THE WATER BALANCE OF AN AGRICULTURAL CATCHMENT III THE WATER BALANCE

The catchment of the Kingston Brook has an area of 57 km², mainly under pasture (56%) and arable crops (36%). Changes of soil water content, measured with a neutron probe from April 1969 to March 1973, were analysed to determine evaporation (summer only) and drainage. From measurements of rainfall and runoff, supplemented by Penman estimates of evaporation (in winter), water storage is estimated month by month. Annual mean values (mm) were: rainfall (559), evaporation (398), runoff (157). During the summer, the measured decrease in soil water storage contributed ca 100 mm to evaporation and drainage and there is evidence of delayed recharge (about 30 mm) during the winter. A linear relation between annual rainfall and annual runoff is interpreted in terms of (i) a fixed catchment storage (125 mm); (ii) a small and nearly constant winter evaporation (ca 49mm); (iii) summer evaporation of 125 mm from storage plus a constant fraction (0.57) of contemporary precipitation. Summer evaporation was restricted by the supply of rain in every year from 1969 to 1976. By estimation, 500 mm of summer rain is needed to maintain potential evaporation, and the deficit at which actual evaporation falls below the potential rate was about 40 mm. Replacing the pasture by cereals would increase runoff by about 10% because of the shorter growing season.     

THE WATER BALANCE OF AN AGRICULTURAL CATCHMENT II. CROP EVAPORATION : SEASONAL AND SOIL FACTORS

Rates of evaporation, soil water deficits and extraction depths for spring cereals and grass were estimated from soil water measurements for a four year period. Estimates of evaporation were also compared with evaporation formulae and evaporimeter values. Evaporation and soil water deficit were highly dependent upon weather, particularly rainfall. Evaporation from grass was noticeably restricted whenever deficits exceed 50 millimeters. Cereals extracted water from a slightly greater depth of soil but developed deficits, up to harvest, very similar to those under grass. Deficits developed in sand and clay soils were also very similar, because water held in large dense clay aggregates is not readily accessible to roots. Although soil type had only minor influence on rates of evaporation, deficits and depths of drying, because of movement of water through fissures, slow drainage and delayed swelling effects soil type has a strong influence on stream behaviour.     

层状土中毛管水上升的实验研究

土壤盐渍化的重要机制是地下水中可溶盐在蒸发作用下不断向表土聚集;次生盐渍土的形成则可能是由于人为抬高地下水位,土壤毛管水重新分配土中盐分所致.不管怎样,防止土壤盐渍化的积极而彻底的方法是消除地下水位抬高的因素.但是,全面而根本的措施,往往限于客观条件不易做到,因此大多数都采取治标措施.例如,利用耕耘增加毛管水上升的困难,以及洪淤压碱等等.     

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.     

覆膜不同开孔程度蒸发条件下土壤水热变化动态研究

覆膜灌溉播前铺膜通常无法覆盖全田面 ,而播种后需为作物破土预留出苗孔 ,覆膜灌溉时还需增设灌水孔 ,对上述各类孔的问题 ,可采用覆膜开孔率来统一表征。到目前为止 ,尚未见国内外关于覆膜开孔蒸发的水热耦合迁移方面的报道。在研制了室内蒸发水热动态观测实验系统的基础上 ,本文根据剖面初始含水率均匀的覆膜开孔蒸发土壤水分和温度的动态观测资料 ,分析了土壤温度变化规律及剖面水分的降低趋势 ,并建立了覆膜开孔蒸发的累积蒸发量、相对累积蒸发量及单位膜孔面积累积蒸发量随时间和不同覆膜开孔率变化的函数关系。分析结果表明 ,不同覆膜开孔率控制下的累积蒸发量与时间平方根之间均符合Gardner提出的裸土蒸发理论关系。该结果发展了传统的Gardner方程 ,拓展了该关系的应用途径 ,对今后进一步研究覆膜开孔蒸发的水热运动规律有重要意义。     

黄土丘陵区农田水分循环特征及土壤水分生态环境

在评价土壤水分性质的基础上,本文系统地讨论了黄土丘陵区农田水分循环特征及作物土壤水分生态环境,并从理论上解释了该区域土壤有效水利用率较高,土壤低温度仍表现出较高的有效性.同时指出,该区域土壤有效水库容大,对作物供水调节能力强;但因气候干旱,土壤水分亏缺严重,土壤只在生育期内对作物供水起调节作用,这是黄土丘陵区土壤水库效应的区域特征.此外,本文将土壤蒸发分为旱季和雨季两个时段,研究表明土壤蒸发最强的时段,在雨季而不在旱季.     

咸水灌溉沙地后的水盐运移规律

实地观测不同深度土壤含水率、基质势、土壤溶液含盐率 ,利用定位通量法和盐分均衡法 ,研究了咸水小畦灌条件下塔克拉玛干沙漠土壤的水盐运移规律。结果表明 :(1)在小畦灌条件下 ,停止灌水后 2 4h ,91.2 5 %的灌水渗入 15 0cm土层以下 ;停止灌水后 72h ,96.6%的灌水渗入 15 0cm土层以下 ,此时 ,0～ 15 0cm土层土壤平均含水率为 0 .0 5 3cm3cm- 3,此后土壤含水率缓慢下降。 (2 )秋季停灌后 ,土壤表面蒸发量在 2～ 6mm。停灌后 2h ,上行水区域在 0～ 2 0cm土层 ;停灌后 60h ,上行水区域在 0～ 90cm ;此后至停灌后 14 4h ,上行水深度稳定在地下 90cm。 (3 )咸水灌溉后 ,80cm以上土层土壤溶液含盐率明显下降 ,以下土壤溶液含盐率变化不明显。停灌 2 4h后 ,0～ 15 0cm土层液相盐分储量开始降低 ,至停灌后 14 4h ,0～ 15 0cm土层液相储盐量相当于灌水前的 5 3 .46%。 (4 )停止灌水后 ,10 0cm× 10 0cm地面日平均积盐量在 13～ 3 5g。     

喷灌条件下不同灌水处理冬小麦的叶水势特征

在喷灌区一季冬小麦的试验表明,冬小麦叶水势值在各生育期每天的1400左右发生明显变化;从生育期看,返青期叶水势低谷值出现较晚,持续时间短且恢复较快;孕穗和开花期低谷出现有提前和持续时间延长趋势;叶水势不完全随生育进程与土壤水分同步变化,原因为同一时期内土壤水分是造成叶水势差异的主导因素,但同时外界条件的变化也对叶水势的变化有一定的影响作用。前期叶水势在黎明前和中午都与浅层土壤水分关系密切;到开花和灌浆期叶水势则变为只在中午与浅层土壤水分关系密切,黎明前更多依赖深层土壤水分状况,表现为与深层土壤水分关系密切。喷灌条件下,当返青后灌水量超过150mm时,灌水量对叶水势的影响减小。     

土壤水空间分布结构的时间稳定性

通过在野外 3 5m× 3 5m的正方形试验区域高密度布置多个电极和定时扫描测定土壤电阻的方法 ,利用反演计算和土壤电阻率与含水量之间的关系 ,计算出了不同时间的土壤含水量在三维空间上的分布。通过对由此获得的含水量资料的分析 ,探讨了土壤水在三维空间上的分布特征及其在时间上的稳定性。结果表明 ,试验区的土壤含水量在三维空间上的分布是非均质的和各向异性的 ,这种非均质性不是随机的而是具有一定的结构或模式 ,这一结构或模式在时间上是相对稳定的。     

Development and evaluation of a soil water evaporation model to assess the effects of soil texture, tillage and crop residue management under field conditions

Abstract. A number of mathematical models to predict soil water evaporation are available in the literature which generally require complex input data. In the present study, a simple parametric model has been developed by coupling existing and newly developed equations to assess soil water evaporation and drainage under field conditions in relation to potential evaporation rate, soil texture, time and depth of tillage and crop residue management. The model has moderate input data requirements and predicts well the effects of tillage and crop residue management practices on soil water loss (evaporation+drainage) with multi-drying and -wetting cycles prevailing under natural conditions. The root mean squares of deviations between observed and predicted cumulative water loss at different periods of study were 0.82, 2.04, 2.31 and 1.74  cm for untreated, residue-mulch, tillage and residue-incorporated treatments, respectively. Simulation analysis on cumulative evaporation and evaporation rate has shown that the evaporation reduction with different combinations of tillage and crop residue followed the order of residue-undercut>residue-mulch>residue-incorporated>tillage. Thus, the magnitude of beneficial effects of crop residues and tillage on soil water evaporation reduction are associated with amount of residues, mode of residue management (mulched or incorporated in the soil) and time and depth of tillage.     

Soil water content measurements deliver reliable estimates of water fluxes: A comparative study in a beech and a spruce stand in the Tharandt forest (Saxony, Germany)

Previous studies have shown that soil water content can vary considerably within homogeneous sites. This small-scale variability of soil water is often neglected when studying water and carbon fluxes in forest ecosystems. In this paper, the small-scale variability of soil water was analyzed at two contrasting eddy-flux sites, a Norway spruce forest and a European beech forest. Simultaneous measurements of precipitation, eddy covariance, and sap flow, from soil water content readings, were used to answer the question of how representative soil water gain is during rainfall and evapotranspiration is during dry periods.Our study demonstrates that the spatial and temporal variability in soil water under spruce and beech was mainly due to the differences in soil properties and root intensity. This can be concluded from the fact that the pattern of soil moisture distribution and flow paths under the trees were generally stable throughout the season. As a tendency, areas with preferred accumulation of rainwater were mainly characterized by maximum soil water depletion. Therefore, the density of the installed water content sensors should correspond to the variability of soil properties as well as rooting intensity. Based on previous studies and our own results, it can be concluded that a horizontal and vertical distance between 10 and 30 cm is best suited for water content sensors to detect preferential flow paths and deliver reliable estimates of soil water balance.Despite the occurrence of preferential flow, we found that the soil water increase during rainstorm periods and the soil water depletion during dry periods can be estimated relatively well when the small-scale variability of soil properties is considered in the experimental setup. In general, the evaporation estimates based on eddy covariance, sap flow, and soil water balance were consistent. However, compared to the spruce site, at the beech site the gap between the evapotranspiration estimates based on eddy covariance and soil water balance were often relatively large. Differences in the spatial extent of these methods can only explain these discrepancies to a certain extent. We suggest that this might be mainly due to the lack of water content sensors in the immediate vicinity of the beech tree trunk. Thus, stemflow-induced wetting and subsequent drying around the trunk could not be monitored in our study. This may result in an underestimation of evaporation from the soil under beech using the soil water balance method compared to the eddy covariance method. Finally, soil water depletion under spruce led to a significant reduction of transpiration when the actual available plant soil capacity (AWC) was <40% of the potential AWC. In contrast to the spruce stand, a reduction of transpiration of beech due to water shortage was not observed.     

Estimating evaporation: A comparison between Penman, Idso-Jackson, and zero-flux methods

A field evaporation—drainage study was conducted to compare three methods of predicting evaporative losses from a bare soil. Two of the methods (modified Penman combination and Idso—Jackson) are dependent only on measurements of atmospheric parameters whereas the third method (plane of zero flux) is dependent only on measurements of soil parameters.A Captina soil profile was wet up and allowed to dry by evaporation and drainage. For the initial two days after infiltration ceased all three methods predicted similar evaporative losses. Differences between the three methods occurred when the soil moisture content at the soil surface controlled the evaporation rates. Under the three drying conditions the three methods behaved somewhat differently in the prediction of the amounts of water evaporated from the soil surface. Lower losses by evaporation were predicted by the Idso—Jackson and zero-flux methods. In the case of the Idso—Jackson method this result was attributed to the influence of clouds on albedo, the impact of wind and the importance of albedo in the predictive equation. For the zero-flux method the decrease in evaporation was due to lower soil water contents and matrix potentials near the surface which resulted in lower transport rates of water to the surface.     

Variability in topsoil texture and carbon content within soil map units and its implications in predicting soil water content for optimum workability

The ability to predict the timing of optimum soil workability depends on knowledge of the extent and structure of variability in main physical characteristics of the soil. Our objectives were to quantify the variability in texture and carbon content within soil map units in a small agriculture-dominated catchment in South-east Norway and to assess implications of variability in texture and carbon content on land management operations, using the predicted maximum water content for optimum workability as an example. Information from three different sources were used: a soil map (1:5000), a large sample grid (100 m spacing, 270 ha extent), and a small sample grid (10 m spacing, 2.25 ha extent). Readily available information on texture and organic matter content from the soil map was found to be of limited use for soil management due to broad textural classes together with deviations from the mapped main textural classes. There were significant differences in clay, silt and sand content between the different soil textural classes on the soil map. Statistical distributions within soil map units were generally either positively or negatively skewed and the coefficient of variation was intermediate, 15–50%. Most of the variation in both grids was spatially correlated. The large grid was dominated by a patchy structure, whilst the small grid showed a systematic trend with a gradual transition indicating fuzzy boundaries between map units in this catchment. The effective range for texture was 16 times larger in the large grid. Implications of variability in texture and carbon content on land management operations were assessed for the maximum water content for optimum workability (Wopt), predicted using pedotransfer functions. Wopt was usually in the same range as the water content at–100 kPa matric potential, indicating that considerable evaporation in addition to drainage is required for obtaining workable conditions in the field. The time required for obtaining the water content was estimated to about 5 days, which is longer than an average length of periods without precipitation in the area, median 3.7 days. Wopt predicted from the soil map deviated strongly from Wopt predicted from the sample grids. Comparing estimates of Wopt from the large grid with measurements in the small grid showed differences corresponding to ±2–3 days of evaporation.     

A model of ammonia volatilization from applied urea. V. The effects of steady-state drainage and evaporation

Rachhpal-Singh & Nye's model of ammonia volatilization is expanded to account for the effects of steady-state water movement by drainage or evaporation when the soil does not dry out to any great extent. The model shows how upward movement of water during evaporation increases volatilization by carrying urea-derived NH₄⁺ and HCO₃^? ions upward, thereby increasing the concentration of ammonia gas at the surface. Conversely, water drainage reduces volatilization by carrying the dissolved solutes into the soil. The model is used to assess the effects on volatilization of evaporating conditions and of irrigation or rainfall.     

Spatial and temporal variability of soil hydraulic conductivity in relation to soil water distribution, using an exponential model.

An internal drainage experiment was conducted to examine soil water content variability in space and in time. It is shown that the flux-gradient model used to describe water flow in field soil profiles, based on the Darcy–Buckingham equation, yielded results of extreme variability and questionable validity. The problem lies in the representativeness of a site due to soil variability, horizontally and vertically in time, which added to the character of the hydraulic conductivity versus soil water content relation, which in many cases can be approximated by exponential functions, leads to coefficients of variation up to 170% in the estimation of soil hydraulic conductivity values.     

蒸发条件下粘土层对土壤水和溶质运移影响的模拟

以土壤水和溶质运移的动力学原理为基础 ,采用数值模拟方法 ,研究了在浅层地下水和蒸发条件下含有粘土层土壤的水和Cl-的运移状况 ,重点探讨了两种粘土的层位和层厚对水和Cl-运移影响的差别及原因。研究结果表明 ,粘土层对土壤的水和溶质运移影响的程度 ,与层状土壤中该粘土及其组合土壤的水力学性质有关。本文模拟的重粘土 (简称Y粘土 )与轻壤土所组成的层状土壤 ,其基本情况为 ,随粘土层层位的升高和层厚加大 ,土壤水分蒸发和地下水补给速率降低 ,Cl-积累减少。而轻粘土 (简称R粘土 )与轻壤土所组成的层状土壤 ,由于它们的导水率曲线在压力水头h约 - 10 0 0cm处相交 ,当h低于此值时 ,R粘土的导水率就大于轻壤土的。因此 ,蒸发、补给速率和Cl-积累强度出现以顶位最高 ,甚至高于均质轻壤土 ,其次为底位 ,最低为中部层位的现象。在蒸发条件下Cl-在剖面中的积聚部位主要是土表。粘土层的存在 ,起到了阻滞作用 ,而阻滞程度则与该粘土水力学性质、层位、厚度和地下水埋深有关     

Soil survey interpretation: Estimating use-potentials of a clay soil under various moisture regimes

Current Dutch soil survey interpretation emphasizes assessment factors which independently define key aspects of soil behavior under actual conditions. For grassland these are: moisture-supply capacity, bearing capacity and drainage status. Practical questions focus on how actual limitations can be overcome. Computer simulation techniques, to be focused on the individual assessment factors, are needed to answer these questions. Soil survey and morphology data were used in this context to: (1) select experimental sites; (2) modify physical monitoring procedures; (3) derive simulation models for swelling soils with macropores; (4) develop simple field methods for characterizing basic physical soil properties and their regional variability, and (5) use soil maps for extrapolating the obtained interpretations.Future soil survey reports should ideally contain basic physical data and the possible ranges for the different assessment factors, expressed by simulation as a function of water management. basic data include hydraulic conductivity, moisture retention, bearing capacity and drainage rate. The latter two were characterized in this study by new field methods. Possible ranges differ for the different assessment factors. For example, an inadequate moisture supply capacity in the growing season can be completely compensated by raising the water table, by sprinkler irrigation or by a combination of both methods. The presented simulation offers a quantitative analysis. The inadequate bearing capacity and drainage status can be compensated for only partly by lower water-table levels in winter and spring.     

砂性层状土柱蒸发过程实验与数值模拟

为了了解不同类型层状土柱蒸发特性,利用砂土和砂黄土2种土壤,设置3种不同厚度分层土柱(11.25、22.5、45 cm)和2种均质对照土柱,测定了土柱蒸发过程中累积蒸发量、相对蒸发速率和剖面含水量的变化;同时利用2种均质土柱排水过程优化的土壤水力参数和Hydrus-1D模型对2种均质土柱和3种不同类型层状土柱蒸发过程进行模拟分析。结果表明,均质砂黄土蒸发第一阶段持续长达34 d,累积蒸发量显著高于均质砂土和其他3种不同类型分层土柱,土柱剖面含水量变化进一步证明表层覆盖砂土可显著抑制土壤蒸发。利用排水过程优化的水力参数,HYDRUS-1D可以较好地模拟层状土柱蒸发过程。研究结果对干旱半干旱区土壤水分管理具有指导意义。     

降雨入渗条件下厚包气带土壤水流通量的模拟与分析

降雨较少的半干旱半湿润地区大范围的厚包气带中土壤水运动的研究越来越受到关注,其中厚包气带对降雨补给浅层地下水特征的影响是重要的研究问题。探究厚包气带情形下降雨对浅层地下水的补给作用和机理可为分析人类干扰情况下水文循环规律的变化、合理地制定水资源开发利用规划提供理论依据。本文以北京市大兴区新凤河流域采育镇由GEOPROBE钻机获得的9.6m厚包气带土壤样品为研究对象,在室内详细测得土壤特性数据的基础上,运用HYSRUS-1D软件模拟55a降雨入渗条件下的一维土壤水流动态过程,分析土壤水流通量的时空变异特征,探讨用较浅层位(2m深度附近)的土壤水流通量来估算深层土壤水渗漏量的可行性,旨在为野外裸地厚包气带条件下评价降雨入渗对浅层地下水的补给提供简捷实用的方法。研究结果表明:该地区多年自然降雨条件下的平均土壤水分深层渗漏量为131.03mm,多年平均降雨入渗补给系数约为0.21。其中,平水年对应的平均降雨入渗补给系数约为0.18。当模拟时段达到55年时,土壤剖面2m以下的入渗通量几乎不随深度变化而变化。这一实际案例的研究结果对该区域宏观水资源管理和评价具有一定的参考价值。     

A SIMPLE COMPUTER MODEL FOR LEACHING IN STRUCTURED SOILS

A layer model for the movement of solutes in soil has been developed in which, as a working approximation, the soil solution is partitioned into mobile and retained phases. Only the mobile solution is displaced during water movement, and equilibrium is assumed to be established between the mobile and retained phases when flow ceases, giving holdback of solute. The model permits the displacement of the mobile solution through an indefinite number of layers when large amounts of rain fall. It can be assumed either that water and solute are stored in the bottom layer and can be withdrawn up the profile by evaporation or that they drain from the bottom layer. Predicted soil nitrate concentrations agreed reasonably well with those measured in a field experiment and indicated some sensitivity to layer thickness. The capacity of the model to predict concentrations in drainage is demonstrated.