Bare Soil Conditions Research Articles

A methodology for measuring drainage and nitrate leaching in unevenly irrigated vegetable crops

The objective of our study was to establish a methodology to determine drainage and nitrate leaching in unevenly irrigated vegetable crops. It was conducted in a tomato crop (Lycopersicum esculentum Mill) with drip irrigation in the summer of 2000 in the Ebro Valley, Spain. Two soil management techniques and two irrigation treatments were evaluated bare soil (S1) and soil mulched with black plastic (S2), with irrigation calculated according to the Crop Evapotranspiration (ETc) for bare soil (R1) and for mulched soil (R2). Volumetric soil water content (θv) was measured weekly to 1 m depth in six positions transverse to the drip line. Drainage was calculated by applying the water balance equation to the data from: (i) all six positions (method 1) and (ii) to the positions located under the plants and between the rows, respectively (method 2). Soil solution was extracted at 1 m depth with porous ceramic cups and analysed for nitrate. Nitrate leached to 1 m depth was calculated as the product of volume of drainage accumulated weekly and the nitrate concentration of the soil solution. Drainage and nitrate leaching were evaluated for two different crop periods crop establishment and crop growth. Method 2 produced results that were not significantly different from those from method 1. However, method 1 was more accurate and identified more differences between treatments. The greater drainage occurred during the crop establishment period, which also favoured the leaching of nitrates previously stored in the soil profile and later applied as fertilizer before planting. During establishment the crop was unable to use all available nitrate and the quality of groundwater deteriorated. The results suggest that further studies are required to adjust crop coefficients (Kc) to the actual needs of tomato crops grown with drip irrigation under bare soil and plastic mulching conditions.

Dynamic change of CO2 flux over bare soil field and its relationship with remotely sensed surface temperature

This study investigated the potential of thermal remote sensing for estimating ecosystem surface CO2 flux. Ecosystem surface CO2 flux was measured by an eddy covariance method for more than 3 years, in conjunction with thermal and optical remote sensing measurements as well as micrometeorological, soil and plant measurements. The soil was Andisol (Hydric Hapludands), a humic volcanic ash soil, which is the major cultivated soil for upland crops in Japan. The soil surface CO2 flux under bare soil conditions was best correlated with the remotely sensed surface temperature, while air temperature was less well correlated and soil temperature and soil water content were poorly correlated. The relationship was well expressed by an exponential Q 10 function (r 2=0.66, RMSE=0.098). The value of Q 10 and the threshold temperature at which the CO2 flux approached zero were estimated to be 1.47 and 10.0°C, respectively. Results suggested that the soil surface temperature had the dominant effect on the microbial respiration as well as on the physical processes determining the CO2 gas transfer at the soil–atmosphere interface. Remotely sensed surface temperature will provide useful information for investigation of CO2 transfer processes near the soil surface, as well as for quantitative assessment of ecosystem surface CO2 flux.

黒ボク畑地におけるＣＯ２フラックスの動態と熱赤外放射測温による土壌面温度との関係

This study investigated the potential of thermal remote sensing for estimating ecosystem surface CO2 flux. Ecosystem surface CO2 flux was measured by the eddy covariance method for more than three years, in conjunction with thermal and optical remote sensing measurements as well as micrometeorological, soil, and plant measurements. The soil was Andisol (Hydric Hapludands), a humic volcanic ash soil, which is the major cultivated soil for upland crops in Japan. The soil surface CO2 flux under bare soil conditions was best correlated with the remotely sensed surface temperature, while air temperature was less well correlated and soil temperature and soil water content were poorly correlated. The relationship was well expressed by an exponential Q10 function (r2=0.70, RMSE=0.095). The value of Q10 and the threshold temperature at which the CO2 flux approached zero were estimated to be 1.31 and 10.0°C, respectively. Results suggested that the soil surface temperature had the dominant effect on the microbial respiration as well as on the physical processes determining the CO2 gas transfer at the soil-atmosphere interface. Remotely sensed surface temperature will provide useful information for investigation of CO2 transfer processes near the soil surface, as well as for quantitative assessment of ecosystem surface CO2 flux.

Evaluation of DSSAT soil-water balance module under cropped and bare soil conditions

The performance of the soil water balance module (SWBM) in the models of DSSAT v3.5 was evaluated against soil moisture data measured in bare soil and dry bean plots, in Paraná, southern Brazil. Under bare soil, the SWBM showed a low performance to simulate soil moisture profiles due to inadequacies of the method used to calculate unsaturated soil water flux. Improved estimates were achieved by modifying the SWBM with the use of Darcy's equation to simulate soil water flux as a function of soil water potential gradient between consecutive soil layers. When used to simulate water balance for the bean crop, the modified SWBM improved soil moisture estimation but underpredicted crop yield. Root water uptake data indicated that assumptions on the original method limited plant water extraction for the soil in the study area. This was corrected by replacing empirical coefficients with measured values of soil hydraulic conductivity at different depths.

Diurnal Covariation in Soil Heat Flux and Net Radiation

Abstract Diurnal variation in soil heat flux is a key constraint on the amount of energy available for sensible and latent heating of the lower troposphere. Many studies have demonstrated that soil heat flux G is strongly correlated with net radiation Rn. However, methods to parameterize G based on this relationship typically do not account for the dependency of G on soil properties and ignore asymmetry in the diurnal variation of G relative to Rn. In this paper, the diurnal behavior of G as a function of Rn is examined for sparse cover and bare soil conditions, focusing on patterns of diurnal variation as well as on the effects of soil moisture and soil type. To this end, information from field data is combined with simulations from a multilayer, diffusion-based soil model over a range of soil conditions and vegetation densities. The results show that a relatively simple function can be used to capture the first-order diurnal covariation between G and Rn. Within this framework, soil moisture exerts an im...

RADIOMETRIC NORMALIZATION OF MULTI–TEMPORAL IMAGES BASED ON IMAGE SOIL LINES

A radiometric normalization procedure utilizing the soil line concept is developed and tested. The routine, referredto as the soil line transformation (SLT) technique, is developed to provide a system for transforming multitemporal imagesto a common reference scene. The routine involves developing soil lines not only for the bare soil images, but also for imageswith vegetative cover, assuming that some pixels within the entire image would represent bare soil conditions orpseudoinvariant features. The procedure uses the soil lines developed for the imagetobetransformed and the referenceimage to form transforming equations using pixels within a specified interval of both soil lines. In this research, the SLTtechnique is evaluated using multitemporal images of crop development. Digital aerial images of two corn (Zea mays L.)fields are used to test the SLT routine based on expected change in pixel intensity due to crop development. The SLT techniqueis also compared against an alternative histogram matching technique based on the percentage of pixels matching theexpected growth pattern away from the soil line. For two fields over two growing seasons, the SLT technique outperformedhistogram matching, resulting in at least 87% of pixels demonstrating the expected growth pattern between the images.

SOILPSI: a potential-driven three-dimensional soil water redistribution model—description and comparative evaluation

ECOPHYS, an individual-based process model for poplar, requires a three-dimensional soil water redistribution model to simulate soil water dynamics, plant uptake, and root growth. SOILPSI is a potential-driven water redistribution model based on the RHIZOS rhizosphere simulator. It expands on RHIZOS by calculating water flux based on water potential, and has a macropore flow mode to allow rapid drainage of the soil. SOILPSI simulates water flux in three dimensions and accounts for slope. SOILPSI was evaluated by comparing model output to soil moisture data collected under bare soil conditions. AMMI analysis of a date×depth matrix of differences between simulated and observed soil moisture content showed that excluding the two shallowest soil layers resulted in a difference matrix that conformed to an additive model. The grand mean predicted values were within 2% of the observed values, and 50 of 56 predicted values were within 5% of the observed values. Better agreements between simulated and observed soil moisture content were observed deeper in the soil profile and later in the season. Agreement between SOILPSI and field conditions was consistently more accurate than RHIZOS. Improving simulation of evaporative flux at the soil surface would improve simulation accuracy in the upper horizons.

Estimation of roughness length and sensible heat flux from WiFS and NOAA AVHRR data

Development of representative NOAA AVHRR data based Land Surface Processes (LSP) parameters, towards incorporation into GCMs in quasi-dynamic mode, was undertaken by using (i) Normalized Difference Vegetation Index (NDVI) for the computation of roughness length (Z o), and (ii) Land Surface Temperature (T s) along with air density, specific heat of air at constant pressure, air temperature and aerodynamic resistance data to compute Sensible Heat Flux (SHF). NDVI was corrected for sensor degradation and atmospheric errors. Surface temperature (T s) was computed for the pixel by weighted mixing of the temperatures computed for the assumed bare soil (T g) and full canopy (T v) conditions, using respective emissivity values in AVHRR channels 4 and 5 and split window algorithm, based on relative normalized distance of NDVI (for the pixel) from NDVI (soil) and NDVI (full canopy). The results reported in this study compared well with the Z o and SHF computed from tower-based meteorological measurements. This paper also discusses Z o computed with 188 m resolution IRS-1C/WiFS data and validation of results obtained from the AVHRR data with those derived from WiFS data

A microwave land emissivity model

Satellite observations using microwave radiometers operating near the window regions are strongly affected by surface emissivity. Presently, the measurements obtained over land are not directly utilized in numerical weather prediction models because of uncertainties in estimating the emissivity. This study develops a new model to quantify the land emissivity over various surface conditions. For surfaces such as snow, deserts, and vegetation, volumetric scattering was calculated using a two‐stream radiative transfer approximation. The reflection and transmission at the surface‐air interface and lower boundary were derived by modifying the Fresnel equations to account for cross‐polarization and surface roughness effects. Several techniques were utilized to compute the optical parameters for the medium, which is used in the radiative transfer solution. In the case of vegetation, geometrical optics is used because the leaf size is typically larger than the wavelength. For snow and deserts, a dense medium theory was adopted to take into account the coherent scattering of closely spaced particles. The emissivity spectra at frequencies between 4.9 and 94 GHz simulated and compared with the ground‐based radiometer measurements for bare soil, grass land, and snow conditions. It is shown that the features including the spectra, variability, and polarization agree well with the measurements. The simulated global distribution of land surface emissivity is also compared with the satellite retrievals from the Advanced Microwave Sounding Unit (AMSU). It is found that the largest discrepancies primarily occur over high latitudes where the snow properties are complex and least understood.

Common Reed Phragmites australis: Control and Effects Upon Biodiversity in Freshwater Nontidal Wetlands

Abstract Phragmites australis (common reed) has expanded in many wetland habitats. Its ability to exclude other plant species has led to both control and eradication programs. This study examined two control methods—herbicide application or a herbicide‐burning combination—for their efficacy and ability to restore plant biodiversity in non‐tidal wetlands. Two Phragmites‐dominated sites received the herbicide glyphosate. One of these sites was burned following herbicide application. Plant and soil macroinvertebrate abundance and diversity were evaluated pre‐treatment and every year for four years post‐treatment using belt transects. The growth of Phragmites propagules—seeds, rhizomes, and rooted shoots—was examined in the greenhouse and under bare, burned, or vegetated soil conditions. Both control programs greatly reduced Phragmites abundance and increased plant biodiversity. Plant re‐growth was quicker on the herbicide‐burn site, with presumably a more rapid return to wetland function. Re‐growth at both sites depended upon a pre‐existing, diverse soil seed bank. There were no directed changes in soil macroinvertebrate abundance or diversity and they appeared unaffected by changes in the plant community. Phragmites seeds survived only on bare soils, while buried rhizomes survived under all soil conditions. This suggests natural seeding of disturbed soils and inadvertent human planting of rhizomes as likely avenues for Phragmites colonization. Herbicide control, with or without burning, can reduce Phragmites abundance and increase plant biodiversity temporarily. These changes do not necessarily lead to a more diverse animal community. Moreover, unless Phragmites is eradicated and further human disturbance is prohibited, it will likely eventually re‐establish dominance.

RELATING INTERFEROMETRIC SIGNATURE OF REPEAT PASS ERS-1 SAR SIGNALS TO DYNAMIC LAND COVER CHANGES

Radar interferometry opens a very wide field of new applications especially to spaceborne Synthetic Aperture Radar (SAR) data. While repeat-pass SAR interferometry provides the possibility of producing topographic maps and geocoded as well as radiometrically calibrated radar images, interferometric coherence due to non-topographic phase change is a powerful tool to analyse the changes taking place in land covers. The degradation of coherence between two images is due to many effects and is not well understood. This degradation is originated either due to changing characteristics of the scatterers between the two images or due to the changes in permittivity driven by moisture migration. It is important to understand the process that determines the degree of coherence between images in repeat-pass SAR interferometry over different surface types. Considering the importance of this aspect, ERS-1 SAR repeat-pass data has been used to derive interferometric correlations between two images and analysis of these correlations with temporal changes, especially in surface soil moisture conditions, provides valuable thematic information. It is quite interesting to note that the changes in phase (represented by interferometric correlation) and intensity of backscatter are not uniform. While change in permittivity driven by moisture migration from bare soil conditions brings in changes in both phase and intensity, volume scattering as a function of canopy geometry, coupled with changes in permitivity from vegetated surfaces is more sensitive to phase than change in backscatter intensity especially between the two passes of 9 days. This reveals the sensitivity of phase information towards extremely dynamic land cover change in terms of soil moisture variations. A quantitative analysis on the interferometric correlation in combination with the backscatter intensity and the backscatter intensity change between the two passes reveals the crucial information which may be used for landuse/land cover classifications and monitoring and assessment of agricultural droughts in tropical regions.

Flattened Residue Effects on Wind Speed and Sediment Transport

Mulching with flattened crop residues is widely used to protect soils from wind erosion. Several wind tunnel and field experiments have shown decreased protection of some soil covers with increasing wind speed. In some studies, sediment transport was enhanced with flattened residue as compared with the bare soil condition. The purpose of this article was to determine the behavior of wind speed and sediment transport when the soil surface is covered with randomly applied, flattened crop residues. A literature review was conducted to evaluate recent insights in turbulent flow properties and related sediment transport. A conceptual model was then developed to explain decreasing soil protection of a certain residue quantity when the free stream wind speed increases. The main reason is the change in turbulent flow properties of the near‐surface wind when no‐erodible roughness elements are added to an otherwise smooth surface. The average wind speed is reduced by the roughness but the probability distribution of instantaneous wind speed becomes wider and positively skewed. If free stream wind speed increases, at a certain moment the changed turbulence will cause more wind gusts that exceed the threshold wind speed for soil particle movement than would occur over a bare surface. But it is likely that this only happens when soil cover is less than 10%. For higher soil covers, increasing wind speed will also cause a decrease in soil protection, but natural wind speeds are normally not sufficient to cause enhanced sediment transport, because average wind speed is sufficiently reduced.

MODIFICATION OF A SOYBEAN MODEL TO IMPROVE SOIL TEMPERATURE AND EMERGENCE DATE PREDICTION

Recent studies have shown that the CROPGRO-Soybean model does not predict soil temperature very well inIowa. This typically gives errors in predicted emergence date, which translates to errors in timing of development andbiomass accumulation during the remainder of the season. In order to improve the model, an energy balance-based soiltemperature model was integrated into the soybean model and compared to the original soil temperature model, whichwas driven primarily by air temperature. In the new model, temperature at the soil surface is estimated from the basicenergy balance equation at the air-soil interface and the soil temperature profile is calculated using the one-dimensionalheat flow equation. The model was calibrated using five years of bare-soil temperature data measured at an experimentalfarm in Ames, Iowa. Validation of the new model using five additional years of bare-soil temperature data from the samelocation gave slightly better predictions of soil temperature in the top 5 cm (RMSE = 3.0, R 2 = 0.86 for validation years),and responded better to surface perturbations than the original model (RMSE = 3.2, R 2 = 0.80 for validation years).Under bare soil conditions, the new model generally gave lower RMSE and higher R 2 values compared to the old modelat all soil depths. The models were also compared for accuracy in predicting emergence date. Experimental data on soiltemperature and emergence for soybeans planted on weekly intervals over an eight-week period were used to test themodels. The new model gave excellent predictions of emergence, with an average error of 0.6 day for the eight weeklyexperiments. The old model had an average error of one day. Under cool conditions, the new model gave more accuratepredictions of emergence dates. However, under warm periods, both models typically gave the same accuracy, and werewithin about one day of the measured emergence date.

Multifrequency Soil Moisture Inversion from SAR Measurements with the Use of IEM

This study focuses on the development of a consistent methodology for soil-moisture inversion from synthetic aperture radar (SAR) data with the use of the integral equation model (IEM), developed by A. K. Fung and colleagues, without the need to prescribe time-varying land-surface attributes as constraining parameters. Specifically, the dependence of backscatter coefficients obtained from synthetic aperture radar (SAR) on the soil dielectric constant, surface-roughness height, and correlation length was investigated. The IEM was used in conjunction with an inversion model to retrieve soil moisture by using multifrequency and multipolarization data (L-, C-, and X-bands) simultaneously. The results were cross validated with gravimetric observations obtained during the Washita '94 field experiment in the Little Washita Watershed, Oklahoma. The average error in the estimated soil moisture was of the order of 3.4%, which is comparable to that expected due to noise in the SAR data. The retrieval algorithm performed very well for low incidence angles and over bare soil fields, and it deteriorated slightly for vegetated areas and overall for very dry soil conditions. Although the original IEM model was developed for bare soil conditions only, one important result of this study was the fact that the retrieval algorithm performed well for vegetated conditions, as demonstrated by the fact that the convergence ratio varied between 92% (dry conditions) and 98% (wet conditions) of all pixels for all days of the experiment. The sensitivity of soil-moisture estimates to spatial aggregation of remote-sensing data before and after the retrieval also was investigated. The results suggest that there is potential to improve the operational utility of high-resolution SAR data for soil-moisture monitoring by compressing the SAR data (preaggregation) to a spatial resolution at least one order of magnitude above that of measurement.

Quantification of runoff in laboratory-scale chambers

Many of the variables that control transport of agrochemicals and pathogens in the field are difficult to measure because parameters such as slope, soil and plant conditions, and rainfall cannot be adequately controlled in the natural environment. This paper describes the design, construction, operation and performance of a system useful for studying surface transport of agrochemicals and pathogens under controlled slope, rainfall and soil conditions. A turntable is used to support and rotate 4 soil chambers under oscillating dripper units capable of simulating rainfall intensities from 1 to 43 mm h −1. Chambers (35 × 100 × 18 cm i.d.) were constructed with an adjustable height discharge gate to collect runoff and three drains to collect leachate. Height adjustable platforms were constructed to support and elevate the chambers up to 20% slope. The chambers were uniformly packed with 35 to 45 kg of soil (bulk density 1.18 – 1.27 g cm −3) and initially saturated with two low intensity rain events. The coefficient of variation of the rainfall delivery over a range of 5 to 43 mm h −1 averaged 7.5%. An experiment to determine the variability between chambers in runoff amount and uniformity indicated that at least one runoff-equilibration cycle is needed to obtain steady state conditions for conducting runoff transport evaluations. Another experiment conducted to evaluate atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)- s-triazine] runoff under simulated crop-residue covered vs bare soil conditions indicated six times more runoff from bare than crop residue covered soil. The system is capable of precise application of simulated rain, the simultaneous collection of runoff and leachate at slopes up to 20% and can be easily modified to meet a wide range of research parameters.

FACTORS UNDERLYING GRAIN YIELD SPATIAL VARIABILITY IN THREE IRRIGATED WHEAT FIELDS

Three irrigated wheat fields in western Yolo County, California, were studied to determine the magnitude and causes of spatial variability in grain yield. Point samples of soil properties were taken on a regular 61 m grid in each field. False color infrared aerial photographs were taken of each field under bare soil conditions and twice during the cropping season. Yield monitor data were collected at harvest. Two of the three fields showed substantial spatial variability in grain yield. Correlation analysis and CART (classification and regression trees) were used to identify factors underlying grain yield spatial variability. Primary causes of variability were soil texture, weed level, and soil phosphate level. Results of the CART analysis were used to subdivide the two fields with high spatial variability into management zones.

Detection and Use of Three Signatures of Soil-Limited Evaporation

The transition from atmosphere-limited to soil-limited evaporation is manifested in increasing surface temperature and albedo, as well as a steadying of near-surface moisture content. Each of these changes can be inferred to varying degree from ground-based and satellite-based remote sensing measurements. An evaporation model (derived for bare soil conditions) that depends on the time (t d ) between rainfall and this transition was tested on three interstorm periods during the first International Satellite Land Surface Climatology Project field experiment in 1987 and 1989. In general, t d calculated from spatially averaged ground-based measurements of albedo, surface temperature, and surface soil moisture fell within 2 to 4 days of the t d estimated from the best fit of the model to the measured evaporation rates. For the dry down spanning mid-September to mid-October 1987, albedo and daytime temperature differences estimated from geostationary operational environmental satellite (GOES) data yielded similar transition times. The effects of vegetation on model utility and on the ability to determine t d from remote sensing are discussed in the context of densely and sparsely vegetated sites within the FIFE experimental area.

Numerical Simulation of the Sensitivity of Summer Monsoon Circulation and Rainfall over India to Land Surface Processes

The influence of soil moisture and vegetation variation on simulation of monsoon circulation and rainfall is investigated. For this purpose a simple land surface parameterization scheme is incorporated in a three-dimensional regional high resolution nested grid atmospheric model. Based on the land surface parameterization scheme, latent heat and sensible heat fluxes are explicitly estimated over the entire domain of the model. Two sensitivity studies are conducted; one with bare dry soil conditions (no latent heat flux from land surface) and the other with realistic representation of the land surface parameters such as soil moisture, vegetation cover and landuse patterns in the numerical simulation. The sensitivity of main monsoon features such as Somali jet, monsoon trough and tropical easterly jet to land surface processes are discussed. Results suggest the necessity of including a detailed land surface parameterization in the realistic short-range weather numerical predictions. An enhanced short-range prediction of hydrological cycle including precipitation was produced by the model, with land surface processes parameterized. This parameterization appears to simulate all the main circulation features associated with the summer monsoon in a realistic manner.

Stochastic analysis of the relationship between topography and the spatial distribution of soil moisture

This paper deals with the issue of the spatial horizontal variability of soil moisture in the root zone of a shallow soil at the large scale. The problem of water flow in the unsaturated zone is formulated so that topography appears explicitly as a forcing for the movement and redistribution of soil moisture. This formulation emphasizes the role of the lateral redistribution of water that is induced by topography. A stochastic theory is developed to relate the statistical distribution of soil moisture to that of elevation. This approach will ultimately facilitate the use of the readily available data sets describing topography for the purpose of defining the large‐scale distribution of soil moisture. The steady state horizontal distribution of soil moisture under homogeneous bare soil conditions is regulated by three distinct factors: topography, climate, and soil properties. First, topography, forces a distribution of soil moisture that tends to mimic the elevation field at large scales. The other two factors are the vertical divergence of water in response to the climate forcing (evaporation) and the capillary resistance to water movement. The climate forcing tends to smooth the spatial distribution of soil moisture. However, the capillary forces exerted by the soil matrix tend to resist displacement of water and hence exert adverse effects against the topography and climate forcings. The variance of the soil moisture distribution increases with the variance of the elevation field and decreases with the correlation scale of the elevation field and the magnitude of the climate forcing. The impact of capillary forces on the vertical fluxes of water is more significant than their impact on the topographically induced horizontal fluxes, owing to the larger hydraulic gradient in the vertical direction resulting from the disparity in scale between the vertical and horizontal directions.

Variations in leaf temperature, plant internal resistance, photon requirement and transpiration of pigeonpea under bare and mulched soil conditions

ABSTRACT. The paper presents the results of an experiment conducted during 1992 and 1993 crop seasons at the farm of Gujarat Agricultural University, Anand on pigeonpea to determine variations in agro-meteorological characteristics of leaf transpiration leaf temperature plant diffusive resistance and quanta were considered at three levels within the crop canopy in mulched and unmulched fields.
 The anlilysis rewaled that leaf temperature is more in unmulched field where transpiration rates are lower than the mulched field. Stomatal resistance and the quantum requirements nearly match in both the treatments. Stomatal conductance attains large values in morning and evening hours.