Mapping Soil Alkalinity and Salinity in Northern Songnen Plain, China with the HJ-1 Hyperspectral Imager Data and Partial Least Squares Regression  
[PERSON]  
1 Heilongjiang Province Key Laboratory of Geographical Environment Monitoring and Spatial Information Service in Cold Regions, Harbin Normal University, Harbin 150025, China; [EMAIL_ADDRESS]  
2 Department of Science, Qiqihar University, Qiqihar 161006, China; [EMAIL_ADDRESS]  
3 Department of Geography, University of South Carolina, Columbia, SC 29208, USA; [EMAIL_ADDRESS]  
4 Department of Geography, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201, USA; [EMAIL_ADDRESS]  
5 Qiqihar Meteorological Bureau, Qiqihar 161006, China; [EMAIL_ADDRESS]  
5 Correspondence: [EMAIL_ADDRESS]; Tel. : +86-158-2830-65825556  
igneous alkaline rocks that contain abundant sodium aluminosilicates, such as orthoclase, plagioclase, albite, sodalite, and nepheline [6]. These igneous alkaline rocks, which serve as parent materials of soil, are closely related to soil salinization and alkalinization in the Songnen Plain [4; 6]. Inside the plain, the land is flat and the slope is gentle, the streams are slow running, the drainage is blocked, and salts are difficult to leach out [7]. Suchomorphic pattern is shaped by Neetectonic movement and favors soil salinization and alkalinization [6]. So, most salt-affected areas correspond to lacustrine depressions, especially in areas where temporary water logging occurs due to insufficient natural drainage [4]. Soil alkalinity and salinity are affected by the monsoon climate greatly. The annual evaporation is larger than precipitation and the humidity is low in Songnen Plain [4; 8]. Salts are accumulated in soil surface by strong evaporation in dry season, whereas they are transported from soil surface to soil bottom by precipitation in the rainy season. As a result, the potential of hydrogen (pH) and electrical conductivity (EC) of the soil surface might be strongly affected by seasonality. Moreover, freeze-thawing process favors soil salinization and alkalinization in the plain [4; 8; 9]. In addition to geomorphic reasons, the human-related factors, such as overgrazing, fertilizers and unreasonable utilization of land and water accelerate the process of soil salinization and alkalinization [4]. Remote sensing technology obtains substantial soil information in large spatial coverage, which could be superior to traditional field surveys that are expensive and time consuming. For this reason, remote sensing data have been applied to monitor saline-alkali soils for decades. A variety of remote sensing data, such as aerial photographs, video images, infrared thermography, multispectral images and microwave images, have been used to identify and monitor salt-affected soils [1]. Multispectral imagery was used to detect different levels of soil alkalinity and salinity with visual interpretation of false-color composites [10; 11]. However, this analytical interpretation is subjective and time consuming. [PERSON] [12] applied fuzzy logic with Landsat Thematic Mapper (TM) images to detect salt types in a salt-affected area of Bolivia. Although numerous multispectral sensors have been successful in distinguishing severely salt-affected soils from non-affected soils [13], they were reported to have limitations in quantitatively estimating soil properties due to their low spectral resolution and the use of conventional classification methods [14]. Some attempts began to estimate soil properties by using multispectral images together with field spectroradiometer data. [PERSON] et al. [15] constructed an index based on the optimal Advanced Land Imaging (ALI) bands with spectral data to estimate soil EC and delineate slight and moderate saline and sodic soil. [PERSON] et al. [16] used stepwise regression based on the optimal Landsat 8 Operational Land Imager (OLI) bands with spectrometer data to estimate soil pH and EC quantitatively in a typical saline-alkali area in Northeast China.