Range Of Soil Moisture Contents Research Articles

Institution of Metal-Organic Frameworks as a Highly Sensitive and Selective Layer In-Field Integrated Soil-Moisture Capacitive Sensor.

The ongoing global industrialization along with the notable world population growth is projected to challenge the global environment as well as pose greater pressure on water and food needs. Foreseeably, an improved irrigation management system is essential and the quest for refined chemical sensors for soil-moisture monitoring is of tremendous importance. Nevertheless, the persisting challenge is to design and construct stable materials with the requisite sensitivity, selectivity, and high performance. Here, we report the introduction of porous metal-organic frameworks (MOFs), as the receptor layer, in capacitive sensors to efficiently sense moisture in two types of soil. Namely, our study unveiled that Cr-soc-MOF-1 offers the best sensitivity (≈24,000 pF) among the other tested MOFs for any given range of soil-moisture content, outperforming several well-known oxide materials. The corresponding increase in the sensitivities for tested MOFs at 500 Hz are ≈450, ≈200, and ≈30% for Cr-soc-MOF-1, Al-ABTC-soc-MOF, and Zr-fum-fcu-MOF, respectively. Markedly, Cr-soc-MOF-1, with its well-known water capacity, manifests an excellent sensitivity of ≈450% in clayey soil, and the analogous response time was 500 s. The noted unique sensing properties of Cr-soc-MOF-1 unveils the great potential of MOFs for soil-moisture sensing application.

三峡库区典型茶园土壤水分对不同降雨模式的响应

PDF HTML阅读 XML下载 导出引用 引用提醒 三峡库区典型茶园土壤水分对不同降雨模式的响应 DOI: 10.5846/stxb202201240232 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（42077065）；国家自然科学基金项目（U224022） Response of soil moisture in typical tea gardens to different rainfall regimes in the Three Gorges Reservoir Area Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:土壤水分是坡面产流和生物地球化学过程的关键控制因素。降水事件可以通过引起土壤剖面不同深度的土壤水分响应，从而影响流域中的径流路径、产流机制和土壤侵蚀过程等。基于三峡库区典型分布的茶园为对象，通过长期定点、高频的气象和水分数据观测，研究不同降雨模式下茶园不同土层深度（0-10、10-20、20-30、30-40cm）土壤水分的时空变化特征，分析茶园不同深度土壤在雨季的水分动态变化规律和对不同降雨模式的响应特征。结果表明：（1）研究区内的降雨和土壤水分含量均表现出明显的季节性特征。降雨在7月达到最大值，土壤水分含量则在8月达到峰值。表明降雨是影响土壤水分含量变化的重要因子，土壤水分对降雨有着明显的响应过程。（2）在相同降雨条件下，土壤含水量具有明显的垂直梯度变化。随着土层深度的增加，土壤水分对降雨的响应逐渐呈现出滞后现象。表层土壤（0-20cm）对降雨的响应较为迅速且幅度更加明显，深层土壤（30-40cm）水分含量变化相对稳定，并且对降雨的响应时间更加滞缓。随着土层深度的增加，土壤水分含量的变化幅度逐渐趋于平稳。（3）土壤水分含量对不同的降雨模式表现出显著差异。在较大雨强条件下，土壤水分变化出现了上升期、平台期及退水期。在小雨强短历时的降雨中，降雨对土壤水分的影响主要表现在土壤表层（0-10cm）。在降雨初期，降雨强度越大，土壤水分入渗越快，土壤的响应深度越深。研究三峡库区典型分布的茶园在不同降雨模式下的土壤水分变化特征，为三峡库区坡地的降水资源优化配置和园地面源污染防控提供理论依据，对区域水土流失治理以及面源污染物防控具有重要意义。 Abstract:Soil moisture is a key control factor for slope runoff generation and biogeochemical processes. Rainfall events can affect runoff paths, runoff mechanisms, and soil erosion processes in watersheds by causing soil moisture responses at different depths in the soil profile. Based on the typical tea gardens in the Three Gorges Reservoir Area, through long-term, fixed-point and high-frequency meteorological and moisture data observations, the temporal and spatial variation characteristics of soil moisture in different soil depths (0-10, 10-20, 20-30 and 30-40cm) of tea gardens under different rainfall regimes was studied, and the dynamic variation of soil moisture in the rainy season at different depths of tea gardens and the response characteristics to different rainfall regimes were analyzed. The results showed that:(1) The rainfall and soil moisture content in the study area showed obvious seasonal characteristics. Rainfall peaked in July and soil moisture levels peaked in August. It showed that rainfall was an important factor affecting the change of soil moisture content, and soil moisture had an obvious response process to rainfall. (2) Under the same rainfall conditions, soil water content had obvious vertical gradient changes. With the increase of soil depth, the response of soil moisture to rainfall gradually showed a hysteresis phenomenon. The topsoil (0-20cm) responds to rainfall more rapidly and the amplitude was more obvious, the change of water content in the deep soil (30-40cm) was relatively stable, and the response time to rainfall was more sluggish. With the increase of soil depth, the variation range of soil moisture content gradually became stable. (3) The soil moisture content showed significant differences with different rainfall regimes. Under the condition of high rainfall intensity, soil moisture changes appeared in the rising period, the plateau period and the retreating period. In the rainfall with small rainfall intensity and short duration, the effect of rainfall on soil moisture was mainly manifested in the soil surface layer (0-10cm). The greater the rainfall intensity, the faster the soil moisture infiltration, and the deeper the soil response depth were exhibited in the early stage of rainfall. Studying the characteristics of soil moisture variation characteristics in typical tea gardens in the Three Gorges Reservoir area under different rainfall patterns will provide a theoretical basis for the optimal allocation of precipitation resources and the prevention and controlling of ground source pollution in the gardens, which is of great significance to regional soil erosion control and the prevention and control of non-point source pollution. 参考文献 相似文献 引证文献

An Improved Exponential Model Considering a Spectrally Effective Moisture Threshold for Proximal Hyperspectral Reflectance Simulation and Soil Salinity Estimation

Soil salinization has become one of the main factors restricting sustainable development of agriculture. Field spectrometry provides a quick way to predict the soil salinization. However, soil moisture content (SMC) seriously interferes with the spectral information of saline soil in arid areas. It is vital to establish a model that is insensitive to SMC for potential in situ field applications. The soil spectral reflectance exponential model (SSREM) has been widely employed for reflectance simulation and SSC inversion. However, its reliability for saline soils with high SMC has not been verified yet. Based on hyperspectral remote sensing data (400~1000 nm) on 459 saline soil samples in Shiyang River Basin of Northwest China, we investigated the role of SMC and SSC in soil spectral reflectance from 29 October 2020 to 22 January 2021. Targeted at saline soils, soil spectral moisture threshold (MT) was introduced to improve the SSREM toward a modified spectral reflectance exponential model (MT-SSREM). The bands that are sensitive to SSC but not sensitive to SMC were obtained based on a method of correlation analysis between original spectra, four kinds of spectral data, and SSC. SSREM and MT-SSREM were finally applied to inversely estimate SSC. Results show that wavelengths at 658~660, 671~685, 938 nm were suitable for SSC estimation. Furthermore, although SSREM was able to simulate the spectral reflectance of most saline soils, its simulation accuracy was low for saline soil samples with high SMC (SMC > MT(i), 400 nm≤i≤1000 nm), while MT-SSREM performed well over the whole range of SMC. The simulated spectral reflectance from MT-SSREM agreed well with the measured reflectance, with the R2 being generally larger than 0.9 and RMSE being less than 0.1. More importantly, MT-SSREM performed substantially better than SSREM for SSC estimation; in the statistical performance of the former case, R2 was in range of 0.60~0.66, RMSE was in range of 0.29~0.33 dS m−1; in the latter case, R2 was in range of 0.10~0.16, RMSE was in the range of 0.26~0.29 dS m−1. MT-SSREM proposed in this study thus provides a new direction for estimating hyperspectral reflectance and SSC under various soil moisture conditions at wavelengths from 400 to 1000 nm. It also provides an approach for SSC and SMC mapping in salinization regions by incorporating remote sensing data, such as GF-5.

Three-Dimensional Model of Soil Water and Heat Transfer in Orchard Root Zone under Water Storage Pit Irrigation

To reveal the water and heat transfer characteristics of the orchard soil under water storage pit irrigation and to regulate the distribution of soil water and heat for improving apple quality and increasing yield, a 3D soil water and heat transfer model of orchards under water storage pit irrigation was established. The model not only considered the influences of root water uptake, precipitation, evaporation, and irrigation, but also simulated the infiltration process of the variable water head in the pit according to the principle of mass conservation and introduced the pit coefficient to simulate the difference in radiation in the pit to describe the influence of the pit on the model. Verify and analyze the simulation results. Results showed that the variation trend of simulated soil moisture and heat was consistent with that of measured data. The mean absolute percentage error, root mean square error, and mean absolute deviation were 3.23%, 0.9460, and 0.6984 for soil temperature and 10.05%, 0.0269, and 0.0214 for water content after irrigation, respectively. The simulation results have high accuracy and show that the soil moisture content centers on the pit with an ellipsoid distribution and tends to be uniform over time. The soil temperature was higher in the 4–5 cm area near the soil surface and the wall of pit, and it remarkably changed with time. The intraday variation of soil temperature was mainly affected by atmospheric temperature, but a certain lag was observed compared with the change of atmospheric temperature. With the increase of the irrigation amount, the distribution range of soil moisture content and water high value area increased, while the average and maximum soil temperature decreased. With the increase of irrigation water temperature to 18–24 h after irrigation, the soil temperature in the ellipsoidal area around the pit remarkably increased. The model established in this paper can be used to simulate the hydrothermal status of the soil in the field under water storage pit irrigation. The results prove that the water storage pit irrigation can effectively improve the hydrothermal status of the middle-deep soil and promote the root system of fruit trees to absorb water.

A comparative study on the role of electro-conductive plastics electrode in direct current electroosmosis

The electrode material is one of the critical factors affecting the electroosmosis efficiency. The electroosmotic test was conducted to compare the current, volume moisture content, and energy consumption of Electro-Conductive Plastics Electrode (ECPE) and metal electrode in the dispose of sludge. The results show that: the current decreases of ECPE is smaller than the metal electrode before 20h, and the difference value of moisture content between cathode and anode of ECPE is smaller when electroosmosis is stable. The energy consumption of ECPE is lower in the range of soil moisture content of 35% - 60%. The ECPE has a particular advantage in energy consumption and uniformity of soil moisture content, but has a disadvantage in residual moisture content as compared with the metal electrode.

The influence of soil matric pressure on the acidity and cation composition of soil solutions extracted by different methods

From the samples of gradually drying (beginning from the field moisture capacity, FMC) soddy-podzolic sandy loam soil (Albic Retisol (Loamic, Ochric)) contaminated with Zn were isolated the soil solutions (SS) by centrifugation, while from the other samples of the soil, moistened to FMC, various energy fractions of the SS were pressed using a hydraulic press. The acidity of the centrifuge solutions extracted from the soil of the experiment variant with the cultivation of plants (“barley”) was, on average, 1 pH higher than on “bare fallow”, and decreased with decreasing soil moisture content. The acidity of the solutions extracted from the soils of the “bare fallow” variant in the same range of soil moisture content practically did not change. It was also found that in more acidic solutions from the “bare fallow” variant the content of Ca, Mg, Mn, Zn and K was 2.4 – 6.9 times higher than the content of these metals in solutions from the “barley” variant. The positive correlation between pF and pH was clearly observed for the pressed solutions from the soils of both variants. Concentrations of Ca, Mg, Sr, Ba,Zn, K and Na in successively pressed fractions of SSs from more acidic soil of the “bare fallow” variant, on average, 3.0 – 8.4 times exceeded their concentrations in similar fractions of solutions of the “barley” variant and showed an inverse correlation with soil moisture (pF value).

Influence of sand gradation on compaction of loess soils

Assessing the interaction among soil organic carbon (OC), Atterberg limits and soil texture, especially sand gradation, in maximum bulk density (BDMAX) can improve prediction of soil compactibility. Our objectives were a) to compare the effect of total OC, complexed OC (COC) and silt + clay (S + C) content on maximum bulk density in cultivated (CULT) and uncultivated (UNCULT) soils, and b) to evaluate how useful it may be to group soils according to Atterberg limits and the sand uniform coefficient (CU) in order to improve BDMAX prediction. Sixteen pairs of CULT and UNCULT soils were sampled in the province of San Luis, Argentina. The standard Proctor test was used to determine BDMAX. The BDMAX was related to OC and S + C in both managements. The BDMAX was better explained by S + C in CULT (R2 = 0.52) and by OC in UNCULT (R2 = 0.62). The COC did not improve prediction compared to OC. The plastic limits of soil, also called Atterberg limits, determine a range of soil moisture contents in which soil has a plastic consistency. Consequently, non-plastic soils are those lacking plastic consistency. In all non-plastic CULT soils, BDMAX was not associated with S + C. However, when these soils were grouped according to CU, the BDMAX was linearly related to S + C, both in well-graded sands (R2 = 0.47) and in poorly graded sands (R2 = 0.27). On the other hand, the relationship between BDMAX and S + C in plastic soils was negative and linear (R2 = 0.96). In coarse-textured soils, soils with a higher CU (well-graded sands) were more easily compacted than soils with a lower CU (poorly graded sands). Therefore, according to this model, in CULT soils with the same S + C content, compaction is 8–10% higher for soils with well-graded sands than those with poorly graded sands.

Evaluation of canola chlorophyll index and leaf nitrogen under wide range of soil moisture

Abstract The paper presents a study on the effect of soil matric suction on the variation of leaf chlorophyll index and nitrogen concentration of canola. Results showed that chlorophyll index increases exponentially with soil matric suction, especially at the late season of canola growing time. At moderate matric suction (200 and 300 kPa soil suction heads), chlorophyll index remains nearly constant, but in drier soil (matric suction >300 kPa), chlorophyll index increases gradually with time. Despite the variation of the total leaf nitrogen with the soil matric suction, it is similar to the variation of the chlorophyll index, but the results showed that the chlorophyll index - nitrogen concentration curve has a demarcated bi-modal shape. We suggest that 2.7% of nitrogen and 69.8 of the chlorophyll index value represent the upper limit of the chlorophyll meter reliability for estimation of canola nitrogen under a wide range of soil moisture levels. These results confirm that the chlorophyll meter can be used as an effective tool for rapid and non-destructive estimation of the relative chlorophyll and nitrogen content in canola leaves at a wide range of soil moisture content, except for nearly wilting coefficient or extremely high drought stress

Evaluation of two end-member-based models for regional land surface evapotranspiration estimation from MODIS data

The importance of accurate estimations of regional and global evapotranspiration (ET) has been recognized worldwide in the fields of hydrology, water resource management, meteorology, and global climate change. Given the different structures of the end-member-based surface energy balance algorithm for land (SEBAL) and surface temperature–vegetation index (Ts–VI) triangle models, which nonetheless use essentially the same definitions of dry and wet pixels, questions arise regarding the nature of the differences between the two models’ estimated sensible heat fluxes and latent heat fluxes and what controls the heat flux differences. This study aims to investigate how the SEBAL model and the Ts–VI triangle method differ from each other in the regional evaporative fraction (EF) and ET estimates through analytical deduction and model applications. Because both the SEBAL model and the Ts–VI triangle model are of limited use when deep-layer soil moisture is moderately to significantly stressed, MODIS remote sensing data from 23 clear-sky overpass times between January 2010 and late October 2011, covering a wide range of soil moisture content and fractional vegetation cover conditions, are acquired to assess the two models over a non-water stressed study area on the North China plain. The results show that the SEBAL model is able to produce satisfactory sensible heat flux (H) and latent heat flux (LE) estimates compared with ground-based large aperture scintillometer measurements at the Yucheng station, with small biases of 4.1W/m2 and 2.3W/m2 and root mean square errors (RMSEs) of 46.4W/m2 and 48.6W/m2, respectively. However, the Ts–VI triangle model produces much worse H and LE estimates, with biases of 98.5W/m2 and −92.2W/m2 and RMSEs of 119.3W/m2 and 115.5W/m2, respectively. Variations of pixel-by-pixel surface available energy and momentum roughness length over the study area are responsible for the differences of the H and LE estimates between the two models. The SEBAL model produces larger EF and ET for most pixels than the Ts–VI triangle method when the same group of dry and wet pixels is applied, especially for pixels characterized by medium to high vegetation fractions or surface soil moisture contents. The Ts–VI triangle method is more sensitive to the surface temperatures of the dry and wet pixels than the SEBAL model. The findings from this study benefit the proper selection of end-member-based models for regional ET estimation and aid in quantifying the resultant uncertainties in the model-derived surface energy components.

Variability of Organic Matter Inputs Affects Soil Moisture and Soil Biological Parameters in a European Detritus Manipulation Experiment

Over the last three decades, increased temperatures and reduced annual precipitation have resulted in significant changes in several Central European deciduous forests. These effects include changes in soil moisture content and detritus production. Within the framework of a detritus manipulation experiment carried out in an old-growth Quercetum petraea–cerris community, we examined how changes in detritus inputs affect soil moisture content and microbial activity within six treatments. CO2 release and microbial enzyme activities are known to be highly sensitive to environmental factors such as soil moisture and detritus inputs. We applied three detritus removal (No Litter, No Roots and No Input) and two detritus addition (Double Litter and Double Wood) treatments. Although the plots received the same amount of precipitation, the various detritus inputs caused significant differences in soil moisture. Treatments excluding living roots had the highest moisture levels, while the treatment excluding only aboveground detritus inputs had the lowest. CO2 release, arylsulphatase activity and saccharase activity showed significant seasonal differences with the highest values occurring in spring. Moisture content had a significant positive correlation with CO2 release, and enzyme activities of the plots were affected by the quantity and quality of detritus inputs. Arylsulphatase activity showed the strongest correlation with soil moisture content (R = 0.62 in the control plot) followed by CO2 release (R = 0.61) and finally saccharase activity (R = 0.42). We observed that there was a remarkably weaker correlation between soil moisture content and the three parameters in the detritus removal treatments (R values between 0.56 and 0.13) than in the Control and detritus addition treatments (R values between 0.72 and 0.42). The correlation between the three parameters of interest and soil moisture content weakens considerably under drought conditions relative to the optimal moisture range of soil moisture content for microbial activity. If the amount of precipitation in the area continues to decrease as anticipated, then litter production and soil microbial activity may be reduced.

Evaluation of Nonnuclear Soil Moisture and Density Devices for Field Quality Control

When new transportation infrastructure is constructed or current infrastructure systems undergo maintenance, sufficient soil strength is critical to a successful construction effort. Currently, soil design specifications are given for a minimum soil density and a specified range of soil moisture content. Quality control is achieved by monitoring the soil density and moisture content throughout the construction process. The nuclear density gauge (NDG) is most commonly employed to determine soil density and moisture content because of its ease of use, speed of readings, and reliability of results. However, potential safety hazards and rigorous user certification requirements have led many agencies to seek alternative devices. This paper focuses on a portion of a much larger study that compared a wide range of compaction control devices; the paper also assesses the performance of devices that measure soil density and moisture content. Several new, commercially available alternatives for the measurement of soil density were tested on various soil types and conditions to determine which device performed best and most consistently. For the same soil types and conditions, several devices and techniques for the determination of soil moisture content were also tested. The combination of the TransTech soil density gauge and the heated frying pan–open flame field moisture content techniques represented the best alternative to the NDG.

Combined Transient Method for Determining Soil Hydraulic Properties in a Wide Pressure Head Range

Simulation of water flow in unsaturated soils requires knowledge of the soil hydraulic properties. Two standard methods for the simultaneous determination of the water retention and hydraulic conductivity function in the laboratory are the multistep-outflow and the evaporation method. The multistep-outflow method provides information primarily in the pressure head range corresponding to relatively moist conditions, whereas the evaporation method has its highest information content in the medium to dry range. In the moisture ranges not covered by the experiments, an extrapolation of the soil hydraulic functions leads to very uncertain and possibly incorrect estimations of hydraulic properties. To obtain reliable estimates of soil hydraulic properties in a wider range of soil moisture contents, we combined both methods in a successive manner. The combined experiment starts with a multistep-outflow experiment, which is directly followed by an evaporation experiment. We tested this experimental design using synthetic data and laboratory measurements evaluated by inverse modeling. In the evaluation of the combined experiment, data points for the retention and conductivity functions were calculated from the evaporation experiment and included in the objective function for the inverse simulation of the multistep-outflow experiment. The combined evaluation led to greatly improved estimates of the hydraulic properties in a wide moisture range, circumvented the unreliable extrapolation beyond the pressure head ranges of the separate experiments, and significantly reduced the model error caused by such extrapolations.

Effects of cattle treading and soil moisture on phosphorus and sediment losses in surface runoff from pasture

Cattle treading is known to influence surface runoff contaminants such as phosphorus (P) and suspended sediment (SS). The objective of this work was to determine how simulated cattle treading affects losses of P and SS in surface runoff from four soils (Brown, Melanic, Pallic and Recent Gley) with contrasting resistance to treading damage and drainage properties. Soils were excavated, placed in boxes (0.8 m long and 0.20 m wide) and one of three moisture treatments were imposed (10, 50 and 90% of available water holding capacity) before surface runoff was induced via simulated rainfall; P fraction and SS were determined in the resulting runoff. The Pallic and Recent Gley soils lost the most P and SS compared with the better structured Brown and Melanic soils. Greater losses of particulate phosphorous (PP) and SS were observed at higher soil moisture contents. In contrast, dissolved P in runoff was greater at lower soil moisture contents, especially dissolved reactive phosphorus (DRP), which accounted for up to 90% of total P losses at 10% soil moisture in the Recent Gley soil. Measurement of soil water soluble P, a surrogate for DRP at a range of soil moisture contents, suggests this could be due to lysis of soil microbial biomass upon drying. Although concentrations and loads of DRP in surface runoff were greatest at 10% moisture, the risk of surface runoff is less than the risk posed by PP and SS losses with treading on wet soils, where surface runoff is more likely. This was particularly true for the Pallic and Recent Gley soils. It is therefore recommended that, to minimise SS and P losses to surface water, care be taken when grazing these soils under wet winter–spring conditions. Although collected under controlled conditions, these data help to explain the wide range of potential SS and P losses that may occur in a field due to hydrologic and soil variation.

A new method for in situ soil gas diffusivity measurement and applications in the monitoring of subsurface CO2 production

Subsurface approaches to soil CO2 monitoring are becoming increasingly important for process studies in terrestrial carbon research. When used in conjunction with a diffusion model to determine CO2 production, subsurface methods require good estimates of effective soil gas diffusivity (De). Outlined here is a novel membrane probe and continuous flow system for in situ soil gas diffusivity measurements. Laboratory tests confirm performance across the range of CO2 diffusivities found in natural soils. Field tests were performed across a range of soil moisture contents by artificially hydrating eight soils over a period of three to seven days. These soils were representative of a range of textural classes in eastern Nova Scotia, Canada. The absolute values of diffusivity, and also the rate at which diffusivity decreased with increasing soil moisture content were typically quite different from model predictions. When applied to subsurface CO2 monitoring at two sites, the site‐specific diffusivity measurements greatly improved the accuracy of CO2 production estimates. We observed a consistent and close correspondence between calculated profile CO2 production and (independently measured) soil CO2 surface flux. The subsurface CO2 production estimates acquired using in situ gas diffusivity measurements allows detailed vertical profile resolution to be constructed over time. The majority of CO2 was generated at shallow depths, but periodic contributions from deeper depths were important, particularly towards the end of the growing season.

A simple method to estimate actual evapotranspiration from a combination of net radiation, vegetation index, and temperature

Satellite remote sensing is a promising technique for estimating global or regional evapotranspiration (ET). A simple and accurate method is essential when estimating ET using remote sensing data. Such a method is investigated by taking advantage of satellite measurements and the extensive ground‐based measurements available at eight enhanced surface facility sites located throughout the Southern Great Plains (SGP) area of the United States from January 2002 to May 2005. Data analysis shows that correlation coefficients between ET and surface net radiation are the highest, followed by temperatures (air temperature or land surface temperature, Ts), and vegetation indices (enhanced vegetation index (EVI) or normalized difference vegetation index (NDVI)). A simple regression equation is proposed to estimate ET using surface net radiation, air or land surface temperatures and vegetation indices. ET can be estimated using daytime‐averaged air temperature and EVI with a root mean square error (RMSE) of ∼30 W m−2 and a correlation coefficient of 0.91 across all sites and years. ET can also be estimated with comparable accuracy using NDVI and Ts. More importantly, the daytime‐averaged ET can also be estimated using only one measurement per day of temperatures (the daytime maximum air temperature or Ts) with comparable accuracy. A sensitivity analysis shows that the proposed method is only slightly sensitive to errors of temperatures, vegetation indices and net surface radiation. An independent validation was made using the measurements colleted by the eddy covariance method at six AmeriFlux sites throughout the United States from 2001 to 2006. The land cover associated with the AmeriFlux sites varies from grassland, to cropland and forest. The results show that ET can be reasonably predicted with a correlation coefficient that varies from 0.84 to 0.95 and a bias that varies from 3 W m−2 to 15 W m−2 and RMSE varying from ∼30 W m−2 to ∼40 W m−2. The positive bias partly comes from the energy imbalance problem encountered in the eddy covariance method. The proposed method can predict ET under a wide range of soil moisture contents and land cover types.

The association between three dominant shrub species and water repellent soils along a range of soil moisture contents in semi‐arid Spain

AbstractSoil water repellency (SWR) is a widespread phenomenon associated with decreased infiltration rates, fingered flow infiltration, and increased runoff. In Spain, research into the causes of SWR has focused on fire‐induced effects. This research investigates the impact of different plant species and soil moisture contents (SMCs) on the water repellency of the surface soil horizon in a semi‐arid shrubland ecosystem. The plant species were selected for their different wax input rates into the soil. In situ (field) SWR was investigated spatially, whereas in the laboratory it was assessed over a soil moisture range. The selected plant species were Stipa tenacissima, Anthyllis cytisoides and Rosmarinus officinalis.In the field, water repellency was found to be limited strictly to an organic soil layer (‘mulch’) between the soil crust and the ectorganic profile (litter, fragmentation and humification layers). When comparing the three species, soil under Stipa was found to be wettable or slightly water repellent, soil under Anthyllis and Rosmarinus severely water repellent. The water repellency of the surface soil appeared to be related to the thickness of the ectorganic profile.The soil organic carbon (SOC) contents correlated with water repellency (r2 = 0·74, p = 0·01). Water repellency displayed a two‐peak distribution over a soil moisture range. The first peak ranged from 22 to 26% SMC (w w−1), the second peak from 45 to 47%. The integrated area in a ‘WDPT versus SMC’ graph was calculated. Soil under Stipa, Anthyllis and Rosmarinus was water repellent in the ratios 1: 2·3: 4·5. The integrated area measurement had a better reproducibility and was more stable than SWR measurements at specific SMCs.It is concluded that, in the investigated ecosystem, SWR occurs without the influence of fire but underneath individual plants (fertile islands) only; plant species affect SWR substantially differently; and SWR occurs over a wide range of SMCs. Appropriate measures of SWR are discussed. Copyright © 2007 John Wiley & Sons, Ltd.

Variation of surface albedo and soil thermal parameters with soil moisture content at a semi-desert site on the western Tibetan Plateau

Almost three years of continuous measurements taken between January 2001 and May 2003 at the Gaize (or Gerze) automatic weather station (32.30 °N, 84.06 °E, 4420 m), a cold semi-desert site on the western Tibetan Plateau, have been used to study seasonal and annual variations of surface albedo and soil thermal parameters, such as thermal conductivity, thermal capacity and thermal diffusivity, and their relationship to soil moisture content. Most of these parameters undergo dramatic seasonal and annual variations. Surface albedo decreases with increasing soil moisture content, showing the typical exponential relation between surface albedo and soil moisture. Soil thermal conductivity increases as a power function of soil moisture content. The diffusivity first increases with increasing soil moisture, reaching its maximum at about 0.25 (volume per volume), then slowly decreases. Soil thermal capacity is rather stable for a wide range of soil moisture content.

Monitoring soil moisture helps refine irrigation management

Soil moisture sensors can be used to determine the appropriate interval between irrigation, depth of wetting, depth of extraction by roots and adequacy of wetting. We tested the performance of soil moisture sensors in different crops. Sensors that read on a continuous basis, such as the Enviroscan device, can provide valuable information that may not be readily evident from periodic measurements. The Watermark blocks responded well throughout the wetting and drying cycles, indicating that they function more consistently over a wider range of soil moisture contents compared with tensiometers and gypsum blocks.

FACTORS AFFECTING COLOR-BASED WEED DETECTION

Color differences between weeds and crops have been used to develop an image-based sensor to detect weedsin wheat and soybean fields for post-emergency herbicide spot spray. Several field factors were found to affect thecorrect-classification rate (CCR) of the sensor. CCR is defined as the ratio between the number of weed-stem pixelscorrectly classified and the total number of weed-stem pixels in an image. The first factor was soil moisture content.Images of weeds and soil at 10 different moisture contents were analyzed using a statistical method. It was found that,within a large variation range of soil moisture content, the values of relative color indices of soil varied within asufficiently narrow range, which did not seriously overlap with the range for weed stems. As a result, variations in soilmoisture content did not cause significant misclassification between soil and weeds. The second factor was illumination.Images of standard RGB primary-color plates were taken at nine different illumination intensities. The effect ofillumination on color indices was found significant. However, the variation ranges of the relative color indices on imagesof the color plates did not overlap, and the variations of the index values followed noticeable patterns, which could beused in color-index calibration. The third factor studied was spatial resolution. Nine different spatial resolutions of weedimages were achieved by taking images at different camera-plant distances. The optimum resolution, at which the CCRwas maximized, was found to be about 0.5 mm. The classifier was tested on field images obtained under natural lightingconditions. The classifier detected 28-day old yellow foxtail and redroot pigweed with CCRs of 40.2% and 54.9%,respectively. The misclassification rate (MCR), which was defined as the ratio between the number of non-weed stempixels misclassified as weed stem and the total number of non-weed stem pixels, was below 0.2% for both weed species.

Importance of environmental factors on the growth of Thermoactinomyces thalpophilus

The germination and growth of endospores from Thermoactinomyces thalpophilus in two soils, a sand silt loam and a silt clay loam was investigated under a range of environmental conditions. A 50-fold increase in the recovery of T. thalpophilus was observed after 28 d growth in silt clay loam soil, when compared to the original inoculum (8.85×10 4 cfu g −1 dry soil), with similar results obtained for this soil over a range of soil moisture contents (5–50% w/w) and at temperatures of 30 and 45°C. In contrast, no increase in the recovery of T. thalpophilus was detected when endospores (2.53×10 5 cfu g −1 dry soil) were inoculated into sand silt loam soil. However, when sterile farmyard manure (2% w/w) was added to sand silt loam soil, a 50-fold increase in the numbers of T. thalpophilus isolated was observed over 28 d incubation. Analysis of the sand silt loam soil showed that organic C and N content, pH, and the C-to-N ratio of the soil had all increased significantly upon addition of farmyard manure, resulting in the growth of T. thalpophilus endospores in a soil which previously could not sustain growth. Addition of manure to the silt clay loam led to a 70% increase in recovery of T. thalpophilus. Growth and respiratory measurements of T. thalpophilus released into sterile manure confirmed the ability of these endospores to show rapid germination, with respiratory activity detected after 7 h, reaching a maximum value (700 μmol CO 2 h −1 g −1 dry manure) 11 h after inoculation, resulting in a 600-fold increase (final recovery, 2.38×10 9 cfu g −1 dry manure) in the recovery of T. thalpophilus. We conclude that T. thalpophilus is capable of growth in soils at temperatures as low as 30°C and under a range of soil moisture conditions. Further, enhanced growth in soils with additions of manure could be observed. These results support the idea that Thermoactinomyces endospores isolated from ancient soils containing agricultural debris may have remained dormant in soil following a brief period of rapid growth immediately following deposition.