Radiobrightness of prairie soil and grassland during dry‐down simulations

Abstract

We present 60‐day summer dry‐down simulations for prairie grassland based upon a one‐dimensional hydrology and radiobrightness models for northern prairie. Our objective is to examine the effect of scaling upon the interpretability of mixed pixel radiobrightnesses. Even for relatively homogeneous regions like North American prairie, there are significant variations in land cover within the relatively large footprints of satellite microwave radiometers. In this paper we specifically address 1.4‐ and 19‐GHz brightness in the presence of subpixel variability in canopy density. Bare soil and a densecanopy grassland can be viewed as extreme examples of prairie land cover. If land cover within any pixel is viewed as a mixture of these two extremes, then the terrain within that pixel can be modeled as some combination of the hydrology and radiobrightness models for bare soil and dense‐canopy grassland. We examined two combination schemes: (1) a homogeneous combination where the dense‐canopy grasses are simply spread uniformly over the pixel to achieve a desired vegetation column density between that of bare soil and dense‐canopy grass, and (2) a tiled combination where the pixel is divided into a region of bare soil and a region of dense‐canopy grassland. We examined H‐polarized, 53° incidence angle, 19.35‐ and 1.4‐GHz pixel brightnesses and found the 19‐GHz brightness to be significantly greater for homogeneous pixels than for tiled pixels throughout the dry‐down period. For example, a 19‐GHz, 50% homogeneous pixel is 50 K brighter than a 50% tiled pixel at the beginning of the dry‐down and 40 K brighter at 60 days. In contrast, the 1.4‐GHz brightnesses are essentially identical for homogeneous and tiled pixels. Within the constraints of our simulation, subpixel variation in canopy density is a significant factor in the quantitative interpretation of the 19‐GHz brightness of prairie grassland but is not a factor in the interpretation of the 1.4‐GHz brightness.