Predicting near-surface moisture content of saline soils from near-infrared reflectance spectra with a modified Gaussian model

Near-surface soil moisture is a spatially highly heterogeneous state variable that affects the hydrologic partitioning of precipitation, net ecosystem exchange, and land-atmosphere interactions. In concert with the rapid advancement of large-scale hyperspectral remote sensing technology over the last decade, numerous methods relating reflectance to near-surface soil moisture have been proposed. Although there is evidence that surface reflectance is conjointly controlled by moisture and salt content, little is known about the impact of soil salinity. To study the effects of soil salinity, near infrared to short-wave infrared reflectance spectra were measured in a controlled laboratory environment for samples representing a wide range of salinity levels. A previously proposed surface moisture prediction method based on geometrical attributes of an inverted Gaussian (IG) function fitted to hyperspectral reflectance curves was revisited. Improvements were proposed by first modifying the geometrical attributes of the IG applied to predict near-surface soil moisture and by concurrently considering multiple geometrical attributes of the IG function as input for trained artificial neural networks (ANNs). Although previously applied linear regression models that relate a single geometrical parameter to soil moisture failed to satisfactorily predict independently measured surface soil moisture, considerable improvements in prediction accuracy were achieved with both geometrical attribute modification and ANN simulations.