Evaluation of time domain reflectometry (TDR) for estimating furrow infiltration

TDR was used to estimate furrow infiltration, which is a key component in furrow irrigation system design and management. Furrow irrigation experiments were conducted on bare and cropped fields consisting of three 40 m long parabolic shaped furrows spaced at 0.8 m on a slope of 0.5%. The centre furrow was taken as the study furrow and the other two provided a buffer to the centre furrow. Altogether, 22 irrigations were conducted during 2004 and 2005 with inflow rates ranging from 0.1 to 0.7 l s⁻¹. TDR probes were installed vertically around the centre furrow at four locations 0.5 (S₁), 13 (S₂), 26 (S₃) and 39.5 m (S₄) from the inlet end. The S₁ and S₃ locations had four TDR probes installed at 0.15, 0.30, 0.45 and 0.60 m depths whereas the S₂ and S₄ locations had two probes each at 0.15 and 0.30 m depths. Soil moisture data collected at 5-min intervals were used to determine the average soil moisture content of the field. The change in moisture content was used to estimate the furrow infiltration which was compared with that measured using an inflow–outflow (IO) method. The performance of the TDR method was studied by calculating the absolute prediction error (APE), root mean square error (RMSE) and index of agreement (I _a). It was found that the TDR-method estimated furrow infiltration well for higher inflow rates and during the initial stages of irrigation. APE decreased and I _a increased with increase in flow rate for both bare and cropped conditions. The APE and RMSE were found to be larger for a cropped field than the bare field when irrigated at the same inflow rate. The accuracy of the TDR-method for estimating total infiltration was improved by using the average field moisture content of 30 or 45 min after the recession phase ceased. These results indicate that TDR can be used to estimate in situ infiltration under furrow irrigation.     

Multi-wire time domain reflectometry (TDR) probe with electrical impedance discontinuities for measuring water content distribution

The adequate estimation of water content distribution in wetted volume is fundamental in determining the number of drippers per plant and their location below the plant canopy in drip irrigation. Measurements of water content distribution are usually made by opening trenches, which is a time-consuming method and sometimes imprecise. Recent scientific developments have created the possibility of monitoring the soil moisture content using electronic sensors. The objective of this research was to develop and test two multi-wire time domain reflectometry (TDR) probes with electrical impedance discontinuities (referred to as the multi-wire probe) for sensing soil profile water content distribution. The experiment was divided in two parts. In part one, the laboratory performance of two multi-wire probe designs was studied and their reliability to monitor the water content variation in a porous media profile was evaluated. The second part was conducted in a 250 l bucket and the soil water content distribution, for an application depth of 15 mm, was evaluated by monitoring over 6 days at discharge rates of 2 and 4 l h⁻¹. The results demonstrated the viability of using multi-wire probes to estimate soil water content distribution with different probe designs and to consistently obtain water content measurement in water dynamic processes. The following conclusion may drawn from the main results: (1) The measured characteristic impedance of the multi-wire probe for different designs was not the same as that geometrically calculated. This was due to the non-ideal probe geometry which provoked signal loss, thus, hindering peak impedance interpretation, mainly for probe 1 design. (2) The use of multi-wire probes in the TDR equipment showed a speedy determination of soil profile water content distribution using a single measurement.     

Use of soil physical characteristics from laboratory measurements or standard series for modelling unsaturated water flow

Soil physical characteristics are important input parameters for simulation modelling of unsaturated flow in soils and associated solute flow. The determination of soil water retention and hydraulic conductivity curves in the laboratory is laborious and expensive. For modelling studies that require characteristics for many soil horizons, such as regional studies or scenario studies, it may be impossible to measure all the necessary characteristics. An alternative would be to use characteristics inferred from readily available soil data by class-pedotransfer functions. In this study such a comparison was made for six sites on sandy soils in the Netherlands using the soil-water model SWACROP with soil physical characteristics from either laboratory measurements or from a standard series as input. For this the simulated pressure head values and moisture content values were compared with measured values at eight different depths using statistical criteria. Furthermore two functional criteria, i.e. the number of workable days and number of days with possible drought, were inferred from simulated pressure head values and again the different results were compared. It was found that simulation results were not significantly different, implying that standard series or class-pedotransfer functions could be used in studies like these for simulating the unsaturated water flow regime in sandy soils on field/farm level or regional level. Differences for specific criteria for individual sites were sometimes substantial and in such cases (at field level) it will make a difference which soil physical characteristics are used.     

Prediction of soil moisture characteristics from mechanical analysis and bulk density data

Soil moisture characteristics can be established directly from the physical properties of soils such as mechanical analysis and bulk densities. Multiple regression equations were worked out by taking first fractions of sand (E₁), silt + clay (E' = E₂ + E₃) and bulk density (P), and second fractions of sand (E₁), silt (E₂), clay (E₃) and bulk density (P) as independent variables to predict the parameter b, the air-entry potential ψ_e and the saturation moisture content _s of the soil-moisture characteristics equation. Regression equations were tested for soils of different textural and structural compositions and showed good agreement between estimated and experimentally determined values.     

Effect of soil moisture regimes on the yield and water use of lentil (Lens culinaris Medic)

Summary The effect of soil moisture regimes on the grain and straw yield, consumptive water use (Cu) and its relation with evaporation from free water surface (Eo), water use efficiency and soil moisture extraction pattern of lentil was studied in a field experiment conducted at the Indian Agricultural Research Institute, New Delhi during the fall-spring season of the crop years 1979–1980 and 1980–1981. The grain and straw yield, consumptive water use rate, Cu/Eo ratio and water use efficiency increased with an increase in irrigation frequency. Consumptive water use rate increased as the crop season advanced and reached its peak value during flowering and grain filling stage. The Cu/Eo ratio attained its minimum values 35 and 105 days after sowing at branching and grain filling stages. Depletion of soil moisture was most from the top 0–30 cm soil layer followed by 30–60 cm soil layer and was least from 90–120 cm soil layer. The pattern of soil moisture depletion was also influenced by soil moisture regime. During the vegetative and flowering stage the percent contribution from the top 0–30 cm soil layer decreased and that from the lower soil layers (30–60, 60–90, and 90–120 cm) increased with an increase in the soil moisture tension, however, the actual amount of moisture depleted from all the soil layers was always higher under low soil moisture tension regime than under high soil moisture tension regime. During the grain development stage the soil moisture treatment had no significant effect on the relative contribution from different soil layers under low and high soil moisture tension as the crop was irrigated at the same time under both these treatments. However, with no irrigation, the percent contribution from top soil layer continued to decrease, and from lower soil layers continued to increase, as the crop advanced from flowering stage to grain development stage.     

Evaluation of soil water storage efficiency for rainfall harvesting on hillslope micro-basins built using time domain reflectometry measurements

Micro-basins are slope management structures built out of earth and stones on hillslopes around cultivated trees (e.g., olive trees) for the harvesting of rainfall and runoff water, and for the rehabilitation of land degraded by water erosion.In this study, the results of an experimental survey for the comparison of soil water content for both inside and outside the micro-basins are analyzed. Measurements are taken after some rainfall events from January to December 2003 in a hilly region of Central Tunisia. The time domain reflectometry technique is used to measure soil moisture in 15 sets of soil profiles (inside and outside) at three different depths. Four different soils are evaluated, i.e., Cambisols, Kastanozems, Arenosols, and Calcisols.The data analysis shows a significant improvement on the water stock obtained by this type of management. The differences in water storage with respect to soil type, depths, and tillage are evident, but strongly connected to farm management. For optimal management conditions an important increase of average water stock is observed; however, for bad or no farm management the amelioration is zero or is even deteriorating the state of vegetation.     

Combining remote sensing and in situ soil moisture data for the application and validation of a distributed water balance model (HIDROMORE)

An application of the FAO56 approach to calculate actual evapotranspiration (AET) and soil moisture is reported, implemented by means of the HIDROMORE computerized tool, which performs spatially distributed calculations of hydrological parameters at watershed scale. The paper describes the application and validation of the model over 1 year in an area located in the central sector of the Duero Basin (Spain), where there is a network of 23 stations for continuous measurement of soil moisture (REMEDHUS; Soil Moisture Measurement Stations Network) distributed over an area of around 1300 km². The application integrated a series of Landsat 7 ETM+ images of 2002, from which the NDVI series (Normalized Difference Vegetation Index) and the map of land covers/uses were derived. Validation consisted of the use of the REMEDHUS soil moisture series and their comparison with the series resulting from the application. Two simulations were performed, with soil parameters values at the surface (0-5 cm depth) and at the mean of the profile scale (0-100 cm depth). The behaviour of the simulated soil moisture was described by means of its correlation with the measured soil moisture (determination coefficient, R² = 0.67 for the surface values and 0.81 for the mean profile values), and the Root Mean Square Error (RMSE), resulting in a range of it for the 23 stations between 0.010 and 0.061 cm³ cm⁻³. The application afforded an underestimation of the soil moisture content, which suggests the need for a redefinition of the limits of the plant available water used in the calculation. The results showed that HIDROMORE is an efficient tool for the characterization of hydrological parameters at global scale in the study zone. The combination of the FAO56 methodology and remote sensing techniques was efficient in the spatially distributed simulation of soil moisture.     

应用时域反射仪快速标定中子水分仪

为提高田间标定效率并减少土壤空间变异对标定方程的影响,利用时域反射仪来快速标定中子水分仪。埋设中子管时边钻孔、边观测土壤岩性,同时用TDR测体积含水量。用中子仪测放射性计数,测量计数除以标准计数得到计数率比,从而得到体积含水量与计数率比关系曲线。试验结果表明:①沙壤和粉沙可以使用同一个标定方程:y=41.989x+14.989,R2=0.8539;②容重不影响标定结果,但是0~30 cm深度不适合使用中子仪测量土壤水分;③研究区内埋深30 cm以下,使用TDR标定中子仪的标定方程为:y=53.139x+10.397,R2=0.968;④TDR适用性分析表明对于研究区内的沙壤和粉沙,TDR可以直接测定其体积含水量,适合对中子仪进行标定。因此,这是一种快速有效的田间标定中子仪新方法。     

Estimation of irrigation return flow from paddy fields considering the soil moisture

The objective of this study was to estimate irrigation return flow in irrigated paddy fields considering the soil moisture. The proposed model was applied to examine its feasibility with regard to the growing period of rice. Simulation results showed a good agreement between the observed and simulated values: root mean square error (RMSE) of 6.05-7.27 mm day⁻¹, coefficient of determination (R²) of 0.72-0.73, and coefficient of efficiency (E) of 0.54-0.55. The estimated average annual irrigation return flow during the period from 1998 to 2001 was 306.2 mm, which was approximately 25.7% of the annual irrigation amounts. Of this annual irrigation return flow, 14.1% was attributable to quick and 11.6% to delayed return flow. These results indicate that considerable amounts of irrigation water in the paddy fields were returned to streams and canals by surface runoff and groundwater discharge. The modeling assessment method proposed in this study can be used to manage agriculture water and estimate irrigation return flow under different hydrological and water management conditions.     

基于水分胁迫系数的枣树园土壤含水率估算

为了实现黄土高原地区滴灌条件下枣树园土壤含水率的小范围快捷监测,根据FAO-56水分胁迫系数的定义和相关计算公式,得到了基于土壤水分胁迫系数的黄土高原地区滴灌条件下枣树根层土壤平均含水率估算公式．2009年4-9月将该公式应用于西北农林科技大学陕西米脂孟岔试验站的枣树试验园,配置了3种不同的土壤含水率控制下限,对枣树2个重要生育期的土壤含水率进行了估算,模拟了水分动态变化过程,并对估算值和实测值进行对比和误差分析．结果表明:采用基于FAO-56水分胁迫系数的计算公式对土壤含水率的动态模拟达到了较高的精度,估算值与实测值之间误差较小:其中开花坐果期各处理(灌水下限为60％,50％,40％的田间持水率)的估算值与实测值之间的相关系数分别为0828 0,0907 3,0935 1;标准误差分别为0055,0093,0068．果实膨大期各处理的相关系数分别为0777 2,0766 7,0905 5;标准误差分别为0057,0092,0079．估算值与实测值之间的相关系数随土壤含水率的增大而减小,随土壤水分胁迫程度的增大而增大,即土壤含水率较高时对公式精度有一定的影响．该方法较适用于黄土高原半干旱地区,对农业用水管理具有一定参考价值．     

Evapotranspiration,pan evaporation and soil water relationships for wheat (Triticum aestivum)

Controlled irrigation experiments were conducted for wheat grown in lysimeters having undisturbed soil profiles and protected from rainfall with transparent plexiglass roofs. Crop evapotranspiration during different crop growth stages and its relationships with Class A pan evaporation and soil water parameters were studied. The actual evapotranspiration during different crop growth stages was greatly influenced by amount and time of irrigation. The ratio of the maximum evapotranspiration and Class A pan evaporation increased linearly from germination to 46 days after sowing and remained constant at 1.45 from 46 to 76 days. Then the ratio decreased linearly towards the crop ripening. The actual evapotranspiration was equal to the maximum evapotranspiration up to the critical value of relative soil water, and then the actual evapotranspiration decreased at a very fast rate with further decrease in relative soil water. The critical value of the relative soil water varied from 0.65 to 0.84 during the crop growth-stage periods late tillering-heading and dough ripe-ripe, respectively.     

Dielectric coated water content reflectometer for improved monitoring of near surface soil moisture in heavily fertilized paddy field

Mat rush (Juncus effusus L.), used for ‘Tatami’ (a traditional Japanese mat), is a type of crop requiring a large amount of fertilizer (450–600 kg ha⁻¹ of N). In a heavily fertilized mat rush paddy field we examined the monitoring of soil water content (θ) by using the water content reflectometer (WCR). WCR sensors with and without coating rods were prepared and tested for their performance in different solutions. In addition, these sensors for Gley Lowland Soils were also calibrated for measuring θ. The results showed that the measured water content using the uncoated WCR, increasing with the EC of the solution, was 1.6 times of that for distilled water while the output for the coated WCR became 1.04 times. The coating prevents conduction losses while it influences the sensitivity of the WCR sensor. The monitoring of θ using both coated and uncoated WCR sensors in a mat rush paddy field was conducted throughout a cropping season. For the coated sensors, water content could be determined accurately even after fertilizer applications, while with the uncoated sensors it was overestimated. Thus, it was concluded that the use of insulated WCR sensors make it possible to accurately monitor the near surface soil moisture in a heavily fertilized paddy field.     

On-farm sampling density and correction requirements for soil moisture determination in irrigated heavy clay soils in the Gezira,central Sudan

Using the neutron scattering technique, with separate calibration for each measuring depth and temperature corrections, an over-sampling experiment with a worst case analysis was conducted in tenant irrigated fields under arid conditions. The purpose was to better understand actual on-farm soil moisture distribution as well as to determine minimum sampling density requirements for water use efficiency calculations in the heavy cracking clay soils of the Gezira irrigation scheme, central Sudan, under inhomogeneous watering conditions. Results show that actual soil moisture inhomogeneities can seriously distort the moisture distribution and water use pictures if the sampling density is too low. In a 2.1 ha end field under Gezira conditions 20 equally spaced neutron probe samples had to be collected from the 30 cm soil depth if the total experimental errors were to be kept within 12.5% of the average moisture content being measured. Sampling density requirements increased to 24, 28 and 33 samples for worst case error limits of 10%, 7.5% and 5% at 30 cm depth. At the agronomically more important lower depths, at or below 70 cm, less than 10 samples only could be afforded with an error of 10% at 70 cm, of 15% at 50 cm and of 20% at 30 cm, the errors typically becoming smaller at larger depths throughout. Credible soil water averages were obtained with this sampling. Field moisture patterns were well recognized when averaging several days of measurements.     

Evaluation of the parameters involved in Gardner's relation between unsaturated hydraulic conductivity and soil water matric suction

An experimental study on evaporation from homogeneous sandy and loamy soils was carried out to define the unsaturated hydraulic conductivity function. Gardner's empirical function k(ψ) (1958) was used in the analysis of experimental data. Parameters involved in the k(ψ) functions obtained were evaluated. The error due to exclusion of the b-factor varied with the depth of water table for both sandy and loamy soils.     

基于土壤入渗水量分布特性的微喷灌系统组合分析

通过显色示踪方法研究了微喷条件下入渗水量分部模式,在此基础上探讨了微喷组合布置形式。试验结果显示喷洒水量分布为2次曲线情况下,入渗后水量分部符合4次曲线;同时分布峰值位置沿半径方向向外侧偏移,而含水率变化量在垂直方向分布差异性较小。依据实际入渗水量分布情况,采用正方形组合方案对系统的灌溉效率进行评价,组合间距为1.1倍有效喷程时,均匀系数最大。基于入渗水量分布进行微喷灌组合将能有效地提高灌水质量和效率。     

Field determination of soil hydraulic properties for simulation of the optimum watertable regime

A field experiment to evaluate accurate cost and time efficient methodologies for determining soil hydraulic properties was done at the NIAB Research Station at Faisalabad, Pakistan. The experiment was performed on a freely draining loamy soil. This soil type is representative of 75% of the topsoil in a tile drainage area known as the Fourth Drainage Project. Redistribution of water was monitored at five locations, for seven depths, following a steady state infiltration for prolonged time. The data were analyzed with Darcian flow analysis, three simplified methods, and two parameter optimization programs to calculate unsaturated hydraulic conductivity. The Darcian flow analysis was used as a reference against which the simplified methods were compared. Two simplified methods produced satisfactory results with less effort. The drawback is that the simplified methods alone do not provide enough information for use in simulation models. The advantage of the two optimization programs — SFIT and RETC — is that they are based on a continuous function which describes complete h() and K() curves. This is a requirement for computer simulation of salt and water movement in the unsaturated soil. The results of the optimizations were evaluated by their correspondence to field measurements and to laboratory measurements and by their ability to simulate soil water flow. Both programs fit the observed field data well, but only the SFIT optimized parameters were suitable for soil water flow simulations.Abbreviations IWASRI International Waterlogging and Salinity Research Institute - NRAP Netherlands Research Assistance Project - NIAB Nuclear Institute for Agriculture and Biology - SSP Soil Survey of Pakistan Revised paper from Field determination of soil hydraulic properties presented in ICID, CIID IDW5, Lahore, Pakistan (1–55 — 1–64).     

Forage quality, water use and nitrogen utilization efficiencies of pearl millet (Pennisetum americanum L.) grown under different soil moisture and nitrogen levels

The increasing scarcity of water for irrigation is becoming the most important problem for producing forage in all arid and semi-arid regions. Pearl millet is a key crop in these regions which needs relatively less water than other crops. In this research, a field study was conducted to identify the best combination of irrigation and nitrogen (N) management to achieve acceptable pearl millet forage both in quantity and quality aspects. Pearl millet was subjected to four irrigation treatments with interaction of N fertilizer (0, 75, 150 and 225 kg ha⁻¹). The irrigation treatments were 40%, 60%, 80% and 100% of total available soil water (I₄₀, I₆₀, I₈₀ and I₁₀₀, respectively). The results showed that increasing moisture stress (from I₄₀ to I₁₀₀) resulted in progressively less total dry matter (TDM), leaf area index (LAI), and nitrogen utilization efficiency (NUzE), while water use efficiency (WUE) and the percentage of crude protein (CP%) increased. The highest TDM and LAI were found to be 21.45 t ha⁻¹ and 8.65, in I₄₀ treatment, respectively. TDM, WUE, CP% and profit responses to N rates were positive. The maximum WUE of 4.19 kg DM/m³ was achieved at I₁₀₀ with 150 kg N ha⁻¹. The results of this research indicate that the maximum profit of forage production was obtained in plots which were fully irrigated (I₄₀) and received 225 kg N ha⁻¹. However, in the situation which water is often limited and not available, application of 150 kg N ha⁻¹ can produce high forage quality and guaranty acceptable benefits for farmers.     

Real-time prediction of soil infiltration characteristics for the management of furrow irrigation

The spatial and temporal variations commonly found in the infiltration characteristic for surface-irrigated fields are a major physical constraint to achieve higher irrigation application efficiencies. Substantial work has been directed towards developing methods to estimate the infiltration characteristics of soil from irrigation advance data. However, none of the existing methods are entirely suitable for use in real-time control. The greatest limitation is that they are data intensive. A new method that uses a model infiltration curve (MIC) is proposed. In this method a scaling process is used to reduce the amount of data required to predict the infiltration characteristics for each furrow and each irrigation event for a whole field. Data from 44 furrow irrigation events from two different fields were used to evaluate the proposed method. Infiltration characteristics calculated using the proposed method were compared to values calculated from the full advance data using the INFILT computer model. The infiltration curves calculated by the proposed method were of similar shape to the INFILT curves and gave similar values for cumulative infiltration up to the irrigation advance time for each furrow. More importantly the statistical properties of the two sets of infiltration characteristics were similar. This suggests that they would return equivalent estimates of irrigation performance for the two fields and that the proposed method could be suitable for use in real-time control.     

Implications of precipitation patterns and antecedent soil water content for leaching of pesticides from arable land

An improved understanding of how precipitation patterns control pesticide leaching from structured soils prone to macropore flow could lead to practical mitigation strategies that would help farmers minimize losses by optimizing application timings. A sensitivity analysis of the macropore flow model MACRO was therefore carried out to examine the influence of antecedent soil water content and precipitation patterns on pesticide leaching to drainage systems and groundwater. One thousand model runs were executed (20 four-year weather data series, 50 application dates per season) for both autumn and spring applications of a hypothetical moderately sorbed and quickly degraded herbicide for one of three national scenarios for pesticide risk assessment in Sweden (Näsbygård, a loamy moraine soil in Scania, southern Sweden). Rapid and direct transport of pesticides in macropores to drainage systems and shallow groundwater was predicted to occur rather infrequently in spring (in 4 of the 20 years) and even more rarely in autumn. For autumn applications, the soil water deficit at application (SWD_tot) and medium-term precipitation (30–90 days after application) were the two most sensitive variables controlling pesticide leaching. For spring applications, total leaching was most closely linked to rainfall the following winter, while short-term precipitation (5 days after application) and the antecedent soil water deficit were the two most important predictors for maximum pesticide concentrations in drainflow. The potential for reducing leaching by restricting applications to periods of low risk was investigated. The results showed that avoiding applications on wet soil in autumn could potentially reduce total pesticide losses by a factor of two to three. Similarly, the risk of acute toxicological effects in surface waters following pesticide applications in spring could be reduced by a factor of 2–3 by avoiding application when 5-day weather forecasts predict precipitation >10 mm.