Fractional Ground Cover Research Articles

Spectral mixture analyses of hyperspectral data acquired using a tethered balloon

Tethered balloon remote sensing platforms can be used to study radiometric issues in terrestrial ecosystems by effectively bridging the spatial gap between measurements made on the ground and those acquired via airplane or satellite. In this study, the Short Wave Aerostat-Mounted Imager (SWAMI) tethered balloon-mounted platform was utilized to evaluate linear and nonlinear spectral mixture analysis (SMA) for a grassland-conifer forest ecotone during the summer of 2003. Hyperspectral measurement of a 74-m diameter ground instantaneous field of view (GIFOV) attained by the SWAMI was studied. Hyperspectral spectra of four common endmembers, bare soil, grass, tree, and shadow, were collected in situ, and images captured via video camera were interpreted into accurate areal ground cover fractions for evaluating the mixture models. The comparison between the SWAMI spectrum and the spectrum derived by combining in situ spectral data with video-derived areal fractions indicated that nonlinear effects occurred in the near infrared (NIR) region, while nonlinear influences were minimal in the visible region. The evaluation of hyperspectral and multispectral mixture models indicated that nonlinear mixture model-derived areal fractions were sensitive to the model input data, while the linear mixture model performed more stably. Areal fractions of bare soil were overestimated in all models due to the increased radiance of bare soil resulting from side scattering of NIR radiation by adjacent grass and trees. Unmixing errors occurred mainly due to multiple scattering as well as close endmember spectral correlation. In addition, though an apparent endmember assemblage could be derived using linear approaches to yield low residual error, the tree and shade endmember fractions calculated using this technique were erroneous and therefore separate treatment of endmembers subject to high amounts of multiple scattering (i.e. shadows and trees) must be done with caution. Including the short wave infrared (SWIR) region in the hyperspectral and multispectral endmember data significantly reduced the Pearson correlation coefficient values among endmember spectra. Therefore, combination of visible, NIR, and SWIR information is likely to further improve the utility of SMA in understanding ecosystem structure and function and may help narrow uncertainties when utilizing remotely sensed data to extrapolate trace glas flux measurements from the canopy scale to the landscape scale.

Evaluation of soil erosion protective cover by crop residues using vegetation indices and spectral mixture analysis of multispectral and hyperspectral data

Crop residues are efficient in reducing erosion and surface water runoff on agricultural soils. Evaluating the crop residue cover fraction and its spatial distribution is important to scientists involved in the modelling of soil erosion and surface runoff, and also to authorities wishing to assess soil conservation adoption by farmers. This study focuses on the evaluation of four remote sensing techniques to estimate the cover fraction of cereal crop residues (i.e., wheat and corn) from multispectral and hyperspectral measurements. These are the Soil Adjusted Corn Residue Index (SACRI), the Crop Residue Index Multiband (CRIM), the Normalized Difference Index (NDI) and the spectral mixture analysis technique (SMA). Field campaigns that were carried out by the FLOODGEN project in Sainte-Angèle-de-Monnoir, Québec, Canada and in the Pays-de-Caux located in the Normandy region of France, allowed us to gather digital photographs, spectra and other measurements to determine the actual ground cover fraction. A linear regression analysis between results derived from Landsat-5 TM simulated field spectra and the actual ground cover fractions showed best results for the CRIM on the Ste-Angèle-de-Monnoir study site ( R 2 = 0.96), and equally good results for the Pays-de-Caux study site ( R 2 = 0.94). Results were not as good when SMA was applied to the same Landsat-5 TM simulated field spectra with R 2 values of 0.70 and 0.68 for both sites, respectively. However, results improved significantly when SMA was applied to the hyperspectral data in which case the R 2 values increased to 0.92 for the Sainte-Angèle-de-Monnoir site and 0.89 for the Pays-de-Caux study site. Results obtained with the NDI and SACRI from both simulated TM and hyperspectral field spectra were not conclusive.

Assessment of soil erosion indicators for maize-based agro-ecosystems in Kenya

Measuring soil loss is costly, must cover a range of field situations, is not standardized, and is season dependent. In addition, use of sparse soil loss data (from other studies) compromises the integrity of many erosion models. Easily assessable soil erosion indicators to monitor the cumulative effect of erosion between tillage/weeding and harvesting called eroding clods, flow surfaces, pre-rills, and rills were surveyed directly after the 1995 rainy season in the Taita Taveta district of Kenya, to assessed the utility of each indicator. Their incidences were modeled using CPA. In the area, 70 maize plots in 11 map units, having considerable variation in altitude, land cover, rainfall, and geomorphology, were surveyed. Soil loss was considered variable between plots due to differences in surface soil, land cover, infrastructure (trash lines, grass strips, and Fanya-juu), crop management, slope, and map unit. The eroding clods indicator proved of little significance because the initial clods cover was unknown; the indicator probably relates better to soil erodibility then to soil loss. Flow surfaces, formed during erosive showers, were less present on fields with a higher ground and canopy cover, if the area of eroding clods was high, and if the topsoil had no loam which reduces chances of sealing; no impact of infrastructure, tillage, and weeding were detected. Fewer pre-rills were present where the fraction of groundcover was high, where Pigeon Peas were not grown (they cause micro-relief and concentrated flows), where weeding ended late (time effect), where more flow surfaces occurred, where Fanya-juu was constructed (less steep slopes), where the top-soil contained little sand (less sediment entrainment), and where maize was intercropped with vegetables (positive canopy cover effects). The model was not map unit specific and had an Adjusted R 2 of 67%. The log-linear relationship indicates that combined positive conditions exponentially reduce the occurrence of pre-rills. The “pre-rill” indicator related best to management-affected site conditions and seems to reflect best the cumulative effects of soil loss over time. Rills were found at 18 sites located in drier areas on sandy–clay soils. The model suggested more rills if the topsoil contains no silt; this makes the soil susceptible to compaction, peptisation when wet, and rill formation.

Soil temperature under forests: a simple model for predicting soil temperature under a range of forest types

A new model (soil temperature under forests—STUF) is described for predicting the seasonal variation of daily average temperature within surface soil layers under a range of forest types from the time of establishment through to harvesting. Daily air temperature for the period of simulation (greater than 1 year) was used to estimate the average annual air temperature, its amplitude, and daily fluctuations. Daily soil temperatures were then predicted from this information using empirical modifications to allow for the effects of canopies of trees, understorey and weeds, mass of the litter layer, and depth at which soil temperature is predicted. Measurements of soil temperature under a range of forest species, ages and management systems were available for 51 sites across southern Australia. The model was parameterised to 31 of these data sets, and the remainder used for validation. The model explained 85% of the variation in observed average daily soil temperature in the latter. Despite the wide range in observed canopy and litter cover, model performance was maintained. Sensitivity analysis indicated that the most important input data required were air temperature, leaf area index and soil depth. For less important inputs such as mass of litter and fraction of ground area covered by weeds or understorey, it may be adequate to use either default values or a scoring procedure for ground cover fractions. The model provides a simple and accurate means to predict daily temperatures in soil in any chosen depth increment. It can be used where accurate daily predictions of average soil temperature are required in a soil profile, for example to generate data to drive a model of organic matter decomposition and mineralisation of nutrients.

Evapotranspiration of a young irrigated olive orchard in southern Spain

This paper assesses the relationship between evapotranspiration (ET) of olive orchards and canopy size, under both dry and wet soil conditions. Measurements of ET of a 4 ha young, uniformly drip irrigated olive cv. ‘Arbequina’ orchard were performed using the eddy covariance and the water balance techniques, for 3 years, while leaf area index (LAI) varied from 0.01 to 1. The two techniques showed excellent agreement. Daily ET varied significantly depending on soil surface wetness. Data with dry soil were used to assess the energy partition as a function of LAI. An analysis for days with and without the irrigation drippers wet spots was used to establish the olive crop coefficient ( K c) under typical “summer” conditions and its relation with LAI and ground cover. The linear model proposed here predicts olive K c of about 0.15 for 5% ground cover, increasing to 0.3 at 25%. The presence of wet spots from drip irrigation increased K c by 37 and 8% for ground cover fractions of 5 and 25%, respectively. Further research is needed to parameterise more sophisticated models of canopy conductance in order to estimate tree transpiration and evaporation from the soil surface independently.

Evapotranspiration of a young irrigated olive orchard in southern Spain

This paper assesses the relationship between evapotranspiration (ET) of olive orchards and canopy size, under both dry and wet soil conditions. Measurements of ET of a 4 ha young, uniformly drip irrigated olive cv. ‘Arbequina’ orchard were performed using the eddy covariance and the water balance techniques, for 3 years, while leaf area index (LAI) varied from 0.01 to 1. The two techniques showed excellent agreement. Daily ET varied significantly depending on soil surface wetness. Data with dry soil were used to assess the energy partition as a function of LAI. An analysis for days with and without the irrigation drippers wet spots was used to establish the olive crop coefficient (Kc) under typical “summer” conditions and its relation with LAI and ground cover. The linear model proposed here predicts olive Kc of about 0.15 for 5% ground cover, increasing to 0.3 at 25%. The presence of wet spots from drip irrigation increased Kc by 37 and 8% for ground cover fractions of 5 and 25%, respectively. Further research is needed to parameterise more sophisticated models of canopy conductance in order to estimate tree transpiration and evaporation from the soil surface independently.

Interception of photosynthetically active radiation and radiation-use efficiency of wheat, field pea and mustard in a semi-arid environment

Fractional ground cover, fractional PAR interception ( f), canopy extinction coefficient ( k) and radiation-use efficiency (RUE) for wheat, pea and mustard were examined in a field experiment conducted over 5 years in a semi-arid environment in Australia. Two crop sequences were compared: fallow–wheat–pea and mustard–wheat–pea. Significant periods of water stress occurred in some years. Complete ground cover was not achieved and f ranged to maxima of 0.77, 0.80 and 0.80 in wheat, pea and mustard, respectively. For wheat, fallow increased f under both average growing season and drought conditions, but pea PAR interception was not influenced by crop sequence. Estimates of k for wheat, pea and mustard were 0.82 (±0.05), 0.76 (±0.03) and 0.68 (±0.12), respectively. Estimated RUE (aboveground biomass) of 1.81 (±0.05), 1.52 (±0.05) and 1.92 (±0.12) g MJ −1 intercepted PAR for wheat, pea and mustard, respectively, was measured over the vegetative phase. Seasonal conditions had minimal impact on k and RUE, and RUE was generally lower than the maxima reported in the literature, but they were similar to others reported in comparable environments and matched predicted unstressed RUE for C 3 plants. This calls into question the application, in simulation models for crops that rarely reach full cover in such arid environments, of water supply or stress factors to RUE values established elsewhere on unstressed crops.

AE—Automation and Emerging Technologies: Novel Imaging Spectroscopy for Grass Sward Characterization

Attempts to improve grassland management may benefit from the use of new sensing techniques, such as imaging spectroscopy. In order to explore the potential of hyperspectral imaging spectroscopy for rapid and objective characterization of grass swards an experimental prototype has been developed. From a height of 1 m, the system detects reflections at high spatial (0·16–1·45 mm 2) and high spectral resolutions (5–17 nm) at wavelengths between 405 and 1659 nm. Detailed information is provided on design, characteristics and test results. Results show that reflection intensity is related to height position in the sward and leaf angle. Grass leaves were recognized accurately. Images of a sports field and a production sward with similar ground cover fractions could be easily distinguished. With the described system, canopy structure can be characterized by ground cover, the distribution of reflection intensity and image texture. Spectral characteristics can be obtained at leaf level.

Environmental controls on ground cover species composition and productivity in a boreal black spruce forest.

Boreal black spruce forests typically have a dense ground cover of bryophytes. The two main bryophyte groups in boreal black spruce forests, feathermoss and Sphagnum, have ecophysiological characteristics that influence the biogeochemical cycles of black spruce forests differently. The objective of this study was to examine the environmental controls of ground cover composition and net primary production (NPP) of feathermoss and Sphagnum in a boreal black spruce forest in central Saskatchewan. The fraction of Sphagnum ground cover was positively correlated to canopy photosynthetically active radiation (PAR) transmittance (r 2=0.48, P=0.03), but the fraction of feathermoss ground cover was negatively correlated to canopy PAR transmittance in plots where Sphagnum was present (r 2=0.87, P<0.0001). Sphagnum presence was inversely correlated (P=0.0001) to water table index, defined as water table depth relative to the peat layer, while feathermoss occurred in a wider range of microenvironments. Average NPP for 1998 was more than three times greater for Sphagnum (77g C m-2 year-1) than feathermoss (24g C m-2 year-1), but the average bryophyte NPP for 1998 was 25g C m-2 year-1 because feathermoss was the dominant ground cover. The large, but differing, peat carbon content of Sphagnum- versus feathermoss-dominated boreal forests and peatlands necessitates the need to accurately quantify fraction ground cover. Additional validation of the empirical models between environmental variables and fraction ground cover of bryophytes is necessary, but the reported relationships offer an approach to model carbon dynamics of bryophytes in boreal forests and peatlands.

Determining transpiration coefficients of Sahelian vegetation barriers

Vegetation barriers along contour lines are an appropriate measure for runoff management. An essential element of their design is minimisation of competition for water between the agricultural crop and the vegetation barrier. This requires the evapotranspiration of the vegetation barrier to be known. In 1998 FAO (the Food and Agricultural Organisation of the United Nations) presented a method to estimate the evapotranspiration of natural vegetation. The method includes a dual crop coefficient approach, distinguishing soil evaporation and canopy transpiration. The transpiration coefficient can be obtained from either the LAI (leaf area index) or the fractional ground cover. This paper reports the results of a study conducted in Burkina Faso in 1998. Transpiration coefficients of three vegetation barriers were obtained by directly measuring transpiration and soil evaporation during the mid-season growing stage. Measured values were compared with those obtained from the FAO estimation method, using both the LAI and the ground cover approach. In most cases the FAO approach overestimated the transpiration coefficient. To overcome overestimation FAO proposes a reduction factor based on the stomatal resistance of vegetation with high stomatal control. This factor is difficult to obtain, however, and complicates the relatively simple method of estimation. Even if this reduction factor is taken into account, an overestimation of transpiration coefficients when using the LAI approach should still be acknowledged.

Characterizing the Spatial Variability of Broadband Albedo in a Semidesert Environment for MODIS Validation

Global data sets on land surface albedo will be one of the core products to be derived from data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS), part of NASA's Earth Observing System. Widespread acceptance of this product by the user communities is dependent, in part, on a comprehensive and rigorous programme of calibration and validation. Since the MODIS albedo product will be produced at a spatial resolution of 1 km, while measurements obtained from field instruments typically relate to areas of only a few tens of meters, this requires an understanding of the spatial variability of land surface albedo and a robust means of scaling up from field to satellite measurements. In this article, we examine these issues for a semidesert environment (the PROVE'97 field site at Jornada, New Mexico, USA). Spatial variations in field measurements of broadband albedo are related to the fractional ground cover of different scene elements (live and senescent vegetation, soil and shadow) via a simple linear mixture model. Information on the fractional ground cover of the scene elements is derived from ground-based hemispherical photography. It is shown that the albedo values predicted by the mixture model are accurate to within 2% of the corresponding measured values. This approach offers considerable potential for the validation of MODIS-derived albedo values through the use of spectral mixture modelling applied to fine spatial resolution satellite sensor images.

On the errors of two estimators of sub-pixel fractional cover when mixing is linear

The authors consider the problem of estimating ground cover at sub-pixel scales from remotely sensed imagery. In particular, they examine two strategies that make use of a set of reference pixels, or training pixels, for which fractional ground cover is already known. These strategies are the so-called classical and inverse methods. The former proceeds by assuming that signals received at a sensor are area-weighted averages of characteristic signals for each ground cover component, estimates those characteristic signals from the training pixels, and predicts fractions for a general pixel to be those that give the best match of the modeled and observed image signals. The latter approach proceeds by direct multivariate regression of ground cover proportions on pixel spectral values. The authors show that when ground cover types are spectrally well separated, and mixing is indeed linear, the difference between the two estimators is much smaller than the prediction error associated with either. This means that it is perfectly acceptable to use standard methods of multivariate regression to perform spectral unmixing. They also show that the inverse estimator can be regarded as a regularized form of the classical estimator and that the supposed optimality of the inverse method may be compromised if the training dataset is not a random subset of the complete set of image pixels.

Influence of Soil Water Stress on Evaporation, Root Absorption, and Internal Water Status of Cotton

Diurnal variations in leaf water potential, diffusion resistance, relative water content, stem diameter, leaf temperature, and energy balance components were measured in cotton (Gossypium hirsutum L. var. Lankart 57) during drought stress under field conditions. A plot of leaf water potential against either relative water content or stem diameter during the 24-hour period yielded a closed hysteresis loop. The relation between cell hydration and evaporation is discussed.Despite low soil water potential in the main root zone, significant plant evaporation rates were maintained. Root absorption rates as a function of soil depth were calculated from water content profiles measured with a neutron probe. The maximal root absorption rate of 3.5 x 10(-3) day(-1) occurred at the 75-centimeter depth, well below the main root zone.Stomatal resistance of individual leaves during the daylight hours remained nearly constant at 2.5 seconds centimeter(-1) even though leaf water potentials approached -30 bars. A growth chamber study indicated stomatal closure occurred at potentials near -16 bars. Possible implications of high soil water stress in relation to stomatal function and growth are discussed. Based on an energy balance method, the actual to potential plant evapotranspiration ratio was 0.43 for the 24-hour period, indicating partial stomatal closure. A surface resistance, r(s), of 4.0 seconds centimeter(-1) was calculated for the incomplete canopy with the use of the energy balance data. Alternatively, a canopy resistance of 1.3 seconds centimeter(-1) was attained from a relationship between leaf area and stomatal resistance of individual leaves. If the soil resistance was assumed to be very large and the canopy resistance was weighted for the fractional ground cover of the crop, the calculated surface resistance was 4.3 seconds centimeter(-1). Under these conditions, the two independent estimates of r(s) were in essential agreement.

Dryland Evaporative Flux in a Subhumid Climate: II. Plant Influences1

AbstractSeasonal variation in three plant factors and in evaporation rates were determined for dryland cotton and grain sorghum in central Texas. Plant factors measured were fractional ground cover, dry matter production, and leaf area index (LAI). When an adequate supply of soil water was available in the rhizosphere, plant factors influenced evaporation rates during most of the first half of the groiwng season until a “threshold” canopy LAI was obtained. A threshold canopy LAI is defined as the minumum LAI necessary to obtain 90% of potential evaporation (Eo) from row crops when the soil evaporation (Es) is small. The threshold LAI was approximately 2.7 for both crops studied. After the threshold canopy LAI was obtained, evaporation was practically independent of plant factors and equal to Eo, as estimated from daily net radiation, until soil water movement to plant roots began to limit evaporation.Es strongly influenced the actual evaporation rates obtained when plant canopy LAIs were below 2.7, To evaluate the influence of plant factors on the resulting plant evaporation (Ep) independent of Es, it was necessary to attempt a rational separation of Es, and Ep. Es was assumed to be equal to the net radiation equivalent of evaporation below the plant canopy when the soil surface was wet. After the soil surface dried sufficiently to limit water movement to the surface, Es was estimated from the measured soil water content near the surface. Ep was estimated using the measured evaporation and Es. The ratio Ep/Eo was found to be related to the LAI according to the equationEp/Eo = −0.21 + 0.70 LAI1/2, 0.1 ≤ LAI ≤ 2.7This nonlinear relation indicates that Ep is larger per unit of leaf area for smaller plants than for larger plants with LAIs approaching 2.7. Sensible heat originating over partially dry surface soil between plant rows was a significant source of the total energy contributing to EpThe physiological stage of growth did not greatly influence the amount of energy used in evaporation as grain sorghum plants approached maturity when an adequate supply of soil water was available.The results imply a possibility for improving water‐use efficiency of dryland crops by using higher plant populations and closer row spacings to decrease Es if the soil water supply and seasonal rainfall is adequate to produce an economic yield.