastic continuous storage function model for enhancement of an event-oriented watershed and channel storage function models which have been used as an official flood forecast model in Korea. For this study, soil moisture accounting component is added to the original storage function model and each hydrologic component, such as surface flow, subsurface flow, groundwater flow and actual evaportranspiration, is simulated as a function of soil water content. And also, ensemble Kalman filtering technique is used for real-time assimilation of measured streamflow from various stream locations in the watershed. Therefore the enhanced model will be able to simulate hydrologic components for long-term period without additional estimation of model parameters and to give more accurate and reliable results than those from the existing deterministic model due to the assimilation of measured streamflow data.

Measurement of the Dielectric Properties of Wyoming Soils Using Electromagnetic Sensors

We developed soil‐specific calibrations for a time domain reflectometry (TDR) sensor, a water content reflectometer (WCR), and an impedance sensor for 19 soil moisture monitoring sites in Wyoming. The main objective was to calibrate the WCR sensors that form a statewide distributed soil moisture network. A secondary objective was to use the TDR and impedance sensor data to better understand the WCR readings. All calibrations were conducted in the laboratory by mixing different amounts of water in the soils to obtain a range of soil water contents and by packing the soils in a bucket. The observed apparent permittivity as a function of soil water content was relatively high for the WCR and impedance sensors (up to 187 and 150% higher, respectively, than TDR). This was attributed to the relatively low operating frequency (&lt;175 and 50 MHz, respectively) of the WCR and impedance devices. Soil texture had little impact on sensor readings. Differences between soils for the WCR and impedance sensors were due to differences in bulk soil electrical conductivity (EC) and dielectric relaxation losses. Relaxation losses were found to be significant at 50 MHz for all soils. Regression equations (R2 &gt; 0.643) were developed for the WCR and impedance sensors that relate the slope of the soil water content vs. permittivity relationship to the bulk soil EC (WCR and impedance sensors) or the imaginary permittivity (impedance sensor).

Ectomycorrhizal fungi enhance nitrogen and phosphorus nutrition of Nothofagus dombeyi under drought conditions by regulating assimilative enzyme activities

Drought stress conditions (DC) reduce plant growth and nutrition, restraining the sustainable reestablishment of Nothofagus dombeyi in temperate south Chilean forest ecosystems. Ectomycorrhizal symbioses have been documented to enhance plant nitrogen (N) and phosphorus (P) uptake under drought, but the regulation of involved assimilative enzymes remains unclear. We studied 1-year-old N. dombeyi (Mirb.) Oerst. plants in association with the ectomycorrhizal fungi Pisolithus tinctorius (Pers.) Coker & Couch. and Descolea antartica Sing. In greenhouse experiments, shoot and root dry weights, mycorrhizal colonization, foliar N and P concentrations, and root enzyme activities [glutamate synthase (glutamine oxoglutarate aminotransferase (GOGAT), EC 1.4.1.13-14), glutamine synthetase (GS, EC 6.3.1.2), glutamate dehydrogenase (GDH, EC 1.4.1.2-4), nitrate reductase (NR, EC 1.6.6.1), and acid phosphomonoesterase (PME, EC 3.1.3.1-2)] were determined as a function of soil-water content. Inoculation of N. dombeyi with P. tinctorius and D. antartica significantly stimulated plant growth and increased plant foliar N and P concentrations, especially under DC. Ectomycorrhizal inoculation increased the activity of all studied enzymes relative to non-mycorrhizal plants under drought. We speculate that GDH is a key enzyme involved in the enhancement of ectomycorrhizal carbon (C) availability by fuelling the tricarboxylic acid (TCA) cycle under conditions of drought-induced carbon deficit. All studied assimilative enzymes of the ectomycorrhizal associations, involved in C, N, and P transfers, are closely interlinked and interdependent. The up-regulation of assimilative enzyme activities by ectomycorrhizal fungal root colonizers acts as a functional mechanism to increase seedling endurance to drought. We insist upon incorporating ectomycorrhizal inoculation in existing Chilean afforestation programs.

Modelling pesticide volatilization after soil application using the mechanistic model Volt'Air

Volatilization of pesticides participates in atmospheric contamination and affects environmental ecosystems including human welfare. Modelling at relevant time and spatial scales is needed to better understand the complex processes involved in pesticide volatilization. Volt'Air-Pesticides has been developed following a two-step procedure to study pesticide volatilization at the field scale and at a quarter time step. Firstly, Volt'Air-NH 3 was adapted by extending the initial transfer of solutes to pesticides and by adding specific calculations for physico-chemical equilibriums as well as for the degradation of pesticides in soil. Secondly, the model was evaluated in terms of 3 pesticides applied on bare soil (atrazine, alachlor, and trifluralin) which display a wide range of volatilization rates. A sensitivity analysis confirmed the relevance of tuning to K h . Then, using Volt'Air-Pesticides, environmental conditions and emission fluxes of the pesticides were compared to fluxes measured under 2 environmental conditions. The model fairly well described water temporal dynamics, soil surface temperature, and energy budget. Overall, Volt'Air-Pesticides estimates of the order of magnitude of the volatilization flux of all three compounds were in good agreement with the field measurements. The model also satisfactorily simulated the decrease in the volatilization rate of the three pesticides during night-time as well as the decrease in the soil surface residue of trifluralin before and after incorporation. However, the timing of the maximum flux rate during the day was not correctly described, thought to be linked to an increased adsorption under dry soil conditions. Thanks to Volt'Air's capacity to deal with pedo-climatic conditions, several existing parameterizations describing adsorption as a function of soil water content could be tested. However, this point requires further investigation. Practically speaking, Volt'Air-Pesticides can be a useful tool to make decision about agricultural practices such as incorporation or for the estimation of overall pesticide volatilization rates, and it holds promise for time specific dynamics.

Spatial variability in the growth of direct-seeded rainfed lowland rice ( Oryza sativa L.) in northeast Thailand

Large within-field variation in rice growth often causes production loss in broadcast-seeded (BC) rainfed lowland rice. The spatial variability of direct-seeded rainfed lowland rice was evaluated in 2004, 2005, and 2007 in on-station experiments at Ubon Ratchathani, northeast Thailand, in relation to soil water content and weed infestation, by adopting semivariogram and block kriging, including comparisons among BC with harrowing (BCH; no weeding), BC with no harrowing (BCNH; no weeding), and row-seeded (RS; interrow weeding once) fields. BCH and BCNH were also compared in 11 farmers’ fields in 2006 and 2007, to assess the effect of harrowing on rice growth and weed infestation. During most of the rice growing periods, flooded and non-flooded portions existed simultaneously in the fields, with different proportions among years and among seeding methods in the on-station experiment. BCH and BCNH rice had large within-field variation in seedling density, heading date, shoot dry matter, grain yield, harvest index, panicle density, and filled spikelet per panicle, as well as in weed infestation, measured by a quick visual estimation. Many of the measured variables (except mean soil water content in RS in 2007, seedling density in BCH in 2005 and 2007, shoot dry matter in BCH and BCNH in 2007, and panicle density in BCH in 2007) were spatially dependent (i.e., data from nearby locations were most similar) by geostatistical analysis. Analysis of correlations using the 420 data sets of BCH plots in 2005 and BCH, BCNH, and RS plots in 2007 revealed a positive correlation between soil water content and grain yield and negative correlations between weed infestation and soil water content and grain yield. Compared with BCH, in 2007 BCNH had much lower grain yield because of lower soil water content after establishment and more weed infestation. BCH had higher grain yield than BCNH in weedy fields in the farmers’ fields experiment. RS with interrow weeding resulted in a smaller coefficient of variation, smaller sill value, and higher grain yield than BCH, due to less weed infestation and a higher proportion of flooded water. These results indicated that reducing the spatial variability in rice growth requires careful field preparation, such as harrowing to level the soil surface and to reduce the uneven distribution of standing water and the variability in soil water content, combined with effective crop and weed management (i.e., harrowing and row-seeding). This is the first study that examined spatial variability in the growth of direct-seeded rice as a function of soil water content and weed infestation in a rainfed lowland environment.

Responses of surface conductance to forest environments in the Far East

The dependence of surface conductance G s on various variables in a forest environment is important for characterising the spatiotemporal variations of water, energy and CO 2 exchange between the forest and the atmosphere. Using a Jarvis-type model, we examined the variation of G s in five different mature forests from three climate zones (boreal, cool- and warm-temperate) in the Far East. First, we applied the model using summertime G s data from each site separately (within-site analysis). We evaluated the maximum surface conductance G smax and parameters related to the response of G s to the environments. We found that these values differed among the locations. Second, we applied the model for pooled G s data from all the sites and calculated a common parameter set (pooled analysis). In the pooled model, the difference in the observed G smax among the sites was expressed as a function of soil water content and the leaf area index. In addition, the responses of G s to radiation, vapour pressure deficit and air temperature in different forests were also closely represented by common response functions. As a result, the pooled model could estimate the variation of G s at each site using one parameter set with similar precision to the within-site models. These results suggest that the surface conductance of the various mature forests had the same maximum value and response properties, although we were not able to verify this. Our new parameterisation concept for pooled G s data should be effective for simultaneous evaluations of the water, energy and CO 2 exchanges of forests over wide regions.

Determination of soil hydrophysical characteristics in the Ivankovo Reservoir watershed

The capillary-sorption potential and the unsaturated hydraulic conductivity of soils as functions of soil water content are derived for forest, meadow, and agricultural ecosystems of the Ivankovo Reservoir watershed. The parameters of van Genuchten-Mualem equations were determined for the same soils based on data on soil particle size distribution and density. Computer code HYDRUS-1D and the obtained data were used to calculate the values of field capacity of soils in forest, meadow, and agricultural ecosystems in the drainage basin of the Ivankovo Reservoir.

Estimation of Osmotic Suction from Electrical Conductivity and Water Content Measurements in Unsaturated Soils

Abstract Salt-affected soils are a common feature of an arid-zone landscape. Osmotic suction arises from the salt content in the soil pore fluid. In this research osmotic potential was obtained by measuring total and matric suction via the filter paper technique. The filter paper technique has been widely adopted for measuring soil water potential in the recent years and now is a U.S. standard. One thousand and twenty samples of clay, silty clay loam, sandy clay loam, and loam having different levels of soil water content, between the field capacity (F.C.) and permanent wilting point (P.W.P.) defined as the soil water content below which plants wilt permanently, and ten levels of soil salinity were tested. The measuring tests were repeated three times and aimed at measuring the osmotic suction. By stepwise multiple regression analysis, four new empirical relations were obtained to estimate the osmotic suction as a function of soil water content and the electrical conductivity of saturation extracts (ECe).The results also indicate that the estimation of osmotic suction using the filter paper technique is inexpensive, relatively simple, and applicable to wide ranges of soil salinity.

Estimating evapotranspiration from small watersheds using a water and energy balance approach

AbstractMeteorological and environmental data measured in semiarid watersheds during the summer monsoon and winter periods were used to study the interrelationships among flux, meteorological and soil water variables, and to evaluate the effects of these variables on the daily estimation of actual evapotranspiration (AET). The relationship between AET and potential evapotranspiration (PET) as a function of soil water content, as suggested by Thornthwaite–Mather and by Morton, was studied to determine its applicability to the study area. Furthermore, multiple linear regression (MLR) analysis was employed to evaluate the order of importance of the meteorological and soil water factors involved.The results of MLR analysis showed that the combined effects of available energy, soil water content and wind speed were responsible for more than 70% of the observed variations in AET during the summer monsoon period. The analyses also indicate that the combined effects of available energy, vapour pressure deficit and wind speed were responsible for more than 70% of the observed variations in AET during the winter period. However, the test results of two different approaches, using the relationships between AET and PET as a function of soil water content, indicated some inadequacy. Copyright © 2007 John Wiley &amp; Sons, Ltd.

A mathematical model for variation in water-retention curves among sandy soils

AbstractEquations were developed to predict soil matric potential as a function of soil water content, texture and bulk density in sandy soils. The equations were based on the additivity hypothesis - that water-retention of a whole soil depends on the proportions of several particle size fractions, each with fixed water-retention characteristics. The new model is an advancement over previously published models in that it embodies three basic properties of water-retention curves: a) matric potential is zero at saturation water content, b) matric potential approaches -∞ as water content approaches zero, and c) volumetric water content in dry soil is proportional to bulk density. Values of model parameters were taken from the literature, or estimated by fitting model predictions to data for sandy soils with low organic matter content. Most of the variation in water-release curves in the calibration data was explained by texture, with negligible effects of bulk density and sand particle size. The model predicted that variation in clay content among soils within the sand and loamy sand textural classes had substantial effects on water-retention curves. An understanding of how variation in texture among sandy soils contributes to matric potential is necessary for interpreting biological activity in arid environments.