Spatial Distribution Of Soil Water Research Articles

Hyperspectral inversion of soil water and salt information based on fractional order derivative technology.

Accurate and efficient acquisition of soil water and salt information is a prerequisite for the improvement and sustainable utilization of saline lands. With the ground field hyperspectral reflectance and the measured soil water-salt content as data sources, we used the fractional order differentiation (FOD) technique to process hyperspectral data (with a step length of 0.25). The optimal FOD order was explored at the correlation level of spectral data and soil water-salt information. We constructed two-dimensional spectral index, support vector machine regression (SVR) and geographically weighted regression (GWR). The inverse model of soil water-salt content was finally evaluated. The results showed that FOD technique could reduce the hyperspectral noise and explore the potential spectral information to a certain extent, improve the correlation between spectrum and characteristics, with the highest correlation coefficients of 0.98, 1.35 and 0.33. The combination of characteristic bands screened by FOD and two-dimensional spectral index were more sensitive to characteristics than one-dimensional bands, with the optimal responses of order 1.5, 1.0 and 0.75. The optimal combinations of bands for maximum absolute correction coefficient of SMC were 570, 1000, 1010, 1020, 1330 and 2140 nm, pH were 550, 1000, 1380 and 2180 nm, and salt content were 600, 990, 1600 and 1710 nm, respectively. Compared with the original spectral reflectance, the validation coefficients of determination (Rp2) of the optimal order estimation models for SMC, pH, and salinity were improved by 1.87, 0.94 and 0.56, respectively. The overall GWR accuracy in the proposed model was better than SVR, where the GWR optimal order estimation models Rp2 were 0.866, 0.904 and 0.647, and the relative per-centage difference were 3.54, 4.25 and 1.86, respectively. The overall spatial distribution of soil water and salt content in the study area was characterized by low in the west and high in the east, with more serious soil alkalinization problems in the northwest and less severe in the northeast. The results would provide scientific basis for the hyperspectral inversion of soil water and salt in the Yellow River Irrigation Area and a new strategy for the implementation and management of precision agriculture in saline soil areas.

Spatial Distribution of Soil Water and Salt in a Slightly Salinized Farmland

It is important to study the mechanisms associated with the spatial distribution of soil water and salt to control soil salinization and promote the sustainable development of farmland. Six plots in a slight farmland with different spatial locations were selected to determine the spatial distribution of soil water and salt and their correlation using the multifractal method. Each plot was applied using the grid method (15 m × 15 m, 3600 m2), where each sampling site was located at the center point coordinates. The 0–20 and 20–40 cm soil layers were sampled.The spatial variability of the soil water and salt were 1.41 and 1.73 fold higher in the upstream farmland than in the downstream farmland. The spatial variability of the soil water and salt was significantly correlated. In addition, the spatial variability of the soil water and salt significantly correlated in the 0–20 and 20–40 cm layers. The spatial distribution of both soil water and salt in the entire soil layer had similar characteristics at this sampling scale. Our results provide a theoretical basis to study the interactive mechanisms associated with the distribution of soil water and salt.

The effect of mulching films on the temporal and spatial distribution of soil water in semiarid hilly area

The effect of mulching films on the temporal and spatial distribution of soil water in semiarid hilly area

Spatial distribution of soil water, plant roots, and water use pattern under different drip fertigation regimes in an apple-soybean intercropping system on the Loess Plateau, China

The irrational utilization of water resources restricts the sustainable development of tree-crop intercropping in the Loess region. Drip fertigation was applied for two years in an intercropping system to save water resources, and improve water use efficiency (WUE) and crop yield. The spatial distribution of soil water, plant roots, and water use pattern were investigated in a drip fertigated intercropping system. A two-factor randomized block design was adopted. Four irrigation levels: 60% (W1), 70% (W2), 80% (W3), and 90% (W4) of field capacity (Fc), together with three nitrogen fertilization treatments [59.40 (F1), 92.00 (F2), and 124.32 kg ha–1 (F3)], were set up with a blank control (CK), which was rain-fed with base fertilizer applied before sowing. The soil water content (SWC) distribution had a “U” shape in the horizontal direction, and the vertical water competition area moved downward as the intercropping year increased. The soybean root length density (RLD) increased as the distance from the tree row increased, but apple showed the reverse trend. The overlap between apple and soybean roots generally decreased as the distance from the tree row increased, but the vertical overlap of roots extended from 0–40 cm (2018) to 0–60 cm (2019). The interspecific competition increased with the increase in intercropping years, resulting in a downward shift of the competition area. Water consumption in 2018 and 2019 increased as the irrigation level rose. The SWC, RLD, soybean yield, and WUE of W3F2 were the highest among the treatments. Excessive irrigation can lead to vigorous vegetative growth, but reduced yields. The negative effects of intense interspecific competition were relieved under drip fertigation. The results suggested that drip irrigation at 80% Fc combined with 92 kg ha–1 N fertilization can improve the yield and WUE of the apple-soybean system on the Loess Plateau.

Estimation of variability in soil water content in a forested critical-zone experimental catchment in Eastern China

Knowledge of soil water content (SWC) dynamics within soil profiles is crucial for the effective management of water and soil resources. This study aims to clarify the temporal variability and stability of SWC in a forested critical-zone experimental catchment, and further to improve the understanding of the temporal and spatial distribution of soil water in a typical hilly catchment in eastern China. The selected Nandadish (NDD) catchment covering 0.79 ha was instrumented with 34 SWC monitoring sites using Frequency Domain Reflectometry. The consecutive high-resolution monitoring data of soil water at different depths of the sites were collected from January 2017 to December 2019. The results showed that the SWC of the shallow layer (0–30 cm) had the strongest variability over time during the three hydrologic years. The interannual variability of SWC showed the opposite regularity with that of the seasonal variability. Specifically, the spatial variability of SWC in the dry years was greater than that in wet years; whilst the temporal stability of SWC in dry seasons was greater than that in rainy seasons. Precipitation and temperature were the two dominant factors influencing the temporal variation of SWC. Precipitation controlled the interannual variation of SWC, while temperature controlled the seasonal variation of SWC. Additionally, soil water had high temporal stability throughout the observation period in NDD catchment, and the most representative point was located at a relatively flat and central place, which can be used to simulate the variability of SWC under different rainfall conditions in the study area. The temporal stability of SWC patterns was controlled by topography, geographic location, throughfall, and the groundwater level in the study area, which was characterized by sloping terrain and forested cover. This research provides scientific bases for the optimum design of ground sampling, and the temporal and spatial prediction for soil moisture in a typical eastern hilly area with forest land uses.

Modeling agro-hydrological processes and analyzing water use in a super-large irrigation district (Hetao) of arid upper Yellow River basin

High water consumption in large irrigation districts has resulted in large river water diversion and serious water use conflicts in the Yellow River basin (YRB). The excessive water applied is also a cause of the dual problems of soil salinity and water logging in the upper YRB, which further negatively impacts crop yields. This study is aimed at modeling and analyzing the agro-ecohydrological processes and water use performance in large irrigation districts of the YRB. The Hetao Irrigation District (Hetao), a super-large irrigation district located in the upper YRB, was selected as a typical research area. A watershed hydrological model (SWAT-AG) with enhanced capacity for simulating regional agro-ecohydrological processes was adopted in this study. We simulated the agro-ecohydrological processes in Hetao and analyzed the regional characteristics of depth to groundwater table, soil moisture and salt content, evapotranspiration and crop yield. Water accounting was further carried out to interpret water cycle processes and evaluate water use performance in Hetao. Results indicated that Hetao was characterized by shallow depth to groundwater table, high soil moisture and high evapotranspiration. For farmland, there was a decreasing trend for both soil moisture and evapotranspiration from the upper to lower regions in Hetao, but this decreasing trend was not found for natural land. Further, results showed soil salt content has strong spatial variations with land use type, crops and shallow groundwater environments. Correspondingly, crop yields presented a decreasing trend from the upper to lower regions of Hetao, with large spatial variabilities closely related to the spatial distribution of soil water and salt conditions across Hetao. It was worth noting that the regional water use efficiency was relatively low in Hetao, especially with large proportions of non-beneficial water consumption to total water consumption. Overall, this study provided a thorough understanding of agro-ecohydrological processes and their regional characteristics, and the current situation of irrigation water use issues in the whole Hetao district, and further provided valuable insights in improving water use and management in large irrigation districts of the upper YRB.

Amelioration of takyric solonetz using drip irrigation with soil‐water‐redistribution medium*

AbstractTakyric solonetz is a highly saline‐sodic soil with poor soil structure and a saturated hydraulic conductivity close to zero. Drip irrigation, soil‐water‐redistribution medium (RM), ridge planting and film mulching have been successfully used to ameliorate such soils. Here, we compared soil moisture and cation distribution during the amelioration with planting years of 1, 3, 4, 5 and 6. A suitable root zone with low electrical conductivity, pH and sodium absorption ratio was quickly and continuously formed in the arid area. An irrigation regime with a decreasing soil matric potential threshold of −5 to −20 kPa in improving years 1–4 and stabilized at −20 kPa was feasible. The wetting front kept moving downward with time, and the dry soil layer in the middle of the profile gradually disappeared after 4 years. In the initial 4 years, the Na+ in the root zone leached mainly to the top layer of ridge slope and furrow. And then the Na+ leached downward. The spatial distribution of soil water and Na+ indicated that soil permeability improved with time, and significant changes occurred after the fifth year.

Dynamics of interspecific water relationship in vertical and horizontal dimensions under a dryland apple-Brassica intercropping system: Quantifying by experiments and the 3D Hi-sAFe model

Separation of water absorption by roots between trees and crops is critical to intercropping in water-limited regions. The current knowledge about interspecific water relationship is in one dimension, i.e., the vertical direction, little is known in both vertical and horizontal directions under different rainfall conditions. Therefore, an experiment was carried out in an apple tree based intercropping system and an apple tree monoculture on China's Loess Plateau during one growing season which started with a high rainfall period followed by a low rainfall period. We investigated the spatial distribution of soil water and plant root, and simulated their water use strategies using the 3D Hi-sAFe model. The root growth of apple trees in the top 60 cm was restricted by intercropping in the cropped area but increased in the top 40 cm in the uncropped area compared with monoculture. In the high rainfall period, compared with the control, the apple trees' water absorption from the uncropped area (0–60 cm) increased by 44.43%, but the water utilization from the cropped area (0–60 cm) decreased by 32.59%. More than 90% of the soil water absorbed by B. napus came from the 0–60 cm soil layer in the cropped area. During the low rainfall period, intercropping promoted an increase of 14.79% in apple trees' water absorption from the 60–100 cm soil layer in the cropped area. The soil water in the 20–60 cm layer became the main source for B. napus and its contribution was 63.51%, but only 20.56% of the water absorbed by apple trees came from here. It can be concluded that, to alleviate water competition, water absorption between species was segregated horizontally during the wet period due to the roots "niche differentiation", while water use complementarity occurred along the vertical axis during the dry period.

Temporal and spatial distribution of soil water and nitrate content affected by surface irrigation and fertilizer rate in silage corn fields

Among surface irrigation systems, long border and furrow are more adaptive to mechanized farming but may cause a non-uniform distribution of water and nutrients. In this study, field experiments were carried out in a flat silage corn field in Hebei, China to investigate the uniformity along the length of border or furrow to understand the spatial and temporal distribution characteristics of soil water, electrical conductivity, and nitrate. This will guide irrigation and fertigation management recommendations, land consolidation, and high standard farmland construction. Border and furrow irrigation were tested using fertilizer rates of 750, 600, 450 and 300 kg/ha. Low quarter distribution uniformity (DUlq) and storage efficiency (E) were quantified to determine the distribution of soil water and soil nitrate content. The results indicate heterogeneity along the length of the border or furrow is weak for soil water content and is moderate for nitrate content, based on the uniformity coefficient (CV). The average low quarter distribution uniformity of soil water (DUlqW) was 96.34, there was a significant effect of irrigation type on DUlqW, and the DUlqW for border irrigation was 0.8% larger than that for furrow irrigation. The average low quarter distribution uniformity of nitrate content DUlqN was 79.04, and there was no significant influence of irrigation type and fertilizer rate on DUlqN. Spatial and temporal distribution analysis showed that the variation of soil water in the 0–60 cm soil layer was larger than that in 60–100 cm soil layer, and the electrical conductivity (EC) and nitrate content gradually decreased with increasing soil depth. There was a decreasing trend in soil EC and nitrate content with decreasing fertilizer rates. The storage efficiency of water (EW) for border irrigation was 56.63, and significantly lower than that for furrow irrigation over the whole growth duration. The nitrate storage efficiency (EN) was 65.47, and there was no significant effect of irrigation type or fertilizer rate on EN. Even with longer borders or furrows of 90 m, the uniformity of water and nitrate along the length of the border or furrow is weak or medium, which can create non-uniform conditions for crop growth. Furrow irrigation may store slightly more water in the top 60 cm of soil compared to border irrigation. Fertilizer rate had no significant effect on the uniformity and distribution of soil water or nitrate.

Interplay between Fingering Instabilities and Initial Soil Moisture in Solute Transport through the Vadose Zone

Modeling water flow and solute transport in the vadose zone is essential to understanding the fate of soil pollutants and their travel times towards groundwater bodies. It also helps design better irrigation strategies to control solute concentrations and fluxes in semiarid and arid regions. Heterogeneity, soil texture and wetting front instabilities determine the flow patterns and solute transport mechanisms in dry soils. When water is already present in the soil, the flow of an infiltration pulse depends on the spatial distribution of soil water and on its mobility. We present numerical simulations of passive solute transport during unstable infiltration of water into sandy soils that are prone to wetting front instability. We study the impact of the initial soil state, in terms of spatial distribution of water content, on the infiltration of a solute-rich water pulse. We generate random fields of initial moisture content with spatial structure, through multigaussian fields with prescribed correlation lengths. We characterize the patterns of water flow and solute transport, as well as the mass fluxes through the soil column. Our results indicate a strong interplay between preferential flow and channeling due to fingering and the spatial distribution of soil water at the beginning of infiltration. Fingering and initial water saturation fields have a strong effect on solute diffusion and dilution into the ambient water during infiltration, suggesting an effective separation between mobile and inmobile transport domains that are controlled by the preferential flow paths due to fingering.

Spatial distribution of soil water content in potato horizontal-ridge profile under various ridge dimensions

Vertical (slopping)-ridge is generally applied to cultivate potato crop in tropical highland area with intensive use of non-organic fertilizer, by which the soil erosion as well as environmental degradation might be significantly accelerated. On the other hand, horizontal (contour)-ridge has been very effective to reduce the soil erosion in potato cropping field, but yet slightly ineffective to support the optimal crop production, due to the waterlogging in the ridge profile. Dimension of the horizontal-ridge is expected to affect water distribution as well as the waterlogged condition, on which a specific study need to be focused, in order to develop an appropriate drainage system on the ridge. This study was aimed to characterize the spatial distribution of soil water in the horizontal-ridge profile under various ridge dimensions. Totally 9-potato-plots of 300 x 300 cm2 with 5% slope were prepared in Serang village, Purbalingga with various dimensions and replications of the horizontal ridges: 30 x 30 x 30 cm3 (G30), 30 x 40 x 30 cm3 (G40), and 30 x 50 x 30 cm3 (G50). Of each plot, the dielectrically (volumetric)-water content of the horizontal-ridge soil within the grid of 80 x 80 cm2 at the depth of 5, 10, and 20 cm were measured every months using EC5 moisture sensors and EM50 data logger, and then averaged. Based on the given grid, the data were plotted in contour pattern and were then analysed using semivariogram (Gstat and GNUplot) to characterize the spatial distribution and variability, respectively. The results showed that the spatial distribution of dielectrically (volumetric)-water contents tended to be inter-correlated within each ridge dimension, and the data were increased with soil depth and ridge dimension increment. Accordingly, the data were spatially-correlated with sill (C, semivariance) ranged from 0.00084-0.00429 cm3 cm−3 and range of influence (a) ranged from 0.624 - 2.809 m, in which the G50 had most representative and stable trend of the spatial variability (C and a values) among others.

Enhancing maize productivity in agroforestry systems through managing competition: lessons from smallholders’ farms, Rift valley, Kenya

Understanding crop performance and productivity in relation to intercropped tree species, tree management and prevailing environmental conditions may assist farmers in managing agroforestry systems appropriately. Our study evaluated the management and spatial effect of six dominant tree species (Eucalyptus spp., Sesbania sesban, Grevillea robusta, Calliandra calothyrsus, Markhamia lutea and Croton macrostachyus) on water availability and crop performance in smallholders’ maize fields in Rift valley, Kenya. Maize performance under C. macrostachyus and M. lutea was also evaluated at on-station experiment in the same area. The smallholder farmers in the study area remain important maize producers in Kenya with an average maize yield of 6.5 tons ha−1 recorded. Maize yield under the dominant tree species in studied smallholder farms showed significant differences (P < 0.001) with leguminous species (C. calothyrsus and S. sesban) recording the highest amount of grain weight. Farmers selectively prune tree they perceive as competitive but still want to maintain them in the farms such as Eucalyptus spp. and G. robusta but miss to equally prune M. lutea and C. macrostachyus which would greatly improve performance of associated crops. In addition to presence of great tree management diversity, the study clearly showed that tree in smallholder farms either have competitive, complementary or balanced-off interaction with crops. Lastly, despite the adequate rainfall throughout the cropping season, dominant tree species in smallholder farms were shown to significantly (P < 0.0001) influence the spatial distribution of soil water.

Spatial distribution of soil water, soil temperature, and plant roots in a drip-irrigated intercropping field with plastic mulch

Intercropping and drip irrigation with plastic mulch are two agricultural practices used worldwide. Coupling of these two practices may further increase crop yields and land and water use efficiencies when an optimal spatial distribution of soil water contents (SWC), soil temperatures, and plant roots is achieved. However, this coupling causes the distribution of SWCs, soil temperatures, and plant roots to be more complex than when only one of these agricultural practices are used. The objective of this study thus was to investigate the effects of different irrigation treatments on spatial distributions of SWCs, soil temperatures, and root growth in a drip-irrigated intercropping field with plastic mulch. Three field experiments with different irrigation treatments (high T1, moderate T2, and low T3) were conducted to evaluate the spatial distribution of SWCs, soil temperatures, and plant roots with respect to dripper lines and plant locations. There were significant differences (p < 0.05) in SWCs in the 0–40 cm soil layer for different irrigation treatments and between different locations. The maximum SWC was measured under the plant/mulch for the T1 treatment, while the minimum SWC was measured under the bare soil surface for the T3 treatment. This was mainly due to the location of drippers and mulch. However, no differences in SWCs were measured in the 60 100 cm soil layer. Significant differences in soil temperatures were measured in the 0 5 cm soil layer between different irrigation treatments and different locations. The soil temperature in the subsoil (15 25 cm) under mulch was higher than under the bare surface. The overlaps of two plant root systems in an intercropping field gradually increased and then decreased during the growing season. The roots in the 0 30 cm soil layer accounted for about 60% 70% of all roots. Higher irrigation rates produced higher root length and weight densities in the 0 30 cm soil layer and lower densities in the 30 100 cm soil layers. Spatial distributions of SWCs, soil temperatures, and plant roots in the intercropping field under drip irrigation were significantly influenced by irrigation treatments and plastic mulch. Collected experimental data may contribute to designing an optimal irrigation program for a drip-irrigated intercropping field with plastic mulch.

Effects of gullies on the spatiotemporal variability of soil-water under different vegetation types on the Loess Plateau

Gullies are pervasive topographic features on the Loess Plateau. We selected two typical gullies in the water-wind erosion region of the plateau to study the effect of gullies on the spatiotemporal variability of soil-water. The distribution and variation of soil-water storage were evaluated using time-series analysis and classical statistical methods. Soil-water storage varied with distance to the gully borders. The spatial average soil-water in the study sites was 300 cm from the border. soil-water in the vertical direction was mostly dependent on the vegetation, and the gullies had little impact. The spatial distribution of soil-water at a slope scale presented different patterns. Strong upland winds increased the evaporation of soil-water, which rapidly dissipated. Soil-water storage at mid-slope near the gullies was as high as 949.9 mm. The bottom of the slopes had soil-water deficits of 4%. Soil-water storage was much higher in an apricot orchard than in shrubland dominated by Caragana korshinskii Kom. Soil evaporation, vegetation type, and the gullies increased the complexity of the pattern of soil-water.

Hillslope-storage and rainfall-amount thresholds as controls of preferential stormflow

Summary Shallow saturated subsurface flow is a dominant runoff mechanism on hillslopes of headwater catchments under humid temperate climate. Its timing and magnitude is significantly affected by the presence of preferential pathways. Reliable prediction of runoff from hillslope soils under such conditions remains a challenge. In this study, a quantitative relationship between rainfall, stormflow, and leakage to bedrock for hillslopes, where lateral preferential runoff represents a dominant part of the overall response, was sought. Combined effects of temporal rainfall distribution and initial hillslope saturation (antecedent moisture conditions) on stormflow, leakage to bedrock, and overall water balance were evaluated by conducting simulations with synthetic rainfall episodes. A two-dimensional dual-continuum model was used to analyze hydrological processes at an experimental hillslope site located in a small forested headwater catchment. Long-term seasonal simulations with natural rainfall indicated that leakage to bedrock occurred mostly as saturated flow during major runoff events. The amount of rainfall needed to initiate stormflow appeared as a dynamic hillslope property, depending on temporal rainfall distribution, initial hillslope storage, and the spatial distribution of soil water within the hillslope. No single valued rainfall threshold responsible for triggering stormflow was found. Rainfall–stormflow as well as rainfall–leakage relationships were found highly nonlinear for low initial hillslope saturations. Temporal rainfall distribution affected the amount of rainfall necessary to initiate stormflow more than it did the amounts of stormflow or leakage to bedrock. In spite of a simple hillslope geometry with constant slope and parallel soil–atmosphere and soil–bedrock interfaces considered in the analysis, the applied model predicted a hysteretic behavior of storage–discharge relationship. The results showed a mutual interplay of components of hillslope water balance exposing a nonlinear character of the hillslope response. The study provided a quantitatively coherent insight in the hydraulic functioning of hillslopes where preferential flow constitutes a dominant part of stormflow.

Quantifying and predicting soil water evaporation as influenced by runoff strip lengths and mulch cover

Soil water evaporation from the cropping surface is a wasteful loss of potentially productive rainwater, thus efficient use of rainwater can help to sustain dryland production. The purpose of this study was to quantify the effect of canopy shading (CS) and mulch levels (ML) on soil water evaporation (Es) from each 1m section of in-field rainwater harvesting (IRWH) and to evaluate the Ritchie (α′) and Stroosnijder (β′) soil evaporation models on the effect of surface treatments. A microlysimetric method was used to measure Es from beneath maize (Zea mays L.) canopy for three consecutive drying cycles across the basin and runoff sections of IRWH on fine sandy loam soil of Bainsvlei Kenilworth ecotope. First, main effects of four runoff strip lengths (RSL) and three ML treatments were statistically analysed on the weighted Es values. Second, the ML treatments were allocated to the main plots and four levels of CS allocated according to lengths of the runoff sections. Third, cumulative Es (∑Es) measurements were used to evaluate empirical equations related to time (α′) and potential evaporation (β′). The two models for Es were compared by considering the effects of surface treatments. A significantly higher Es was observed from a bare (ML0%) treatment compared with either of two mulched treatments viz. mulch level 39% and 96% cover (ML39% and ML96%); no significant differences were found between the mulched treatments. The insignificant effect of RSL treatments on Es implied the dynamics of spatial distribution of soil water and energy that influenced evaporation were as a result of green mulch or shading cover (CS) on Es beneath the canopy. Less suppressive Es properties were developed from bare surface and efficient Es restriction was found under high mulch and shading cover treatments. The α′ and β′ values ranged from 2.34 to 4.26mmd−0.5 and from 1.38 to 2.06mmd−0.5, respectively. In all the treatments the simulated ∑Es was underestimated by the Ritchie model and overestimated by the Stroosnijder model. The main effect of shading was due to the dominant effect of energy limited evaporation (stage-1), while the mulched treatments were mainly driven by soil limited stage (stage-2) of evaporation. The Ritchie model performed well to estimate ∑Es from the basin area and the potential Stroosnijder model from the unshaded runoff strips. The microclimate of the cropping system changed according to surface treatments that highly influenced the Es losses in IRWH of dryland production.

Evaluation of the Precision Agricultural Landscape Modeling System (PALMS) in the Semiarid Texas Southern High Plains

Accurate models to simulate the soil water balance in semiarid cropping systems are needed to evaluate management practices for soil and water conservation in both irrigated and dryland production systems. The objective of this study was to evaluate the application of the Precision Agricultural Landscape Modeling System (PALMS) model to simulate soil water content throughout the growing season for several years and for three major soil series of the semiarid Texas Southern High Plains (SHP). Accuracy of the model was evaluated by comparing measured and calculated values of soil water content and using root mean squared difference (RMSD), squared bias (SB), squared difference between standard deviations (SDSD), and lack of correlation weighted by the standard deviation (LCS). Different versions of the model were obtained by modifying soil hydraulic properties, including saturated hydraulic conductivity (Ks) and residual (θr) and saturated (θs) soil volumetric water content, which were calculated using Rosetta pedotransfer functions. These modifications were combined with updated routines of the soil water solver in PALMS to account for rapid infiltration into dry soils that often occur in the SHP. Field studies were conducted across a wide range of soil and water conditions in the SHP. Soil water content was measured by neutron attenuation and gravimetrically throughout the growing seasons at each location to compare absolute values and the spatial distribution of soil water with PALMS calculated values. Use of Rosetta calculated soil hydraulic properties improved PALMS soil water calculation from 1% - 13% of measured soil volumetric water content (θv) depending on soil type. Large-scale models such as PALMS have the potential to more realistically represent management effects on soil water availability in agricultural fields. Improvements in PALMS soil water calculations indicated that the model may be useful to assess long-term implications of management practices designed to conserve irrigation water and maximize the profitability of dryland and irrigated cropping systems in the SHP.

Sensitivity of EM38 in determining soil water distribution in an irrigated wheat field

Geonics EM38 is a portable, non-invasive equipment that induces an electrical current in the soil for rapid measurement of apparent electrical conductivity (EC a) in the field. We used an EM38 in a wheat field to evaluate the effects of systematic variation in soil water content (within three replicate plots of four irrigation treatments) and seasonal variation in soil temperature on EC a. The effective depth of sensing of EM38 could be varied by using it in both vertical and horizontal dipole modes and by placing it at various heights above the ground. Accumulated water within various soil depths was measured with a neutron probe throughout the season. Values of EC a over the season for a soil depth related linearly or nonlinearly with soil water within that depth with a coefficient of determination ( R 2) of 0.70–0.81. EC a values were also influenced by variation in soil temperature within 5–25 cm depth (range 10.1–29.3 °C) and air temperature (range 14.2–34.0 °C), but to a smaller extent than soil water. The overall relationship between EC a, soil water and soil temperature improved considerably when multiple regression was used ( R 2 = 0.82–0.98). Good correspondence between maps of EC a and soil water content over the experimental field suggests that EC a maps could be useful in determining spatial distribution of soil water within crop fields so that variable rather than uniform quantity of irrigation water can be applied to improve water use efficiency.

Detection of soil water in macropores of undisturbed soil using microfocus X-ray tube computerized tomography (μCT)

Minimal information has been garnered regarding the spatial distribution of soil water in relation to pores and the soil matrix. Destructive layer-by-layer reconstructions derived from polished section methodology exclude any data of water in the soil. In contrast, microfocus X-ray tube computerized tomography generates images of the internal structure of the soil with a resolution down to 1 μm, at the same time creating a visual image of the spatial distribution of water in undisturbed soils. As X-rays pass through the soil, some radiation is absorbed, some is scattered, and some is transmitted. Using advanced microfocus computerized tomography (μCT) which ensures controlled and stable output intensity for X-ray emissions and thus a constant focal spot size and spatial resolution, the resulting pattern of radiation detects to a 0.5% contrast difference. While 2D X-ray imaging is sufficient in many cases, 3D images derived from X-ray irradiation of a soil sample can reveal complex inner structures in more comprehensive format, providing information on the causal connection of water and soil structure. Using the X-ray Feinfocus Y.FOX System and related programmes, two- and three-dimensional images of two different soils (Haplic Luvisol and Stagnic Anthrosol) at field capacity (pF 1.8) have been produced which show films of water which are associated with the pore surfaces. The mean thickness of the water films was 10.6 μm in the Stagnic Anthrosol and 3.0 μm in the Haplic Luvisol. These results were unexpected in pores >50 μm since at field capacity only the adhesive water should be present which would create water films in the range of nanometres. Myriads of colloidal dispersed nanoparticles, detected with dark field microscopy and SEM, seem to be the source of the adhesion and cohesion, causing micro-rheological effects which lead to water films of up to 30 μm in pores. Additionally, nanoparticles correlated to the clay content (fine clay) appear to conglomerate in the water films, presumably forming surface protuberances on the films of varying extent.

Thermal Neutron Computed Tomography of Soil Water and Plant Roots

Neutron radiography is a noninvasive imaging technique that measures the attenuation of thermal neutrons, as is done with x‐ray and γ‐ray radiography, to characterize the internal composition of materials. Neutron and x‐ray imaging are complementary techniques, with neutron imaging especially well suited for materials containing H atoms and other low‐atomic‐weight attenuating materials. Although neutron computed tomography (NCT) techniques are routinely used in engineering, relatively little is known about their application to soils. We developed new techniques that use thermal neutron attenuation to measure the spatial and temporal distribution of water in soils and near roots at near 0.5‐mm spatial resolution or higher. The neutron source was a Mark II Triga Reactor at McClellan Nuclear Radiation Center in Sacramento, CA. After calibration using both deuterated and regular water, the effects of beam hardening and neutron scattering could be corrected for, provided that the total path length for a soil–water mixture does not exceed 1.0 cm, limiting soil sample thickness to about 2.5 cm. Using regular water, for a wide range of soil water content values, experiments demonstrated that NCT is sensitive to small changes in soil volumetric water content, allowing estimation of the spatial distribution of soil water, roots, and root water uptake. Although the spatial resolution of the applied NCT system was 80 μm, an error analysis showed that the averaging measurement volume should be not less than about 0.5 mm for the uncertainty in volumetric water content to be minimized to near 0.01 m3 m−3 A single root water uptake experiment with a corn (Zea mays L.) seedling demonstrated the successful application of NCT, with images showing spatially variable soil water content gradients in the rhizosphere and bulk soil.