Observations and Modeling of a Pine Forest Floor at L-Band

Abstract

Abstract-Recent studies of passive L-band observations over forests have shown that the average canopy transmissivity in temperate coniferous and deciduous forests is on the order of 0.4-0.5. Although the canopy would therefore be expected to transmit a reasonable amount of ground emission, the total emission observed above the canopy shows very little variation with varying soil moisture content. Moist litter present on the forest floor is known to obscure the soil emission. Therefore, more knowledge on the L-band radiative properties of litter and understory layers is needed to better understand the emission of the whole forest system. In order to contribute toward this issue, a field experiment was conducted in a pine forest in southwest France. Radiometric observations were done on the canopy and on different configurations of the forest floor, following sequential stripping of each forest floor layer. In combination with a long-term data set of abovecanopy observations, this resulted in emissivity values of bare soil, soil-litter, soil-litter-grass, and soil-litter-grass-canopy configurations for a range of soil and litter moisture values. Calculations involved the use of the Wilheit and L-MEB models. The sensitivity to soil moisture was found to be substantially suppressed by the presence of a grass understory and litter. This corroborates the low correlation between soil moisture and L-band brightness temperature measured above the canopy. Several results of recent modeling and laboratory studies are also confirmed by this paper, which is, to our knowledge, the first to use in situ experimental data in this context.

Impact of Conifer Forest Litter on Microwave Emission at L-Band

“…Over the past few years, some attention has been placed on understanding the microwave signatures of deciduous and coniferous forest floors [8]- [10], [16], [17]. Several field experiments have demonstrated the significance of a forest litter layer in SM retrieval from L-band brightness data [5], [8], [20].…”

“…Laboratory observations of the dielectric properties of litter were made by Kleshchenko et al [21] and Demontoux et al [22]. Grant et al [10] made in situ field observations of forest floor layers in their undisturbed state by sequentially stripping each layer. This was the first investigation to use in situ experimental data in this context.…”

“…In the study by Grant et al [10], observations of the forest floor were made with ground-based instruments located below the canopy. Microwave measurements in this investigation were acquired from above the conifer canopy.…”

Comparison of Two Bare-Soil Reflectivity Models and Validation With L-Band Radiometer Measurements

“…The gradual dielectric transition from air to soil is represented using a semiempirical effective medium approach. As demonstrated in [24]- [26], a similar approach can also be used for modeling the reflectivity of soils covered with sparse vegetation or litter, provided that scattering is not dominant.…”

Evaluation and Improvement of SMOS and SMAP Soil Moisture Products for Soils with High Organic Matter over a Forested Area in Northeast China

“…Second, a depth of approximately 3 cm litter is presented in this study area. As previous experimental studies [41,42] found, the sensitivity of the above-canopy TB to soil moisture may be substantially attenuated by the presence of litter when the soil is wet. Models for correcting the effects of that layer overlaying the soil have been developed, such as by Della Vecchia et al for needle litter [13], by Schwank et al for deciduous leaf litter [11], and lately by Bircher et al for boreal forest organic layer [26].…”