Intercepted fog water (fog drip) by forest canopy, rainwater, shallow soil water and groundwater were collected during January 2002 and December 2003 for stable isotopic analysis at a tropical seasonal rain forest site in Xishuangbanna, Southwest China. The object of the study is to determine whether the fog interception has influence on soil water and groundwater. Bottle-funnel collectors were used to determine daily amount of intercepted fog water during fog-only events in the rain forest, and fog drip samples were collected monthly interval. Rainwater samples were collected monthly interval on an event basis, by using a collector consisting of a stainless steel funnel mounted on the top of a 72 m high meteorological tower. Rainwater sample was also collected at each rain event when rainfall exceeded 10 mm at the weather station. Shallow soil samples were obtained randomly within the forest site from cores collected to a depth of 20 cm beneath the canopy on each sampling day. Groundwater was collected monthly from the hand-dug wells located in the forest. The stable hydrogen (δD) and oxygen isotope composition (δ¹⁸O) of fog drip, rainwater, shallow soil water and groundwater was determined from an isotope ratio mass spectrometer. The results indicated that the fog drip water was thought to contain water that has been evaporated and recycled terrestrial meteoric water (evaporation of nearby surface waters and canopy interception). The rain was isotopically more depleted. The fog drip was consistently more enriched than the rain during both the dry season and rainy season. The shallow soil water commonly had composition between the fog drip and rain, suggesting that the shallow soil water is a mixture of the two waters. However, the soil water collected in dry season appeared to contain more fog drip water than that collected in rainy season. The detection of fog drip water in the shallow soil indicates that fog drip may in dry season be an important factor in growth and survivability of understory species and seedlings. Therefore, neglect of the contribution from horizontal precipitation (fog drip) will make calculations of the water balance inaccurate in the forest. The groundwaters in both seasons were characterized by an isotopic composition similar to that of rain. The stable isotopic compositions of groundwater collected during both seasons also showed little seasonal variations, indicating that the groundwater has a constant and homogeneous source. These waters mainly located on the LMWL (Local Meteoric Water Line) and plotted in the rain isotopic field, suggesting that they were not affected by evaporation during infiltration in the recharge area and were recharged by rainwater. The groundwaters also displayed less fluctuation in isotopic compositions than the rain and fog drip, indicating a long residence time for these groundwaters. These variations in isotopic compositions of groundwater are probably the result of local recharge events since groundwater is believed to be recharged only by rainwater. Comparison of the δ18O of groundwater with rainwater indicated that the seasonal response of groundwater lags behind the seasonal change in rainwater because the groundwater has a greater residence time than soil water. For example, the rainwater reached its greatest δ18O value in April 2003 when the δ18O value of groundwater was still near their minimum values for the year, and reached its lowest δ18O value in the September when the groundwater was near their maximum. This isotopic pattern suggests fog drip water does not play a significant role as a source of recharge for the groundwater. This groundwater was thought to be recharged solely by rainwater. Our results also demonstrate the importance of understanding the impacts of climate factors, and have important implications for ecologists and hydrologists interested in fog-inundated ecosystems and the plants which inhabit them.
Further detailed studies which should cover the entire water cycle and water use by plants in this ecosystem are warranted to investigate the importance of the fog drip to the forest. The Xishuangbanna area is suitable for these studies since dense fog almost occurs every day and little rainfall falls during dry season (November to April).