Abstract
The extent, timing, and magnitude of soil moisture in wetlands (the hydropattern) is a primary physical control on biogeochemical processes in desert environments. However, determining playa hydropatterns is challenged by the remoteness of desert basin sites and by the difficulty in determining soil moisture from remotely sensed data at fine spatial and temporal scales (hundreds of meters to kilometers, and hours to days). Therefore, we developed a new, reflectance-based soil moisture index (continuum-removed water index, or CRWI) that can be determined via hyperspectral imaging from drone-borne platforms. We compared its efficacy at remotely determining soil moisture content to existing hyperspectral and multispectral soil moisture indices. CRWI varies linearly with in situ soil moisture content (R2 = 0.89, p < 0.001) and is comparatively insensitive to soil clay content (R2 = 0.4, p = 0.01), soil salinity (R2 = 0.82, p < 0.001), and soil grain size distribution (R2 = 0.67, p < 0.001). CRWI is negatively correlated with clay content, indicating it is not sensitive to hydrated mineral absorption features. CRWI has stronger correlation with surface soil moisture than other hyperspectral and multispectral indices (R2 = 0.69, p < 0.001 for WISOIL at this site). Drone-borne reflectance measurements allow monitoring of soil moisture conditions at the Alvord Desert playa test site over hectare-scale soil plots at measurement cadences of minutes to hours. CRWI measurements can be used to determine surface soil moisture at a range of desert sites to inform management decisions and to better reveal ecosystem processes in water-limited environments.
Highlights
The extent, timing, and magnitude of inundation in terrestrial wetlands, called the hydropattern, is a primary physical control on biogeochemical processes in surface environments ranging from coastal plains to desert playas [1]
CRWI measurements and NDWI have a small, but Remote Sens. 2021, 13,significant, positive linear correlation (R2 = 0.009, p = 0.0001), while CRWI measurements and NDVI have a small, but negative correlation (R2 = 0.006, p = 0.002). These results suggest that drone-borne reflectance spectroscopy can be used. These results suggest that drone-borne reflectance spectroscopy can be used to calculate the continuum-removed water index (CRWI), which in turn, can be used to to calculate the continuum-removed water index (CRWI), which in turn, can be used to measure the time-transgressive distribution of soil moisture in barren ground soils, even measure the time-transgressive distribution of soil moisture in barren ground soils, even those with high concentrations of hydrated clay minerals
For that reason, coupled with the higher correlation coefficient between CWRI and soil moisture content than between WISOIL and soil moisture content (0.89 vs. 0.69), we suggest that CRWI may be a more suitable soil moisture index for measurement of near-surface water content in sandy and/or clay-rich soils similar to those of the Alvord playa
Summary
The extent, timing, and magnitude of inundation in terrestrial wetlands, called the hydropattern, is a primary physical control on biogeochemical processes in surface environments ranging from coastal plains to desert playas [1]. The spatial extent, temporal duration, and concentration of soil moisture are three key components of dryland playa hydropatterns that directly control microbial and floral biodiversity and community functioning in these environments [3,4]. Determining playa hydropattern directly is challenged by the remoteness of desert basin sites and by the difficulty in determining soil moisture content from remotely sensed data at spatial and temporal scales suitable for evaluating rapidly changing processes in small basins. Playa hydropatterns change over meter length scales, within regions hundreds of meters to a few kilometers wide, over daily to sub-daily timescales
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