Application of multivariate geostatistics in delineating management zones within a gravelly vineyard using geo-electrical sensors

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Abstract

In gravelly soils, surveys are generally time-consuming, labour-intensive and costly. This limits the possibility of adopting an appropriate sampling to determine within-field spatial variability. The potential use of electro-magnetic induction scans (EMI) to measure apparent electrical conductivity (EC) and improve the estimate accuracy of sparsely sampled primary variables was assessed in a 5-ha gravelly soil vineyard in Valpolicella, north-eastern Italy. EC was measured using a Geonics EM38DD operating in both horizontal and vertical mode. Geo-electrical investigations were also done in 18 positions with the electrical resistivity tomography (ERT) method to obtain high-resolution images of the soil profile. The spatial variability of soil properties and their relationships with EC in horizontal and vertical mode was estimated using multivariate geostatistical techniques. Spatial dependence between EC and physical soil properties (particle-size distribution) was explored with factorial kriging analysis (FKA) that could isolate and display sources of variation acting at different spatial scales, expressed as regionalised factors, which was followed by fuzzy c-means classification for zoning the vineyard.

There was a generally close relationship between EC and the measured physical properties: EC was negatively correlated with the coarser texture components (gravel and sand) and positively with the finer ones (clay and silt). EC measurements were also consistent with ERT profiles, evidencing the presence of gravelly parent material, with low electrical conductivity, variably distributed in the 3 dimensions and affecting vine rooting depth. FKA isolated two significant regionalised factors which, with an acceptable loss of information, give a concise description of the soil physical variability at the different selected spatial scales. These factors, used in fuzzy c-means classification, allowed the delineation of zones to be managed separately. The results prove that EM38DD could be advantageously used to map soil spatial variability in gravelly soils, even if ground-truth soil samples are obligatory to understand and interpret the EC measurements.

Introduction

Efficient methods for accurately measuring within-field variations in soil properties are crucial for precision viticulture (Bramley, 2005). Sampling at discrete places has been the traditional means of obtaining information about the soil, but soil surveys are generally time-consuming, labour-intensive and costly, especially in the gravelly soils characterising some of the most important terroirs in the world. The large numbers of samples required in gravelly areas in order to attain a good representation of the soil properties (Buchter et al., 1994) limit the possibility of adopting an appropriate sampling intensity to determine the spatial variability within vineyards.

The potential use of ancillary data that can be intensively recorded, such as soil bulk electrical conductivity (EC) measured by electro-magnetic induction (EMI) surveys, has been well examined over the last decade. This is because data are relatively easy and inexpensive to collect (Blackmer et al., 1995, Mulla, 1997). If the sparse and more intensive data are spatially correlated, then the additional information from the ancillary data can be used to improve the estimate precision of the sparsely sampled primary variable. Several scientists have used EMI surveys to characterise soil salinity (Rhoades et al., 1999a) and nutrients (Kaffka et al., 2005), texture (Triantafilis and Lesch, 2005), bulk density related (Rhoades et al., 1999b) and many other properties (Corwin and Lesch, 2005). EMI investigations were also applied to identify morphological features such as depth to boulder clay (Brus et al., 1992) or clay pan (Sudduth et al., 1995). Although EMI is useful for looking at lateral spatial variation, it gives limited information on how conductivity varies with depth because the relationship between a specific earth domain and a particular EC reading is poorly quantified (Pellerin and Wannamaker, 2005). To improve the characterisation of the soil profile, the EMI method can be coupled with electrical resistivity measurements to increase the vertical resolution of subsurface electrical images (Rizzo et al., 2004). Few papers have used different geo-electrical techniques in an integrated way (De Benedetto et al., 2008).

Modelling the relationships between primary soil variables and EC is essential to assess and describe the spatial variability within a vineyard with sufficient precision and then identify management zones. The task is not generally easy, because EC depends on many soil properties over different spatial scales, in a very complex and non-linear way. Moreover, difficulties increase when sampling intensity is reduced by unfavourable soil conditions such those in gravelly soils. Several methods have been proposed to incorporate secondary information. A number of “hybrid” interpolation techniques, combining geostatistical technique of (co)kriging with exhaustive secondary information, have been developed and tested to improve primary variable precision (Goovaerts, 2000, McBratney et al., 2000, Frogbrook and Oliver, 2001). Kriging with external drift (Royle and Berliner, 1999, Wackernagel, 2003) is a non-stationary geostatistical technique, based on a model assumed for the conditional distribution of the primary variable and taking into account the linear relationship between primary and auxiliary data. Hierarchical spatial regression models (Triantafilis and Lesch, 2005) and regression kriging (Hengl et al., 2004) have been used as an alternative to cokriging. Another technique is an approximation of multivariate extension of kriging, known as collocated cokriging, which has proved to be well-suited to merging types of information with different resolution (Castrignanò et al., 2008).

Recent research on precision viticulture has focused on the use of management zones, which are defined as subfield regions within which the effects on the crop of seasonal differences in weather, soil, management, etc. are expected to be more or less uniform (Lark, 1998). For this purpose it is often useful to define classes from a set of multivariate spatial data stored in a geographical information system (GIS). Cluster analysis procedures have been effectively used to divide a field into potential management zones (Stafford et al., 1999). This methodology groups similar individuals into distinct classes called “clusters” in the N-dimensional character space defined by the N properties measured for each individual (Lark, 1998). Existing traditional clustering techniques produce natural groupings of the data only in the attribute space without any reference to geographical position. Spatial classification obtained by clustering does not account for the spatial correlation between observations and takes little account of gradual change, either from one class to another or within any one class. On the contrary, a multivariate geostatistical technique, called factor kriging analysis (FKA) (Castrignanò et al., 2000, Bocchi et al., 2000, Casa and Castrignanò, 2008), treats multivariate indices of spatial variation (regionalised factors) as continua in a joint attribute and geographical space.

The objective of this work was to propose a procedure, based on multivariate geostatistics, to build maps of soil attributes and classify gravelly vineyards in zones to be differently managed.

Section snippets

Study site

The study site is a 5-ha vineyard at San Pietro in Cariano, Valpolicella (north-eastern Italy; 45°31′ N 10°53′ E, 145 m a.s.l.), located in a DOC (controlled denomination of origin) area producing Valpolicella and Amarone wines. The climate is sub-humid, with mean annual rainfall of about 850 mm distributed fairly uniformly throughout the year. From December to February the temperature rarely falls below zero, while maximum temperatures in summer vary from 25 to 30 °C during the day and 18 to 20 °C

Results and discussion

Gravel in the top layer ranged from 192 g kg−1 to 751 g kg−1, with an average of 474 g kg−1 (Table 1). Gravel of 20–100 mm diameter was the most representative fraction (67% of the total gravel), while stones >100 mm in diameter were not found in the majority of samples. Gravel content sharply increased with depth, with values higher than 600 g kg−1 in layers deeper than 40 cm (Table 2). Gravel fraction >100 mm also increased with depth, reaching a content of 117 g kg−1 in the 80–100 cm layer. This fraction

Conclusions

In this work, multivariate geostatistical analysis has allowed the relationship between EC and some soil physical properties to be described and, coupled with fuzzy c-means classification, to delineate potential management zones. The utility of EMI mapping comes from the relationships that often exist between EC and a variety of soil properties. Spatial variation of soil properties could therefore be advantageously inferred, using ancillary data, which are less expensive to obtain. In gravelly

Acknowledgements

The authors are very grateful to Dr. G. Morelli of Geostudi Astier, Livorno, for his valuable work in the realisation of the geo-electric measures. The authors also wish to thank the Pule Vinery for hosting the experiment and Marco Marconi for his support in the field operations.

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