A method for spatial prediction of daily soil water status for precise irrigation scheduling

Available water holding capacity (AWC) and field capacity (FC) maps have been produced using regression models of high resolution apparent electrical conductivity (EC_a) data against AWC (adj. R² = 0.76) and FC (adj. R² = 0.77). A daily time step has been added to field capacity maps to spatially predict soil water status on any day using data obtained from a wireless soil moisture sensing network which transmitted hourly logged data from embedded time domain transmission (TDT) sensors in EC_a-defined management zones. In addition, regular time domain reflectometry (TDR) monitoring of 50 positions in the study area was used to assess spatial variability within each zone and overall temporal stability of soil moisture patterns. Spatial variability of soil moisture within each zone at any one time was significant (coefficient of variation [% CV] of volumetric soil moisture content (θ) = 3-16%), while temporal stability of this pattern was moderate to strong (bivariate correlation, R = 0.52-0.95), suggesting an intrinsic soil and topographic control. Therefore, predictive ability of this method for spatial characterisation of soil water status, at this site, was limited by the ability of the sensor network to account for the spatial variability of the soil moisture pattern within each zone. Significant variability of soil moisture within each EC_a-defined zone is thought to be due to the variable nature of the young alluvial soils at this site, as well as micro-topographic effects on water movement, such as low-lying ponding areas. In summary, this paper develops a method for predicting daily soil water status in EC_a-defined zones; digital information available for uploading to a software-controlled automated variable rate irrigation system with the aim of improved water use efficiency. Accuracy of prediction is determined by the extent to which spatial variability is predicted within as well as between EC_a-defined zones.     

Soil water content and water balance in rainfed fields in Northeast Thailand

Northeast Thailand has a semi-humid tropical climate which is characterized by dry and rainy seasons. In order to stabilize crop production, it may be necessary to develop new water resources, such as soil moisture and groundwater, instead of rainfed resources. This is because rainfed agriculture has already been unsuccessfully tried in many areas of this region. In this study, we investigate the soil water content in rainfed fields in Khon Kaen in Northeast Thailand, where rice and sugarcane were planted, over a 1-year period that contained both dry and rainy seasons, and estimate the actual evapotranspiration (ET_a) using micrometeorological data. In addition, we assess the water balance from the results of the soil water content investigation and the actual evapotranspiration. Although the soil water content at depths above 0.6 m in both the lower and the sloping fields gradually decreased during the dry season, the soil water content at a depth of 1.0 m was under almost constant wet conditions. Two-dimensional profiles of the soil water content demonstrated that at the end of the dry season, the soil layers below a depth of 0.4 m showed a soil water content of more than 0.10-0.15 m³ m⁻³, thus suggesting that water was supplied to the sugarcane from those layers. The range in ET_a rates was almost the same as that in the previous study. The average ET_a rates were 3.7 mm d⁻¹ for the lower field and 4.2 mm d⁻¹ for the sloping field. In the dry season, an upward water flow of 373 mm (equivalent to a flux of 1.9 mm d⁻¹) was estimated from outside the profile. The source of this upward water flow was the sandy clay (SC) layer below a depth of 1 m. It was this soil water supply from the SC layer that allowed the sugarcane to grow without irrigation.     

Effect of sustained saline irrigation on soil salinity and crop yields

Summary Field studies were conducted for a period of ten years (1974 to 1984) on Typic Ustochrept to determine the sustained effects of saline irrigation water electrical conductivity (EC _iw ) 3.2 dS/m, sodium adsorption ratio (SAR) 21 (mmol/1)^1/2 and residual sodium carbonate (RSC) 4me/1, on the build up of salinity in the soil profile and yield of crops grown under fixed rice-wheat and maize/millet-wheat rotations. Saline waters were continuously used with and without the addition of gypsum (at the rate needed to reduce RSC to zero) applied at each irrigation. In maize/millet-wheat rotation, two additional treatments viz. (i) irrigation with 50% extra water over and above the normal 6 cm irrigation, and (ii) irrigation with good water and saline water alternately, were also kept. The results showed that salinity increased rapidly in the profile during the initial years but after five years (1979–1984) the average soluble salt concentration in 0–90 cm soil profile did not appreciably vary and the mean EC _e values under saline water treatment remained almost similar to EC _iw , under both the crop rotations.Saline water irrigation increased pH and Na saturation of the soil, reduced water infiltration rate and decreased yields of maize, rice and wheat. The differences in the build up of salinity and ESP of the soil under the two cropping sequences seemed to be related with the differences in leaching that occurred under rice-wheat and maize/millet-wheat rotations. Application of gypsum increased the removal of Na from the profile, appreciably decreased the pH and Na saturation and improved water infiltration rate and raised crop yields. Application of non-saline and saline waters alternately was found to be a useful practice but irrigation with 50% extra water to meet the leaching requirement did not control salinity and hence lowered crop yields.     

Predicting soil salinity in response to different irrigation practices, soil types and rainfall scenarios

A model was developed to predict rootzone salinity under different irrigation practices on different soil types, with similar rainfall but different monthly distributions. A rootzone daily water and salt balance was performed using eight scenarios: two soil types (coarse textured vs. fine textured), two multi-year series of actual rainfall data and two irrigation practices (surface with fixed number of irrigations and ET-based sprinkler irrigation). All factors influenced the mean electrical conductivity (EC) of the rootzone in the growing season (EC_eS): (i) Surface irrigation led to lower EC_eS than sprinkler irrigation; (ii) Winter-concentrated rainfall caused lower EC_eS than rainfall distributed uniformly throughout the year; and (iii) Coarser-textured soil usually resulted in lower EC_eS than the finer textured. The EC_eS was related to the total precipitation of the hydrologic year and to the annual leaching fraction (LF) but surprisingly not to the seasonal LF. In most cases, the model predicted lower EC_eS than the FAO steady-state approach. Therefore, considering these site-specific features could lead to lower leaching requirements and the safe use of higher salinity water.     

Shoot growth and fruit development of muskmelon under saline and non-saline soil water deficit

In irrigated agriculture, the production of biomass and marketable yield depend largely on the quantity and salinity of the irrigation water. The sensitivity of field-grown muskmelon (Cucumis melo L. cv. Galia) to water deficit was compared, using non-saline (EC_i= 1.2 dS m^–1) and saline (EC_i=6.3 dS m^–1) water. Drip irrigation was applied at 2-day intervals at seven different water application rates for each water quality, including a late water-stress treatment. Neutron scattering measurements showed that the soil layers below the root zone remained dry throughout the experiment, indicating negligible deep percolation. Thus, the sum of the seasonal amount of applied water and the change in soil moisture approximated the cumulative evapotranspiration (ET). Gradual buildup of water and salt stresses resulted in small treatment effects on the size of the vegetative cover and large effects on leaf deterioration and fruit production. Crop responses to salinity may result from an osmotic component of the soil water potential or from other salt effects on the crop physiology. Relating plant data to cumulative ET allowed a distinction to be made between the effect on water availability and specific salinity effects. The relation between fruit fresh weight and ET was not sensitive to EC_i. The slopes for fruit dry weights were also insensitive to EC_i but the intercept was larger for saline treatments. At any given ET saline water increased fruit number, increased fruit dry matter content and decreased fruit netting, in comparison with non-saline water. The combination of salinity and soil-water deficit was detrimental to fruit quality. Saline soil-water deficit decreased the percentage of marketable (netted) fruit and caused an early end to the period of marketable fruit production. Non-saline soil-water deficit increased the percentage of marketable fruit and had no effect on the duration of the production period. Late non-saline water stress caused a pronounced increase in the percentage of marketable fruit.     

Soil salinity related to physical soil characteristics and irrigation management in four Mediterranean irrigation districts

Irrigated agriculture is threatened by soil salinity in numerous arid and semiarid areas of the Mediterranean basin. The objective of this work was to quantify soil salinity through electromagnetic induction (EMI) techniques and relate it to the physical characteristics and irrigation management of four Mediterranean irrigation districts located in Morocco, Spain, Tunisia and Turkey. The volume and salinity of the main water inputs (irrigation and precipitation) and outputs (crop evapotranspiration and drainage) were measured or estimated in each district. Soil salinity (EC_e) maps were obtained through electromagnetic induction surveys (EC_a readings) and district-specific EC_a-EC_e calibrations. Gravimetric soil water content (WC) and soil saturation percentage (SP) were also measured in the soil calibration samples. The EC_a-EC_e calibration equations were highly significant (P < 0.001) in all districts. EC_a was not significantly correlated (P > 0.1) with WC, and was only significantly correlated (P < 0.1) with soil texture (estimated by SP) in Spain. Hence, EC_a mainly depended upon EC_e, so that the maps developed could be used effectively to assess soil salinity and its spatial variability. The surface-weighted average EC_e values were low to moderate, and ranked the districts in the order: Tunisia (3.4 dS m⁻¹) > Morocco (2.2 dS m⁻¹) > Spain (1.4 dS m⁻¹) > Turkey (0.45 dS m⁻¹). Soil salinity was mainly affected by irrigation water salinity and irrigation efficiency. Drainage water salinity at the exit of each district was mostly affected by soil salinity and irrigation efficiency, with values very high in Tunisia (9.0 dS m⁻¹), high in Spain (4.6 dS m⁻¹), moderate in Morocco (estimated at 2.6 dS m⁻¹), and low in Turkey (1.4 dS m⁻¹). Salt loads in drainage waters, calculated from their salinity (EC_dw) and volume (Q), were highest in Tunisia (very high Q and very high EC_dw), intermediate in Turkey (extremely high Q and low EC_dw) and lowest in Spain (very low Q and high EC_dw) (there were no Q data for Morocco). Reduction of these high drainage volumes through sound irrigation management would be the most efficient way to control the off-site salt-pollution caused by these Mediterranean irrigation districts.     

Prediction of the soil-depth salinity-trend in a vineyard after sustained irrigation with saline water

Remote sensing combined with an ability to look deeper than the soil surface is currently high in demand. This study was conducted through scaling down the amount of soil data from a saline irrigation water experiment to see if one can still capture the essential soil salinity depth trends within the data, to a level that can enhance the ability of remote methods. A saline irrigation experiment with 6 water qualities was conducted for 8 years on 1.2 ha of vineyard land near Robertson in the Western Cape Province of South Africa. Soil water was sampled at regular intervals at 5 depths between 0.15 and 1.2 m with suction cup lysimeters at a fixed time following each irrigation. Electrical conductivity of the soil water (EC_sw) was determined after sampling. Data collected over the full 8-year period were investigated for depth trends in EC_sw, seeking trend lines with lowest polynomial order that were still significantly predict the salinity profile. At all treatment levels a first order polynomial equation, fitted to the salinity profiles, significantly predicted the salinity trends. The EC_sw value at only two depths could therefore be used to calculate total salt accumulation and soil water quality below the root zone. The implication is that considerable value can be obtained from minimal measurements both in estimating salt accumulation in the soil profile and predicting water quality in return flow from saline irrigation.     

Comparison and sensitivity of measurement techniques for spatial distribution of soil salinity

In Khorezm, a district of Uzbekistan situated in the Aral Sea Basin, soil salinization is an important driver of soil degradation in irrigated agriculture. The main objective of this study was to identify techniques that enable rapid estimation of soil salinity. Therefore, bulk electrical conductivity of the soil (EC_a-meas) was measured with three different devices (2P, 4P, and CM-138) and electrical conductivity of the soil paste (EC_p-meas) was measured with the so-called 2XP device. These measurements were compared with independent estimates of EC_a-calc and EC_p-calc based on laboratory measurements of the saturated extract, EC_e, of soil samples from the same sites. Soil salinity could be assessed satisfactorily with all four devices. EC_p-meas could be well reproduced by the 2XP device (R ² = 0.76), whereas EC_a-meas estimates using 2P, 4P, and CM-138 in the field were less accurate (R ² < 0.50). The sensitivity of all devices to the main ions Cl⁻ and Ca^{2 +} suggests that the measuring principles are similar for all instruments. The devices can therefore be used interchangeably. Field assessment of soil salinity was considerably enhanced by the use of CM-138, because large areas can be quickly assessed, which may be desirable in spite of the lower accuracy.     

Evaluation of crop water stress index for LEPA irrigated corn

This study was designed to evaluate the crop water stress index (CWSI) for low-energy precision application (LEPA) irrigated corn (Zea mays L.) grown on slowly-permeable Pullman clay loam soil (fine, mixed, Torrertic Paleustoll) during the 1992 growing season at Bushland, Tex. The effects of six different irrigation levels (100%, 80%, 60%, 40%, 20%, and 0% replenishment of soil water depleted from the 1.5-m soil profile depth) on corn yields and the resulting CWSI were investigated. Irrigations were applied in 25 mm increments to maintain the soil water in the 100% treatment within 60–80% of the “plant extractable soil water” using LEPA technology, which wets alternate furrows only. The 1992 growing season was slightly wetter than normal. Thus, irrigation water use was less than normal, but the corn dry matter and grain yield were still significantly increased by irrigation. The yield, water use, and water use efficiency of fully irrigated corn were 1.246 kg/m², 786 mm, and 1.34 kg/m³, respectively. CWSI was calculated from measurements of infrared canopy temperatures, ambient air temperatures, and vapor pressure deficit values for the six irrigation levels. A “non-water-stressed baseline” equation for corn was developed using the diurnal infrared canopy temperature measurements as T _c–T _a = 1.06–2.56 VPD, where T _c was the canopy temperature (°C), Ta was the air temperature (°C) and VPD was the vapor pressure deficit (kPa). Trends in CWSI values were consistent with the soil water contents induced by the deficit irrigations. Both the dry matter and grain yields decreased with increased soil water deficit. Minimal yield reductions were observed at a threshold CWSI value of 0.33 or less for corn. The CWSI was useful for evaluating crop water stress in corn and should be a valuable tool to assist irrigation decision making together with soil water measurements and/or evapotranspiration models. Received: 19 May 1998     

Quantitative evaluation of soil salinity and its spatial distribution using electromagnetic induction method

In the Lower Yellow River Delta, soil salinity is a problem due to the presence of a shallow, saline water table and marine sediments. Spatial information on soil salinity at the field level is increasingly needed, particularly for better soil management and crop allocation in this area. In this paper, a mobile electromagnetic induction (EMI) system including EM38 and EM31 is employed to perform field electromagnetic (EM) survey, and fast determination and quantitative evaluation of the spatial pattern of soil salinity is discussed using the field EM survey data. Optimal operation modes of EM38 and EM31 are determined to establish multiple linear regression models for estimating salinity from apparent soil electrical conductivity (EC_a). Spatial trend and semivariogram are illustrated and spatial distribution of field salinity status is further visualized and quantitatified. The results suggest that EC_a (EM38 and EM31) data is highly correlated with salinity, and that the interpretation precision of soil salinity at various layers can be improved using EM_38h and EM_31h (where h represents the horizontal mode of EM measurement). Both EM_38h and EM_31h exhibit significant geographic trend. Nested spherical models fit the semivariance of EM_38h and EM_31h better than single spherical models. Spatial autocorrelation of EM_31h is stronger than that of EM_38h, and short-range variation is the chief constitute of spatial heterogeneity for both EM_38h and EM_31h. Quantitative classification shows that soil salinity exhibits the trend of accumulation in the root zone. In 0-1.0 m solum, heavy salinized and saline soils are the predominant soil types, accounting for 54% and 41% of total survey area, respectively. The area of light and moderate salinized soils is comparatively small, which accounts for only 0.4% and 4.6%, respectively.     

Soil–water and solute movement under precision irrigation: knowledge gaps for managing sustainable root zones

Precision irrigation involves the accurate and precise application of water to meet the specific requirements of individual plants or management units and minimize adverse environmental impact. Under precision irrigation applications, water and associated solute movement will vary spatially within the root zone and excess water application will not necessarily result in deep drainage and leaching of salt below the root zone. This paper estimates that 10% of the irrigated land area (producing as much as 40% of the total annual revenue from irrigated land) could be adversely affected by root zone salinity resulting from the adoption of precision irrigation within Australia. The cost of increases in root zone salinisation due to inappropriate irrigation management in the Murray and Murrumbidgee irrigation areas was estimated at AUD 245 million (in 2000/01) or 13.5% of the revenue from these cropping systems. A review of soil–water and solute movement under precision irrigation systems highlights the gaps in current knowledge including the mismatch between the data required by complex, process-based soil–water or solute simulation models and the data that is easily available from soil survey and routine soil analyses. Other major knowledge gaps identified include: (a) effect of root distribution, surface evaporation and plant transpiration on soil wetted patterns, (b) accuracy and adequacy of using simple mean values of root zone soil salinity levels to estimate the effect of salt on the plant, (c) fate of solutes during a single irrigation and during multiple irrigation cycles, and (d) effect of soil heterogeneity on the distribution of water and solutes in relation to placement of water. Opportunities for research investment are identified across a broad range of areas including: (a) requirements for soil characterisation, (b) irrigation management effects, (c) agronomic responses to variable water and salt distributions in the root zone, (d) potential to scale or evaluate impacts at various scales, (e) requirements for simplified soil–water and solute modelling tools, and (f) the need to build skills and capacity in soil–water and solute modelling.     

Comparative affects of blending,intera/inter-seasonal cyclic uses of alkali and good quality waters on soil properties and yields of paddy and wheat

Degradation of soils irrigated with the ground waters having residual alkalinity constitutes a major threat to irrigated agriculture in semi-arid parts especially the South Asia. Paddy–wheat has come to stay as the major crop rotation in the afflicted areas, which is either irrigated solely with alkali waters (AW) or combined with good quality water supplies through canal networks. Therefore, to develop appropriate conjunctive use strategies for the latter situations, response of paddy and wheat was evaluated to the combined use of a good quality water (GW, EC_iw 0.5, RSC nil) and that having residual alkalinity (AW, EC_w 2.3 dS m⁻¹, RSC 11.3 mequiv L⁻¹, SAR_w 15 mmol L^0.5) for 6 years (1997–2003) in lysimeters (2.0 m deep, 0.9 m i.d., with drainage outlets at the bottom) filled in with a sandy loam soil (pH 7.8, ESP 5.3). Increase in soil pH (8.71), salinity (3.8 dS m⁻¹) and sodicity (ESP 27.3) as a consequence of irrigation with alkali water markedly affected the yields of both the crops. The sustainability yield index (SYI) was 0.522 and 0.793 for paddy and wheat, respectively, indicating the sensitivity of the former to the use of alkali water. Keeping the AW input to be similar through irrigations, the SYI for paddy with blending of GW and AW in the ratio of 2:1, 1:1 and 1:2 was 0.732, 0.708 and 0.678, respectively, when compared with 0.751, 0.729 and 0.701 under intera-seasonal cyclic uses. Similarly, the SYI of wheat ranged between 0.821–0.907 and 0.853–0.949 with blending and cyclic uses of the two waters, indicating thereby a yield advantage with the latter. When the two waters were rotated inter-seasonally, the dilution effects of monsoon rains helped to induce greater use of AW for paddy. The overall deterioration in soil properties under different modes was related to proportion of AW applied. It was concluded that the alternating good quality and alkali waters could be a better way to alleviate sodicity problems caused with the use of alkali water alone.     

Artificial neural network and time series models for predicting soil salt and water content

Volumetric water content of a silt loam soil (fluvo-aquic soil) in North China Plain was measured in situ by L-520 neutron probe (made in China) at three depths in the crop rootzone during a lysimeter experiment from 2001 to 2006. The electrical conductivity of the soil water (EC_sw) was measured by salinity sensors buried in the soil during the same period at 10, 20, 45 and 70 cm depth below soil surface. These data were used to test two mathematical procedures to predict water content and soil water salinity at depths of interest: all the available data were divided into training and testing datasets, then back propagation neural networks (BPNNs) were optimized by sensitivity analysis to minimizing the performance error, and then were finally used to predict soil water and EC_sw. In order to meet with the prerequisite of autoregressive integrated moving average (ARIMA) model, firstly, original soil water content and EC_sw time series were likewise transformed to obtain stationary series. Subsequently, the transformed time series were used to conduct analysis in frequency domain to obtain the parameters of the ARIMA models for the purposes of using the ARIMA model to predict soil water content and EC_sw. Based on the statistical parameters used to assess model performance, the BPNN model performed better in predicting the average water content than the ARIMA model: coefficient of determination (R²) = 0.8987, sum of squares error (SSE) = 0.000009, and mean absolute error (MAE) = 0.000967 for BPNN as compared to R² = 0.8867, SSE = 0.000043, MAE = 0.002211 for ARIMA. The BPNN model also performed better than the ARIMA model in predicting average EC_sw of soil profile. However, the ARIMA model performed better than the BPNN models in predicting soil water content at the depth of 20 cm and EC_sw at the depth of 10 cm below soil surface. Overall, the model developed by BPNN network showed its advantage of less parameter input, nonlinearity, simple model structure and good prediction of soil EC_sw and water content, and it gave an alternative method in forecasting soil water and salt dynamics to those based on deterministic models based on Richards’ equation and Darcy's law provided climatic, cropping patterns, salinity of the irrigation water and irrigation management are very similar from one year to the next.     

Nitrogen fixation in a white clover-grass pasture irrigated with saline groundwater

Nitrogen (N₂) fixation in an irrigated white clover-grass sward was estimated using the ¹⁵N isotope dilution technique following the addition of K¹⁵NO₃ at 0.5 gN m^–2 and 80 atom % ¹⁵N in a field study during the 1990–91 season. Two water salinity treatments (channel water; EC_w = 0.07 and groundwater; 2.4 dS m^–1) and four irrigation frequencies were included in a factorial design with four replicates. The channel water treatments were irrigated when pan evaporation minus rainfall equalled 50 mm, whereas the groundwater treatments were irrigated at deficits of 40, 50, 65 or 80 mm. Cumulative dry matter of the clover was significantly less in treatments irrigated with saline groundwater compared to channel water at day 164, and soil salinities (EC_e) increased on average from 2.3 to 5.07 dS m^–1. In contrast, salinity of the irrigation water had no effect on the cumulative yield of grass. Cumulative dry matter of the grass and clover were not affected by groundwater irrigation frequency. Total N accumulation by the grass did not differ significantly between treatments. However, total N accumulation in white clover was significantly less (P < 0.05) in all treatments irrigated with groundwater compared to channel water. Neither the N concentrations of the grass nor the clover differed significantly between the salinity treatments. Salinity and irrigation frequency had no effect on the proportion of clover N (P_atm) derived from N₂ fixation. The values of P_atm were high throughout, and increased progressively from 0.78 at day 39 to 0.91 at day 164 (P < 0.01). However, the yield of fixed N was lower in clover when watered with groundwater compared to channel water (P < 0.01). Thus low to moderate soil salinity did not affect the symbiotic dependence of clover, but the yield of biologically-fixed N was depressed through a reduction in the dry matter yield of the legume.     

Effects of water infiltration and storage in cultivated soil on surface irrigation

Surface irrigation analysis and design require the knowledge of the variation of the cumulative infiltration water Z (L) (per unit area) into the soil as a function of the infiltration time t (T). The purpose of this study is to evaluate water infiltration and storage under surface irrigation in an alluvial clay soil cultivated with grape yield, and to determine if partially wetted furrow irrigation has more efficient water storage and infiltration than traditional border irrigation. The two irrigation components considered were wet (WT) and dry (DT) treatments, at which water applied when available soil water reached 65% and 50%, and the traditional border irrigation control. Empirical power form equations were obtained for measured advance and recession times along the furrow length during the irrigation stages of advance, storage, depletion and recession. The infiltration (cumulative depth, Z and rate, I) was functioned to opportunity time (t_o) in minute for WT and DT treatments as: Z_WT = 0.528 t_o^0.6, Z_DT = 1.2 t_o^0.501, I_WT = 19 t_o^−0.4, and I_DT = 36 t_o^−0.498. The irrigation efficiency and soil water distribution have been evaluated using linear distribution and relative schedule depth. Coefficient of variation (CV) was 5.2 and 9.5% for WT and DT under furrow irrigation system comparing with 7.8% in border, respectively. Water was deeply percolated as 11.88 and 19.2% for wet and dry furrow treatments, respectively, compared with 12.8% for control, with no deficit in the irrigated area. Partially wetted furrow irrigation had greater water-efficiency and grape yield than both dry furrow and traditional border irrigations, where application efficiency achieved as 88.1% for wet furrow irrigation that achieved high grape fruit yield (30.71 Mg/ha) and water use efficiency 11.9 kg/m³.     

紫鹊界梯田区不同土地利用类型土壤入渗特征研究

采用压力仪和盘式入渗仪测定了紫鹊界梯田区域不同土地利用类型土壤的水分入渗特征并应用多种计算理论推算其水分运移参数。结果表明,林地和退耕地土壤持水能力比旱田和水田强,在相同吸力条件下林地和退耕地的含水率平均比旱田和水田分别高28%和55%;不同土地利用类型对土壤水分累积入渗量、土壤吸渗率和导水率有着较大的影响,其值均按照林地、退耕地、水田、旱地的顺序依次增大;研究结果初步揭示了紫鹊界土地利用类型对梯田区土壤水分运移机理影响。     

Irrigation with brackish water under desert conditions I. Problems and solutions in production of onions (Allium cepa L.)

To identify the problems and suggest solutions for onion production under brackish water irrigation in a desert environment, a series of trials with brackish water (electrical conductivity, EC_i = 4.4 dS/m) and fresh water (EC_i = 1.2 dS/m) was conducted, using both sprinkler and drip irrigation systems.Under sprinkler irrigation with brackish water the mean electrical conductivity of the saturated soil extract ($\text{EC}$_e) was about 6.0 dS/m and the yield reduction was 60%. With drip irrigation, the $\text{EC}$_e under the drippers was about 5.0 dS/m and the yield reduction was 30%. Sprinkler irrigation affected yield through a reduction in both bulb size and bulb number per unit area. Drip irrigation affected the bulb number only. In the latter system seedling death occurred during the first 40 days following field emergence. Yield reduction was completely prevented by germinating and establishing the field with freshwater irrigation before transferring to brackish water irrigation, 45 days after sowing.With the sprinkler system, onion yield with brackish water irrigation could be increased by either increasing the sowing density or by alternating between brackish and fresh water irrigation.     

Drainage and desalinization of heavy clay soil in Portugal

The Leziria Grande area consists mainly of poorly drained, saline clay soils of marine origin. Three experimental fields were laid out to find whether subsurface drainage can be effective in lowering the groundwater table and improving desalinization.Subsurface drainage results in a lower groundwater table than does surface drainage. With increasing spacing, the groundwater remains at a higher level for longer periods, which is expressed here by the sum of exceedances of the groundwater table above 30 cm during winter.Soil salinity, expressed as EC_1:2, and sodicity, expressed as E.S.P., decreased during the first 3 years, in which precipitation varied between 600 and 750 mm and the average drain outflow was about 250 mm. The leaching efficiency decreased with time, indicating that the removal of salt is a slow process in fine-textured soil.Application of gypsum lowered the E.S.P. The infiltration rate and the drain outflow increased. Although the total amount of salts in the drainwater was 40% higher than for the untreated plots, no lower EC_1:2 values were found. This is ascribed to spatial variability in soil salinity.     

Monitoring scale-specific and temporal variation in electromagnetic conductivity images

In semiarid and arid landscapes, irrigation sustains agricultural activity but because of increasing demands on water resources there is a need to make gains in efficiency. As such spatial variation of soil properties such as clay and salinity needs to be understood because they strongly influence soil moisture availability. One way is to use electromagnetic induction because apparent soil electrical conductivity (EC_a) is related to volumetric soil moisture (θ), clay and salinity (EC_e). However, depth-specific variation has not been explored. Our aim is to generate electromagnetic conductivity images (EMCIs) by inverting DUALEM-421 EC_a and show how true electrical conductivity (σ) can be correlated with θ, clay, EC_e and bulk density (ρ) on different days post-irrigation (i.e., 1, 4 and 12 days). Two-dimensional multi-resolution analysis (MRA) is used to show how spatio-temporal variation in σ is scale-specific and how soil properties influence σ at different scales. We study this beneath a pivot irrigated alfalfa crop. We found that σ on days 1 and 4 was correlated with θ (Pearson’s r = 0.79 and 0.61) and clay (0.86 and 0.80) and the dominant scale of variation occurred at 9.3–18.7 m (50.21 % of total variation), >74.7 m (23.18 %) and 4.7–9.3 m (16.29 %). Between 9.3–18.7 and 4.7–9.3 m the variation may be a function of the cutter width (8 m), while >74.7 m may be change in clay and EC_e and gantry spacing (~48 m). The sprinkler spacing (1.2 and 1.6 m) explains short-scale variation at 1.2–2.3 m.     

Estimating in situ soil–water retention and field water capacity in two contrasting soil textures

A priori knowledge of the in situ soil field water capacity (FWC) and the soil-water retention curve for soils is important for the effective irrigation management and scheduling of many crops. The primary objective of this study was to estimate the in situ FWC using the soil-water retention curve developed from volumetric water content (θ), and water potential (ψ) data collected in the field by means of soil moisture sensors in two contrasting-textured soils. The two study soils were Lihen sandy loam and Savage clay loam. Six metal frames 117 cm × 117 cm × 30 cm high were inserted into the soil to a depth of 5–10 cm at approximately 40 m intervals on a 200 m transect. Two Time Domain Reflectrometry (TDR) sensors were installed in the center of the frame and two Watermark (WM) sensors were installed in the SW corner at 15 and 30 cm depths to continuously monitor soil θ and ψ, respectively. A neutron probe (NP) access tube was installed in the NE corner of each frame to measure soil θ used for TDR calibration. The upper 50–60 cm of soil inside each frame was saturated with intermittent application of approximately 18–20 cm of water. Frames were then covered with plastic tarps. The Campbell and Gardner equations best fit the soil–water retention curves for sandy loam and clay loam soils, respectively. Based on the relationship between soil ψ and elapsed time following cessation of infiltration, we calculated that the field capacity time (t _FWC) were reached at approximately 50 and 450 h, respectively, for sandy loam and clay loam soils. Soil-water retention curves showed that θ values at FWC (θ _FWC) were approximately 0.228 and 0.344 m³ m⁻³, respectively, for sandy loam and clay loam soils. The estimated θ _FWC values were within the range of the measured θ _FWC values from the NP and gravimetric methods. The TDR and WM sensors provided accurate in situ soil–water retention data from simultaneous soil θ and ψ measurements that can be used in soil-water processes, irrigation scheduling, modeling and chemical transport.