International Journal of Applied Earth Observation and Geoinformation
An evaluation of supervised classifiers for indirectly detecting salt-affected areas at irrigation scheme level
Introduction
The term salinity is used to describe the processes and impacts of salt and water, while also being a measure of the amount of salt in soil or water (McGhie and Ryan, 2005). Soil salinity is defined as the accumulation of soluble salts in the soil (Al-Khaier, 2003). Salt accumulation can occur naturally (primary salinity) or as a result of human interference (secondary salinity). Human activities such as vegetation clearing, irrigation and landscape reshaping through earth works can, for example, increase the volume of water and salt in the soil and change how they move and where they accumulate (Ghassemi et al., 1995, McGhie and Ryan, 2005, Metternicht and Zinck, 2009). Determining salinity risk goes hand in hand with the understanding of groundwater movement, since this controls the mobility and transfer of salt deposits, among other landscape factors (Spies and Woodgate, 2005).
According to Hillel (2000) soil salinity is a severe environmental hazard that affects the growth of various crops. Although salinity might not be as dramatic or damaging as earthquakes or landslides, it is an environmental hazard that greatly reduces crop yields and agricultural production (Ghassemi et al., 1995, Metternicht and Zinck, 2009, Umali, 1993, Zinck, 2000). Salt accumulation also has additional secondary negative impacts, such as the devaluation of farm properties, land degradation, eutrophication of rivers, damage to infrastructure, increased soil erosion and engineering difficulties (Metternicht and Zinck, 2003).
It is estimated that nearly one billion hectares (ha), which is equivalent to about 7% of the earth’s continental extent, is associated with salt-affected areas (Ghassemi et al., 1995). According to the Food and Agriculture Organization of the United Nations (FAO), of the 230 million ha land available for irrigation, 45 million ha (19.5%) are salt affected (FAO, 2016). Most of these affected areas are the consequence of human activities, in particular, irrigation (Metternicht and Zinck, 2003). This is an alarming statistic and Abbas et al. (2013) estimates that, at global scale, soil salinization is increasing at a rate of up to 2 million ha per annum.
Proactive monitoring of salt accumulation is needed to keep its negative effects under control. Salinity monitoring involves the identification of areas where salts concentrate and the detection of temporal and spatial changes in this occurrence (Zinck, 2000). Remotely sensed data can contribute a great deal to monitoring these processes because of its ability to capture information in both spatial and temporal scales (Abbas et al., 2013). Bastiaanssen et al. (2000) states that remote sensing has the potential to predict soil salinity, perform diagnosis and asses its impact. Compared to solely relying on regular field surveys for monitoring salt accumulation, the synthesis of remote sensing with field surveys can potentially save labour, time and effort (Eldiery et al., 2005, Metternicht, 1996).
Soil salinity can be detected from remotely sensed data either directly or indirectly (Bastiaanssen, 1998, Mougenot et al., 1993). When studying the spectral properties of bare soil directly, indicators of salt accumulation includes white salt crusts, puffy soil, dark greasy surfaces, dehydrated cracks and coarser topsoil (Goodall et al., 2000, McGhie and Ryan, 2005, Metternicht and Zinck, 2003). The main limitation of the direct approach is that farming practices such as tillage and irrigation compromises the spectral properties of soils (Metternicht and Zinck, 2003, Zhang et al., 2011), particularly in highly dynamic irrigation schemes.
Techniques other than the direct approach are considered indirect methods for detecting salt accumulation. The aim of the indirect approach is to map the effect that salt accumulation has on land cover. Most indirect indicators are related to vegetation types and growth. An excess of salt in a plants’ root environment leads to a reduced ability to take up water. The salts reduce the osmotic potential of water and hinder the movement of water from the soil into the root. If saline conditions persist, the small amount of salt that enters a plant along with water, accumulates over time and becomes toxic (Hillel, 2000, McGhie and Ryan, 2005). These effects lead to spotty/uneven growth, plant wilting, blue-green tinge and moisture stress.
Several authors have successfully applied the indirect approach to identify plant stress caused by salt accumulation (Abood et al., 2011, Fernández-Buces et al., 2006, Koshal, 2010, Lenney et al., 1996a, Lobell et al., 2010, Peñuelas et al., 1997, Wiegand et al., 1994, Zhang et al., 2011). In all of these cases vegetation indices (VIs) (e.g. NDVI; EVI and SAVI) were employed. The use of VIs is, however, not without complications. Plant species with a high salt tolerance can lead to ambiguity in the response of vegetation indices (Aldakheel, 2011); poor farming practices and soil preparation can lead to poor vegetation responses which can be misinterpreted as being caused by salinity conditions (Furby et al., 1995); and using VIs in study areas with a high bare ground backscatter/noise can influence some of the vegetation indices negatively (Dehni and Lounis, 2012, Douaoui et al., 2006). Other indirect indicators of salt accumulation include soil types (García Rodríguez et al., 2007) elevation and terrain data (Caccetta et al., 2000, Farifteha et al., 2006, Furby et al., 1995, Jenkin, 1981, McFarlane et al., 2004) and image texture (Metternicht and Zinck, 2003).
Most applications of remote sensing for monitoring salt accumulation have been carried out in areas were salt accumulation occurs on a grand scale. As a result, the majority of the satellite imagery used for the identification of salt accumulation has medium to low resolutions. Landsat (30 m) (Abdelfattah et al., 2009, Aldakheel, 2011, Al-Khaier, 2003, Caccetta et al., 2000, Dehni and Lounis, 2012, Elnaggar and Noller, 2010, Fernández-Buces et al., 2006, Furby et al., 1995, Gao and Liu, 2008, García Rodríguez et al., 2007, Howari, 2003, Lenney et al., 1996a, Mohamed et al., 2011); IRS (20 m) (Abbas and Khan, 2007, Abbas et al., 2013, Dwivedi and Sreenivas, 1998, Dwivedi et al., 2001, Eldiery et al., 2005, Khan et al., 2001, Koshal, 2010) and MODIS (250mn) (Lobell et al., 2010) are most commonly used. However, agricultural activities in irrigation schemes are normally diverse in nature as many different crops are usually planted, often on a rotational basis. This poses a unique challenge for detecting salt accumulation, especially if it occurs in small patches within relatively small fields. Such areas will be difficult to detect using medium to low resolution (e.g. 20–250 m) satellite images. Few studies on the use of very high resolution (VHR) imagery for indirect salt accumulation monitoring exist. Exceptions include Douaoui and El Ghadiri (2015), and the unpublished work of Abood et al. (2011) who used WorldView-2 (WV2) (2 m) imagery; as well as Howari and Goodell (2009), Eldiery et al. (2005), Dwivedi et al. (2009) and Allbed et al. (2014) who used Ikonos (4 m) imagery. Other high resolution imagery used for detecting soil salinity include airborne multispectral (Hick and Russell, 1990, Howari, 2003, Wiegand et al., 1994) and hyperspectral imagery (Dehaan and Taylor, 2003, Dehaan and Taylor, 2002, Dutkiewicz et al., 2009, Farifteh, 2009, Naumann et al., 2009, Schmid et al., 2009). Hyperspectral imagery in particular hold key advantages over standard multispectral imagery due to its rich spectral information (Metternicht and Zinck, 2009). Due to limited coverage and low availability of these images, they were not considered for this study.
Supervised classification is a proven tool for successfully identifying salt accumulation. The maximum likelihood classification is the most commonly applied supervised method for the identification of salt accumulation. Its application stretches across various satellite platforms, namely, Landsat (Abbas et al., 2013, Castaneda and Herrero, 2009, García Rodríguez et al., 2007, Howari, 2003, Iqbal, 2011, Wu et al., 2008), LISS II (Abbas et al., 2013) and Ikonos (Dwivedi et al., 2009, Howari and Goodell, 2009). However, to our knowledge, no research has been done to evaluate the ability of different supervised classifiers for detecting salt-affected areas using VHR imagery in highly complex and dynamic irrigated areas.
The aim of this paper, which forms part of a Water Research Commission (WRC) project (WRC, 2010), is thus to evaluate a range of indirect indicators—derived from SPOT-5 imagery for detecting and classifying salt-affected areas at irrigation scheme level. Several supervised image classification techniques are evaluated, namely maximum likelihood (ML), support vector machines (SVM), nearest neighbour (NN), decision trees (DT), and random forests (RF). The techniques are evaluated on two contrasting irrigation schemes in South Africa (Vaalharts and Breede River) to determine whether any of the classifiers consistently produce superior results and to identify the indirect indicators that can reliably be used to detect salt accumulation. The results are discussed and interpreted in context of finding operational solutions for salt accumulation monitoring at irrigation scheme level.
Section snippets
Study area
The Vaalharts irrigation scheme, situated on the borders of the Northern Cape, North West and the Free State provinces, near the towns of Jan Kempdorp, Hartswater and Pampierstad, was selected as the first study area (Fig. 1). The scheme covers 29181 ha and is one of the largest irrigation schemes in South Africa (Van Rensburg et al., 2012). The area borders two plateaus on the east and west of the Harts River Valley. The valley slopes towards the south with very little topographical changes due
Spectral analysis
Fig. 2 shows the spectral profiles of salt-affected (EC > 4.0 ds/m) and unaffected (EC < 4.0 ds/m) crops in the Vaalharts irrigation scheme. Compared to healthy crops, salt-affected crops generally have higher reflectance in the blue to red region of the electromagnetic spectrum, while reflectance is lower in the near and shortwave infrared regions. This suggests that vegetation in salt-affected areas experience weaker growth (vegetation vigour) than in unaffected areas. This result is in accordance
Conclusions
This study examined a number of indirect indicators for identifying salt-affected areas in cultivated fields at irrigation scheme level. Given that such areas tend to occur in small patches within fields, 2.5 m pansharpened SPOT-5 high resolution satellite imagery was evaluated in two distinctly different South African irrigation schemes (Vaalharts and Breede River).
A series of regression analyses were carried out to evaluate the continuous relationships between the observed in situ salinity
Acknowledgements
This work forms part of a larger project titled “Methodology to monitor the status of water logging and salt affected soils on selected irrigation schemes in South Africa” which was initiated and funded by the Water Research Commission (WRC) of South Africa (contract number K5/1880//4). More information about this project is available in the 2009/2010 WRC Knowledge Review (ISBN 978-1-4312-0004-7) available at www.wrc.org.za. The authors also acknowledge and thank the project leader, Dr Piet
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