MONITORING AND MAPPING SOIL ICE/WATER PHASE TRANSITIONS IN ARID REGIONS

Water resources in many arid regions are inadequate to support increasing populations and may become more so due to onrushing climate changes. Water in these regions comes from three main sources: precipitation, meltwater streams from nearby mountains, and shallow and deep aquifers, and it makes sense to monitor each of these using remote sensing for mapping where possible. It is important to track the changing extent of ground ice – seasonal as well as permafrost – as one measure of the effect of climate change.

We are developing a method with which to map near-surface ground ice thermally, using time series of long-wave infrared (8-12 μm) satellite images to detect freeze-up (or thaw) from the signature “zero curtain.” The zero curtain is a well understood phenomenon that occurs when ground temperatures seasonally cool (or warm) through 0°C but stall for a week or more as available energy is used to freeze (or melt) groundwater in Fall (or Spring) instead of changing its temperature. Detecting the zero curtain is not the only way to infer the presence of ground ice: for example, surficial soil moisture has been be detected with side-looking RADAR backscatter because the dielectric constant differs for soil, ice and water. However, the zero curtain is a distinctive signature. Our method appears to be effective as thermal imagers survey Earth’s surface daily (e.g., MODIS) or more frequently (e.g., AVHRR), albeit at low resolution (km scale). We have measured ground temperature profiles in the Atacama Andes (Barrancas Blancas on Ojos de Salado) and in the San Juan Province of the Argentinean Andes for periods of up to 6 years, and obtained daily and 8-day MODIS thermal images with which we can detect and verify the zero curtain. Mapping ice-rich permafrost is an obvious goal, but in the Andes the coarse km-scale spatial resolution has prevented us from detecting the small patches of intermittent permafrost mapped in the field. We have therefore tested the MODIS data over coarsely mapped ice-rich permafrost regions in the Mongolian steppes. Factors that appear to limit the detection of the zero curtain from space include vegetation, the depth to permafrost and the active layer, as well as the tradeoff between spatial and temporal resolution available from current spaceborne sensors.