A Wetness Index Using Terrain-Corrected Surface Temperature and Normalized Difference Vegetation Index Derived from Standard MODIS Products: An Evaluation of Its Use in a Humid Forest-Dominated Region of Eastern Canada

Quazi K Hassan; Charles P-A Bourque; Fan-Rui Meng; Roger M Cox

doi:10.3390/s7102028

A Wetness Index Using Terrain-Corrected Surface Temperature and Normalized Difference Vegetation Index Derived from Standard MODIS Products: An Evaluation of Its Use in a Humid Forest-Dominated Region of Eastern Canada

Sensors (Basel). 2007 Oct 1;7(10):2028-2048. doi: 10.3390/s7102028.

Authors

Quazi K Hassan¹, Charles P-A Bourque², Fan-Rui Meng³, Roger M Cox⁴

Affiliations

¹ Faculty of Forestry and Environmental Management, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 6C2. q.k.hassan@unb.ca.
² Faculty of Forestry and Environmental Management, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 6C2. cbourque@unb.ca.
³ Faculty of Forestry and Environmental Management, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 6C2. fmeng@unb.ca.
⁴ Natural Resources Canada - Canadian Forest Service, Atlantic Forestry Centre, Fredericton, New Brunswick, Canada E3B 5L7. rcox@nrcan.gc.ca.

Abstract

In this paper we develop a method to estimate land-surface water content in amostly forest-dominated (humid) and topographically-varied region of eastern Canada. Theapproach is centered on a temperature-vegetation wetness index (TVWI) that uses standard 8-day MODIS-based image composites of land surface temperature (T_S) and surface reflectanceas primary input. In an attempt to improve estimates of TVWI in high elevation areas, terrain-induced variations in T_S are removed by applying grid, digital elevation model-basedcalculations of vertical atmospheric pressure to calculations of surface potential temperature(θS). Here, θ_S corrects T_S to the temperature value to what it would be at mean sea level (i.e.,~101.3 kPa) in a neutral atmosphere. The vegetation component of the TVWI uses 8-daycomposites of surface reflectance in the calculation of normalized difference vegetation index(NDVI) values. TVWI and corresponding wet and dry edges are based on an interpretation ofscatterplots generated by plotting θ_S as a function of NDVI. A comparison of spatially-averaged field measurements of volumetric soil water content (VSWC) and TVWI for the 2003-2005 period revealed that variation with time to both was similar in magnitudes. Growing season, point mean measurements of VSWC and TVWI were 31.0% and 28.8% for 2003, 28.6% and 29.4% for 2004, and 40.0% and 38.4% for 2005, respectively. An evaluation of the long-term spatial distribution of land-surface wetness generated with the new θ_S-NDVI function and a process-based model of soil water content showed a strong relationship (i.e., r² = 95.7%).

Keywords: MODIS standard products; normalized difference vegetation index; potential temperature; soils; surface; temperature-vegetation wetness index; vegetation; water content..