Differences In Soil Water Content Research Articles

Long-term soil water content dynamics under different land uses in a small agricultural catchment

Abstract Longer term monitoring of soil water content at a catchment scale is a key to understanding its dynamics, which can assist stakeholders in decision making processes, such as land use change or irrigation programs. Soil water monitoring in agriculturally dominated catchments can help in developing soil water retention measurements, for assessment of land use change, or adaptation of specific land management systems to climate change. The present study was carried out in the Pannonian region (Upper-Balaton, Hungary) on Cambisols and Calcisols between 2015 and 2021. Soil water content (SWC) dynamics were investigated under different land use types (vineyard, grassland, and forest) at three depths (15, 40, and 70 cm). The meteorological data show a continuous decrease in cumulative precipitation over time during the study with an average of 26% decrease observed between 2016 and 2020, while average air temperatures were similar for all the studied years. Corresponding to the lower precipitation amounts, a clear decrease in the average SWC was observed at all the land use sites, with 13.4%, 37.7%, and 29.3% lower average SWC for the grassland, forest, and vineyard sites, respectively, from 2016 to 2020 (measured at the 15 cm depth of the soil). Significant differences in SWC were observed between the annual and seasonal numbers within a given land use (p < 0.05). The lowest average SWC was observed at the grassland (11.7%) and the highest at the vineyard (28.3%). The data showed an increasing average soil temperature, with an average 6.3% higher value in 2020 compared to 2016. The grassland showed the highest (11.3 °C) and the forest soil the lowest (9.7 °C) average soil temperatures during the monitoring period. The grassland had the highest number of days with the SWC below the wilting point, while the forest had the highest number of days with the SWC optimal for the plants.

A New Non-Inserted and Portable FDR Instrument for Measuring Surface Soil Water Content

Soil water content (SWC) is a vital parameter for understanding crop growth and the soil nutrient water cycle. Monitoring SWC without inserting sensors into the soil, which can break the soil structure, has previously been a significant challenge for scientists. In this study, we developed a non-inserted portable frequency domain reflection (NIP-FDR) instrument to monitor SWC continuously and automatically. The working technique of this instrument was based on the improved adjustable high-frequency oscillation method originating from the frequency domain reflection principle. Compared to the control SWC measurement instrument, the difference in SWC at 0–10 cm, 10–20 cm, and 20–30 cm depth was within 1%, 3%, and 15%, respectively, and the mean variation of SWC was less than 5% in the indoor measurements. In the field verification experiment conducted in the summer of 2020, the mean error of SWC measurements at a depth of 0–20 cm was 5%, while we failed to compare SWC at a depth of 20–30 cm due to low variability in the SWC measurement at this depth during the summer measurement period. This pioneer NIP-FDR was able to effectively monitor surface SWC, especially at depths of 0–20 cm.

A Comparative Analysis of Root Growth Modules in HYDRUS for SWC of Rice under Deficit Drip Irrigation

Root distribution during rice cultivation is a governing factor that considerably affects soil water content (SWC) and root water uptake (RWU). In this study, the effects of activating root growth (using growth function) and assigning a constant average root depth (no growth during simulation) on SWC and RWU for rice cultivation under four deficit drip irrigation treatments (T90, T80, T70, and T60) were compared in the HYDRUS-2D/3D model version 3.03. A secondary objective was to investigate the effect of applied deficit irrigation treatments on grain yield, irrigation water use efficiency (IWUE), and growth traits of rice. The simulated DI system was designed to reflect a representative field experiment implemented in El-Fayoum Governorate, Egypt, during two successive seasons during 2017 and 2018. The deficit treatments (T90, T80, T70, and T60) used in the current study represent scenarios at which the first irrigation event was applied when the pre-irrigation average SWC within the upper 60 cm of soil depth was equal to 90%, 80%, 70%, and 60% of plant-available water, respectively. Simulation results showed that as water deficiency increased, SWC in the simulation domain decreased, and thereby, RWU decreased. The average SWC within the root zone during rice-growing season under different deficit treatments was slightly higher when activating root growth function than when considering constant average root depth. Cumulative RWU fluxes for the case of no growth were slightly higher than for the case of root growth function for T90, T80, and T70 accounting for 1289.50, 1179.30, and 1073.10 cm2, respectively. Average SWC during the growth season (24 h after the first irrigation event, mid-season, and 24 h after the last irrigation event) between the two cases of root growth was strongly correlated for T90, T80, T70, and T60, where r2 equaled 0.918, 0.902, 0.892, and 0.876, respectively. ANOVA test showed that there was no significant difference for SWC between treatments for the case of assigning root growth function while the difference in SWC among treatments was significant for the case of the constant average root depth, where p-values equaled 0.0893 and 0.0433, respectively. Experimental results showed that as water deficiency decreased, IWUE increased. IWUE equaled 1.65, 1.58, 1.31, and 1.21 kg m−3 for T90, T80, T70, and T60, respectively. Moreover, higher grain yield and growth traits of rice (plant height, tillers number plant−1, panicles length, panicle weight, and grain number panicles−1) were obtained corresponding to T90 as compared with other treatments. Activating the root growth module in HYDRUS simulations can lead to more precise simulation results for specific dates within different growth stages. Therefore, the root growth module is a powerful tool for accurately investigating the change in SWC during simulation. Users of older versions of HYDRUS-2D/3D (version 2.05 and earlier) should consider the limitations of these versions for irrigation scheduling.

Changes in Deep Soil Water Content in the Process of Large-Scale Apple Tree Planting on the Loess Tableland of China

Soil water has become a major limiting factor in agriculture and forestry development on the Loess Plateau of China. In the past 20–30 years, large areas of apple orchards have been built in this region, which have resulted in excessive consumption of deep soil water and soil desiccation. To evaluate the effects of orchard development on deep soil water content (SWC), a meta-analysis of 162 sampling sites on the loess tableland from 44 peer-reviewed publications was conducted in this study. The results showed that the deep SWC in orchards depended on stand age, planting density and annual precipitation. In regions with 550–600 mm precipitation, the orchard with lower planting density showed no soil desiccation in young and early fruiting stages, while deep soil (>2 m) desiccation occurred in full fruiting and old orchards. The effect of planting density on deep SWC varied with stand age. There were significant differences in SWC among different planting densities in early fruiting orchards (p < 0.05), in which soil desiccation occurred in orchards with higher planting density. However, with the continuous consumption of soil water by apple trees, deep soil desiccation occurred in old orchards regardless of planting density. Further, affected by the spatial variation of annual precipitation, deep SWC in orchards significantly decreased with annual precipitation from 650 to 500 mm among the 44 study sites (p < 0.05). Our results suggest that the planting density should be reasonably regulated on the level of annual precipitation, and apple trees need to be pruned appropriately with a goal of moderate productivity, so as to achieve the sustainable use of regional water resources, food security and economic development.

Impact of tussock grasses removal on soil water content dynamics of a tropical mountain hillslope

AbstractLand use impacts on soil water content (SWC) are scarcely studied in mountain humid ecosystems, such as the Andean páramos, despite its influence on ecohydrological processes. Our objective was to analyze the impacts of extensive grazing on SWC of an Andean páramo hillslope with native tussock grasses. Along two parallel transects in a hillslope, we monitored SWC through pairs of SWC sensors placed at different soil depths (Ah horizon= 10 –35 cm; C horizon= 65 – 75 cm). The tussock grasses were cutout from one transect (Experimental) as an emulation of extensive grazing (without soil destruction), whereas the second transect (Control) remained intact. Both transects were hourly monitored for 10 months (post‐intervention period). Post‐intervention period was compared with a pre‐intervention period of similar precipitation characteristics to minimize the influence of precipitation in the interpretation of the results. We analyzed both periods through differences in SWC between the pairs of sensors (Experimental minus Control) and differences in SWC dynamic of the pairs of sensors through linear regressions (slope and intercept values). Results suggest that the emulation did not change the mean SWC, but caused a reduction of SWC dynamic. Causes of SWC attenuation were a lower interception and transpiration, a higher amount of effective rainfall infiltrated, and similar evaporation from the soil than under unchanged conditions. These observations were done at all the soil depths. Probably, extensive grazing does not negatively affect the hydrological functioning of páramo ecosystems, particularly its water yield, as long as the soil remains undisturbed.

Evaluating the effects of biodegradable film mulching on soil water dynamics in a drip-irrigated field

Drip irrigation with plastic film mulching (PM) is an important agricultural practice used to conserve soil water and produce higher crop yields worldwide. However, the plastic film is usually made from non-degradable polyethylene materials, which may lead to the incorporation of plastic into soils and cause a series of negative environmental effects. As such, biodegradable film mulching (BM) can represent a valuable alternative to PM. A two-year experiment was carried out during the 2014 and 2015 seasons to evaluate the effects of PM, BM, and no film mulching (NM) on the soil water balance in the corn field. The two-dimensional numerical HYDRUS-2D model was calibrated and validated using experimental data collected in 2014 and 2015, respectively, under BM, PM, and NM and then used to evaluate soil water contents (SWC), water fluxes, and soil water balances in two-dimensional soil profiles. The results of numerical simulations were in good agreement with measurements, with the mean relative errors for the calibration and validation periods for BM, PM and NM of 11.2%, 10.7%, and 11.7%, respectively. The results showed that SWCs were similar in the BM and PM scenarios during early and middle crop growth periods. Significant differences in SWCs were observed only after the average fraction of the disintegrated area of the biodegradable film was equal or higher than about 40% (during the late crop growth period). The results also showed that mulching mainly affects SWCs in the top 0–20 cm soil layer. There were significant differences in cumulative evaporation and water fluxes between different treatments, but an insignificant difference in root water uptake between PM and BM. Cumulative evaporation for BM increased by 30.5% compared to PM and decreased by 41.9% compared to NM. Corresponding cumulative water fluxes at the 80 cm depth increased and decreased by 6.6% and 48.8%, respectively. Two-dimensional simulations of soil water content distributions revealed that the “water stress” area in the soil profile (with SWCs lower than readily available water) before irrigation was 1.79 and 0.75 times larger for BM than for PM and NM, respectively. SWCs in the surface soil layer of BM also increased after rainfall during the late crop growth period, which improved the rainwater use efficiency by 8.2% compared to PM. Both experiments and simulations revealed that BM has more comparable soil moisture dynamics to PM than to NM.

Gap thinning improves soil water content, changes the vertical water distribution, and decreases the fluctuation

Although it is clear that gap thinning significantly increases the soil water content (SWC) of the topsoil, less is known about whether and how this treatment affects deeper layers. From December 2008 to April 2012, we monitored the SWC at depths of 10, 20, 30, 45, 60, and 90 cm in gap creation treatments (small gap size of 30 m2, intermediate gap size of 80 m2, and unthinned plots) in a typical pine plantation in the eastern Tibetan Plateau. Among gap treatments, differences in SWC and its coefficient of variation (CV) at each depth and the soil water content proportion (SWCP) of the whole soil profile at specific depths were compared. Gap thinning improved SWC and decreased the CV at each depth. The SWCPs in thinned plots were lower at depths from 10 to 30 cm compared with unthinned plots but higher at depths of 45 and 60 cm. Also, in each season, the patterns were similar to the general results. In conclusion, gap thinning improves the SWC, changes the vertical soil water distribution, and decreases the SWC heterogeneity. The soil water conditions in intermediate gaps are more appropriate for local forest restoration.

Characterizing Spatial Variability in Soil Water Content for Precision Irrigation Management

Among one of the many challenges in implementing precision irrigation is to obtain an accurate characterization of the soil water content (SWC) across spatially variable fields along the crop growing season. The accuracy of characterizing SWC has been tested primarily on a small-scale and has received little attention from the scientific community at the field scale. Hence, the objective of this study was to assess the characterization of the spatial distribution of soil water content at the field scale by the apparent electrical conductivity (ECa). In evaluating the current aim, ECa survey was compared against repeated measurements of SWC at five depths using neutron probe. Results showed that mean SWC was different across ECa derived management zones, which indicates that on a macro-scale, soil ECa could effectively characterize the mean differences in SWC across management zones. Results also showed that deep ECa (0–150 cm) survey outperformed shallow survey (0–75 cm). Considering other soil properties, such as organic matter content and salt content, further improved the relationship between SWC and ECa.

Exploring the effects of the “Grain for Green” program on the differences in soil water in the semi-arid Loess Plateau of China

Land-use/vegetation is an essential variable on controlling the heterogeneities and distribution pattern of soil water content (SWC). Along with large-scale re-vegetation in arid and semi-arid areas, the variations of SWC ascribed to the land-use conversion have been considered as an important factor to assess the re-vegetation efforts. In this study, soil water data at the depth of 0–300cm were obtained by field observation in two eco-hydrologic zones in Loess Plateau of China. The mean annual precipitation was 350–400mm (zone-A) and 500–550mm (zone-B), respectively. The differences in SWC were analyzed among different land-use types; and the SWC in different restoration years was also discussed using the method of space replacing time. Results indicated that (1) the differences in SWC between the two zones were lower in re-vegetated lands (2.76%, forestland and 4.22%, for shrub land) than in abandoned farmland (5.85%). The differences in re-vegetated lands diminished gradually as the soil depth (0–300cm) increased, whereas the abandoned farmland represented an opposite trend. (2) Although the variation trends of SWC in abandoned farmland of the two zones were different as the restoration years increased, the differences in SWC between abandoned farmland and native grassland both diminished gradually. This meant the soil water deficit relative to native grassland would be alleviated as the progress of abandoned farmland. (3) As the restoration years increased, the differences in SWC between re-vegetated lands and native grassland increased gradually because of the continuous decrease of SWC in re-vegetated lands. The consequence will be that the soil water deficit in the re-vegetation lands is aggravated continually. From the perspective of soil water restoration and conservation in semi-arid areas, the abandoned farmland should be a more reasonable method than re-vegetated lands covered with introduced plants.

Characterizing spatial variability in soil water content for precision irrigation management

One among many challenges in implementing precision irrigation is the reliable characterization of the soil water content (SWC) across spatially variable fields. For this purpose, commercial retailers are employing apparent soil electrical conductivity (ECa) to create irrigation prescription maps. The accuracy of this method at the field scale has received little attention from the scientific community. Hence, the objective of this study was to characterize spatial distribution of soil water content at the field scale for the purpose of precision irrigation management. Results showed mean SWC to be different across ECa derived management zones, indicating that soil ECa was able to characterize mean differences in SWC across management zones.

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.

The effects of a dry sand layer on groundwater recharge in extremely arid areas: field study in the western Hexi Corridor of northwestern China

Evaporation capacity is an important factor that cannot be ignored when judging whether extreme precipitation events will produce groundwater recharge. The evaporation layer’s role in groundwater recharge was evaluated using a lysimeter simulation experiment in the desert area of Dunhuang, in the western part of the Hexi Corridor in northwestern China’s Gansu Province. The annual precipitation in the study area is extremely low, averaging 38.87 mm during the 60-year study period, and daily pan evaporation amounts to 2,486 mm. Three simulated precipitation regimes (normal, 10 mm; ordinary annual maximum, 21 mm; and extreme, 31 mm) were used in the lysimeter simulation to allow monitoring of water movement and weighing to detect evaporative losses. The differences in soil-water content to a depth of 50 cm in the soil profile significantly affected rainfall infiltration during the initial stages of rainfall events. It was found that the presence of a dry 50-cm-deep sand layer was the key factor for “potential recharge” after the three rainfall events. Daily precipitation events less than 20 mm did not produce groundwater recharge because of the barrier effect created by the dry sand. Infiltration totaled 0.68 mm and penetrated to a depth below 50 cm with 31 mm of rainfall, representing potential recharge equivalent to 1.7 % of the rainfall. This suggests that only extreme precipitation events offer the possibility of recharge of groundwater in this extremely arid area.

Responses of ecosystem carbon dioxide fluxes to soil moisture fluctuations in a moist Kenyan savanna

Abstract:Measurements were conducted within a fence-exclosure between February 2008 and July 2009 to investigate the influence of soil moisture on ecosystem CO2 fluxes in a Themeda triandra-dominated grassland of a humid Kenyan savanna. Rainout shelters were constructed to reduce ambient rainfall by 0%, 10% and 20% respectively to attain variable soil water content (SWC) during plant growth. SWC within the top 30 cm layer, above-ground biomass, soil and plant nitrogen (N) concentrations were assessed monthly alongside CO2 fluxes. Net ecosystem CO2 exchange (NEE) and ecosystem respiration (Reco) were measured with closed chambers while carbon (C) partitioning during the wet and dry seasons were assessed through pulse 13C labelling. There were significant seasonal and between plot differences in SWC, above-ground biomass, canopy light utilization efficiency (α), CO2 fluxes and C allocation pattern resulting from differences in SWC. The ecosystem was a net C sink during the wet and C neutral during the dry seasons. The study showed strong seasonal fluctuations in ecosystem CO2 fluxes and underscores the significant role of the savanna grasslands in regional C balance due to its expansive nature. The savanna grassland is however vulnerable to low soil moisture, with significant reduction in CO2 uptake during drought.

Changes in soil water content under annual, perennial, and shrub-based pastures in an intermittently dry, summer-rainfall environment

Soil water content (SWC) was monitored in an intermittently dry environment in 2003–08, for the following pasture types: perennial ryegrass (Lolium perenne cv. Skippy), lucerne (Medicago sativa cv. Venus), phalaris (Phalaris aquatica cv. Atlas PG), a lucerne/phalaris mixture, digit grass (Digitaria eriantha ssp. eriantha cv. Premier), and old man saltbush (Atriplex nummalaria). Perennial ryegrass and phalaris pastures persisted until late winter–early spring 2005 and, after that time, were maintained as degraded annual grass pastures and bare fallows, respectively. For all pasture types, mean SWC was generally higher for the 0–0.9 m soil depth than the 0.9–2.1 m (63 v. 51 mm of water per 0.2 m soil layer). At a soil depth of 0–0.9 m, few significant differences in SWC occurred among pasture types. However, significant differences among pasture types were recorded in SWC at depths of 0.9–2.1 m for these perennial-based pastures with low herbaceous plant densities. At this depth the SWC of lucerne/phalaris was lower (P &lt; 0.05) than that of perennial ryegrass and phalaris pasture types in March 2005 (Day 500), and that of the degraded annual grass pasture in August 2006 (Day 1000) and December 2007 (Day 1500). Overall, maximum extractable water was highest (P &lt; 0.05) for digit grass and old man saltbush pasture types (~180 mm) and lowest for the bare fallow (99 mm). Estimates of root depth were highest (2.0 m) for the lucerne/phalaris pasture type.

Analysis of Multiple Precipitation Products and Preliminary Assessment of Their Impact on Global Land Data Assimilation System Land Surface States

Abstract Precipitation is arguably the most important meteorological forcing variable in land surface modeling. Many types of precipitation datasets exist (with various pros and cons) and include those from atmospheric data assimilation systems, satellites, rain gauges, ground radar, and merged products. These datasets are being evaluated in order to choose the most suitable precipitation forcing for real-time and retrospective simulations of the Global Land Data Assimilation System (GLDAS). This paper first presents results of a comparison for the period from March 2002 to February 2003. Later, GLDAS simulations 14 months in duration are analyzed to diagnose impacts on GLDAS land surface states when using the Mosaic land surface model (LSM). A comparison of seasonal total precipitation for the continental United States (CONUS) illustrates that the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) has the closest agreement with a CPC rain gauge dataset for all seasons except winter. The European Centre for Medium-Range Weather Forecasts (ECMWF) model performs the best of the modeling systems. The satellite-only products [the Tropical Rainfall Measuring Mission (TRMM) Real-time Multi-satellite Precipitation Analysis and the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN)] suffer from a few deficiencies—most notably an overestimation of summertime precipitation in the central United States (200–400 mm). CMAP is the most closely correlated with daily rain gauge data for the spring, fall, and winter seasons, while the satellite-only estimates perform best in summer. GLDAS land surface states are sensitive to different precipitation forcing where percent differences in volumetric soil water content (SWC) between simulations ranged from −75% to +100%. The percent differences in SWC are generally 25%–75% less than the percent precipitation differences, indicating that GLDAS and specifically the Mosaic LSM act to generally “damp” precipitation differences. Areas where the percent changes are equivalent to the percent precipitation changes, however, are evident. Soil temperature spread between GLDAS runs was considerable and ranged up to ±3.0 K with the largest impact in the western United States.

Late-summer soil water content on clear-cut reforestation areas two decades after site preparation in Finnish Lapland

Site preparation has been widely used on reforestation areas in Finnish Lapland due to the high soil moisture and low temperature. The long-term effect of site preparation on soil volumetric water content was studied in ploughed ridges and the untreated intermediate areas of prescribed burning, patch scarification, disk trenching and ploughing plots in eight reforestation experiments 19 years later. Soil water content (SWC) profile measurements were performed with a capacitance probe in the mineral topsoil in the dry late summer of 1993. The mean SWC (2.5–15.0 cm layer) in August in the mineral soil of the untreated intermediate areas varied from 0.13 to 0.20 m 3 m −3 among the experimental areas, and in ploughed ridges from 0.09 to 0.19 m 3 m −3. In intermediate areas, no significant differences in SWC were found between the site preparation methods. Soil in and under the ploughed ridges was significantly drier down to a depth of 17.5 cm than the soil at the same depth in the adjacent untreated intermediate areas. SWC was higher on formerly Norway spruce ( Picea abies L. Karst.) sites than on Scots pine ( Pinus sylvestris L.) sites. The proportion of pine and spruce in the previous tree generation, topography, and soil organic matter content and particle size showed significant correlation with SWC. The basal area of the planted pine saplings significantly affected SWC in the root zone, suggesting that the transpiration and rainfall interception of trees should be taken into account in soil moisture studies on forested sites.

Tillage effects on water storage during fallow, and on barley root growth and yield in two contrasting soils of the semi-arid Segarra region in Spain

In semi-arid areas under rainfed agriculture water is the most limiting factor of crop production. To investigate the best way to perform fallow and its effect on soil water content (SWC) and root growth in a barley ( Hordeum vulgare L.) crop after fallow, an experiment was conducted on two soils in La Segarra, a semi-arid area in the Ebro Valley (Spain). Fallow was a traditional system used in these areas to capture out-of-season rainfall to supplement that of the growing period, usually lasting 16 months, from July to October of the next year. Soil A was a loamy fine Fluventic Xerochrept (Haplic Calcisol, FAO) of 120 cm depth and Soil B was a loamy Lithic Xeric Torriorthent (Calcaric Regosol, FAO) of only 30 cm depth. The experiment was continued for four fallow-crop cycles in Soil A and for two in Soil B. In Soil A, three tillage systems were compared: subsoil tillage (ST), minimum tillage (MT) and no-tillage (NT). In Soil B, only MT and NT were compared. In the fields cropped to barley, SWC and root length density (LV) were measured at important developmental stages during the season, lasting from October to June. In the fallow fields SWC was also monitored. Here, evaporation (EV), water storage (WS) and water storage efficiency (WSE) were calculated using a simplified balance approach. The fallow period was split in two 8-month sub-periods: July–February (infiltration) and March–October evaporation (EV). In Soil A, values of WSE were in the range 10–18% in 1992–1993, 1993–1994 and 1994–1995 fallow, but fell to 3% in 1995–1996. Among tillage systems, NT showed significantly greater WSE in the July–February sub-period of 1992–1993 and 1993–1994 fallow, but significantly lower WSE in the March–October sub-period, due to greater EV under NT. Consequently, no differences in total WSE were found between tillage systems. In Soil B, WSE was low, about 3–7%, and there were no difference between tillage systems. During the crop period, the differences in SWC and LV between tillage systems were small. Regarding yields, the best tillage system depended on the year. NT is potentially the best system for executing fallow, but residues of the preceding crop must be left spread over the soil.

Soil-Water Extraction Profiles of Cotton (Gossypium hirsutum) and Weed Species

Soil-water profiles of cotton, several weeds, and bare soil were compared in research conducted in 1989 and 1990 near Perkins, OK. Neutron probe access tubes were installed prior to plant establishment to facilitate nondestructive volumetric water content determinations at selected depths throughout the season. Planting was in a double-circle pattern concentric around each neutron probe access tube. Phenological and soil-water data were collected weekly. As plants began to senesce, aboveground biomass yield data were collected. In 1989, rainfall frequently replenished soil-water in the upper soil profile. The plants used the recharge water to varying degrees, particularly late in the season. Common cocklebur and johnsongrass extracted soil-water from greater depths than cotton and velvetleaf throughout the season. In 1990, the largest differences detected between bare soil and weeds early in the season were measured for common cocklebur, devil's-claw, and silverleaf nightshade. Later in the season, however, there was little difference in soil-water content in the upper soil profiles for all species. Johnsongrass emerged later than the other species in 1990 which affected its relative soil-water extraction.

Small-scale heterogeneity and soil-moisture variability in the unsaturated zone

The distribution of water within a soil profile can only be partly explained by the time distribution and rate of surface-water input. Observed differences in soil moisture within the unsaturated zone result from the interaction of surface-water inputs with spatially inhomogeneous soil characteristics. Water which initially percolates vertically is differentially impeded as a result of subtle textural changes in the soil, and is then preferentially retained in such zones of transition, causing large differences in soil-water content to occur. The scale of this vertical variability is of tenths of metres, whilst lateral variability of soil moisture reflects textural changes over a few metres. The observed influence of small-scale heterogeneity on soil-water content suggests that the conventional assumptions of isotropicity and homogeneity of the textural and hydraulic properties of porous media used in drainage basin and hillslope hydrological models need scrutiny, even for single stratigraphic units.