Determining soil water‐balance components using an irrigated grass lysimeter in NE Austria

The water balance of a certain soil profile in a certain time interval is subjected to changes of soil water content within the respective profile, and fluxes at its upper and lower boundary. Weighing lysimeters are valuable instruments for water‐balance studies. Typically, mass changes—thus, changes of soil profile water content—are detected by a weighing system, while seepage water outflow is measured by a tipping bucket, and precipitation data originate from a rain gauge. Hence, evapotranspiration as unknown component can be determined by solving a simple water‐balance equation. However, using separately measured precipitation data may cause implausible (negative) evapotranspiration. In this study, change of soil profile water content, seepage water, precipitation, and evapotranspiration were determined directly from 10‐min lysimeter data from 2011. Precipitation measured with the lysimeter was in total 20% greater than rain‐gauge values. Even dew formation was measured and considered as water‐balance component; however, its total amount was rather low. Evapotranspiration calculated on daily and hourly base indicated a sound correlation with measured data, but measured values were considerably smaller. Both calculated and measured dew amount were of the same magnitude. Comparison of lysimeter evapotranspiration with daily calculations (neglecting dew) and hourly computation (considering dew) delivered similar results. Generally, the utilized lysimeter facility and the specific data management provided sound water‐balance components with high accuracy and temporal resolution, respectively.     

芒萁根系对崩岗红土层土壤抗剪强度的影响

为探究植物根系对崩岗红土层土壤抗剪强度的作用机制,采取室内直剪试验,研究不同含水率条件下芒萁（Dicranopteris dichotoma）根系对红土层抗剪强度的影响。结果表明：（1）不同根重密度土体的抗剪强度总体随含水率增加而下降,含根土体的抗剪强度大于素土。（2）黏聚力随着含水率的增加均呈先增大后减小的趋势,而随根重密度的增加呈增大的趋势,且增量逐渐减小。（3）内摩擦角与土壤含水率之间呈线性负相关,但与根重密度之间无明显关系。（4）含水率对抗剪强度的影响大于根重密度,可用含水率和根重密度模拟根土复合体的抗剪强度（NSE=0.84）。综上,根系的作用可增加崩壁红土层土壤的抗剪强度,但高含水率条件下根系的增强效应降低,可通过减少水分注入以增加崩壁根土复合体的稳定性。     

利用土壤表层含水量序列预测深层含水量的研究

土壤剖面含水量的预测对于灌溉、防治水土流失、改善生态环境等一系列环境过程具有重要意义。根据每天测量红壤不同层次含水量,利用时间序列分析方法,依据表层10cm含水量序列预测20cm,30cm,40cm和60cm土层含水量。结果表明,各不同土层含水量之间呈极显著性相关;利用分布滞后模型根据10cm土层含水量预报各深层土壤含水量,模型的滞后时间随着被预报土层深度的增加而增加,预报模型相对误差不超过6%,最大相对误差不超过10%。10 cm土层含水量分别联合20 cm,30 cm,40 cm和60 cm土层含水量,利用自回归分布滞后模型对相应各土层含水量模拟预报,缩短了滞后时间,模型表达式更简洁,精度仍然较高。     

利用热脉冲技术测量土壤表层含水率

准确测定表层土壤水分对陆地-大气间水热交换研究具有重要意义。由于对土壤结构影响轻微,热脉冲技术在原位监测含水率方面具有较大优越性,但目前田间应用集中在5 cm以下土层。该研究利用多针热脉冲传感器测定土壤容积热容量,然后基于热脉冲含水率法和热脉冲含水率变化法分别得到了3、9、21和39 mm的土壤含水率。结果表明,与烘干法含水率比较,热脉冲含水率变化法含水率在4个深度的均方根误差分别为0.022、0.006、0.004和0.006 m3/m3,均小于相应深度上热脉冲含水率法含水率的均方根误差。另外,热脉冲含水率变化法也降低了4个热脉冲传感器测定含水率的变异性。因此,热脉冲技术能够监测表层的土壤水分动态,表层土壤含水率的均方根误差在0.022 m3/m3以内。     

Performance of a non‐nuclear resonant frequency capacitance probe. III. error analysis

Abstract

All techniques used to measure soil water content have errors associated with them. These errors are further propagated when soil volumetric water contents are numerically integrated over a number of depth intervals to estimate the soil water storage for the entire profile. Previous studies had field‐tested the performance of a commercial capacitance probe (Troxler Sentry 200‐AP). The probe was calibrated and the measured data on crop water use were compared with that reported previously in literature using other instruments. However, the errors associated with the field estimates of soil water storage and crop water use measured with the Sentry 200‐AP capacitance probe were not evaluated. The objective of this study was to analyze these associated errors. Equations were derived to evaluate the errors incurred in calculating the volumetric water content using the fitted field calibration curve for the Sentry 200‐AP capacitance probe and in the estimates of the crop water use. This study suggests that the error in making in situ measurements of soil water content in heterogeneous soil systems using the Sentry 200‐AP capacitance probe depends on the degree of saturation at the point of measurement in the soil profile. The Sentry 200‐AP capacitance probe produces errors of >5% in measuring soil volumetric water contents of >35% in heterogeneous soil systems.     

Performance of pedotransfer functions in predicting soil water characteristics of soils in Norway

Abstract

Pedotransfer functions (PTFs), predicting the soil water retention curve (SWRC) from basic soil physical properties, need to be validated on arable soils in Norway. In this study we compared the performance of PTFs developed by Riley (1996), Rawls and Brakensiek (1989), Vereecken et al. (1989), Wösten et al. (1999) and Schaap et al. (2001). We compared SWRCs calculated using textural composition, organic matter content (SOM) and bulk density as input to these PTFs to pairs of measured water content and matric potential. The measured SWRCs and PTF input data were from 540 soil horizons on agricultural land in Norway. We used various statistical indicators to evaluate the PTFs, including an integrated index by Donatelli et al. (2004). The Riley PTFs showed good overall performance. The soil specific version of Riley is preferred over the layer specific, as the latter may introduce a negative change in water content with increasing matric potential (h). Among the parameter PTFs, Wösten's continuous PTF showed the overall best performance, closely followed by Rawls&B and Vereecken. The ANN-based continuous PTF of Schaap showed poorer performance than its regression based counterparts. Systematic errors related to both particle size and SOM caused the class PTFs to perform poorly; these PTFs do not use SOM as input, and are therefore inappropriate for soils in Norway, being highly variable in SOM. The PTF performance showed little difference between soil groups. Water contents in the dry range of the SWRC were generally better predicted than water contents in the wet range. Pedotransfer functions that included both SOM and measured bulk density as input, i.e. Wösten, Vereecken and Rawls&B, performed best in the wet range.     

Comparison of two computer models for predicting soil water in a tropical monsoon climate

A physical soil water model, based on Darcy's Law and the continuity equation, and a water budget model were tested for three crops grown in the humid coastal zone of northeastern Brazil. Both models required daily information on precipitation, class A pan evaporation, and used Soil Conservation Service curves to estimate runoff. Crop canopy and rooting characteristics, needed on a daily basis, were similar for both models. Both models predicted soil water contents that compared favorably with measured values, and predicted evapotranspiration and drainage were probably superior to estimates from water balance calculations based on periodic measurements of soil water content and suction. The water budget model would have been improved if the “field capacity” had been based on the highest measured water contents in the rainy season. The models were more sensitive to rooting depth than to root distribution. Rate of canopy development had little effect on predicted drainage and evapotranspiration, but significantly changed the ratio of evaporation to transpiration.     

陕北丘陵区陡坡柠条林地与荒坡土壤水分变化研究

通过定点土壤水分测定与对比分析，研究了陕北丘陵区陡坡柠条林地与荒坡的土壤水分亏缺状况，年际年内动态变化规律，干燥化特征及其自然降水的补偿能力。结果表明，柠条林地与荒坡0-10m土层贮水量仅相当于田间持水量的26．2％～41．2％。荒坡地0-10m土层贮水量相当于田间持水量的39．8％～41．2％。土壤贮水量的分布是阳坡〈半阳坡〈阴坡，上坡位〈下坡位。年际间土壤水分的变异程度随土壤深度的增加而减弱，土壤贮水量的变化主要发生在2m以上土层内。土壤贮水量具有明显的季节变化特征，但滞后于降雨量变化。生长季内，柠条地与荒坡的土壤平均贮水量之间差异显著，土壤越深，其含水量变化程度越小。两种利用方式的土壤剖面都产生了不同程度的干化层。相比而言，柠条林地深层土壤干燥化强度明显大于荒坡地。丰水年柠条林雨水补偿深度仅为1．0m，荒坡也仅为1．2m。柠条林丰水年的雨水补偿深度比干旱年可增加60cm以上，5m土层贮水增量增加3倍以上。     

Water retention and hydraulic conductivity of a loamy sand soil as influenced by crop rotation and fertilization

Measurements are reported of soil organic carbon content, dry bulk density, water retention characteristics, and saturated hydraulic conductivity of a sandy loam soil with two different crop rotations and two levels of fertilization. The water retention characteristics were fitted to the van Genuchten equation. Values of unsaturated hydraulic conductivity were estimated by calculation. It was found that crop rotation has much larger effects on these soil physical properties than fertilization. Water retention and hydraulic conductivity are greater when mustard, and clover with grass are included in the crop rotation, but only at water contents greater than 0,10 and 0, 13 kg kg^?1respectívely.     

Wasserhaushalt und Bodenwasserverteilung bei einer Futtermaiskultur in Südchile

Water balance and distribution in the soil on maize in southern Chile A water balance on maize in a Dystrandept in southern Chile was determined. With raingauges and stemflow rings the amount of rainfall was divided into throughfall, stemflow and interception. The variation with time of the water content in the soil was determinated by tensiometers and gypsum-blocks located in the soil in and between plant rows. The water distribution on the soil surface was very heterogeneous depending on the plant development. The stemflow increased with plant growth and it is the principal factor of the irregular distribution of water in the soil profile. In the plant rows and up to a medium soil depth the matrix potential was greater than between rows. Small precipitations only have a significant effectiveness on the soil because of stemflow.     

SIMULATION OF THE WATER BALANCE OF SOIL COLUMNS AND FALLOW SOILS

A model is described that predicts the evaporation of water from, and the distribution of water in, a soil column evaporating into a constant environment. It is based on a numerical solution of the flow equation and requires only the initial water distribution in the column, the equilibrium (air-dry) water content at the soil surface and the relationship between volumetric water content and diffusivity. The model predictions show good agreement with a published analytical solution and with experimental results. Modifications to the model that allow for rewetting of the sod by rainfall, and changes in atmospheric conditions above the soil, enable predictions to be made of the water balance of a fallow field. In general, good agreement was obtained with the measured distribution of water deficits in the soil profile, although the predicted water content of the surface 2.5 cm of soil showed systematic differences from the measured values. The reasons for this are discussed.     

WATER RETENTION BY OSMOTIC SWELLING OF CERTAIN COLLOIDAL CLAYS WITH VARYING IONIC COMPOSITION

Experimental data and theoretical calculations of water retention by osmotic swelling of colloidal homoionic Na- and Ca-montmorillonites, vermiculites, and kaolinites were carried out at several levels of electrolyte concentration, in order to evaluate their respective contributions to the physical response of a soil system to changes in its chemical environment. Similar experiments were conducted for mixed-ion Na-Ca-montmorillonite systems to establish whether a threshold value exists for cation proportions in the exchange positions, at which the physical properties of the system show a sudden change. The results are projected as relationships between the chemical treatment and moisture contents at three equilibrium pressures. Theoretical calculations, based on the diffuse double-layer model, predict that montmorillonite systems with a certain form of cation saturation and salt content should have greater swelling pressure or higher capability for water retention than vermiculites, which have greater values than kaolinites under similar chemical conditions. This was confirmed by the experimental data, although measured values were consistently higher than those predicted by theory. The studies on mixed-ion montmorillonites show that mutual existence of the two cation species in the clay-water system produces a nearlinear relationship between exchangeable Na percentage and the moisture retention at a particular equilibrium pressure.     

东北黑土区TDR测定农田土壤含水量的室内标定

土壤水分标定是利用TDR监测农田土壤水分过程中的必要环节.以东北黑土区农田土壤剖面深度上的4种典型介质（黑土层、母质层、过渡层、砂砾层）为试验对象,利用“由湿到干”的土柱试验方法,用烘干法对TDR进行室内标定研究.结果表明：1）TDR内置土壤标定曲线测定的含水量明显低于烘干法测量的含水量.在TDR法土壤含水量标定时,指数关系比线性关系具有更高的标定精度.2）单一介质的TDR法土壤含水量标定曲线对自身介质的标定精度高（最大绝对误差3.2％）.3）混合介质的TDR法土壤含水量标定曲线的标定精度较高（最大绝对误差6.6％）,可用于不同介质TDR法土壤含水量标定.本研究结果可用于研究区及类似区域修订TDR法含水量测定结果.     

松嫩平原西部不同土地利用方式盐渍化效应研究

在2003年6月~10月选择农田、草地和碱斑地作为典型的土地利用方式,对0~120 cm土层水盐时空动态进行了对比分析,结果发现:农田和草地土壤剖面含水量都有“上升—下降—上升”的趋势,但是在8月中旬之前农田含水量高于草地,而碱斑地含水量的变化呈“U”型。三种土地利用方式土壤剖面含盐量的变化趋势与含水量相反;土壤剖面6月7日含盐量最大,10月17日最小;农田和碱斑地土壤剖面各层含盐量在8月8日开始上升,而草地在7月7日已开始上升;从垂直剖面看,农田各个时刻土壤含盐量都随剖面深度的增加而上升,碱斑地则随着剖面深度的增加而下降,草地在各个时刻土壤剖面含盐量均呈先上升后下降型,在土壤剖面有峰存在。采样期内,草地有良好的蓄水作用,而农田土壤剖面含盐量下降幅度最大,水分、盐分在土壤剖面运移剧烈。     

Effects of Long-Term Inorganic and Organic Fertilization on Soil Micronutrient Status

Soil micronutrients were studied on loess soil with an 18-year long-term experiment. The results indicated that total soil iron and copper contents were similar under all treatments, but total soil manganese and zinc contents were significantly greater at the surface soil in the fertilized plots than in the controls, and total manganese contents were significantly greater in the whole soil profile under manure plus inorganic fertilizers than under controls. Generally, application of inorganic fertilizers had no effects on available soil micronutrient contents. The straw plus inorganic fertilizers significantly increased available manganese content at surface soil and available iron in subsurface soils. However, manure plus inorganic fertilizers significantly augmented soil-available iron contents throughout the profile, and raised available manganese, copper, and zinc contents, respectively, at surface soil relative to controls. The results suggest that long-term input of organic amendments alter the properties of soil and increase its plant-available micronutrient contents.     

The interrelationship between the cultivation of crops and soil‐strength dynamics

It is well accepted that the penetration resistance of soils is, among others factors, highly sensitive to the moisture status of the soil. This study tested the hypothesis of whether the dewatering of a soil by crops of varying dewatering capacities significantly affects the soil's penetration resistance and whether this contributes to an exceedance of the commonly accepted root‐growth threshold already in the range of plant‐available water. During a 22‐month period between March 2002 and December 2003, the soil water content of a former lignite strip mine in E Germany was studied. The soil had been restored with Saalian glacial till. Plots contained two different crops, a 3 y–old stand of lucerne (Medicago sativa L.) and a 7 y–old stand of wild rye (Secale multicaule L.). Soil water contents under the two crops were converted on the basis of the water‐retention characteristics into water tensions, allowing an investigation of the changes in the measured water content in the wider context of the water availability to the crops. During both growing seasons, the water tension under lucerne exceeded the permanent‐wilting point (10^4.2 hPa) for up to 20 weeks between 0 and 90 cm, which is equal to a predicted penetration resistance of >15 MPa. Water tensions under the wild rye rose only up to a maximum of 10³ hPa for the same period, so that the predicted penetration‐resistance values remained constantly <5 MPa. Our findings demonstrate that the dewatering by plants during the growing seasons affects the actual strength of the soil, which can lead to the exceeding of the commonly accepted root‐growth threshold.     

Temporal stability in soil water content patterns across agricultural fields

When a field or a small watershed is repeatedly surveyed for soil water content, locations can often be identified where soil water contents are either consistently larger or consistently less than the study area average. This phenomenon has been called temporal stability, time stability, temporal persistence, or rank stability in spatial patterns of soil water contents. Temporal stability is of considerable interest in terms of facilitating upscaling of observed soil water contents to obtain average values across the observation area, improving soil water monitoring strategies, and correcting the monitoring results for missing data. The objective of this work was to contribute to the existing knowledge base on temporal stability in soil water patterns using frequent multi-depth measurements with Multisensor Capacitance Probes (MCPs) installed in a coarse-texture soil under multi-year corn production. Water contents at 10, 30, 50, and 80 cm depths were measured every 10 min for 20 months of continuous observation from May 2001 to December 2002. The MCPs revealed temporal stability in soil water content patterns. Temporal stability was found to increase with depth. The statistical hypothesis could not be rejected (P < 0.0001) that data collected each 10 min, each 2 h, each day, and each week had the same temporal stability. The locations that were best for estimating the average water contents were different for different depths. The best three locations for the whole observation period were the same as the best locations for a month of observations in about 60% of the cases. Temporal stability for a specific location and depth could serve as a good predictor of the utility of this location for estimating the area-average soil water content for that depth. Temporal stability could be efficiently used to correct area-average water contents for missing data. Soil water contents can be upscaled and efficiently monitored using the temporal stability of soil water content patterns.     

Modelling and measurement of the nitrogen cycle in a vegetable field in Switzerland II. Validation of the nitrogen model with field data

Field experiments were conducted over a period of two years on a commercial vegetable farm for calibration and validation of the nitrogen model (tables 1 and 2). Yield and nitrogen uptake of the crops were measured at least five times during crop growth. Soil water tension and nitrate concentration in the soil solution were assessed by frequent sampling of different soil layers. The volumetric soil water content was calculated from continuously measured water potential by means of a field desorption curve. The model was calibrated using a transfer factor for rapid and slow water transport corresponding to changes in water content observed during a leaching experiment in winter 1987/88. Other required input parameters were taken from existing knowledge about the growth of vegetable crops, an evaluation of the soil profile to assess water transport in the soil, and the results of an incubation experiment for the estimation of the net nitrogen-mineralisation. Yield and nitrate uptake of different vegetables are predicted with an accuracy of about 15% for yield and of about 20 kg N/ha for nitrate uptake (table 4, figure 1). Measurement of water and nitrate transport under fennel and a fallow plot (figure 2) reveals that the simulated net nitrogen-mineralisation is overestimated by about 30 percent, and that simulation of rooting depth and evapotranspiration should be improved. Nitrate leaching during summer takes place only after periods of very heavy rain. The accuracy of the agreement between simulated and measured data is within 10% for the water content, and within 25% for the nitrate concentration in the soil solution, while measured nitrate leaching exceeded the simulated values by up to 80 kg N/ha. The precision of the model seems to be satisfactory in view of the large amount of nitrogen and water cycled in a vegetable field (table 5). However with respect to water management improvements are necessary. Further validation of the model on other sites is required before a predictive model for practical purposes can be elaborated.     

砒砂岩-黄土沟谷土壤含水量的时空变化

以晋、陕、蒙交界地区常见的砒砂岩-黄土丘陵沟谷为研究对象,分别于2005年7月和lO月对沟谷不同部位土壤含水量进行了测定.结果表明,其时空变化特征明显.(1)沟谷不同部位土壤平均含水量大小为7月份:沟谷底部14.44%,阳坡顶部9.29%,阴坡顶部9.01%,阴坡中部8.25%,阳坡中部6.36%;10月份:沟谷底部9.96%,阴坡顶部9.81%,阳坡中部9.48%,阴坡中部9.09%,阳坡顶部8.06%.(2)砒砂岩平均含水量季节变幅小,属稳定型;黄土及有冲积层的黄土平均含水量季节变化较大,属波动型.(3)在砒砂岩剖面上,泥岩含水量显著大于砂岩,砒砂岩坡面平缓的地段贮水能力比陡的地段大.(4)在黄土剖面上,明显出现土壤干层现象,在夏季、秋季降雨较少的情况下,土壤水分会严重降低,土壤水分处于负补偿状态,土壤干层距地表更深.在半干旱地区的砒砂岩-黄土丘陵沟壑区,黄土剖面土壤干层的发生是一个自然现象,随降雨量的多寡,呈消长状态.