Estimation of water percolation by different methods using TDR

Detailed knowledge on water percolation into the soil in irrigated areas is fundamental for solving problems of drainage, pollution and the recharge of underground aquifers. The aim of this study was to evaluate the percolation estimated by time-domain-reflectometry (TDR) in a drainage lysimeter. We used Darcy's law with K(θ) functions determined by field and laboratory methods and by the change in water storage in the soil profile at 16 points of moisture measurement at different time intervals. A sandy clay soil was saturated and covered with plastic sheet to prevent evaporation and an internal drainage trial in a drainage lysimeter was installed. The relationship between the observed and estimated percolation values was evaluated by linear regression analysis. The results suggest that percolation in the field or laboratory can be estimated based on continuous monitoring with TDR, and at short time intervals, of the variations in soil water storage. The precision and accuracy of this approach are similar to those of the lysimeter and it has advantages over the other evaluated methods, of which the most relevant are the possibility of estimating percolation in short time intervals and exemption from the predetermination of soil hydraulic properties such as water retention and hydraulic conductivity. The estimates obtained by the Darcy-Buckingham equation for percolation levels using function K(θ) predicted by the method of Hillel et al. (1972) provided compatible water percolation estimates with those obtained in the lysimeter at time intervals greater than 1 h. The methods of Libardi et al. (1980), Sisson et al. (1980) and van Genuchten (1980) underestimated water percolation.

Water storage variability in a vineyard soil in the southern highlands of Santa Catarina state

In the subtropical regions of southern Brazil, rainfall distribution is uneven, which results in temporal variability of soil water storage. For grapes, water is generally available in excess and water deficiency occurs only occasionally. Furthermore, on the Southern Plateau of Santa Catarina, there are differences in soil properties, which results in high spatial variability. These two factors affect the composition of wine grapes. Spatio-temporal analyses are therefore useful in the selection of cultural practices as well as of adequate soils for vineyards. In this way, well-suited areas can produce grapes with a more appropriate composition for the production of quality wines. The aim of this study was to evaluate the spatio-temporal variability of water storage in a Cambisol during the growth cycle of a Cabernet Sauvignon vineyard and its relation to selected soil properties. The experimental area consisted of a commercial 8-year-old vineyard in São Joaquim, Santa Catarina, Brazil. A sampling grid with five rows and seven points per row, spaced 12 m apart, was outlined on an area of 3,456 m². Soil samples were collected with an auger at these points, 0.30 m away from the grapevines, in the 0.00-0.30 m layer, to determine gravimetric soil moisture. Measurements were taken once a week from December 2008 to April 2009, and every two weeks from December 2009 to March 2010. In December 2008, undisturbed soil samples were collected to determine bulk density, macro- and microporosity, and disturbed samples were used to quantify particle size distribution and organic carbon content. Results were subjected to descriptive analysis and semivariogram analysis, calculating the mean relative difference and the Pearson correlation. The average water storage in a Cambisol under grapevine on ridges had variable spatial dependence, i.e., the lower the average water storage, the higher the range of spatial dependence. Water storage had a stable spatial pattern during the trial period, indicating that the points with lower water storage or points with higher water storage during a certain period maintain these conditions throughout the experimental period. The relative difference is a simple method to identify positions that represent the average soil water storage more adequately at any time for a given area.

Evaluation of physical quality indices of a soil under a seasonal semideciduous forest

The concept of soil quality is currently the subject of great discussion due to the interaction of soil with the environment (soil-plant-atmosphere) and practices of human intervention. However, concepts of soil quality relate quality to agricultural productivity, but assessment of soil quality in an agronomic context may be different from its assessment in natural areas. The aim of this study was to assess physical quality indices, the S index, soil aeration capacity (ACt/Pt), and water storage capacity (FC/Pt) of the soil from a permanent plot in the Caetetus Ecological Reserve (Galia, São Paulo, Brazil) under a seasonal semideciduous forest and compare them with the reference values for soil physical quality found in the literature. Water retention curves were used for that purpose. The S values found were higher than the proposed limit for soil physical quality (0.035). The A and E horizons showed the highest values because their sandy texture leads to a high slope of the water retention curve. The B horizons showed the lowest S values because their natural density leads to a lower slope of the water retention curve. The values found for ACt/Pt and FC/Pt were higher and lower than the idealized limits. The values obtained from these indices under natural vegetation can provide reference values for soils with similar properties that undergo changes due to anthropic activities. All the indices evaluated were effective in differentiating the effects of soil horizons in the natural hydro-physical functioning of the soils under study.

Stabilization of soil hydraulic properties under a long term no-till system

The area under the no-tillage system (NT) has been increasing over the last few years. Some authors indicate that stabilization of soil physical properties is reached after some years under NT while other authors debate this. The objective of this study was to determine the effect of the last crop in the rotation sequence (1st year: maize, 2nd year: soybean, 3rd year: wheat/soybean) on soil pore configuration and hydraulic properties in two different soils (site 1: loam, site 2: sandy loam) from the Argentinean Pampas region under long-term NT treatments in order to determine if stabilization of soil physical properties is reached apart from a specific time in the crop sequence. In addition, we compared two procedures for evaluating water-conducting macroporosities, and evaluated the efficiency of the pedotransfer function ROSETTA in estimating the parameters of the van Genuchten-Mualem (VGM) model in these soils. Soil pore configuration and hydraulic properties were not stable and changed according to the crop sequence and the last crop grown in both sites. For both sites, saturated hydraulic conductivity, K0, water-conducting macroporosity, εma, and flow-weighted mean pore radius, R0ma, increased from the 1st to the 2nd year of the crop sequence, and this was attributed to the creation of water-conducting macropores by the maize roots. The VGM model adequately described the water retention curve (WRC) for these soils, but not the hydraulic conductivity (K) vs tension (h) curve. The ROSETTA function failed in the estimation of these parameters. In summary, mean values of K0 ranged from 0.74 to 3.88 cm h-1. In studies on NT effects on soil physical properties, the crop effect must be considered.

Index of soil physical quality of hardsetting soils on the brazilian coast

Many soils have a hard-setting behavior, also known as cohesive or "coesos". In such soils, the penetration resistance increases markedly when dry and decreases considerably when moist, creating serious limitations for plant emergence and growth. To evaluate the level of structure degradation in hard-setting soils with different texture classes and to create an index for assessing soil hardness levels in hard-setting soils, six soil representative profiles were selected in the field in various regions of Brazil. The following indices were tested: S, which measures soil physical quality, and H , which analyzes the degree of hardness and the effective stress in the soil during drying. Both indices were calculated using previously described functions based on data from the water-retention curves for the soils. The hard-setting values identified in different soils of the Brazilian Coastal Tablelands have distinct compaction (hardness) levels and can be satisfactorily measured by the H index. The S index was adequate for evaluating the structural characteristics of the hard-setting soils, classifying them as suitable or poor for cultivation, but only when the moisture level of the soil was near the inflection point. The H index showed that increases in density in hard-setting soils result from increases in effective stress and not from the soil texture. Values for Bd > 1.48 kg dm-3 classify the soil as hard-setting, and the structural organization is considered "poor".

Quantification of the least limiting water range in an oxisol using two methodological strategies

The least limiting water range (LLWR) has been used as an indicator of soil physical quality as it represents, in a single parameter, the soil physical properties directly linked to plant growth, with the exception of temperature. The usual procedure for obtaining the LLWR involves determination of the water retention curve (WRC) and the soil resistance to penetration curve (SRC) in soil samples with undisturbed structure in the laboratory. Determination of the WRC and SRC using field measurements (in situ ) is preferable, but requires appropriate instrumentation. The objective of this study was to determine the LLWR from the data collected for determination of WRC and SRC in situ using portable electronic instruments, and to compare those determinations with the ones made in the laboratory. Samples were taken from the 0.0-0.1 m layer of a Latossolo Vermelho distrófico (Oxisol). Two methods were used for quantification of the LLWR: the traditional, with measurements made in soil samples with undisturbed structure; and in situ , with measurements of water content (θ), soil water potential (Ψ), and soil resistance to penetration (SR) through the use of sensors. The in situ measurements of θ, Ψ and SR were taken over a period of four days of soil drying. At the same time, samples with undisturbed structure were collected for determination of bulk density (BD). Due to the limitations of measurement of Ψ by tensiometer, additional determinations of θ were made with a psychrometer (in the laboratory) at the Ψ of -1500 kPa. The results show that it is possible to determine the LLWR by the θ, Ψ and SR measurements using the suggested approach and instrumentation. The quality of fit of the SRC was similar in both strategies. In contrast, the θ and Ψ in situ measurements, associated with those measured with a psychrometer, produced a better WRC description. The estimates of the LLWR were similar in both methodological strategies. The quantification of LLWR in situ can be achieved in 10 % of the time required for the traditional method.

EFFECT OF WATER AVAILABILITY ON SOIL MICROBIAL BIOMASS IN SECONDARY FOREST IN EASTERN AMAZONIA

Soil microbial biomass (SMB) plays an important role in nutrient cycling in agroecosystems, and is limited by several factors, such as soil water availability. This study assessed the effects of soil water availability on microbial biomass and its variation over time in the Latossolo Amarelo concrecionário of a secondary forest in eastern Amazonia. The fumigation-extraction method was used to estimate the soil microbial biomass carbon and nitrogen content (SMBC and SMBN). An adaptation of the fumigation-incubation method was used to determine basal respiration (CO2-SMB). The metabolic quotient (qCO2) and ratio of microbial carbon:organic carbon (CMIC:CORG) were calculated based on those results. Soil moisture was generally significantly lower during the dry season and in the control plots. Irrigation raised soil moisture to levels close to those observed during the rainy season, but had no significant effect on SMB. The variables did not vary on a seasonal basis, except for the microbial C/N ratio that suggested the occurrence of seasonal shifts in the structure of the microbial community.

Water absorbent polymer in sugarcane crop

The water absorbent polymer effect on vegetative growth and production of Theoretical Recovery Sugar (TRS) of sugarcane cv. RB 86 7515 was evaluated on two field tests installed in randomized blocks, with four treatments and five repetitions. The polymer doses were 0; 4; 8 and 12 g m-1 of furrow (test 1) and 0; 1.4; 2.8 and 4.2 g m-1 of furrow (test 2). Test 1 (dec/2007 to may/2009) was implanted in a Distroferric Red Argisol soil in Presidente Prudente - State of São Paulo (SP), Brazil; and the test 2 (Aug/2008 to Aug/2009) was implanted in a Red Yellow Argisol soil in Lucélia - State of São Paulo (SP), Brazil. In test 2, there were no significant differences for any evaluated parameters. In both tests the polymer doses equal to or less than 4 g m-1 of furrow showed no significant effect on the evaluated parameters. In test 1, the polymer doses of 8 and 12 g m-1 of the conditioning polymer increased the number of tillers in stage II of development and led to the largest amount of straw. The gross income per hectare has positive relation with the polymer doses. The polymer had no significant effect on the sugarcane stems productivity and technological parameters.

Drought resistance of sugar-cane crop for different levels of water availability in the soil

Soil water availability is the main cause of reduced productivity, and the early development period most sensitive to water deficit. This study aimed to evaluate the drought resistance of the varieties of sugar-cane RB867515 and SP81-3250 during the early development using different levels of water deficit on four soil depths. The experiment was conducted at the Department of Biosystems at Escola Superior de Agricultura "Luiz de Queiroz" (ESALQ/USP) in a greenhouse in soil classified as Oxisol, sandy loam texture (Series "Sertãozinho"). Once exhausted the level of available water in the soil, the dry strength of the studied strains are relatively low. Water balance with values less than -13 mm cause a significant decrease in the final population of plants, regardless of the variety, and values below -35 mm, leads to the death of all plants.

Incorporation of water vapour transfer in the JULES Land Surface Model: implications for key soil variables and land surface fluxes

This study focuses on the mechanisms underlying water and heat transfer in upper soil layers, and their effects on soil physical prognostic variables and the individual components of the energy balance. The skill of the JULES (Joint UK Land Environment Simulator) land surface model (LSM) to simulate key soil variables, such as soil moisture content and surface temperature, and fluxes such as evaporation, is investigated. The Richards equation for soil water transfer, as used in most LSMs, was updated by incorporating isothermal and thermal water vapour transfer. The model was tested for three sites representative of semi-arid and temperate arid climates: the Jornada site (New Mexico, USA), Griffith site (Australia) and Audubon site (Arizona, USA). Water vapour flux was found to contribute significantly to the water and heat transfer in the upper soil layers. This was mainly due to isothermal vapour diffusion; thermal vapour flux also played a role at the Jornada site just after rainfall events. Inclusion of water vapour flux had an effect on the diurnal evolution of evaporation, soil moisture content and surface temperature. The incorporation of additional processes, such as water vapour flux among others, into LSMs may improve the coupling between the upper soil layers and the atmosphere, which in turn could increase the reliability of weather and climate predictions.

An easily implemented agro-hydrological procedure with dynamic root simulation for water transfer in the crop-soil system: validation and application

Models for water transfer in the crop-soil system are key components of agro-hydrological models for irrigation, fertilizer and pesticide practices. Many of the hydrological models for water transfer in the crop-soil system are either too approximate due to oversimplified algorithms or employ complex numerical schemes. In this paper we developed a simple and sufficiently accurate algorithm which can be easily adopted in agro-hydrological models for the simulation of water dynamics. We used a dual crop coefficient approach proposed by the FAO for estimating potential evaporation and transpiration, and a dynamic model for calculating relative root length distribution on a daily basis. In a small time step of 0.001 d, we implemented algorithms separately for actual evaporation, root water uptake and soil water content redistribution by decoupling these processes. The Richards equation describing soil water movement was solved using an integration strategy over the soil layers instead of complex numerical schemes. This drastically simplified the procedures of modeling soil water and led to much shorter computer codes. The validity of the proposed model was tested against data from field experiments on two contrasting soils cropped with wheat. Good agreement was achieved between measurement and simulation of soil water content in various depths collected at intervals during crop growth. This indicates that the model is satisfactory in simulating water transfer in the crop-soil system, and therefore can reliably be adopted in agro-hydrological models. Finally we demonstrated how the developed model could be used to study the effect of changes in the environment such as lowering the groundwater table caused by the construction of a motorway on crop transpiration. (c) 2009 Elsevier B.V. All rights reserved.

A universal agro-hydrological model for water and nitrogen cycles in the soil-crop system SMCR_N: critical update and further validation

Agro-hydrological models have widely been used for optimizing resources use and minimizing environmental consequences in agriculture. SMCRN is a recently developed sophisticated model which simulates crop response to nitrogen fertilizer for a wide range of crops, and the associated leaching of nitrate from arable soils. In this paper, we describe the improvements of this model by replacing the existing approximate hydrological cascade algorithm with a new simple and explicit algorithm for the basic soil water flow equation, which not only enhanced the model performance in hydrological simulation, but also was essential to extend the model application to the situations where the capillary flow is important. As a result, the updated SMCRN model could be used for more accurate study of water dynamics in the soil-crop system. The success of the model update was demonstrated by the simulated results that the updated model consistently out-performed the original model in drainage simulations and in predicting time course soil water content in different layers in the soil-wheat system. Tests of the updated SMCRN model against data from 4 field crop experiments showed that crop nitrogen offtakes and soil mineral nitrogen in the top 90 cm were in a good agreement with the measured values, indicating that the model could make more reliable predictions of nitrogen fate in the crop-soil system, and thus provides a useful platform to assess the impacts of nitrogen fertilizer on crop yield and nitrogen leaching from different production systems. (C) 2010 Elsevier B.V. All rights reserved.

Least limiting water range in soil under crop rotations and chiseling

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Water table effects on bean yield and nitrate distribution in the soil profile

In order to evaluate the bean yield under different water table levels as well as the moisture and nitrate distribution in the soil profile, a field experiment was carried out at the experimental area from the College of Agronomic Sciences - UNESP, Botucatu, SP, Brazil. Beans were grown in field lysimeters and subjected to five water table depths:30; 40; 50; 60 and 70 cm. The moisture in the soil profile was gravimetrically determined through samples obtained at 10; 20; 30; 40; 50; 60 and 70cm of depth. The water table depths of 30cm and 40cm showed the highest productivities (3,228.4 kg.ha-1 and 3,422.1 kg.ha-1, respectively), showing no statistical differences between each other. The highest productivity was related to the two most elevated water table levels (30 and 40cm), which provided the highest moisture average values on basis of volume in the soil profile (33.3 e 31%) as well as the consumptive use of water (416 and 396 mm). The nitrate content during the bean cycle at the extraction depth of 60cm has been under the safe drinking limit of 10 mg.1-1 for water table depths of 30; 40; 50 and 60cm, showing the denitrification effectiveness as a way of controlling water table from nitrate pollution. The water table handling allowed the attainment of high bean productivity levels, as well as the reduction of the nitrate level.