Geospatial estimation of soil moisture in rain-fed paddy fields using SCS-CN-based model

Paddy fields are characterized by standing water and saturation condition during the entire crop growth period. However, in sub-humid and semi-arid areas, scarce rainfall and intermittent dry spells often cause soil moisture depletion resulting in unsaturated condition in the fields. These distinctive characteristics of the paddy fields have significant influence on the runoff generation and soil moisture retention characteristics of the watershed. In this study, the objective is to extend the application of the Soil Conservation Services Curve Number (SCS-CN)-based models for the geospatial and temporal simulation of soil moisture to paddy field-dominated agricultural watersheds in the water scarce areas. Different SCS-CN-based models, integrated with the soil moisture balance equation, are used to estimate the surface runoff and soil moisture content wherein, the spatial variation in the soil hydraulic characteristics is used to calculate the geospatial variation in soil moisture content. Physical significance of the terms initial abstraction (I_a) and potential maximum retention (S) in these models and their influence on the estimation of runoff and soil moisture are analysed in detail. A new SCS-CN-based model for soil moisture simulation (SCS-CN-SMS), to improve the soil moisture estimation, is proposed in this paper. The proposed model is built up on the soil moisture balance equation to account for the effect of ponding condition and soil moisture variation between the dry and saturation condition. The method is tested with 3 years observed surface runoff data and crop production statistics from a part of the Gandeshwari sub-watershed in West Bengal, India. The entire study area is divided into cells of 20 m × 20 m. Various components of the soil moisture balance equation are estimated for each cell as a function of the soil moisture content. Remote Sensing Technique and Geographic Information System (GIS) are used to extract and integrate the spatially distributed land use and soil characteristics. The Hortonion overland flow concept adopted in the SCS-CN method is used to estimate the soil hydraulic characteristics of each cell in which the curve number is used to infer the spatial variation of the land use and soil characteristics. Even though the original SCS-CN method and the existing modified versions are efficient for runoff estimation, these models are found to be inappropriate for the estimation of soil moisture distribution. On the other hand, the proposed SCS-CN-SMS model gives better results for both runoff and soil moisture simulation and is, therefore, more suitable for the hydrological modeling of paddy field-dominated agricultural watersheds.     

基于GIS/RS的不同土地利用类型重金属面源污染比较

针对不同土地利用类型,采样测定土壤重金属质量比背景值;基于DEM数据和遥感影像数据,结合空间信息技术（GIS/RS）,利用RUSLE模型和SCS-CN模型分别核算流域内不同土地利用类型的土壤流失量和地表径流量,在验证模型准确性的基础上,估算重金属非点源污染负荷。研究结果表明：研究区域不同土地利用类型的重金属质量比背景值差异较大,重金属Cu和Pb在工矿用地、草地和住宅用地的背景值较高,在有林地的背景值较低。综合估算随2种主要迁移途径输出的重金属面源污染负荷,重金属Cu总污染负荷量大约为651913kg/a,重金属Pb总污染负荷量大约为268004kg/a。不同土地利用类型中,随土壤侵蚀输出的重金属污染负荷,以疏林地和草地的贡献率最大,有林地和水浇地最小;随地表径流输出的重金属污染负荷,以草地和疏林地的贡献率最大,有林地和旱地最小。     

石漠化治理区不同植被类型浅层土壤水分对降雨的响应

【目的】揭示石漠化治理区不同植被类型浅层土壤水分入渗过程的差异性,分析降雨对土壤水分的补给特征,因地制宜地开展灌溉工作。【方法】以花椒地、金银花地、火龙果地、荒地为研究对象,监测了地区降雨量和植被0~10、10~25 cm土层土壤含水率,并计算了土壤储水量、滞后时间、补给速率、补给效率等指标。【结果】①4种植被浅层土壤含水率变化趋势与降雨一致,6、8、9月为二者的峰值期;浅层土壤含水率为火龙果地最高,而后依次为荒地、金银花地、花椒地;火龙果地土壤含水率变异系数最小,荒地最大。②植被土壤含水率对小雨量降雨事件的响应较小,仅火龙果地增长11.97%;随降雨量级增大,土壤含水率增长率为火龙果地(7.89%~17.94%),其次为金银花地(0~45.09%)、荒地(0~59.86%)、花椒地(0~126.95%);火龙果地浅层土壤含水率增长率在大雨事件中最小,其他3种样地均为小雨量时增长率小,大雨量时增长率大。③不同植被浅层土壤水分对降雨响应时间有显著差异,大雨量条件下响应快于小、中雨量,0~10 cm土层优于10~25 cm土层;平均滞后时间为荒地0.3 h、火龙果地0.5 h、花椒地0.9 h、金银花地3.0 h;补给效率为火龙果地(64.87%)>荒地(38.16%)>花椒地(31.94%)>金银花地(29.23%)。【结论】丰水期,对火龙果地适当减少人为灌溉,增加地表覆盖以减轻水土流失;对金银花地、花椒地可采取相应保墒措施提高土壤对降雨的利用效率且在雨量较小时增加灌溉,提高入渗量。     

Cautionary notes on the use of pedotransfer functions for estimating soil hydraulic properties

The performance of published pedotransfer functions was evaluated in terms of predicted soil water content, pressure heads, and drainage fluxes for a layered profile. The pedotransfer functions developed by Vereecken et al. (1989), Vereecken et al. (1990) were used to determine parameters of the soil hydraulic functions θ(h) and K(h) which were then used as input to SWATRER, a transient one-dimensional finite difference soil water model with root uptake capability. The SWATRER model was used to simulate the hydraulic response of a multi-layered soil profile under natural climatic boundary conditions for a period of one year. The simulations were repeated by replacing the indirectly estimated water retention characteristic by (1) local-scale, and (2) field-scale mean observed θ(h) relationships. Soil moisture contents and pressure heads simulated at different depths in the soil profile were compared to measured values using these three different sets of hydraulic functions. Drainage fluxes at one meter below ground surface have also been simulated using the same three sets of hydraulic functions. Results show that simulations based on indirectly estimated moisture retention characteristics (obtained from pedotransfer functions) overpredict the observed moisture contents throughout the whole soil profile, but predict the pressure heads at shallow depths reasonably good. The results also show that the predicted drainage fluxes based on estimated retention functions are about four times as high compared to the drainage fluxes simulated using measured retention curves.     

A new stochastic optimization model for deficit irrigation

Deficit irrigation has been suggested as a way to increase system benefits, at the cost of individual benefits, by decreasing the crop water allocation and increasing the total irrigated land. Deterministic methods are common for determining optimal irrigation schedules with deficit irrigation because considering the inherent uncertainty in crop water demands while including the lower and upper bounds on soil moisture availability is a hard problem. To deal with this, a constraint state formulation for stochastic control of the weekly deficit irrigation strategy is proposed. This stochastic formulation is based on the first and second moment analysis of the stochastic soil moisture state variable, considering soil moisture as bounded between a maximum value and a minimum value. As a result, an optimal deficit irrigation scheduling is determined using this explicit stochastic model that does not require discretization of system variables. According to the results, if irrigation strategy is based on deterministic predictions, achievement of high, long-term expected relative net benefits by decreased crop water allocation and increased irrigated land may have a higher failure probability.     

植被恢复类型对土壤物理性质的影响研究

应用空间代替时间的方法研究了盐池县天然草地、人工封育草地、撂荒地、退耕还草地、固定沙地5种不同植被恢复类型的土壤物理性质和水分变化。结果表明,灌草结合的人工封育草地、退耕还林地和固定沙地在降低土壤密度方面优于草本为主的天然草地和撂荒地。不同植被恢复类型的土壤孔隙度差异显著,根系垂直分布较好的植被对深层土壤有较好的改良作用。封育、退耕还林等人工植被恢复措施能够降低土壤密度,增加孔隙度,改善土壤结构和物理性状,但在改良土壤质地方面还需要时间的累积。不同植被恢复类型的土壤水分变化迥异,这与土壤密度、孔隙度和质地等物理性质以及植被类型和分布密切相关。     

Soil properties and their spatial pattern in an oasis on the lower reaches of the Tarim River, northwest China

In this study, classical, geostatistical methods and a geographical information system have been used to identify soil properties including soil organic carbon (SOC), total nitrogen (total N), salt content (SC) and soil moisture (SM), and their spatial variation as well as their relationships with groundwater, land use, and soil texture. The data came from 36 soil samples from 0 to 20 cm depth, and 36 groundwater samples in the Tikanlik Oasis on the lower reaches of the Tarim River, northwest China. The objective was to provide a scientific basis for understanding the heterogeneity of the spatial distribution of soil properties on a large scale with the goal of accelerating the sustainable development of agriculture in the oasis. The results showed that the SC and SM of the surface soil varied greatly, but the SOC and total N varied very little. A possible reason for the low variability of SOC and total N was the relatively consistent human activities within the typical agricultural oasis with more than 75% land used as cropland and orchards, because there was no significant effect of land use types on soil total N and SOC. Geostatistical analysis and the kriging estimator showed that SC and SM had a strong spatial autocorrelation. One-way ANOVA and LSD suggested that land use was the main structural indicator that resulted in the spatial autocorrelation of SC and SM within the 0-20 cm soil depth through its significant differences in soil texture, irrigation and groundwater. In particular, croplands and orchards had significantly higher SM, silt and clay particle percentages and lower SC compared to other land uses due to regular irrigation. Additionally, irrigation could effectively relieve the degree of the hazard from soil salt and soil water stress in cropland.     

Effect of drainage conditions on winter wheat production

To forecast the suitability of land for winter wheat, soil investigations were directed towards the evaluation of the effect of drainage conditions on productivity. Experiments were carried out on two soil types in Mugello (Italy): a well-drained soil and a poorly drained soil. Measurements of plants, atmosphere and water-table data were taken to simulate soil water contents and yields according to the model of Feddes et al. (1978). Computed and measured soil water contents and yields showed good agreement. On the poorly drained soil, root development was limited by the groundwater table and the final productivity, expressed as dry biomass, resulted in a 50% yield depression compared with that on the well drained soil.     

基于水平尺度扩展的土壤水分介电传感技术

设计了一种水平放置管式结构的介电水分传感器测量系统,通过水平往复移动扫描,测量一定深度下水平剖面的土壤水分。水分传感器的测量原理为频域(FD)外缘场阻抗法,该系统实现了土壤水平剖面含水率的线扫描测量,其最大扫描长度为380 cm。为了检验该测量系统的技术性能与可应用性,结合农田滴灌试验给出了2个点源激励下土壤入渗的应用测试案例。试验结果显示了不同质地土壤的入渗差异并记录了点源激励下土壤入渗过程的时间、空间三维数据。     

土壤湿度指数在黄土高原的适宜性评价

利用1992—2000年间黄土高原地区逐月降雨量、实测土壤湿度数据,结合GIS遥感技术、相关分析等方法,对基于TU-Wien变化检测算法,从ERS散射计数据反演获取的土壤湿度指数(SWI)或表层土壤湿度(SSM),从点和面2个尺度进行了验证。结果表明,在不同的土地利用、土壤质地和地形条件下,SWI或SSM与降水呈正相关(P<0.01)。时间序列方面,降雨、实测值和SWI或SSM基本上呈现出相同的变化趋势;空间分布上,降水和SWI具有相似的分布特征。在点尺度上,选取的7个站点的SWI与降水呈正相关(P<0.01),SWI和实测值的相关性随土层深度加深而下降,仅在10cm处达到显著水平(P<0.01)。对表层10cm的实测数据而言,降水和SWI的相关性要好于其与实测值的相关性。在黄土高原区域尺度上,SWI或SSM能够较为准确地揭示该地区表层土壤湿度的演变和空间分布特征。有长达20年(1992—2011年)之久的数据积累,并且可以免费及时获取的SWI或SSM,对于大范围实测土壤湿度数据匮乏的黄土高原地区而言,是一种具有较高利用价值的土壤湿度监测数据产品。     

The soil water balance of rainfed and irrigated oats,Italian rye grass and rye using the CropSyst model

Crop growth models have been used in simulating the soil water balance for purposes of irrigation management and yield predictions. The application of CropSyst, a cropping systems simulation model, was evaluated for Cedara, South Africa. Simulations included soil water balance of fallow land and rainfed and irrigated winter crops [oats (Avena sativa), Italian ryegrass (Lolium multiflorum) and rye (Secale cereale)]; and irrigation scheduling of the winter crops. Soil, plant, weather and management inputs were used for the soil water balance simulations. Model crop parameters were used from past experiments or obtained from model documentation, with a slight modification to account for varietal differences. The fallow land soil water simulations were more accurate for dry than for wet soil. For all three winter crops, the model consistently over-estimated the soil water content in the upper layers, with a good agreement for the deeper layers until a large precipitation event occurred to which the model responded more slowly than that observed. Simulations using model-scheduled irrigation based on 0.4 and 0.6 maximum allowable depletion criteria indicated that the observed applied irrigation in the field was more than that required. Soil water depletion and accumulated transpiration simulations were similar in both the observed and model-scheduled irrigations, but total soil evaporation and percolation were greater in the case of the observed than the model-scheduled irrigations. Irrigation scheduling using crop growth models may assist in avoiding over- or under-application of irrigation applications by ensuring efficient utilization of rain and irrigation.     

不同种植年限压砂地土壤水分入渗过程及模型分析

为探究不同种植年限对土壤水分入渗的影响,利用室内垂直一维土柱入渗试验分析了不同种植年限(2,5,10,20,30,40 a)压砂地土壤水分入渗速率、累积入渗量和湿润锋运移规律,借助4种入渗模型对土壤水分入渗过程进行拟合并评价其适用性,通过一维代数模型及相关参数预测土壤剖面含水率分布特征.结果表明:随种植年限增加,土壤累积入渗量、入渗速率及湿润锋运移距离均减小,入渗时间为300 min时,5,10,20,30,40 a的累积入渗量相较于2 a分别减少了27.01%,43.32%,53.22%,54.79%和54.68%,湿润锋运移距离减小了24.84%,38.38%,48.56%,52.67%和54.00%.4种入渗模型的R²从大到小依次为Horton模型、通用经验模型、Kostiakov模型、Philip模型.入渗完成后同一深度土壤剖面含水率呈现出随种植年限增大而减小的趋势,一维代数模型拟合的不同种植年限土壤剖面含水率值与实测值间的MAE均位于0.48%~2.09%,RMSE为0.52%~2.13%,D均大于0.782,t检验P值均大于0.05,说明其可以较好地拟合不同种植年限压砂地水分分布状况.压砂地种植年限越长,水分入渗能力越差、土壤剖面含水率越低,建议通过加强农田管理、休耕等措施实现压砂地可持续发展.     

滴灌对农田N2O排放影响的研究进展

灌溉作为调控土壤水分状况的重要措施,直接影响农田温室气体的排放过程。滴灌是一种高效节水灌溉技术,其对温室气体排放的影响受到广泛关注。通过查阅大量文献,综合分析了滴灌对农田土壤N2O排放影响的研究进展与发展趋势。文献资料显示,滴灌通过调控土壤湿度和温度环境,改变土壤微生物菌群和土壤中气体传输速度,进而影响土壤N2O的产生以及排放速率;与其他灌溉方式相比,滴灌不破坏土壤结构,土壤内部水、肥、气、热条件稳定,适宜于作物生长,有利于土壤有机氮的矿化。但目前滴灌条件下温室气体排放的空间异质性和多种温室气体的同步定量研究等方面仍存在一些不足(例如:滴灌条件下土壤干燥区和湿润区N2O排放通量间差异研究和不同种类温室气体的同期影响研究)。今后,要加强监测滴灌下多种温室气体同期排放和不同土壤区域N2O排放差异,加强从分子水平探究滴灌模式下土壤微生物对N2O气体产生过程的作用机理等方面的研究,为构建环境友好型农业模式提供科学依据。     

Investigating water availability for introducing an additional crop yield in dry season on hill land at Rubirizi, Rwanda

The study explores the potential of introducing an additional crop during dry season in Rwanda, comparing the efficiency of in situ soil moisture conservation techniques to sustain rain-fed agriculture. Comparative study of in situ soil moisture conservation techniques in bench terraces and unterraced field with maize crop had been conducted from June 2007 to October 2007. Bench terrace increased the average soil moisture content in 90 cm soil depth by more than 50% than that of unterraced land. Within the bench terraced field compartment bund and ridges and furrows increased soil moisture by 19.5% and 27.9% higher than plain bed. In terms of efficiency of moisture conservation, ridges and furrows performed well with 85.8% followed by compartment bund with 75.9% in terraced field. Unterraced field conserved moisture very poorly with 13.9% efficiency inferring importance of bench terraces for soil moisture conservation. No maize grain yield was recorded in all the techniques because soil water depleted to 60% and above from the beginning of the cropping period inferring the need of supplementary irrigation. Analysis of rainfall, crop water demand and in situ moisture conservation reveals exciting opportunities for water productivity enhancements by integrating components of water management within the context of rain-fed farming through water harvesting and supplemental or microirrigation for dry spell mitigation. Detailed analysis is needed for feasibility of lift irrigation with different crops under different altitudes to derive suitable policy for hill land irrigation.     

Mapping the total volumetric irrigation water requirements in England and Wales

The net volumetric (m³) irrigation water requirements for the main crop categories currently irrigated in England and Wales have been calculated and mapped within a geographic information system (GIS). The procedure developed by Knox et al. (1996, Agric. Water Manage., 31: 1–15) for maincrop potatoes (Solanum tuberosum) was extended to cater for the other crops currently irrigated. The annual irrigation needs (mm) for the eight major irrigated crop categories, grown on three contrasting soil types at 11 representative weather stations, were determined using a daily water balance irrigation scheduling model. The results were correlated with existing national datasets of climate, current land use, soils and irrigation practice, to generate volumetric (m³) irrigation water requirement maps at 2 km resolution.The total net volumetric irrigation water requirements for a UK ‘design’ dry year (defined as the requirement with a 20% probability of exceedance) are estimated to be 140 × 10⁶ m³ for the eight main crop categories currently irrigated and the 1994 cropping pattern. Previous theoretical dry year demand estimates, using scheduling models and large agroclimatic areas, were 109 × 10⁶ m³ and 222 × 10⁶ m³. The irrigation demand for other crops grown in the open would typically add another 4%.The procedure has been validated nationally, by comparing the calculated dry year demand for 1990 against government irrigation survey returns for 1990, for each crop category, and regionally against National Rivers Authority (NRA) abstraction records for 1990, for each NRA Region. The estimates obtained agree well with the reported distribution between crops and between regions.The most recent actual ‘dry’ year for which comparative data are available is 1990. It is estimated that the dry year requirements for the 1990 land use would have been 148 × 10⁶ m³. Although farmer demand, actual abstractions and crop requirements are not necessarily the same, irrigation survey returns to the Government indicated that 134 × 10⁶ m³ were actually applied, and the NRA estimated from meter returns that 138 × 10⁶ m³ were abstracted. It is noted, however, that some abstraction restrictions were in force, the scope of the data is slightly different and all figures contain inaccuracies. Potential applications for improving irrigation demand management and water conservation at regional and catchment levels are discussed with reference to two contrasting regions.     

Erosion/productivity modelling of maize farming in the Philippine uplands: Part I: parameterising the Agricultural Production Systems Simulator

This paper describes the parameterisation of the Agricultural Production Systems Simulator (APSIM) model to simulate open-field farming and intercropping of maize with leguminous shrub hedgerows. Whenever possible, parameters for the model were determined from measured or standard values for the environment of the field trials, while other parameters were derived from previous modelling experience in tropical environments. The remaining parameters were derived using step-wise calibration, where one or two parameters were calibrated against closely related measured data. Once parameterised, APSIM gave acceptable predictions of maize yields and soil loss from open-field farming and hedgerow intercropping. The version of APSIM described in this paper is used to simulate maize yields and soil erosion from open-field farming and hedgerow intercropping in the second paper in this series (Nelson et al., this issue). In the third paper, Nelson et al. (this issue) use cost–benefit analysis to compare the economic viability of hedgerow intercropping relative to traditional open-field farming of maize in relatively inaccessible upland areas.     

Simulating and mapping agricultural land productivity: An application to Macadamia nut

A procedure for simulating and mapping land productivity was developed. The procedure used only existing data such as soil surveys based on soil taxonomy, weather records and yield either from published sources or collected from producers. It provides a means of assessing quantitatively the likelihood of introducing new crops to a region. The need to estimate benefits from crop inputs modification such as irrigation and fertilizer can also be met by this procedure. It can also be used to help select crop performance test sites for maximum extrapolation of test results to a large area.     

Obtaining soil physical field data for simulating soil moisture regimes and associated potato growth

Soil moisture regimes under potatoes were monitored during two growing seasons in a sandy loam and a silty clay loam soil in The Netherlands. Measured moisture contents were used to validate those calculated using the simulation model SWATRE. The model requires hydraulic conductivity, moisture retention, rooting-depth and soil cover data, which were obtained in this study. Measured groundwater levels formed the lower boundary condition and precipitation and potential evapotranspiration the upper boundary condition of the model flow system. Calculated moisture contents agreed well with measured values, but only when in situ moisture retention curves were used and when the effects of cracking in the silty clay loam soil were expressed by modifying the ?-θ function. The moisture supply capacity of the sandy loam soil was highest, particularly in the first growing season and this was interpreted as a major reason for the corresponding higher yields. Simulation can be used for predicting the soil moisture supply capacity in the context of land evaluation, when soil-cover and rooting depth are standardized, for example by simulating growth of a grass crop.     

节水灌溉水稻需水新特点

1982年以来,于麦仁店灌溉试验站进行了控制土壤含水量水稻灌溉技术及水稻需水量的试验研究,其成果表明,采用节水高产的控制灌溉技术后,水稻耗水量大幅度下降,生理生态需水规律发生显著变化。文章从稻田土壤水分等生态环境的改善,以及水稻自身生理调节机能和适应能力变化等方面,分析了水稻节水机理,并对彭曼法在节水灌溉水稻需水量计算中的应用进行了探讨,提出了应力系数K_s的计算公式。     

利用盘式负压仪测定土壤导水率的计算方法对比

土壤导水率作为重要的水力特征参数之一,准确测量和计算不仅有助于促进土壤非饱和带的水分运动过程理论研究,同时可为合理确定农田灌排技术参数提供科学依据.为了比较分析在盘式入渗仪下不同导水率计算方法的适用性,针对2种土地类型(菜地、茶园)进行了4个负压水头(-9,-6,-3,0 cm)、2个盘径(10,20 cm)的入渗试验,并通过不同计算方法计算导水率.结果表明:盘径对导水率影响不具有统计学意义,且不同计算方法结果趋于一致,因此在野外缺水条件下,可考虑选用小圆盘进行试验;对于不同的土地类型,建议选择不同的方法测量,作为耕作地的菜地,计算导水率时建议使用稳态流方法,而茶园导水率的测定则推荐使用瞬态流方法;在相同负压下,不同土地利用方式对导水率的影响具有统计学意义,且对于不同的计算方法,2种土地类型在4个负压下表现的导水率变化规律一致.