Drained Upper Limit Research Articles

Soil water retention capacity surpasses climate humidity in determining soil organic carbon content but not plant production in the steppe zone of Northern China

Soil hydraulic properties determine water transport, distribution and storage in soils, thus affect ecosystem functioning. Soil water retention parameters, such as saturation water content (SAT), water drained upper limit (DUL) and lower limit (DLL), characterize soil water-holding capacity, thus are important for plant growth and carbon and nutrient cycling in grassland ecosystem. However, we are still unclear to what a content these soil water retention parameters may regulate plant production and soil organic matter content in natural ecosystems in semi-arid grassland environment. We investigated plant species richness (SR) and aboveground biomass (AGB), soil organic carbon (SOC) and nitrogen (SON) contents, and soil water retention and other physical parameters in the steppe ecosystems along a climate transect, which spans the desert steppe, typical steppe and meadow steppe zones in Northern China. We analyzed the relative importance of climate factors, soil water retention parameters and other soil physical parameters in regulating plant AGB and SOC and SON contents using partial correlation analysis, and quantified the contribution of each separate and combined climate and soil physical parameters in predicting plant AGB and SOC and SON contents using geographic detectors method. We found that (i) plant SR and AGB, soil SAT and DUL, and SOC and SON are all significantly inter-correlated, and increased with the increase of climate humidity. (ii) Climate, but not soil physical parameters, was the most preponderant factor in predicting plant SR and AGB. (iii) Soil physical parameters were more important than climate parameters in regulating soil organic matter content; soil DUL was the best predictor to SOC content. Our results suggest that soil water retention parameters especially soil DUL, may play a greater role than climate factors in modulating SOC content, though climate is more predominant in regulating plant SR and production. Our results imply that we need to be cautious in estimating SOC content based on climate factors and vegetation biomass or remotely-sensed vegetation indices, and that soil physical properties need also to be incorporated along with vegetation parameters for improving SOC estimation at a regional scale.

Upland Rootzone Soil Water Deficit Regulates Streamflow in a Catchment Dominated by North American Tallgrass Prairie

Intermittent tallgrass prairie streams depend on surface runoff and are highly susceptible to hydrological disturbances such as droughts. The objective of this study was to investigate the timing of intermittent streamflow pulses and upstream rootzone soil water deficit in a watershed dominated by tallgrass prairie. The study was conducted from July to December 2021 in the Kings Creek watershed located within the Konza Prairie Biological station, Kansas, USA. Hourly precipitation and soil moisture observations in the 0–10, 10–30, and 30–50 cm depth were obtained from a hydrological network consisting of 16 monitoring stations across the Kings Creek watershed. Rootzone soil water storage (S) was computed at hourly time steps as the sum of the soil water storage of each soil layer. A drained upper limit (DUL) was estimated as the soil moisture remaining 24 h after the soil had been thoroughly wetted during large (~100 mm) rainfall events. A lower limit (LL) was estimated as the lowest rootzone soil water storage during the study period. Hourly soil water deficit (D) was computed as D = (DUL − S)/(DUL − LL). The study period had 19 precipitation events totaling 436 mm, and only 14 out of the 19 precipitation events exceeded a common canopy and litter interception threshold of 4 mm for tallgrass prairies in this region. Only two precipitation events resulted in measurable streamflow, and the inception of these two streamflow events was associated with a negative weighted soil water deficit (i.e., S > DUL). This pilot study revealed that upland rootzone soil water deficit plays a major role controlling the timing of streamflow in the Kings Creek watershed and possibly in other catchment areas with intermittent prairie streams.

Pattern analysis of Australia soil profiles for plant available water capacity

Plant available water capacity (PAWC), defined as the amount of water held in the soil between the drained upper limit (DUL) and crop lower limit (CLL), is a major determinant for crop yield in dryland cropping areas. However, measured soil profile data quantifying the physical soil parameters that determine PAWC are limited. Choosing a ‘representative’ soil profile in order to parameterise a deterministic model is often subjective and may not be appropriate, leading to biased conclusions. To help support these deterministic crop modelling studies, we analysed soil profile data in Australia available in the APSoil database; a database of field and laboratory measured physical soil properties used to parameterise the Agricultural Production Systems sIMulator (APSIM) model. Specifically, we developed typical soil pattern profiles utilizing wheat for CLL using statistical shape analysis and cluster analysis. Our method allows modellers to reduce the number of soil profile parameter choices using a statistically sound basis. We implement our method by grouping the APSoil soil profiles into five clusters, whereby each cluster is represented by a more general soil profile. The representative soil profiles reflect impact of the soil texture on PAWC. The five clusters and representative soil profiles can then be used in future modelling studies to investigate the impact of soil variation on systems performance. In turn, soil scientists will have a plausible and quantifiable collection of soil profiles to choose from for future studies.

Bioclimatic variables as important spatial predictors of soil hydraulic properties across Australia's agricultural region

Soil water directly or indirectly affects almost all ecological processes. Soil available water capacity (AWC), the difference between field capacity, or drained upper limit (DUL), and wilting point, or lower limit (LL15), and saturated water content (SAT) are among the most important soil hydraulic properties controlling soil water dynamics. These properties vary across space and are expensive to measure directly. It is difficult to obtain reliable estimates of soil hydraulic properties at an appropriate scale for water and land management. Here we modelled LL15, DUL, SAT and AWC measurements from 1127 whole-soil profiles across Australian agricultural areas with the Random Forest machine learning model using 19 bioclimatic and 15 topographical covariates. The amount of variance explained by the model reached up to R2 = 0.69 depending on the property and soil depth assessed. For all soil hydraulic properties, the bioclimatic variables alone contributed to more than 90% of the explained variance. Particularly, temperature of driest and wettest quarter, and precipitation of warmest month were the three most influential variables. Using the derived models, we also mapped the four hydraulic properties across Australian agricultural areas in six sequential depths down to 2 m at a spatial resolution of 90 m. Moreover, we combined our mapping of AWC with existing products via an ensemble model averaging approach which proved to be more accurate than each of the three contributing products. Our results uncover the significant role of bioclimatic variables in regulating soil hydraulic properties, providing a benchmark assessment of soil hydraulic properties in agricultural regions for efficient water-related land management.

Farming on the fringe: Shallow groundwater dynamics and irrigation scheduling for maize and wheat in Bangladesh’s coastal delta

Further efforts are needed to combat poverty and agricultural productivity problems in the delta region of Bangladesh. Sustainable intensification of crop production through irrigation and production of cash crops such as maize and wheat might be a promising option to increase income and diversify food production. Only limited research has however been conducted on the potential of using surface water from canals as an irrigation source for maize and wheat production in the delta region. To better understand the contribution of shallow groundwater to crop production and number of irrigations needed for maize and wheat in this unique coastal environment, we conducted multi-locational trials on farmers' fields over three rabi seasons. In addition to soil moisture and salinity, we recorded the depth and salinity of the shallow water table throughout these experiments. Maize in particular requires considerable capital investment for seeds, fertilizer, irrigation and labor. Although farmers express wide interest in maize – which can be sold as a profitable cash crop into Bangladesh's expanding poultry feed industry – many of them are reluctant to invest in fertilizer because of the high entry costs. We therefore also investigated the profitability of growing maize under low and high (recommended) fertilizer regimens. Volumetric soil moisture at sowing and during the grain filling phase or at maturity indicated that there is ample supply of water in the profile. Most measurements were above the drained upper limit (DUL). We attributed this to the generally shallow water table depths, which never exceeded 2.75 m at any location, but generally stayed between 1–2 m depth throughout the season. The region’s soil texture classes (clay loams, silt loams and silty clay loams) are all conducive for capillary rise of water into the rooting zone. Consequently, irrigation had a significant effect on maize yield in the driest winter only, whereas for wheat, it had no effect. The key for a successful maize and wheat production in the delta region of Bangladesh is to ensure a good crop establishment, which can be achieved with a starter and an additional irrigation at crown root initiation for wheat and at V6-8 for maize. Maize however is not always profitable. Compared to low fertilizer rates, higher rates reduced losses in low yielding site-years and increased profits in high-yielding site years. This indicates that it is advisable for farmers not to reduce fertilizer rates. Low-risk financial credit with rationally structured interest rates that allow farmers to invest in maize could potentially offset these constraints.

Soil Water Extraction for Several Dryland Crops

Core Ideas Crops differ in depth of soil water extraction. Crops did not differ in lower limit of water availability. Wheat ends the growing season with a drier soil profile. Proso millet ends the growing season with a wetter soil profile. Extractable available soil water may aid in designing successful rotational sequences. Dryland cropping decisions would benefit from information about soil water extraction by various candidate crops. The objectives of this experiment were to: (i) quantify average soil water extraction by depth in the soil profile for winter wheat (Triticum aestivum L.), corn (Zea mays L.), proso millet (Panicum milliaceum L.) , and dry pea (Pisum sativum L.), and (ii) verify previously published values of drained upper limit (DUL) and lower limit (LL) of water extraction for each crop grown on a silt loam soil in northeastern Colorado. Soil water contents at planting and physiological maturity were measured over a 21‐yr period. Average ending soil water was least at all measurement depths for wheat and greatest for millet. The greatest total profile water extraction was seen for wheat (141 mm) and the least for pea (46 mm). Soil water extraction occurred, on average, from the 0‐ to 180‐cm profile for wheat, 0‐ to 150‐cm profile for corn, 0‐ to 120‐cm profile for millet, and 0‐ to 90‐cm profile for pea. When soil water was plentiful at planting and followed by dry growing season conditions, millet extracted soil water from the entire 0‐ to 180‐cm profile. Crop rotational sequences utilizing shallow rooted crops (such as millet and pea) that do not fully extract soil water at lower depths will allow for greater soil water availability to subsequent crops such as wheat and corn that are able to explore the lower soil profile more effectively for soil water.

Characterisation of the soil pore system in relation to its hydraulic functions in two South African aeolian soil groups

The characterisation of soil pores has a prime importance in soil hydrology particularly for soil water management in both irrigated and dryland agriculture. Hence, this study describes the soil pore system in the Clovelly and Hutton soil forms and its roles in soil hydrology by quantifying the different functional pore classes. The soil pore size–pore volume (PS-PV) response curve, which shows the relationship between cumulative pore volume to the corresponding pore sizes, was estimated from the soil water retention curve (SWRC) by applying the capillary theory. The SWRC was measured in the laboratory using a hanging water column and pressure plate apparatus. Saturated hydraulic conductivity was determined in situ with a constant head permeameter, whereas unsaturated hydraulic conductivity (KL) and diffusivity (Df) were estimated from the SWRC by the van Genuchten conductivity model. The classification of soil pore sizes given in the literature was modified based on the concept of field water capacity. Therefore, pore sizes greater than the drained upper limit (DUL) were classified as transmission pores. Similarly, the pore size between the DUL and the lower limit (LL) were classified as storage pores, and pore sizes smaller than the LL were residuals and bonding pores collectively. Soil water retention increased non-linearly as pore sizes decreased and the opposite was true for KL and Df. Generally, macropores were important for water movement, whereas micropores were responsible for water retention. The findings are important for soil water management of cultivated aeolian soils in semi-arid and arid regions of sub-Saharan Africa. The proposed method to classify soil pores on the basis of their function also gives a consistent and realistic value compared with the fixed boundary classes, although the method requires further development.

Biochar In Situ Decreased Bulk Density and Improved Soil-Water Relations and Indicators in Southeastern US Coastal Plain Ultisols

Biochar may improve soil physical properties for crop growth, but multiyear, multicrop field studies are lacking. To determine the effects of biochar on soil physical properties, we applied 0, 10, 20, 40, and 80 Mg ha biochar with/without NPK fertilizer to the surface 15 cm of 1 × 1 m plots in a single association of fine-loamy, siliceous, subactive, thermic Oxyaquic and Aquic Paleudults under a 2-year corn-winter wheat–double-crop soybean rotation. After 3 years, we sampled soil to 7.6 cm, measured bulk density and water retention, and then derived pore-size distribution and related physical and water retention model parameters. Fertilizer had little to no effect. Among the statistically significant results, biochar increased structural porosity (3- to 59-μm effective pore diameter [EPD]) but neither matrix- (0.2- to 3-μm EPD) nor macro (EPD >59 μm) porosity. Biochar ≥40 Mg ha decreased bulk density 16%; 80 Mg ha increased total porosity 14%. However, it also increased water content at −1,500 kPa 22.5%. Biochar ≥40 Mg ha increased the drained upper limit (DUL) by 15%; relative field capacity, 3%; and total and structural plant-available water (PAW: held between the DUL and −1,500 kPa), 7 and 18%, respectively. Increases were greatest at −10 kPa and least at −33 kPa. At −10 kPa, 80 Mg ha biochar increased total PAW 4.0-mm equivalent depth compared with 5.7 mm for structural PAW, approximately 0.5-day demand for actively growing corn. Modeled saturated water content increased with total porosity. Biochar improved plant-soil-water relations, but required high rates.

MODELLING DEEP DRAINAGE RATES OF IRRIGATION STRATEGIES UNDER CROPPING SEQUENCE IN SUBHUMID, SUBTROPICAL AUSTRALIA

Water availability is a major concern for irrigated vegetable cropping in the Lockyer Valley area of Australia. Soil water balance and in particular deep drainage were modelled for a representative site of this area for a range of irrigation scenarios over 12 years of cropping sequence. A scenario that refilled the soil to the drained upper limit (DUL) resulted in 530 mm yr−1 of irrigation, 673 mm yr−1 of transpiration and 84 mm yr−1 of deep drainage, whereas maintaining the soil to DUL+25% and DUL+50%, with soil moisture between DUL and saturation (75 and 50% free water respectively), resulted in similar transpiration but with increased irrigation (up to 717 mm yr−1) and deep drainage (up to 262 mm yr−1). For a given amount of available water, irrigations at fixed intervals were found to be less efficient (reduced crop transpiration) than irrigation scheduled according to a soil water deficit. This highlights the importance of moisture monitoring to optimize irrigation use efficiency. The study suggests solutions to improve irrigation efficiency by minimizing deep drainage under cropping sequences. Copyright © 2013 John Wiley & Sons, Ltd. La disponibilité limitée en eau d'irrigation pose un problème pour les cultures maraichères de la Lockyer Valley (Australie). Différents régimes d'irrigation ont été simulés au cours de 12 années de séquence pour un site représentatif de cette région. Le régime maintenant le sol à la capacité au champ (DUL) a nécessité 530 mm d'irrigation par an en moyenne, et a permis aux cultures d'utiliser 673 mm/an (transpiration), avec un drainage de 84 mm/an. Avec une irrigation (‘DUL+25%’ et ‘ DUL+50%’) maintenant l'humidité du sol entre capacité au champ et saturation (respectivement 75 et 50% d'eau libre), la quantité d'eau utilisée a augmenté (irrigation de 717 mm/an en moyenne) ainsi que le drainage (262 mm/an), sans qu'aucun bénéfice notable pour les cultures n'ait été simulé en terme de quantité d'eau transpirée. D'autre part, des irrigations à pas de temps réguliers se sont avérées moins efficaces (transpiration réduite) qu'un régime opportuniste, avec des irrigations régulées en fonction de l'état hydrique du sol. Cette étude renforce ainsi l'importance de contrôler l'état hydrique du sol pour adapter le régime d'irrigation. Les résultats suggèrent également que l'efficacité de l'irrigation sous des séquence peut être améliorée en minimisant le drainage profond. Copyright © 2013 John Wiley & Sons, Ltd.

Improving Simulation of Soil Water Balance Using Lysimeter Observations in a Semiarid Climate

Water balance simulation in cropping systems is a very useful tool to study how water can be used efficiently. However this requires that models simulate an accurate water balance. Comparing model results with field observations will provide information on the performance of the models. The objective of this study was to test the performance of DSSAT model in simulating the water balance by comparing the simulations with observed measurements. The soil water balance in DSSAT uses a one dimensional “tipping bucket” soil water balance approach where available soil water is determined by the drained upper limit (DUL), lower limit (LL) and saturated water content (SAT). A continuous weighing lysimeter was used to get the observed values of drainage and evapotranspiration (ET). An automated agrometeorological weather station close to the lisymeter was also used to record the climatic data. The model simulated accurately the soil water content after the optimization of the soil parameters. However it was found the inability of the model to capture small changes in daily drainage and ET. For that reason simulated cumulative values had larger errors as the time passed by. These results suggested the need to compare outputs of DSSAT and some hydrological model that simulates soil water movement with a more mechanistic approach. The comparison of the two models will allow us to find which mechanism can be modified or incorporated in DSSAT model to improve the simulations.

Simulation analysis of lucerne–wheat crop rotation on the Loess Plateau of Northern China

The Agricultural Production System Simulator (APSIM) was parameterised and tested against datasets from two field experiments being conducted on Heilu soil at the Qingyang Research Station, Gansu, China as to investigate long-term lucerne productivity and management options of reducing impact of lucerne on winter wheat yield in a lucerne–wheat rotation system. With minimal parameterisation and configuration of the APSIM-Lucerne module, APSIM was able to simulate phenological development and seasonal growth of winter-dormant lucerne cultivar, Longdong compared with the observed data. Flowering date was accurately simulated using the established relationship between accumulated thermal time and mean photoperiod. After the APSIM-Lucerne module was configured for the seasonal variation in RUE (radiation use efficiency), the model simulated lucerne seasonal biomass production over three growing seasons in the continuous lucerne treatment with a root mean squared deviation (RMSD) of 1132 kg/ha (30% of the mean observed biomass). In the treatment where lucerne was removed in August 2001 and two winter wheat crops were sown and harvested in 2001/2002 and 2002/2003 growing seasons, APSIM simulated winter wheat crop biomass in both growing seasons with a RMSD of 1420 kg/ha (20% of the mean observed crop biomass). Wheat grain yield was simulated with a RMSD of 918 kg/ha (27% of the mean observed grain yield). Using measurements of drained upper limit (DUL) and lower limit (LL), and standard soil evaporation and runoff parameters, the model was able to simulate soil water dynamics and water use by lucerne in the lucerne-fallow, continuous lucerne and lucerne–wheat treatments. The long-term simulation suggested that under local climatic conditions, lucerne could produce 11,000 kg/ha biomass annually. The simulation also indicated that integrating lucerne with annual cropping could potentially reduce runoff and early removal improved soil water storage prior to sowing winter wheat and optimise wheat yield following lucerne in a lucerne–wheat rotation system. The findings from these long-term simulations suggest there is need to develop management strategies when lucerne is integrated with annual cropping system to improve soil water use and reduce runoff. There is also need to consider a balance between sustaining wheat yield and providing feed for livestock when developing management strategies for timing of lucerne removal and cropping option in lucerne-based rotation systems. The successful test of APSIM will give local researchers confidence to use the tool exploring cropping system issues in Northern China.

The hydrology of Vertosols used for cotton production: II. Pedotransfer functions to predict hydraulic properties

Using a range of earlier published results and a recently published dataset, pedotransfer functions (PTFs) were developed to predict some hydraulic properties of Vertosols. A fitting approach using neural networks was employed with good results to predict the soil water characteristic curve. The developed functions are complex due to the large numbers of parameters, but moisture contents are predicted to within 5%. Other PTFs to predict the moisture content at the drained upper limit (DUL) and lower limit (LL), and bulk density in the normal shrinkage curve, were developed using multiple linear regression. The PTFs to predict the soil water characteristic curve, DUL and LL, and the bulk density in the normal shrinkage zone were mainly based on total clay, sand, and silt contents and bulk density, with minor contributions of ECEC and total carbon content. PTFs for unsaturated hydraulic conductivities were also developed using multi-linear regression and were mainly dependent on silt contents and ESP values. The mean error in these predictions was 2.76 mm/h, which is reasonable for predictions at the field and farm scale where inherent soil variability can cause larger variation. The developed PTFs can be used to predict parameters needed in crop modelling tools such as OZCOT to simulate cotton development on Vertosols. Some further examples of the use of the PTFs for management of irrigation are given.

Comparison of measured changes in seasonal soil water content by rainfed maize-bean intercrop and component cropping systems in a semi-arid region of southern Africa

Seasonal water content fluctuation within the effective root zone was monitored during the growing season for a maize-bean intercrop (IMB), sole maize (SM) and sole bean (SB) in Free State Province, Republic of South Africa. Comparisons were undertaken for progressive depths of extraction 0–300 mm; 300–600 mm and 600–900 mm respectively. These enabled the understanding of water extraction behavior of the cropping systems within the different soil layers including the topsoil surface normally influenced by soil surface evaporation. Additive intercrops have been known to conserve water, largely due to the early high leaf area index and the higher total leaf area. In this study, the combined effect of the intercrop components seemed to lower the total water demand by the intercrop compared to the sole crops. During the two seasons (2000/2001 and 2001/2002) the drained upper limit (DUL) and crop lower limits (CLL) were determined. The maize-bean intercrop, sole maize and sole bean had CLL of 141 mm/m, 149 mm/m and 159 mm/m respectively. The DUL was 262 mm/m for the site and therefore the potential plant extractable soil water for the cropping systems were: 121 mm/m (IMB); 114 mm/m (SM) and 103 mm/m (SB). Overall, the intercrop did not have significantly different total soil water extraction during both seasons, although it was additive, showing that it had higher water to biomass conversion.

DYNAMIC NEAREST-NEIGHBOR METHOD FOR ESTIMATING SOIL WATER PARAMETERS

For simulation of soil water dynamics, the lower limit (LL) of plant water availability and the drained upper limit(DUL) are required but are generally not available. Methods that estimate these properties using soil survey data are in demand.The objective of this study was to develop a self-calibrating, non-parametric method to estimate DUL and LL. Usingthe K-nearest-neighbor (NN) method and a large dataset of field-measured soil-water retention parameters, textures, organiccarbon, and bulk density, we developed and demonstrated its use to estimate DUL and LL. R2 was 79% and 73% for DULand LL, respectively. Estimated root mean squared error (RMSE) values were the lowest for potential extractable soil water(PESW) (0.024 cm3 cm-3) and about the same for DUL (0.035 cm3 cm-3) and LL (0.037 cm3 cm-3). Overall, LL and DULvalues of coarser soils (i.e., sand) were more accurately estimated than for fine-textured (i.e., clay loam) soils using acombination of clay, sand, and organic carbon. The NN method does not require prior assumptions as to the relationshipsamong soil water parameters and texture variables, and it is valid for a wide range of textures with no apparent limitationslike many existing methods. Based on this study, the NN method provides an efficient tool for estimating soil water retentionparameters with a high degree of accuracy through a database of clay, sand, and organic matter.

Contributions of soil and crop factors to plant available soil water capacity of annual crops on Black and Grey Vertosols

Improved methods for field measurements of plant available soil water capacity (PAWC) of Black and Grey Vertosols in Australia’s north-eastern grain region were employed to characterise 83 soil–crop combinations over 7 depth intervals to 180 cm. Soil sub-order was shown to influence all components of PAWC (means of 224 and 182 mm in Black and Grey Vertosols, respectively) with drained upper limit (DUL), bulk density (BD), and crop lower limits (CLL) showing clear separation between soil sub-orders and a trend with soil depth. In addition to soil sub-order and soil depth effects, CLL showed crop effects such that expected PAWC of various crops when adjusted for soil sub-orders were: cotton 240 mm; wheat 233 mm; sorghum 225 mm; fababean 209 mm; chickpea 197 mm; barley 191 mm; and mungbean 130 mm. A total of 549 measured CLL values were used to develop a predictive model for estimating CLL from the soil sub-order, depth, DUL, and crop by predicting a CLL as a function of DUL and a depth-dependent variable for each crop–soil sub-order. The model CLL = DUL * (a + b * DUL) explained 85% of observed variation in the measured data with no significant bias between observed and predicted data. While properly measured data would be more reliable than estimated data, where specific site accuracy is less critical, this model may be used to estimate PAWC with an acceptable degree of accuracy.

Soil and plant resistances to water uptake byVicia faba L.

The hydraulic resistivity ofVicia faba L. roots grown in soil was estimated from steady state measurements of transpiration rate and leaf and soil water potentials. Root and stem axial resistivities, estimated from xylem vessel radii, were negligible. Root radial resistivity was estimated to be 1.3×1012 sm−1. This root radial resistivity value was used to estimate, root resistance to water uptake for a field crop ofVicia faba. Previously published results were used for root distribution and soil water contents at the drained upper limit (DUL) and the lower limit (LL) of extractable soil water. Soil resistance to water uptake was estimated from single root theory using the steady rate solution. At the DUL, root resistance was about 105 times greater than soil resistance. At the LL, soil resistance exceeded root resistance for depths less than 0.3 m, but for depths greater than this soil resistance was smaller than root resistance. Estimates of possible uptake rates at given leaf water potentials indicated that overall soil resistance had a negligible influence upon uptake, even at the LL. The reliability of this result is examined in detail. It is concluded that over the complete range of extractable soil water contents soil resistanceper se would not have limited water use by this crop. This conclusion may also be valid for a wide range of soil and crop combinations.

Relations between available and extractable soil water and evapotranspiration from a bean crop

The accuracy of ‘available’ and ‘extractable’ soil water estimates was investigated using irrigated and unirrigated beans ( Vicia faba) grown in an alluvial silt loam in Canterbury, New Zealand. Available water capacity was defined as the difference between soil water contents in the root zone at the drained upper limit (DUL) and at the lower limit (LL) as estimated by laboratory procedures. Extractable water capacity was specified as the difference between field estimates of DUL and LL for the whole profile affected by roots. DUL was estimated in the laboratory by equilibrating soil cores at matric potentials at −10, −20 or −30 kPa, and in the field by neutron moderation. Laboratory estimates of LL were made from soil samples equilibrated at −1.5 MPa matric potential. In the field LL was measured by neutron moderation on plots where evaporation had apparently ceased due to drought stress. When compared at intervals down the profile laboratory estimates of DUL and LL showed poor agreement with field observations. However, the final estimates of available and extractable water capacities were similar because of compensatory inaccuracies in the laboratory estimates. Furthermore, field measurements of evapotranspiration, using neutron moderation and tensiometry, indicated that the accuracy of the available water estimates was much reduced by upward fluxes of water into the rooting zone. These fluxes resulted in water extraction to at least 1.0 m although the apparent maximum rooting depth (measured by counting roots washed from soil cores) was only 0.7 m. Particular attention was paid to the influence of subsoil textural variability, which is pronounced in such soils. Laboratory and field estimates of the LL had to be carefully matched texturally before relevant comparisons could be made. Problems associated with subsoil textural variability affected laboratory methods of DUL estimation more than field methods.