ASTER User's Guide - NASA

ASTER Users Handbook156 km 3 . The surface layer of Lake Tahoe deepens during the fall and winter. Complete verticalmixing only occurs every few years. Due to its large thermal mass, Lake Tahoe does not freezein winter. There are approximately 63 streams flowing into the lake and only one river flowingout of the lake. Lake Tahoe is renowned for its high water clarity. However, the water clarity hasbeen steadily declining from a maximum secchi depth of 35 m in the sixties to its current valueof ~20 m. Research by University of California (UC), Davis has identified that the decline is inpart due to increased algal growth facilitated by an increase in the amount of nitrogen andphosphorus entering the lake and, in part, due to accumulation of small suspended inorganicparticulates derived from accelerated basin-wide erosion and atmospheric inputs.11.2.3 Field MeasurementsIn order to validate the data from the MODIS and ASTER instruments, the Jet PropulsionLaboratory (JPL) and UC Davis (UCD) are currently maintaining four surface sampling stationson Lake Tahoe (Hook et al. 2002). The four stations (rafts/buoys) are referred to as TR1, TR2,TR3 and TR4 (Figure 20). Each raft/buoy has a single custom-built self-calibrating radiometerfor measuring the skin temperature and several bulk temperature sensors. The radiometer ismounted on a pole approximately 1m above the surface of the water that extends beyond the raft(Figure 21).Figure 21: Raft Measurements.The radiometer is orientated such that it measures the skin temperature of the water directlybeneath it. The radiometer is contained in a single box that is 13 cm wide, 43 cm long, and 23 cm46